Public Switched Telephone Network: PSTN

Public Switched Telephone Network

The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks. It consists oftelephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by switching centers, thus allowing any telephone in the world to communicate with any other. Originally a network of fixed-lineanalog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones.

The technical operation of the PSTN utilizes standards created by the ITU-T. These standards allow different networks in different countries tointerconnect seamlessly. There is also a single global address space for telephone numbers based on the E.163 and E.164 standards. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone.

The PSTN is composed of telephone exchanges networked togethor to foem a nationwide telephone communication system.It is public because the system is available to anyone who can afford the service.All calls are switched,that is,a caller's conversation is broken into piecves and these pieces are sent simultaneously over many connections to reach a reciever at the other end.The individual pieces are switched from one telephone device to another until they reach their final destination at the recieving end.All phones in the PSTN are networked in that any phone can make a call to anuy other phone because all the local phone system around the country are connected to each other.

PSTN Operators

The task of building the networks and selling services to customers fell to the network operators. The first company to be incorporated to provide PSTN services was the Bell Telephone Company in the United States.

In some countries however, the job of providing telephone networks fell to government as the investment required was very large and the provision of telephone service was increasingly becoming an essential public utility. For example, the General Post Office in the United Kingdom brought together a number of private companies to form a single nationalised company.

In recent decades however, these state monopolies were broken up or sold off through privatization.

Component of the public switched telephone network

A Private Branch Exchange (PBX) is a call switching device owned and operated by a private organization in order to allow them to better control calls after they reach the company. It operates very similarly to a telephone company's switching device, except it does not perform certain functions that by law can only be performed by the phone company.

The national exchange is the connection from the regional telephone providers to the long-distance telephone providers. This exchange is where your area code comes from. The North American Numbering Plan contains a list of Numbering Plan Areas corresponding to various areas of the United States.

The telephone set is an interface device that provides the interface to the Public Switched Telephone Network. You can pick up and dial a phone number to make a call. The telephone set performs the dialing function, converts the sound of your voice to electricity and converts the electricity on the line back into the voice of the party you are calling.

Central Office (CO) Locations

Early telephone systems started on the roof of the local central office and radiated outward in a spider web of connections between the central office, businesses and residentialsubscribers. To get everyone connected, the phone company sometimes created 'party lines' where multiple phones in different locations were on the same phone circuit.

PSTN Architecture

Conceptual View – Screen Phone Connectivity:

In simple terms, a telephone (a screen phone) will have a data interface to the CO/PSTN, connecting to the SS7 network. You can think of this interface to the phone as being an ISDN-style interface, but only using the I-frame call control data channel. But, since ISDN has never grabbed hold in the U.S., in more modern terms you could think of a DSL or cable VoIP modem that has an Ethernet connection to the phone, rather than a two-wire analog connection. The Ethernet connection carries signaling data as well as digitized audio, and conveys MultiMedia data from intervening or end systems

For the first scenario, let’s assume that the called device is not compatible with the Next Generation MultiMedia, and must be treated as an analog phone call.

The destination CO receives the call setup request, determines that the called device is not capable of an Internet communication, and accepts the call as an analog call.

The called device is a Next Generation device capable of an Internet MultiMedia connection.

The underlying operation can be fairly simple, although more sophisticated arrangements offering additional benefits are possible. In the above illustration, a screen phone goes off-hook and dials a number. The local CO (or appropriate SS7 node) forwards the call setup message to the remote CO serving the called number. The destination CO responds as to whether or not the called system is capable of an Internet connection. If so, the destination CO forwards the call setup message to the end device (which might be a phone, a voice mail system, etc.). The call setup message includes the IP address of the calling phone. Upon receipt of the call setup message, the called phone learns both the telephone number and the IP address of caller. The called phone can then respond to the caller with high-speed MultiMedia information across the Internet, as shown below.

One of the virtues of this system is that the PSTN maintains call control. Among other things, this means that calls can be transferred transparently from analog to MultiMedia and back without the users having to know anything about the types of phone service or types of phones – the intelligence is built into the network, and users only need to know what they want to do. MultiMedia phones will offer users screen buttons to activate these operations, analog phone users will flash the switchhook, as they have always done – it’s just automatic.

The audio portion of a telephone call can be transmitted over the PSTN, providing guaranteed high quality audio, and all the other features in a “carrier class” environment, while transmitting the visual and Multimedia aspects over the Internet. MultiMedia telephone service will also allow carriers to offer the same intrinsic value and appeal as a computer with a graphical browser for accessing the Internet. This technology is available for use between land-based and cellular telephones interchangeably.

Switching Techniques

Message passing

Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, andinterprocess communication. In this model, processes or objects can send and receive messages (comprising zero or more bytes, complex data structures, or even segments of code) to other processes. By waiting for messages, processes can also synchronize.

Packet switching

Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams (sequences of packets) over a shared network. When traversing network adapters, switches, routers and other network nodes, packets are buffered and queued, resulting in variable delay and throughput depending on the traffic load in the network.

Circuit Switching

Circuit-switching systems are ideal for communications that require data to be transmitted in real-time. Packet-switching networks are more efficient if some amount of delay is acceptable.

Circuit-switching networks are sometimes called connection-orientednetworks. Note, however, that although packet switching is essentially connectionless, a packet switching network can be made connection-oriented by using a higher-level protocol. TCP, for example, makes IP networks

Advantages of PSTN

All signaling conducted on PSTN
Signaling data is secure
No dependency on DNS (recall that, recently, hackers crashed the entire worldwide DNS system)
PSTN controls call
User just dials a call -- calls automatically complete best way
Voice portion can ride across PSTN, high quality audio, no jitter or dropouts
Can use standard billing systems & call rating schemes