SOHO Networking

SOHO NetworkingNetwork Basics

A network is a set of nodes connected together by a shared medium. A node can be a computer or a connecting device. A computer on a network is also called a host. The medium may be referred to a cable or a radio wave.

SOHO (Small Office/Home Office) networks are small-scale networks suitable for small-and-medium enterprises which make up more than 90% of businesses in Hong Kong.

1. Network Size

WAN (Wide Area Network): A computer network covering a wide geographical area, involving a vast array of computers. The internet can be considered as a public WAN.

MAN (Metropolitan Area Network): A large computer network spanning a city. LAN (Local Area Network): A computer network covering a local or small area, such as

home, a room, or a school.

2. Needs of Network

Sharing of data: Folders and files in a computer can be shared out to allow authorized users to access from other computers. In most organizations, files are centrally stored a computer called file server. A file server has a larger storage capacity. Storing data centrally has the following advantages:(1) User can always access the most updated version of data which may be modified by

different users.(2) Data can be treated with more intensive care. For instance, more frequent backup,

automatic file duplication using another hard disk. Sharing of hardware: On a network, hardware, like a printer or CD-ROM driver can be

shared out so that it can be accessed by other computers on the network. This saves the cost in purchasing many hardware.

Sharing of software: Some software is designed to work in a networked environment, for instance, the network version of a DBMS. The DBMS must be installed in a database server which stores the database files.

Ease of communication: On a network, users may communicate with others through e-mails, notice boards, chat rooms, newsgroups, videoconferencing etc.

Internet sharing: Internet sharing means simultaneous use of one Internet connection by more than one computer within an organization.

Remote access / Remote control

3. Network Topology (not too important) Bus topology: All devices are connected to a central cable, called bus or backbone. Bus is

easy to install. It requires the least amount of cable compared with other topologies.

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Star topology: All devices are connected to a central node (switch or hub). All data transfer between computers on the network must pass through the cable concentrator.

Ring topology: All nodes are connected to one another in a shape of a closed loop. Data travels around the ring in one direction, either clockwise or anti-clockwise, until it reaches the destination. Ring topology is commonly used in mainframe systems.

Comparison between above topologies:Topologies Advantages DisadvantagesBus Require least actual wires Difficult to trouble-shoot because the

cable connection extends to all nodes If the backbone is broken, the network

failsStar Easy to maintain

Flexible in adding or removing nodes If a node fails, the network can still

operate

If the central node fails, the entire network fails

Require more cabling The performance would deteriorate if

there are too many nodesRing Can support large number of nodes Failure in any node will cause failure of

the entire network Expensive

4. Networking Medium

Coaxial cableA coaxial cable consists of a central copper wire surrounded by an insulating layer, a wire mesh and an outer plastic jacket.

The wire mesh shields the core from external electromagnetic interference (EMI) due to other electric currents. Shielding also stops the cable from interfering neighboring cables, eliminating cross-talk. The effective length of a coaxial cable can be as great as 50 meter. Coaxial cables are standard medium in cable TV network for broadband Internet accesses with transmission rate up to 10 Mbps.

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Twisted pair cableA twisted pair cable consists of several pairs of copper wires. The twists are designed to cancel the magnetic fields due to both wires so that adjacent cables would not produce EMF that affects one another. This eliminates cross-talk between a set of cables.

There are several grades of UTP, with Category 5-grade (Cat-5) being the most popular. The maximum effective transmission length of a UTP cable is 100 meter. Each Cat-5 UTP consists of four pairs of wiresCat-5 UTP is a standard cable used on computer networks. The Cat-5 UTP has a speed up to 1 Gbps (1000 Mbps).The connectors at both ends of a twisted pair cable that is used on a standard Ethernet network are called RJ-45 connectors. The connectors for phone lines (used in ordinary phone network) are called RJ-11 connectors, which are smaller in size.

Optical fiberOptical fibers use a glass strand that carries light signals instead of electrical signals. In an optical fiber, one end of the glass core is a light source and the other end is a light sensor. Therefore, light can pass in only one direction for any single glass core. In order to avoid light from crossing from one glass core to another, each glass core is surrounded by a layer of material called cladding.

Unlike all other media, optical fiber does not emit electromagnetic waves. Therefore, signals cannot be intercepted easily. This makes optical fibers highly secure. On the other hand, since optical fibers have less attenuation, they can span miles of distance. For this reason, optical fibers are often used across floors in a building or between buildings as network backbone to provide high speed connection.However, Use of optical fiber expensive since it is the most costly to install and maintain.

Radio wavesRadio waves are used in wireless technology which is now the favorite for homes and small offices. But in serious business like banking, wireless technology is hesitated to implement because radio waves can be intercepted easily.

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Advantages Disadvantages Easy and inexpensive to install Flexible in location of workstations Flexible in number of workstations

Comparatively slow in data transmission Data are more easily intercepted

At present, there are three wireless standards: 802.11a, 802.11b and 802.11g.

Standard Max. Speed Frequency Remark802.11a 54 Mbps 5 GHz Most reliable, but incompatible with the more

widely used 802.11b802.11b 11 Mbps 2.4 GHz Most widely used802.11g 54 Mbps 2.4 GHz Backward compatible with 802.11b

5. Basic Network Devices

ServerA server is a computer that performs tasks for other computers and manages the network, e.g. enforcing security, sharing of files, sharing of hardware, etc. Most servers can run using more than one processor to increase the speed of processing.Data must be placed onto the RAM before processed by the CPU. If there is not enough RAM, data not in immediate use will be moved from the RAM to hard disk to make room for new data by a process called swapping. Too many swapping will reduce the performance of the server. Therefore, the more is the RAM, the better would the server perform. Heavy-duty servers may install more than 1 GB of RAM.Domain controller (for logon management), backup server and Network Attached Storage (NAS) server are common kinds of server. NAS server is a specialized file server that has no mouse, keyboard, or monitor.

Uninterruptible Power Supply (UPS)An UPS provides temporary power supply to the computer when the main power fails. It is usually connected between the mains power supply and a server. An UPS includes a battery and built-in charging circuit. During times of good power, the battery is recharged. When the main power fails, the UPS will provide power to the server.

Advantages of using UPS:The UPS allows access to the server in the event of a power failure.Sudden power down may cause damages to data files. The UPS alerts the administrator so that the server can be shut down in an orderly fashion to prevent data loss.Power fluctuation or sudden power down may cause damages to hardware components within a computer. UPS can be used a power buffer to avoid hardware damage.

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WorkstationsA workstation is a computer that allows users to make access to the resources on the network. On a server-based network, workstations are also called clients, which send requests to the servers. A workstation may be physically connected to a network through cables or through radio waves as in the case of a wireless network.All computers, including servers and workstations, must be installed with a NIC to connect to a network. Each NIC has a universal unique identifier, called MAC address, which is a six-byte code burnt permanently to the ROM of the NIC.

RepeaterAs a signal travels along a cable, it would become weaker and weaker. This phenomenon is called attenuation. Therefore, each kind of cable has its own effective transmission length. A repeater takes a weak signal from one side, amplifies the signal, and then sends it out from the other side.

Repeaters do not understand the signals. They merely increase the strength of the signals. Repeaters cannot help direct traffic, nor can they be used to join two networks.

HubA hub provides a common connection for the network cables. It is also called a cable concentrator. Each cable connects the NIC of a computer to a port of the hub. A hub may consist of multiple ports.

The signals entering a hub are echoed to all other ports, that is a phenomenon called broadcasting. Since all the cables would be occupied whenever a host transmits its signals, a hub-based network would easily get overloaded. Note that as each signal carries a receiver address, only the specified node will handle the signal. Others will ignore the broadcast.

SwitchSimilar to a hub, a switch is a cable concentrator that joins the network cables together. Unlike a hub, a switch does not echo signals to all other ports. Instead, it keeps a table of the MAC address for each port and is able to forward the data to the correct port, similar to a bridge. We say that a switch establishes a connecting path dynamically between any two nodes. Therefore, there are less "collisions".

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Also, a switch allows several pairs of nodes to transmit at the same time. As a result, switch-based networks are faster than those hub-based. As switches have become less expensive, more and more networks adopt 100% switched approach.

BridgeA bridge is designed to connect two network segments to form a network within an organization. Unlike a hub which is a dumb device, a bridge records the MAC addresses of the nodes in each segment in a forwarding table and examines the data. Therefore, a bridge can confine the traffic within each segment. For this reason, a bridge can also be used to divide a larger network into smaller segments to improve the efficiency of the network.

RouterA router is a connecting device that routes data from one network to another. It is inserted among two or more networks. Each router keeps a routing table internally and is able to determine the best path for signals. Each port in a router is either connected to a network or another router.

A router is vital equipment that connects a Local Area Network (LAN) to the Internet. Unlike a switch, a router keeps a table of IP address.

Packet Switching Network

Packet switching is a common kind of network transmission model today. When a computer sends data over the network, the data is divided into small packets. Each packet contains the data, as well as the destination, origin, and the sequence information used to reassemble the data at the destination. During transmission, each packet will be directed by a series of routers across the Internet. Finally, when the packets reach the receiver, they are recombined to form the original message.

1. Packet Structure

Structure of a packet:

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Receiver addressSender addressSequencing InformationTime-to-live (TTL)

Error control

The receiver address is to make sure that the packet can be sent to the recipient.

The sender address is necessary to acknowledge receipt of packet. It is useful if an error occurs in transmission so that re-transmission is needed.

Since packets may reach the recipient out of order, the sequencing information helps to re-assemble the packets into the correct order.

TTL information is to avoid any undelivered packet moving on a network forever. When a packet is created, TTL is initialized to 64, say. This value would be decreased by 1 when the packet reaches a router. If the value of TTL falls to zero, the packet would be discarded.

Error-control information ensures that errors in transmission can be detected, and even recovered. If the errors cannot be recovered, the recipient will ask for re-transmission.

2. Benefits of Packet Switching

Packet switching can avoid monopolization of transmission line. If a very large un-split unit of data is transmitting continuously along a transmission line, it would occupy all the available bandwidth so that other computers cannot communicate. By splitting the data, packets from different nodes can be interleaved and the transmission line would be equally shared.

Also, since packets from the same sender may use different paths, the loading of the transmission line can be evenly distributed.

Since there is always more than one path, the transmission will not complete fail due to the failure of part of the nodes.

If an error occurs, re-transmitting a very large unit of data is inefficient. By splitting the data into smaller packets, only those with errors need re-transmission. Then, error recovery is much easier.

3. Transport Control Protocol

TCP/IP is not a single protocol. Instead, it is a protocol suite that consists of a set of protocols. One of the two major components is Transport Control Protocol (TCP). TCP is responsible for breaking a message into packets and re-assembling them at the destination. It also re-sends packets which have errors during transmission.

4. Internet Protocol

Another major component is Internet Protocol (IP). Internet Protocol handles the IP addresses used in packets through the network. In the sender computer, IP receives data from the TCP, adds sequencing information, addresses and other control information. The reverse process will be done by the IP of the receiver computer.

5. Pros and Cons of TCP/IP

Pros: As TCP/IP is the actual implementation of the concept of packet switching, the advantages of TCP/IP are similar to those of packet switching. Besides, TCP/IP also allows computers of different hardware and software to communicate as long as the operating systems supporting TCP/IP.

Cons: The major disadvantage of TCP/IP is that it is not designed for transmitting real-time signals, like live voice or video. Since packets may arrive out of sequence and it is impossible to re-transmit erratic packets of real-time signals, the quality of conversation may be reduced.

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IP Address

On a TCP/IP network, every device has a unique identifier called IP address, in additional to the MAC address. IP address is logical and dynamic while MAC address is physical and fixed.

On a LAN, each device is assigned with a unique internal IP address. There are two ways to assign IP address: Manually assigning static IP addresses to devices. Making use of Dynamic Host Configuration Protocol (DHCP) which assigns dynamic IP

addresses automatically. This saves a lot of administrative efforts.

1. Structure of an IP Address The IP address consists of 4 bytes, each byte separated by a period (called dot). The

number in each part is between 0 and 255The total possible combination is 256 256 256 256But, a considerable portion is reserved for special purposes.

An IP address is logically divided into two parts: the prefix Net ID and the suffix Host ID.

The Net ID identifies a network, while the Host ID identifies a computer on that network. For instance, since the subscribers of an ISP belong to the IPS's network, their global IP addresses use the same Net ID. Therefore, for any given global IP address, one can tell the ISP and, therefore, its location.Such design can make routing efficient. If the IP addresses of all computers on the network are stored, this table will be very large. Instead, a router only stores the addresses of some major networks, such as the ISP's network. Subsequent routing is done by these networks.Also, this design allows a large network to be broken down into smaller ones - a process known as subnetting. This makes network management easier. For instance, a school can be subnetted to form a network accessible by students and another for administration.

2. Classification of Global IP Addresses Class A

In a class A network, the leading byte specifies the network identifier, and the remaining 3 bytes specify a particular workstation identifier.

The leftmost bit of the Net ID must be "0", therefore, a class A Net ID values range between 0.0.0.0 and 127.0.0.0However, 0.0.0.0 and 127.0.0.0 are reserved for some special purposes, so that there are only 126 possible Net ID’s that can be used. Within the possible values, 10.0.0.0 (10/8 prefix) is especially used for private networks.Class A addresses are suitable but not restricted in large size network.

Class BIn a class B network, the leading 2 bytes specify the network identifier, and the remaining 2 bytes specify a particular workstation identifier.

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The leftmost two bits of the Net ID must be "10", therefore, a class B Net ID values range between 128.0.0.0 and 191.255.0.0Theoretically there are 214 = 16,384 possible Net ID’s that can be used. Within the possible values, the range from 172.16.0.0 to 172.31.255.255 (172.16/12 prefix) are especially used for private networks.Class B addresses are suitable for medium size network.

Class CIn a class C network, the leading 3 bytes specify the network identifier, and the remaining byte specify a particular workstation identifier.

The leftmost three bits of the Net ID must be "110", therefore, a class C Net ID values range between 192.0.0.0 and 223.255.0.0Theoretically there are 221 = 2,097,152 possible Net ID’s that can be used. Within the possible values, the range from 192.168.0.0 to 192.168.255.255 (192.168/16 prefix) are especially used for private networks.Class C addresses are suitable for small size network.

Classless IP addressesThe current IP addresses are called classless addresses, which started from 1993Before then, IP addresses were classful. The size of those networks was not specified by the subnet mask. Rather, networks were classified into above three pre-defined sizes.The major problem of classful addressing is the inefficient use of the IP address space. For instance, there are 127 Class A networks, but it is unlikely that a site contains 16 millions of computers. Rather, only a small percentage of the available Host ID is used.The problem of the classful address is due to its fixed boundary between Net ID and Host ID. The problem is solved by classless addressing that uses subnet mask to divide an IP address at arbitrary position.For example, 192.168.57.32/26 gives the meaning that Net ID has 26 bits while the Host ID has 32 – 26 = 6 bits, that has possible range between 000000 and 111111Thus, the network can identify and install at most 26 – 2 (since all 0’s and all 1’s are reserved for special purposes) = 62 hosts.

3. Subnet MaskIn order to specify the boundary between Net ID and Host ID by the network devices, additional information called subnet mask is stored with each IP address. A subnet mask determines which part of an IP address denotes the network and which part denotes the host in an efficient way.Like an IP address, a subnet mask is 32 bits long. But, each subnet mask is composed of contiguous l's followed by contiguous 0's.

How does a computer work out the Net ID from an IP address and its subnet mask?

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Consider an IP address 202.123.150.17 with a subnet mask 255.255.240.0The binary notation of the two numbers are 11001010 01111011 10010110 00010001 and 11111111 11111111 11110000 00000000To find out the NetID, the computer will carry out a logical AND operation as follows:

11001010 01111011 10010110 00010001 (IP address)AND 11111111 11111111 11110000 00000000 (Subnet mask)Result 11001010 01111011 10010000 00000000 (NetID)

In the above example, the number of zero-bits in subnet mask is 12In other words, the maximum size of the above network is 212 – 2 = 4094 hosts.

4. Special IP addressesSome IP addresses are reserved and cannot be assigned to a host: Network Address

The address of a network is the NetID followed by a number of 0's that make up the total length of 32 bits. For instance, the IP address 128.10.0.0/16 is used to denote a network and not to the host computers attached to that network.

Broadcasting AddressSometimes, it is necessary to broadcast a packet to all hosts on another network. This process is called directed broadcasting. The IP address used in directed broadcasting is made up of the target Net ID followed by a number of l's that make up a total length of 32 bits. For instance, when the packet with address 128.10.255.255 reaches the network 128.10.0.0/16, all the hosts there will receive the packet.On the other hand, a TCP/IP network allows broadcasting to all the hosts within the network, a process called limited broadcasting. This is used during the startup of a computer which does not yet know the network NetID. The IP address for limited broadcasting is all 1's, i.e255.255.255.255.

This Computer AddressAs a computer boots, it is not assigned an IP address. But, in order to communicate, all packets must contain the addresses of the sender and the recipient, even at the initial connection. To solve this problem, IP reserves the address that consists of all zero, i.e0.0.0.0, to mean this computer.

Loopback AddressLoopback address is used by programmers to test the communication capability of a network application. Rather using two computers, a programmer can use loopback address to send packets from one program to another program within the same computer. During loopback testing, no packets ever leave a computer.

Ethernet (further information for students, out of syllabus)

Currently, Ethernet is the most popular LAN technology. Alternative solutions, such as Wi-Fi (wireless fidelity, also called wireless LAN or WLAN), powerline networking and phoneline networking, are getting more and more popular, especially in home networking.

Several versions of Ethernet have been developed. The name of each version consists of three fields in the following forms:

nBase-x or nBroad-x

The first field n can be 10, 100 or 1000 which represents the speed in Mbps.

The second field (Base or Broad) represents the mode of signal transmission: Base for baseband transmission, Broad for broadband transmission. In baseband transmission, only a small portion of the frequency spectrum is used and only one signal can be transmitted at a time. The advantages of baseband are that signals can be transmitted in digital form - modems are not necessary. On the other hand, broadband transmission uses a wide spectrum of frequency. The signals are analog. Therefore, modems are required at both ends. The advantage of broadband is that it allows multiple messages to transmit at a time to achieve high bandwidth. Broadband is mainly used in WAN, for instance, Internet access.

The last field x mainly represents the transmission medium: T for twisted pair cable, F for optical

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fiber. But, for early version, the last field is related to the maximum length. For example, 10Base-T represents 10 Mbps baseband using twisted pair cable; 100Base-F represents 100 Mbps baseband using optical fiber.

Do not confuse Ethernet with TCP/IP or topology. Ethernet is a standard that specifies the technology (mainly networking hardware) used, while TCP/IP is a protocol standard and topology is a connecting configuration. The distinguishing feature of Ethernet is that it uses the CSMA/CD access method to handle simultaneous demands on a shared medium. Any computer ready to send data must listen to the channel to ensure that it is free. When a collision occurs, it would wait a random period of time and then re-send the data.

Network Operating SystemServers are installed with a sophisticated Network Operating System (NOS) which manages the network and entertains requests from clients. Common NOS are: UNIX, Linux, Windows Server 2003 and Windows Server 2007Some of the features of Network Operating System are: Provide basic operating system features such as support for protocols, automatic hardware

detection and support multi-processing of applications. Security features such as authentication, authorization, logon restrictions and access control. Provide name and directory services. Provide file, print, web services, back-up and replication services. User management and support for logon and logoff, remote access; system management,

administration and auditing tools with graphic interfaces. Provide fault-tolerant system. Provides the ability to share files, devices and hardware throughout the network. Enables resources sharing over the network.

The operating system used in a workstation is quite different from that used in a server. A workstation may use a desktop operating system as designed for standalone personal computer, together with some networking software. The networking software enables a workstation to communicate with the server and other computers and handles the task of sending and receiving data over the network. It includes drivers for the NIC and programs for protocols. Common NOS are: Linux, Windows NT, Windows 2000, Windows XP and Windows Vista.

The operating system for workstations may come from different vendors, i.e. it is not necessary for all computers to use the same as in order .to communicate.

Home Network Applications

A home network is similar to the network of your school or a business, except that it comprises smaller amount of nodes and less sophisticated software and connecting devices.

1. Sharing an Internet Connection

Since Internet sharing requires a router installed and computers connected to form a network, the popularity of Internet sharing implies that home networks has become a trend.

2. Smart Home

The networking technology and the technology of controlling home appliances have been combined to enable people to manage their homes from a computer, even remotely. This is known as home automation system or smart home.

Some of the things you can do with a smart home are: Automatic light control

The system may turn lights on and off at predefined times. Control home appliances remotely

For instance, you can control the heating/cooling system, curtains and windows through a panel inside the house or a Web browser.

Control access to a homeThe system may allow access to a home through electronic key pads, smart cards or biometric authentication system, such as fingerprint scanner.

Monitor home remotely

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With a Web cam connected to the home network, you can monitor your home over the Internet while you are not at home.

Enhance home securityThe system can be connected to sensors that monitor movement (get alerts when a door opens that should not be opening) and sound (such as glass breaking).

Automatic alertThe system can alert you changes in environment such as temperature, brightness and moisture (alerting you to such things as flooding).

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Network Models

There are two fundamental models of network: Peer-to-Peer and Client-Server.

1. Peer-to-Peer

In a peer-to-peer network, computers communicate with each other as equals. Each computer is responsible for sharing its own resources available to other computers. setting up and maintaining its own security for those resources. accessing the network resources it needs.

Pros and cons of peer-to-peer model:

Advantages:(1) Less expensive hardware can be used.(2) Easy to set up.(3) Setting up a server is not required.(4) Expensive operating system is not required.(5) Smaller chance of failure of the entire network.

Disadvantages:(1) Network is insecure.(2) Difficult to backup all data.(3) Difficult to maintain version control.(4) Since each workstation maintains its own set of user accounts, users need to remember

the username and password for every workstation.

2. Client-Server

In a client-server network, all network resources are centrally managed by one or more servers.

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ServersServers are responsible for making available, managing and enforcing the security of shared resources. Sometimes, a server may host one or more services. According to the services they provide, servers can be classified into the following:(1) Authentication server (Domain controller) -- maintains a security database which

keeps all the users’ account and controls their access level. The server authenticates users when they login the system.

(2) File server -- A file server provides a centralized location for storing files. Shared files are usually stored in a shared disk drive. Security control includes who can make access to each directory and what kinds of access.

(3) Print server -- A print server allows the shared printer to be accessed by authorized users and manages the printing sequence of all the print jobs.

(4) Web server -- A Web server stores the contents of web pages created by authorized users. It entertains requests from other users on the Internet by delivering the web pages. Each web server must have a unique public IP address.

(5) Proxy server -- A proxy server improves the efficiency of Internet access by temporarily storing copies of web pages visited by previous users of the same LAN. It also can be used to filter web pages to prevent users from accessing some certain sites.

(6) Mail server -- A mail server provides e-mail services to users. Each user has two mailboxes that store the incoming emails and copies of the outgoing emails.

(7) Application server -- An application server provides remote computing for clients. A typical example is a database server.

ClientsA client, also called workstation, is a computer that requests services from a server. A client uses a desktop as with appropriate networking software, which allows the client to access the resources from the server.

Pros and cons of client-server model:

Advantages:(1) Very secure.(2) Better workstation performance because dedicated servers handle all the resource sharing

tasks.(3) Centralized backup.(4) More reliable because many servers have built-in redundancy solutions for hardware

failure.(5) Easier user management.(6) User can log in from any workstation using the same user name and password.

Disadvantages:(1) Require professional administration.(2) More hardware-intensive because a dedicate server is an expensive computer with lots of

memory, disk space and hot swap components.(3) Expensive network operating system

Network Performance

The performance of a network refers to the efficiency in data transmission and the amount of tasks it can handle without being overloaded. Network performance can be greatly reduced if a bottleneck occurs.

A bottleneck is part of a network where the capacity cannot meet the requirement of data flow. Bottlenecks may be caused by a slow server, a cable of low bandwidth, poor network design, or adoption of a poor technology etc.

1. Server Performance

The performance of a server is mainly determined by the efficiency of its hardware and the amount of services it hosts. Efficiency of Hardware

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The efficiency of a server can be raised by improving the following:(1) number of CPUs and their clock speeds(2) memory size and speed(3) disk drive capacity and speed(4) cache memory size

Amount of ServicesIt is quite often for a server to host more than one service. For instance, a file server may also be an authentication server that handles user login and manages security. It is important not to overload a server, causing bottleneck.To increase the performance of a server, besides improving the hardware as mentioned above, one may consider reducing the services hosted by the server. This implies increasing the number of servers.

2. Network Traffic Bandwidth

Bandwidth indicates the maximum rate of data transfer of a network channel. It is determined by the NICs, cable concentrator and type of cable. For instance, a 10 Mbps Ethernet NIC limits the channel capacity to 10 Mbps, even though other devices can operate at higher speeds.To improve the bandwidth of an Ethernet, a switch should be used rather than a hub. Also, Cat-5 cables should be used in place of Cat-3 cables.

TrafficNetwork traffic represents the amount of data flow in each second. Clearly, network traffic is higher if there are more users. But, high network traffic may also be caused by a hub which echoes the signals to all other ports, creating collisions which leads to re-transmission.

ThroughputNetwork throughput means the amount of useful data flow in each second. High network traffic does not necessarily means high throughput, as in the case of a hub-based network.

The backbone of a network can be considered as the circuit (including a switch and cables) that links up smaller network segments. The traffic along the backbone is the most busy.

As a rule of thumb, the bandwidth of the backbone should be 10 times higher than the segments connected to it. For example, if the bandwidth of the segment is 10 Mbps, the bandwidth of the backbone should be 100 Mbps.

Switches instead of hubs should be used in backbone. Fiber-optic cable is the best choice if budget allows.

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Network Planning1. Need Analysis

Before building up a network, a need analysis should be conducted in order to collect enough information for designing the network. The needs should then be translated to a set of requirements and specifications.A simple need analysis may involve the following questions: How many computers will be in the network? What kind of services and applications (e.g. network printing, etc.) will run on the

network? What kind of files will be stored in the server? How much storage space is required? How much bandwidth is required? (e.g. video streaming may require higher bandwidth) What is the budget? Will the plan cater for future expansion? What is the coverage of the network? (a room or a floor, or a building)

2. Network DesignDesign of the network include: The choice of network media (e.g. CAT5 UTP cable or optical fiber) The choice of network devices (e.g. hub, switch, wireless access point, router) Location of the backbone Number of nodes and their locations Number of workstations and servers (and types of server) The need of Internet access and firewall The choice of network operating system

3. Network LayoutA network diagram can be used to illustrate the layout of a network. Example 1 (Home networking)

This example assumes that the family has two computers and broadband Internet access. Since a wireless LAN is preferred, a wireless router is chosen. The initial configuration of the router requires connecting a computer physically to one of its RJ45 ports. Therefore, a NIC must be installed in one of the computers. The peer-to-peer model is adopted.Equipment QuantityWorkstations 2Wireless router (with firewall capability) 1Wireless LAN card 1NIC 1

Example 2 (A small office with 15 users)Assume that the office occupies one floor with 15 users sharing a broadband Internet access. Two servers will be installed, with redundant services, like domain controller. Out of the 15 users, five of them require faster access to the network resources.Equipment QuantityServers 2Workstations with NICs 15Router 1Firewall 1

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8-port 10 Mbps switches 216-port 100 Mbps switches 1

Example 3 (A medium-sized office in two floors)Assume that the office occupies two floors, each having 10 workstations. Two servers will be installed in the upper floor, with redundant services for the domain controller. An optical fiber and two 16-port switches are used to form the backbone that connects the two floors. All the workstations in the lower floor can be connected to one of the switches. An 8-port switch is used to connect the remaining computers in the upper floor.Equipment QuantityServers 2Workstations with NICs 20Router 1Firewall 18-port 10 Mbps switches 116-port 100 Mbps switches 2

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Network Monitoring

The purpose of network monitoring is to ensure that the network is healthy enough to function continuously. Most network operating systems provide tools for monitoring a network. The aspects that worth monitoring include server performance, network statistics, hard disk performance, memory usage and CPU utilization.

1. Server Performance

The performance of a server is indicated by: Number of queued commands

Queued commands are commands awaiting execution. If the number of queued commands is too high, it may indicate that there is a bottleneck in the server. Additional server and using multiple processors may be necessary.

Quality of server sessionsA server session records the connection between the server and a client. It shows how the connection is broken: Is it a normal logoff, an error or server timeout? If the connection is broken due to server timeout, it may indicate that the server is overloaded.

2. Network Statistics

Network statistics can tell the overall performance of a network. Error rate

Error rate refers to the number of collisions per second. Re-transmission will take place after a collision. But, the waiting time before re-transmission will increase greatly with additional failed attempts, and this would greatly reduce the overall network performance.

Network utilizationNetwork utilization measures the actual bits transmitted per second compared with the maximum bandwidth. A network is known to be saturated if the utilization is close to 100%.

3. Hard Disk Performance

The performance of hard disk is indicated by: Disk space remaining Throughput of the hard disk

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The throughput of a hard disk is the amount of data transferred per second. It indicates the speed at which requests are serviced.

Number of queued requestsThe number of queued requests indicates how busy a hard disk is. Too many queue requests means that the hard disk is overloaded.

4. Memory Usage

Memory usage is indicated by the rate of memory swapping.Swapping occurs when data not in use are moved from the main memory onto the hard disk to make room for new data. If there are too many swapping, the system's response will slow down and even come to a halt.

5. CPU Utilization

The utilization of a CPU is the average percentage of CPU activity. If the utilization of a server is constantly higher than 90%, the machine is most likely overloaded by requests from clients. However, this may also be caused by low memory, improper configuration of devices etc.

Troubleshooting

Troubleshooting means to diagnose and fix problems. Common problems on a network may be response too slow unable to detect the network from a workstation unable to login unable to use resources of the network, like files, printers etc. unable to connect to the Internet unable to execute certain programs

Steps of troubleshooting:

1. Establishing the symptoms

Establish the symptoms of the problem and get problem information from the affected user or event logs.

2. Identifying the affected area

Determine the scope of affected area. The problem may involve a single workstation, a network segment or the entire network.

3. Establishing what has changed

If a network that functions properly suddenly fails, it is often that something has changed. Ask yourself: Has any new software or hardware been added? Have positions of hardware on the network been changed?

The answers to these questions will give you cue to establish the cause. For instance, if you know that a workstation has been moved to a new location on the network, you should suspect a cabling related issue.

4. Selecting the probable cause

The focus of this stage is to select the most probable cause. For instance, if a user is unable to log onto the network, the following are probable causes: invalid logon faulty cable bad network card the account has been locked

Selecting the probable cause requires isolating the problem. There are tools that help you confirm the cause and locate the affected area.

The following techniques may be used in isolating the problem: removing cables connected to the hub or switch one by one shutting down services of a server one after the other pulling affected system out of the network and check whether it can still work as a

standalone machine

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Rebooting the network (powering off and then on connecting devices and server) in order to isolate a problem should be the last resort.

The tools in troubleshooting a network include: A cable tester is a device used in checking whether a cable is working properly or not. An event viewer is a utility that stores security, system and application activities in log

files. The utility records security threats, like failed attempts to log on or access data. It also records activities carried out by applications and the operating system, providing information about how well the server is running and whether all hardware are working properly.

5. Implementing the solution

After selecting the probable cause, the next step is to plan the solution. Sometimes, the process of isolating problem may resume the operation partially. The affected area could then be corrected by replacing with a new hardware or by reconfiguring the server.

If the solution requires powering down a server to replace hardware, you will need to plan the time to shut down the system making backups of the system.

It is important that solution should be implemented with only one fix at a time. Trying too many solutions at once will make it very difficult to determine exactly what has corrected the problem and why.

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6. Testing the results

After implementing the solution, it is necessary to verify that the solution really addressed the problem. Any changes to the network should be tested and verified to be functional.

7. Documenting the solution

The purpose of documentation is to provide a quick reference in case the problem arises again in future.

Network Maintenance

1. Security Threats

A network which is lack of security may have the following consequences: Company secrets fallen into wrong hands Disclosure of personal information Information may be modified or destroyed Network cannot function The network becomes a center that spreads virus

There are two types of security threats: External threats and Internal threats.

2. External Threats

External threats are mainly due to hackers who breach the security of a network. Once they gain access, they may control a server and do whatever they like. The attacks by hackers include the following: Front-door attack

For example, using a program to generate passwords until a match occurs (cracking). Backdoor attack

Making use of the bugs in a NOS and gaining control over the system. Wireless attack

Accessing through wireless connection.

There are several ways to work against front-door attacks: firewall, restricting access to server and dial-back. Firewall

A firewall is a software, or a combination of software and hardware, designed to prevent unauthorized access. It sits between the router and the organization's network (i.e. between the Internet and the network) and decides whether it is safe to let messages pass through to the network.One of the techniques used by firewalls is packet filtering, which compares the IP addresses of the incoming or outgoing packets against a table of IP address before allowing them to cross the firewall.The table of IP addresses in a firewall is dynamic. It means that new information can be added from time to time. For example, a firewall is able to blacklist those IP addresses related to unusual activities, such as password guessing.Another technique is to use network address translator (NAT) to hide the IP addresses of computers inside a network. This prevents computers inside the network from communicating directly with external computers.

Restricting access to a serverIn addition to using a firewall, it is possible to limit access to a server to certain machines only, based on the Ethernet MAC addresses. In that case, the server can only be accessed by computers inside the network.

Dial-backThis is designed for enhancing the security of remote access to a LAN using modem. When users want to connect, they dial the system, request access. The remote access system then terminates the connection, dials the preprogrammed phone number to make the real connection.

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Backdoor attacks refer to those illegal access to a network by taking the advantages of problems in the NOS, called security holes. These problems may enable programs called worm (a kind of malicious software) to enter a network. Some worm may install a "backdoor" to the victim system that will allow access by hackers. Some may attack the victim network by overloading the computational resources or consuming the bandwidth.

It is quite often that once a security hole is discovered, worms would spread quickly over the Internet. This is known as an outbreak. Very soon, many networks all over the world would be harmed.

The best preventive solution is to regularly update the NOS with security-related patches provided by the vendors.

Wireless signals travel through air and are easy to intercept by any suitably equipped receiving device. Preventing wireless signals interception is difficult. The technology used to provide confidentiality for wireless communication is mainly encryption. However, there are two problems associated with wireless security: The encryption capability of many wireless devices is non-activated by default. As a result,

many wireless networks, especially those home networks, can be broken in fairly easily. The basic security of wireless network is WEP (Wired equivalent privacy). But, WEP has

security flaws and there are reports that WEP has been broken.

In establishing a wireless LAN, it is very important to activate encryption. Also, a strong user authentication should be implemented on the LAN so that even the encryption has been broken, resources cannot be accessed easily. For a peer-to-peer network, the directory should be shared out only when needed.

3. Internal Threats

Internal threats occur when internal users access files that they are not supposed to access. An example of internal threat is that the files stored in teachers' folders are unfortunately accessible by some students.

Internal threats may be caused by improper account setting, users leaving their seats without logging off, users writing their passwords on a piece of paper etc. Account security

Account security refers to managing user accounts properly to enhance the security of a network. The following are general steps:(1) Set up groups on the network and assign access rights for each group. Assign users into

different groups. The highest access rights on a network belongs to the administrative group, in which users have full control over the network system.

(2) Most networks have a default user account called "guest". But, this is the frequent target of hackers. So, remove this account immediately. Also, delete those accounts for testing purposes, like "test", "temp".

(3) Rename the default administrative account.(4) Set password policies to force user to change password regularly.

Change password regularly. Previous used password cannot be reused. Each password should consist of at least eight characters, with mixture of case

sensitive letters and numbers. Use a long password. (But don’t over-demand, otherwise, users tend to write it

down on paper.)(5) Watch for incorrect password attempts. If too many attempts occur, the system should

lock out the user account, preventing further attempts.(6) Establish times of day that a user is allowed to log on.

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Directory permissionDirectory permission refers to rights assigned to users to allow them to carry out certain tasks within that directory. Different users will have different permissions for the same directory. The permissions include:(1) create only: Users are allowed to add a new file to the directory, but restricted from

seeing, editing, or deleting existing files. It is similar to a mailbox on a street corner: You can put only new things on it.

(2) read only: Users are allowed to open a file for viewing, but cannot change the file.(3) change: Users can do whatever they like with the files, except they cannot give other

user access to the directory.(4) full control: Users can do whatever they like with the files and grant other users access

to the directory

Network Maintenance

Even the most fault-tolerant network may fail, and backups allow you to restore the data resources. A complete backup plan should be designed to cover all the workstations and servers.

1. Full and Incremental Backups

The backup plan should include a combination of full and incremental backups. A full backup copies all directories and associated files from a hard disk to the backup media. It can take extremely long time and use extensive system resources -- this can slow down the network.

An incremental backup is used to backup only those files modified since the last full backup. Usually, full backup is done once every week and incremental backup is done every night (except that full backup has been done on the same day).

2. Backup Medium

The backup medium may be magnetic tape or optical disk. Determine the appropriate medium based on the requirements for speed, reliability, and storage duration.

3. Location for Storing Backup Copies

The backup copies should be stored securely in a safe location away from the network system. They should be locked physically in a fire-resistant place.

4. Configure the Scheduled Backups

Backup can be scheduled after busy hours. For those backup performed automatically, it is important to confirm that the scheduled procedures are actually working.

5. Test the Recover Ability

It is possible that a scheduled backup does not actually work after a certain period of time, for reasons like degeneration of backup medium. Regularly test the ability to recover. This will help to discover any hidden problems.

6. Document the procedures in restoration

The steps in restoring from backup copies must be clearly written so that when the system fails, other people can carry out the task successfully.

Fault-tolerant Features

A fault-tolerant system means that even a part fails, the whole system can still operate. It guarantees the uptime of a network and provides continued data availability.

Fault-tolerant requires redundant components that can take over automatically when there is a failure. The redundant components are also referred to as hot swap components, meaning that they can be replaced without turning off the computer. Redundancy applies to disks, power supplies, servers.

1. RAID System

A RAID (Redundant Array of Inexpensive Disks) system uses a set of hard disks that appear to a user as if they form a large logical disk for fault tolerant and/or improving performance.

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In a RAID system, data is spread across many hard disks. Therefore, a single file in a RAID system may have several parts stored in different hard disks.

A single disk drive is limited by a set of disk heads. Multiple disks perform more quickly because several sets of disk heads can work independently and send the data simultaneously to the controller where the data are assembled.

There are several approaches. In all cases except RAID-0, if one of the drives in a RAID system fails, the surviving drives have the information needed to reconstruct what is missing from the failed disk. This process is done automatically while users are alerted to replace the failed disk.

2. Different levels of RAID System

Several levels of RAID have been developed, RAID-0 up to RAID-5, with RAID-5 being the most popular. Take an example of a 4-disk RAID system for explanation. In all cases, the file to be stored is broken up into many equal segments which are stored separately in the disks.

In RAID-0, all disks are used for data storage. Since every disk is used in parallel to read and write data, RAID-0 is the most efficient (fastest), but it does not offer fault tolerance. It allows 4 parallel reads and 4 parallel writes. It is useful in extensive I/O applications.

In RAID-1, fault tolerance is accomplished by redundantly storing the same data in two locations. Our 4-disk RAID system allows two parallel writes and two parallel reads across Disk 1 and Disk 2. However, it is not as efficient as RAID-0.

In RAID-5, instead of storing data in full redundancy as in RAID-1, a mathematical formula combines the three segments in Disks 1 to 3 to form a new segment known as ECC (Error Correcting Code) which is then stored in Disk 4. Assume that Disk 3 fails, the lost data in the first segment can be recovered from the segments in Disk 1, Disk 2 and the ECC in Disk 4. Recovering data from other failed disk is similar. Since it takes a longer time to write the ECC, storing ECC in different disks could spread the delay over all disks. This avoids bottleneck to occur.

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Exercise

1. Most networks consist of one or more servers and a number of clients.(a) State the differences in roles between a server and a client.(b) Describe the general differences between a dedicated server and an ordinary workstation in

terms ofi) operating systemii) number of processorsiii) disk interfaceiv) bus capability

(c) State a network-related hardware whichi) can be found in both a server and a workstation.ii) connects a network to the Internet.iii) divides in large network into smaller segments.iv) is used in amplifying the signals on a coaxial cable.

2. Some dedicated servers have more than one processor.(a) What are the requirements in hardware and software in order to make use of multiple

processors?(b) Explain why doubling the number of processors may NOT double the overall processing

speed.(c) Can a backup server increase the processing speed? Explain.

3. The computers in a university are scattered in several buildings. A network is set up to link these computers together so that students are able to access their stored documents from any computer. A tape is dedicated for storing the backup copies of such documents.(a) Identify the difference between a computer on a network and a standalone computer in

terms ofi) the software installedii) the hardware.

(b) State the location where the documents of the students are stored.(c) State TWO advantages to the students of storing documents on a network.

In the university, signals transmitted along UTP cables will become weaker and weaker.(d) What do we call this phenomenon?(e) Suggest TWO different ways so that the above problem can be solved by the university.

4. UTP cables are commonly used on Ethernet networks.(a) What is the maximum effective transmission length of a UTP cable?(b) On a network that uses hubs and UTP cables, what is the maximum distance between any

two computers?(c) Optical fiber is regarded as the most secure networking medium. Explain briefly.(d) Besides higher security, state TWO advantages and ONE disadvantage of optical fiber over

UTP cable.

5. A hub is a connecting device used on a network. It can also be used as a repeater.(a) Under what conditions is a repeater needed?(b) In designing a network layout, what should you take into account when you plan the

locations of the hubs?

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(c) A hub is not a very efficient connecting device. Explain this by describing how it relays the information from one workstation to another.

A large hub-based network is usually split into smaller segments by bridges.(d) Describe how a bridge can split a network into smaller segments.(e) Explain why it is necessary to split a large hub-based network.

A switch does similar tasks as a hub, but is regarded to be smarter and having better performance.(f) Which task of a switch is similar to a hub?(g) Explain why a switch performs better than a hub.

Routers can be found on some networks.(h) State ONE function of the routers on these networks.

6. The connection between a computer and a printer can be done by using either a parallel printer cable or a USB cable.(a) Describe the differences between the above two cables in terms of

i) number of wiresii) rate of data transmissioniii) cost

(b) Connection of devices to a computer using USB cables is regarded to be convenient. Suggest TWO reasons for this.

(c) Besides using cables, state ONE other mean of connection between a computer and a printer.

(d) Sometimes a printer is connected directly to the hub of a network. Suggest the cable for this kind of connection.

(e) Can a printer which is connected to a computer be accessed by other computers on a network? If yes, briefly describe the procedures needed.

7. Network Adapter or NIC is an important hardware on a network.(a) State TWO components on a network where a NIC can be found.(b) NIC is known to be 10/1 00 Mbps.

i) What does Mbps stand for?ii) What do the numbers indicate?

(c) All NICs carry a MAC address.i) What is the purpose of this address?ii) State the characteristics of this address.

The following is the result shown on the screen when a network utility of a workstation is executed:

Physical Address 00-02-3F-24-11-7DDHCP Enabled YesAuto-configuration Enabled YesIP Address 192.168.1.33Subnet Mask 255.255.255.0Default Gateway 192.168.1.1DHCP Server 192.168.1.1DNS Server 202.130.97.65

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(d) What is the MAC address of the NIC?(e) How many bytes are used in the MAC address?(f) Which of the above data can be modified by users through configuration?(g) A traditional NIC is known to convert between parallel and serial data transmissions.

describe in details the role of a NIC in this aspect.

8. TCP/IP is the protocol suite used on the Internet. Messages sent over the Internet are broken into packets. Each packet carries the IP addresses of the sender and receiver.(a) State the meaning of protocol and identify the difference between protocol and protocol

suite?(b) TCP/IP consists of TCP, IP and some other protocols. State THREE other protocols that

are compatible with TCP/IP.(c) Give TWO examples of operating systems that support TCP/IP.(d) Give the full name for TCP and describe TWO functions carried out by TCP.(e) Give the full name for IP and describe ONE function carried out by IP.(f) The IPv4 global IP address will be used up one day.

i) Calculate the theoretical maximum IP addresses.ii) State one solution to this problem.

9. On an IP-based network, all the computers are assigned with a unique IP address.(a) Name TWO other devices on the network that also have an IP address.(b) Discuss TWO methods in assigning an IP address to a computer or device, giving an

example of use in each case.(c) Describe the format of an IP address. Hence, determine the theoretical maximum number

of IP addresses.(d) Are IP addresses always unique? Explain your answer by giving an appropriate example.(e) Some IP addresses are reserved for special purposes. Give THREE examples of such IP

addresses.

10. On a TCP/IP intranet, the IP addresses of workstations are assigned automatically by an DHCP server.(a) Give the full name for DHCP.(b) State TWO advantages of using DHCP.(c) Suppose there are only ten workstations on the network. State which part of the IP

addresses of these computers are different from one another.(d) Name TWO devices whose IP addresses should not be assigned by DHCP.(e) Besides IP address, a computer also has computer name and MAC address.

i) Give the full name for MAC address.ii) Which of the above (IP address, MAC address, computer name) is permanent?

Explain.iii) When the NIC of the computer is changed, which of the above (IP address, MAC

address, computer name) would be affected?iv) The IP address of a workstation is regarded as dynamic when compared to the

computer name. Explain.

11. A network with 50 workstations is connected to the Internet using the Internet services provided by a cable TV company.(a) What is the role of the cable TV company?

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(b) What kind of Internet access is provided by the cable TV company?(c) State TWO pieces of hardware that can be found on the network in order that all

workstations on the network can access the Internet. Describe the functions of each hardware.

(d) State the hardware and system software that a workstation on the network needs in order to access the Internet. Briefly describe their functions.

The network is assigned a static global IP address while all workstations are assigned an internal IP address.(e) State the most efficient way to assign internal IP addresses for the workstations.(f) Discuss how incoming messages that carry the same global IP address can be directed

correctly to the workstations.(g) Besides the internal IP address, each computer also has a MAC address. Explain why IP

address is necessary while computers can be identified by their MAC address.

12. Mr. Lee is going to set up a home network, which consists of two desktop computers and a notebook computer, using wireless technology.(a) Suggest a hardware that connects the computers together.(b) What are the requirements in hardware and software for these computers so that they can

communicate with each other?(c) Suggest a protocol suite that Mr. Lee should use. Explain briefly.

Mr. Lee is going to connect his network to the Internet. Currently, home users can access the Internet using broadband or dial-up access.(d) Which of the above Internet accesses would you recommend to Mr. Lee? Explain your

answer by stating TWO reasons.(e) Suggest a hardware that Mr. Lee needs for all his computers to access the Internet at the

same time. Explain your answer.

Mr. Lee has assigned IP addresses for all his computers internally.(f) What should Mr. Lee take into account to avoid conflict?(g) Explain why these addresses cannot be used on the Internet.(h) When the computers are connected to the Internet, they are assigned with a dynamic global

IP address.i) What is the meaning of dynamic in this sense?ii) State how the global IP address is assigned to the computers.

13. The diagram shows two networks, A and B, located in the same city and connected by a long-distance communication link Y.

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(a) State the topology used oni) network Aii) network B

(b) Suggest the networking media used oni) network Aii) network B

(c) Name the device Z which connects the wires in network B together.(d) Explain whether the networks A and B should use the same protocol.(e) Classify the networks A, Band C in terms of the geographical area covered.(f) Name the device X and describe its function.(g) Explain why UTP cables cannot be used in Y.(h) Suggest a medium for Y if

i) security is of high concernii) it is easily availableiii) laying physical cable is impossible

14. Patrick is a freelance graphic designer. He has a SOHO network that connects several computers together and is Internet accessible. When an utility program in a workstation is executed, the following information are shown on the screen.

Physical Address 00-11-11-0F-49-7CDHCP Enabled YesAuto-configuration Enabled YesIP Address 192.168.1.34Subnet Mask 255.255.255.0Default Gateway 192.168.1.1DHCP Server 192.168.1.1DNS Server 202.130.97.65

202.130.97.66

(a) Name the protocol suite used by Patrick's network. Explain briefly.(b) There are two addresses found on the list.

i) Determine their lengths.ii) Discuss their differences in terms of

(1) permanence.(2) dependence on hardware.(3) their relative levels in the OSI model.

(c) Given that a subnet mask divides an IP address into NetID and HostID. Write down the NetID and HostID of the workstation.

(d) What is the maximum number of hosts on the network?(e) Suppose the utility that gives the above information is executed in another workstation of

the same network. Which piece(s) of information will have a value different from above?(f) The global IP address assigned by the ISP to Patrick's network is 202.130.90.42. By

comparing it with the value of the DNS server in the above table, write down a subnet mask used by the ISP.

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15. The diagram shows how a user is connected to a private local area network using a public phone line.

(a) Name device X and discuss its functions.(b) State ONE advantage and TWO disadvantages of this kind of connection.(c) For the connection to be successful, state the software installed in

i) the user's computer,ii) the server of the network.

(d) What is the topology adopted by network A?(e) Name the device Y and suggest the networking medium used in network A.(f) State THREE advantages and ONE disadvantage of the topology used by network A.

16. ABC Company is running two different businesses which are located on two floors (8th and 10th floors) of a commercial building. There are about 200 computers on each floor. A network that uses purely hubs and UTP cables is set up to link up all the computers of the company as shown.

The speed of the network is getting slow. Suppose you are invited to examine the network and suggest improvement.(a) Do you agree that the network is a local area network? Explain.(b) Do you think that the networking standard of the workstations should be upgraded?

Explain.(c) What is the network topology being used by the company? Explain.

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(d) Should the network topology be modified for speed improvement? Explain.

Although there is connection between the two floors, communication mainly occurs between computers on the same floor.(e) Can hubs limit the data transmission within each floor? Explain.(f) Hence, account for the poor performance of the network.(g) What is your recommendation for improvement? State your suggested hardware, if any.

17. Maria is studying in Canada. She is coming back to Hong Kong during the summer vacation. In the airport of Canada, she wants to use her notebook computer to send an e-mail to her father before she takes the airplane.(a) Maria finds that after her computer is turned on, it is connected to a network automatically,

without any cable.i) State the networking medium used in the connection.ii) State TWO limitations of using wireless connection compared to cabled connection.iii) What connecting devices for wireless connection can be found in

(1) Maria's computer, and(2) the network of the airport?

iv) The system in airport allows passengers to use HTTP only. What kind of e-mail facility is being used by Maria?

(b) When Maria returns to Hong Kong, she needs to access the file server of her school in Canada. The school has a virtual private network (VPN).i) Besides using VPN, state an alternative way for Maria to connect to the file server.

State the medium needed.ii) Explain why it is not economical to connect using the alternative connection.iii) Besides the cost, state TWO other disadvantages of the alternative connection.iv) The school does not provide an FTP site for download. Give TWO reasons to support

the decision.

18. Some network operating systems include a function known as Active Directory that keeps the accounts of all users and manages all the security related tasks.(a) Explain why a network needs to keep accounts of its users.(b) Discuss what happens if the Active Directory fails.(c) The Active Directory controls access to the network resources.

i) State TWO examples of network resources.ii) Discuss how the Active Directory determines who is the right person for a certain

resource.(d) Besides Activity Directory, there are other services on Windows-based networks. Name

FOUR of them.

19. A small trading company with 20 staff has set up an Ethernet network that links up 15 computers together. The network does not include any dedicated server and each computer has a login system.(a) State two types of networking media that can be used on Ethernet networks.(b) For each of the networking media in (a), state whether a central connecting device is

necessary.(c) A user has changed her login password in a computer. State whether the user can use the

same password when she logs in other computers. Explain.(d) The manager requires the staff to save all their files in a particular computer.

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i) State the procedures that should be carried out by the manager before the staff can access this computer.

ii) State TWO advantages and TWO disadvantages of this policy.

The company is going to purchase a laser printer which will be shared over the network. The manager is deciding whether he should buy a printer with a network interface or not.(e) State TWO advantages and ONE disadvantage of a printer with a network interface.

20. An organization has 100 computers connected to form a network. One of these computers acts as a dedicated server.(a) What is the role of the other 99 computers?(b) State the major difference in hardware between the server and other computers on the

network.(c) Determine the number of cables connected to the server if

i) bus topology,ii) star topology is used.

The performance of the network is not very satisfactory. The following remedial plans are being considered:

Plan A: Upgrade the hardware of the old server.Plan B: Add one more server to the network. The new server will carry out the same set of

tasks as the old one.Plan C: Add one more server to the network. The tasks are redistributed so that there is no

overlapping tasks between the old and new servers.

(d) State THREE aspects of hardware that should be considered in Plan A.(e) Comment on the effectiveness of

i) Plan B,ii) Plan C.

(f) If the server is upgraded and the network still has no significant improvement, state THREE other aspects of the network that should be reviewed.

21. A school has set up a LAN from which students having logged in may surf the Internet. All students are offered an e-mail account with the domain name of the school. The mail boxes are kept by the school.(a) The school needs an ISP. From the given information, state TWO services provided by the

ISP.(b) Write down the servers that support the services of

i) logging-inii) improving the response when students surf the Internetiii) sending and receiving e-mailsiv) translating the domain name of e-mail addresses of students into the IP address of the

LANFor each of these services, state whether it is hosted by the school or not.

(c) The school is hosting her web site which includes hundreds of web pages.i) State an alternative way for the school to host the web pages.ii) Discuss TWO advantages of hosting a web site by the school herself.iii) State the service provided by a server for students to upload files to the web site.

(d) Students are able to reserve books in the school library using a browser.

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i) State whether pure HTML can handle the book reservation or not. Explain briefly.ii) State the server that enables book reservation to be carried out.

22. Paul has set up a SOHO software house which designs business computer programs. Most of his clients are oversea companies.(a) In order to develop programs in different platforms, Paul has installed several computers

that use different operating systems. A peer-to-peer network is set up to link all these computers together.i) Peer-to-peer networks are considered insecure. Explain this in terms of the policy of

user accounts in such networks.ii) It is not easy to perform backup in peer-to-peer networks. Explain this by describing

how Paul performs backup for his network.iii) Despite the disadvantages mentioned above, Paul finds that the peer-to-peer network

is more suitable for his work than a client/server one. Give TWO reasons for his choice.

(b) The communication of Paul with the clients is mainly through e-mails. Sometimes, it is necessary for Paul to talk to the clients face-to-face.i) Can Paul deliver his products to the clients through e-mails? Explain briefly.ii) State TWO disadvantages of using e-mail for Paul to communicate with the clients.iii) Discuss how Paul can talk to the clients without leaving his home office or using

phone calls, stating its advantages over using e-mail.

23. Dick has installed a network monitoring software for the Ethernet network of his company. The software reports figures on the traffic and bandwidth of the network. But, it cannot tell the throughput of the network.(a) For a channel on the network, explain the meanings of

i) traffic,ii) bandwidth andiii) throughput.

(b) Discuss how the utilization of a network can be calculated from the figures on traffic. Hence, explain what a saturated network is.

(c) It is possible that an Ethernet network has high traffic but low throughput.i) Give a reason for this.ii) Suggest how this can be improved.

(d) Networks with a low traffic may be due to a bottleneck.i) A poorly performed server is a cause of bottleneck. Discuss TWO figures for

measuring the performance of a server.ii) State TWO other possible causes of bottlenecks.

24. Eason is the engineer of a network supplier which also provides technical advices to clients who ask for help.(a) State THREE common problems occurred on a network.(b) State THREE causes of problems on a network.(c) Before a site visit, Eason will talk to the clients over telephone and ask them to try some

simple actions, like resetting the network.i) State the most simple way to reset a network.ii) State TWO other simple actions that Eason will ask the client to carry out.

(d) State THREE tools that Eason will use to identify the problems of a network.

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25. There are two basic security models in use today:Model 1 Shared-level security

Each shared resource is assigned a password. To access the resource, the user enters the correct password and is granted access.

Model 2 User-level securityEach shared resource has permissions associated with it. A user is granted to access that resource based on the permissions granted to that user.

(a) Suppose the network administrator has changed the password of a shared resource in the first model. State the subsequent task that he/she needs to carry out.

(b) Hence, or otherwise, explain why networks using the first model is considered insecure.(c) Compare the number of passwords that a user needs to remember in the two models?(d) Compare the two security models in terms of

i) user authenticationii) implementing hierarchical structure of users (similar to the rank of staff in a

company)iii) the ease in tracing the usage of a resourceiv) the workload of the network administrator

26. A school has set up a network which is connected to the Internet via a local ISP. A firewall is installed to protect the network of the school.(a) State the connecting device that joins the network of the school to that of the ISP.(b) By stating the consequences that may occur to an unprotected network, explain why

protection by firewall is necessary.(c) With the help of a diagram, describe the best location of the firewall.(d) The firewall is able to blacklist certain web sites.

i) Discuss what information the firewall should store to achieve this.ii) Explain whether the information should be stored in ROM or not.

(e) The firewall may limit the traffic to certain applications only.i) Give an example of such applicationsii) Discuss how the firewall is able to achieve this technically.

(f) The firewall is able to hide the computers of the network so that they are invisible to outside.i) Discuss the purpose of hiding computers.ii) Explain how the firewall is able to achieve this.

27. A government office consists of 30 staff working inside. A server-based network has been set up to link 20 desktop computers and two laser printers using UTP cables. Some staff connect their notebook computers to the network using wireless technology.(a) Someone said that wireless connection is not as secure as cable connection. Do you agree?

Comment on this statement.(b) The laser printers have USB connections.

i) Discuss TWO ways that the printers can be connected to the network.ii) Suppose the laser printers are accessible by the desktop computers. Can they be

accessed by the notebook computers? Explain briefly.(c) Staff of different ranks have different access rights to the resources on the network.

i) Besides printers, name TWO other resources that can be shared on a network.ii) Describe how to manage the access rights to the resources.

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(d) Staff are required to change their passwords regularly.i) When users change their passwords, they need to enter their new passwords twice.

Explain why such verification applies to password only but not to most other information supplied by the users.

ii) State TWO other common password policies.iii) Policies for password should not be too loose or too demanding. With the aid of an

example, explain why if the password policies are too demanding, it would be more likely for security threats to occur.

28. A large company has a network with several file servers spread out over multiple floors.(a) There are two schemes for backup using tapes:

Scheme A Single backup driveThis scheme uses a centralized backup drive. All servers are connected to the same drive over the network.

Scheme B Individual backup driveThis scheme involves a backup drive installed for each server.

Compare, with reasons, the two schemes in terms ofi) time required to complete the backup processii) traffic flow over the network iii) ease in administration

(b) A backup plan is drafted. This plan includes a combination of two backups:Type 1 Full backup

Complete backup of the servers. This backup is performed on each Sunday.Type 2 Incremental backup

Backup all files modified since the last full backup. This backup is performed nightly.

i) Give TWO reasons to explain why backups are performed at night or on Sunday.ii) Suggest ONE reason to explain why full backup is not performed everyday.iii) Suppose a disaster occurs to a server on a Wednesday morning and is discovered

immediately. Discuss how the system can be recovered using a new computer.(c) The log of backup is a file that. records information of backup performed, including when

the backup has been carried out and the types of backup used.i) State the drawback if the log of backup is stored inside the hard disk of a computer.ii) Suggest a location to store the log of backup.

(d) Suggest how to ensure that the backups can recover the system when disaster occurs.

29. Suppose you are the network administrator of a corporate network. You are going to create users accounts of different ranks which have different access rights to the shared files on the network.(a) Discuss how user accounts are created to have different ranks.(b) For each shared directory, you may set permissions for different ranks of accounts. State

THREE permissions that you can apply to a shared directory.(c) Discuss how you would fight against the following threats to the network:

i) power failure or power fluctuationii) server disk crashiii) virusiv) hackers on the Internetv) natural disaster, like fire, bomb attacks

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30. RAID system is common in dedicated servers. It consists of several hard disks working together and appears as a single hard disk.(a) State TWO advantages of using a RAID system.(b) A student says that since there are several hard disks, the chance of failure is higher than

using just one hard disk. Do you agree? Explain briefly.

Dedicated servers used on a network are incorporated with fault-tolerant feature.(c) Explain the meaning of fault-tolerant.(d) Besides RAID, give TWO examples of fault-tolerant features.

Suggested Answers

1. (a) A server provides services to other computers.A client requests services from the server.

(b) i) A dedicated server is installed a sophisticated network operating system. A client is installed an ordinary desktop operating system with networking software.

ii) A server can support multiple processors; a workstation usually supports single processor.

iii) Disk interface that can handle simultaneous access to more than one hard disk, such as SCSI, is more commonly used in server.

iv) A faster and wider bus is used in server.(c) i) network adapter (or NIC)

ii) routeriii) bridgeiv) repeater

2. (a) The motherboard should be built with slots for multiple processors.The operating system should support multiple processors

(b) Not all programs or instructions can benefit from multi-processing.(c) No. A backup server provides redundancy only. The same task is normally not handled

simultaneously by two servers.

3. (a) i) A workstation must be installed with networking software.ii) A workstation must be installed with a network adapter (NIC).

(b) file server(c) Advantage:

(1) The document can be accessed from anywhere in the campus.(2) The files are treated with more intensive care, like backup.(3) It can offer better version control.

(d) attenuation(e) Use repeaters or hubs.Replace the UTP cables with optical fibers.

4. (a) 100 m(b) 200 m(c) A optical fiber does not emit EM waves. Therefore, signals on a optical fiber cannot be

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intercepted/tapped easily.(d) Advantage:

(1) longer effective transmission length(2) higher bandwidthDisadvantage:(1) higher cost in the medium(2) higher cost in installation and maintenance

5. (a) If the distance between workstations is too long, exceeding the effective transmission length of the cables, signals will be too weak to transmit. In that case, a repeater is necessary to amplify the signals.

(b) The mains power supply (power points) for powering the hubs.(c) A hub echoes and broadcasts the signals input from one port to all other ports of the hubs.

This creates collisions and slows down the data transmission rate (reducing bandwidth).(d) A bridge keeps a forwarding table that records the MAC address of nodes on both sides. It

examines each packet and determine whether it would forward it to the other segment or not.

(e) A large hub-based network will have too many collisions, making data transmission inefficient. A bridge can reduce the network traffic (and thus the collisions) by confining the traffic within each network segment.

(f) A switch is a cable concentrator.(g) A switch keeps a table of the MAC addresses of nodes. It examines the packets and

establishes a virtual connecting path between two stations rather than broadcasting the signals to all ports. Therefore, network traffic is reduced.

(h) A router is used to connect the network to another network (or to the Internet).

6. (a) i) The number of wires in printer cables are several times more than USB cables.ii) The rate of data transmission in printer cables is lower..iii) Printer cables are more expensive.

(b) USB cables are plug and play; There are less pins involved; The cables are thinner.(c) Using the IrDA port that uses infrared radiation; Using Bluetooth that uses radio waves.(d) UTP cable(e) Yes. In the computer that connects to the printer, share the printer so that it can be accessed

by other users on the network.

7. (a) Workstation, server, some connecting devices (e.g. switch, access point, router)(b) i) Mbps stands for mega bits of data transmitted per second

ii) A 10/100 Mbps NIC means that the NIC can transmit data at speeds of 10 Mbps or 100 Mbps, depending on other devices and networking medium of the channel.

(c) i) MAC address is used to identify a network interface card on a network.ii) It is unique universally; it cannot be modified

(d) 00-02-3F-24-11-7D(e) Six bytes(f) All of them except the Physical address.(g) The buses on the motherboard, connecting the memory and CPU, are parallel; The cables

that connect the computers on a network are serial; As a NIC is the interface between buses

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and cables, it converts between parallel and serial data transmissions.8. (a) Protocol is a set of rules governing how data are transmitted and received.

Protocol suite is a set of protocols(b) FTP, HTTP, DNS, SMTP, Telnet, etc.(c) Any two popular operating systems: Windows, Linux, Mac OS.(d) Transmission Control Protocol

(1) Breaking a message into packets(2) Re-assembling the packets into message(3) Re-sends packets which have errors

(e) Internet Protocol(1) Adding the IP address to the packets(2) Removing the IP address from the packets(3) Routing packets through the network

(f) i) 256 256 256 256 = 4,295 109

ii) Increase the number of bits. e.g. IPv6 uses 128 bits.

9. (a) Routers, print server. (Hubs and switches are not acceptable answers)(b) Assigned manually: e.g. Web server, routers have fixed IP address.

Assigned automatically by DHCP: e.g. workstations on the network(c) An IP address consists of four numbers, between o and 255, separated by periods. The

maximum number of IP address is 256 256 256 256 = 4,295 109(d) IP addresses used internally inside a LAN are unique inside the network. But, two networks

may use the same set of internal IP addresses for their workstations. So, internal IP addresses are not unique when more than one network is considered.

(e) Internal, e.g. 10.x.x.x, or 192.168.x.xLimited broadcasting, e.g. 256.256.256.256.This computer address, e.g. 0.0.0.0.Loopback, e.g. 127.0.0.1

10 (a) Dynamic Host Configuration Protocol(b) Advantage:

(1) Efficient (fast) in allocating IP addresses to devices(2) Save efforts in manually assigning the addresses(3) Conflicts in IP addresses can be avoided

(c) Their IP addresses differ only in the last decimal number.(d) Router, printer, Web server(e) i) Medium Access Control address

ii) MAC address is permanent. It is burnt onto the ROM of the NIC.iii) MAC addressiv) IP address is assigned dynamically by DHCP. Computer name is set by the user and

remains unchanged unless it is changed by the user.

11. (a) ISP (Internet Service Provider) establishes a connection between the Internet backbone and the customer's network.

(b) Broadband Internet Access(c) Router: connects the network of organization to the network of the cable TV company

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Cable modem: converts between analog and digital signals(d) Hardware: NIC, UTP cable. The NIC is used to send and receive packets.

System software: desktop as with networking software (software for TCP/IP). The software is to communicate with other computers.

(e) Use DHCP to assign IP addresses to workstation automatically.(f) An arbitrary port number is assigned to each bi-directional communication between an

application program on the network and the remote program. Each packet that moves in and out of the network carries a port number. The port numbers and their associated internal IP address are kept in the table of the NAT. Therefore, incoming messages that carries the port number can be directed to the correct workstation.

(g) MAC address is a physical address burnt permanently in the NIC. However, it depends on the hardware used. IP address is logical and is designed to provide seamless communication independent of hardware used.

12. (a) Wireless access point(b) Software: networking software

Hardware: wireless NIC(c) TCP/IP; because it is the most common standard and can be used on the Internet.(d) Broadband access

Reason:(1) higher data transmission speed(2) always-on connection: no need to dial-up(3) no need to pay for PNETS

(e) Router with NAT. The router connects his network to the network of an ISP. The NAT allows different computer to share a global IP address.

(f) Internal IP address must be unique on the network(g) There are computers using the same set of internal IP addresses in other networks.(h) i) Dynamic means that the IP address may be changed when the computer accesses the

Internet next time.ii) It is assigned by the DHCP of the ISP.

13. (a) i) Busii) Star

(b) i) Coaxial cableii) UTP cable

(c) Hub / switch / wireless access point(d) They should use the same protocol so that they can communicate with each other(e) A, B: Local Area Network

C: Metropolitan Area Network(f) Router. Use to direct traffic and connect two LANs(g) The length of UTP cable cannot exceed 100 meter.(h) i) optical fiber

ii) public phone linesiii) microwaves, satellite, radio waves

14. (a) The protocol suite is TCP/IP because IP address is used.

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(b) i) MAC: 6 bytesIP address: 4 bytes

ii) (1) MAC address is permanently burnt onto the ROM but IP address is dynamically assigned by the DHCP

(2) IP address is independent of hardware(3) IP address is at a higher level than MAC address

(c) NetID: 192.168.1HostID: 34

(d) 256 – 2 = 254(e) Physical address and IP address(f) 255.255.0.0. This is because the global IP address and the IP address of the DNS server

both belong to the ISP and they are different in the last two bytes.Alternative answers: 255.255.192.0, 255.255.128.0

15. (a) Modem. It is used to convert digital signal (computer) to analog (phone line) and vice versa.

(b) Advantage: easily availableDisadvantage. slow; insecure

(c) i) user computer: remote access clientii) server: remote access server;

(d) star topology(e) Y is a hub / switch.

The network media is UTP cable(f) Advantage:

(1) Easy to maintain(2) Flexible in adding or removing nodes(3) Reliable: if a node fails, the network can still operateDisadvantage:(1) If the hub fails, the entire network fails(2) Require more cables than other network

16. (a) Yes. Because WAN technology is not used, or UTP cables cannot span great distances, or there is only one single network.

(b) No. Ethernet is already in use. Currently there is no better choice for LAN.(c) Star, because hubs are used.(d) No, because other topologies are unreliable (failure of a node will cause total failure of the

network)(e) No, because hubs broadcast the signals from a port to all other ports. It is not smart enough

to know the location of the computers.(f) Firstly, hubs are half-duplex. Secondly, as a hub broadcasts the signals, this increases the

chance of collisions. Computers have to wait for retransmission when collision occurs. This slows down the data transmission.

(g) Split the network into two smaller segments to confine data flow within each segment. Hardware needed is a bridge.Or, use switches to replace the hubs to enhance performance.

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17. (a) i) Radio wavesii) Insecure -- the signals can be intercepted and/or her computer can be hacked; Data

transmission speed is slower.iii) (1) wireless NIC

(2) wireless access point(b) i) Remote access using modem dial-up.

Medium: public telephone linesii) Long-distance phone calls are expensive.iii) Disadvantages:

(1) insecure(2) slow

iv) Insecure -- more easily hacked than VPNLess functions than VPN

18. (a) Authentication; to stop unauthorized person to use the network; to control unauthorized access to the network resources

(b) No one can log on the network; all the resources cannot be used.(c) i) Files, hardware. (e.g. printer)

ii) Users are classified into groups which have different access levels to the shared resources.

(d) E-mail services, printer sharing, Web hosting, ftp, telnet, remote access services etc.

19. (a) Coaxial cable, UTP cable, optical fiber(b) Central connecting device is not needed for coaxial cable, but is needed for UTP cable and

optical fiber.(c) No, because different workstations keep different sets of user accounts. Therefore,

changing the password in one computer will not affect the password in other computers.(d) i) Directories must be created in the computer and shared out

ii) Advantage:(1) Easier to perform backup(2) Easier to keep version controlDisadvantage:(1) The shared files can be accessed by everyone(2) Access of the shared files is slower than access of local computers(3) The computer that shares out file would be slowed down

(e) Advantage:(1) The printer would handle the tasks of sharing by itself. This would not slow down any

computer.(2) The printer can be used without requiring a particular computer to be turned on.(3) Accessing the printer is faster, because the printer is not require to handle other

resource sharing tasks.Disadvantage:(1) It is more expensive(2) Configuration is required such as assigning it with a static IP address. The setup

procedures are more difficult than using plug-and-play connection.

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20. (a) Clients(b) The clients request services from the server which then entertains the clients.(c) i) one

ii) one(d) Number of CPU, memory size and speed, disk drive capacity, speed and interface, cache

memory size.(e) i) Plan B is not effective because a task cannot be split into two and handled by two

servers separatelyii) Plan C is effective. The load is shared. However, redundancy should be introduced to

the new server.(f) Other aspects should be reviewed:

(1) Circuit capacity(2) Bandwidth of backbone(3) Protocols used(4) Network operating system

21. (a) Internet access, provide a fixed IP address for the mail server(b) i) Authentication server (hosted by school)

ii) Proxy server (hosted by school)iii) Mail server (hosted by school)iv) DNS server (not hosted by school)

(c) i) Use commercial web hosting servicesii) Advantage:

(1) No worries about storage size and usage(2) Information can be kept confidential(3) Web pages can be updated without uploading(4) Full control of the Web server(5) Full bandwidth, i.e. bandwidth is not shared with other users

iii) FTP(d) i) No, because the book reservation involves database access.

ii) Database server, or application server

22. (a) i) There is no precise secure level for different groups; users administer their own security so that it is not unified; files shared out can be accessed by almost all users.

ii) Paul has to perform backup in all computers individuallyiii) Reasons:

(1) easy to set up(2) less expensive(3) no need to administer the servers(4) chance of failure of the entire network is low

(b) i) Yes, through e-mail attachmentii) Time delay in response; no face-to-face contact; mainly in text; insecureiii) Videoconferencing; can talk face-to-face; can use text to chat instantly; can write and

draw on electronic white board.

23. (a) i) The rate of data transmission over the cable.

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ii) The capacity of the cable, i.e. the maximum rate of data transmissioniii) The rate of useful data transmitted over the cable

(b) Utilization = traffic / bandwidthA network is saturated if the utilization is close to 100%

(c) i) Collision often occurs so that data have to be re-transmitted frequently.ii) Replace the hubs with switches

(d) i) The rate of data read from and written to a server;The amount of queued commands (commands waiting to be processed);Quality of server sessions

ii) Circuit bandwidth;protocols adopted;topology used;network operating system

24. (a) Common problems occurred on a network:(1) Response is too slow(2) Unable to detect the network(3) Unable to login(4) Unable to use resources(5) Unable to connect to the Internet(6) Unable to execute certain programs

(b) Causes:(1) Failure of hardware(2) Incorrect software configuration(3) Poor network design

(c) i) Power off and then power on the switches, routers and other connecting devicesii) Reboot the computers;

Make sure that the cables are securely connected;Replace certain cables

(d) Network monitoring software (Protocol analyzer); cable tester; event log

25. (a) The network admin has to inform all the users concerned.(b) Since changing a password is not easy, passwords will be rarely changed. Also, the

password is known to everyone using the same resource, it is difficult to trace how the password is leaked.

(c) In Model 1, a user needs to remember the passwords for all the shared resources.In Model 2, a user needs to remember his log-in password only.

(d) i) Model 1 does not require authentication on particular user because all users use the same password.Model 2 requires authentication because different users have different permissions.

ii) Hierarchical structure of users requires setting up user groups. Since only user-level security can set up groups for different users, hierarchical structure of users cannot be implemented in Model 1, but it is possible in Model 2.

iii) It is more difficult to track the usage of a resource in Model 1 than Model 2.iv) The workload of network administrator is greater in Model 1 because he has to

manage the passwords of all the resources, and he needs to inform all users after

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changing a password.

26. (a) Router(b) Prevent unauthorized access over the Internet. Otherwise, important information of an

organization may be disclosed, modified or destroyed; or the network cannot function at all etc.

(c) The firewall should be located between the router and the network.

(d) i) IP addresses of those blacklisted sitesii) No. The information should be updated from time to time.

(e) i) Web browser (HTTP), FTP software, SMTPii) Using port number, which identifies the applications. Traffic may be disallowed by

blocking such ports.(f) i) By hiding computers, it is more difficult for hackers to locate a particular computer

and browse the files in that computer.ii) Computers inside the network uses internal IP address. Whenever a connection of a

computer with the Internet, address translation is handled by the firewall or NAT

27. (a) Agree: Wireless signals can be intercepted. Connection may be made without physically working on the workstation inside an organization;Disagree: Shared resources are protected by setting strict access rights; Authentication is required to control connection using wireless technology; Data are encrypted when sending through the radio carrier.

(b) i) Ways to connect printers to the network:(1) Connect to a hardware print server (which is a box of size about a credit card)(2) Connect to a computer which shares out the printer

ii) Yes, because the permission is usually based on users (user-level) but not the computer(c) i) File, fax modem, CD-ROM drive

ii) Assign users to different groups and then assign permissions to the resources in groups.

(d) i) A password is invisible while it is being typedii) A password should be at least 6 characters in length; Case sensitive; Mixture of letters

and digits; Cannot be the same as the previous one; Common names are not allowed.iii) If the password policy is too demanding, for instance, requiring users to use minimum

length of 20 characters, users will tend to write down their passwords on a piece of paper.

28. (a) i) The time required to complete backup is more in Scheme A, because there is only one backup drive to write the data onto the tapes.

ii) The amount of traffic flow over the network is more in Scheme A, because data have to move from different servers to the tape drive located in a particular floor.

iii) Scheme A is easier to administer because there is only one tape drive involved.

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(b) i) Reasons:(1) The traffic of the network is not affected.(2) Most files are not in use since the files being backup cannot be opened or

accessed by users.(3) Data to be backup have been saved on the files.

ii) Full backup is lengthyiii) The incremental backups on Monday,

Tuesday are used together with the most recent full backup (on Sunday).(c) i) The log file would be damaged if disaster occurs in the computer.

ii) It should be stored in the backup tapes(d) Carry out simulation (rehearsal) of disaster regularly to see whether the backups can be

used to recover the system.

29. (a) Create several user groups each representing a rank. Grant users to different groups.(b) Possible permissions:

(1) Not accessible(2) Read only(3) Read, write and delete (change) but cannot change(4) Full control

(c) i) UPSii) RAID / backupiii) install anti-virus softwareiv) install firewallv) store the backup tape off-site

30. (a) Advantages:(1) Speed of disk access is higher(2) Provide fault-tolerance: When one of the disks fails, the data in other disks can be

used to recover the loss data.(b) Although the chance of having one disk failed is increased by having more disks, RAID

can prevent total data loss and can keep the system running even one of them fails. The chance of having two or more disks failed at the same time is very low. So, the chance of having system failure is much lower.

(c) Fault-tolerant means that even part of the system fails, the whole system can still operate(d) UPS, backup server

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