Question

A data transmission technology uses CRC of eight bit to detect errors in packet headers with generator polynomial is g(x)= X8+X2+X+1. The header contains the following fields in order for left to right:

• GFC, 4 bits;
• VPI, 8 bits;
• VCI, 16 bits;
• Type, 3 bits;
• CLP, 2 bits;
• CRC, 8 bits.
  

(a) Find the last 8 bits of the header if the GFC, VPI, Type, and CLP fields are all zero and the VCI field is 00000000 00010111.

(b) Under what condition would errors be undetectable, give an example.

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Answer 1

A crc is a form of integrity checksum. It is a powerful method for detecting errors in the received data is by grouping the bytes of data into a block and calculating a Cyclic Redundancy Check (CRC). This is usually done by the data link protocol and calculated CRC is appended to the end of the data link layer frame.

The CRC is calculated by performing a modulo 2 division of the data by a generator polynomial and recording the remainder after division.

Three polynomials are in common use they are:

CRC-16 = x16 + x15 + x2+ 1 (used in HDLC)

CRC-CCITT = x16 + x12 + x5 + 1

CRC-32 = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 (used in Ethernet)