What's in a PSTN?

What's exactly in a PSTN. What is is made up of? What are some of the equipment used in a PSTN? I have seen those exchange buildings, those dull (and sometimes unamed) concrete buildings with very few windows, if any. Yet, they are able to connect to netwoks all over the world. Just pick up any land line and dial the country code, followed by the area code, then number (of course do watch your next telephone bill). All this was before the advent of the Internet and VoIP.

Could anyone tell me which companies produce industrial grade PSTN equipment (preferbaly 1980s and early 1990s)? I hope some Telecom experts can answer this question.

Suren,

What I meant was an explanation of how it works. Anyway, cooladi gave a very comprehensive answer. Thanks. Keep those answers flowing!

Short for Public Switched Telephone Network, (PSTN) which refers to the international telephone system based on copper wires carrying analog voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI.
Telephone service carried by the PSTN is often called plain old telephone service (POTS

The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks, in much the same way that the Internet is the network of the world's public IP-based packet-switched networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and now includes mobile as well as fixed telephones.

The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 addresses (known more commonly as telephone numbers) for addressing.

The PSTN was the earliest example of traffic engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang (1878鈥?929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and personnel required to deliver a specific level of service.

In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology of the Internet.

Only the very oldest parts of the telephone network still use analogue technology for anything other than the last mile loop to the end user, and in recent years digital services have been increasingly rolled out to end users using services such as DSL, ISDN and Cable modem systems.

Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way to go before this transition can be made. The QoS guarantee is one aspect that needs to be improved on in the Voice over IP (VoIP) technology.

There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large private branch exchange (PBX) system.

Although the network was created using analog voice connections through manual switchboards, automated telephone exchanges replaced most switchboards, and later digital switch technologies were used. Most switches now use digital circuits between exchanges, with analog voice used to connect to many telephones.

The basic digital circuit in the PSTN is a 64-kilobits-per-second channel, originally designed by Bell Labs, called Digital Signal 0 (DS0). 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 (PCM). 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.

The DS0s are the basic granularity at which switching takes place in a telephone exchange. DS0s are also known as timeslots because they are multiplexed together using time-division multiplexing (TDM). Multiple DS0s are multiplexed together on higher capacity circuits, such that 24 DS0s make a DS1 signal, which when carried on copper is the well-known, T-carrier system, T1 (the European equivalent is an E1, containing 32 64 kbit/s channels). In modern networks, this multiplexing is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.

The timeslots are conveyed from the initial multiplexer to the exchange over a set of equipment collectively known as the access network. The access network and inter-exchange transport of the PSTN use synchronous optical transmission (SONET and SDH) technology, although some parts still use the older PDH technology.

Within the access network, there are a number of reference points defined. Most of these are of interest mainly to ISDN but one 鈥?the V reference point 鈥?is of more general interest. This is the reference point between a primary multiplexer and an exchange. The protocols at this reference point were standardised in ETSI areas as the V5 interface.

PSTN is Public Switched Telephone Network - At one time in the past this was the entire telephone network however, since the realization of data only traffic this is no longer true. PSTN refers to the core network of exchange carriers that route traffic through it's digital switch network. this network only carries DS0(64kps) circuit traffic, any data in this network is Pulse-Code modulated into this DSO signal from the Subscriber Loop Carriers. Essentially anything outside the LEC central office switchs is considered facilities and can be carrying a multitude of diffirent information and is ususally routed around the PSTN. Anything above DS0 level (DS1-3, OCn) along with RF circuits are termed Multiplexed Transport Networks. Even the central office environment is becoming more and more facility with Customer Co-Location lease space, where the telephone companies business customers re-routes its customers away from the Traditional telephone network onto there own networks with TCP/IP and VoIP and RF technologies. Hence any 'Station to Station' land line telephone call has to routed though a PSTN.
This is a major drawback to VoIP because the nations 911 services are PSTN controlled and are not accessable from outside it. Similiar with Celluar, WIreless, RF services, while the interface between client and user may be of these technologies the client must still route there traffic through PSTN to complete calls. Hence the Telephone companies will never die!