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# CS601 Data Communication Assignment#05 Solution

ASSIGNMENT # 5

SEMESTER Spring-2010

CS601 – Data Communication

Q : 1)   What is the error detection method used by the upper-layer protocols ?

Solution:

The mechanism of error detection is by REDUNDANCY i.e. an extra bit is added (redundant) to the information and discarded as on the accurate reception.  Any discrepancy is indicated as an error and suitable error correction mechanism can be utilized.  This error detection is called Redundancy Check.

There are 4 types of redundancy checks which are as under:

Vertical Redundancy Check    (VRC)

Longitudinal Redundancy check    (LRC)

Cyclic Redundancy Check    (CRC)

Checksum

First three methods are implemented in the physical layer and the fourth one is used by the upper layers.

CHECKSUM:

For transmission the data is subdivided into equal segments of ‘n’ bits.

These segments are added and then their complement is found out.

This complement is appended to the end of the original data as redundancy bits called Checksum.

The appended data is transmitted over the network.

The receiver subdivides data unit as above and adds the segments together and complement the result.

If the intended data unit is intact, total value found by adding the segments and the checksum field should be zero.

If the result is not zero, the unit contains an error and the receiver rejects it.

Q : 2)   Calculate the VRC and LRC for the following bit pattern using even parity:

← 0100110 1110001 1100101 0000000

Solution:

 DATA VRC 0100110 01001101 1110001 11100010 1100101 11001010 0000000 00000000

 DATA 0100110 1110001 1100101 0000000 LRC à 0110010

Q : 3)   Find the checksum for the following bit sequence. Assume a 16-bit segment size.

1001101101010101

0001010101110011

Solution:

 1001101101010101 0001010101110011 10011011 00010101 01010101 01110011 Sum  à  11110000 Sum  à  10001000 Checksum  à  00001111 Checksum  à  01110111 100110110101010100001111 000101010111001101110111

Views: 447

### Replies to This Discussion

Q1
Error Control

Network is responsible for transmission of data from one device to another device. The end to end transfer of data from a transmitting application to a receiving application involves many steps, each subject to error. With the error control process, we can be confident that the transmitted and received data are identical. Data can be corrupted during transmission. For reliable communication, error must be detected and corrected.

Error control is the process of detecting and correcting both the bit level and packet level errors.
Check sum

Check sum is the third method for error detection mechanism. Checksum is used in the upper layers, while Parity checking and CRC is used in the physical layer. Checksum is also on the concept of redundancy.

In the checksum mechanism two operations to perform.

Checksum generator

Sender uses checksum generator mechanism. First data unit is divided into equal segments of n bits. Then all segments are added together using 1’s complement. Then it complements ones again. It becomes Checksum and sends along with data unit.

Exp:

If 16 bits 10001010 00100011 is to be sent to receiver.

So the checksum is added to the data unit and sends to the receiver. Final data unit is 10001010 00100011 01010000.

Checksum checker

Receiver receives the data unit and divides into segments of equal size of segments. All segments are added using 1’s complement. The result is completed once again. If the result is zero, data will be accepted, otherwise rejected.

Exp:

The final data is nonzero then it is rejected.

Q2
vrc

0100110

0100110 1

1110001

1110001 0

1100101

1100101 0

0000000

0000000 0
By Aqeel Haider

0100110

1110001

1100101

0000000
-----------------
0110010 LRC
bY Aqeel Haider

Q3
1001101101010101
0001010101110011
-------------------------
1011000011001000 sum

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