How systems differ: Telephone system: Electronic switching, redundancy, and automatic recovery reduce outages

Telephone customers expect and usually obtain good quality service at any time under any condition. This high degree of availability is met even though any given telephone call involves numerous pieces of equipment. The major elements in a local call are the equipment at a customer's location, the wire pair connecting the customer to the local switching system, and the switching system itself. For toll calls, the local switch in a system is interconnected in a network of other switching and transmission systems. Failure anywhere in the physically distributed network can cause failure to originate a call, failure to complete the call to the called party, or unexpected cutoff.