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Forouzan Data communications and Networking 17 In a cyclic code, if a codeword is cyclically shifted (rotated), the result is another codeword.

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Addressing ( MAC physical address) 2.

Framing 3. Flow control (Regulating the flow of data so that slow receivers are not swamped ) 4.

Each frame contains a frame header, a payload field for holding the packet, and a frame trailer Frame Header contains the physical address of the next receiving node Computer networks Andrew S.

Forouzan Data communications and Networking 4 The data link layer needs to pack bits into frames, so that each frame is distinguishable from another.

The simple act of inserting a letter into an envelope separates one piece of information from another; the envelope serves as the delimiter.

Two consecutive flag bytes indicate the end of one frame and the start of the next.

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It may happen that the flag byte occurs in the data, especially when binary data such as photographs or songs are being transmitted.

One way to solve this problem is to have the senders data link layer insert a special escape byte (ESC) just before each accidental flag byte in the data.

The data link layer on the receiving end removes the escape bytes before giving the data to the network layer.

Framing can be also be done at the bit level, so frames can contain an arbitrary number of bits made up of units of any size.

Each frame begins and ends with a special bit pattern, 01111110 or 0x7E in hexadecimal.

Assume for the moment that the receiver can tell whether a frame that it receives contains correct or faulty information (using error detection and correction techniques) The usual way to ensure reliable delivery is to provide the sender with some feedback about what is happening at the other end of the line.

Typically, the protocol calls for the receiver to send back special control frames bearing positive or negative acknowledgements about the incoming frames.

On the other hand, a negative acknowledgement means that something has gone wrong and the frame must be retransmitted again.

To be able to detect or correct errors, we need to send some extra bits with our data.

These redundant bits are added by the sender and removed by the receiver.

Forouzan Data communications and Networking 14 The most common and least expensive.

How does it work A k-bit dataword is changed to an n-bit codeword where n k 1.

The extra bit, called the parity bit, is selected to make the total number of 1s in the codeword even Note: there is either even parity or odd parity.

Since the number of 1s is odd then the parity bit is equal to 1 and the codeword for this dataword is 10111.

It can detect burst errors only if the total number of errors in each data unit is odd.

A better approach is the two-dimensional parity check which organizes the data units into table (rows and columns).

The parity bit for each column is Calculated and added to the table as a new row (column parity).