Circuit switched networks have been used in the past in the traditional telecommunication networks for phone calls. Circuit switching has following parameters:

• reserved bandwidth
• dedicated connection
• protected bandwidth
• synchronized

It has been used in the analogue telephone network to establish temporary circuits (phone calls) or permanent circuits between peers, sites. Before packet switched networks became mainstream, this has been the first way for Telecoms for making profit out of circuits. Circuit switched network are reliable and expensive, have limited bandwidth, and available bandwidth can not be temporarily reserved elsewhere. Once a phone call (circuit) from location A (Russia) to location B (China) has been established, this bandwidth is reserved for the time the phone call (circuit) is active. Circuit switched networks also need a clock. So beside the circuit switched network, there is a need of a clock for the whole network. Operating a clock for a circuit switched comes with some additional complexity. The clock has to be synchronized across the whole circuit switched domain. Synchronization is a prerequisite for traditional circuit switching. Once the network is out of time there are errors could be f.e. in telephony the other end can not be comprehended, in video the receiving picture is distorted. A circuit is a dedicated point to point connection.

ATM or Frame Relay is a cicuit switched network. I have been operating ATM and Frame Relay networks in the past. If provisioning a path for a customer in ATM backbone there were parameters that needed to be know before the provisioning could be done. Parameters like

• icd (initial cell rate)
• cdr (committed data rate)
• cbs (committed burst rate)
• pd (peak data rate)

Only to name a few. There are a lot more parameter VC (virtual circuit) could have. Also ATM networks (I speak mainly about circuit switching) were able even in the past to establish "real time" communication for video and audio. PVC is a permanent virtual circuit, a PVC can not be automatically re-routed, and a SPVC (soft permanent virtual circuits) can be re-routed. Also there are PNNI (Private Network-to-Network Interface) and NNI (Network-to-Network Interface).

At the current moment of time there are rarely circuit switched networks out there, they are being replaced by packet switched networks which are more suited for today needs. Circuit switched technology compared to nowadays packet switched technology has been far more reliable and advanced, even today. With superior signalling and debugging capabilities nowadays ethernet networks do not have like this. This advantage comes at some cost. Operating circuit switched networks has been far more complex than operating packet switched network, it did not scale well as data communication came to circuit switching besides video and audio. Operating packet switched networks is still not easy if done right it needs also careful planing and knowledge of the used technology. But in the end I can say there is the old war, you probably know form editors, Emacs vs vim. The same war is in telecommunication ISO vs RFC.

ISO - is a standard, reliable, done once and for all the time, not scalable, internationally supported, only one way of doing it, expensive RFC - not a standard, good enough, can be rewritten at all times, scalable, different vendors, different implementations that mostly work together.

Circuit switching is a ISO domain, old telecoms, reliable, expensive, done once a internationally working, out of the box. Best comparison ever for me is NSAP vs IPv4 and IPv6. If you do not know what NSAP is?