Frame check sequence

The all important information field is of variable length in the HDLC protocol, although other protocols use a fixed length. The condition of this field is highly conditioned by the fields that have already gone before. The length of the field is contained in the control field and may vary from packet to packet. The penultimate field is the frame check sequence which is a number... 

Cyclic redundancy checking (CRC) will normally be carried out as part of installation tests. The CRC can be used to detect accidental alteration of data during transmission or storage. The CRC technique is used to protect blocks of data called Frames. Using this technique, the dansmitter appends an extra n-bit sequence to every frame called a frame check sequence (FCS). The FCS holds redundant information about the frame that helps the dansmitter detect errors in the frame. The CRC is one of the most used techniques for error detection in data communications. 

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