Packet Switching
Circuit switching wastes a reserved path whenever nobody is talking. Computers are silent most of the time and then send data in quick bursts, so reserving a path for them would waste almost all of it. Packet switching reserves nothing at all.
1Packet switching
No path is set up in advance, and no link belongs to anyone. Every link carries packets from many conversations, one after another.
2What is in a packet
A packet is a small piece of the message with a label on the front, called its header. The header says where the packet came from, where it is going, and which piece of the message it is.
3Store and forward
When a packet reaches a switch, the switch stores it for a moment, reads its header, and then forwards it along whichever link is free towards its destination. This is called store and forward, and packet switching uses it at every switch.
Press Send. Then let others send at the same time, or cut a link, and send again. Drag sideways to turn.
Simplified: here the switches take turns sending packets over the two routes, and the top route is the busier one, so its packets wait longer. Real switches choose from moment to moment.
Send A’s message on its own and look at what B receives: YOU, SEE, FIVE, AT. The packets on the busier top route waited longer at each switch, so they arrived later. It does not matter. Each packet carries its number, and B puts them back as SEE YOU AT FIVE.
Think of moving house by courier
4Advantages and disadvantages
- Links are shared, so nothing is wasted while someone is silent.
- No time is spent setting up a path before sending.
- If a link breaks, packets simply take another route.
- Many conversations can use the network at the same time.
- Packets can arrive out of order, and must be put back together.
- Packets may be delayed when switches are busy.
- A lost packet has to be sent again.
The Internet uses packet switching. Every web page, message and video you receive arrives as packets.
5From the board papers
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Packet switching.
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Packet switching. Each switch stores a packet, then forwards it.
(a) In packet switching, a dedicated path is established for the entire communication session.
(b) Circuit switching is more efficient than packet switching for networks with bursty data traffic.
(c) Packet switching allows multiple communications to share the same network resources simultaneously.
(d) Circuit switching breaks data into packets and sends them independently.KV pre-board 2025-26, Chennai region
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(c). (a) describes circuit switching, (d) describes packet switching, and (b) is backwards: with data sent in bursts, a reserved circuit sits idle most of the time, so packet switching is the more efficient one.
6Check yourself
Packets of one message arrive at the receiver out of order. How does the receiver rebuild the message?
In the lab, the link from S2 to S4 is cut. What happens to A's packets?