DIGITAL DATA COMMUNICATION TECHNIQUES Networks and Communication Department Chapter 6

Lecture Contents Communication Types: Transmission Types: HDX. FDX. Parallel Serial Synchronous. Asynchronous. 5-Dec-18 Networks and Communication Department

Communications Types 5-Dec-18 Networks and Communication Department

Half Duplex (HDX) : A half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying. 5-Dec-18 Networks and Communication Department

Full Duplex (FDX) : A full-duplex (FDX) system, or sometimes called double-duplex, allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time. 5-Dec-18 Networks and Communication Department

FDX vs. HDX : There are several benefits to using full-duplex over half-duplex: Firstly, time is not wasted, since no frames need to be retransmitted, as there are no collisions. Secondly, the full data capacity is available in both directions because the send and receive functions are separated. Thirdly, stations (or nodes) do not have to wait until others complete their transmission, since there is only one transmitter for each twisted pair 5-Dec-18 Networks and Communication Department

Transmission Types 5-Dec-18 Networks and Communication Department

Transmission Types 5-Dec-18 Networks and Communication Department

Parallel transmissions: Transfer of one (byte) with all bits are sent at the same time, each using a separate link n wires are used to send n bits at one time Advantage: speed Disadvantage: cost; limited to short distances 5-Dec-18 Networks and Communication Department

Parallel transmissions: 5-Dec-18 Networks and Communication Department

Serial transmissions: Transfer of one (byte) with one bit at a time using only one single link Advantage: reduced cost Disadvantage: requires conversion devices Techniques: Asynchronous Synchronous 5-Dec-18 Networks and Communication Department

Serial transmissions: 5-Dec-18 Networks and Communication Department

Asynchronous Transmission 5-Dec-18 Networks and Communication Department

Asynchronous Transmission Transfer of data with start and stop bits and a variable time interval between data units. Timing is unimportant Start bit alerts receiver that new group of data is arriving. Stop bit alerts receiver that byte is finished. Synchronization achieved through start/stop bits with each byte received. Serial transmissions 5-Dec-18 Networks and Communication Department

Asynchronous Transmission The beginning of a character is signaled by a start bit with a value of binary 0. followed by the 5 to 8 bits that actually make up the character. Then the data bits are usually followed by a parity bit, set by the transmitter ,The receiver uses this bit for error detection. The final element is a stop element, which is a binary 1 Serial transmissions 5-Dec-18 Networks and Communication Department

Asynchronous Transmission Serial transmissions Requires additional overhead (start/stop bits) Slower Cheap and effective Ideal for low-speed communication when gaps may occur during transmission (ex: keyboard) 5-Dec-18 Networks and Communication Department

Synchronous Transmission 5-Dec-18 Networks and Communication Department

Synchronous Transmission Serial transmissions Requires constant timing relationship Bit stream is combined into longer frames, possibly containing multiple bytes Any gaps between bursts are filled in with a special sequence of 0s and 1s indicating idle Advantage: speed, no gaps or extra bits more efficient than asynchronous 5-Dec-18 Networks and Communication Department

Synchronous Transmission Serial transmissions block of data transmitted, sent as frames in a steady stream without start and stop codes. clocks must be synchronized can use separate clock line or embed clock signal in data need to indicate start and end of block use preamble and post-amble (flags) 5-Dec-18 Networks and Communication Department

Transmissions Types: In serial transmission a character is converted from parallel to serial form when transmitting and from serial to parallel form when receiving. 5-Dec-18 Networks and Communication Department

Transmissions Types: 5-Dec-18 Networks and Communication Department