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Network Protocols and Packet Switching: Question 8

Syllabus 14.2

Structured A2 5 marks

Two offices, one in London and one in Leeds, are connected by a traditional landline telephone network. An employee in the London office picks up a handset and dials a colleague's extension in the Leeds office, and the two have a single, continuous phone conversation.

(a) Describe how circuit switching would be used to connect and carry this phone call between the two offices. [3]

(b) State one drawback of using circuit switching, rather than packet switching, for this phone call. [2]

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Worked solution

Part (a): Setting up and using the circuit

Before the London employee’s call can actually begin, the telephone network first finds and sets up a single, continuous, dedicated path, a circuit, made up of connected links running all the way from the London handset to the Leeds handset.

Once this path has been set up, it is reserved exclusively for this one call: no other call can use any part of that same path while the conversation is happening. The path stays open, ready to carry the conversation in both directions, for the entire duration of the call, including any pauses or silences where neither person is speaking. Only once the call ends. One of the handsets is put down, is the path closed and released, freeing its links up so the network can use them to set up other, new calls.

[3 marks]: [1] for describing a single, dedicated path being set up before the call starts, [1] for stating the path is reserved exclusively for this call for its whole duration, [1] for stating the path is closed/released once the call ends.

Part (b): A drawback of circuit switching here

Because the dedicated path stays reserved for the entire call, including any silent pauses where nothing is actually being transmitted. The network’s capacity along that path is wasted during those idle moments. No other call, and no other data, can use that path even though nothing is actively being carried along it at that moment. Packet switching does not have this particular drawback, because its shared links can carry packets belonging to many different communications at the same time, rather than reserving a whole path for just one.

[2 marks]: [1] for identifying that capacity is wasted/unusable by others during idle periods of the call, [1] for correctly explaining why. The path is reserved for the whole call regardless of whether it is actively being used at every moment.

Final answers

  • (a) A single dedicated path (circuit) is set up before the call starts, reserved exclusively for this call for its whole duration, and closed/released once the call ends.
  • (b) The reserved path wastes network capacity during silent pauses in the call, since it cannot be shared with any other call or data even while nothing is being transmitted.