-DigitalDataComm - New.pptVIP

Data and Computer Communications Eighth Edition by William Stallings New Concepts timing Asynchronous/ synchronous transmission clock Idle state overhead Preamble/postamble Single-bit/burst error Error detection/correction Error-detecting code Error-correction code codeword Forward error correction (FEC) Frame check sequence (FCS) Half/full duplex transmission Parity bit Even/odd parity topology Point-to-point connection Multipoint connection Cyclic redundancy check (CRC) Digital Data Communications Techniques A conversation forms a two-way communication link; there is a measure of symmetry between the two parties, and messages pass to and fro. There is a continual stimulus-response, cyclic action; remarks call up other remarks, and the behavior of the two individuals becomes concerted, co-operative, and directed toward some goal. This is true communication. —On Human Communication, Colin Cherry Asynchronous and Synchronous Transmission two solutions to synchronizing clocks asynchronous transmission synchronous transmission timing problems require a mechanism to synchronize the transmitter and receiver receiver samples stream at bit intervals if sender’s or receiver’s clocks are misaligned or one of them drifts, sampling occurs at wrong time Asynchronous Transmission Asynchronous - Behavior simple cheap overhead of 2 or 3 bits per char (~20%) good for data with large gaps (keyboard) Synchronous Transmission block of data transmitted sent as a frame clocks must be synchronized can use separate clock line can embed clock signal in data (eg, Manchester code for digital data or a phase shift for analog data) need to indicate start and end of block use preamble and postamble more efficient (lower overhead) than async Types of Errors an error occurs when a bit is altered between transmission and reception single-bit errors only one bit altered caused by white noise burst errors contiguous sequence of bits in which first, last, and any number of intermediate bits ar