06-DigitalDataComm - Data and Computer Communications...

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Unformatted text preview: Data and Computer Communications Communications Chapter 6 – Digital Data Chapter Communications Techniques Communications Eighth Edition by William Stallings Lecture slides by Lawrie Brown Digital Data Communications Techniques 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 Synchronous timing problems require a mechanism to timing synchronize the transmitter and receiver synchronize receiver samples stream at bit intervals iif clocks not aligned and drifting will sample at f wrong time after sufficient bits are sent wrong two solutions to synchronizing clocks asynchronous transmission synchronous transmission Asynchronous Transmission Asynchronous Asynchronous - Behavior Asynchronous simple cheap overhead of 2 or 3 bits per char (~20%) good for data with large gaps (keyboard) Synchronous Transmission Synchronous block of data transmitted sent as a frame clocks must be synchronized need to indicate start and end of block can use separate clock line or embed clock signal in data use preamble and postamble more efficient (lower overhead) than async Types of Error Types an error occurs when a bit is altered between an transmission and reception transmission single bit errors only one bit altered caused by white noise burst errors contiguous sequence of B bits in which first last and contiguous any number of intermediate bits in error any caused by impulse noise or by fading in wireless effect greater at higher data rates Error Detection Error will have errors detect using error-detecting code added by transmitter recalculated and checked by receiver still chance of undetected error parity parity bit set so character has even (even parity parity) or odd (odd parity) number of ones parity) even number of bit errors goes undetected Error Detection Process Error Cyclic Redundancy Check Cyclic one of most common and powerful checks for block of for k bits transmitter generates an n bit frame check sequence (FCS) transmits k+n bits which is exactly divisible transmits k+n by some number by receiver divides frame by that number if no remainder, assume no error for math, see Stallings chapter 6 Error Correction Error correction of detected errors usually requires correction data block to be retransmitted data not appropriate for wireless applications bit error rate is high causing lots of retransmissions when propagation delay long (satellite) compared with when frame transmission time, resulting in retransmission of frame in error plus many subsequent frames frame iinstead need to correct errors on basis of bits nstead received received error correction provides this Error Correction Process Error How Error Correction Works How adds redundancy to transmitted message can deduce original despite some errors eg. block error correction code map k bit input onto an n bit codeword map bit each distinctly different iif get error assume codeword sent was f closest to that received closest for math, see Stallings chapter 6 means have reduced effective data rate Line Configuration - Topology Line physical arrangement of stations on physical medium medium point to point - two stations • such as between two routers / computers such multi point - multiple stations • traditionally mainframe computer and terminals • now typically a local area network (LAN) Line Configuration - Topology Line Line Configuration - Duplex Line classify data exchange as half or full duplex half duplex (two-way alternate) only one station may transmit at a time requires one data path full duplex (two-way simultaneous) simultaneous transmission and reception between simultaneous two stations two requires two data paths • separate media or frequencies used for each direction or echo canceling Summary Summary asynchronous verses synchronous asynchronous transmission transmission error detection and correction line configuration issues ...
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