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CCIE must know: PSTN protocol details

PSTN protocol details

PSTN, namely Public Switched Telephone Network, is a Switched Network used for global voice communication, which has about 800 million users and is the world’s largest telecommunications Network at present. This network is used in everyday life, such as when we make landline calls or dial up the Internet on a telephone line at home. It is important to note that the PSTN network was originally designed to transmit voice data.
PSTN — Public Switch Telephone Network (PSTN) is a Telephone Network commonly used in our daily life. As we all know, PSTN is a circuit-switched network based on analog technology. Among many wan interconnection technologies, PSTN has the lowest communication cost, the worst data transmission quality and speed, and the low network resource utilization rate.
It also means plain old Telephone service (POTS). It is a collection of all the circuit-switched telephone networks that have operated since Alexander Graham Bell invented the telephone. The public switched telephone network is now technically fully digital, in addition to the last link between the user and the local telephone switchboard. In its relationship with the Internet, the PSTN provides a significant portion of the Internet’s long-distance infrastructure. Internet service providers (ISPs) pay device owners to use the PSTN’s long-distance infrastructure and to share circuits through information exchange among many users. Internet users would then only have to pay for Internet service providers. PSTN is a circuitswitched service based on standard telephone lines used as a connection method to connect remote endpoints. Typical applications include connections between remote endpoints and local Lans and dial-up Internet access for remote users.
PSTN can be composed of two parts, one is exchange system; One part is the transmission system. The switching system consists of the telephone exchange, while the transmission system consists of the transmission equipment and cables. With the growth of user demand, these two components are constantly evolving and improving to meet user demand.

  1. The development of the exchange system probably goes through the following stages.
    In the era of manual exchange, it is done manually. A long time ago, when you made a phone call, you would get through to an operator and the operator would transfer you.
    In the era of automatic switching, step – by – step and vertical – and – horizontal switches were produced.
    In the era of semi-electronic switching, electronic technology is introduced in the control part of the switch.
    In the era of air – point switching, program-controlled switches were born, but analog signals were still transmitted.
    In the era of digital switching, with the successful application of PCM pulse code modulation technology, the DIGITAL program-controlled switch also came into being, in which digital signals were transmitted.
  2. PSTN transmission equipment developed from carrier multiplexing equipment to SDH equipment, and the cable also developed from copper wire to optical fiber.
    The PSTN provides an analog, proprietary channel connected by a number of telephone exchanges. When two host or router devices need to connect through THE PSTN, modems must be used at the network access sides on both ends to realize the analog/digital and digital/analog conversion of signals. From the perspective of the OSI seven-layer model, PSTN can be seen as a simple extension of the physical layer, without providing users with traffic control, error control and other services. Moreover, since THE PSTN is a circuit-switched mode, the entire bandwidth of a path from inception to release can only be used by devices at both ends of the path, even if there is no data to be transferred between them. Therefore, this circuit switching method cannot make full use of the network bandwidth.
    PSTN Network Access mode:
    PSTN access to the network is relatively simple and flexible, there are usually the following:
  3. Access the network through ordinary dial-up telephone lines. Just connect the Modem on the original telephone line of both parties, and then connect the Modem with the corresponding Internet device. Most internet-connected devices, such as PCS or routers, provide a number of serial ports, and rS-232 and other serial interface specifications are used between the serial port and Modem. The cost of this kind of connection is more economical, the charging price is the same as that of ordinary telephone, and can be applied to the occasion of less frequent communication.
  4. Access the network by renting a special telephone line. Compared with ordinary dial-up telephone lines, leased telephone lines can provide higher communication rates and data transmission quality, but the corresponding cost is higher than the former method. Special line access method and the use of ordinary dial-up line access method is not very different, but save the process of dial-up connection.
  5. PSTN is transferred to the public switched data network (X.25 or Frame-Relay, etc.) by ordinary dial-up or leased private telephone line. Using this method to connect to remote locations is a better remote method, because the public switched data network provides users with reliable connection-oriented virtual circuit services, which are much more reliable and faster than the PSTN.
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