Page 110 - DCAP406_DCAP_207_Computer Networks

Unit 7: Data Link Layer

from a string of 0s, of the same length as the divisor, replaces the divisor in that step of the Notes
process. For string of 0s, of the same length as the divisor, replaces the divisor in that step of the
process. For example, if the divisor is four bits long, it is replaced by four 0s. (Remember, we are
dealing with bit patterns, not with quantitative values; 0000 is not the same as 0). This restriction
means that, at any step, the leftmost subtraction will be either 0 – 0 or 1 – 1, both of which equal
0. So, after subtraction, the leftmost bit of the remainder. Note that only the first bit of the
remainder is dropped – if the second bit is also 0. It is retained, and the dividend/remainder for
the next step will begin with 0. This process repeats units the entire dividend has been used.

The CRC Checker

A CRC checker functions exactly like the generator. After receiving the data appended with the
CRC, it does the same modulo – 2 divisions. If the remainder is all 0s, the CRC is dropped and
the data accepted; otherwise, the received stream of bits is discarded and data are resent. Figure
shows the same process of division in the receiver. We assume that there is no error. The
remainder is therefore all 0s and the data are accepted.

Figure 7.6: CRC Checker

Self Assessment

Fill in the blanks:

1. The data link layer receives a raw bit stream from the ……………………. layer that may
not be error free.
2. Some of the examples of …………………. check are audio storage and playback devices
such as audio CD’s.
3. CRC codes are also called as …………………………….. codes.
4. ………………………… consists of Even Parity and Odd Parity Method.

7.2.2 Error Correction Code

An error-correcting code (ecc) or forward error correction (fec) code is a system of adding
redundant data, or parity data, to a message, such that it can be recovered by a receiver even
when a number of errors (up to the capability of the code being used) were introduced, either
during the process of transmission, or on storage. Since the receiver does not have to ask the

LOVELY PROFESSIONAL UNIVERSITY 103