Circuit Switching

OK, we’ve discussed packet-switching, so now it’s time to cover its less exciting cousin―circuit switching. Most LECs still use it to complete landline calls. A circuit refers to a dedicated communication channel set up for the duration of a transmission, be it voice or data. Once that transmission is over, which is signaled by hanging up the phone, it’s taken down. You pick up the handset and the dial tone, delivered from the LEC’s central office (local exchange), is its way of saying, “OK, I’ve just reserved a transmission line for you. Now it’s time to enter the number you’d like to reach.” What happens if you don’t dial a number within the LEC’s designated timeframe? You get a fast busy signal, the LEC’s way of saying, “I reckon you don’t need the circuit, so I’m taking it down.”

Local calls usually remain circuit-switched throughout, and when callers place a long distance call by adding a 1 at the beginning of the called number, the central office looks up which long distance carrier you selected when you set up your service, and connects to it; from there, it becomes packet-switched.

Circuit Switching designates a single, dedicated path from point A to point B. Packet Switching means the data transmitted breaks into multiple packets and each can travel different routes. Each packet has a VCI (virtual circuit identifier) that specifies the address of the destination. The packets reassemble in the proper sequence once they arrive.

Early cellular service, including 2G and 3G, was for voice communications, and used circuit-switching.