Mumbai University > Information Technology > Sem4 > CN

Marks: 5M

Year: Dec 2016

• Data transmission in the physical layer means moving bits in the form of a signal from the source to the destination. The physical layer provides bit synchronization to ensure that the sender and receiver use the same bit durations and timing. The data link layer, on the other hand, needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of framing.

• A good design must make it easy for a receiver to find the start of new frames while using little of the channel bandwidth. The four different methods used for framing are:

  1.Byte count.

  2.Flag bytes with byte stuffing.

  3.Flag bits with bit stuffing.

  4.Physical layer coding violations.

  1.Byte Count: The first framing method uses a field in the header to specify the number of bytes in the frame. When the data link layer at the destination sees the byte count, it knows how many bytes follow and hence where the end of the frame is. This technique is shown in Fig.3.1 for four small example frames of sizes 5, 5, 8,and 8 bytes, respectively.

Figure 1. A byte stream a) Without errors b) With one error

• The trouble with this algorithm is that the count can be garbled by a transmission error. For example, if the byte count of 5 in the second frame of Figure 1 (b) becomes a 7 due to a single bit flip, the destination will get out of synchronization. It will then be unable to locate the correct start of the next frame.