2.0 OBJECTIVES
By the end of this unit, you should be able to:
• discuss the meaning data communication.
• mention the types of data communication
• state the network topologies
• discuss the communication media
• explain the communication networks
• define international communication.
3.0 MAIN CONTENT
The unit for measuring the speed of data transmission is known as baud.
It refers to the number of signal elements transmitted each second.
Common speeds available to terminal users accessing remote systems are 300 and 1200 bauds. Faster transmission speeds of 2400, 4800, 9600 and even 24,000 and 96,000 bauds are also available. The speed of transmission depends upon the band-width of the channel being used to transmit the data; the bandwidth is measured in Hertz or cycle per second. The number of bits transmitted per second may not be same as a baud for this very reason.
Serials vs. Parallel Data Transmission
Within a computer, it is usual to employ parallel data paths which transmit 8, 16, and 32 bits simultaneously. Parallel data buses are employed to achieve much higher transmission speeds where the cost of additional wires or tracks on circuit boards is not significant. While the parallel interfaces allocate dedicated functions to the wires, the serial lines have to carry data and accompanying information multiplexed in a bit-by-bit form according to a communication protocol. It is common to employ parallel data transfer buses between computers and printers (18 pins, 24 pins, etc.) and disk drives to achieve higher speed and accuracy.
Serial transmission is preferable for all long-distance data transmission as the cost of an interface for a long parallel transmission cable becomes prohibitive.
Communication Channels
Data transmission occurs in one of three modes:
Simplex
Half Duplex (HDX)
Full Duplex (FDX)
The choice for selection of transmission mode rests upon host computer system being accessed. Simplex transmission is suitable only for device such as printers which never transmit information. In full duplex mode, data can be transmitted in both directions along the telecommunication channels simultaneously. In full duplex mode a key pressed on the keyboard results in a series of bits
which is transmitted down the channels to the host computer and then 'echoed back' for display on the screen or console which, in turn, serves as a check on the character actually received by the computer. In half-duplex mode data can be transmitted along the channels in one direction
assurance that the host computer has received the transmitted data correctly.
Communication Modes
Information may be sent on a line in one of the two modes -asynchronous or synchronous. Asynchronous or start-stop transmission is simpler and is used usually when terminals access remote computer systems. In this case a start code is prefixed and a stop code is suffixed to each character being transmitted. Thus an ASCII character of 8 bits is transmitted as a string of 10 or 11 bits; so a speed of transmission of 300 baud is roughly equivalent to 30 characters per second (cps). In Synchronous transmission, data octets are transmitted in a continuous sequence without start and stop pulses. Each set of synchronous character in a continuous starting of bits which are delimited by using a number of synchronization characters at the beginning of the block and by counting at groups of 8 bits octets after the final synchronous character.
Communication connections
There are various ways by which terminals and computer systems can be connected to other computer systems. Dial-up access employs a telephone line for dialling up the host computer system directly.
Although cost effective, this method cannot be used for fast and accurate transmission of data. A leased line may be used to transmit large amount of data at a very high speed. A data network, is a practical alternate for remote access. The charges for using such a network are usually dependent on the amount of data transmitted and not on the distance.
Switching Techniques
Computers connected to the transmission lines may establish a path by either circuit switching, packet switching or message switching. Circuit switching connects the two machines via a line and this line is used exclusively by the two machines as long as they communicate. In packet switching, blocks of messages to be transmitted between machines are formed into a packet with sources and destination addresses synchronizing, error detection and control bits and placed on the channel. Packets are routed using the address information. In message switching, all packets are sent to a central computer by all other machines. The central computer, stores and forwards the messages to the appropriate destination addresses. It is more economical to use packet switching for data communication.
Data transmission protocols are sets of commonly agreed rules that are followed to interconnect and communicate between computers in a network. A protocol defines the communication procedures and encoding used to interconnect the systems. A number of such protocols are now available and are in vogue. A universally used standard method of interconnecting user terminals to computers is RS 232-C (proposed by Electronic Industries Association, USA). The RS 232-C interface consists of 25 connections, voltage levels, signal transmission rates, timing information and control information. In 1976, CCITT (The International Telegraphy and Telephonic Consultative Committee) also introduced the X.25 standard for the interface between terminal and host computer in a packet switched data network. CCITT introduced another standard X.75 in 1978, for the interlinking of packet-switched data network.
International Standard Organization (ISO) has suggested a layered approach where each layer addresses itself to one aspect of the communication problem. This approach allows each layer to be independently developed. CCITT X.25 standard defines the first three layers of ISO. This standard has been integrated in the network architecture of many vendors. A few important standards and specifications available for various aspects of data transmissions are given below (McGoven, 1989):
Physical Transmission Links: RS-232-C, RS-422, RS-423 &
X.21Data Link Control (DLC): Async, Bisync, SDLC , ISO HDLC
Communication Path Control: X.25 Packet Switching Procedures
System and User Control: IBM SNA, DEC DNA, ISO OSI Standards
Network Topologies
Network topologies address issues like number of network nodes, concentration of terminals and devices to various locations and how effectively they could be interconnected. Different patterns of interconnections amongst computers in a network are known as network topologies. Computers in different locations may be interconnected in a Mesh Network having multiple message paths between nodes; Star Network having dedicated channels between each station and the control hub. All communication between stations must pass through the hub;
Bus Network having a linear topology and station attached by tabs; Tree
transmission path between any two stations; and Ring Network in which each node is connected to its two adjacent nodes and messages are circulated around the closed ring. A loop network is a ring network in which one master station control transmission. Each of these topologies has their advantages and disadvantages. The local requirements and computer configurations involved may be considered while choosing a specific topology (Stallings, 1984).