Introduction and Historical Perspective
The traditional technology for connecting customer sites to telephone exchanges or central office switching points is twisted pair copper telephone cables. Now, applications for telecommunications systems include telephony, Internet access, wide area networking for teleworkers and Video On Demand.
This report discusses the traditional and new uses of twisted pairs, including the technologies of analogue telephony and ISDN.
Twisted Pair Copper
Until the 1940s, telephone signals were carried from exchanges to customer sites via aerial wires on poles. Typically this involved a pair of wires for each phone, but in rural areas a single wire was also used, with the path for return current being the Earth. Underground telephone wiring began with paper-insulated copper wires - with each pair gently twisted - in lead-sheathed cable. The paper and lead was replaced by polyethylene insulation and sheathing in the 1950s and 60s. Some paper insulated cables are still in service, but the great majority of telephone connections in developed countries now use plastic insulation and are laid in ducts, rather than being buried directly in the soil.
These cables were designed for voice frequency performance, but can be used to carry higher frequency signals to carry digital information. Basic Rate ISDN was the first of these technologies, and Primary Rate ISDN and the various 'xDSL' Digital Subscriber Line technologies - such as ADSL (Asymmetrical Digital Subscriber Line) - are methods of achieving high-capacity digital communications over a twisted pair of copper wires.
'Last Mile' Technologies Other Than XDSL and ISDN
There are four other prominent technologies suitable for making the 'last mile' link from a telecommunications carrier's switching point to tens of thousands of customer sites in a particular service area. Each of these is capable of bi-directional digital communication, and so could carry voice telephony as well as data for Internet and unidirectional applications such as subscription TV and asymmetrical applications such as Video On Demand:
- Hybrid Fiber Coaxial (HFC) cable systems are being widely deployed in many developed countries.
- Fiber To The Curb (FTTC) - which uses VDSL over twisted pair copper for the last few hundred meters - and is beginning to be commercially deployed. FTTC and VDSL are sometimes referred to as 'Switched Digital Video' or SDV.
- LMDS (Local Multipoint Distribution System) microwave systems that are expected to become technically and commercially feasible ca. 2000. These use 'super high frequency' 30 Gigahertz bi-directional radio communications over ranges of a 1 to 5 kilometers.
The term 'Full Service Network' is often used to denote a single technical approach for providing all telecommunications services to a given area, possibly including the functionality of multi-channel broadcast video and HDTV (High Definition Television). The aim is to use a single infrastructure -such as FTTC, FTTH or HFC - and highly automated management systems so that a wide and extendable range of services, from telephony to broadband Internet and VOD (video-on-demand), can be reliably provided with minimal labor. Telecommunications carriers are particularly interested in developing an FSN (Full Services Network) architecture which will serve all the needs of customers for several decades and so remove the need to maintain and/or expand their existing twisted pair copper Customer Access Network.
The term 'last mile' loosely refers to the distance from a telecommunications carrier's switching point to the customer's site. In central business districts this may be a kilometer or two. In urban areas and regional towns and cities, the distance may be five to ten kilometers. In rural areas, the distance may be much greater. While some of these technologies - such as twisted pair copper and microwave - may be suitable for distances of more than fifteen kilometers in rural areas, the focus of the following discussion is on 'last mile' technologies in built-up areas, including to any customer site within 10 or 15 kilometers of the switching point.
The term 'switching point' is used here to refer to a physical telephone exchange, or its equivalent for broadband digital communications. Increasingly these small exchanges are being replaced by fewer larger ones, and the 'switching point' is a building or substantial installation which connects to the carrier's backbone network via redundant fiber cables. With standard telephony, this point is either an exchange or a Remote Access Unit of a distant telephone exchange.
Voice telephony is assumed to be the basic service offering, but most of these technologies provide digital communications as well. So the term 'switching point' may refer not just to a telephone exchange or Remote Access Unit, but to a router or other form of data switch.
WLL - Wireless Local Loop
The provision of telephony and low to moderate rate data services via radio technologies, such as GSM, CDMA, 3G or LMDS is often referred to as 'Wireless Local Loop' or WLL. ('Loop' in this context is a generalized term for the telecommunications carrier's method of connecting to customer premises).
This report does not concentrate on these WLL approaches, but on the use of existing twisted pair cables, fiber, HFC cable and microwave approaches which provide broadband two-way communication.
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