Byte Count Framing in data link layer

Byte Count Framing

Byte count framing is a character-oriented framing. That means in this method, data to be carried is 8-bit characters from one of the coding systems like ASCII. While framing, it is important to mention the start point and end point of each frame. It is important to insert frame delimiters so that at the receiver side receiver’s data link layer will be able to correctly identify the boundaries of each frame to control the flow and to control the errors in the data transmission.

The byte count method uses a field in the header of a frame to specify the size of each frame. The site itself will be used as a frame delimiter.

To explain the concept of byte count framing let me use the above diagram. In this diagram, L3 data is network layer data in the form of packets. Let us say for efficient data transmission data link layer divides the data into two units, frame one F1 and frame two F2. For simplicity, we will assume that frame one contains 5 characters 1,2,3,4 and 5 and frame two contains 4 characters 6,7,8 and 9.

In the byte count method data link layer inserts the byte count in the frame header. So, in this example, as there are 5 bytes in frame one; byte count 6 is inserted in the byte count field in the header. In Frame F2, there are 4 bytes. So, the byte count will be equal to 5 i.e., the total number of bytes including the byte count field. Therefore 5 will be inserted as a byte count. The data link layer gives these two frames to the physical layer which then encodes data to signals and transmits it to the receiver.

At the receiver side physical layer will decode these signals to a bit stream and this bit stream is given to a data link layer. The first byte that is byte count is equal to 6. The data link layer understands there must be 6 bytes in the frame. The data link layer will count bytes to know the end of the frame. The next bite is 5 i.e. byte count of the next frame. The data link layer knows how many bytes follow this byte and hence where the end of the frame is.

However, the byte count method has a drawback. When data is transmitted from one end to the other end, there is always the possibility that data bits may get corrupted due to noise. If even one bit of a byte count is changed whole byte count will be changed.

For example, if one bit of byte count 6 is changed. 6 may become 4, as 6 can be represented in binary as

00000110 = 6

one-bit change may make it

0000100 = 4

which is binary 4. In this case receiver’s data link layer will consider the size of frame 1 to be equal to 4 and therefore will not be able to locate the start of the next frame correctly. Therefore, the data link layer will lose the frame synchronization.

To summarise we can say the byte count method is the character-oriented method which is an easy method of framing. But it has a drawback. When the byte count is changed due to noise the receiver’s data link layer will lose the frame synchronization and therefore will not be able to locate the start of the next frame.