Текст №2

(1) Data Highway

An optical trunk cable comprises a bundle of optical fibers around a thicker strengthening wire, contained in layers of protective sheaths. Each fiber has a core, through which light travels, and a cladding, which contains the light in the core. Both are made from silicon glass, with small amounts of boron or germanium added to improve transmission properties. A plastic sheath around the cladding ensures that no stray light passes into other fibers.

(2) Light Pipes

Optical fibers can transmit digital data in the form of up to 2 billion pulses of laser light a second. This makes them the ideal medium for carrying the rapidly increasing numbers of telephone calls, fax messages, and computer information traveling from place to place. The glass they are made of is so clear that signals can travel for tens of miles before they have to be amplified – ten times farther than traditional copper cables.

A fiber is in fact made up of two concentric layers of ultra-pure bubble-free glass. The cylindrical core is surrounded by a cladding drawn from glass with a different refractive index. Laser light shone into the core is confined in a process called total internal reflection – rays hitting the boundary between the two layers at a shallow enough angle are reflected rather than escaping.

Because fibers are so thin – narrower even than human hair – they can be bent quite sharply before light “leaks” out. input pulse.

(3) Narrow Cables

A pulse of light sent down an optical fiber with a wide core can travel along many alternative paths [A], some involving many more reflections than others. Over long distances the pulse becomes spread out and “blurred”, eventually merging with the edges of entertainment.

However, in a narrow-core fiber [B], the pulse has only one possible path – straight down the center. Blurring of the pulse is greatly reduced and clear signals can therefore be sent over longer distances in such fibers.

(4) Blanket Coverage

A proposed new worldwide telecommunication system based on transmitters in space will have the ability to connect two people anywhere on the globe [E]. The system shown will include 77 satellites, uniformly spaced, 475 miles above the earth and linked by digital signals to form a cellular network.

Subscribers to the system will be able to communicate with any telephone on the terrestrial networks. A call will be routed directly to a satellite from handsets, earphones, or even solar powered phone booths.

(5) Patterns of use

Radio space is very limited, with demands on it from many different users, so that only a small range of frequencies is available in each country for cellular telephones [D].

Each hexagonal cell has a base station, which is assigned a portion of the limited radio channels available. All the channels are assigned over a pattern of 8 cells, and because the transmitters have such low power – and therefore range – this pattern can be repeated to let an entire country be covered with a small number of channels.

The number of local users determines cell size. Cells in a major city may be as small as 330 ft wide, enabling the available channels to be reused more often. The process of changing frequencies as a user crosses a cell boundary during a call is highly complex, and involves more unheard radio traffic between computers.

The base station at first dealing with the call constantly monitors the strength of the phone signal [1]. As the user walks or drives away from the base station, the strength of the signal from his phone decreases. When it falls below a critical level, the base station sends a digital message, alerting the central exchange [2], which instructs nearby bases to measure the strength of the signal reaching them. The exchange then tells the phone to return to a channel on the cell receiving the strongest signal [3], and the conversation is resumed [4].