F451 - packet switching

• Protocols in which messages are split up into packets before they are sent. 
• Each packet is then transmitted individually and can even follow different routes to its destination. 
• Once all the packages forming a message arrive at the destination, they are recompiled into the original message.

• A typical upper bound on packet size is 1000 octets.
• If a station has a longer message to send it is broken into a series of small packets. Each packet contains part of the user's data and some control information.
• The control information should at least contain:
• Destination Address
• Source Address
• Store and forward - packets are received, stored briefly (buffered) and past on to the next node.

Advantages:

• Line efficiency
• Single node to node link can be shared by many packets over time.
• Packets queued and transmitted as fast as possible.
• Data rate conversion
• Each station connects to the local node at its own speed.
• Node buffer data if required to equalize rates.
• Packets are accepted even when network is busy
• Delivery may slow down
• Priorities can be used.

Switching technique:

• Packets are handled in two ways:
• Datagram
• Virtual circuit

Datagram:

• More flexible
• Routing can be used to avoid congested parts of the network.
• No call setup phase
• Better if few packets

Virtual circuits:

• Network can provide sequencing and error control.
• Packets are forwarded more quickly
• No routing decisions to make
• Less reliable
• Loss of a node looses all circuits through that node