Packets

• Data is broken down into smaller more manageable pieces called packets.
• Special control information is added in order to:
• disassemble packets
• reassemble packets
• check for errors

Types of data sent includes

• Can contain information such as messages or files.
• Computer control data and commands and requests.
• Session control codes such as error correction and retransmission requests.

• Original block of data is converted to a packet at the Transport layer.

Packet Components

• Header
1. Alert signal to indicate packet is being transmitted
2. Source address.
3. Destination address.
4. Clock synchronization information.
• Data
1. Contains actual data being sent.
2. Varies from 512 to 4096 bytes (4K), depending on the network
• Trailer
1. Content varies by protocol.
2. Usually contains a CRC.

Packet Creation

• Look at the example on pp. 201 - 204
• Begins at the Application layer where data is generated.
• Each layer subsequently adds information to the packet; the corresponding layer on the receiving machine reads the information.
• Transport layer breaks the data into packets and adds sequencing information needed to reassemble data at the other end => the structure of the packets is defined by the common protocol being used between the two computers.
• Data is passed through the Physical layer to the cable.

Packet Addressing

• every NIC sees all packets sent on its cable segment but only interrupts the computer if the packet address matches the computer's address
• a broadcast type address gets attention of all computers on the network