Computer Networks and Internets with Internet Applications 4

Computer Networks and Internets with Internet Applications, 4 e By Douglas E. Comer Lecture Power. Points By Lami Kaya, LKaya@ieee. org © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Chapter 5 Local Asynchronous Communication © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Topics Covered • • • 5. 1 Introduction 5. 2 The Need For Asynchronous Communication 5. 3 Using Electric Current To Send Bits 5. 4 Standards For Communication 5. 5 Baud Rate, Framing, And Errors 5. 6 Half And Full Duplex Asynchronous Communication 5. 7 Limitations Of Real Hardware 5. 8 Hardware Bandwidth And The Transmission Of Bits 5. 9 The Effect Of Noise On Communication 5. 10 Significance For Data Networking © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 1 Introduction • Digital devices, such as computers use bits to represent data – Transmitting data across a NW means sending bits • Physically, communication system can use – electric current – radio waves – or light to transfer information This hapter shows • How bits can be encoded • Discusses mechanism for sending characters • Introduces channel measurements: – bandwidth – and delay © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 2 The Need For Asynchronous Communication • Asynchronous means if a sender/receiver – – do not need to coordinate before data can be sent a sender can wait arbitrarily long between transmissions it can transmit whenever data becomes ready a receiver must be ready to accept data whenever it arrives • Useful when two devices operating at different speeds • Electrical signal does not contain information for a receiver to determine where individual bits begin/end – To ensure meaningful exchange send • Start bit before character • One or more stop bits after character © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 3 Using Electric Current To Send Bits • How to encode bits? – Each bit represented by a voltage, such as • 0 negative voltage • 1 positive voltage • Or otherwise – Figure 5. 1 shows a waveform diagram • visual representation of how an electrical signal varies over time © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

© 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 7

5. 4 Standards For Communication Questions to be answered • How long should sender hold a voltage on the wire for a single bit? • What is the maximum rate at which HW can change the voltage? • How can we be sure HW from different vendors work together? – Specifications for communication systems are standardized by • International Telecommunication Union (ITU) • Electronic Industries Association (EIA) • Institute for Electrical and Electronic Engineers (IEEE) – Standard documents answer questions about a particular technology • Timing of signals • Electrical details of voltage and current • One such popular standard produced by EIA is RS-232 – Serial transmission; bits travel one after another on the wire © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

© 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 9

5. 5 Baud Rate, Framing, And Errors • Baud rate – number of changes in the signal per second • Number of bits may be sent together as a signal/symbol Bit rate = baud rate x number of bits per signal/symbol bits per signal/second (bps) • If sending/receiving HW are not configured to use the same speed – framing errors occur © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 6 Half And Full Duplex Asynchronous Communication • Communication modes – Simplex one-way – Half-duplex; one-way at a time – Full-duplex; two-way anytime • Signals sent over a standard cable are used for – Data transfer – Control • Connectors used – Data Communication Equipment (DCE) – Data Terminal Equipment (DTE) © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

© 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 12

5. 7 Limitations Of Real Hardware • How fast a HW can transmit bits across a wire? • In practice, no electronic device can produce an exact voltage or change from one voltage to another instantly • No wire conducts electricity perfectly: – Signal loses energy • attenuation – Noise/interference may exist • errors © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

© 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved. 14

5. 8 Hardware Bandwidth And The Transmission Of Bits Each transmission system has a limited bandwidth (BW) • BW is the maximum rate that HW can change a signal • BW is measured in cycles per second or Hertz (Hz) • BW limitations arise from physical properties and energy • Every physical transmission sys has a limited BW • Nyquist discovered (1920 s) a relationship between the BW and bit rate of a transmission system – Maximum data rate that can be achieved for a BW of B is 2 B. – If system uses K possible values of voltages instead of two Maximum data rate = 2 B log 2 K bps © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 9 The Effect Of Noise On Communication • Nyquist’s theorem provides and absolute (theoretical) maximum bound that can not be achieved in practice • A real communication system is subject to small amount of background interference – called noise • Shannon extended Nyquist’s theorem (1948) that takes into account noise in a transmission system C = B log 2 (1 + S/N) bps • C: Capacity; effective limit on the channel capacity • S: average signal power • N: average noise power © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 10 Significance For Data Networking (1) Nyquist and Shannon theorems that have consequences for engineers: • Nyquist's work has provided an incentive to explore complex ways to encode bits on signals: – Nyquist's theorem encourages engineers to explore ways to encode bits on a signal – because a clever encoding allows more bits to be transmitted • Shannon's Theorem informs engineers that no amount of clever encoding can overcome the laws of physics – that place a fundamental limit on the rate that can be achieved in a real communication system © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

5. 10 Significance For Data Networking (2) Shannon's Theorem helps explain how fast one can send data across a voice telephone call: • Ex: voice telephone system has a S/N ratio of approximately 30 d. B and a BW of approximately 3000 Hz • The maximum number of bps is limited to: C = 3000 log ( 1 + 1000 ) = approximately 30, 000 bps • faster speeds will only be possible if S/N ratio can been improved • How can dial-up modems achieve higher throughput than Shannon Theorem? one possibility compression – Compression only works if the data has been encoded inefficiently – Ex, when an 8 -bit ASCII encoding is used to transfer email that contains only upper and lower case English letters and digits • only 62 of the 256 possible 8 -bit values are actually used • If the same data is encoded in 6 -bit characters, 25% fewer bits are needed. © 2007 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.