The two major components relating to a WAN are the Protocol and the Media. The media are the actual path ways that the protocol travels on. The network protocol defines the format in. which data are sent through the media. Protocols are rules governing the initiation and *maintenance of data flows. These are implemented in different ways for different types of equipment and networks. Information professionals are most likely to encounter these when setting up their sat-ware and hardware for online searching. In such situations, all should deal with such parameters as:
- bit rate
- stop bits
- flow control
Besides the above, other important parameters are word length used, and whether transmission is synchronous or asynchronous. The significance of these parameters is summarised below
Line bit rate is measured in bits per second (bps) and also referred to as baud rate. The speed at which data can be transmitted is determined by the service being accessed, and the quality and characteristics of the telecommunications link.
Parity: The parity bit is a rudimentary form of error checking which can be employed when a word length of 7 bits is in use. The full range of values is: odd, even, ignore; mark (always set to 1); none (word length of bits in use).
If odd parity is selected the computer assigns a value to the parity bit so that the numbers of '1' bits in each character is odd, for even parity, the parity is set so that the number of bits is even.
Stop bits: These are a means of keeping the transmitting and receiving computers in step with one another. Normally one stop bit is used.
Flow Control: When data is flowing into a computer at a rate greater than that at which it can be printed, displayed or stored to disk, the connection between the transmitting and receiving computers may be severed. This is avoided by using X-ON X-OFF. If the receive buffer is nearly full, the computer sends an X-OFF signal to the transmitting system to tell it to temporarily suspend transmission. When the receive buffer has emptied, an X-ON signal is sent and the transmission is resumed. X-OFF and X-ON can be sent from keyboard by pressing control/s and control/q respectively.
Echoplex: Echo ON/OFF is sometimes referred to as echoplex. In other words, the characters are displayed on the screen either as they are typed in or after being echoed back by the remote computer.
Duplex: Data flows in both directions at the same time are referred to as duplex mode.
Word Length: This defines the number of bits that make up a character, (i.e.,) 7 or 8 bits. The most common codes are ASCII (American Standard Code for Information Interchange) and EBCDIC, Telex uses the Band of code which has a word length of 5 bits, thus limiting the number of characters that can be generated.
In synchronous communication, data is sent from one machine to another in a continuous stream. The sending and receiving computers must be in step with each other through out the transmission. Synchronous transmission is used for high speed data transmission, generally between two mainframes.
In asynchronous mode of transmission data is sent character by character. This form of communication is used by most information systems attached to the public telephone network.