Circuit Switching
Circuit Switching
The phone system uses a technique called circuit switching
1. Once a call has been completed, the user sees a set of virtual wires” between communicating endpoints.
2. The user sends a continuous stream of data, which the channel guarantees to deliver at a known rate.
3. Data transmission handled elegantly using TDM or FDM. Note that TDM/FDM work well because the data rate is predictable | the voice grade signal is sampled using PCM generating a steady stream of bits.
4. Call setup required before any data can be sent, allowing network to set up the path, allocate subchannels, etc. Call setup also used to decide who to charge for the call.
5. Call termination required when parties complete call, allowing the network to reclaim resources. At this point, a billing record is saved somewhere that records where the call was made, its duration, etc.
Advantages of circuit switching:
1. Fixed bandwidth, guaranteed capacity (e.g., no congestion).
2. Low-varience end-to-end delay (e.g., delay nearly constant).
Drawbacks:
1. Connection setup introduces delay before communication can begin.
2. User pays for circuit, even when not sending any data.
3. Other users cannot use bandwidth of other circuits that are not actually being used(e.g., in most conversations, only one person speaks at a time. Thus, half the underlying bandwidth is wasted!