How do the international numbering systems work? How was it established? And, why is it important?
Description:
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.
What is PSTN ?
The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the wired phone system over which land-line telephone calls are made. The PSTN relies on circuit switching. To connect one phone to another, the phone call is routed through numerous switches operating on a local, regional, national or international level. The connection established between the two phones is called a circuit.
In the early days, phone calls traveled as analog signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. That's why you needed operators' assistance in making calls. The operators sat at a switchboard, literally connecting one piece of copper wire to another so that the call could travel across town or across the country. Long-distance calls were comparatively expensive, because you were renting the use of a very long piece of copper wire every time you made a call.
Beginning in the 1960s, voice calls began to be digitized and manual switching was replaced by automated electronic switching [source: Wireless Center]. Digital voice signals can share the same wire with many other phone calls. The advent of fiber-optic cables now allows thousand of calls to share the same line. But fiber-optic and other high-bandwidth cables haven't changed the basic nature of circuit switching, which still requires a connection -- or circuit -- to remain open for the length of the phone call.
Routing calls requires multiple switching offices. The phone number itself is a coded map for routing the call. In the United States, for example, we have 10-digit phone numbers.
- The first three digits are the area code or national destination code (NDC), which helps route the call to the right regional switching station.
- The next three digits are the exchange, which represents the smallest amount of circuits that can be bundled on the same switch. In other words, when you make a call to another user in your same exchange -- maybe a neighbor around the corner -- the call doesn't have to be routed onto another switch.
- The last four digits of the phone number represent the subscriber number, which is tied to your specific address and phone lines.
Within a company or larger organization, each employee or department might have its own extension. Extensions from the main phone number are routed through something called a private branch exchange (PBX) that operates on the premises.
To make an international call requires further instructions. The call needs to be routed through your long-distance phone carrier to another country's long-distance phone carrier. To signal such a switch, you have to dial two separate numbers, your country's exit code (or international access code) and the corresponding country code of the place you're calling.
Almost all exit codes are either 00 or 011, although there are a few exceptions like Cuba (119) and Nigeria (009). Country codes are one- to three-digit prefixes that are assigned to specific countries or groups of countries. For example, the country code for the United States is 1, but the United States shares that country code with Canada and several smaller island nations like Jamaica, Puerto Rico and Guam. The country code for Mexico is 52 and Saudi Arabia is 966.
The Telecommunication Standardization Sector
The Telecommunication Standardization Sector is part of the International Telecommunication Union (ITU), an agency of the United Nations. The ITU, based in Geneva, Switzerland, works with 191 member countries to develop and implement global communications technology.
The specific responsibility of the Telecommunication Standardization Sector (ITU-T) is to research and recommend standards and protocols relating to voice and data transmissions over landline and mobile networks. This includes everything from streaming video on cell phones to Voice over IP (VoIP) to SMS to international call rates.
When it was established in 1925, the ITU-T was called the Comité Consultatif International Téléphonique et Télégraphique (CCITT). The CCITT was responsible for breakthrough work in standardizing fax data transmissions, modems, data compression, packet-switching and e-mail. The CCITT became the ITU-T in 1993 as part of a new strategic plan to respond quicker to the ever-changing technological landscape [source: International Telecommunication Union].
The ITU-T is best known for its recommendations. As their name implies, recommendations aren't laws or regulated standards, but merely suggestions for the best way to make telecommunications technology and networks run smoothly. Recommendations are established by 13 study groups within the ITU-T. Each study group is comprised of international managers and rapporteurs (appointees) from the public and private sector. A typical study group might include a CTO from Israel, a researcher from Germany and an American policymaker from the Federal Communications Commission.
Study groups are assigned a handful of questions, which they research over the course of two to four years before publishing their recommendations. Questions resemble topics. Current examples of questions include:
- Optical fiber cable network maintenance
- Traffic engineering for mobile communications
- Voice and video IP applications over cable television networks
- Real-time audio, video and data communication over packet-switched networks.
The ITU-T also organizes focus groups, which are smaller research units within a study group working on a specific problem or question. Recommendations are published online and are free to the public as well as private industry and government agencies.
Regulation of the PSTN
In most countries, the central government has a regulator dedicated to monitoring the provision of PSTN services in that country. Their tasks may be for example to ensure that end customers are not over-charged for services where monopolies may exist. They may also regulate the prices charged between the operators to carry each others' traffic.
Technology in the PSTN
Network topology:
The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls, connections to other countries, direct dialling and so on. The model developed by the US and Canada was adopted by other nations, with adaptations for local markets.
The original concept was that the telephone exchanges are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing. This reduced the number of connecting trunks required between operators over long distances and also kept local traffic separate.
However, in modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers.
U.S. PSTN telephone switch hierarchy.
Digital channels:
As described above, most automated telephone exchanges now use digital switching rather than mechanical or analog switching. The trunks connecting the exchanges are also digital, called circuits or channels. However analog two-wire circuits are still used to connect the last mile from the exchange to the telephone in the home (also called the local loop). To carry a typical phone call from a calling party to a called party, the analog audio signal is digitized at an 8 kHz sample rate with 8-bit resolution using a special type of nonlinearpulse code modulation known as G.711. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a call set up protocol (usually ISUP) between the telephone exchanges under an overall routing strategy.
The call is carried over the PSTN using a 64 kbit/s channel, originally designed by Bell Labs. The name given to this channel is Digital Signal 0 (DS0). The DS0 circuit is the basic granularity of circuit switching in a telephone exchange. A DS0 is also known as a time slot because DS0s are aggregated in time-division multiplexing (TDM) equipment to form higher capacity communication links.
A Digital Signal 1 (DS1) circuit carries 24 DS0s on a North American or Japanese T-carrier (T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on an E-carrier (E1) line used in most other countries. In modern networks, the multiplexing function is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.
These aggregated circuits 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 use synchronous optical transmission, for example, SONET andSynchronous Digital Hierarchy (SDH) technologies, 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 standardized in ETSI areas as the V5 interface.
Tivadar Puskás, inventor of the telephone exchange.
Impact on IP standards :
Voice quality over PSTN networks was used as the benchmark for the development of the Telecommunications Industry Association's TIA-TSB-116 standard on voice-quality recommendations for IP telephony, to determine acceptable levels of audio delay and echo
A Video on Telephone & PSTN :
