FRAMING

 

v  FRAMING:

·         In order to provide services to the network layer, the data link layer must use the services from the physical layer.

·         The physical layer sends the bit stream to the data link layer.

·         The no. of bits received may be different from the no. of bits transmitted.

·         The data link layer convert the bit stream into data frames and compute the checksum for each frame.

·         At the destination, the check sum is recomputed.

·         If the recomputed check sum is different from the one contained in the frame.

·         An error has occurred and the data link layer deals with the errors.

·         To mark the start and end of each frame, we use four methods. They are

1.      Character count.

2.      Flag bytes with byte stuffing.

3.      Starting and ending flags with bit stuffing.

4.      Physical layer coding violations.

1. Character count:

This method uses a field in the header to specify the number of characters in the frame.

·         The first framing method uses a field in the header to specify the number of characters in the frame. When the data link layer at the destination sees the character count, it knows how many characters follow and hence where the end of the frame is. This technique is shown in Fig.1(a) for four frames of sizes 5, 5, 8, and 8 characters, respectively.

 

           

1.(a) With out error                     1.(b) with error

             

·         For example, if the characters count of 5 in the second frame becomes a 7.

·         The destination will go out of synchronize and will be unable to locate the start of the next frame.

·         The destination does not know how many characters to skip over to get to the start of the re- transmission. For this reason, the character count is rarely used.

             

2. Flag bytes with byte stuffing/character stuffing:

·         In this method, each frame start and end with special bytes. Most protocols have used the same byte called a flag byte as FLAG at both starting &ending of the frame.

·         The second framing method gets around the problem of resynchronization after an error by having each frame start and end with special bytes. In the past, the starting and ending bytes were different, but in recent years most protocols have used the same byte, called a flag byte, as both the starting and ending delimiter, as shown in  3.2(a) as FLAG.

·          In this way, if the receiver ever loses synchronization, it can just search for the flag byte to find the end of the current frame. Two consecutive flag bytes indicate the end of one frame and start of the next one.

·          Even if the receiver ever loses synchronization, it can just search for the flag byte to find the end of the current frame.

Fig . (a) A frame delimited by flag bytes. (b) Four examples ofbyte sequences before and after byte stuffing.

·         The sender’s data link layer inserts a special escape byte “ESC“just before each flag byte the data. At the receiving end, the data link layer removes the escape byte before the data are given to the network layer. This technique is called” byte stuffing” or “character stuffing”.

·           A framing flag byte can be identified by the absence or presence of an escape byte in the data. If an escape byte occurs in the middle of data that, too, is stuffed with an escape byte.

·           Any single escape byte is part of an escape sequence, where as a doubled one indicates that a single escape occurred naturally in the data. Examples of the byte sequences before & after byte stuffing.

·           A major disadvantage of this framing method is What happens if an escape byte occurs in the middle of the original data already. The transmission takes place by stuffed with an escape byte. Thus, any single escape byte is part of an escape sequence, whereas a doubled one indicates that a single escape occurred naturally in the data.

·           It is used for 8-bit character only. Not all character codes use 8-bit characters, some use 16-bit characters, so a new technique had to developed to allow sized characters.

3. Starting and ending flags, with bit stuffing:

·          In this method, each frame begins and ends with a special bit pattern, 01111110 a flag byte.

·          When the sender’s data link layer encounters five consecutive 1’s in the data, it automatically stuffs a 0 bit into outgoing bit stream.

·          This bit stuffing is similar to byte stuffing.

·         When the receiver sees five consecutive incoming 1 bits, followed by a 0 bit, it automatically destuffs (i.e., deletes) the 0 bit. Just as byte stuffing is completely transparent to the network layer in both computers, so is bit stuffing. If the user data contain the flag pattern, 01111110, this flag is transmitted as 011111010 but stored in the receiver's memory as 01111110.

·          The boundary between two frames can be recognized by the flag pattern.

·          If the receiver losses its track the receives has to scan the input for flag sequences.

·          The flag sequences occur only at frame boundaries and never with in the data.

 

 

4. Physical layer coding violations:

·         This framing method is used in the networks in which physical medium contains some redundancy. A 1 bit is a high –low pair & a 0 bit is a low-high pair. It means that every data bit has a transmission in the middle. It makes easy for the receiver to locate the bit boundaries.