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#
数据通信信道
数据为了传递，必须通过一些东西：电话线、电缆或空气都是传输介质，即信道。但是在数据被传送前，它必须被转化成适于通信的形式。
数据通信线路可以以两种配置方式连接：点到点和多点线路连接。点到点线路直接连接发送和接收设备，而多点线路连接多个设备，而不仅仅是一个发送方设备和一个接收方设备。
微型机之间以及与其他设备连接有两种方法：通过电缆和空气，数据可以被转化为通信的三种基本形式是：电脉冲或电荷，电磁波和光波。
具体地，有五种技术可用来传送数据。它们是：电话线(双绞线)、同轴电缆、光缆、微波和卫星。
电话线 由一对或多对18到24规格的铜线组成的便宜的多重导体电缆。铜线绞合着以提高防止电磁和无线电频率干扰的能力。这些电缆，可以是屏敞或非敞屏的，作为电话线用于低速通信中。由于其窄的带宽，它仅被用于基带网络中。你看到的架在电线杆上的绝大多数电话线是双绞线，它们是由数以百计的铜线组成的电缆。比绞线对于各类电的干扰很敏感，这就限制了数据不被窜改而传送的实际距离。双绞线被用于传递声音和数据已有好多年了，然而它们正被更技术先进和可靠的介质所逐步代替。
同轴电缆 同轴电缆是一种有厚绝缘层的铜线，可以携带大量的数据――大约每秒100万位，这个绝缘层是由一层金属筛网和厚的橡胶或塑胶的绝缘材料组成。根据连接的电话的数目，同轴电缆的传输容量是双绞线的80倍。同轴电缆是局域网最常用的主要的通信介质，而局域网中所有计算机间的通信是局限在有限的地理区域内，比如一座大楼。
同轴电缆也用于海底电话线。
光缆 由金属包裹层和外层保护层包裹着的中央玻璃光导纤维绳组成的一种传输介质。它是以调制的从激光或光发射二板管中发射的光的形式来传送数字信号。在光缆中，当光脉冲通过玻璃管时数据就被传送。根据可连接的电话数目，光缆的传输容量是双绞线2万倍，然则它却非常小。实际上，光缆管的直径仅仅是人的头发丝的一半。尽管受传送信息距离的限制，光缆还是具有一些优点。这样的光缆不受电子的干扰，这就使得它们更安全。它们比同轴电缆轻还便宜，而且传输数据更可靠。光缆用光束以光的速度而不是电脉冲的速度传输数据，这就使得它们比铜缆快得多。光缆正迅速地取代双绞线的电话线。
微波 不使用电线或电缆，微波系统使用大气作为传输介质传送信号。微波是以直线穿过空气的高频率的无线电波。由于微波不能随地球表面的弯曲度弯曲，因此只能传送很短的距离。这样，对于一个城市中或者是一个大学校园内的两个大楼之间，微波是很好的传送数据的介质。对于较长距离，微波必须通过“盘子”，即天线放大。这些天线可以被安装在塔止，高楼止，以及山顶上。每个塔的设备接收进来的信号，增强信号的强度然后将信号发送给下一站。
卫星 卫星通信指的是利用轨道通信卫星传送从一个地面站发来的信息到另外一个或多个地面站。卫星通信是通信领域的研究成果，其目的就是以尽可能少的代价获得不断增加的范围和容量。卫星在地球表面22，000英里的轨道上以精确位置和速度旋转着。这就是使得它们看起来是静止的，因此可以将一个地面站转发器发送来的信息放大和传送到下一站。通信卫星的主要优点是一个卫星可以覆盖的面积的数量。它也具有其他的特点：远距离通信，转发站的费用独立于通信距离，以广播方式运作，易于多路访问，承受大量的信息，能够传送不同类型的服务，独立的发送、接收和监控。在特定轨道放置三颗卫星就可以覆盖整个地球表面，而且还有些重叠。其惟一缺点是恶劣的天气有时可影响数据的传送。
