What is Framing?

As the physical layer merely accepts and transmits a flow of bits without any look upon to meaning or structure, it is upto the data link layer to generate and recognize frame borders. This can be talented by attaching unusual bit patterns to the starting and end of the frame. If these bit patterns can by chance occur in data, extraordinary care must be taken to make sure these patterns are not wrongly interpreted as frame delimiters. The 4 framing methods that are widely used are

• Character count
• Starting and ending characters by character stuffing
• Starting and ending flags, with bit stuffing
• Physical layer coding violations

Character Count

This method uses a field in the header to specify the number of characters in the frame. When the data link layer at the dest. sees the character count and it knows how many characters follow, and thus where the end of the frame is. The drawback is that if the count is garbled by a transmission error, the dest. will mislay synchronization and will be unable to locate the start of the next frame. as a result, this method is rarely used.

Character stuffing

In the second method, each frame starts with the ASCII character sequence DLE STX and ends with the sequence DLE ETX.(Here DLE is Data Link Escape, STX is Start of TeXt and ETX is End of TeXt.) This method overcomes the limitations of the character count method. If the destination ever loses synchronization than  it  has to look only for DLE STX and DLE ETX characters. On the other hand, binary data is being transmitted then there exists a possibility of the characters DLE STX and DLE ETX occurring in the data. Because this can interfere with the framing, a procedure called character stuffing is used. The sender ends data link layer inserts an ASCII DLE character just before the DLE character in the data. The receiver ends data link layer removes this DLE before this data is given to the network layer. Yet character filling is closely associated with eight bit characters and this is a major hurdle in transmitting random sized characters.

Bit stuffing

The 3rd method allows data frames to contain an random number of bits and allows character codes with an random number of bits per character. At the start and end of every frame is a flag byte consisting of the particular bit pattern 01111110.Every time the sender's data link layer encounters five successive 1s in the data, it automatically insert a zero bit into the outgoing bit stream. This procedure is called bit stuffing. When the receiver sees 5 successive 1s in the incoming data stream, followed by a 0 bit, it automatically destuffs the 0 bit. The boundary among two frames can be find out by locating the flag pattern.

Physical layer coding violations

The final framing process is physical layer coding violations and is valid to networks in which the encoding on the physical medium having some redundancy. In such cases generally, a one bit is a high-low pair and a zero bit is a low-high pair. The combinations of low- low and high-high which are not used for data may be used for marking frame limits.

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