Question

a. Critically analyse the role of flow and error control in a computer network. In your explanation appropriate figures and examples are expected.

b. Calculate the CRC for the X binary data shown as shown below, when the divisor is 1101.

(A)2 = (200 + X)10 X = 1001100

c. Consider a scenario where a frame would be sent after appending the checksum to the original data. How would you prove that the frame received is error free?

Answer 1

a)

FLOW CONTROL	ERROR CONTROL
It refers to the proper tansmission of data from sender to receiver.	It refers to the error-free delivery of data to the receiver
Feedback based flow control and rate -based flow control are the approaches to achieve the proper flow control.	This mechanism detects and corrects error in the data.
Protocols for flow control Stop & Wait ARQ , Go Back N ARQ & Selective Repeat ARQ	Parity Checking , CRC and CheckSum are the approaches to detect the error where as Hamming Code are the ways to correct error .
Flow control is maintained at DLL and Transport Layer when a sender sends the frames faster than the receiver can accept.In this case , even the data is received without any error .So inorder to overcome the speed mismatch of sender and receiver a mechanism known as Flow control is used.	This is also maintained at DDL & Transport Layer for error detection and correction of frames delivered to receiver.The error occured in the frame may be a single bit error or brust error.Single bit error refers to exchange of 0 ->1 & 1->0 where as brust error include duplicate packet , packet loss, ACK lost .

c)

CheckSum: It is a error detection method

Working Scenario :

Step1: At Sender Side

i) If m bit checksum is used , the data unit to be transmitted is divided into segments of m bits.

ii) All the m bits added, SUM is then complemented using 1's complement arithmetic , value so obtained is called as CheckSum.

Step2:Data along with the CheckSum value is transmitted into the receiver.

Step3: At Receiver Side,

i) If m bit checksum is being used , the received data unit is divided into segments of m bits.

ii) All the m bits segments are added along with the checksum value , the value so obtained is complemented and result is checked.

Two Cases would be possible :

Case1: Result =0 ,Receiver assumes no error occured during the transmission and accepts data.

Case2:Result !=0 ,Receiver assumes error occured during the transmission and discards the data and asks for retransmission.