Data Communications and Computer Networks 1/10 Data Transmission Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: Feb 8 2001 Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/10 Transmission mode Simplex Simplex Transmission mode Half-duplex Transmission mode Half-duplex Full-duplex Full-duplex Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 1 Kasetsart University
Data Communications and Computer Networks 3/10 Simplex Display Computer Unidirectional Hi…. pager Transmitter Data are only transmitted in one direction � Only one of the two devices on a link can transmit; the � other can only receive Applied Network Research Group Department of Computer Engineering, Kasetsart University 4/10 Half-duplex S 1 Transmit from S 1 to S 2 at time T 1 S 2 Transmit from S 2 to S 1 at time T 2 Transmitter/Receiver Transmitter/Receiver Both stations may transmit; but only one at a time (alternate � in transmitting) Example devices : Walkie-Talkies � Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 2 Kasetsart University
Data Communications and Computer Networks 5/10 Full-duplex Simulteneous send and receive data from each other S 1 S 2 Transmitter/Receiver Transmitter/Receiver Full-Duplex operation can occur in two ways : � � two separate transmission path � channel is divided between signals traveling in opposite directions Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/10 Line Configurations Point-to-Point Point-to-Point Line configurations Line configurations Multipoint Multipoint Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 3 Kasetsart University
Data Communications and Computer Networks 7/10 Point-to-Point A direct link A direct link between two devices between two devices Station Station A transmission medium is point-to-point if it provides a � direct (dedicated) link between two devices (only two devices share the medium) The entire capacity of the link is reserved for the � transmission between those two devices Applied Network Research Group Department of Computer Engineering, Kasetsart University 8/10 Multipoint Station Station Station A link is shared between several devices Station Station More than two devices share the same medium � The capacity of the link is shared � Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 4 Kasetsart University
Data Communications and Computer Networks 9/10 Data Transmission Parallel Parallel Data Transmission Data Transmission Serial Serial Asynchronous Asynchronous Synchronous Synchronous Applied Network Research Group Department of Computer Engineering, Kasetsart University 10/10 Parallel Transmission 0 1 0 Need eight data lines 1 for sending each byte 1 0 1 1 Simultaneous transmission results high speed � Required more data lines, hence high cost � Usually limited to short distances because its costs and � crosstalk Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 5 Kasetsart University
Data Communications and Computer Networks 11/10 Serial Transmission A group of bits are sent one after another 0101101110 0100101110 0000101110 0110101110 Need only one data line One bit follows another, only one data line for each � direction The sender needs a parallel-to-serial converter and the � receiver needs a serial-to-parallel converter. Applied Network Research Group Department of Computer Engineering, Kasetsart University 12/10 Serial Transmission Mode Asynchronous Asynchronous avoid timing problem by not sending long stream of bits � Serial data are transmitted one character at a time � Serial synchronization is maintained within each character with � start and stop codes Synchronous Synchronous a block of bits is transmitted in a steady stream without � start and stop codes synchronization is maintain by a separate clock line or � embedding clocking info in the signal Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 6 Kasetsart University
Data Communications and Computer Networks 13/10 Asynchronous Transmission line idle line idle start bit parity bit stop bit Direction of transmission 7-8 data bits 1-2 bits Idle state : no character is being transmitted � Start bit : the beginning of a character (1 bit) � Date bits : bit to be transmitted (7-8 bits) � Parity bit : for error detection (None, Even or Odd) � Stop bit : end of byte (1, 1.5 or 2 bits) � Applied Network Research Group Department of Computer Engineering, Kasetsart University 14/10 Asynchronous Data Stream Varying (unpredictable) time interval between each character No synchronization at the byte level � But within each byte, the synchronization with start and � stop bit is required Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 7 Kasetsart University
Data Communications and Computer Networks 15/10 Bit missed start stop 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 idle 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 ideal sampling 6 7 1 2 3 4 5 8 9 10 11 12 13 14 10 30.8 51.7 72.5 93.3 114.2 135 155.8 176.7 197.5 218.3 239.3 260 280.8 missed! 20 ms bit duration and 48Hz receiver clock Applied Network Research Group Department of Computer Engineering, Kasetsart University 16/10 Synchronous Transmission : � But their clock is somehow be synchronized! � provides a separate clock line, or � embedded clocking information in the digital signal � Need another level of synchronization � flag to determine the beginning and end of the block, normally called preamble and postamble preamble data postamble Two categories of synchronous transmission : • Byte-Oriented Protocols • Bit-Oriented Protocols Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 8 Kasetsart University
Data Communications and Computer Networks 17/10 Byte-Oriented protocol � View each frame as a collection of bytes � exemplified by the BISYNC (Binary Synchronous Protocol) protocol developed by IBM in the late 1960s � also DDCMP used in DECNET, IMP-IMP in ARPANET � BISYNC example : SYN SYN SOH header STX data ETX CRC ETX or DLE might appear …. ETX DLE … in the data portion …. DLE ETX DLE DLE … escaping the ETX by also escaping a DLE preceding it with the a DLE with an extra DLE (Byte stuffing technique) Applied Network Research Group Department of Computer Engineering, Kasetsart University 18/10 Bit-Oriented protocol � View each frame as a collection of bits � SDLC (Synchronous Data Link Control Protocol) developed by IBM is an example � later standardized by the OSI as the HDLC (High-Level Data Link Control Protocol) � PPP is an example of HDLC variation opening flag data closing flag 01111110 01111101101111100 01111110 additional bit inserted (bit stuffing technique) 01111110 might occur in the the body of frame any time five consecutive 1s have been sent, a 0 is insert before sending the next bit Applied Network Research Group Department of Computer Engineering, Kasetsart University Department of Computer Engineering 9 Kasetsart University
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