Transmission Techniques:

1.  Asynchronous: Little blocks of bits (generally bytes) are sent at a time without any time relation among successive bytes. When no transmission occurs a default state is maintained equivalent to bit 1. Due to arbitrary delay between successive bytes, the time occurrences of the clock  pulses at  the receiving end  need to  be matched for each byte. This is achieved by giving 2 extra bits start and stop.

Start bit: It is prefixed to each byte and equals zero. Thus it ensures a transition from 1 to 0 at onset of transmission of byte.Here,the leading edge of start bit  is used as a reference for generating clock pulses at necessary sampling instants. Therefore each onset of a byte results in resynchronization of receiver clock.

Stop bit: To ensure that transition from 1 to 0 is always present at beginning of a byte it is essential that default state be one. But there may be two bytes one immediately following the other and if last bit of first byte is 0 then transition from one to zero will not occur. for that reason a stop bit is suffixed to each byte equalling one. It's duration is typically 1,1.5 and 2 bits.

Asynchronous transmission is simple and cheap but requires an overhead of 3 bits i.e. for 7 bit code 2 (start, stop bits) +1 parity bit implying 30% overhead. However % can be reduced by sending larger blocks of data but then timing errors between receiver and sender cannot be tolerated beyond [50/no.  of bits  in  block]  %  (assuming sampling is done at middle of bit interval). It will not only end result in incorrect sampling but also skewed bit count i.e. a data bit could be mistaken for stop bit if receiver's clock is faster.

2.  Synchronous - bigger blocks of bits are successfully transmitted. Blocks of data are either considered as sequence of bits or bytes. To avoid timing drift clocks at two ends need to be synchronized. This can done in 2 different ways:

1.  Provide a separate clock line between receiver and transmitter. OR

2.  Clocking information is embedded in data signal to be precise biphase coding for digital signals.

Still another level of synchronization is required so that receiver determines beginning or end of block of data. thus each and every block begins with a start code and ends with a stop code. These are in generally known as flag that is exclusive sequence of fixed number of bits. In addition some of control characters encompass data within these flags. Data+control information is called a frame. Since any arbitrary bit pattern can be transmitted there is no assurance that bit pattern for flag will not appear inside the frame thus destroying frame level synchronization. For  avoiding this we use bit stuffing

Bit Stuffing: Suppose our flag bits are 01111110 (six 1's). So the transmitter will always insert an extra 0 bit after each occurrence of five 1's (except for flags). After detecting a initial flag the receiver monitors the bit stream. If pattern of five 1's occurs, the sixth is examined and if it is 0 it is deleted else if it is 1 and next is 0 the combination is accepted as a flag. in the same way byte stuffing is used for byte oriented transmission. In this we can use an escape sequence to prefix a byte similar to flag and 2 escape sequences if byte is itself a escape sequence.

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