Module 5 2010

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Module 5 2010

  1. 1. CUTAJAR & CUTAJAR ©2010
  2. 2.  Students should be able to ◦ understand the basics of transmission methods in communication ◦ distinguish between different categories of networks ◦ appreciate the purpose of a protocol in communication ◦ appreciate the wide range Internet related technical terms and Internet applications Communication & Networks 2
  3. 3. Cutajar & Cutajar
  4. 4.  Networks are an interconnection of computers. These computers can be linked together using a wide variety of different cabling types, and for a wide variety of different purposes. The basic reasons why computers are networked are ◦ to share resources (files, printers, modems, fax machines) ◦ to share application software (distributed programs) ◦ increase productivity (make it easier to share data amongst users) ◦ provide fast communication between users. Communication & Networks 4
  5. 5. There are FOUR basic elements involved in communications:1. The SENDER which initiates the communication.2. The MEDIUM which is the mechanism by which communication is conveyed to the receiver3. The RECEIVER which receives the communication4. The MESSAGE, which is the information content that is transferred between the sender and receiver via the medium. message Communication Medium Source Sink Transmitter Receiver Noise SENDER MEDIUM RECEIVER Communication & Networks 5
  6. 6.  Originally, communications depended on codes transmitted by visual systems such as mirrors, flags and smoke. Modern Communications make use of electrical or optic signals to communicate between one side and the other. ◦ Electrical data communication systems transmit codes by switching electrical currents or voltages. ◦ Optic data communication systems transmit codes by switching light pulses through an optic fibre. Communication & Networks 6
  7. 7.  Samuel F.B. Morse perfected the telegraph,The first mass data communication system based on electrical power. Telegraph used the Morse code to transmit messages from one operator to another.This code was very difficult to automize Communication & Networks 7
  8. 8.  Codes: Standard (agreed-on-in advance) interpretations between signalling elements and characters. Some Codes are used to represent characters within a computer.  Signalling Elements: Representations of characters that are transmitted over the transmission lines.  Characters: Letters, signs and symbols on input/output devices. Signaling elementsCharacters Encoding Decoding Characters Communication & Networks 8
  9. 9.  Emile Baudot developed one of the most successful codes, suited for machine encoding and decoding. However, it was limited because it could only use five signalling elements per character. He introduced the LTRS and FIGS to double his character set. Communication & Networks 9
  10. 10.  ASCII: “as-key” is a code developed by ANSI. It uses 7 bits to represent 128 characters. It is the most popular code today. They are loaded in a PC using the ANSI.SYS file.“A” 100 0001 65“a” 110 0001 97“0” 011 0000 48Internally the 8th bit, which is used intransmission as the parity bit, is used toextend the character set to 256 characters,called the Extended ASCII Set.Another Code called EBCDIC is an 8th bitalternative to ASCII Communication & Networks 10
  11. 11.  Unicode has the explicit aim of transcending the limitations of traditional character encodings, such as those defined by the ASCII code, which find wide usage in various countries of the world, but remain largely incompatible with each other. Many traditional character encodings share a common problem in that they allow bilingual computer processing (usually using Latin characters and the local script), but not multilingual computer processing (computer processing of arbitrary scripts mixed with each other). Communication & Networks 11
  12. 12. Computer A Computer B Application User-to-User communication Application Process Process Computer-to-Computer communicationCommunication Communication Subsystem Subsystem Computer-to-Network communications Data Communication network Communication & Networks 12
  13. 13.  Irrespective of the type of data communications facilities being used, in most applications data is transmitted between computers in a bit-serial mode whilst inside a computer data is transferred in a word-parallel mode. It is thus necessary to perform a parallel-to-serial conversion at the transmitter and vice-versa at the receiver Bit Serial Word Data Parallel-to-Serial Serial-to-Parallel Parallel conversion conversion Word Data Parallel Data Communication & Networks 13
  14. 14.  Once data is transmitted outside of a computer, there is a much increased probability that bits are received in error. It is therefore important to provide a means to correct the data in case of error ( Error Control ) The rate at which data is transferred between two computers must also be controlled so as to assure that all the information is received. ( Flow Control ) If an intermediate network is involved, establishing a communications path across a network is also necessary ( Routing ). Communication & Networks 14
  15. 15.  If only two computers are involved and both are in the same room or office, then the transmission facility can comprise just a simple point-to-point wire link. Computer A Computer B Application Application Process Process Communication Communication Subsystem Subsystem Communication & Networks 15
  16. 16.  If the two computers are located in different parts of a town or country, public carrier facilities must be used. Normally this will involve the Public Switched Telephone Network (PSTN) which requires a device known as modem to transmit data. Application Application Process Computer A Process Computer B Communication Communication Subsystem Subsystem Modem PSTN Modem Communication & Networks 16
  17. 17.  In standards, data processing equipment (computers) are known as Data Terminal Equipment ( DTE ). Modems are known as Data Circuit termination Equipment ( DCE ). It is probably easier to remember it as Data Communication Equipment, but this is not the official name. Network DTE DCE DCE DTE Communication & Networks 17
  18. 18. When more than two computers Floor 1are involved, a switched Site-widecommunication facility (network) (Backbone)is normally provided to enable the MANcomputers to communicate with Floor 2each other. If all the computersare installed within the same LAN Abuilding, it is possible to installone’s own network. Suchnetworks are known as Local Area Floor 3 LAN BNetworks or LANs andinterconnect various LANS by Terminalsmeans of a Metropolitan AreaNetwork or MAN Bridges LAN C Transceivers Communication & Networks 18
  19. 19.  When individual local area networks are located in different sites, the public carrier facilities must again be used. The resulting network is known as a Wide Area Network or WAN Private Branch Intelligent Exchange Leased Lines SITE BSITE A MultiplexerVoice Voice PBX PBX IMUX PSTN IMUX DSE DSEData Data Company-wide Data Switching backbone network Equipment Communication & Networks 19
  20. 20. The Public Service Networks provide a public switched data services whichhave been designed specifically for data transmission rather than voice.Consequently, distributed networks use a Public Switched Data Network(PSDN). Computer Computer Communication Subsystem Terminal PSDN Controller Interface Standards TC Computer Communication & Networks 20
  21. 21.  Alternatively, many public carriers are now converting their existing public switched telephone networks to enable data to be transmitted without the need of modems. The resulting networks, which operate in all digital mode are known as Integrated Services Digital Networks (ISDN) referring to both voice and data. Voice Voice NTE NTE Data Data ISDN Network Termination Voice Equipment NTE Data Communication & Networks 21
  22. 22.  Till now we have considered only intranetworking, in the sense that communication is always within the same LAN or WAN. In some applications however, communication is also needed between separate networks such as LAN-WAN-LAN connections. This type of communication, is known as internetworking or internet. Gateway Satellite Earth Station PSDN PSDN LAN LAN LAN LAN Communication & Networks 22
  23. 23.  Until recently computer industry standards were concerned primarily with either the internal operation of a computer or the connection of a local peripheral device. This resulted in communication subsystems offered by manufacturers only enabled their own computers to exchange information. Such systems are known as closed systems. Initially, the services provided by most public carriers were concerned primarily with data transmission, and device interfacing with the network. This resulted in interface standards of multi-vendor systems. Communication & Networks 23
  24. 24.  In contrast to the closed system, the various international bodies concerned with public carrier networks have formulated agreed standards for connecting devices to these networks: ◦ V-Series Recommendations: DTE-Modem-PSTN connections ◦ X-Series Recommendations: DTE to PSDN connections ◦ I-Series Recommendations: DTE to ISDN connections Additionally they devised higher level standards concerned with the format (syntax) and control of the of information (data) between systems. Consequently equipment from different manufacturers could be exchanged as long as it adheres to these standards The resulting system is known as open system or open system interconnection environment (OSIE) Communication & Networks 24
  25. 25. To overcome the complexity of the communication subsystem,the ISO (International Standards Organisation) has adopted alayered approach for the reference model. The completesubsystem was broken down in layers, each of which performs awell defined function. Conceptually these layers can beconsidered as performing one of two generic functions:- ◦ Network dependent functions ◦ Application oriented functions Application Oriented Network Dependent Communication & Networks 25
  26. 26. There exist 3 operational environments:a. The Network environment: This is concerned with protocols and standards relating to different types of underlying communication networks.b. The OSI environment: This embraces the network environment and adds additional application oriented protocols and standards to allow the end system to communicate with one another in an open way.c. The Real system environment: This is concerned with the manufacturers own proprietary software and services which have been developed to perform a particular distributed information processing task. Communication & Networks 26
  27. 27. Computer A Computer BApplication Process Application Process Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Data Communication NetworkNetwork Environment OSI Environment Real System Environment Open System Interconnection Communication & Networks 27
  28. 28. End User Application Process Distributed information service Application Layer : FTP, Information Interchange, job transfer Presentation Layer : Syntax negotiations, data representation transformations Session Layer : Dialogue and Synchronization control for applications Transport Layer : End to End message transfer ( connection management, error control, fragmentation and flow control ). Network Layer : Network Routing, addressing, call setup and clearing Data Link Layer : Datalink control ( framing, data transparency, error control ) Physical Layer :Mechanical and Electrical network interface definitions Physical connection to the network terminating equipmentData Communication Network : The real physical network carrying messages Communication & Networks 28
  29. 29. Provides the following services in the form of normal function calls: Identification of the intended communication partner(s) by name or by address Determination of the current availability of the partner Establishment of authority to communicate Agreement on privacy (encryption) mechanism Authentication of partner Selection of dialogue discipline, including initialisation and release procedures Agreement on responsibility of error correction Identification of constraints Communication & Networks 29
  30. 30.  This layer is responsible for the syntax of the data transfer, transforming from abstract data syntax to transfer or concrete syntax: Anecdote - Language translator. Issues handled by this layer are data encryption and decryption, and key transfer for such a job. Communication & Networks 30
  31. 31.  This layer is used for the organisation andsynchronisation of messages and setting up andclearing a dialogue between two peer computers. Optional services offered by this layer are: ◦ Interaction management - Duplex/Half Duplex ◦ Synchronisation - If messages are too long establishes synchronisation points ◦ Exception Reporting - Reports on non recoverable exceptions Communication & Networks 31
  32. 32.  This is one of the most important Layers and interfaces the network-dependent protocols to the application oriented layers and provides a message transfer facility which is thus network independent. Two classes of functions exist in this layer: ◦ Class 0 - basic connection and data transfer ◦ Class 4 - full error control and flow control Communication & Networks 32
  33. 33.  The function of these layers varies from network to network and the three layers which are included here are: ◦ Network Layer: This is responsible for establishing and clearing a network wide connection, the routing of messages (addressing) and flow control of traffic in the network. ◦ Link Layer: This layer is responsible for a reliable information transfer using error detection and retransmission where needed.  Two types of services exist:  Connectionless - Self contained message entities or Datagrams  Connection Oriented - Virtual Circuit ◦ Physical Layer: Responsible for the DCE - DTE connection - It provides the link layer a means of transmitting a serial bit stream between two pieces of equipment. Communication & Networks 33
  34. 34.  Prior and concurrently with the ISO standards activity, the End-user/Application process United states Department of ISO Defense has funded research Layers which resulted in an File transfer Protocol (FTP) internetwork known as Remote terminal protocol (TELNET) ARPANET which was extended Name Server Protocol (NSP) (5-7) to incorporate other internets Simple Network Management Protocol (SNMP) to form the now well know Internet. (4) TCP UDP The internet Protocol Suite IP known as Transmission Control (1-3) IEEE802.X / X.25 Protocol / Internet Protocol (TCP/IP) or the User Datagram Protocol (UDP/IP) has thus been developed LAN / WAN Communication & Networks 34
  35. 35. Cutajar & Cutajar
  36. 36.  In practice, transmission can occur in one of three modes, namely, Simplex, Half-Duplex and Full-Duplex modes • Simplex:Transmission in one direction only • Half-Duplex:Transmission in both directions but not at the same time • Full-Duplex:Transmission in both directions simultaneouslyHalf-Duplex Communication Communication & Networks 36
  37. 37.  In practice, transmitted electrical signals are attenuated ( reduced ) and distorted ( misshapen ) by the transmission medium, so that at some stage the reciever is unable to discriminate between the binary 1 and 0 signals.Distortion and attenuation Transmitted Data 0 1 0 0 1 1 0 1depend strongly on : Transmitted Signal• The transmission medium, time• The bit rate of the data being Typical Receivedtransmitted, Signal time•The distance between two Sampling Instantscommunicating devices. Received Data 0 1 0 0 1 0 0 1 Transmitting Electrical Signals Communication & Networks 37
  38. 38.  The type of transmission medium is important, since it determines the maximum number of bits that can be transmitted per second ( bps ) according to the maximum bandwidth provided by the medium. The most commonly used media are: ◦ Two wire open lines ◦ Twisted Pair cables ◦ Coaxial Cables ◦ Optic Fibers Communication & Networks 38
  39. 39.  Simplest form of transmission medium maximum distance: 50 m , maximum speed: 19.2 Kbps Working on Current or Voltage sensing Normally used for DTE-DCE connections Types available: multicore cable or flat ribbon cable Care must be taken to avoid cross coupling (capacitive coupling between the two wires ) - crosstalk Open structure makes it susceptible to the pickup of spurious noise signals caused by electromagnetic radiation - picked up by just one wire Communication & Networks 39
  40. 40.  Has a much better noise immunity ( symmetrical pickup ) and reducedcrosstalk Types available: UTP ( Unshielded ) and STP ( Shielded ) Twisted Pairs:  Because a wire acts as an antenna, several techniques are used to reduce Braided Plastic Metal electromagnetic interference (EMI). Most Jacket Shield wires are shielded, and some wires are also twisted at 90º angles every so often. The twists serve to additionally suppress EMI. The attenuation of twisted wire pairs rises rapidly with increasing frequency, and the amount of crosstalk between adjacent pairs also increases with frequency. Twisted Pair Communication & Networks 40
  41. 41.  used in token ring (4 or 16MBps), 10BaseT (Ethernet 10MBps), 100BaseT (100Mbps) reasonably cheap reasonably easy to terminate [special crimp connector tools are necessary for reliable operation] UTP often already installed in buildings UTP is prone to interference, and skin effect which limits speed and distances low to medium capacity medium to high loss category 2 = up to 1Mbps (Telephone wiring) category 3 = up to 10Mbps (Ethernet and 10BaseT) category 5 = 100MBps (supports 10BaseT and 100BaseT) Communication & Networks 41
  42. 42.  No skin effect and radiation effects at high frequencies maximum distance: 600 m , maximum speed 10 Mbps Applicable to both point to point and multipoint topologies limited only by the maximum transmission frequency through copper Communication & Networks 42
  43. 43.  medium capacity Ethernet systems (10Mbps) slighter dearer than UTP more difficult to terminate not as subject to interference as UTP care when bending and installing is needed 10Base2 uses RG-58AU (also called Thin-Net or Cheaper Net) 10Base5 uses a thicker solid core coaxial cable (also called Thick-Net) Communication & Networks 43
  44. 44.  Carries information in the form of a fluctuating beam of light in a glass fibre. ( light waves have a much higher maximum transmission frequency then electrical waves ) Maximum distance : a few Kilometres, maximum speed: 100 Mbps Immune to electromagnetic radiation : thus can be employed in electrically noisy environments Types available: ◦ Multimode Stepped index ◦ Multimode Graded index ◦ Monomode Stepped index. Reinforcing Sheath Material Cladding Optical Fiber Individual Fiber Jacket Communication & Networks 44
  45. 45.  relatively expensive used for backbones [linking LAN’s together] or FDDI rings (100Mbps) high capacity [100Mbps] immune to electromagnetic interference and degrading low loss difficult to join (renders it more secure) connectors are expensive long distance Communication & Networks 45
  46. 46. cladding jacket Uses the principle of total interface internal refraction: when light passes from a more dense to a lighter dense medium  core Optical Optical transmitter receiver Unrefracted ray Impulse response Normal 2 The pulse is widened since not all theLess dense medium n2 Refracted ray rays starting at the same point takeMore dense medium n1 the same path and thus arrive at different time intervals 1 Incident ray Communication & Networks 46
  47. 47.  Advantages ◦ Multimode step-index fibers are inexpensive and simple to manufacture. ◦ It is easy to couple light into and out of multimode step-index fibers; they have a relatively large source-to-fiber aperture. Disadvantages ◦ Light rays take many different paths down the fiber, which results in large differences in their propagation times. Because of this, rays traveling down this type of fiber have a tendency to spread out. Consequently, a pulse of light propagating down a multi-mode step-index fiber is distorted more than with the other types of fibers. ◦ The bandwidth and rate of information transfer possible with this type of cable are less than the other types. Communication & Networks 47
  48. 48. cladding jacket interface The refractive index of thecore is decreased outwardly  so as to provide a gradual core Optical Optical change in direction of the transmitter receiver incident light Impulse response Decreasing refractive Index Communication & Networks 48
  49. 49.  Essentially, there are no outstanding advantages or disadvantages of this type of fiber. Multimode graded- index fibers are easier to couple light into and out of than single-mode step-index fibers but more difficult than multimode step-index fibers. Distortion due to multiple propagation paths is greater than in single- mode step-index fibers but less than in multimode step-index fibers. Graded-index fibers are easier to manufacture than single-mode step-index fibers but more difficult than multimode step-index fibers. The multi-mode graded-index fiber is considered an intermediate fiber compared to the other types. Communication & Networks 49
  50. 50. cladding jacket interface  core Optical Optical transmitter receiver Impulse response Here light travels directly to destination or with some total internal refraction. The power of the light source must be higher because of the small acceptance angle. Thus lasers are normally used as light sources instead of LED’s or ILD’s. Communication & Networks 50
  51. 51.  There is minimum dispersion. Because all rays propagating down the fiber take approximately the same path, they take approximately the same amount of time to travel down the cable. Consequently, a pulse of light entering the cable can be reproduced at the receiving end very accurately. Because of the high accuracy in reproducing transmitted pulses at the receive end, larger bandwidths and higher information transmission rates are possible with single- mode step-index fibers than with the other types of fibers. Communication & Networks 51
  52. 52.  Because the central core is very small, it is difficult to couple light into and out of this type of fiber. The source- to-fiber aperture is the smallest of all the fiber types. Again, because of the small central core, a highly directive light source such as a laser is required to couple light into a single-mode step-index fiber. Single-mode step-index fibers are expensive and difficult to manufacture. Communication & Networks 52
  53. 53.  Terrestial Microwaves ◦ These are used in remote places where cables are difficult to reach ◦ Maximum distance: 50 Km. Radio ◦ Lower frequency radio transmission is also used in place of fixed wire links over more modest distances using ground-based transmitters and receivers such as wi-fi. F1 F2 F3 F1 Radio field coverage of base stationF2 F3 F1 F2 F3 Fixed network Base Station F1 F2 F3 F1 User computers 53 Communication & Networks
  54. 54.  Any signal carried on a transmission medium Transmitted Data 0 1 0 0 1 1 0 1 will be affected by Transmitted Signal attenuation and noise. time Caused by Attenuation time Line (system) noise time Combined received Signal time Sampling Instants Received Data 0 1 0 0 1 0 0 1 Bit error Communication & Networks 54
  55. 55.  As a signal propagates along a transmission medium (line) its amplitude decreases due to signal attenuation. For long cables , amplifiers - also known as repeaters must be inserted at intervals along the cable to restore the received signal to its original level. Attenuation increases with frequency and since a signal comprises a range of frequencies amplifiers must be designed to amplify different frequency signals by varying amounts. Alterenatively equalizers are used to equalize the attenuation across a defined band of frequencies. Communication & Networks 55
  56. 56.  The frequency of a channel is limited by the bandwidth of the physical circuit. The bandwidth of a channel is the range of frequencies that the circuit can pass without heavy attenuation. Signals whose frequency is out of this region are attenuated Gain EXAMPLE : Telephone Line 1 Bandwidth Bandwidth 3000Hz frequency 0 fL= 300Hz fH= 3300Hz Lower Cutoff frequency Upper Cutoff frequency Communication & Networks 56
  57. 57.  In the absence of a signal, a transmission line will ideally have zero electrical signal present. In practice, however, there will be random perturbations on the line. This is known as the line noise level. In the limit, as a transmitted signal becomes attenuated, its level is reduced to that of the line (background) noise. ◦ Impulse Noise is caused by impulses of electrical energy associated with external activity. ◦ Thermal Noise is caused by the thermal agitation of electrons in the transmission line material. This type is also known as White noise. An important parameter associated with a transmission medium, therefore, is the ratio of the power in a received signal, S, to the power in the noise level, N. The ratio S/N is known as the signal-to-noise ratio and is normally expressed in bB. Communication & Networks 57
  58. 58.  The bit rate is the number of bits (1’s or 0’s) transmitted per second whilst Baud rate is the number (or frequency) of signalling elements per second. Nyquist showed that the maximum data transfer rate C of a line of bandwidth B, assuming M levels per signalling element is given by: C = 2.B.log2M bps. The Bandwidth is a measure of frequency which takes into account a whole wave cycle. So if with had just 2 possible levels per signaling element with would have 11 1 10 a maximum bit rate of 2.B. 01 0 With 4 levels per signaling element, 2 bits can be sent 00 per signaling element and thus the bit rate becomes 2.B.2 Communication & Networks 58
  59. 59.  A modem to be used with a PSTN uses an AM-PSK modulation scheme with eight levels per signalling element. If the bandwidth of the PSTN is 3100 Hz, deduce maximum data transfer rate. C = 2.B.log2M = 2 x 3100 x log28 = 2 x 3100 x 3 Therefore C = 18600 bps In fact the data transfer rate will be less than this because of other effects such as noise. Communication & Networks 59
  60. 60.  The voltage inside a digital computer systems are mainly TTL (Transistor Transistor Logic) with two nominal voltages – a 0V represents the logic level 0 and 5V represents the logic level 1 In practice there are two ranges to represent such levels – voltages below 0.8V are considered a 0 and all voltages above 2V are considered as 1, 5.0 V 1 representation 2.0 V Intermediate 0.8 V 0 representation 0.2 V Internal binary representation (TTL) Communication & Networks 60
  61. 61. Cutajar & Cutajar
  62. 62.  Although the analogue PSTN was designed specifically for voice communications, it is also possible to transmit data using a modem. In the case of ISDN, calls can be set up and data transmitted directly with a much higher bit rate. In the case of leased circuits, although in some circumstances it is still necessary to use leased PSTN lines – and hence modems – in most cases leased circuits are now all-digital. Communication & Networks 62
  63. 63. It is necessary to convert the binary data into a formcompatible with a speech signal at the sending end of theline and to reconvert this signal back into its binary format the receiver. The circuit that performs the firstconversion is called a modulator whilst the inverse functionis performed by a demodulator. DTE PSTN Modem Telephone Communication & Networks 63
  64. 64.  Various types of modulation are employed for converting signals into a form suitable for transmission on a PSTN. ◦ Amplitude Modulation (AM) ◦ Frequency Modulation (FM) ◦ Phase Modulation (PM) In converting binary signals keying is used and thus the modulation techniques used are: ◦ Amplitude Shift Keying (ASK) Data ◦ Frequency Shift Keying (FSK) Carrier Communication & Networks 64
  65. 65. The level or amplitude of a single frequency audio tone(carrier) switched or keyed between two levels at a ratedetermined by the transmitted binary data signal.Although the simplest type it is too much affected bysignal attenuation. 1 0 1 1 0 0 1 0 Binary signal AM Communication & Networks 65
  66. 66. The frequency of a fixed amplitude carrier signal ischanged according to the binary stream to be transmitted.Since only two frequencies ( audio tones ) are used forbinary data, this type of modulation is also known as digitalFM or frequency-shift keying (FSK). 1 0 1 1 0 0 1 0 Binary signal FM Communication & Networks 66
  67. 67.  Let us consider we can share the bandwidth of a particular medium by different channels, using modulation. The bandwidth occupied by a particular channel depends on the type of modulation used and the maximum bit rate of the channel. Bandwidth determined by the bit rate and modulation method used Signal Level Frequency F0 F1 Communication & Networks 67
  68. 68.  All the information relating to calls – voice and data – associated with most public carrier networks is now transmitted between the switching exchanges within the network in digital form. The resulting network is then known as an integrated services digital network or ISDN since the user can readily transmit data with voice without the use of modems. Voice transmissions are limited to a maximum bandwidth of less than 4KHz. To convert such signals into digital form, the Shannon’s sampling theorem states that their amplitude must be sampled at a minimum rate of twice the highest frequency component. Hence to convert a 4Khz voice signal into digital form, it must be sampled at 8000 times per second. Digital Communication & Networks 68
  69. 69. Analogue voice signal Time (A) (A) SamplingSampling circuit clock (B) (B) Pulse amplitude (C) modulated signal (PAM) (C) Quantization and companding (D) (D) Digitized voice signal Communication & Networks 69
  70. 70.  Voice communication tends to be short duration but Circuit Message Packet continuous. Computer Switching Switching Switching communication tends to be in burst with long periods of no transmission. Because of these differences, voice is often transmitted over a fixed, dedicated channel or circuit while data is normally transmitted in an occasional packet, as needed, over a temporary or shared channel. Communication & Networks 70
  71. 71.  Placing a phone call builds a physical path or circuit from your phone to the receivers. When you hang up, the circuit is broken and intermediate channels are then available for other circuits to be built for other phone calls. The circuit from sender to receiver is dedicated during the communication interval, so no intermediate storage is required. However, the sender must wait for the circuit to the receiver to be constructed before transmission can start. Delay is a function of the time required to acquire exclusive use of the channel. Communication & Networks 71
  72. 72.  The communication channel is shared, with a message occupying the complete channel during transmission. The entire message is sent at once to an intermediate switch so there is no wait for circuit construction all the way to the receiver. However, the switch must be able to store and forward the entire message, placing an upper limit on the size of message that can be transmitted to the lowest switch capacity along the path. Because a message occupies the complete channel during transmission, large messages can cause considerable delay for other users waiting to send messages. Also, since errors occasionally occur and large messages are more likely to contain an error than small ones, handling errors by resending the message is potentially very costly. Communication & Networks 72
  73. 73.  The channel is again shared. The message is broken up by the sender into smaller packets of a maximum size that can be handled by the intermediate switches. The switch stores each packet and forwards to another switch along the way or to the receiver if directly connected. Switches can receive and send packets simultaneously, unlike message switching which must receive the entire message before forwarding. This reduces the overall time required to receive the complete message since initial packets can be sent on the communications channel without waiting for the complete message. When errors occur only the bad packet must be corrected (usually by resending) rather than the complete message. Since the channel is shared, no one user has exclusive control, other users packets can be multiplexed onto the same channel, small packets reduce the delay for other users sharing the channel. Communication & Networks 73
  74. 74. Cutajar & Cutajar
  75. 75.  Here we are concerned with the mode of operation of the different types of computer network that are used to interconnect a distributed community and their various interface standards and protocols. When the computers are distributed over a localized area – such as a building – the network used is known as a Local Area Network (LAN). Many LAN’s are linked together to form a Metropolitan Area Network (MAN). When the computers are distributed over a wider geographical area – such as a country – the network is known as a Wide Area Network (WAN) Communication & Networks 75
  76. 76.  LANs are used to interconnect distributed communities of computer-based DTEs located within say a single establishment. LANs are also referred to as private data networks as they are normally installed and maintained by a single organization. There are two quite different types of LAN: ◦ Wired LANs ◦ Wireless LANs We shall consider mostly the first type of LAN Communication & Networks 76
  77. 77.  The most common network topologies found are: ◦ Mesh - sometimes referred to as distributed or network ◦ Star – All computers connected to a central node. ◦ Bus – A common bus cable links all computers ◦ Ring – All computers are linked to form a ring of computers Communication & Networks 77
  78. 78.  Most WANs, such as the PSTN, use a mesh (sometimes referred to as a network), However, with LANs the limited physical separation of the DTEs permits simpler topologies as the other four mentioned. There are two types of mesh topologies: full mesh and partial mesh:  Full mesh topology occurs when every node has a circuit connecting it to every other node in a network. Full mesh is very expensive to implement but yields the greatest amount of redundancy, so in the event that one of those nodes fails, network traffic can be directed to any of the other nodes. Full mesh is usually reserved for backbone networks.  Partial mesh topology is less expensive to implement and yields less redundancy than full mesh topology. With partial mesh, some nodes are organized in a full mesh scheme but others are only connected to one or two in the network. Partial mesh topology is commonly found in peripheral networks connected to a full meshed backbone. Communication & Networks 78
  79. 79.  The best example of a LAN based on a star topology is the digital Private Automatic Branch Exchange (PABX). The need of modems are eliminated in modern PABXs by the use of digital-witching techniques within the exchange and are therefore referred to private digital exchanges (PDXs) Communication & Networks 79
  80. 80.  Typically, with a bus topology the network cable is routed through all those locations that have a DTE to be connected to the network and a physical connection (tap) is made. Appropriate medium access control (MAC) circuitry and algorithms are then used to share the available transmission bus among the various DTEs attached. Bus extenders are used to link various bus sections Bus Bus extender Communication & Networks 80
  81. 81.  With a ring topology, the network cable passes from one DTE to another until the DTEs are interconnected in the form of a loop or ring. The ring is unidirectional in operation and appropriate MAC algorithms ensure the correct shared use of the ring. DTE Communication & Networks 81
  82. 82.  When a communication path is established between two DTEs through a star network, the central controlling node ensures that the transmission path between the two DTEs is reserved for the duration of the call. However, with both ring and bus topologies this control is distributed among the DTEs attached to the common transmission path. Two most common techniques adopted are: ◦ Carrier Sense Multiple-Access (CSMA) for bus topologies. It’s my ◦ Control Token for bus or ring networks turn ◦ Slotted versions of the above two. Communication & Networks 82
  83. 83.  Carrier Sensing Multiple Access with Collision Detection (CSMA/CD): In this method if a collision is detected between two transmitting DTEs, transmission is aborted and after a certain back-off time, retransmission is attempted . Communication & Networks 83
  84. 84.  In CSMA, two DTEs can attempt to transmit a frame over the cable at the same time, causing data from both sources to get corrupted (collision). To reduce this possibility, before transmitting, the source DTE senses the cable to check if a carrier is already present on the common line (frame in transit). If a carrier is sensed (CS), the DTE defers the transmission until the passing frame has been transmitted. A A A A C Communication & Networks 84
  85. 85.  All DTEs are connected directly to the same cable, which is said to operate in Multiple Access (MA) mode.  To transmit data the sending DTE first encapsulates the data in a frame headed with the destination address. The frame is then broadcast on the bus. All stations listen to the broadcast and compare the destination with their A B C own address. If it A C A C matches, they continue copying all the data in the frame. D E Communication & Networks 85
  86. 86. A t = t B Even so, two DTEs wishing to transmit a frame simultaneously sense no carrier and start transmitting simultaneously. A t = tp -t B A DTE monitors the data signal on the cable when transmitting the contents of a frame on the cable. If the transmitted and monitored signals are different, a collision is assumed to have occurred – Collision Detection. A t = tp B To ensure that the colliding parties are all aware of the collision a random bit pattern (jam sequence) is sent by the DTE detecting the collision. A t = 2tp B The stations involved back-off for a certain random time and then retry the transmission. tp = worst case delay Communication & Networks 86
  87. 87.  In the event of a collision, retransmission of the frame is attempted up to a defined maximum number of tries known as the attempt limit. Since overloading the network leads to the network breakdown, the MAC unit tries to adjust the load by progressively increasing the time delay between repeated retransmission attempts. The scheduling of retransmissions is controlled by a process called truncated binary exponential backoff. When transmission of the jam sequence is over, and assuming the attempt limit has not been reached, the MAC unit backs off a random integral number R of slot times which is given by: 0  R  2K where K = min{N, backoff limit} Thus the backoff range doubles with every attempt until the backoff limit is reached. Communication & Networks 87
  88. 88. Set Status to frame ready for NOT OK transmission ? Format frame Compute and wait Yes for transmission backoff time No Yes Carrier Attempts limit signal on ? reached? Complete Notransmission and Start transmittingset status to OK after interframe gap Transmit jam sequence Increment attempts No Yes Collision detected ? Communication & Networks 88
  89. 89.  Another way of controlling A access to a shared transmission medium is by a control token (permission). D Token-ring B This token is passed from one D B Token DTE to another according to a defined set of rules. A DTE may transmit a frame only when it is C in possession of the token and, A after it has transmitted the Token frame, it passes the token on to D Token-ring B allow another DTE to access D B the transmission medium. C Communication & Networks 89
  90. 90.  The frame is repeated (that is, each bit is received and then transmitted) by all DTEs in the ring until it circulates back to the initiating DTE, where it is removed. In addition to repeating the frame, the intended recipient retains a copy of the frame and indicate that it has done so by setting the response bits at the end of the frame. A Sender DTE releases the token in one of two ways: ◦ The token is released only after the frame comes back and the response bits are received. ◦ The token is released after transmission of the last bit of the frame ( early token release ) Communication & Networks 90
  91. 91.  Monitoring functions within the May I have active DTEs connected to the another token physical medium provide the please ? basis for initialization and recovery, both of the connection and the logical ring and from loss of token. Although the monitoring functions are normally replicated among all the DTEs on the medium, only one DTE at a time May I have another ring please ? carries the responsibility for recovery and reinitialization. Communication & Networks 91
  92. 92.  The Physical medium need not be a ring topology; a token can also be used to control access to a bus network. Thus we can have: ◦ A token ring and ◦ A token bus. Physical Logical Communication & Networks 92
  93. 93. S  After reading the data the receiving DTE modifies the pair of response bits. S 01 11  If the DTE is inoperable, the response bits ACK remain unchanged.  The Sender reads back the frame, checks the S response bits and releases the token. 11 Inoperable11 8 8 N 11 DESTINATION SOURCE 10 11 ADDRESS ADDRESS DATA NAK Monitor Passed Bit S (Acknowledge) Response bits: 00 Busy Start of Packet 01 Accepted 10 Rejected 11 Ignored (not working) 00 11 Busy Communication & Networks 93
  94. 94.  The monitor node, after initializing the ring with a fixed number of empty slots, ensures that the number of bits in the ring remain constant. The monitor passed bit is used by the monitor to detect whether a DTE fails to release the slot after transmitting the frame. The monitor node is the vulnerable node of the ring network. Frame segmentation and monitor vulnerability are the weak points of this type of network. Monitor passed bit = 1 Empty Slot Monitor Monitor Monitor passed bit = 0 Monitor passed bit = 1 Communication & Networks 94
  95. 95.  There is a single token and only the possessor of the token can transmit a frame. All DTEs that can initiate the transmission of a frame are linked in the form of a logical ring. P=F P =A P=B  The token is passed physically using S=B S=C S=D the bus around the logical ring. A B C  On receipt of the token from its predecessor (upstream neighbor) on the logical ring, a DTE may transmit any waiting frames up to a defined F E D maximum. logical  It then passes the token to its known ring P=E P=D P=C successor (downstream neighbor) on S =A S=F S=E the logical ring. Communication & Networks 95
  96. 96.  The three MAC standards together with their associated physical media specifications are contained in the following IEEE standards documents: ISO RM  IEEE 802..3 CSMA/CD bus Network Layer  IEEE 802.4 Token bus Logical link  IEEE 802.5 Token ring control 802.2 Data link Layer  IEEE 802..11 Wireless Medium access control 802.3 802.4 802.5 802.11 IEEE 802 Physical Layer Physical Transmission Medium Communication & Networks 96
  97. 97. Cutajar & Cutajar
  98. 98.  To ensure that the information received by the receiver is the same as that transmitted by the transmitter there must be a way for the receiver to deduce , to a high probability when the received information contains errors. Furthermore, should errors be detected, a mechanism is needed to obtain a (hopefully) correct copy of the information. There are two approaches for achieving this: ◦ Forward error control: in which each transmitted character or frame contain additional (redundant) information so that the receiver can, not only detect when errors are present but also determine where in the received bit stream the errors are. The data can thus be corrected. ◦ Backward error control: in which each character or frame includes only sufficient additional information to enable the receiver to detect when errors are present but not their location. A retransmission control scheme is then used to request another hopefully correct copy. Communication & Networks 98
  99. 99. The most common method used for detecting bit errors with asynchronousand character oriented transmission is the parity bit method. With thismethod the transmitter adds an additional bit – the parity bit – to eachtransmitted character prior to transmission.The parity bit used is a functionof the bits that make up the character being transmitted, such that it can berecomputed by the receiver to verify the correctness of the characterreceived. Transmitted character Start bit Stop bits Parity bit 1001001 1 (even parity) 1001001 0 (odd parity) Communication & Networks 99
  100. 100. To compute the parity bit for a character, the number of 1 bits in the codefor the character are added together (modulo 2) and the parity bit is thenchosen so that the total number of bits (including the parity bit itself) iseither even (even parity) or odd (odd parity). (EXAMPLE 1001001)(1) B0 (1)(0)  (1) B1  (0) B2 (0)  Odd Parity (0) B3  (0) (0) (1) B4 B5  (0)  Even Parity (0) B6 (1) (1) Communication & Networks 100
  101. 101.  Here when blocks of characters are being transmitted, an extension to the error detecting capabilities obtained by the use of a single parity bit per character can be achieved , using an additional set of parity bits computed from the complete block of characters in the frame. In addition to the standard parity check (transverse or row parity), an extra bit is computed for each bit position (longitudinal or column parity ). Communication & Networks 101
  102. 102. P B6 B5 B4 B3 B2 B1 B0 P B6 B5 B4 B3 B2 B1 B0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 STX 1 0 1 0 1 0 0 0 0 1 0 0 0 1 1 0 1 0 1 0 1 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 1 1 0 1 0 1 0 1 1 0 1 0 0 1 0 0 0 0 0 Frame 0 1 0 0 0 0 0 0 1 0 1 0 1 1 0 1 Contents 1 1 0 0 1 0 1 1 1 0 0 0 0 0 1 1 0 1 0 0 0 0 0 0 1 1 0 0 0 0 0 1 1 1 1 0 0 0 1 1 1 0 0 0 0 0 1 1 ETX 1 1 0 0 0 0 0 1 BCC Column Undetected Parity Error Row (even) CombinationParity Example (odd) Communication & Networks 102
  103. 103.  An alternative to retransmission of the blocks of data after an error has been detected, is to build sufficient redundancy into the code to enable the receiver to correct the error. The technique of detecting and correcting the errors using an error correction code is known as Forward error correction. The particular advantage of forward error correction is evident when there is a long propagation delay, and thus since retransmission of the message is remote, a lot of time is saved. This means that a continuous stream of data can be transmitted with only a few interruptions for retransmissions. An error correcting code can normally detect more errors than it can correct. This scheme can detect single and double bit errors. Communication & Networks 103
  104. 104.  In this case the most common alternative is based on the use of polynomial codes. Simply said, The transmitter divides the message in binary by another number (Generating Polynomial) and appends the remainder to the tail of the message. The receiver performs the same operation to check if it obtains the same remainder. If the remainders agree, the message is assumed to be correct. The computed check digits are referred to as the frame check sequence (FCS) or the cyclic redundancy check (CRC) digits. Communication & Networks 104
  105. 105. Cutajar & Cutajar
  106. 106.  Error control is only one component of a data link protocol. Another important and related component is Flow control. As the name implies, it is concerned with controlling the rate of transmission of frames on a link so that the receiver always has sufficient buffer storage resources to accept them prior to processing. Enough !! Communication & Networks 106
  107. 107.  A flow control facility is often invoked to ensure that a terminal does not send any further characters until an overload condition has been cleared. This mechanism is achieved by the computer sending a special control character X-OFF to the controlling device within the terminal instructioning it to cease transmission. When the overload condition Computer Terminal ends and the computer becomes available to accept further characters, it returns a companion control character X-ON to inform the terminal X-OFF control device that it may restart sending characters. X-ON This is known as handshaking. Communication & Networks 107
  108. 108. In practice there are two basic types of ARQ: Idle RQ: used with character-oriented data transmission schemes, implemented in either: ◦ Implicit Request or ◦ Explicit Request. Continuous RQ: used with bit-oriented transmission schemes and employs either: ◦ Selective repeat or ◦ Go-back-N retransmission strategies. Communication & Networks 108
  109. 109.  The idle RQ error control scheme has been defined to enable blocks of printable and formatting control chacters to be reliably transferred – ie, to a high probability, without error or replication and in the same sequence as they were submitted. The information ( I-frames ) is transmitted here between the sender (primary [P]) and the receiver (secondary [S]) DTE’s across a serial data link. It operates in a half-duplex mode since the primary after sending and I-frame, must wait until it receives an indication from the scondary as to whether the frame was correctly received or not. The primary then either sends the next frame, if the previous frame was correctly received, or retransmits a copy of the previous frame if it was not. Communication & Networks 109
  110. 110.  There are two ways of implementing this sheme. In implicit retransmission S only acknowledges correctly received frames and P interprets the absence of an acknowledgement as an indication that the previous frame was corrupted. Alternatively, in explicit request, when S detects that a frame has been corrupted, it returns a negative acknowledgement to request another copy of the frame. ? message OK message Implicit message OK message NOT OK Explicit Communication & Networks 110
  111. 111. The following can be noted from the following slides : P can have only one I-frame outstanding ( awaiting an acknowledgement or ACK-frame) at a time; On receipt of an error-free I-frame, S returns an ACK-frame to P; On receipt of an error-free ACK frame, P can transmit another I-frame ; When P initiates the transmission of an I-frame it starts a timer; If S receives an I-frame or P receives an ACK-frame cantaining transmission errors, the frame is discarded; If P does not receive an ACK-frame within a predefined time interval (the timeout interval), then P retransmits the waiting I-frame; If an ACK-frame is corrupted, then S receives another copy of the frame and hence this is discarded by S; Communication & Networks 111
  112. 112. Note that:  P can have only one I-frame outstanding ( awaiting an ACK-frame) at a time;  On receipt of an error-free I-frame, S returns an ACK-frame to P;  On receipt of an error-free ACK frame, P can transmit another I-frame ;  When P initiates the transmission of an I-frame it starts a timer; stop stop Timerstart start start I(N) I(N+1) I(N+2) Primary P I(N) I(N+1) I(N+2) ACK(N) ACK(N+1) I(N) I(N+1) Secondary S Communication & Networks 112
  113. 113.  If S receives an I-frame or P receives an ACK-frame cantaining transmission errors, the frame is discarded. expired stop Timerstart start I(N) I(N) Primary P I(N) I(N) ACK(N) I(N) I(N) Secondary S Communication & Networks 113
  114. 114.  If P does not receive an ACK-frame within a predefined time interval (the timeout interval), then P assumes that the message has not been received correctly and retransmits the waiting I-frame. If an ACK-frame is corrupted, then S receives another copy of the frame and hence this is discarded by S; expired stop Timerstart start I(N) I(N) Primary P I(N) I(N) ACK(N) ACK(N) I(N) I(N) Secondary S Duplicated Message (discarded) Communication & Networks 114
  115. 115.  As with implicit acknowledgement sheme, on receipt of an error free I-frame, S returns an ACK-frame to P; On receipt of an ACK-frame, P stops the timer and can then initiate the transmission of another I-frame. If S receives an I-frame containing transmission errors, the frame is discarded an it returns a NAK ( negative acknowledgement) frame. If P does not receive an ACK-frame ( or NAK-frame) within the timeout interval, P retransmits the waiting I- frame. Communication & Networks 115
  116. 116.  If S receives an I-frame containing transmission errors, the frame is discarded an it returns a NAK (negative acknowledgement) frame. stop stop Timerstart start start I(N) I(N) I(N+1) Primary P I(N) I(N) I(N+1) NAK(N) ACK(N) I(N) I(N) Secondary S Communication & Networks 116
  117. 117.  Since with the idle RQ scheme the primary must wait for an acknowledgement after sending a frame, it is also known as Stop-and- Wait. With both schemes however, it is possible for S to receive two or more copies a of a particular I-frame (duplicates). In ordeer for S to discriminate between the next vaild I-frame and a duplicate, each frame transmitted contains a unique identifier known as sequence number (N, N+1 etc). To enable P to resynchronize, S returns an ACK-frame for each correctly received frame with the related I-frame identifier within it. The sequence number carried in each I-frame is known as the send sequence number or N(S), and the sequence number in each ACK and NAK frame as the receive sequence number N(R) Communication & Networks 117
  118. 118. In continuous RQ, the primary continues to send messages without waiting for acknowledge ment. If something goes wrong there are two possible retransmission schemes: Selective Repeat: where only the message in error is retransmitted. This requires a certain amount of storage space on the receiver side, to be able to re-order the message sequence one the retransmitted messages arrives. Go-Back-N: where all the messages from the erroneous message onwards are retransmitted. This requires no storage space on the receiver side. Communication & Networks 118
  119. 119. N N+1 N+2 N+3 N+4 N+5 N+5 V(S) N+4 N+4 N+4 N+1 N+2 N+1 N+3 N+2 N+3 N+2 N+3 N+2 N+3 N+2 time N N N N+1 N+1 N+1 N+1Primary (P) I(N) I(N+1) I(N+2) I(N+3) I(N+4) I(N+1) I(N+2)Secondary (S) I(N) I(N+2) I(N+3) I(N+4) I(N+1) Discarded frames N N N N N N+1 N N+1 N+1 N+1 N+1 N+1 N+2 V(R) Communication & Networks 119
  120. 120. N N+1 N+2 N+3 N+4 N+5 N+5 V(S) N+4 N+1 N+2 N+1 N+3 N+2 N+3 N+2 N+1 N+4 N+1 time N N N N+1 N+1 N+3 N+4Primary (P) I(N) I(N+1) I(N+2) I(N+3) I(N+4) I(N+1)Secondary (S) I(N) I(N+2) I(N+3) I(N+4) I(N+1) N N+2 N+2 N+2 N+2 N+3 N+3 N+3 N+4 N+4 N+1 N N+1 N+1 N+1 N+1 N+1 N+5 V(R) Communication & Networks 120
  121. 121. Cutajar & Cutajar
  122. 122.  The distances which can be covered by a single LAN Network are limited and frequently there is a requirement to driven extend this range. This maybe due to: ◦ Partitioning the whole network into groups of separate entities for security reasons or to improve the Multivendor OS driven Integration performance of the network. ◦ Coupling together existing entities and form a new cohesive structure. These may have been installed as separate initiatives aimed at resolving unique Application driven requirements and thus be from different vendors. Thus OSI MODEL we speak of multivendor integration. The approach Application taken in the integration of these computers can take Interoperability Presentation various viewpoints. Session Multivendor Transport Internetworking + Interoperability = Integration Internetworking Network Data link Physical Communication & Networks 122
  123. 123.  Each layer acts as though it is communicating with its corresponding layer on the other end. A B  In reality, data is passed from one layer down to the USER USER 7 7 next lower layer at Data Application Layer 6 Presentation Layer 6 the sending Headers AH Data 5 Session Layer 5 computer, till its PH AH Data Tails 4 Transport Layer 4 finally transmitted SH PH AH Data 3 Network Layer 3 TH SH PH AH Data onto the network 2 Data Link Layer 2 NH TH SH PH AH Data NT cable by the DH NH TH SH PH AH Data NT DT 1 Physical Layer 1 Physical Layer. FH DH NH TH SH PH AH Data NT DT FT As the data is passed down to a lower layer, it is encapsulated into a larger unit (in effect, each layer adds its own layer information to that which it receives from a higher layer). At the receiving end, the message is passed upwards to the desired layer, and as it passes upwards through each layer, the encapsulation information is stripped off . Communication & Networks 123
  124. 124.  Summary of Repeater features ◦ increase traffic on segments ◦ have distance limitations ◦ limitations on the number that can be used ◦ propagate errors in the network ◦ cannot be administered or controlled via remote access ◦ cannot loop back to itself (must be unique single paths) ◦ no traffic isolation or filtering Repeaters also allow isolation of segments in the event of failures or fault conditions. Disconnecting one Repeater Repeater side of a repeater effectively isolates the associated segments from the network. Communication & Networks 124

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