Data Transmission Textbook Chapter 2 p 19 to

Data Transmission Textbook: Chapter 2 (p. 19 to p. 26) Data Communications (E&T 2760): Data Transmission 1

Introduction l Covers some basic concepts and terminologies. Data Communications (E&T 2760): Data Transmission 2

Agenda l Communication Modes – simplex, half duplex and full duplex l l l Parallel and Serial Data Transmission Asynchronous and Synchronous Transmission Bit Synchronization Data Frame Synchronization – character oriented and bit oriented Data Communications (E&T 2760): Data Transmission 3

Vocabulary l Protocol – a set of rules to govern an activity Data Communications (E&T 2760): Data Transmission 4

Communication Modes 1) Simplex - one way communication - e. g. broadcasting - seldom used in data communication because the receiver has no way to tell the transmitter to re-transmit – in case of errors Data Communications (E&T 2760): Data Transmission 5

Communication Modes 2) Half Duplex - two way-communication, but only one direction at a time – on a common channel - e. g. fax - need a protocol to ensure that when one station is in transmit mode, the other is in receive mode Data Communications (E&T 2760): Data Transmission 6

Communication Modes 3) Full Duplex - simultaneous two-way communication using two separate channels Data Communications (E&T 2760): Data Transmission 7

Source: Duck, Bishop & Read, Data Communications for Engineers, Addison-Wesley Data Communications (E&T 2760): Data Transmission 8

Parallel and Serial Transmission l Parallel Transmission – the native format inside a computer as the data bus is usually 8, 16, 32, … bits wide – e. g. Centronics Printer Interface, SCSI (Small Computer System Interface) – advantage: fast – disadvantage: expensive cables and connectors Data Communications (E&T 2760): Data Transmission 9

Parallel and Serial Transmission l Serial Transmission – in the transmitter, data are converted from parallel to serial form and then sent out bit by bit along a single line – in the receiver, the received bit streams are converted back to words of 8, 16, 32, … bits wide – e. g. COM, PS/2, USB, LAN – advantage: less expensive cables and connectors, because of fewer wires – disadvantage: lower speed Data Communications (E&T 2760): Data Transmission 10

Asynchronous Serial Transmission l l When there is no data to send, the line remains in an idle state (MARK). When a character (usually 7 or 8 bits long) is to be sent, a Start bit (SPACE of one bit duration long) is sent out first. Then, the data bits of the character is sent out one by one, followed with an optional Parity check bit. Finally, a Stop bit (Mark of one to two bits duration long) is transmitted. Data Communications (E&T 2760): Data Transmission 11

l l The line then goes back to the idle state or the Start bit of the next character. The transmitter and the receiver use separate clocks which are of similar frequency. The Start, Stop bits are used to adjust the phase of the receiver clock so that the received bits can be sampled right at the centres. If the total number of bits per character is 10 (including Start, Stop bits), the transmitter and receiver clocks can differ not more than 5% for reliable detection. – Read example 2. 1 on p. 23 and p. 24 of Textbook l Inefficient use of the communication channel as Start, Stop bits are attached to each character. Data Communications (E&T 2760): Data Transmission 12

Asynchronous Serial Transmission Data Communications (E&T 2760): Data Transmission 13

Receiving errors in asynchronous transmission Source: Duck, Bishop & Read, Data Communications for Engineers, Addison-Wesley Data Communications (E&T 2760): Data Transmission 14

Synchronous Serial Transmission l The transmitter and receiver clocks are synchronized either by: – sending the clock signal from one end to the other end – embedding the clock signal into the transmitted data through proper Line Codes l More efficient use of the communication channel as no Start, Stop bits is required. Data Communications (E&T 2760): Data Transmission 15

Synchronous Serial Transmission Source: Stein, Introduction to Data Communications, Delmar Publishers Inc. Data Communications (E&T 2760): Data Transmission 16

Bit Synchronization l l Mechanism which enables the receiver to sample the received bits at their centre positions In asynchronous transmission: – by means of Start and Stop bits (oriented) – error occurs if transmitter and receiver clocks are slightly different and word length is long l In synchronous transmission: – by means of clock recovery circuit at the receiver (bitoriented) – less chance of getting errors, provided that clock recovery is good Data Communications (E&T 2760): Data Transmission 17

Data Frame l l In data communications, data are usually divided into several packets (i. e. frames) before being transmitted out Reasons: – in case of transmission errors, only the error packets need to be re-transmitted – instead of all data. – sharing of communication circuits – by means of medium access protocols like TDM, CSMA/CD, … etc. Data Communications (E&T 2760): Data Transmission 18

Frame Synchronization l l Mechanism which enables the receiver to identify the start and end of each data frame (i. e. data packet) Character oriented – use special characters to indicate start and end of each frame l l e. g. start of frame: DLE SOH e. g. end of frame: DLE ETX – apply character stuffing in case a DLE character occurs inside the frame – e. g. BISYNC Data Communications (E&T 2760): Data Transmission 19

BISYNC Data Frame Source: Stein, Introduction to Data Communications, Delmar Publishers Inc. Data Communications (E&T 2760): Data Transmission 20

Frame Synchronization l Bit oriented – use special flag pattern (preamble) to delimit each frame l e. g. HDLC: flag pattern = 01111110 – apply bit stuffing in case the flag pattern appears inside the frame Data Communications (E&T 2760): Data Transmission 21

Ethernet Data Frame Data Communications (E&T 2760): Data Transmission 22

Where to Get More Information l Textbook – Chapter 2, p. 19 to p. 26 l Exercises – Chapter 2, p. 39, Q 2. 1, 2. 2, 2. 3, 2. 8 Data Communications (E&T 2760): Data Transmission 23