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Line Set-Up/Connectivity

A very basic protocol issue involves the manner in which the circuit is set up between devices. There are three alternatives: simplex, half-duplex and full duplex (Figure 7-4).


Figure 7.4  Simplex, Half Duplex (HDX) and Full Duplex (FDX) Transmission.

Simplex
transmission is unidirectional. The information flows in one direction across the circuit, with no capability to support a response in the other direction. Simplex transmission generally involves dedicated circuits. Simplex circuits are analogous to escalators, doorbells, fire alarms, and security systems. Contemporary applications for simplex circuits, although rare, include remote station printers, card readers, and a few alarm systems (fire and smoke alarms). Generally speaking, simplex transmission is conducted across dedicated circuits of low capacity.
Half-Duplex (HDX)
transmission operates in both directions, although not simultaneously. HDX is generally used for relatively low-speed transmission, usually involving two-wire, analog circuits provided on a circuit-switched basis through the PSTN. As the circuit must be turned around in order to support the change in direction of the conversation, it tends to limit the speed and agreeability of conversational data communications. Line turnaround time is a limiting factor, being in the range of 50 to 500 milliseconds (thousands of a second), depending on the length of the circuit.
In many environments, HDX is the predominant transmission mode, although high-performance networks are becoming increasingly available and cost-effective. Examples of HDX application include line printers, polling of buffers, and modem communications (many modems can support FDX, as well). HDX is used extensively in transaction-based communications, such as credit card verification and ATM (Automatic Teller Machine) networks. Such applications are not seriously affected by delays associated with line turnaround.
Full Duplex (FDX)
is a fully bidirectional transmission mode in which communications is supported in both directions, simultaneously. FDX typically requires two simplex circuits, one operating in each direction. FDX circuits are generally characterized as being four-wire, high-capacity, dedicated circuits, most of which are multichannel in nature. All wideband and broadband circuits are FDX in nature, as are most multichannel circuits. FDX circuits are sometimes used to connect half-duplex terminals in order to avoid issues of line turnaround. More typical examples of FDX applications include channel links between host processors, channel links between controllers/concentrators and hosts, and other applications involving the interconnection of substantial computing systems. Carrier services that deliver FDX capabilities include DDS, E/T-carrier, and broadband services such as Frame Relay, SMDS and ATM, all of which will be discussed in later chapters.

Transmission Mode: Transmission Method

There are two basic methods of data transmission, asynchronous and synchronous.

Asynchronous

Asynchronous, or character-framed, transmission, is a method which grew out of telegraphy and teletypewriting. From Latin and Greek, it translates as not together with time; in other words, it is not synchronous. Asynchronous transmission is a start-stop method of transmission in which each computer value (letter, number, or control character) is preceded by a start bit, which alerts the receiving terminal to the transmission of something worthy of its attention. The transmitted computer value is succeeded by a stop bit, which advises the receiving terminal that the transmission of that set of information is ended; some asynchronous protocols make use of two stop bits.

PCs, teletypes and other devices which make use of asynchronous transmission frame, or surround, each byte of information with start and stop bits, which are interpreted by the receiving terminal and subsequently stripped away. The inclusion of start and stop bits adds two or three bits of overhead to the transmission of each 8-bit byte. Additionally, asynchronous transmission adds a parity checking bit for relatively poor error control. The framing of the data with these three or four bits of control information yields an overhead, or inefficiency, factor of 20% to 30%.

Asynchronous transmission can be characterized as start-stop (not synchronized) transmission of one character at a time at a variable speed. Additionally, overhead is high and error control is poor.

Synchronous

From Latin and Greek orgins, synchronous translates as together with time. Such transmission is message-framed and overcomes the inefficiencies of asynchronous, start-stop, transmission for high-speed data communications applications. Rather than surrounding each character with start and stop bits, a relatively large set of data is framed or blocked, with one or more synchronization bits or bit patterns being used to synchronize the receiving terminal on the rate of transmission. Through the receipt of the synchronizing bits, or clocking pulses, the receiving device can match its speed of receipt to the rate of transmission across the circuit. Each bit and byte of data and control information can be separately distinguished, as the device knows when to expect what information, and in what sequence. As the large block or frame of data is surrounded by only a few framing bits and synchronizing bits, the overhead is much reduced, the efficiency of transmission is much increased and the effective throughput is much greater.

Error control is quite sophisticated and reliable, involving statistical sampling techniques and mathematical calculations performed on the set of data. Synchronous transmission can be characterized as transmission of multiple characters at a time, organized into character sets and presented in blocks or frames. The transmission is synchronized, and takes place at a predetermined and relatively high rate of speed. Further, error control is excellent and overhead is relatively low.


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