Introduction and Historical Perspective
While the telephone handset has changed little in half a century, previously isolated national telephone networks have evolved into a sophisticated, reliable global network. Despite the growth and immense flexibility of the Internet, telephony will remain a vital part of electronic communications for the foreseeable future. Competition in a multi-carrier environment will help to bring costs down as fast as they should fall according to the rapid advances in the underlying technology. Before such a competitive environment can be properly achieved, the difficult problems of number portability must be solved. Here we describe the most important technical characteristics of the telephone network, and how it differs from the Internet.
The global circuit-switched telephone network has grown incrementally from isolated, analogue, manually switched systems to form a single global network, in which calls to almost anywhere can be made by simply dialing the appropriate number - even from a telephone handset which is fifty years old. This ubiquitous and generally reliable network (at least in developed countries) is surely one of humanity's greatest achievements.
Despite the simple network interface of twisted pair telephony, with its user interface of a simple analogue phone with pulse or tone dialing, the network itself is extremely complex. To transport audio signals reliably in two directions at once, between any two telephones on Earth, and to be able to set up, close down and bill such calls within seconds, requires a high degree of standardization between the systems of different countries and carriers.
In fact, such standardization is incomplete in many areas, and quite a few older analogue exchanges, with a variety of signaling techniques, are still in use alongside the relatively standardized digital exchanges that make up the bulk of the network.
For simplicity, this discussion focuses on that digital network - and then presents an idealized view of it - as an introduction to the key characteristics of the telephone network. However, the reality of implementation is far more complex. A good example of a modern, elegantly designed telephone network is the national network in Australia. With just several hundred exchanges and only two 'layers' of exchanges (local and transit - while some other networks may have up to four layers), and with the integration of local, long-distance and international services, it is relatively simple and modern compared to that mandated by the more complex regulatory and commercial structure and the larger customer base of the United States.
The task of planning, installing, managing and expanding this modern network is a huge one, but it is far simpler than maintaining what it replaced: thousands of exchanges of over fourteen different types, most of which were small rural exchanges which had a variety of signaling and billing systems which were incompatible with each other and with the core network.
Telephone technology has evolved in a totally different manner from the light-weight, highly flexible, equipment and protocols of the Internet. If the telephone network was designed from scratch for the twenty-first century, it would probably be very different. However it has evolved for over a century and the result is a stable, relatively inflexible, expensive and 'heavy-weight' technological solution which is based on exchange technology of the 1980s and fiber technology of the 1990s.
Fortunately, many important telephone exchange functions can now be performed by highly reliable versions of personal computers with specialized hardware and software - as developed internally by international carrier World exchange. Another sign of this trend away from 'mainframe' equipment is the inclusion of telephony functions in routers, such as voice-compression, IP gateways and SS7 signaling. These developments enable smaller companies to inter-operate with the big carriers -with rapid installation, a lower cost structure and greater flexibility. So the 'mainframe' like structure of exchanges as described below is no longer the only, or necessarily the best, way of providing telephone services.
A Circuit-Switched Network
The telephone network's primary purpose is to provide a call-by-call service, in which one telephone is linked via a bi-directional audio or digital path to another telephone . This is very different from the purpose of the Internet - a packet-switched network - in which each connected computer has a permanent or temporary link to the Internet, and where that link enables the sending and reception of packets of data to any other connected computer, with each packet being an independent item of traffic, rather than part of a 'call' which must be set up. The Internet closely resembles the postal system, handling packets of up to 1500 bytes of data. Packet delivery is not guaranteed, but higher level protocols are available to ensure retransmissions of packets which are not received correctly.
The telephone network provides a continuous bi-directional 'pipe' to and from the other phone, which continually sends numbers or audio signals between them. The customer end of the telephone network may be a standard analogue POTS telephone (Plain Old Telephone Service) connected by several kilometers of twisted pair copper, or it could be another technology such BR-ISDN or a GSM mobile phone.
The inter-exchange network is typically made of SDH (Synchronous Digital Hierarchy, or SONET in North America) optical fiber links. Each fiber may carry 622 Mb/s, 2.4Gb/s or even higher rates, which is likely to be split into sub-streams which serve a variety of purposes. Carriers initially constructed this inter-exchange network, and its international equivalents - which include geostationary satellites - to carry primarily circuit-switched telephone traffic. These fiber links are permanent installations and the assignment of capacity within them is flexible (in multiples such as 2 and 34 Mb/s), but would typically change little from one week to the next.
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