Circuit switching, packet switching

Computer Science A2 · Communication and internet technologies. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

In circuit switching a dedicated path is set up between the two ends before any data is sent. The path is kept for the whole exchange and released at the end. All data follows the same route and arrives in order.

In packet switching the message is split into packets. Each packet has a header with the source address, destination address and a sequence number. Packets share the links with other users and may take different routes.

A router reads the destination address of each packet, looks it up in its routing table and forwards the packet towards its destination. The receiving host puts the packets back in order and asks for any missing ones to be sent again.

Rules to remember

Circuit switching benefits: the full bandwidth is reserved, the data rate is constant, the data arrives in order with no delay once the circuit is set up.
Circuit switching drawbacks: time is needed to set up the circuit, the reserved path is wasted when nothing is sent, and if a link fails the connection is lost.
Circuit switching is used for a traditional landline telephone call and other real-time uses that need a steady rate.
Packet switching benefits: links are shared efficiently, packets can be sent round a failed or busy route, and only a lost or damaged packet is resent.
Packet switching drawbacks: packets can be delayed, lost or arrive out of order, each one carries a header as overhead, and they must be reassembled.
Packet switching suits data sent in bursts, such as web pages and email; it is how the internet works.
A routing table entry links a destination network to the next hop (where to send the packet next).

Common mistake

Students say that the router reassembles the message. A router only forwards each packet separately; it is the destination host that puts the packets back in order using their sequence numbers.

Worked example

A message is sent as four packets with sequence numbers 1 to 4 across a packet-switched network. They arrive in the order 3, 1, 4 and packet 2 does not arrive. What does the receiving host do?

The packets took different routes, so arriving out of order is normal.
The host uses the sequence numbers to place them as 1, _, 3, 4 and sees that 2 is missing.
It asks the sender to send packet 2 again; only that packet is resent, not the whole message.
When packet 2 arrives the host reassembles 1, 2, 3, 4 and removes the headers.

Answer: It reorders the packets by sequence number, requests packet 2 again and then rebuilds the message.

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