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INTERNATIONAL DIPLOMA IN COMPUTER STUDIES
COMPUTER NETWORK (C1006)
ASSIGNMENT
TERM 1 2010
Instructions:
Answer ALL questions.
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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
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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