Date Communications Channels

　　To get here to there, data must move through something. A telephone line, cable, or the atmosphere are all transmission media, or channels. But before the data can be communicated, it must be converted into a form suitable for communication.

　　Data communications lines can be connected in two types of configurations: point-to-point and multidrop. A point-to-point line directly connects the sending and the receiving devices, and a multidrop line connects many devices, not just one sending device and one receiving device.

　　The two ways of connecting microcomputers with each other and with other equipments are through the cable and through the air. There are three basic forms into which data can be converted for communication: electrical pulses or charges, electromagnetic waves, and pulses of light.

　　Specifically, five kinds of technology are used to transmit data. These are telephone lines (twisted pair), coaxial cable, fiber-optic cable, microwave, and satellite.

　　Telephone Lines Inexpensive, multiple-conductor cable comprised of one or more pairs of 18 to 24 gauge copper strands. The strands are twisted to improve protection against electromagnetic and radio frequency interference. The cable, which may be either shielded or unshielded, is used in low-speed communications, as telephone cable. It is used only in baseband networks because of its narrow bandwidth. Most telephone lines you see strung on poles consist of cables made up of hundreds of copper wires are twisted pairs. Twisted pairs are susceptible to a variety of types of electrical interference (noise), which limits the practical distance that data can be transmitted without being garbled. Twisted pairs have been used for years for voice and data transmission, however they are now being phased out by more technically advanced and reliable media. #p#

　　Coaxial Cable Coaxial cable is a type of thickly insulated copper wire that can carry a larger volume of data—about 100 million bits per second, the insulation is composed of a nonconductive material covered by a layer of woven wire mesh and heavy-duty rubber or plastic. In terms of number of telephone connections, a coaxial cable has 80 times the transmission capacity of twisted pair. Coaxial cables are most often used as the primary communications medium for local connected network in which all computer communication is within a limited geographic area, such as in the same building.

　　Coaxial cable is also used for undersea telephone lines.

　　Fiber-Optic Cable A transmission medium composed of a central glass optical fiber cable surrounded by cladding and an outer protective sheath. It transmits digital signals in the form of modulated light from a laser or LED (light-emitting diode). In fiber-optic cable, data is transmitted as pulses of light through tubes of glass. In terms of number of telephone connections, fiber-optic cable has 20,000 times the transmission capacity of twisted pair. However, it is significantly smaller. Indeed, a fiber-optic tube can be half the diameter of a human hair. Although limited in the distance they can carry information, fiber-optic cables have several advantages. Such cables are immune to electronic interference, which makes them more secure. They are also lighter and less expensive than coaxial cable and are more reliable at transmitting data. They transmit information using beams of light at light speeds instead of pulses of electricity, making them far faster than copper cable. Fiber-optic cable is rapidly replacing twisted-pair telephone lines.#p#

　　Microwave Instead of using wire or cables, microwave systems can use the atmosphere as the medium through which to transmit signals. Microwaves are high-frequency radio waves that travel in straight lines through the air. Because the waves cannot bend with the curvature of the earth, they can be transmitted only over short distances. Thus, microwave is a good medium for sending data between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of "dishes", or antennas. These can be installed on towers, high buildings, and mountaintops. Each tower facility receives incoming traffic, boosts the signal strength, and sends the signal to the next station.

　　Satellites Satellite communications refers to the utilization of geostationary orbiting satellites to relay the transmission received from one earth station to one or more earth stations. They are the outcome of research in the area of communications whose objective is to achieve ever-increasing ranges and capacities with the lowest possible costs. Orbiting about 22,000 miles above the earth, satellites rotate at a precise point and speed above the earth. This makes them appear stationary so they can amplify and relay microwave signals from one transmitter on the ground to another. The primary advantage of satellite communication is the amount of area that can be covered by a single satellite. It also has other features: long communication distance, and the cost of station building is independent of the communication distance, operating in broadcasting mode, easy for multiple access, sustaining heavy traffic, able to transport different types of service, independent sending and receiving, and monitoring. Three satellites placed in particular orbits can cover the entire surface of the earth, with some overlap. Their only drawback is that bad weather can sometimes interrupt the flow of data.#p#

　　數據通信信道

　　數據為了傳遞，必須通過一些東西：電話線、電纜或空氣都是傳輸介質，即信道。但是在數據被傳送前，它必須被轉化成適于通信的形式。

　　數據通信線路可以以兩種配置方式連接：點到點和多點線路連接。點到點線路直接連接發送和接收設備，而多點線路連接多個設備，而不僅僅是一個發送方設備和一個接收方設備。

　　微型機之間以及與其他設備連接有兩種方法：通過電纜和空氣，數據可以被轉化為通信的三種基本形式是：電脈沖或電荷，電磁波和光波。

　　具體地，有五種技術可用來傳送數據。它們是：電話線(雙絞線)、同軸電纜、光纜、微波和衛星。

　　電話線 由一對或多對18到24規格的銅線組成的便宜的多重導體電纜。銅線絞合著以提高防止電磁和無線電頻率干擾的能力。這些電纜，可以是屏敞或非敞屏的，作為電話線用于低速通信中。由于其窄的帶寬，它僅被用于基帶網絡中。你看到的架在電線桿上的絕大多數電話線是雙絞線，它們是由數以百計的銅線組成的電纜。比絞線對于各類電的干擾很敏感，這就限制了數據不被竄改而傳送的實際距離。雙絞線被用于傳遞聲音和數據已有好多年了，然而它們正被更技術先進和可靠的介質所逐步代替。

　　同軸電纜 同軸電纜是一種有厚絕緣層的銅線，可以攜帶大量的數據――大約每秒100萬位，這個絕緣層是由一層金屬篩網和厚的橡膠或塑膠的絕緣材料組成。根據連接的電話的數目，同軸電纜的傳輸容量是雙絞線的80倍。同軸電纜是局域網最常用的主要的通信介質，而局域網中所有計算機間的通信是局限在有限的地理區域內，比如一座大樓。

　　同軸電纜也用于海底電話線。

　　光纜 由金屬包裹層和外層保護層包裹著的中央玻璃光導纖維繩組成的一種傳輸介質。它是以調制的從激光或光發射二板管中發射的光的形式來傳送數字信號。在光纜中，當光脈沖通過玻璃管時數據就被傳送。根據可連接的電話數目，光纜的傳輸容量是雙絞線2萬倍，然則它卻非常小。實際上，光纜管的直徑僅僅是人的頭發絲的一半。盡管受傳送信息距離的限制，光纜還是具有一些優點。這樣的光纜不受電子的干擾，這就使得它們更安全。它們比同軸電纜輕還便宜，而且傳輸數據更可靠。光纜用光束以光的速度而不是電脈沖的速度傳輸數據，這就使得它們比銅纜快得多。光纜正迅速地取代雙絞線的電話線。

　　微波 不使用電線或電纜，微波系統使用大氣作為傳輸介質傳送信號。微波是以直線穿過空氣的高頻率的無線電波。由于微波不能隨地球表面的彎曲度彎曲，因此只能傳送很短的距離。這樣，對于一個城市中或者是一個大學校園內的兩個大樓之間，微波是很好的傳送數據的介質。對于較長距離，微波必須通過“盤子”，即天線放大。這些天線可以被安裝在塔止，高樓止，以及山頂上。每個塔的設備接收進來的信號，增強信號的強度然后將信號發送給下一站。

　　衛星 衛星通信指的是利用軌道通信衛星傳送從一個地面站發來的信息到另外一個或多個地面站。衛星通信是通信領域的研究成果，其目的就是以盡可能少的代價獲得不斷增加的范圍和容量。衛星在地球表面22，000英里的軌道上以精確位置和速度旋轉著。這就是使得它們看起來是靜止的，因此可以將一個地面站轉發器發送來的信息放大和傳送到下一站。通信衛星的主要優點是一個衛星可以覆蓋的面積的數量。它也具有其他的特點：遠距離通信，轉發站的費用獨立于通信距離，以廣播方式運作，易于多路訪問，承受大量的信息，能夠傳送不同類型的服務，獨立的發送、接收和監控。在特定軌道放置三顆衛星就可以覆蓋整個地球表面，而且還有些重疊。其惟一缺點是惡劣的天氣有時可影響數據的傳送。