Jaringan telepon umum
Jaringan telepon umum atawa public switched telephone network (PSTN) ngarupakeun jaringan tina jaringan-jaringan telepon anu diwangun ku circuit-switched pikeun kaperluan publik sadunya. PSTN awalna ngarupakeun jaringan tina sistem telepon analog sambungan-tetep. Kiwari PSTN ampir digital sakabéhna, sarta ngawengku ogé telepon mobil.
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Architecture and context[édit | sunting sumber]
The PSTN was the earliest example of traffic engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang (1878–1929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and the number of 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 oldest parts of the telephone network still use analog 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, FTTP 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 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).
Early history[édit | sunting sumber]
The first telephones had no network but were in private use, wired together in pairs. Users who wanted to talk to different people had as many telephones as necessary for the purpose. A user who wished to speak, whistled into the transmitter until the other party heard. Soon, however, a bell was added for signalling, and then a switchhook, and telephones took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a local telephone exchange, and the exchanges were wired together with trunks. Networks were connected together in a hierarchical manner until they spanned cities, countries, continents and oceans. This was the beginning of the PSTN, though the term was unknown for many decades.
Automation introduced pulse dialing between the phone and the exchange, and then among exchanges, followed by more sophisticated address signaling including multi-frequency, culminating in the SS7 network that connected most exchanges by the end of the 20th century.
Digital Channel[édit | sunting sumber]
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 two-wire circuits still used to connect to most 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 into a DS1 signal, carrying 24 DS0s on a North American or Japanese T1 line, or 32 DS0s (30 for calls plus two for framing and signalling) on an E1 line used in most other countries. 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 – the V reference point – 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.