1.9 Public Switched Telephone Network

A call is an information unit in the PSTN.

The PSTN has a global addressing scheme to uniquely identify an end device; an end device is commonly referred to as a telephone, while a more generic term is Customer Premise Equipment (CPE).

The global addressing scheme is known as E.164 addressing. It is a hierarchical addressing scheme that identifies the country code at the top level followed by the city or area code, and finally the number assigned to a subscriber.

Nodes in the PSTN are called switches, which are connected by Intermachine Trunks (IMTs), also known as trunk groups.

From a protocol architecture point of view, and using the OSI reference model, PSTN can be simply summed up as consisting of application layer, network layer, and physical layer.

The application layer enables the telephone service, the network layer handles addressing and routing, while the physical transmission system carries the actual signal for voice communication.

1.9 Public Switched Telephone Network

From a service architecture perspective, it provides the service model of blocked calls- cleared mode using circuit switching.

Circuit switching means that for a call requesting to be connected, a dedicated path is to be established instantaneously on demand from the source to the destination. The dedicated path is in fact a dedicated circuit with a specific bandwidth allocated—this value is 4 kilohertz (kHz) in an analog circuit and 64 kbps in a wire line digital circuit. The bandwidth of the circuit cannot be used by any other calls as long as this call is actively using it.