Computer Net Interoperability
Computer Net Interoperability CECS 474
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This 15 page Class Notes was uploaded by Zackary Cronin on Monday October 5, 2015. The Class Notes belongs to CECS 474 at California State University - Long Beach taught by Tracy Maples in Fall. Since its upload, it has received 79 views. For similar materials see /class/218744/cecs-474-california-state-university-long-beach in Computer Science and Engineering at California State University - Long Beach.
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Date Created: 10/05/15
CECS 474 Computer Network Interoperability CHAPTER 5 Local Asynchronous Communication RS232 Computers are digital thus they use binary digits bits to represent data Defn Communication is said to be asynchronous if the sender and receiver do not need to coordinate before the data can be sent Advantage A sender can wait arbitrarily long between transmissions the sender can transmit Whenever the data is ready to send Using Electric Current to Send Bits Example Digital Encoding Medium Copper Wire Energy Form Electric Current Encoding Negative voltage encodes 1 Positive voltage encodes 0 Dr Tracy Bradley Maples F all 2002 1 manage a 7 I me Ia1n 01 Fig 51 A waveform diagram The Xaxis corresponds to time The y is corresponds to voltage m Txazy EndlzyMszS H mm Standards for Communication Encoding details are specified by a standard Several organizations produce networking standards 0 IEEE Institute for Electrical and Electronic Engineers 0 ITU International Telecommunications Union 0 EIA Electronic Industries Association Encoding standards specify both 1 the timing of signals and 2 the electrical details of voltage and current Dr Tracy Bradley Maples Fall 2002 The RS232C Standard RS232C is the most widely accepted way to transfer characters across copper wires between a computer and a device such as a modem keyboard or terminal RS232 defines serial asynchronous communication RS232 specifies the physical connection as well as the electrical details Specified by the EIA Voltage is 15 or 15 Cable limited to 50 feet Latest EIA standard is RS422 ITU standard is V24 It specifies the transfer of characters usually 7bit Example use connection to a keyboard or mouse Via the serial port on a PC Dr Tracy Bradley Maples F all 2002 4 Asynchronous Communication 0 Sender and receiver must agree on Number of bits per character Duration of each bit 0 Receiver Does not know when a character will arrive May wait forever 0 To ensure meaningful exchange send Start bit before each character One or more stop bits after each character Dr Tracy Bradley Maples F all 2002 a I me idle want u 1 I o I Oslop Idl Fi ggeSJ Voltage on awire as a characteris transmitted using RS 232 Startbit 7 Same as 0 a Notpart ofdata Stop bit 7 Same as 1 r Follows data m Txacy ndlzyszlzs H mm Baud Rate Framing and Errors The duration of a bit in RS232 is determined by the baud rate Defn The baud rate of transmission hardware is the number of changes in the signal per second that the hardware generates Examples Typical baud rates 96 Kbaud 144 Kbaud 288 Kbaud For RS232 it is a very simple scheme the baud rate is exactly equal to the number of bits per second 288 Kbaud 288 kbits per second The duration of a bit lbaud rate To make RS232 more general manufacturers design each piece of hardware to operate at a variety of baud rates 0 Sender and receiver must agree on the baud rate 0 Receiver samples the signal to verify agreement 0 Disagreement results in a framing error Dr Tracy Bradley Maples F all 2002 Full Duplex Asynchronous Communication Note All electric circuits require a minimum of two wires 0 The current ows out on one o The current ows back on the other often called the grounaO When RS232 is used with twisted pair wiring one wire carries the signal and the other is ground For coax the signal travels down the center conductor and the ground returns on the shield Defn Transfer in which information is sent one direction is called half duplex or simplex transmission Defn Transfer in which information can be sent simultaneously in two direction is called full duplex transmission Full Duplex requires that each side have a transmitter and a receiver Dr Tracy Bradley Maples F all 2002 FullsDuplex Communication Figure 53 Minimal wiring for fullduplex RS232 communication Transmitter on one side is connectedto the receiver on the other Separate wires are needed to carry cunent in each direction A common ground wire can be used for both RS232 de nes a 25 pin connector ca11ed DBZS and speci es how to use each ofthe 25 wires for control or data Examnle For a computer Iranth on pin 2 receive on pin 3 For a modem 39n39ansmit on pin 3 receive on pin 2 The ground wire uses pin 7 Dr Tncyandieywhdesaui m2 9 Limitations of Real Hardware Because of the limitations of electrical transmission eg energy RS dissipates as it travels resistance and interference distortion occur 232 muxt han 39 39 onions R87232 will Take multiple samples per hit Tolerate less than full Voltage Unfortunately this means that we can not use electrical current for long distance transmissions mm 54 m vlhmmwu rr me wlmgr mm m r w dam is H mm A m rquot Mm wluu m Hm WW llmn m W 39 m D TracyEradleyMaplesGallZ Z 10 Performance Issues Review Recall o Propagation Delay Determined by physics Time required for a signal to travel across the medium 0 Bandwidth Electrical property of a physical transmission system Maximum times per second the signal can change Fundamental Performance Measures 0 Delay The amount of time required for a bit of data to travel from one end to the other Usually the same as the propagation delay in the underlying hardware 0 Throughput The number of bits per second that can be transmitted Related to the underlying hardware bandwidth Dr Tracy Bradley Maples F all 2002 ll Digital Throughput vs Bandwidth The relationship between digital throughput and bandwidth is given by Nyguist39s Theorem D 2 B log K where o D is the maximum data rate 0 B is the hardware bandwidth 0 K is the number of values used to encode the data Example 1 RS232 K 2 because RS232 only uses two values 15 or 15 volts D2Blog222B Example 2 Phaseshift Encoding Suppose K 8 the number of possible shifts D2Blog282B36B Dr Tracy Bradley Maples F all 2002 12 The Bad News Physics tells us that real systems emit and absorb energy eg thermal Engineers call unwanted energy noise In Nyquist39s Theorem a noisefree system is assumed Nyquist39s Theorem only works in theory We turn to Shannon39s theorem to correct for noise Dr Tracy Bradley Maples F all 2002 13 Digital Throughput vs Bandwidth Again The gal relationship between digital throughput and bandwidth is given by Shannon39s Theorem C B log 1 SN where o C is the effective channel capacity in bits per second 0 B is the hardware bandwidth 0 S is the average power signal 0 N is the noise 0 SN is the signaltonoise ratio Example Conventional Telephone System 0 Engineered for voice 0 Bandwidth is 3000 Hz 0 Signaltonoise ratio is approximately 1000 0 Effective capacity 3000 log 1 1000 30000 bps Conclusion Dialup modems have little hope of exceeding 288 Kbps Dr Tracy Bradley Maples F all 2002 14 Conclusions 0 Nyquist39s Theorem means that finding a way to encode more bits per cycle improves the data rate 0 Shannon39s Theorem means that no amount of clever engineering can overcome the fundamental physical limits of a real transmission system Dr Tracy Bradley Maples F all 2002 15