Computer Network Asg

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INTERNATIONAL DIPLOMA IN COMPUTER STUDIES

COMPUTER NETWORK (C1006)

ASSIGNMENT

TERM 1 2010

Instructions:

Answer ALL questions.

Marks will be awarded for good presentation and thoroughness in your approach

NO marks will be awarded for the entire assignment if any part of it is found to be

copied directly from printed materials or from another student.

Complete this cover sheet and attach it to your assignment.

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I declare that:

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The implication of plagiarism has been explained to me by my institution

This assignment is all my own work and I have acknowledged any use of thepublished and unpublished works of other people.

Students signature:. Date:

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T110 C1006 ASG QP Page 1 of 13


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Question 1

With aid of proper diagrams, explain how the following types of multiplex devices work.

Frequency Division MultiplexingMultiplexing is the set of techniques that allows the simultaneously transmission ofmultiple transmission of multiple signals across a single data link. In a multiplexedsystem, n lines share the bandwidth of one link.

This figure shows the basic format of a multiplexed system. The lines on the leftdirect their transmission streams to a multiplexer (MUX), which combines them into asingle stream (many to one). At the receiving end, that stream is fed into ademultiplexer (DEMUX), which separates the stream back into its componenttransmissions (one to many) and directs them to their corresponding lines.

The word link refers to the physical path. The word channel refers to the portion of alink that carries a transmission between a given pair of lines. One link can have manychannels.

Classification of Multiplexing:

There are three basic multiplexing techniques:1. Frequency Division Multiplexing (FDM)

2. Time Division Multiplexing (TDM)

3. Wavelength Division Multiplexing (WDM)

Frequency Division Multiplexing (FDM)

FDM is an analog multiplexing technique that combines analog

signals. This technique is the oldest multiplexing technique. In

FDM, signals generated by each sending device modulate different

carrier frequencies. The multiplexer is attached to a high-speed

communications line. A corresponding multiplexer, or demultiplexer,

is on the end of the high-speed line and separates the multiplexed



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signals. These modulated signals are then combined into a single

composite signal that can be transported by the link. Carrier

frequencies are separated by sufficient bandwidth to accommodate

the modulated signal. Channels can be separated by strips of

unused bandwidth ---- guard bands --- to prevent signals from

overlapping. Carrier frequencies must not interfere with the original

data frequencies.

Multiplexing Process:

Carrier frequencies must not interfere with the original data

frequencies. Each source generates a signal of similar frequency

range. Inside the multiplexer, these similar signals modulates

different carrier frequencies. The resulting modulated signals are

then combined into a single composite signal that it sent out over a

media link that has enough bandwidth to accommodate it.



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Demultiplexing Process:

The demultiplexer uses a series of filters to decompose the

multiplexer signal into a constituent component signals. The

individual signals are then passed to a demodulator that separates

them from their carriers and passes them to the output lines.

Time Division Multiplexing TDM is a digital process that allows several connections to share the

high bandwidth of a link.

Each connection occupies a portion of time in the link.



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Classification of TDM1. Synchronous Time Division Multiplexing

2. Statistical Time Division Multiplexing

Synchronous Time Division MultiplexingThis is the original time division multiplexing. In synchronous TDM, the

data flow of each input connection is divided into units, where each input

occupies one input time slot. Each input unit becomes one output unit and

occupies one output time slot. The duration of an output time slot is n

times shorter than the duration of an input time slot. If an input time slot

is T s, the output time slot is T/n s, where n is the number of connections.

In synchronous TDM, a round of data units from each input connection is

collected into a frame. Time slots are grouped into frames. A frame

consists of one complete cycle of time slots, with one slot dedicated to

each sending device. T-1 and ISDN telephone lines are common examples

of synchronous time division multiplexing.



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Interleaving

TDM can be visualized as two fast rotating switches, one on themultiplexing side and the other on the demultiplexing side. The switches

are synchronized and rotate at the same speed, but in opposite directions.

On the multiplexing side, as the switch opens in front of a connection, that

the connection has the opportunity to send a unit onto the path. This

process is called interleaving. On the demultiplexing side, as the switch

opens in front of a connection, that connection has the opportunity to

receive a unit from the path.

Fig: TDM Multiplexing



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Fig: TDM Demultiplexing

Empty slotsThis TDM is not as efficient as it could be. If a source does not have data to send, thecorresponding slot in the output frame is empty.

Frame SynchronizingThe implementation of TDM is not as simple as that of FDM. Synchronization between

multiplexer and demultiplexer is a major issue. If the multiplexer and demultiplexerare not synchronized, a bit belonging to one channel may be received by the wrongchannel. For this reason, one or more synchronization bits are usually added to thebeginning of each frame. These bits are called framing bits, follow a pattern frame toframe, that allows the demultiplexer to synchronize with the incoming stream so thatit can separate the time slots accurately.

Statistical Time Division Multiplexing

In statistical time division multiplexing, slots are dynamically allocated to

improve

bandwidth efficiency. The number of slots in each frame is less than the number

of input lines. The multiplexer checks each input line in round-robin fashion, it

allocates a slot for an input line if the line has data to send; otherwise it skips the

line and checks the next line. No slot is left empty as long as there are data to

be sent by any input line. A slot needs to carry data as well as the address of the

destination. There is no fixed relationship between the inputs and outputs



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The protocols involved

Diagram

Solution

As A5 transmits How are you to B2, when the enter key is pressed from node A5, the message (how are

you) is converted into binary codes as computers communicate by using codes (binary codes) not English

alphabets. The detailed explanation of the events taking place from the sender to the receiver of the

message is presented in the following section with respect to different networking concepts and processes.

2.1 Creation of Binary CodesThe How are you message that is sent from A5 to B2 must have to be converted to binary digits as

computers understand and communicate using binary codes. These binary codes are in different formats,

the commonly used is the ASCII (American Standard Code for Information Interchange).

Each of the English alphabets have a binary representation in the ASCII, and when this is converted it is

transmitted to the receiving host based on the window sized that both nodes agree upon.

The conversion of the message to be transmitted to binary codes or digits is done at the physical layer of

the OSI (Open Systems Interconnection) model.

2.2 Packet Switching technologyA packet is information or data that is to be sent over a network from one location to another, in packet

switching, communication is discrete in form of packets. Each packet is of a limited size and can hold up to

a certain number of octets of user data. Larger messages are broken into smaller chunks so that they can befitted into packets. In addition to user data, each packet carries additional information (in form of a header)

to enable the network to route it to its final destination.

The packet How are you is handed over from node to node across the LAN A to LAN B. Each of these

nodes receives and temporarily stores the packet, until the next node is ready to receive it, and then passes

it onto the next node. This technique is called store-and-forward and overcomes one of the limitations of

circuit switching. A packet-switched network has a much higher capacity for accepting further connections.

Additional connections are usually not blocked but simply slow down existing connections, because they

increase the overall number of packets in the network and hence increase the delivery time of each packet.

Two variations of packet switching exist: virtual circuit and datagram.

The virtual circuit method (also known as connection-oriented) is closer to circuit switching. Here a

complete route is worked out prior to sending data packets. The route is established by sending a

connection request packet along the route to the intended destination. This packet informs the intermediate



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nodes about the connection and the established route so that they will know how to route subsequent

packets. The result is a circuit somewhat similar to those in circuit switching, except that it uses packets as

its basic unit of communication. Hence it is called a virtual circuit.

Each packet carries a virtual circuit identifier which enables a node to determine to which virtual circuit it

belongs and hence how it should be handled. (The virtual circuit identifier is essential because multiple

virtual circuits may pass through the same node at the same time.) Because the route is fixed for the

duration of the call, the nodes spend no effort in determining how to route packets.

The advantage of the datagram approach is that because there is no circuit, congestion and faulty nodes can

be avoided by choosing a different route. Also, connections can be established more quickly because of

reduced overheads. This makes datagram better suited than virtual circuits for brief connections. For

example, database transactions in banking systems are of this nature, where each transaction involves only

a few packets.

The advantage of the virtual circuit approach is that because no separate routing is required for each packet,they are likely to reach their destination more quickly; this leads to improved throughput. Furthermore,

packets always arrive in order. Virtual circuits are better suited to long connections that involve the transfer

of large amounts of data (e.g., transfer of large files).

2.3 Ethernet AddressingEthernet is a contention media access method that allows all hosts on a network to share the same

bandwidth of a link. Ethernet is popular because its readily scalable, meaning that its comparatively easy

to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet, into an existing network

infrastructure. Ethernet networking uses Carrier Sense Multiple Access with Collision Detection(CSMA/CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit

at the same time on the network medium

When host A5 transmits over the network to host B2, it first checks for the presence of a digital signal on

the wire. If all is clear (no other host is transmitting), the host will then proceed with its transmission. But it

doesnt stop there. The transmitting host (host A5) constantly monitors the wire to make sure no other hosts

begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that

causes all nodes on the segment to stop sending data (think busy signal). The nodes respond to that jam

signal by waiting a while before attempting to transmit again. Backup algorithms determine when the

colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit

will then timeout.

When a collision occurs on an Ethernet LAN, the following happens:

- A jam signal informs all devices that a collision occurred.



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- The collision invokes a random backup algorithm.

- Each device on the Ethernet segment stops transmitting for a short time until the timers expires.

- All hosts have equal priority to transmit after the timers have expired.

It uses the Media Access Control (MAC) address burned into each and every Ethernet network interface

card (NIC). The MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal format.

Figure 1.19 shows the 48-bit MAC addresses and how the bits are divided.

Ethernet addressing using MAC addresses

The organizationally unique identifier (OUI) is assigned by the IEEE to an organization. Its composed of

24 bits, or 3 bytes. The organization, in turn, assigns a globally administered address (24 bits, or 3 bytes)

that is unique (supposedly, againno guarantees) to each and every adapter it manufactures. Look closely

at the figure. The high-order bit is the Individual/Group (I/G) bit.

2.4 IP addressing and DNS

IP addressing: IP address is shown in a group of numbers separated by dots. Each device to which isconnected to the internet has its own IP address. Therefore IP address is use to recognize it to the world and

play, the main part in TCP/IP protocol. Domain name system was the full meaning of the 3-letter words

(DNS). Similarly DNS seems like a converter because its convert its domain name into IP addresses. Its

special task is to go-between the IP addresses. DNs works each moment a domain name was typed in a

browser it will automatically passed to domain name system (DNS).

Frame formats and changes in the Header and Tail

The usual frame format of how are you is figuratively presented below:

Packet header: The packet header usually contains the source address which is A5 and destination address

(B2) of the packet.

Data section: The data section consists of the actual data being sent. The sizes of this section can very

depending on the network type.



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Packet trailer: The packet trailer contains information to verify the validity of the packet. Using a cyclic

redundancy check (CRC) usually does this. The CRC is a number on the packet calculated by the sending

computer and added to the trailer. When the receiving computer gets the packet, it recalculates the CRC

and compares it to the one in the trailer. If the CRCs match, it accepts the packet as undamaged. If CRCs

dont match, the receiving computer requests that the packet be re-sent.

2.4 The protocols involvedThere are several protocols involved in the process of sending How are you from host A5 in LAN A to

host B2 in LAN 2. Some of these protocols are discussed earlier in the section above, the common

protocols are:

- TCP/IP

- CSMA/CD

Diagram

References:



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1. Concepts of Topology, viewed on 4th 2010-06-04 http://whatis.techtarget.com/definition/network-

topologies.html

2. Understanding Local Area Networks Protocols, 4th 06 2010

http://learnat.sait.ab.ca/ict/txt_information/Intro2dcRev2/page19.html#WANs

3. Understanding networking technologies, by Clayton Coulter 1997

4. Wireless Local Area Network Guide, Hewlett Packard (HP) Small and Medium Business,

retrieved on 2nd June, 2009 23:42 at

http://www.hp.com/sbso/productivity/howto/wireless_lan/do_it.html

http://whatis.techtarget.com/definition/network-topologies.htmlhttp://whatis.techtarget.com/definition/network-topologies.htmlhttp://learnat.sait.ab.ca/ict/txt_information/Intro2dcRev2/page19.html#WANshttp://www.hp.com/sbso/productivity/howto/wireless_lan/do_it.htmlhttp://whatis.techtarget.com/definition/network-topologies.htmlhttp://whatis.techtarget.com/definition/network-topologies.htmlhttp://learnat.sait.ab.ca/ict/txt_information/Intro2dcRev2/page19.html#WANshttp://www.hp.com/sbso/productivity/howto/wireless_lan/do_it.html

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