Telecom Networks and Cables

DISCLAIMER

For most of its lifetime, the PSTN has relied on copper wiring to carry voice signals to and from the local exchange and the customer’s telephone. But while it’s estimated that around 60% of all telecommunications traffic still travels over copper, cabling based on glass fibres – fibre optic cable – is making rapid inroads, thanks to its ability to support faster bit-rates, its robust nature and its huge bandwidth. Fibre can already carry 26,000 times more traffic than a standard copper line, but if that’s not enough, new multiplexing technology known as Wave Division Multiplexing (WDM) is now boosting capacity fibre optic cables by as much as 100 times.

Home Truths

Throughout the world, most homes are connected to the telecommunications operators’ Central Office (effectively, the local phone exchange) via an analogue twisted pair line. Disconnect the wire from the back of your home phone and you’ll probably find a configuration telecoms engineers call ‘quad’ – four wires held in place by a plastic RJ-11 connector. The inner pair of wires corresponds to Line One and the outer pair to Line Two. Each wire in the pair has a name (tip and ring), and the two together provide a complete connection to the local exchange – the reason why this connection is known as the Local Loop.

Each pair of local loop wires transmits voice, tones and power. The tones consist of the Dual-Tone Multi-Frequency (DTMF) tones generated by pressing the buttons on a touch-tone keypad, as well as information tones which are generated at the exchange. The power is used to ring your phone and to keep it working even when the domestic electricity supply fails (otherwise, how could you call the power company to complain?). On an ordinary phone line, even data from a fax machine or a modem is translated into voice energy - which sounds like static - before it's sent over the line.

Fair Exchange

The connection between telecommunications exchanges in industrialised nations has long since been converted from analogue technology to digital, which is much better suited to processing the large amounts of mixed information now travelling over the PSTN. Carrying data comes naturally to digital systems, since the information is already coded into a language the system understands. To move voice across the network, digital systems sample the voice signal, convert it into binary – that is, to a series of 1s and 0s – and send the signal as data using a protocol called Signalling System 7 (SS7). When the data reaches the exchange at the other end of the connection, the data is turned back into voice and sent as analogue signal down the so-called ‘last mile’ to the subscriber’s home.

Despite widespread deployment of fibre optic cable in the PSTN, almost all local loop connections remain copper-based. While the dream of an all-fibre network – often called ‘fibre-to-the-home’ – will probably one day become a reality, for the moment carriers’ huge investments in their copper infrastructure, the high price of fibre, the relatively low traffic levels generated by residential users, and the lack of ‘killer’ applications to drive consumer demand for costly higher-speed home connections are all conspiring to keep the local loop a copper-based affair, at least for the next few years.

Fibre Links

The advent of commercial fibre optics in the mid-1970s revolutionised the way telecoms traffic was carried. For the first time, mainline trunks could carry not just thousands of simultaneous calls, but hundreds of thousands. In addition, carrying signals as photonic energy – pulses of light – down hair-thin strands of pure glass cuts signal attenuation dramatically, allowing signals to be carried as far as 2,000km without the need for a repeater station.

Optical fibre based networks can move information much more quickly than conventional copper wire networks, supporting data rates of up to 10Gbps in contrast to the 64kbps over a standard residential telephone connection. Although fibre remains relatively expensive compared to standard copper wiring, it has a long lifespan and requires very little maintenance. At the same time it’s easier to manage, being free of problems like crosstalk and interference, and much more secure, since unlike metal-based links fibre cables do not emit radiation which outsiders can tap to decode the signals on the line.

For these reasons, fibre-based infrastructure is certain to play a key role in the high-capacity broadband networks of the future which will be used to deliver fast Internet access and fast data transfer, as well as a wide range of new multimedia applications such as full-motion video, audio clips and real-time videoconferencing.

The widespread use of fibre in undersea cables, inter-city trunks and increasingly in metropolitan area networks and fibre-to-the-kerb schemes has dramatically cut the cost of carrying telecoms traffic. Once the cost of installing large trunks has been amortised, operators can carry calls for tiny fractions of a cent on major routes, a factor which is now helping to drive down the cost of international calls.

Undersea Trunks

The last ten years has seen tremendous growth in high-capacity submarine fibre optic trunks, which have boosted capacity on international routes to an unprecedented level and made the cost of carrying international traffic almost negligible. With many more high profile projects now underway, including Project Oxygen, Global Crossing, and FLAG (Fibre-optic Link Around the Globe) analysts predict that by 2003 more than US$ 56 billion will be invested in the fibre optic undersea market with close to a million route kilometres in place.

Because of their lower cost and longer lifespan, undersea fibre optic cables have now largely taken over from satellites as the principal means of delivering international traffic.

New Copper Technologies

While fibre wins hands down when it comes to absolute bandwidth, copper still comes up trumps on cost and – most importantly – installed base. With copper lines still servicing almost every home on the world-wide telephone network (there were 834 million fixed lines world wide by the end of 1998), carriers have a big incentive to find new ways to take advantage of this sprawling, costly asset. It has been estimated that the value of all the copper currently installed in the US phone network comes to around US$100 billion, while a 1994 estimate on the cost of replacing BT’s copper network came in at close to US$18 billion.

To date, the most promising of all the technologies developed to enhance the performance of existing copper networks is so-called xDSL. DSL stands for Digital Subscriber Line, with the ‘x’ referring to the fact that there are several flavours of the technology (HDSL, ADSL, VDSL – see Cabling Terminology, below), suited to different applications and network configurations.

For the moment, Asymmetric DSL (ADSL) seems to have taken the running, with many large operators now in the process of launching services which have the potential to expand network access capacity by a factor of as much as 50. As its name suggests, ADSL supports asymmetrical traffic flows down copper wires – that is, fast bit-streams of up to 8Mbps in the downstream (telephone exchange to customer) direction, and slower rates of up to 1Mbps in the upstream (customer to exchange direction). High downstream capacity makes ADSL well–suited for new technologies like high-speed Internet connection and near-video-on-demand, where the bulk of the traffic flow is to the customer, in the form of Web pages, movies or interactive games.

New Fibre Technologies

The most important new development in fibre optic cabling is undoubtedly a complex technology known as Wave Division Multiplexing (WDM). WDM systems effectively split a single beam of white light travelling along a fibre optic cable into its component optical wavelengths, and then allocate a separate data channel to each ‘colour’ of light.

Using advanced narrow and constant wavelength lasers, very precise optical filtering and switching, and complex, software-controlled monitoring systems, Wave Division Multiplexers can greatly increase the bandwidth of a fibre optic cable by allocating the total capacity of the fibre’s original traffic-carrying capability onto each individual wavelength. Dense Wave Division Multiplexing (DWDM) is an advanced mechanism for even further increasing the total number of separate optical wavelengths per fibre (by as much as 32 times) using present-day technology.

To date, the high cost of WDM and DWDM systems has largely limited their use to point-to-point high capacity long-haul telecommunications backbones, such as the 30,000km-plus SEA-ME-WE 3 (South East Asia-Middle-East-Western Europe) undersea cable project, MCI WorldCom’s pan European network, and Sprint’s US long distance network.

Increasingly, however, new developments are making WDM and DWDM technology more cost-effective on shorter routes like regional and inter-city networks, where there is a high volume of traffic and a demand for optimum service quality.

Other Cabling Options

Cable television operators are increasingly getting into the business of offering mixed telecommunications and entertainment packages over their high-speed direct-to-home coaxial cable networks, which can deliver up to 80 times the capacity of standard copper twisted pair.

To provide optimum-grade voice telephony, most favour so-called hybrid fibre/coax networks, using fibre to carry voice signals and the coax to deliver TV and Internet connectivity. With fast cable modems expected to be widely available as early as next year, supporting always-on Internet connectivity at speeds of up to 2Mbps and the crucial two-way capabilities needed to support future online services like interactive TV and online games, cable TV networks could pose a serious threat to telcos’ copper networks in the early years of the next decade.

For the moment, however, most cable operators need to pull a lot more fibre in order to upgrade existing broadcasting-oriented networks to carry a mix of traffic – an expensive proposition which could yet delay the widespread and affordable launch of bundled communications offerings from the cable operators.

Some Cabling Terminology

Attenuation - the decrease in power of a signal transmitted over a wire, measured in decibels. As attenuation increases, the signal decreases. To counter attenuation, telecommunications carriers use repeater stations which boost and regenerate the signal along the cable at regular intervals.

Copper Twisted Pair -The mainstay of the world’s telecommunication networks, copper twisted pair comprises two copper wires twisted together to reduce the electrical coupling between them and the amount of electrical noise they pick up. Standard copper twisted pair supports data rates of up to 64kbps.

Crosstalk - on cabling, the spillover of a signal from one channel to another, causing interference

Coaxial Cable - Coaxial cable consists of a core of copper wire – either solid or stranded – which is surrounded by an external shield of woven copper braid or metallic foil. The braid and the central conductor share the same axis – hence the name coaxial. Plastic insulation is used to separate the inner and outer conductors, while another layer of insulation covers the outer braiding. In coaxial cable, the outer conductor shields the inner conductor from outside electrical signals while also reducing the radiation of interior signals.

Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between equipment than twisted pair cable. The two types of coaxial cabling are: thick coaxial and thin coaxial. Coaxial cable is used for the delivery of cable television, and is also a popular type of cabling for computer networks.

Dial-up Line - A communications circuit established by dialling a destination over the PSTN.

Fibre Optic Cable - Fibre optic cable consists of strands of glass fibre surrounded by a coating of Teflon. Kevlar or stainless steel fibres are often incorporated into the cable to provide added strength. Signals are transmitted along the cable as pulses of light, rather than electricity. Fibre optic cable has several advantages over copper wiring, including complete freedom from electrical interference, a small, compact diameter, and the potential for carrying very large amounts of data at very high speeds. Although signals on copper cabling and fibre optic cable travel at around the same speed, light meets less resistance as it moves along the cable, allowing signals to travel much further without attenuation.

HFC - Hybrid Fibre/Coaxial systems, often used by cable TV operators who are beginning to use this very high-capacity combination to offer a mix of voice, online services and television.

Leased Line - A communications circuit reserved for the exclusive use of the subscriber. Frequently used by businesses on a point-to-point basis to connect remote offices, since the subscriber pays a flat rate for the line rental, rather than charges for the individual calls travelling over the connection. Also called Leased Circuit and Private Line.

Photonics - the science relating to optical fibre-based switching, multiplexing and amplification equipment such as Wave Division Multiplexing and Dense Wave Division Multiplexing. It lies at the heart of all-optical networks, which have the potential to transport, route and deliver much greater amounts of bandwidth more cost-effectively than existing electro-optical systems.

Pulse-code Modulation (PCM) - a common method of digitising voice signals. The bandwidth required for a single digitised voice channel is 64kbps.

Quad - Quad is the telephone wiring which comes into your home. It comprises four parallel twisted pair wires bunched into a single cable.

RJ-11 - The small, usually transparent, 4-pin plastic connector commonly used to connect voice telephony equipment to the network.

xDSL - Digital Subscriber Line. There are many kinds of DSL technology, including ADSL (Asymmetric DSL - up to 8Mbps), HDSL (High Bit-Rate DSL - up to 64kbps) and VDSL (Very High Data Rate DSL - up to 52Mbps over very short distances).

For further information, please contact the Telecom 99 + Interactive 99 Secretariat at +41 22 730 6161 (phone) or +41 22 730 6444 (fax), or see the Telecom web site at www.itu.int/Telecom.

For media representatives, please contact the Telecom 99 Press Service at +41 22 730 6463/6402 (phone) or +41 22 730 6577/6578 (fax)

These backgrounders are posted free of copyright and may be used in part or in full by anyone who so desires.