Coding

In all digital communications channels, computers transmit data and information in forms of binary codes. Both sender and receiver of the data and information should have a standard for both to understand them.

A coding scheme for communications is a binary system, as in the computer systems. The system consists of groups of bits (0 or 1) that represent characters. In computer systems, a byte is a group of bits and represents a character. In data communications, a byte is the same, but some codes use different number of bits such as 5, 7, 8 or 9.

Two predominant coding schemes ASCII and EBCDIC. ASCII refers America Standard Code for Information Interchange. It is the most popular code for data communications and is the standard code on most communications terminals. Among two types of ASCII, a 7-bit code can make 128 character combinations, and an 8-bit can do 256 combinations. EBCDIC refers Extended Binary Coded Decimal Interchange Code. It is IBM's standard information code, and has 8 bits for a character.

Each types of communications media has different transmission speed. The bandwidth is a measure of the transmission rate of communications channels.

• Baseband: Digital signals are commonly called baseband signals. Baseband is a communications technique in which digital signals are placed onto the transmission line without change in modulation. It transmits up to a couple of miles, and does not require the complex modems. Typical Token Ring and Ethernet use baseband signals.

• Broadband: Broadband is a technique for transmitting large amounts of data, voice and video over long distances simultaneously by modulating each signal onto a different frequency. Using the FDM (Frequency division multiplexing) technique, several streams of data can be transmitted simultaneously.
  Broadband is the bandwidth used for direct communication between very high-speed computers (e.g., large mainframe computers). This bandwidth includes microwave, satellite, coaxial cable, and fiber-optic media.

Two forms of data movement exist: parallel data transmission and serial data transmission.

• Parallel Transmission: Parallel data transmission involves the concurrent flow of bits of data through separate communications lines. This pattern resembles the flow of automobile traffic on a multilane highway. Internal transfer of binary data in a computer uses a parallel mode. If the computer uses a 32-bit internal structure, all the 32 bits of data are transferred simultaneously on 32 lane connections. \ Parallel data transmission is commonly used for interactions between a computer and its printing unit. The printer usually located close to the computer, because parallel cables need many wires and may not work stably in long distance.

• Serial Data Transmission: Most data transmitted over telephone lines use a serial pattern. That is, each individual bit of information travels along its own communications path; the bits flow in a continuous stream along the communications channel. This pattern is analogous to the flow of traffic down a one-lane residential street.
  Serial transmission is typically slower than parallel transmission, because data are sent sequentially in a bit-by-bit fashion.

Besides the previously mentioned ways in which data may travel, there are three directional modes of travel in data transmission.

• Simplex Communication: Simplex communication is a mode in which data only flows in one direction. Because most modern communications require a two-way interchange of data and information, this mode of transmission is not as popular as it once was. However, one current usage of simplex communications in business involves certain point-of-sale terminals in which sales data is entered without a corresponding reply.

• Half-duplex Communication: Half-duplex communication adds an ability for a two-way flow of data between computer terminals. In this directional mode, data travels in two directions, but not simultaneously. Data can only move in one direction when data is not being received from the other direction. This mode is commonly used for linking computers together over telephone lines.

• Full-duplex Communication: The fastest directional mode of communication is full-duplex communication. Here, data is transmitted in both directions simultaneously on the same channel. Thus, this type of communication can be thought of as similar to automobile traffic on a two-lane road. Full-duplex communication is made possible by devices called multiplexers. Full-duplex communication is primarily limited to mainframe computers because of the expensive hardware required to support this directional mode.

Another way of classifying data communications flow is as synchronous or asynchronous.

• Synchronous Transmission: Large volumes of information can be transmitted at a single time with synchronous transmission. This type of transmission involves the simultaneous flow of several bytes of data. Because a large block of data being sent synchronously cannot be interrupted, a synchronized clock is necessary to carefully schedule the transmission of information. This special communications equipment is expensive; but this cost can be made up in part by faster, less expensive transmission of information.

• Asynchronous Transmission: Conversely, asynchronous transmission involves the sending and receiving of one byte of data at a time. This type of transmission is most often used by microcomputers and other systems characterized by slow speeds.