Main Factors Affecting Data Transmission
There are several factors that affect data transmission. They include speed or bandwidth, serial or parallel transmission, direction of data flow, modes of transmission data, and protocols.
Bandwidth The different communications channels have different data transmission speeds. This bit-per-second transmission capability of a channel is called its bandwidth. Bandwidth may be of three types: voiceband, medium band, and broadband. Voiceband is the bandwidth of a standard telephone line and used often for microcomputer transmission, the bps is 300-9600. Medium band is the bandwidth of special liased lines used mainly with minicomputers and mainframe computers, the bps is 56,000/264 million. Broadband is the bandwidth that includes microwave, satellite, coaxial cable, and fiber-optic channels. It is used for very high-speed computers whose processors communicate directly with each other. It is in the range of 56,000/30 billion bps.
Serial or Parallel Transmission Data travels in two ways: serially and in parallel. In serial data transmission, bits flow in a serial or continuous stream, like cars crossing a one-lane bridge. Each bit travels on its own communications line. Serial transmission is the way most data is sent over telephones lines. Thus, the plug-in board making up the serial connector in a microcomputer's modem is usually called a serial port. More technical names for the serial port are RS-232C connector and asynchronous communications port. With parallel data transmission, bits flow through separate lines simultaneously. In other words, they resemble cars moving together at the same speed on a multilane freeway. Parallel transmission is typically limited to communications over short distances and is not used over telephone lines. It is, however, a standard methods of sending data from a computer's CPU to a printer.
Direction of Data Transmission There are three directions or modes of data flow in a data communications system: simplex communication, half-duplex communication, and full-duplex communication. Simplex communication resembles the movement of cars on a one-way street. Data travels in one direction only. It is not frequently used in data communication systems today. One instance in which it is used may be in point-of sale (POS) terminals in which data is being entered only. In half-duplex communication, data flows in both directions, but not simultaneously. That is, data flows in only one direction at any one time. This resembles traffic on a one-lane bridge. Half-duplex is very common and is frequently used for linking microcomputers by telephone lines to other microcomputers, minicomputers, and mainframes. Thus, when you dial into an electronic bulletin board through your microcomputer, you may well be using half-duplex communication. In full-duplex communication, data is transmitted back and forth at the same time, like traffic on a two-way street. It is clearly the fastest and most efficient form of two-way communication. However, it requires special equipment and is used primarily for mainframe communications. An example is the weekly sales figures that a supermarket or regional office sends to its corporate headquarters in another place.
Modes of Transmitting Data Data may be sent by asynchronous or synchronous transmission. In asynchronous transmission, the method frequently used with microcomputers, data is sent and received one byte a time. Asynchronous transmission is often used for terminals with slow speeds. Its advantage is that the data can be transmitted whenever convenient for the sender. Its disadvantage is a relatively slow rate of data transfer. Synchronous transmission is used to transfer great quantities of information by sending several bytes or a block at a time. For the data transmission to occur, the sending and receiving of the blocks of bytes must occur at carefully timed intervals. Thus, the system requires a synchronized clock. Its advantage is that data can be sent very quickly. Its disadvantage is the cost of the required equipment.
Protocols For data transmission to be successful, sender and receiver must follows a set of communication rules for the exchange of information. These rules for exchanging data between computers are known as the line protocol. A communication software package like Crosstalk helps define the protocol, such as speeds and modes, for connecting with another microcomputer. TCP/IP ( Transmission Control Protocol and Internet Protocol) are the two standard protocols for communications on the Internet.
TCP/IP is the "language" of the Internet. It is a networking technology developed by the United States Government Defense Advanced Research Project Agency (DARP) in the 1970s. It is most commonly employed to provide access to the Internet but can be and is used by many people to create a LAN that may or may not connect to the Internet. In many aspects TCP/IP is a client/server-type LAN, but many manufacturers of TCP/IP software have applications that allow the "clients" to serve files or even applications. TCP/IP is truly an open systems protocol. This means that no one manufacturer creates the product—any computer running TCP/IP software can connect to anyone else who has TCP/IP software (provided the user has an account and security permissions), regardless of who made the particular version of software.
When different types of microcomputers are connected in a network, the protocols can become very complex. Obviously, for the connections to work, these network protocols must adhere to certain standards. The first commercially available set of standards was IBM's Systems Network Architecture (SNA). This works for IBM's own equipment, but other machines won't necessarily communicate with them. The International Standards Organization has defined a set of communications protocols called the Open Systems Interconnection (OSI). The purpose of the OSI model is to identify functions provided by any network. It separates each network's functions into seven "layers" of protocols, or communication rules. When two network systems communicate, their corresponding layers may exchange data. This assumes that the microcomputers and other equipment on each network have implemented the same functions and interfaces.#p#