DATA COMMUNICATION PRINCIPLES Bit Stuffing Bit Stuffing The

DATA COMMUNICATION PRINCIPLES : Bit Stuffing

Bit Stuffing The objective of bit stuffing is to prevent the occurrence of the Flag pattern in PDU fields where it is not supposed to occur. Since the protocol information consists of fixed fields, one may argue that bit stuffing is not needed in the protocol information field. In practice however, bit stuffing may be done all over the PDU excluding the Flag because of its simplicity as compared to a selective bit stuffing. If the Flag pattern occurs (or is due to occur) at an unwanted place, it is altered in a known way. The following example can be used to illustrate the concept. Suppose a string of 8 alternating '1's and '0's is used as a Flag (1010). Bit stuffing can be done by inserting a ‘ 1’ whenever the following 7 bit pattern occurs in data field 1010101. If this is agreed by the transmitter and the receiver, then the bit stuffing consists of the following two-step mechanism.

At the sending end, the bit stuffing mechanism monitors the PDU data field. Whenever 1010101 occurs, it adds a '1' to make it 10101011. This is done regardless of the actual value of the bit in data. The bit stuffing mechanism at the receiver also monitors data continuously. Whenever the pattern 10101011 occurs, the last '1' is removed from it. In this way, the layer 3 data is delivered without any alteration. Many physical layer protocols also allow or require transmission of data block by block. Synchronization is needed for such PHY protocols as well. Many PHY protocols, however, treat a character as the transmission block. The synchronous transmission of a single character is not practical due to its small size. The phenomenon of placing and extracting the timing information on a character-by-character basis is called 'asynchronous transmission'.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press