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Wednesday, October 27, 2010


CS 610 Computer Networks, Fall 2010
Assignment # 01

Your assignment must be uploaded / submitted before or on November 01, 2010.
Marks = 20
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Please view the assignment submission process document provided to you by the
Virtual University to upload the assignment.
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Please note that your assignment will not be graded if: 
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•  It is in some format other than .doc

The assignment has been designed to enable you:
•  Understanding of CSMA/CD and Data Stuffing. 

Assignment Statements:
Question # 1: Explain CSMA/CD with a real example in your own
Question # 2: What do you mean by data stuffing, Can you differentiate
the control characters from frame delimiters?
Important Note: It is better to write your own understanding rather than
coping from any source.  

It is Collision Detection for Ethernet. Basically if two devices try to communicate with the same third device at the same time they will fail. Then they will start a random algorithm in which it will count down to retransmit.

• Short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.
• CSMA/CD is a type of contention protocol. Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics. The CSMA/CD method is internationally standardized in IEEE 802.3 and ISO 8802.3.


CS 610 Idea Solution:
Question # 1: Explain CSMA/CD with a real example in your own
The CSMA/CD protocol functions somewhat like a conference in a dark room. Everyone
has same priority to speak something. Everyone around the table must listen for a period
of quiet before speaking (Carrier Sense). Once  a space o ccurs ever yone has an equal
chance to say something (Multiple Access). It two people start talking at the same time
no one can understand what they speak and th ey detect that fact and quit speaking
(Collision Detection).
To translate this into Ethernet terms, each interface must wait until there is no signal on
the channel, and then it can begin transmitting. If some other interface is transmitting
there will be a signal on the channel, which is called carrier. All other interfaces must
wait until carrier ceases before trying to transmit, and this process is called Carrier Sense.
All Ethernet interfaces are equal in their ability to send frames onto the network. No one
gets a higher priority than anyone else, and democracy reigns. This is what is meant by
Multiple Access. Since signals take a finite time to travel from one end of an Ethernet
system to the other, the first bits of a transmitted frame do not reach all parts of the
network simultaneously. Therefore, it's possible for two interfaces to sense that the
network is idle and to start transmitting their frames simultaneously. When this happens,
the Ethernet system has a way to sense the "collision" of signals and to stop the
transmission and resend the frames. This is called Collision Detect.
The CSMA/CD protocol is designed to provide fair access to the shared channel so that
all stations get a chance to use the network. After  every packet transmission all stations
use the CSMA/CD protocol to determine which station gets to use the Ethernet channel
Solve it by yourself

CSMA/CD (Carrier Sense Multiple Access / Collision Detection) is the protocol used in
Ethernet networks to ensure that only on e network node is transmitting on the network
wire at any one time.
Carrier Sense
means that every Ethernet device listens to the Ethernet wire before it
attempts to transmit.  If the Ethernet device senses that another device is transmitting, it
will wait to transmit.
Multiple Access
means that more than one Ethernet device can be sensing (listening and
waiting to transmit) at a time.
Collision Detection
means that when multiple Ethernet devices accidentally transmit at
the same time, they are able to detect this erro r.
Question # 2: What do you mean by data stuffing, Can you differentiate
the control characters from frame delimiters?
Whenever a system sends data to another system it sends it through frames. It uses
control characters soh (start of header) and eot (end of header) to indicate the start and
end of a data frame. They actually play management role in frames. Sometimes the
control characters may appear in data  and as a par t of data they will be misinterpreted as
framing data.
In general to distinguish between data being sent and control information such as frame
delimiters network systems arrange for the sending side to change the d ata  slightly before
it is sent because systems usually insert data or b ytes to change data for transmission, the
technique is known as Data Stuffing.
The benefit of Data Stuffing is that the confusion of control characters in the data is
Control characters are used as frame delimiters. For e.g. the fr ame used the control
character soh in the start of frame to tell the  receiver that the frame is start from there and
the control character eot is used at the end of the  frame to tell the receiver that the frame
is ended. These control characters are called frame delimiters.
Solve it by yourself

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