Coaxial Cable

Developed in 1936, the coaxial cable uses cable conductors to carry a large number of circuits. The modern coaxial cable consists of copper tubes 0.95cm in diameter. Each has a thin copper wire held exactly in the centre of the tube by plastic disc insulators about 2.5 cm (= 1 inch) apart. The tube and the wire have the same centre; that is, they are coaxial. The copper tubes shield the transmitted signal from electrical interference and prevent energy losses by radiation. A cable, consisting of up to 22 coaxial tubes arranged in tight rings sheathed in polyethylene and lead, can carry 132,000 messages simultaneously.



Fibre-Optic Cable

Coaxial cables are increasingly being replaced by optical glass fibres. Messages are digitally coded into pulses of light and transmitted over great distances by these slender fibres. A fibre cable may contain up to 50 fibre pairs, each pair carrying up to 4,000 voice circuits. The basis of the new fibre optics technology, the laser, exploits the visible region of the electromagnetic spectrum, where frequencies are thousands of times higher than in radio and thus able to carry much larger volumes of information.

Another advantage of optical-fibre systems is the great distances that a signal can travel before a repeater is needed to regenerate it. Fibre-optic repeaters are currently separated by about 100 km, compared to about 1.5 km for electrical systems. Newly developed optical-fibre amplifiers can extend this distance even farther.


Microwave Relay

In this method of transmission microwaves are relayed from station to station. Microwaves are high-frequency radio waves lying between infrared waves and conventional radio waves. Because the transmission of microwaves requires a clear line of sight between sending and receiving stations, the average distance between relay stations is about 40 km. As many as 600 telephone conversations can be transmitted over one microwave relay channel.



Satellite Telephony

In 1969 the first global telephone relay network was completed with a series of satellites in stationary orbits 35,880 km above the earth. These satellites are powered by solar energy cells. Calls transmitted from an earth antenna are amplified and retransmitted to distant ground stations. The integration of satellite and terrestrial facilities allows calls to be routed between continents as easily as between domestic points. Thanks in large part to digitization of transmissions, satellites of the global Intelsat series can relay up to 33,000 calls simultaneously as well as several television channels.

One satellite would not serve for a call from New York to Hong Kong, for example, but two would. Even considering the expense of a satellite such a path is cheaper to install and maintain per channel than the equivalent path using coaxial cables on the ocean floor. Consequently, as much use is made as possible of satellite links in long distance.

Satellites do have one serious shortcoming, however. Because of the satellite's distance and the finite speed of radio waves, there is a noticeable lag in conversational responses. Because of this, many calls will only use a satellite for one direction of transmission (say from New York to San Francisco) and will use a ground microwave or coaxial link for the opposite direction. The participants in a call from New York to Hong Kong might be annoyed if carried over a two satellite link in both directions because they would find it difficult to interrupt—which is a normal occurrence in speech. They would also be bothered by the long time (over a second) it took the other party to respond after each had finished speaking.

A combination of microwave, coaxial cable, fibre-optic, and satellite paths now link the major cities of the world.


  Coaxial Cable Fibre-Optic Cable Microwave Relay Satellite telephony




(simultaneous calls



Some types of DSL


The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users.

ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a  small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media  intensive Web data need lots of downstream bandwidth, but user requests and  responses are small and require little upstream bandwidth. Using ADSL, up  to 6.1 megabits per second of data can be sent downstream and up to 768  Kbps upstream. The high downstream bandwidth means that your telephone line  will be able to bring motion video, audio, and 3-D images to your computer  or hooked-in TV set. In addition, a small portion of the downstream  bandwidth can be devoted to voice rather data, and you can hold phone  conversations without requiring a separate line.

Unlike a similar service over your cable TV line, using ADSL, you won't be  competing for bandwidth with neighbors in your area. In many cases, your  existing telephone lines will work with ADSL. In some areas, they may need upgrading.



CDSL (Consumer DSL) is a trademarked version of DSL that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) but has the advantage that a "splitter" does not need to be installed at the user's end.

Rockwell, which owns the technology and makes a chipset for it, believes that phone companies should be able to deliver it in the $40-45 a month price range. CDSL uses its own carrier technology rather than DMT or CAP ADSL technology.



The earliest variation of DSL to be widely used has been HDSL (High bit-rate DSL) which is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main  characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. For this reason, the maximum  data rate is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair as can be carried on a T1 line in North America or an E1 line in Europe (2,320 Kbps).



VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its  effectiveness in some environments is not yet determined. A number of  standards organizations are working on it.