TRANSMISSION MODES

Data from the transmitter to the receiver can be executed in different ways.

It can be parallel or serial. The serial data transmission scheme is again subdivided into asynchronous, synchronous, and isochronous. The differ-ent modes of transmission are shown in Figure 1.3.

1.7.1 Parallel

Whereas a message consisting of n bits requires n clocks for sending the same to the receiver, it requires a single clock for parallel transmis-sion. Thus, in essence, parallel transmission is much faster than serial

Direction of data

Station 2

Station 2

Station 2 Station 1

Station 1 Station 1

Station 2

Station 3 Direction of data at time t1

Direction of data at time t₂

Direction of data at all time

Direction of data

Direction of data Station 1

FIGURE 1.2 Different transmission modes. (From B. A. Forouzan. Data Commu-nications and Networking, 4th Edition, Special Indian Edition. Tata McGraw Hill Companies Inc., New Delhi, India, p. 6, 2006.)

Data Communication 7

transmission; however, the cost of laying n wires would entail extra cost.

Thus, parallel transmission is undertaken only if the distance between the transmitter and receiver is not considerable.

1.7.2 serial

Serial transmission involves transmission of bits of a message in serial manner—one after the other, i.e., only one communication channel is required. When the distance between the transmitter and the receiver is considerable, it makes sense to transmit information serially because in such cases parallel transmission would entail considerable cost, and laying of wires may sometimes be difficult. Serial transmission, as already men-tioned, can be of three types—the kind of serial transmission depends on the specific requirements.

1.7.3 asynChronous

Although termed asynchronous communication, it is basically a synchro-nous type of communication where synchronization is maintained for each character—each character consisting of 5–8 data bits. The receiver resyn-chronizes at the beginning of each character frame. Synchronization basi-cally means agreeing or coinciding exactly in time scale. Asynchronous communication is called start–stop type transmission because framing of each character is between a start and stop bit(s). A character starts with a start bit (which is always a logic 0), followed by the bits of the data (with LSB being sent first), the parity bit, and lastly one, one and a half, or two stop bits. The stop bit(s) is/are always 1 or high state. Synchronization is achieved at the receiver on receiving the high to low transition of the start pulse. The stop bit provides a minimum guard band or buffer period between two characters. An asynchronous frame format is shown in Figure 1.4.

Data transmission

Parallel Serial

Asynchronous Synchronous Isochronous

FIGURE 1.3 Different data transfer schemes. (From B. A. Forouzan. Data Commu-nications and Networking, 4th Edition, Special Indian Edition. Tata McGraw Hill Companies Inc., New Delhi, India, p. 131, 2006.)

An idle line (i.e., no transmission) is always a stream of 1’s. When a character is being sent down the line, it starts with a start bit of status 0.

Thus, the start bit is identified by the receiver with a high to low transition at which time the bits of data follow, and lastly the stop bit(s). Since the stop bit is always 1 in nature and assuming a second character immedi-ately follows the first, then again there would be high to low transition at the receiver (due to the stop bit of the first character and the start bit of the second character). If no second character is sent, the line continues in the idle line high state after completion of the stop bit.

An example of an asynchronous data transmission is an operator typing data into the computer (a real-time transmission). Since the typing speed can never remain constant, the number of idle line 1’s will vary.

For reliable communication, be it asynchronous or synchronous, a pre-agreed set of rules has to be obeyed by both the transmitter and the receiver, called protocol. The major elements comprising a protocol are syntax (refers to format and signal levels), semantic (refers to control information for proper coordination and error handling), and timing (refers to speed matching and sequencing). In the present case of asynchronous communication, the protocols are clock speed, character frame length, sig-nal level, number of start and stop bits, and type of parity bit (odd or even).

A separate clock line is normally not taken from the transmitter to receiver in asynchronous transmission. However, the receiver clock is designed to be as close to the transmitter clock value. If these two clocks differ somewhat, a clock slippage may occur. There can be underslipping or overslipping. The former occurs if the transmitter clock is slower than the receiver clock, while if the transmitter clock is faster than the receiver clock, overslipping would occur. Thus, for the case of overslipping, the received data is being sampled at a rate slower than the rate at which the data bits are received from the transmitter. In this case, as the samples are being analyzed and stored in the receiver memory, a time will be reached at which a data bit will be completely skipped.

Asynchronous transmission is undertaken when the data is sporadic or intermittent in nature and also the volume of data to be handled is not huge.

It is simple and cheap but its overhead is somewhat quite high—about Electronic Communications Systems, 6th Edition. Pearson Inc., New Delhi, India, p. 165, 2004.)

Data Communication 9

2–3 bits for each 8 bits of data. This comes to about 20–25% of overhead bits and hence it affects throughput (it is the number of actual data bits sent in unit time) in a big way. A framing error can occur if the sudden appear-ance of noise causes a change of idle line condition from 1 to 0, which the receiver would assume inadvertently to be a valid start pulse.

1.7.4 synChronous

In synchronous transmission, a huge chunk of data is transported from trans-mitter to receiver at a fast rate in predefined frames. While transmission is character-by-character type in asynchronous transmission, it is frame-by-frame type in synchronous transmission. In this transmission scheme, it is best to insert clock information in the data signal itself. One such example is Manchester coding. Thus, irrespective of data signal pattern, there would always be a level transition in each period. At the receiving end side, this level transition is utilized to generate the clock signal that would be totally in synchronism with the received data. Thus, even if there is any change in the data rate during transmission, no loss of synchronism would occur, which is realized by employing PLL (phase locked loop). A synchronous frame for-mat is shown in Figure 1.5. The frame contains several fields. The first field is a synchronous pattern, also called a flag field. The synchronous character places the receiver in the character mode and conditions it to receive data bits byte-wise. In case of BSC (Binary Synchronous Communications), two synchronous characters are sent, one after the other, to avoid misinterpret-ing any random data byte to be a synchronous character.

Synchronous transmission becomes more and more efficient as the amount of data to be transferred increases. Efficiency is the ratio of information bits to total transmitted bits. In this case, the percentage of overhead bits is less than 1% assuming 1000 character block of data having approximately 48 overhead bits in the form of synchronous and control bits.

1.7.5 isoChronous

In synchronous transmission, frames travel down the line in fixed time slots. Here, frames are synchronized for transmission. It fails if there is an

8 bits 8 bits 8 bits 8 bits

FIGURE 1.5 Synchronous data frame format.

uneven delay between frames, which occurs in real-time audio and video.

TV transmissions involve sending 30 images per second. TV viewing must also have to be at the same rate. Isochronous transmission guarantees such transmissions such that images arrive at the same rate for the purpose of viewing. A major part of the bandwidth is allocated to a single or two devices for data transfer in isochronous mode. This method is used for high-speed data transfer. A packet consists of a maximum of 123 bytes in this mode, and there is no limit to the maximum number of packets that can be sent.