1. Packets, Frames & Error Detection

2. Packet Concepts A packet is a small block of data. Networks which use packets are called packet networks or packet- switched networks. Packets are quite small compared to the overall data transfer in which it is included.

3. Why Packets? Because a packet is very small, a transmission failure on a single packet does not affect the entire data transmission. The packet can just be resent! Packets are efficient; packets can take independent routes on a network and still arrive at their destination properly. A constant data stream is not necessary.

4. Why Packets (cont.) When a computer sends data on a network, no other computer can send data on the same segment. If a machine is sending a large, constant file, it would block access to the network for the other machines. This is unacceptable!

5. Packets and Multiplexing Packets should ideally be used in a TDM system because it allows all systems to send data on the shared medium. Packets do not have to be used in a FDM (WDM) system because each system can send a full data stream independent of the others.

6. Packets and Framing A RS-232 transmission is a form of framing; a start bit and a stop bit forms the frame, where the 7 bits constitute the packet. But a 7 bit number is too small for a packet. Usually, it’s on the order of 1000 bytes. So some other, offical form of framing needs to be used with an RS-232 communication.

7. Framing (cont.) One example of framing uses the unprintable ASCII characters, soh and eot.