Packet Formats

The format of a packet in packet switching network can vary significantly from one network to another. Generally header includes all related control information. In some cases, control information is communicated through a special control packets. Fig. 11.8 shows typical packet format.

Fig. 11.8. Typical packet format. A packet contains 3 major fields.

1. Header. It contains sub fields in addition to the necessary address fields. Other than the to and from address field, the following are the useful control information.

(a) Op code. It designates whether the packet is a message (text) packet or control packet.

(b) A sequence number (Seq) to reassemble messages at the destination node, detect faults and facilitates recovery procedures.

(c) Byte count. Used to indicate the length of a packet.

Fig. 11.8. Typical packet format. A packet contains 3 major fields.

1. Header. It contains sub fields in addition to the necessary address fields. Other than the to and from address field, the following are the useful control information.

(a) Op code. It designates whether the packet is a message (text) packet or control packet.

(b) A sequence number (Seq) to reassemble messages at the destination node, detect faults and facilitates recovery procedures.

(c) Byte count. Used to indicate the length of a packet.

2. Data. A portion of a data stream to be transferred in the data field. Some packets may not contain a message field if they are being used strictly for control purposes.

3. CRC. The cyclic redundancy checks (CRC) field contains a set of parity bits that cover overlapping fields of message bits. The fields overlap in such a way that small numbers of errors are always detected. The probability of not detecting the occurance of 2 large number of errors is 1 in 2M, where M is the number of bits in the cheek code.

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