Communications Link

The cost for linking widely scattered computers is substantial, so it is worthwhile to examine the communication options. Telephone lines are the most convenient communications channel because an executive system is ready in place, but there are many other options. A communications link is the physical medium used for transmission.

👉Type Of Communication Links

There are several kinds of communications links. Some may be familiar to you already.

• Wire pairs
  One of the most common communications media is the wire pair, also know as the twisted pair. Wire pairs are wires twisted together to from a cable, which is then insulated. Wire pair are inexpensive. Further, they are often used because they had already been installed in a building for other purposes or because they are already in use in telephone system. However, they are susceptible to electrical interference, or noise. Noise is anything that causes distortion in the signal when it is received. High-voltage equipment and even the sun can be sources of noise.
  
  
• Coaxial Cables
  Know for sending a strong signal, a coaxial cable is a single conductor wire within a shielded enclosure. Bundles of cables can be laid underground or undersea. These cables can transmit data much faster than wire pairs and are less prone to noise.
  
  
• Fiber Optics
  Traditionally, most phone lines transmitted data electrically over wires made of metal, usually copper. These metal wire had to be protected from water and other corrosive substances. Fiber optics technology eliminates this requirement. Instead of using electricity to send data, fiber optics use light. The cable are made of glass fibers, each thinner than a human hair, that can guide light beams for miles. Fiber optics transmits data faster than some technologies, yet the materials are substantially lighter and less expensive than wire cables. It can also send and receive a wider assortment of data frequencies at one time. The range of frequencies that a device can handle is known as its bandwidth; bandwidth is a measure of the capacity of the link. The broadbandwidth of fiber optics translates into promising multi media possibilities, since fiber optics is well suited for handling all types of data-voice, pictures, music, and video-at the same time.
• Satellite Transmission
  The basic components of satellite transmission are earth stations, which send and receive signals, and a satellite component called a transponder. The transponder receives the transmission from an earth station, amplifies the signal, changes the frequency, and retransmits the data to a receiving earth station. ( The frequency is changed so that the weaker incoming signals will not be impaired by the stronger outgoing signals). This entire process takes a matter of a few seconds.

  

  Microwave and satellite transmission

  
  
  If a signal must travel thousands of miles, satellites are usually part of the link. A message being sent around the world probably travels by cable or some other physical link only as far as the nearest satellite earth transmission station. From there it is beamed to a satellite, which sends it back to earth to another transmission station near the data destination. Communications satellites are launched into space where they are suspended about 22,300 miles above the earth. Why 22,300 miles? that is where satellites reach geosynchronous orbit-the orbit that allows them to remain positioned over the same spot on the earth.
  
  
• Mixing and Matching
  A network system is not limited to one kind of link and, in fact, often work in various combinations, especially over long distances. An office worker who needs data from a company computer on the opposite coast will most likely use wire pairs in the phone line, followed by microwave and satellite transmission. Astonishingly, the trip across the country and back, with a brief stop to pick up the data, may take only seconds.

👉Protocols

A protocol is a set  of rules for the exchange of data between a terminal and a computer or between two computers. A protocol is embedded in the network software. Think of protocol as a sort of pre-communication to make sure everything is in order before a massage or data is sent. Protocol are handled by software related to the network, so that users need only worry about their own data.

• Protocol Communications

  Two devices must be able to ask each other questions (Are you ready to receive a message? Did you get my last message? Is there trouble at your end?) and to keep each other informed ( i am sending data now). In addition, the two devices must agree on how data is to be transferred, including data transmission speed and duplex setting. But this must be done in a formal way. When communication is desired among computers from different vendors ( or even different models from the same vendor), the software development can be a nightmare because different vendors use different protocols. Standards would help.

  

  Satellite dishes

  
  

• Setting Standards

  Standards are important in the computer industry; it saves money we can all coordinate effectively. Nowhere is this more obvious than in data communication system, where many components must "come together". But it is hard to get people to agree to a standard.
  
  
  Communications standards exist, however, and are constantly evolving and being updated for new communications forms. Standards provide a framework for how data is transmitted. The Internal Standards Organization (ISO), based in Geneva, Switzerland, has defined a set of communications protocols called the Open System International (OSI) model. The OSI model has been endorsed by the united nation. As we will discuss shortly, particular types of protocols are used for local area networks.

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