Q.1 Write case study on wide area network? “A wide area network (WAN) is a telecommunication network that covers a broad area (i.e., any network that links across metropolitan, regional, or national boundaries).”Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations. In essence this mode of telecommunication allows a business to effectively carry out its daily function regardless of location.

This is in contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area.

Design options

The textbook definition of a WAN is a computer network spanning regions, countries, or even the world. However, in terms of the application of computer networking protocols and concepts, it may be best to view WANs as computer networking technologies used to transmit data over long distances, and between different LANs, WANs and other localized computer networking architectures. This distinction stems from the fact that common LAN technologies operating at Layer 1/2 (such as the forms of Ethernet or Wifi) are often geared towards physically localized networks, and thus cannot transmit data over tens, hundreds or even thousands of miles or kilometers.

WANs necessarily do not just connect physically disparate LANs. A CAN, for example, may have a localised backbone of a WAN technology, which connects different

LANs within a campus. This could be to facilitate higher bandwidth applications, or provide better functionality for users in the CAN.

WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization's LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects the LAN on one side with a second router within the LAN on the other. Leased lines can be very expensive. Instead of using leased lines, WANs can also be built using less costly circuit switching or packet switching methods. Network protocols including TCP/IP deliver transport and addressing functions. Protocols including Packet over SONET/SDH, MPLS, ATM and Frame relay are often used by service providers to deliver the links that are used in WANs. X.25 was an important early WAN protocol, and is often considered to be the "grandfather" of Frame Relay as many of the underlying protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay.

Connection technology options

Several options are available for WAN connectivity:

Option: Description Advantages Disadvantages Bandwidth range

Sample protocols used

Leased line

Point-to-Point connection between two computers or Local Area Networks (LANs)

Most secure ExpensivePPP, HDLC, SDLC, HNAS

Circuit switching

A dedicated circuit path is created between end points. Less Expensive Call Setup 28 - 144

kbit/s PPP, ISDN

Packet switching

Devices transport packets via a shared single point-to-point or point-to-multipoint link across a carrier internework. Variable length packets are transmitted over Permanent Virtual Circuits (PVC) or Switched Virtual Circuits (SVC)

Shared media across link

X.25 Frame-Relay

Cell relay

Similar to packet switching, but uses fixed length cells instead of variable length packets. Data is divided into fixed-length cells.

Best for simultaneous use of voice and data

Overhead can be considerable

ATM

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Q.2. Write a case study on STOP AND WAIT protocol?

Stop-and-wait is a method used in telecommunications to send information between two connected devices. It ensures that information is not lost due to dropped packets and that packets are received in the correct order.

It is the simplest kind of automatic repeat-request (ARQ) method. A stop-and-wait ARQ sender sends one frame at a time; it is a special case of the general sliding window protocol with both transmit and receive window sizes equal to 1. After sending each frame, the sender doesn't send any further frames until it receives an acknowledgement (ACK) signal. After receiving a good frame, the receiver sends an ACK. If the ACK does not reach the sender before a certain time, known as the timeout, the sender sends the same frame again.

Typically the transmitter adds a redundancy check number to the end of each frame. The receiver uses the redundancy check number to check for possible damage. If

“ In the stop-and-wait method of flow control, the sender sends one frame and waits for an acknowledgement before sending the next frame.”

the receiver sees that the frame is good, it sends an ACK. If the receiver sees that the frame is damaged, the receiver discards it and does not send an ACK -- pretending that the frame was completely lost, not merely damaged.

One problem is where the ACK sent by the receiver is damaged or lost. In this case, the sender doesn't receive the ACK, times out, and sends the frame again. Now the receiver has two copies of the same frame, and doesn't know if the second one is a duplicate frame or the next frame of the sequence carrying identical data.

Another problem is when the transmission medium has such a long latency that the sender's timeout runs out before the frame reaches the receiver. In this case the sender resends the same packet. Eventually the receiver gets two copies of the same frame, and sends an ACK for each one. The sender, waiting for a single ACK, receives two ACKs, which may cause problems if it assumes that the second ACK is for the next frame in the sequence.

To avoid these problems, the most common solution is to define a 1 bit sequence number in the header of the frame. This sequence number alternates (from 0 to 1) in subsequent frames. When the receiver sends an ACK, it includes the sequence number of the next packet it expects. This way, the receiver can detect duplicated frames by checking if the frame sequence numbers alternate. If two subsequent frames have the same sequence number, they are duplicates, and the second frame is discarded. Similarly, if two subsequent ACKs reference the same sequence number, they are acknowledging the same frame.

Stop-and-wait ARQ is inefficient compared to other ARQs, because the time between packets, if the ACK and the data are received successfully, is twice the transit time (assuming the turnaround time can be zero). The throughput on the channel is a fraction of what it could be. To solve this problem, one can send more than one packet at a time with a larger sequence number and use one ACK for a set. This is what is done in Go-Back-N ARQ and the Selective Repeat ARQ.

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Q.3. Write a case study on sliding window protocol?

In the sliding window method of flow control, the sender can transmit several frames before needing an acknowledgement. Fames can be sent one after another, meaning that the link can carry several frames at once and its capacity can be used efficiently. The receiver acknowledges only some of the frames, using a single ack to confirm the receipt of multiple data frames.

A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP).

Conceptually, each portion of the transmission (packets in most data link layers, but bytes in TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to place received packets in the correct order, discarding duplicate packets and identifying missing ones. The problem with this is that there is no limit of the size of the sequence numbers that can be required.

By placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence numbers.

For the highest possible throughput, it is important that the transmitter is not forced to stop sending by the sliding window protocol earlier than one round-trip delay time (RTT). The limit on the amount of data that it can send before stopping to wait for

“ In the Sliding window method of flow control, several frames can be transit at a time.”

an acknowledgment should be larger than the bandwidth-delay product of communications link. If it is not, the protocol will limit the effective bandwidth of the link.

Conceptually, the sliding window of the sender shrinks from the left when frames of data are sent. The sliding window of the sender expands to the right when acknowledgements are received.

Go-Back-N

Go-Back-N ARQ is the sliding window protocol with wt>1, but a fixed wr=1. The receiver refuses to accept any packet but the next one in sequence. If a packet is lost in transit, following packets are ignored until the missing packet is retransmitted, a minimum loss of one round trip time. For this reason, it is inefficient on links that suffer frequent packet loss.

Selective repeat

The most general case of the sliding window protocol is Selective Repeat ARQ. This requires a much more capable receiver, which can accept packets with sequence numbers higher than the current nr and store them until the gap is filled in.

The advantage, however, is that it is not necessary to discard following correct data for one round-trip time before the transmitter can be informed that a retransmission is required. This is therefore preferred for links with low reliability and/or a high bandwidth-delay product.

The window size wr need only be larger than the number of consecutive lost packets that can be tolerated. Thus, small values are popular; wr=2 is common.

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Q.4. Write a brief note on IEEE 802.3, 802.4, 802.5?

Ethernet: IEEE 802.3

IEEE 802.3 supports a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation and Xerox. This was

called “Ethernet.”

Ethernet protocols refer to the family of local-area network (LAN) covered by the IEEE 802.3. In the Ethernet standard, there are two modes of operation: half-duplex and full-duplex modes. In the half duplex mode, data are transmitted using the popular Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) protocol on a shared medium. The main disadvantages of the half-duplex are the efficiency and distance limitation, in which the link distance is limited by the minimum MAC frame size. Therefore, the carrier extension technique is used to ensure the minimum frame size of 512 bytes in Gigabit Ethernet to achieve a reasonable link distance.

Four data rates are currently defined for operation over optical fiber and twisted-pair cables:

10 Mbps - 10Base-T Ethernet (IEEE 802.3) 100 Mbps - Fast Ethernet (IEEE 802.3u) 1000 Mbps - Gigabit Ethernet (IEEE 802.3z) 10-Gigabit - 10 Gbps Ethernet (IEEE 802.3ae).

The Ethernet system consists of three basic elements: 1. the physical medium used to carry Ethernet signals between computers, 2. a set of medium access control rules embedded in each Ethernet interface, and 3. an Ethernet frame that consists of a standardized set of bits used to carry data over the system.

As with all IEEE 802 protocols, the ISO data link layer is divided into two IEEE 802 sublayers, the Media Access Control (MAC) sub layer and the MAC-client sublayer. The IEEE 802.3 physical layer corresponds to the ISO physical layer.

The MAC sub-layer has two primary responsibilities:

Data encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception

Media access control, including initiation of frame transmission and recovery from transmission failure

The MAC-client sub-layer may be one of the following:

Logical Link Control (LLC), which provides the interface between the Ethernet MAC and the upper layers in the protocol stack of the end station. The LLC sublayer is defined by IEEE 802.2 standards.

Access to the shared channel is determined by the medium access control (MAC) mechanism embedded in the Ethernet interface located in each station. The medium access control mechanism is based on a system called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

Protocol Structure - Ethernet: IEEE 802.3 Local Area Network protocolsThe basic IEEE 802.3 Ethernet MAC Data Frame for 10/100Mbps Ethernet:

7 1 6 6 2 46-1500bytes 4

Pre SFD DA SA Length Type Data unit + pad FCS

Preamble (PRE) - 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.

Start-of-frame delimiter (SFD) - 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.

Destination address (DA) - 6 bytes. The DA field identifies which station(s) should receive the frame..

Source addresses (SA) - 6 bytes. The SA field identifies the sending station. Length/Type- 2 bytes. This field indicates either the number of MAC-client data

bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format.

Data- Is a sequence of n bytes (46=< n =<1500) of any value. (The total frame minimum is 64bytes.)

Frame check sequence (FCS) - 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

Token bus: IEEE 802.4

Local area network have a direct application in factory automation and process control, where the nods are computers controlling the manufacturing process .in this type of application, real time processing with minimum delay is needed. Processing must be occur at the same speed as the objects moving along the assembly line. Ethernet (IEEE 802.3) is not a suitable protocol for this purpose because the number of collisions is not predictable and the delay in sending data from the control center to the computers along the assembly line resembles a bus topology and not a ring

Token bus combine feature of Ethernet and token ring. It combines the physical configuration of Ethernet and the collision free feature of the token ring. Token bus is a physical bus that operates as a logical ring using tokens. Token bus is limited to factory automation and process control and has no commercial application in data communication.

Token bus was standardized by IEEE standard 802.4. It is mainly used for industrial applications. Token bus was used by GM (General Motors) for their Manufacturing Automation Protocol (MAP) standardization effort. This is an application of the concepts used in token ring networks. The main difference is that the endpoints of the bus do not meet to form a physical ring. The IEEE 802.4 Working Group is disbanded (FMS).

Token Ring: IEEE 802.5

Token Ring as defined in IEEE 802.5 is originated from the IBM Token Ring LAN technologies. Both are based on the Token Passing technologies. While them differ in minor ways but generally compatible with each other.

Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit. If a node receiving the token has no information to send, it seizes the token, alters 1 bit of the token (which turns the token into a start-of-frame sequence), appends the information that it wants to transmit, and sends this information to the next station on the ring. While the information frame is circling the ring, no token is on the network, which means that other stations wanting to transmit must wait. Therefore, collisions cannot occur in Token Ring networks.

The information frame circulates the ring until it reaches the intended destination station, which copies the information for further processing. The information frame continues to circle the ring and is finally removed when it reaches the sending station. The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination.

“ Token ring allow each station to send one frame per turn.”

Unlike Ethernet CSMA/CD networks, token-passing networks are deterministic, which means that it is possible to calculate the maximum time that will pass before any end station will be capable of transmitting. This feature and several reliability features make Token Ring networks ideal for applications in which delay must be predictable and robust network operation is important.

Protocol Structure - Token Ring: IEEE 802.5 LAN Protocol

1 2 3 9 15bytes

SDEL AC FC Destination address Source address

Route information 0-30 bytes

Information (LLC or MAC) variable

FCS (4 bytes) EDEL (1) FS(1)  

SDEL / EDEL - Starting Delimiter / Ending Delimiter. Both the SDEL and EDEL have intentional Manchester code violations in certain bit positions so that the start and end of a frame can never be accidentally recognized in the middle of other data.

AC - Access control field contains the Priority fields. FC - Frame control field indicates whether the frame contains data or control

information Destination address - Destination station address. Source address - Source station address. Route information - The field with routing control, route descriptor and routing

type information. Information - The Information field may be LLC or MAC. FCS - Frame check sequence. Frame status - Contains bits that may be set on by the recipient of the frame to

signal recognition of the address and whether the frame was successfully copied.

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Q.5. Explain FDDI in detail?

Fiber Distributed Data Interface (FDDI)

FDDI is a set of ANSI protocols for sending digital data over fiber optic cable. FDDI networks are token-passing (similar to IEEE 802.5 Token Ring protocol) and dual-ring networks, and support data rates of up to 100 Mbps. FDDI networks are typically used as backbones technology because of its support for high bandwidth and great distance. A related copper specification similar to FDDI protocols, called Copper Distributed Data Interface (CDDI), has also been defined to provide 100-Mbps service over twisted-pair copper.

An extension to FDDI, called FDDI-2, supports the transmission of voice and video information as well as data. Another variation of FDDI called FDDI Full Duplex Technology (FFDT) uses the same network infrastructure but can potentially support data rates up to 200 Mbps.

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle, the primary purpose of the dual rings is to provide superior reliability and robustness.

FDDI's four specifications are the Media Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT) specifications. The MAC specification defines how the medium is accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms. The PHY specification defines data encoding/decoding procedures, clocking requirements, and framing, among other functions. The PMD specification defines the characteristics of the transmission medium,

including fiber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT specification defines FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection.

Protocol Structure - FDDI: Fiber Distributed Data Interface

2 6 6 0-30 Variable 4bytes

Frame control

Destination address

Source address

Route information

Information

FCS

Frame control - The frame control structure is as follows:

C L F F Z Z Z Z

C - Class bit: 0 Asynchronous frame; 1 Synchronous frame/ L - Address length bit: 0 16 bits (never); 1 48 bits (always). FF - Format bits. ZZZZ - Control bits.

Destination address - The address structure is as follows:

I/G U/L Address bits

Source address - The address structure is as follows:

I/G RII Address bits

I/G - Individual/group address: 0 Group address; 1 Individual address. RII - Routing information indicator: 0 RI absent; 1 RI present.

Route Information - The structure of the route information is as follows:

3 5 1 6 1 16 16 16

RT LTH D LF R RD1 RD2 RDn

RC - Routing control (16 bits). RDn - Route descriptor (16 bits). RT - Routing type (3 bits). LTH - Length (5 bits). D - Direction bit (1 bit). LF - Largest frame (6 bits). R - reserved (1 bit).

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Q.6. Explain various Network Topologies?

TOPOLOGY

The term topology refers a way a network is laid out, either physically or logically.Two or more devices connect to a link; two or more link forms a topology. There are basic five topologies are possible

Mesh topology Star Topology Tree Topology Bus Topology Ring Topology

Mesh

The dedicated means that the link carries traffic only between the two devices it connects. A fully connected mesh network therefore has n (n-1)/2 physical channels to link n devices. To accommodate that many links, every device on the network must have n-1 input/output ports.

Advantages:- The use of dedicated links guarantees that each connection can carry its own

data load. A mesh topology is robust. It also provide the privacy and security of the network.

Disadvantages:-

“In a Mesh Topology, every device has a dedicated point to point link to every other device.”

The main disadvantages of mesh are related to the amount of cabling and the number of I/O ports required.

Star

The devices are not directly connected to each other. Unlike a mesh topology, a star topology does not allow direct traffic between two devices. The controller acts as an exchange: if one device wants to send data to another, it sends data to the controller, which then relays the data to the other connected devices. A star topology is less expansive than a mesh topology. In a star, each device needs only one link and one I/O post to connect it to any number of others. It includes robustness.

Advantages:- A star topology is less expansive than a mesh topology. In a star, each device

needs only one link and one I/O post to connect it to any number of others. It includes robustness. If one link fails, only that link is affected. All other links

remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working. It can be used for monitor link problems and bypass defective lines.

It is easy to install and configure.Disadvantages:-

If the central controller hub fails. Then the network gets destroyed.

Tree

HUB

“ In a star topology, each device has a dedicated point to point link only to a central controller, usually called a hub” HUB

However, not every device plugs directly into the central hub. The central hub in a tree is an active hub. An active hub contains a repeater, which is a hardware device that regenerates the received bit pattern before sending them out.

Advantages:- It allows more devices to be attached to a single central hub and can therefore

increase the distance a signal can travel between those devices. It allows the network to isolate and prioritize communications from different

computers.

A good example of tree topology can be seen in cable TV technology where the main cable from the main office is divided into main branches and each branch is divided into smaller branches and so on. The hubs are used when the cable are divided.

As in a star, nodes in a tree are linked to a central hub that controls the traffic to the network. However, not every device plugs directly into the central hub. The central hub in a tree is an active hub. An active hub contains a repeater, which is a hardware device that regenerates the received bit pattern before sending them out.

“A tree topology is a variation of star topology. As in a star, nodes in a tree are linked to a central hub that controls the traffic to the network.” HUB

Bus

Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of the cable to create a contact with the metallic core.

Advantages:- A bus topology includes easy of installation. Backbone cable can be laid along the

most efficient path, and then connected to the nodes by drop lines. A bus uses less cabling then of other topologies.

Disadvantages:- It includes difficult reconfiguration and fault isolation. A fault or break in the bus cable stops all the transmission.

“A bus topology, on the other hand, is multipoint. One long cable act as a backbone to link all the devices in the network.”

Ring

A signal is passed along the ring in one direction from device to device, until it reaches its reaches its destination.. Each device in a ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bit and passes them along. A ring is relatively easy to install and reconfiguration.

Advantages:- A ring is relatively easy to install and reconfigure. Each device is linked only to its

immediate neighbors. Fault isolation is also simplified. Generally in a ring, a signal is circulating at all

times.

Disadvantages:-

Unidirectional traffic is a disadvantage of ring topology. A break in the ring can disable the entire network.

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“In a ring topology, each device has a dedicated point to point line configuration only with the two devices on either side it.”

Q.7. Establishing and studying the various parameters of a home LAN network?

Installing a Wireless RouterOne wireless router supports one WLAN. Use a wireless router on your network if:

you are building your first home network, or you want to re-build your home network to be all-wireless, or you want to keep your WLAN installation as simple as possible

Try to install your wireless router in a central location within the home. The way Wi-Fi networking works, computers closer to the router (generally in the same room or in "line of sight") realize better network speed than computers further away.

Connect the wireless router to a power outlet and optionally to a source of Internet connectivity. All wireless routers support broadband modems, and some support phone line connections to dial-up Internet service. If you need dial-up support, be sure to purchase a router having an RS-232 serial port. Finally, because wireless routers contain a built-in access point, you're also free to connect a wired router, switch, or hub.

Next, choose your network name. In Wi-Fi networking, the network name is often called the SSID. Your router and all computers on the WLAN must share the same SSID. Although your router shipped with a default name set by the manufacturer, it's best to change it for security reasons. Consult product documentation to find the network name for your particular wireless router, and follow this general advice for setting your SSID.

Last, follow the router documentation to enable WEP security, turn on firewall features, and set any other recommended parameters.

Installing a Wireless Access PointOne wireless access point supports one WLAN. Use a wireless access point on your home network if:

you don't need the extra features a wireless router provides AND you are extending an existing wired Ethernet home network, or you have (or plan to have) four or more wireless computers scattered throughout

the home

Install your access point in a central location, if possible. Connect power and a dial-up Internet connection, if desired. Also cable the access point to your LAN router, switch or hub. See the diagram in the Page 3 sidebar for details.

We won't have a firewall to configure, of course, but we still must set a network name and enable WEP on your access point at this stage.

Configuring the Wireless Adapters Configure your adapters after setting up the wireless router or access point (if you have one). Insert the adapters into your computers as explained in your product documentation. Wi-Fi adapters require TCP/IP be installed on the host computer.

Manufacturers each provide configuration utilities for their adapters. On the Windows operating system, for example, adapters generally have their own graphic user interface (GUI) accessible from the Start Menu or taskbar after the hardware is installed. Here's where you set the network name (SSID) and turn on WEP. You can also set a few other parameters as described in the next section. Remember, all of your wireless adapters must use the same parameter settings for your WLAN to function properly.

Configuring an Ad-Hoc Home WLAN Every Wi-Fi adapter requires you to choose between infrastructure mode (called "access point" mode in some configuration tools) and ad-hoc ("peer to peer") mode. When using a wireless access point or router, set every wireless adapter for infrastructure mode. In this mode, wireless adapters automatically detect and set their WLAN channel number to match the access point (router).

Alternatively, set all wireless adapters to use ad hoc mode. When you enable this mode, you'll see a separate setting for channel number. All adapters on your ad hoc wireless LAN need matching channel numbers.

Ad-hoc home WLAN configurations work fine in homes with only a few computers situated fairly close to each other. You can also use this configuration as a fallback option if your access point or router breaks:

Configuring Software Internet Connection Sharing As shown in the diagram, you can share an Internet connection across an ad hoc wireless network. To do this, designate one of your computers as the host (effectively a substitute for a router). That computer will keep the modem connection and must obviously be powered on whenever the network is in use. Microsoft Windows offers a feature called Internet Connection Sharing (ICS) that works with ad hoc WLANs.

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Q.8. Explain Routers, Bridges, and Switches and gateways?

Routers

Network router is a device or a piece of software in a computer that forwards and routes data packets along networks. A network router connects at least two networks, commonly two LANs or WANs or a LAN and its ISP network. A router is often included as part of a network switch. A router is located at any where one network meets another, including each point-of-presence on the Internet. A router has two key jobs:

The router ensures that information doesn't go where it's not needed. This is crucial for keeping large volumes of data from clogging the network.

The router makes sure that information does make it to the intended destination.

In performing these two jobs, a router joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. This process is known as routing.

Routing is a function associated with the Network layer (layer 3) in the Open Systems Interconnection (OSI) model. Routers use network layer protocol headers, such as IP header where the source and destination addresses are included and routing tables to determine the best path to forward the packets. For the communication among routers and decide the best route between any two hosts, routing protocols such as ICMP are used.

Actually, routers are specialized computers that send messages speeding to their destinations along thousands of possible pathways. One of the tools a router uses to decide which path a packet should go is a routing table. A routing table contains a collection of information, including:

Information on which connections lead to particular groups of addresses Priorities for connections to be used Rules for handling both routine and special cases of traffic

Information in the routing tables can be static (with routes manually entered by the network administrator) or dynamic (where routers communicate to exchange connection and route information using various routing protocols).

A routing table can be as simple as a few lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages.

As the number of networks attached to one another grows, the routing table for handling traffic among them grows, and the processing power of the router is increased.

Packets routed by routers to their destinations

Bridges

Bridges operate in both the physical and data link layer of the OSI modal. Bridges can divide a large network into smaller segments. They can also relay frames between two originally separate LAN’s. Bridges contain logic that allows them to keep the traffic for each segment separate. In this way, they filter traffic, a fact that makes them useful for controlling congestion and isolating problem links. Bridges can also provide security through this partitioning of traffic. When a frame enters a bridge, the bridge not only regenerates the signal but checks the address of the destination and forwards the new copy only to the segment to which the address belongs. As a bridge encounters a packet, it reads the address contained in

the frame and compares that address with a table of all the stations on both segments. When it finds a match, it discovers to which segment the station belongs and relays the packet only to that segment.

Types of Bridge

To select between segments, a bridge must have a look up table that contains the physical address of every station connected to it. The table indicates to which the segment each station belongs.

Simple Bridge

Simple bridges are the most primitive and least expansive type of bridge. A simple bridge links two segments and contains a table that lists the addresses of all the stations included in each of them. What makes it primitive is that these addresses must be entered manually. Before a simple bridge can be used, an operator must sit down and enter the addresses of every station. Whenever a new station is added, the table must be modified. If a station is removed, the newly invalid address must be deleted.

Multiport bridge

A multiport bridge can be used to connect more than two LAN’s. The bridge has three tables, each one holding the physical addresses of stations reachable through the corresponding port.

Transport Bridge

A transparent, or learning, bridge builds its table of station addresses on its own as it performs its bridge function. When the transparent bridge is first installed, its table is empty. As it encounters each packet, it looks a both the destination and the source addresses. It checks the destination to decide where to send the packet. If it is not recognize the destination address, it relays the packet to all of the stations on both segments. It uses the source address to build its table. As it reads the source address, it notes which side the packet came from and associates that addresses with the segment to which it belongs.

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Switch

“Switches are hardware or software devices which are capable of creating temporary connection between two or more devices linked to the switch not to the each other.”

Switch is a network exchange facility operating at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Reference Model. Classified by working protocols, there are two-layer switch, three-layer switch, four-layer switch and multiple-layer switch. Switch also can be classified into managed switch and unmanaged switch.

Generally, three-layer switch and above has management function (managed switch).

Unlike hubs, switches prevent promiscuous sniffing. In a switched network environment, Javvin Packet Analyzer (or any other packet analyzer) is limited to capturing broadcast and multicast packets and the traffic sent or received by the PC on which it is running.

However, most modern switches (management switches) support "port mirroring", which is a feature that allows you to configure the switch to redirect the traffic that occurs on some or all ports to a designated monitoring port on the switch. With this feature, you can monitor the entire LAN segment in switched network environment. Please refer to the documentation coming with your switch for the availability information about this feature and configuration instructions.

If your switch dose not support "port mirroring", you can install Javvin Packet Analyzer on a workstation connected to the same hub as your Internet gateway, or on your Internet gateway (if acceptable), thus you can monitor all network traffic between your intranet and the Internet.

Configuring a switch

Javvin Packet Analyzer should be installed on the host/server connected with the switch’s mirror port (span port).

Mirror port configuration:

Mirror the way out port to the management port (mirror port), in this way the entire data transmitted into/out of LAN can be monitored.

Mirror all way out ports to the management port (mirror port), in this way not only the entire data transmitted into/out of LAN but also the communication among hosts in LAN can be monitored. (Recommend)

The following are two examples for CISCO switch using the "monitor" command in configuration mode:

Format:

#monitor session number source interface mod_number/port_number#monitor session number destination interface mod_number/port_number

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another.

CROSS-BAR SWITCH

A switch may be directly manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a light switch. Automatically operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a

machine tool is in a position to accept another work piece. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is a temperature-operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism. Some switches are used to isolate electric power from a system.

Gateway

In telecommunications, the term gateway has the following meaning:

In a communications network, a network node equipped for interfacing with another network that uses different protocols.

o A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolators, or signal translators as necessary to provide system interoperability. It also requires the establishment of mutually acceptable administrative procedures between both networks.

o A protocol translation/mapping gateway interconnects networks with different network protocol technologies by performing the required protocol conversions.

Loosely, a computer or computer program configured to perform the tasks of a gateway. For a specific case, see default gateway.

“ Gateways, also called protocol converters, can operate at any network layer. The activities of a gateway are more complex than that of the router or switch as it communicates using more than one protocol.”

A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes, while the nodes that connect the networks in between are gateways. For example, the computers that control traffic between company networks or the computers used by internet service providers (ISPs) to connect users to the internet are gateway nodes.

In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.

Internet-to-Orbit Gateway

An Internet to orbit gateway (I2O) is a machine that acts as a connector between computers or devices connected to the Internet and computer systems orbiting the earth, like satellites or even manned spacecrafts. Such connection is made when the I2O establishes a stable link between the spacecraft and a computer or a network of computers on the Internet, such link can be control signals, audio frequency, or even visible spectrum signals.

Project HERMES is the first project to have brought this kind of machine into operation. The HERMES-A/MINOTAUR Space Flight Control Center became operative on June 6, 2009 and was operated by representatives of 34 countries on the UNOOSA Symposium of Small Satellites for Sustainable Development in Graz, Austria on

September 10, 2009. Project HERMES is an initiative of the Ecuadorian Civilian Space Agency and has a maximum coverage of 22,000 km, HERMES-A is supposed to be the first gateway of a network of five covering all South America. HERMES-A/MINOTAUR is not only capable of data transmission but voice also.

Project GENSO is an initiative from NASA and ESA, and it is expected to begin operations on April 2010, it is supposed to have worldwide coverage.

Cloud Gateway

A Cloud storage gateway is a network appliance or server which resides at the customer premises and translates cloud storage APIs such as SOAP or REST to block-based storage protocols such as iSCSI or Fiber Channel or file-based interfaces such as NFS or CIFS. Cloud storage gateways enable companies to integrate cloud storage into applications without moving the applications into the cloud. In addition they simplify data protection.

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