06-12-2012, 02:49 PM
Bit and Byte Stuffing
Bit and Byte.ppt (Size: 159.5 KB / Downloads: 107)
Synchronous versus Asynchronous Transmissions
There exists a hierarchy of synchronization tasks:
Bit level : recognizing the start and end of each bit
Character or byte level : recognizing the start and end of each character (or small unit of data)
Block or message level : recognize the start and end of each large unit of data (in networks this is a frame).
A fundamental requirement of digital data communications is that the receiver knows the starting time and the duration of each bit.
Asynchronous transmission :: each character (or byte) is treated independently for clock (bit) and character (byte) synchronization purposes and the receiver resynchronizes at the start of each character received.
Synchronous transmission :: the complete frame is transmitted as a contiguous string of bits and the receiver endeavors to keep in synchronism with the incoming bit stream for the duration of the frame.
Synchronous Transmissions
More efficient, i.e., less overhead
Blocks of characters transmitted without start and stop codes
The transmitted stream is suitably encoded so the receiver can stay in “synch” by:
Using a separate clock line
Embedding clocking information into data (e.g. biphase coding).
Byte Stuffing[HDLC Example]
Also referred to as character stuffing.
ASCII characters are used as framing delimiters (e.g. DLE STX and DLE ETX)
The problem occurs when these character patterns occur within the “transparent” data.
Solution: sender stuffs an extra DLE into the data stream just before each occurrence of an “accidental” DLE in the data stream.
The data link layer on the receiving end unstuffs the DLE before giving the data to the network layer.
Bit Stuffing
Each frame begins and ends with a special bit pattern called a flag byte [01111110]. {Note this is 7E in hex}
Whenever sender data link layer encounters five consecutive ones in the data stream, it automatically stuffs a 0 bit into the outgoing stream.
When the receiver sees five consecutive incoming ones followed by a 0 bit, it automatically destuffs the 0 bit before sending the data to the network layer.