CS 610 Computer Networks, Fall 2010
Assignment # 01
Deadline
Assignment # 01
Deadline
Your assignment must be uploaded / submitted before or on November 01, 2010.
Marks = 20
Upload Instructions
Please view the assignment submission process document provided to you by the
Virtual University to upload the assignment.
Rules for Marking
Please note that your assignment will not be graded if:
• It is submitted after due date
• The file you uploaded does not open
• The file you uploaded is copied from some one else or from internet
• It is in some format other than .doc
Objective
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
words?
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.
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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.
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CS 610 Idea Solution: |
Question # 1: Explain CSMA/CD with a real example in your own |
words? |
Solution: |
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 |
next. |
Note: |
Solve it by yourself |
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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. |
OR |
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? |
Solution: |
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 |
removed. |
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. |
Note: |
Solve it by yourself |
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