PSTN(Circuit Switch): PSTN is a circuit switched network is one where a dedicated connection (circuit or channel)    must    be   set   up   between   two   nodes   before   they may communicate.   For   the   duration   of   the communication, that connection may only be used by the same two nodes, and when the communication has ceased, the connection must be explicitly cancelled.

The basic digital circuit in the PSTN is a 64-kilobits-per-second channel, originally designed by Bell Labs, called a "DS0" or Digital Signal 0. To carry a typical phone call from a calling party to a called party, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a signaling protocol (SS7) between the telephone exchanges under an overall routing strategy.

PSTN, the Public Switched Telephone Network, is a circuit-switched network that is used primarily for voice communications worldwide, with more than 800 million subscribers. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital and also includes mobile as well as fixed telephones. The basic digital circuit in the PSTN is a 64-kilobit-per-second channel, known as "DS0" or Digital Signal 0. DS0's are also known as timeslots because they are multiplexed together in a time-division fashion. To carry a typical phone call from a calling party to a called party, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation. Multiple DS0's can be multiplexed together on higher capacity circuits, such that 24 DS0's make a DS1 signal or T1 (the European equivalent is an E1, containing 32 64 kbit/s channels). For more than a hundred years, the PSTN was the only bearer network available for telephony. Today, the mobile telephone over wireless access network, which is carried through the PSTN trunking network, is becoming increasingly popular. Other bearer networks for voice transmission include integrated service digital network (ISDN), Digital Subscriber Line (DSL), Asynchronous Transfer Mode (ATM), frame relay and the Internet VOIP.

T-1 & E-1 Circuit:

T-1 is a digital circuit that uses the DS-1 (Digital Signaling level 1) signaling format to transmit voice/data over the PSTN network at 1.544 Mbps. T-1 can carry up to 24 uncompressed digital channels of 64 Kbps (DS0) for voice or data.

E-1 is the European equivalent of the T-1, except E-1 carries information at the rate of 2.048 E-1 is used to transmit 30 64Kbps digital channels (DS0) for voice or data calls, plus a 64Kbps channel for signaling, and a 64Kbps channel for framing and maintenance. T1/E1 circuit is a dedicated circuit and is always composed of two parts: the local loop and the carrier circuit.

A T1/E1 circuit is the first multiplexed level of the digital signaling multiplexing scheme. T1s use what is called a Stratum 3 clock to maintain what is called clocking on the line.