Unit 1 Lecture 4

Serial Transmission In Serial Transmission, the bits of a byte are serially transmitted one after other. The shift register is used for serial transmission. The byte to be transmitted is the first stored in a shift register. Then these bits are shifted from MSB to LSB bit by bit in synchronization with the clock. Bits are shifted right by one by one position per clock cycle. The bit which forced out of the shift register is transmitted. Hence LSB is transmitted first. As an advantage only one wire is used in serial transmission between the transmitter and the receiver.

Serial Transmission

Some examples of serial Communication Devices Morse code telegraphy RS-232 (low-speed, implemented by serial ports) RS-422 RS-423 RS-485 Ethernet SONET and SDH T-1, E-1 Lines

Advantages of serial transmission Only one wire is required Reduction in cost due to less number of conductor wires It is the most used method for long distance data transfers. Disadvantages of serial transmission Since there is only one line of transmission therefore the speed of transmission is quite slow. If we have to increase the speed of data transfer then it is necessary to increase the clock frequency. But there is a limit of clock frequency.

Types of serial transmission? Serial transmission has two categories: Synchronous data transmission Asynchronous data transmission

Asynchronous Data Transmission It is called asynchronous because timing of the signal is not an issue. The information can be sent & received as per mutual agreement of the sender & receiver. asynchronous transmission is at byte level only because transmission of bits is always synchronized. Start Stop Data Byte 1 Idle time Idle

Asynchronous transmission is eased by two bits, namely start bit as ‘0’ & stop bit as ‘1’. We send ‘0’ bit to start the communication & ‘1’ bit to stop the transmission. There can be a time delay between communication of two bytes. Only one byte is sent at a time. After a gap of time next byte is transmitted. The transmitter and receiver may function at different clock frequencies. Asynchronous transmission is mostly used by asynchronous modems.

Operation Before signalling will work, the sender and receiver must agree on the signalling parameters: Full or half-duplex operation The number of bits per character the order in which the bits are sent The speed or bits per second of the line (often incorrectly referred to as the Baud rate). Some systems use automatic speed detection. Whether to use or not use parity Odd or even parity, if used The number of stop bits sent must be chosen (the number sent must be at least what the receiver needs)

Asynchronous Transmission

Why it is called Asynchronous? This mechanism is called as Asynchronous because at the byte level the sender and receiver do not have to be synchronized. However within each byte, the receiver should still be synchronized with the incoming bit stream. This means that some synchronization is required only for the duration of single byte.

Advantages of Asynchronous transmission Asynchronous transmission has many advantages these are: It is a flexible data transmission method. Synchronization between the transmitter and receiver is not necessary. It is possible to transmit signals from the sources having different bit rates. The transmission can commence as soon as the data byte to be transmitted becomes available. This mode of transmission is easy to implement. It is an effective scheme. It is a cheap scheme in terms of money.

Disadvantages of Asynchronous Transmission Additional bits called start and stop bits are required to be used. The timing error may take place because it is difficult to determine synchronicity. It has slower transmission rate.

Synchronous transmission Synchronous events means events which happen at the same clock rate. Synchronous transmission is carried out under the control of a common master clock. Here the bits which are being transmitted as synchronized to the same reference clock. The stream of bits is combined into bigger frames which may comprise more than one byte. Each byte is transmitted without gap between the next byte. No start and stop bits are used instead the bytes are transmitted as a block in a continuous stream of bits.

The receivers operates at the same clock frequency as that of transmitter. The data is sent in blocks ( we can call these blocks as frames or packets) spaced by fixed time intervals. It is the duty of the receiver to separate the bits sent in group There is an inter block idle time which also is filled with idle characters. Ways to find the solution to the problem of error in transmission because of loss in synchronization is to include re-synchronization of the clocks and use of check digit to ensure the byte is correctly interpreted and received

Synchronous transmission The Partitions shown between bytes are just the logical partitions.

Advantages of Synchronous transmission The main advantage is speed. The speed of transmission is much higher than that asynchronous transmission. This is due to the absence of gaps between the data units and absence of start stop bits. Timing errors are reduced due to synchronization.

Disadvantages of Synchronous Transmission The timing is very important. The accuracy of the received data is dependent entirely on the ability of the receiver to count the received bits accurately. The transmitter and receiver have to operate at the same clock frequency. This requires proper synchronization which makes the system complicated.

Comparison of Asynchronous & Synchronous Transmission SNo Asynchronous transmission Synchronous transmission 1 In asynchronous transmission, information is transmitted character by character. In synchronous transmission, blocks of characters are transmitted at high-speed on the transmission line.

2 At the beginning of a character, a start bit is sent. The nature of the start signal is standardized and is ‘understood’ by the receiving end equipment which prepares to receive the coded character. The start signal is followed by the bits of coded character. The start signal is followed by the bits of coded character. If ASCII code is used, the seven coded bits and a parity bit are sent. Following the character code is stop signal. A distinctive synchronization pattern is sent at the beginning of the block. The synchronization pattern is followed by codes to identify sender and receiver and the message characters. The message is terminated by an end of message character followed by a check character to aid detection of any transmission error.

3 It is cheap. It is expensive. 4 It takes more time in data transmission. It takes less time. 5 It does not require any local storage at the terminal end. It needs for local buffer storages at the two ends of the line to assemble blocks. 6 It does not require any synchronized clocks. It requires accurately synchronized clocks at both ends. 7 The transmission line is idle during the time intervals between transmitting characters. Synchronous transmission is efficient utilization of the transmission line. 8 It is used in low-speed communication like the connection of a terminal to a computer. It is used in high-speed applications like the transmission of data from one computer to another.

Comparison between Serial & Parallel Transmission SNo Parallel Transmission Serial Transmission 1 This type of transmission permits the transmission of a n group of bits simultaneously. In this transmission, all the bits are transmitted one after another. 2 Separate lines are used for transmitting each group. Just one line is used for transmission. 3 This transmission is suitable for short distance. This transmission is suitable for long distances. 4 This transmission is expensive. It is cheaper in comparison.

5 This transmission is not very reliable. This transmission is more reliable. 6 Transmission circuitry is simple. Transmission circuitry is complex due to sending and receiving devices. 7 Its main advantage is speed. It is slower in comparison to parallel transmission.

Data to be send = 12 Mbyte = 12 x 106 bytes = 12 x 106 x 8 bits Example 1. A company must transmit Parallel 12 M bytes file in 25 or fewer minutes. What is the minimum speed (bits per second) necessary to send the entire file? Assume that each byte is 8 bits and error rates and overhead are not significant factor. Calculate the minimum speed. Solution: Data to be send = 12 Mbyte = 12 x 106 bytes = 12 x 106 x 8 bits Given time = 25 minutes = 25 x 60 seconds = 1500 sec Now it is required to send 96 x 106 bits in 1500 seconds resulting in data rate of (96 x 106 )bps/ 1500 = 6.4 x 104 bps

Advantages Disadvantages Asynchronous transmission Simple, doesn't require synchronization of both communication sides Cheap, because asynchronous transmission requires less hardware Setup is faster than other transmissions, so well suited for applications where messages are generated at irregular intervals, for example data entry from the keyboard, and the speed depends on different applications. Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes and thus carry no useful information Synchronous transmission Lower overhead and thus, greater throughput Slightly more complex Hardware is more expensive