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Error Correcting methods

The most popular error correcting methods are:

1. Automatic Repeat Request (ARQ)
2. Forward Error Correction (FEC)
3. Go back ‘n’
4. Hamming Code
   

Automatic Repeat Request (ARQ)

A.R.Q. stands for Automatic Repeat request. It is also called as PAR (Positive Acknowledgement with Retransmission). Protocols in which the sender waits for a positive acknowledgement before advancing to the next data item are often called PAR or A. R. Q. Like protocol 2, A.R.Q. protocol also transmits data only in one direction. Although it can handle lost frames by timing out, it requires the time out interval to be long enough to prevent premature time outs. If the sender times out too early, while the acknowledgement is still on the way, it will send duplicate.

When the previous acknowledgement finally does arrive the sender will mistakenly think that the just-sent frame is the one being acknowledged and will not realize that there is acknowledgement frame somewhere “in the pipe”. If the next frame is sent is lost completely but the extra acknowledgement arrives correctly, the sender will not attempt to retransmit the lost frame and the protocol will fail. In later protocol, the acknowledgement frames will contain information to prevent just this sort of trouble. For the time being it is assumed that a strict alternation of the sender and receiver and the acknowledgement frames will just be dummies.

Hamming Code

Transmission error are doing to be a fact of life for many years to come. While errors are rare on the digital part, they are still common on the local loops. Further more, wireless communication is becoming more common and the error rates here are orders of magnitude worse than on the interoffice trunks.

Networks designers have developed two basic strategies for dealing with errors. One way is to include enough redundant information along with each block of data sent to enable the receiver to deduce what the transmitted character have been. The other way is to include only enough redundancy to allow the receiver to deduce that an error occurred, but not which error and have it request a retransmission. The former strategy uses error correcting codes and the latter uses error detecting codes.

The error-detecting and error-correcting properties of a code depend on its hamming distance. The number of bit positions in which two code words differ is called the Hamming distance. An n-bit unit containing data and check-bits is often referred to as n-bit codeword.

To detect errors, you need a distance d+1 code because with such a code there is no way that d-single bit errors can change a valid code word into another valid word. When the receiver sees an invalid code-word, it can tell that a transmission error has occurred. Similarly, to corrected errors, you need a distance 2d+1 code because that way the legal code words are so far apart that even with d change, the original code word is still closer than an any other code word, so it can be uniquely determine.

Modem Error correction and data compression standard

It is important to make sure that your modem can sent data accurately, without errors and rapidly. When you speak on a phone, you can sometimes listen other noises, which can correct data transmission and cause transmission errors. To get over this “Electrical noise” on the telephone line, modems should include errors correction features. The two most popular system are MNP(Micro Com Networking Protocol) and V.42.

Data compression is also an important consideration when buyer a modem. It compresses data as it is being transmitted. The most common form of data compression is V.42 bits. This can quadruple the effective rate at which the data is sent by the modem. The faster you send data, the more the modem is prone to noise, so error correction is vital for high speed modem but not necessary for low speed modems.

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