Jaimin chp-2 - 2011 batch


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GTU-MCA-SEM IV - Fundamentals of Networking

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Jaimin chp-2 - 2011 batch

  1. 1. The Physical Layer Chapter 2
  2. 2. The Theoretical Basis for Data Communication <ul><li>Fourier Analysis </li></ul><ul><li>Bandwidth-Limited Signals </li></ul><ul><li>Maximum Data Rate of a Channel </li></ul>
  3. 3. Bandwidth-Limited Signals <ul><li>A binary signal and its root-mean-square Fourier amplitudes. </li></ul><ul><li>(b) – (c) Successive approximations to the original signal. </li></ul>
  4. 4. Bandwidth-Limited Signals (2) <ul><li>(d) – (e) Successive approximations to the original signal. </li></ul>
  5. 5. Bandwidth-Limited Signals (3) <ul><li>Relation between data rate and harmonics. </li></ul>
  6. 6. Guided Transmission Data <ul><li>Magnetic Media </li></ul><ul><li>Twisted Pair </li></ul><ul><li>Coaxial Cable </li></ul><ul><li>Fiber Optics </li></ul>
  7. 7. Transmission Media (1) <ul><li>Magnetic media </li></ul><ul><ul><li>Tapes, diskettes </li></ul></ul><ul><ul><li>High bandwidth </li></ul></ul><ul><ul><li>A 8 mm tape = 7 GB  A 50*50*50 Cm box = 1000 tapes =7000 GB </li></ul></ul><ul><ul><li>7000GB/24 Hrs= 648 Mbps 7000GB/1Hr=15Gbps </li></ul></ul><ul><ul><li>Sometimes it's cheaper and faster to load a box of tapes in your car! </li></ul></ul><ul><ul><li>Problem: Delay ! </li></ul></ul><ul><li>Twisted pair (1) </li></ul><ul><ul><li>Simply two wires twisted together – thickness=1mm </li></ul></ul><ul><ul><li>The twisting cuts down on electrical interference . </li></ul></ul><ul><ul><li>Heavily used in the phone system </li></ul></ul><ul><ul><li>Typical office has four pairs for phones. </li></ul></ul><ul><ul><li>Until some Kilometers/ Some Mbps </li></ul></ul><ul><ul><li>For Analog and Digital </li></ul></ul>
  8. 8. Transmission Media (2) <ul><li>Twisted pair (2) </li></ul><ul><ul><li>Bandwidth depends on thickness and distance </li></ul></ul><ul><ul><li>Need repeater for long distances </li></ul></ul><ul><ul><li>Category 3 and 5 - with 5 having more twists and better insulation. </li></ul></ul><ul><ul><li>Popular by UTP (Unshielded Twisted Pair) </li></ul></ul><ul><ul><li>Cat3 Cat 5 </li></ul></ul>
  9. 9. Twisted Pair <ul><li>Waves from different twists cancel out , so the wire radiates less effectively. The more is the number of twists per cm lesser is the radiation. </li></ul><ul><li>They run for several Km without amplification </li></ul><ul><li>For longer distances repeaters are required. </li></ul><ul><li>Cat3 - 16 MHz </li></ul><ul><li>Cat5 - 100 MHz </li></ul><ul><li>Cat6 – 250 MHz for Gigabit Ethernets </li></ul><ul><li>Cat7 – 600 MHz </li></ul><ul><li>All these wirings are referred to as UTP (Unshielded Twisted Pair). </li></ul>
  10. 10. Twisted Pair <ul><li>(a) Category 3 UTP. </li></ul><ul><li>(b) Category 5 UTP. </li></ul>
  11. 11. Transmission Media (3) <ul><li>Baseband Coaxial cable </li></ul><ul><ul><li>Used for digital transmissions (called baseband.) </li></ul></ul><ul><ul><li>Good noise immunity. </li></ul></ul><ul><ul><li>Data rates as high as 2 Gbps for 1 Km distance. </li></ul></ul><ul><ul><li>Now being replaced by fiber. </li></ul></ul><ul><li>Broadband Coaxial cable </li></ul><ul><ul><li>Used for analog transmissions (called broadband.) </li></ul></ul><ul><ul><li>Can run 300 MHz for long distances. </li></ul></ul><ul><ul><li>Analog signaling has better S/N than digital signaling. </li></ul></ul><ul><ul><li>Interfaces must convert digital signals to analog and vice versa. </li></ul></ul><ul><ul><li>Designed for long distances - can use amplifiers. </li></ul></ul>
  12. 12. Coaxial Cable <ul><li>A coaxial cable. </li></ul><ul><li>Better shielding hence better noise immunity </li></ul><ul><li>High bandwidth up to 1GHz </li></ul><ul><li>Earlier used on long distance telephone lines (short distance is twisted pair), now replaced with optical fiber </li></ul><ul><li>Now used largely in cable TV and MANs. </li></ul>
  13. 13. Transmission Media (4) <ul><li>Fiber Optic (1) </li></ul><ul><ul><li>Transmission of light through fiber </li></ul></ul><ul><ul><li>Bandwidth more than 50,000 Gbps ! </li></ul></ul><ul><ul><li>But now restricted to 1Gbps! </li></ul></ul><ul><ul><li>Reason: Electrical and optical signal conversion </li></ul></ul><ul><ul><li>Including 3 components: </li></ul></ul><ul><ul><ul><li>Light source: Pulse of light=1, absence of light=0 </li></ul></ul></ul><ul><ul><ul><li>Transition medium: an ultra-thin fiber of glass </li></ul></ul></ul><ul><ul><ul><li>detector: generate an electrical pulse when light falls on it </li></ul></ul></ul><ul><ul><li>Similar to coax (without braid) </li></ul></ul>
  14. 14. Transmission Media (5) <ul><li>Fiber Optic (Conti…) </li></ul><ul><ul><li>Thickness of core: 8~10 microns or 50 microns </li></ul></ul><ul><ul><li>Two typically light sources: </li></ul></ul><ul><ul><ul><li>LED (Light Emitting Diode) </li></ul></ul></ul><ul><ul><ul><li>response time=1ns  data rate = 1Gbps </li></ul></ul></ul><ul><ul><ul><li>2. Semiconductor laser </li></ul></ul></ul>
  15. 15. Transmission Media (6) <ul><li>Fiber Optic (Conti…) </li></ul><ul><ul><li>Properties include total internal reflection and attenuation of particular frequencies. </li></ul></ul><ul><ul><li>Fiber Optic Networks - can be used for LANs and long-haul. </li></ul></ul><ul><ul><li>A fiber-optic LAN </li></ul></ul>
  16. 16. Fiber Optics <ul><li>(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. </li></ul><ul><li>(b) Light trapped by total internal reflection. </li></ul>
  17. 17. Transmission of Light through Fiber <ul><li>Attenuation of light through fiber in the infrared region. </li></ul>
  18. 18. Fiber Cables <ul><li>(a) Side view of a single fiber. </li></ul><ul><li>(b) End view of a sheath with three fibers. </li></ul>
  19. 19. Fiber Cables (2) <ul><li>A comparison of semiconductor diodes and LEDs as light sources. </li></ul><ul><li>Attenuation of light thru glass depends upon the wavelength of the light and the physical properties of the glass. </li></ul>
  20. 20. Fiber Optic Networks <ul><li>A fiber optic ring with active repeaters. </li></ul>
  21. 21. Ring with Active vs passive interface <ul><li>In active, since signals are being regenerated, computer-to-computer links can be km long, with virtually no limit on the total length of the ring </li></ul><ul><li>In passive, signal looses light at each juncture, hence number of computers and total length are greatly restricted. </li></ul>
  22. 22. Fiber Optic Networks (2) <ul><li>A passive star connection in a fiber optics network. </li></ul>
  23. 23. Transmission Media (7) <ul><li>Comparison of fiber optic and copper wire </li></ul>Copper Fiber Lower Higher Bandwidth 5 Km 30 KM Distance between repeaters High Low Interference - Smaller/Lighter Physical Bi-directional Uni-directional Flow
  24. 24. Transmission Media (8) <ul><li>Connector </li></ul><ul><li>Repeater </li></ul><ul><ul><li>Signal Regeneration </li></ul></ul><ul><ul><li>Clean up </li></ul></ul><ul><ul><li>Amplify </li></ul></ul><ul><ul><li>Distance Extension </li></ul></ul><ul><li>Hub </li></ul><ul><ul><li>Repeater functionality, plus... </li></ul></ul><ul><ul><ul><li>Concentration Point </li></ul></ul></ul><ul><ul><ul><li>Signal Distribution Device </li></ul></ul></ul><ul><ul><ul><li>Management Functions </li></ul></ul></ul>
  25. 25. Wireless Transmission <ul><li>The Electromagnetic Spectrum </li></ul><ul><li>Radio Transmission </li></ul><ul><li>Microwave Transmission </li></ul><ul><li>Infrared and Millimeter Waves </li></ul><ul><li>Lightwave Transmission </li></ul>
  26. 26. Wireless Transmission <ul><li>When electrons move, they create electromagnetic waves that travel thru space. </li></ul><ul><li>When an antenna of appropriate size is attached to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away. All wireless communication is based on this principle. </li></ul>
  27. 27. The Electromagnetic Spectrum <ul><li>The electromagnetic spectrum and its uses for communication. </li></ul>
  28. 28. Higher Frequency waves <ul><li>UV, X-ray and gamma rays can carry more information but, </li></ul><ul><ul><li>They are hard to produce and modulate </li></ul></ul><ul><ul><li>Do not propagate well thru buildings </li></ul></ul><ul><ul><li>And, are dangerous to living things </li></ul></ul>
  29. 29. Radio Waves <ul><li>Are easy to generate , can travel long distances and can penetrate buildings easily. </li></ul><ul><li>Are Omni-directional, i.e. they travel in all directions </li></ul><ul><ul><li>Adv : transmitter and receiver do not have to be aligned </li></ul></ul><ul><ul><li>Disadv : interference of signals : less secure : Govt license required to use particular frequency band </li></ul></ul><ul><li>The properties of RW are frequency dependent </li></ul><ul><ul><li>At low frequency : pass thru obstacles well but the power falls off sharply with distance from the source. </li></ul></ul><ul><ul><li>At high frequency : they travel in straight lines , bounce off obstcales, and absorbed by rain </li></ul></ul>
  30. 30. Radio Transmission <ul><li>(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. </li></ul><ul><li>(b) In the HF band, they bounce off the ionosphere. </li></ul>
  31. 31. Microwave transmission <ul><li>Above 100 MHz, the waves travel in nearly straight lines . </li></ul><ul><li>They do not pass thru buildings well </li></ul><ul><li>Concentrating all energy into a single beam gives a much higher SNR (signal-to-noise ratio) but, </li></ul><ul><li>The transmitting and receiving antennas must be aligned properly. </li></ul>
  32. 32. Microwave transmission <ul><li>Since MW travel in a straight line, if the towers are too far apart, the earth will get in the way, hence </li></ul><ul><li>Repeaters are required periodically. </li></ul><ul><li>The higher the towers are, the farther apart they can be. </li></ul>
  33. 33. Multi-path fading in MW <ul><li>Even though MW travel in a straight line .. there is some divergence in the space. </li></ul><ul><li>Some waves may be refracted off low-lying atmospheric layers and may take slightly longer to arrive than the direct waves. The delayed waves may arrive out of phase with the direct wave and thus cancel the signal. This effect is called multi-path fading. </li></ul><ul><li>It is a serious problem and is weather and frequency dependent. </li></ul><ul><li>The only solution is to do away with such frequencies and keep some frequencies spare to be used under such circumstances. </li></ul>
  34. 34. MW vs Fiber <ul><li>No right of way is needed for MW. </li></ul><ul><li>MW is relatively inexpensive as compared to fiber. </li></ul>
  35. 35. Application of MW <ul><li>Short range Networking </li></ul><ul><li>Example : WLL : Wireless Local Loop </li></ul>
  36. 36. Infrared and Millimeter waves <ul><li>For short range </li></ul><ul><li>Directional </li></ul><ul><li>Do not pass thru solid objects </li></ul><ul><li>Because of above properties .. No eavesdropping .. Hence secure .. No govt license reqd </li></ul>
  37. 37. Application of Infrared <ul><li>Applications of Infrared </li></ul><ul><ul><li>Remote control Home- appliances </li></ul></ul><ul><li>Applications of Millimeter </li></ul><ul><ul><li>Wireless Local Loop </li></ul></ul>
  38. 38. Completely different approach <ul><li>A completely different approach to allocating frequencies is not to allocate at all . Let everyone transmit at will but regulate the power so that stations have such a short range that they do not interfere with each other. </li></ul>
  39. 39. The ISM(Industrial, Scientific and Medical) band <ul><li>Low power, hence short range so that no interference from each other </li></ul><ul><li>For unlicensed usage : </li></ul><ul><ul><li>Garage door openers, </li></ul></ul><ul><ul><li>Cordless phones, </li></ul></ul><ul><ul><li>Radio-controlled toys, </li></ul></ul><ul><ul><li>Wireless mouse, </li></ul></ul><ul><ul><li>And numerous other wireless household devices use the ISM band </li></ul></ul>
  40. 40. Politics of the Electromagnetic Spectrum <ul><li>The ISM bands in the United States. </li></ul>FHSS 802.11 HR-DSSS 802.11a OFDM 802.11 a/g Device<1W power ex: Wireless KB,mouse,webcam etc
  41. 41. Lightwave Transmission <ul><li>Convection currents can interfere with laser communication systems. </li></ul><ul><li>A bidirectional system with two lasers is pictured here. </li></ul>
  42. 42. Communication Satellites <ul><li>Geostationary Satellites </li></ul><ul><li>Medium-Earth Orbit Satellites </li></ul><ul><li>Low-Earth Orbit Satellites </li></ul><ul><li>Satellites versus Fiber </li></ul>
  43. 43. Communication Satellites <ul><li>Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage. </li></ul>
  44. 44. Communication Satellites (2) <ul><li>The principal satellite bands. </li></ul>
  45. 45. Communication Satellites (3) <ul><li>VSATs using a hub. </li></ul>
  46. 46. Low-Earth Orbit Satellites Iridium <ul><li>(a) The Iridium satellites from six necklaces around the earth. </li></ul><ul><li>(b) 1628 moving cells cover the earth. </li></ul>
  47. 47. Globalstar <ul><li>(a) Relaying in space. </li></ul><ul><li>(b) Relaying on the ground. </li></ul>
  48. 48. Public Switched Telephone System <ul><li>Structure of the Telephone System </li></ul><ul><li>The Politics of Telephones </li></ul><ul><li>The Local Loop: Modems, ADSL and Wireless </li></ul><ul><li>Trunks and Multiplexing </li></ul><ul><li>Switching </li></ul>
  49. 49. Public Switched Telephone System (PSTN) (1) <ul><li>For connecting computers in near distances (in a company) run a cable between them=LAN </li></ul><ul><li>For long distances and more computers use existing telecommunication facilities = PSTN </li></ul><ul><ul><li>A cable running  faster than 10 9 bps </li></ul></ul><ul><ul><li>A dial-up line has max. 56 Kpds </li></ul></ul><ul><ul><li>Difference factor = 20,000 !! </li></ul></ul><ul><li>Transmission of voice and data on system </li></ul><ul><li>Computer system designer try to figure out how to use PSTN efficiently </li></ul>
  50. 50. PSTN (2) <ul><li>Telephone structures </li></ul><ul><ul><li>Fully interconnected </li></ul></ul><ul><ul><li>Centralized switch </li></ul></ul><ul><ul><li>Two-level hierarchy </li></ul></ul>
  51. 51. PSTN (3) <ul><li>A typical circuit route for a medium-distance call </li></ul><ul><li>Major Components of the Telephone System </li></ul><ul><ul><li>Local loops: Analog twisted pairs going to houses and businesses </li></ul></ul><ul><ul><li>Trunks: Digital fiber optics connecting the switching offices </li></ul></ul><ul><ul><li>Switching offices: Where calls are moved from one trunk to another </li></ul></ul>
  52. 52. PSTN (4) <ul><li>The use of analog and digital signals has pros and cons </li></ul>Analog Digital Signals Originally Increasingly Attenuation/Noise Low High Amplification/Regeneration Hard Easy Information loss Some Little Maintain - Easier/cheaper
  53. 53. PSTN (5) <ul><li>Transmission Impairments: </li></ul><ul><ul><li>Attenuation - the loss of energy as the signal propagates </li></ul></ul><ul><ul><li>Delay Distortion - different frequencies travel at different speeds so the wave form spreads out. </li></ul></ul><ul><ul><li>Noise - unwanted energy that combines with the signal - difficult to tell the signal from the noise. </li></ul></ul><ul><li>Modem </li></ul><ul><ul><li>A device that converts digital data to and from an analog signal for transmission over phone lines. </li></ul></ul>
  54. 54. PSTN (6) <ul><li>Modem </li></ul><ul><ul><li>Amplitude modulation: Two different amplitudes of sine wave are used to represent 1's and 0's. </li></ul></ul><ul><ul><li>Frequency modulation: Two (or more) different frequencies, close to the carrier frequency, are used. </li></ul></ul><ul><ul><li>Phase modulation: The phase of the sine wave is changed by some fixed amount. </li></ul></ul>Binary Signal
  55. 55. PSTN (7) <ul><li>The cost of a wire is independent of the bandwidth of that wire - costs come from installation and maintenance of the physical space (digging) </li></ul><ul><li>Frequency Division Multiplexing (FDM): </li></ul><ul><ul><li>The frequency spectrum is divided up among the logical channels - each user hangs on to a particular frequency. Note that this is analog stuff. </li></ul></ul>
  56. 56. PSTN (8) <ul><li>Wavelength Division Multiplexing (WDM): </li></ul><ul><ul><li>The same as FDM, but applied to fibers. There's great potential for fibers since the bandwidth is so huge (25,000 GHz). </li></ul></ul><ul><li>Time Division Multiplexing (TDM): </li></ul><ul><ul><li>In TDM, the users take turns, each one having exclusive use of the medium in a round robin fashion. TDM can be all digital . </li></ul></ul><ul><ul><li>Ex. T1=24 channels, each 8 bits =192 bits +1  1.544 Mbps </li></ul></ul>
  57. 57. PSTN (9) <ul><li>Switching </li></ul><ul><ul><li>Circuit Switching: A physical connection (electrical, optical, radio) is established from the caller phone to the calle phone. This happens BEFORE any data is sent. </li></ul></ul><ul><ul><li>Message Switching: The connection is determined only when there is actual data (a message) ready to be sent. The whole message is re-collected at each switch and then forwarded on to the next switch. This method is called store-and-forward . This method may tie up routers for long periods of time - not good for interactive traffic. </li></ul></ul><ul><ul><li>Packet Switching : Divides the message up into blocks ( packets ). Therefore packets use the transmission lines for only a short time period - allows for interactive traffic. </li></ul></ul>
  58. 58. <ul><li>Understanding Network Techniques </li></ul><ul><ul><li>Point-to-Point link is a dedicated line which provides an uninterrupted connection between the devices </li></ul></ul><ul><ul><ul><ul><li>Disadvantage : impractical due to long distance </li></ul></ul></ul></ul><ul><ul><ul><li>Alternative to Point-to-Point link are </li></ul></ul></ul><ul><ul><ul><ul><li>Switched Networks </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Circuit Switching </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Message Switching </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Packet Switching </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Cell Switching </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>Broadcast Networks </li></ul></ul></ul></ul>
  59. 59. <ul><li>A circuit switching network is one that establishes a circuit (or channel) between nodes and terminals before the users may communicate, as if the nodes were physically connected with an electrical circuit. </li></ul><ul><li>In circuit-switched networks, network resources are static, set in “copper” if you will, from the sender to receiver before the start of the transfer, thus creating a “circuit”. </li></ul><ul><li>The resources remain dedicated to the circuit during the entire transfer and the entire message follows the same path. </li></ul><ul><li>Advantage is that it provides for non-stop transfer without requiring packets and without most of the overhead traffic usually needed, making maximal and optimal use of available bandwidth for that communication. </li></ul><ul><li>Disadvantage of inflexibility tends to reserve it for specialized applications, particularly with the overwhelming proliferation of internet-related technology. </li></ul><ul><li>Examples of circuit switched networks </li></ul><ul><li>Public Switched Telephone Network (PSTN) </li></ul><ul><li>ISDN B-channel </li></ul><ul><li>Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD) service in cellular systems such as GSM </li></ul><ul><li>X.21 (Used in the German DATEX-L and Scandinavian DATEX circuit switched data network) </li></ul>
  60. 60. <ul><li>In a circuit-switched network, before communication can occur between two devices, a circuit is established between them. This is shown as a thick blue line for the conduit of data from Device  A  to Device  B , and a matching purple line from  B  back to  A . Once set up, all communication between these devices takes place over this circuit, even though there are other possible ways that data could conceivably be passed over the network of devices between them. </li></ul><ul><li>Circuit switching </li></ul><ul><li>3 phases </li></ul><ul><li>step 1 Establish a circuit </li></ul><ul><ul><ul><li>step 2 Actual data transfer </li></ul></ul></ul><ul><li>step 3 circuit disconnect </li></ul>
  61. 61. <ul><li>In this network type, no specific path is used for data transfer. </li></ul><ul><li>Instead, the data is chopped up into small pieces called  packets  and sent over the network. </li></ul><ul><li>The packets can be routed, combined or fragmented, as required to get them to their eventual destination. </li></ul><ul><li>On the receiving end, the process is reversed—the data is read from the packets and re-assembled into the form of the original data. </li></ul><ul><li>A packet-switched network is more analogous to the postal system than it is to the telephone system (though the comparison isn't perfect.) </li></ul>Packet switching
  62. 62. Packet Switching Communication broken down in to packets allows many users to share the same data path
  63. 63. <ul><li>Packet = Small unit of data </li></ul><ul><li>Packets are routed through network Communication broken down in to packets allow many users to share the same data path </li></ul><ul><li>In message switching network, message length can vary and can be thousands of bits long while on packet switching network messages are divided into packets are of same length </li></ul><ul><li>Pipelining </li></ul><ul><ul><li>Computer does not have to wait for instructions – when it completes execution of one anthemion, next is ready & waiting –similarly packets sent on a communication line as soon as the line is available </li></ul></ul>
  64. 65. <ul><li>Packet switching used 1 of the 3 methods listed below </li></ul><ul><li>Datagram </li></ul><ul><ul><li>Each packet is treated independently </li></ul></ul><ul><ul><li>packet sent to the same destination may not follow the same path </li></ul></ul><ul><ul><li>Packet arrived in wrong sequence </li></ul></ul><ul><ul><li>Responsibility of receiver to reorder </li></ul></ul><ul><ul><ul><li>when this process is used it is called datagram service </li></ul></ul></ul><ul><ul><ul><li>Flexible-quicker-reliable </li></ul></ul></ul><ul><li>SVC – Switched Virtual circuit </li></ul><ul><ul><ul><li>Logical connection is established between two nodes before any packet sent </li></ul></ul></ul><ul><ul><ul><li>connection is established by a call set up request sent by sender - SVC session establishment </li></ul></ul></ul><ul><ul><ul><li>Circuit will allocated for entire session </li></ul></ul></ul><ul><ul><ul><li>Packets have code attached – that identities the virtual circuit used </li></ul></ul></ul><ul><ul><ul><li>Route is determined before the packet is sent-so “no sorting. Decision necessary” </li></ul></ul></ul><ul><ul><ul><li>Svc is not equal to dedicated path it simply means that circuit is determined prior to data transmission </li></ul></ul></ul><ul><ul><ul><li>Call-clearing svc is dissolved at the end of the session is called call-clearing </li></ul></ul></ul>
  65. 66. <ul><li>PVC Permanent Virtual Circuit </li></ul><ul><li>When two nodes require an almost continuous connection than we use PVC </li></ul><ul><li>Circuit is allocated permanently between two nodes, so “ No call setup is needed ” </li></ul><ul><li>like leased line, PVC provides single continuous line </li></ul><ul><li>criteria to select which method to select depends Upon </li></ul><ul><ul><li>Length of time needed for transmission </li></ul></ul><ul><ul><li>number of packets to be sent </li></ul></ul><ul><ul><ul><li>For short messages use -> datagram </li></ul></ul></ul><ul><ul><ul><li>for long messages use -> SVC </li></ul></ul></ul><ul><ul><ul><li>Large amount of dater over long tome use -> PVC </li></ul></ul></ul>Advantage: The ability to have many devices communicate simultaneously without dedicated data paths is one reason why packet switching is becoming predominant today. Disadvantages: of packet switching compared to circuit switching. One is that since all data does not take the same, predictable path between devices, it is possible that some pieces of data may get lost in transit , or show up in the incorrect order. In some situations this does not matter, while in others it is very important indeed. Advantages and Disadvantages of Packet Switching Networks
  66. 67. Circuit switching Packet switching Circuit switching is old and expensive, and it is what PSTN uses. Packet switching is more modern. VoIP uses packet switching technology. The resources remain dedicated to the circuit during the entire transfer and the entire message follows the same path. There is no circuit dedication. The cost is shared. During the entire transfer the entire message follows the same path. In packet-switched networks, the message is broken into packets, each of which can take a different route to the destination where the packets are recompiled into the original message. Circuit-switching is more reliable than packet-switching. When you have a circuit dedicated for a session, you are sure to get all information across. When you use a circuit which is open for other services, then there is a big possibility of congestion (which is for a network what a traffic jam is for the road), and hence the delays or even packet loss. 
  67. 68. <ul><li>In telecommunications,  message switching  was the precursor of packet switching, where messages were routed in their entirety, one hop at a time . </li></ul><ul><li>It was first introduced by Leonard Kleinrock in 1961. </li></ul><ul><li>A form of store-and-forward network. </li></ul><ul><li>Data is transmitted into the network and stored in a switch. </li></ul><ul><li>The network transfers the data from switch to switch when it is convenient to do so, as such the data is not transferred in real-time. </li></ul><ul><li>Blocking can not occur, however, long delays can happen. The source and destination terminal need not be compatible, since conversions are done by the message switching networks. </li></ul><ul><li>Examples </li></ul><ul><li>  UUCP Unix-to-Unix Copy Program ,E-mail are examples of message switching systems. </li></ul>Message switching
  68. 69. <ul><li>Emphasis is on the messages moving across the network rather than on the actual circuit used </li></ul><ul><li>does not require a dedicated circuit, so the message is stored until the receiver connects to the network </li></ul><ul><li>The process of accepting message, storing them and then passing them on the appropriate network node is called store-and-forward (S-F) </li></ul><ul><li>Message passes through several intermediate nodes, so delay is introduced </li></ul>Message switching (continue...)
  69. 72. PSTN (10) <ul><ul><li>(a) Circuit switching. </li></ul></ul><ul><ul><li>(b) Packet switching. </li></ul></ul><ul><li>comparison </li></ul>
  70. 73. Structure of the Telephone System <ul><li>(a) Fully-interconnected network. </li></ul><ul><li>(b) Centralized switch. </li></ul><ul><li>(c) Two-level hierarchy. </li></ul>
  71. 74. Structure of the Telephone System (2) <ul><li>A typical circuit route for a medium-distance call. </li></ul>
  72. 75. Major Components of the Telephone System <ul><li>Local loops </li></ul><ul><ul><li>Analog twisted pairs going to houses and businesses </li></ul></ul><ul><li>Trunks </li></ul><ul><ul><li>Digital fiber optics connecting the switching offices </li></ul></ul><ul><li>Switching offices </li></ul><ul><ul><li>Where calls are moved from one trunk to another </li></ul></ul>
  73. 76. The Politics of Telephones <ul><li>The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it. </li></ul>
  74. 77. The Local Loop: Modems, ADSL, and Wireless <ul><li>The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs. </li></ul>
  75. 78. Modems <ul><li>(a) A binary signal </li></ul><ul><li>(b) Amplitude modulation </li></ul><ul><li>(c) Frequency modulation </li></ul><ul><li>(d) Phase modulation </li></ul>
  76. 79. Modems (2) <ul><li>(a) QPSK. </li></ul><ul><li>(b) QAM-16. </li></ul><ul><li>(c) QAM-64. </li></ul>
  77. 80. Modems (3) <ul><li>(a) V.32 for 9600 bps. </li></ul><ul><li>(b) V32 bis for 14,400 bps. </li></ul>(a) (b)
  78. 81. Digital Subscriber Lines <ul><li>Bandwidth versus distanced over category 3 UTP for DSL. </li></ul>
  79. 82. Digital Subscriber Lines (2) <ul><li>Operation of ADSL using discrete multitone modulation. </li></ul>
  80. 83. Digital Subscriber Lines (3) <ul><li>A typical ADSL equipment configuration. </li></ul>
  81. 84. Wireless Local Loops <ul><li>Architecture of an LMDS system. </li></ul>
  82. 85. Frequency Division Multiplexing <ul><li>(a) The original bandwidths. </li></ul><ul><li>(b) The bandwidths raised in frequency. </li></ul><ul><li>(b) The multiplexed channel. </li></ul>
  83. 86. Wavelength Division Multiplexing <ul><li>Wavelength division multiplexing. </li></ul>
  84. 87. Time Division Multiplexing <ul><li>The T1 carrier (1.544 Mbps). </li></ul>
  85. 88. Time Division Multiplexing (2) <ul><li>Delta modulation. </li></ul>
  86. 89. Time Division Multiplexing (3) <ul><li>Multiplexing T1 streams into higher carriers. </li></ul>
  87. 90. Time Division Multiplexing (4) <ul><li>Two back-to-back SONET frames. </li></ul>
  88. 91. Time Division Multiplexing (5) <ul><li>SONET and SDH multiplex rates. </li></ul>
  89. 92. Circuit Switching <ul><li>(a) Circuit switching. </li></ul><ul><li>(b) Packet switching. </li></ul>
  90. 93. Message Switching <ul><li>(a) Circuit switching (b) Message switching (c) Packet switching </li></ul>
  91. 94. Packet Switching <ul><li>A comparison of circuit switched and packet-switched networks. </li></ul>
  92. 95. The Mobile Telephone System <ul><li>First-Generation Mobile Phones: Analog Voice </li></ul><ul><li>Second-Generation Mobile Phones: Digital Voice </li></ul><ul><li>Third-Generation Mobile Phones: Digital Voice and Data </li></ul>
  93. 96. 2.6.1 First-Generation Mobile Phones: Analog Voice <ul><li>IMTS - Improved Mobile Telephone System. </li></ul><ul><li>It, too, used a high-powered (200-watt) transmitter, on top of a hill, </li></ul><ul><li>two frequencies, one for sending and one for receiving, </li></ul><ul><li>push-to-talk button was no longer needed. Since all communication from the mobile telephones went inbound on a different channel than the outbound signals, the mobile users could not hear each other (unlike the push-to-talk system used in taxis). </li></ul><ul><li>IMTS supported 23 channels spread out from 150 MHz to 450 MHz </li></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Due to the small number of channels, users often had to wait a long time before getting a dial tone. </li></ul></ul><ul><ul><li>Also, due to the large power of the hilltop transmitter, adjacent systems had to be several hundred kilometres apart to avoid interference. </li></ul></ul><ul><ul><li>limited capacity made the system impractical. </li></ul></ul><ul><li>All that changed with AMPS (Advanced Mobile Phone System), invented by Bell Labs and first installed in the United States in 1982. </li></ul>
  94. 97. AMPS (Advanced Mobile Phone System) <ul><li>In all mobile phone systems, a geographic region is divided up into cells, which is why the devices are sometimes called cell phones. </li></ul><ul><li>In AMPS, the cells are typically 10 to 20 km across; </li></ul><ul><li>in digital systems, the cells are smaller. </li></ul><ul><li>Each cell uses some set of frequencies not used by any of its neighbours. </li></ul><ul><li>The key idea that gives cellular systems far more capacity than previous systems is the use of relatively small cells and the reuse of transmission frequencies in nearby (but not adjacent) cells. </li></ul><ul><li>Whereas an IMTS system 100 km across can have one call on each frequency, an AMPS system might have 100 10-km cells in the same area and be able to have 10 to 15 calls on each frequency, in widely separated cells. </li></ul><ul><li>Thus, the cellular design increases the system capacity by at least an order of magnitude, more as the cells get smaller. Furthermore, smaller cells mean that less power is needed, which leads to smaller and cheaper transmitters and handsets. </li></ul><ul><li>Hand-held telephones put out 0.6 watts; transmitters in cars are 3 watts, the maximum allowed by the FCC. </li></ul>
  95. 98. Advanced Mobile Phone System <ul><li>(a) Frequencies are not reused in adjacent cells. </li></ul><ul><li>(b) To add more users, smaller cells can be used. </li></ul>
  96. 99. Switching to a new channel: Soft & Hard handoff <ul><li>Handoffs can be done in two ways. Takes 300 ms. </li></ul><ul><li>soft handoff, the telephone is acquired by the new base station before the previous one signs off. </li></ul><ul><ul><li>In this way there is no loss of continuity. </li></ul></ul><ul><ul><li>The downside here is that the telephone needs to be able to tune to two frequencies at the same time (the old one and the new one). </li></ul></ul><ul><ul><li>Neither first nor second generation devices can do this. </li></ul></ul><ul><li>hard handoff, the old base station drops the telephone before the new one acquires it. </li></ul><ul><ul><li>If the new one is unable to acquire it (e.g., because there is no available frequency), the call is disconnected abruptly. Users tend to notice this, but it is inevitable occasionally with the current design. </li></ul></ul>
  97. 100. Channels in AMPS <ul><li>The AMPS system uses 832 full-duplex channels , each consisting of a pair of simplex channels. </li></ul><ul><li>There are 832 simplex transmission channels from 824 to 849 MHz and 832 simplex receive channels from 869 to 894 MHz . </li></ul><ul><li>Each of these simplex channels is 30 kHz wide. </li></ul><ul><li>Thus, AMPS uses FDM to separate the channels. </li></ul><ul><li>In the 800-MHz band, radio waves are about 40 cm long and travel in straight lines . </li></ul><ul><li>They are absorbed by trees and plants and bounce off the ground and buildings. </li></ul><ul><li>It is possible that a signal sent by a mobile telephone will reach the base station by the direct path, but also slightly later after bouncing off the ground or a building. </li></ul><ul><li>This may lead to an echo or signal distortion (multipath fading). Sometimes, it is even possible to hear a distant conversation that has bounced several times </li></ul>
  98. 101. Channel Categories in AMPS <ul><li>The 832 channels are divided into four categories: </li></ul><ul><li>Control (base to mobile) to manage the system </li></ul><ul><li>Paging (base to mobile) to alert users to calls for them </li></ul><ul><li>Access (bidirectional) for call setup and channel assignment </li></ul><ul><li>Data (bidirectional) for voice, fax, or data </li></ul>
  99. 102. 2.6.2 Second-Generation Mobile Phones: Digital Voice <ul><li>No worldwide standardization during the first generation, there was also no standardization during the second, either. </li></ul><ul><li>Four systems are in use now: </li></ul><ul><ul><li>D-AMPS </li></ul></ul><ul><ul><li>GSM </li></ul></ul><ul><ul><li>CDMA </li></ul></ul><ul><ul><li>PDC(Japan only). </li></ul></ul>
  100. 103. D-AMPS: The Digital Advanced Mobile Phone System <ul><li>D-AMPS and is fully digital . </li></ul><ul><li>D-AMPS was carefully designed to co-exist with AMPS so that both first- and second-generation mobile phones could operate simultaneously in the same cell. </li></ul><ul><li>In particular, D-AMPS uses the same 30 kHz channels as AMPS and at the same frequencies so that one channel can be analog and the adjacent ones can be digital. </li></ul><ul><li>Depending on the mix of phones in a cell, the cell's MTSO determines which channels are analog and which are digital, and it can change channel types dynamically as the mix of phones in a cell changes. </li></ul><ul><li>new frequency band was made available to handle the expected increased load. </li></ul><ul><ul><li>The upstream channels were i 1850–1910 MHz range, </li></ul></ul><ul><ul><li>corresponding downstream channels were in the 1930–1990 MHz range, </li></ul></ul><ul><li>In this band, the waves are 16 cm long, so a standard ¼-wave antenna is only 4 cm long, leading to smaller phones . </li></ul><ul><li>On a D-AMPS mobile phone, the voice signal picked up by the microphone is digitized and compressed compression is done in the telephone, rather than in the base station or end office, to reduce the number of bits sent over the air link. </li></ul><ul><li>huge gain from doing digitization and compression in the handset , so much so that in D-AMPS, three users can share a single frequency pair using time division multiplexing. </li></ul>
  101. 104. D-AMPS Digital Advanced Mobile Phone System <ul><li>(a) A D-AMPS channel with three users. </li></ul><ul><li>(b) A D-AMPS channel with six users. With better compression algorithm </li></ul>
  102. 105. GSM—The Global System for Mobile Communications <ul><li>To a first approx. GSM is similar to D-AMPS. Both are cellular systems. </li></ul><ul><li>In both systems, frequency division multiplexing is used , with each mobile transmitting on one frequency and receiving on a higher frequency (80 MHz higher for D-AMPS, 55 MHz higher for GSM). </li></ul><ul><li>Also in both systems, a single frequency pair is split by time-division multiplexing into time slots shared by multiple mobiles. However, the GSM channels are much wider than the AMPS channels (200 kHz versus 30 kHz) and hold relatively few additional users (8 versus 3), giving GSM a much higher data rate per user than D-AMPS . </li></ul><ul><li>Main properties of GSM. </li></ul><ul><ul><li>Each frequency band is 200 kHz wide, as shown in Fig. 2-43 . </li></ul></ul><ul><ul><li>A GSM system has 124 pairs of simplex channels. </li></ul></ul><ul><ul><li>Each simplex channel is 200 kHz wide and supports eight separate connections on it, using time division multiplexing. </li></ul></ul><ul><ul><li>Each currently active station is assigned one time slot on one channel pair. </li></ul></ul>
  103. 106. Communication in GSM <ul><li>Theoretically, 992 channels can be supported in each cell, but many of them are not available, to avoid frequency conflicts with neighboring cells. </li></ul><ul><li>In Fig. 2-43 , the eight shaded time slots all belong to the same connection, four of them in each direction. </li></ul><ul><li>Transmitting and receiving does not happen in the same time slot because the GSM radios cannot transmit and receive at the same time and it takes time to switch from one to the other. </li></ul><ul><li>If the mobile station assigned to 890.4/935.4 MHz and time slot 2 wanted to transmit to the base station, it would use the lower four shaded slots (and the ones following them in time), putting some data in each slot until all the data had been sent. </li></ul>
  104. 107. GSM Global System for Mobile Communications <ul><li>GSM uses 124 frequency channels, each of which uses an eight-slot TDM system </li></ul>
  105. 108. <ul><li>As can be seen from next slide, </li></ul><ul><ul><li>eight data frames make up a TDM frame and </li></ul></ul><ul><ul><li>26 TDM frames make up a 120-msec multiframe. Of the 26 TDM frames in a multiframe, slot 12 is used for control and slot 25 is reserved for future use, so only 24 are available for user traffic. </li></ul></ul><ul><ul><li>However, in addition to the 26-slot multiframe shown in Fig. 2-44 , a 51-slot multiframe (not shown) is also used. Some of these slots are used to hold several control channels used to manage the system. </li></ul></ul><ul><li>Broadcast control channel : is a continuous stream of output from the base station containing the base station's identity and the channel status. </li></ul><ul><ul><li>All mobile stations monitor their signal strength to see when they have moved into a new cell. </li></ul></ul><ul><li>Dedicated control channel: is used for location updating, registration, and call setup. </li></ul><ul><ul><li>In particular, each base station maintains a database of mobile stations currently under its jurisdiction. </li></ul></ul><ul><ul><li>Information needed to maintain this database is sent on the dedicated control channel. </li></ul></ul><ul><li>Common control channel, </li></ul><ul><ul><li>which is split up into three logical subchannels. </li></ul></ul><ul><ul><ul><li>The first of these subchannels is the paging channel , which the base station uses to announce incoming calls. Each mobile station monitors it continuously to watch for calls it should answer. </li></ul></ul></ul><ul><ul><ul><li>The second is the random access channel, which allows users to request a slot on the dedicated control channel. If two requests collide, they are garbled and have to be retried later. Using the dedicated control channel slot, the station can set up a call. </li></ul></ul></ul><ul><ul><ul><li>The assigned slot is announced on the third subchannel, the access grant channel. </li></ul></ul></ul>
  106. 109. GSM (2) <ul><li>A portion of the GSM framing structure. </li></ul>
  107. 116. CDMA—Code Division Multiple Access <ul><li>D-AMPS and GSM are fairly conventional systems. </li></ul><ul><li>both FDM and TDM to divide the spectrum into channels and the channels into time slots. </li></ul><ul><li>However, there is a third kid on the block, CDMA (Code Division Multiple Access), which works completely differently. </li></ul><ul><li>viewed as the best technical solution for the third-generation mobile systems. </li></ul><ul><li>CDMA is completely different from AMPS, D-AMPS, and GSM. </li></ul><ul><li>Instead of dividing the allowed frequency range into a few hundred narrow channels, CDMA allows each station to transmit over the entire frequency spectrum all the time. </li></ul><ul><li>Multiple simultaneous transmissions are separated using coding theory. </li></ul><ul><li>CDMA is comparable to everybody being in the middle of the room talking at once, but with each pair in a different language. The French-speaking couple just hones in on the French, rejecting everything that is not French as noise. </li></ul><ul><li>Thus, the key to CDMA is to be able to extract the desired signal while rejecting everything else as random noise. </li></ul>
  108. 117. <ul><li>In CDMA, each bit time is subdivided into m short intervals called chips . Typically, there are 64 or 128 chips per bit, but in the example given below we will use 8 chips/bit for simplicity. </li></ul><ul><li>Each station is assigned a unique m-bit code called a chip sequence . </li></ul><ul><li>To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the one's complement of its chip sequence. No other patterns are permitted. Thus, for m = 8, if station A is assigned the chip sequence 00011011, it sends a 1 bit by sending 00011011 and a 0 bit by sending 11100100. </li></ul><ul><li>Increasing the amount of information to be sent from b bits/sec to mb chips/sec can only be done if the bandwidth available is increased by a factor of m, making CDMA a form of spread spectrum communication (assuming no changes in the modulation or encoding techniques). </li></ul><ul><li>If we have a 1-MHz band available for 100 stations, with FDM each one would have 10 kHz and could send at 10 kbps (assuming 1 bit per Hz). With CDMA, each station uses the full 1 MHz, so the chip rate is 1 megachip per second. </li></ul><ul><li>With fewer than 100 chips per bit, the effective bandwidth per station is higher for CDMA than FDM, and the channel allocation problem is also solved. </li></ul><ul><ul><li>For pedagogical purposes, it is more convenient to use a bipolar notation, with binary 0 being -1 and binary 1 being +1. We will show chip sequences in parentheses, so a 1 bit for station A now becomes (-1 -1 -1 +1 +1 -1 +1 +1). In Fig. 2-45(a) we show the binary chip sequences assigned to four example stations. In Fig. 2-45(b) we show them in our bipolar notation. </li></ul></ul>
  109. 118. CDMA – Code Division Multiple Access <ul><li>(a) Binary chip sequences for four stations </li></ul><ul><li>(b) Bipolar chip sequences </li></ul><ul><li>(c) Six examples of transmissions </li></ul><ul><li>(d) Recovery of station C’s signal </li></ul>
  110. 119. Third-Generation Mobile Phones: Digital Voice and Data <ul><li>Basic services an IMT-2000 network should provide </li></ul><ul><li>High-quality voice transmission </li></ul><ul><li>Messaging (replace e-mail, fax, SMS, chat, etc.) </li></ul><ul><li>Multimedia (music, videos, films, TV, etc.) </li></ul><ul><li>Internet access (web surfing, w/multimedia.) </li></ul><ul><li>Several proposals were made: </li></ul><ul><ul><li>W-CDMA (Wideband CDMA) - by Ericsson. – </li></ul></ul><ul><ul><ul><li>EU prefers this as Ericsson is Sweden based - UMTS (Universel Mobile Télécommunications System). </li></ul></ul></ul><ul><ul><li>CDMA2000 by Qualcomm in US </li></ul></ul><ul><li>Recent Options in use today serves as stop-gap measure until 3G arrives is called 2.5G and they are: </li></ul><ul><ul><li>EDGE (Enhanced Data rates for GSM Evolution), which is just GSM with more bits per baud. </li></ul></ul><ul><ul><ul><li>more bits per baud also means more errors per baud </li></ul></ul></ul><ul><ul><ul><li>so EDGE has nine different schemes for modulation and error correction, differing on how much of the bandwidth is devoted to fixing the errors introduced by the higher speed. </li></ul></ul></ul><ul><ul><li>GPRS (General Packet Radio Service), which is an overlay packet network on top of D-AMPS or GSM. </li></ul></ul>
  111. 120. 2.7 Cable Television <ul><li>Community Antenna Television </li></ul><ul><li>Internet over Cable </li></ul><ul><li>Spectrum Allocation </li></ul><ul><li>Cable Modems </li></ul><ul><li>ADSL versus Cable </li></ul>
  112. 121. Community Antenna Television <ul><li>An early cable television system. </li></ul>The system initially consisted of a big antenna on top of a hill to pluck the television signal out of the air, an amplifier, called the head end , to strengthen it, and a coaxial cable to deliver it to people's houses
  113. 122. Internet over Cable <ul><li>Cable television </li></ul>
  114. 123. Cable and Conventional internet issues: <ul><li>However, there is another difference between the HFC system of Fig. 2-47(a) and the telephone system of next slide Fig. 2-47(b) that is much harder to remove. </li></ul><ul><li>Down in the neighbourhoods, a single cable is shared by many houses , </li></ul><ul><li>whereas in the telephone system, every house has its own private local loop(see next slide) . </li></ul><ul><li>When used for television broadcasting, this sharing does not play a role. All the programs are broadcast on the cable and it does not matter whether there are 10 viewers or 10,000 viewers. </li></ul><ul><li>But ,when the same cable is used for Internet access, it matters a lot if there are 10 users or 10,000. </li></ul><ul><li>For ex: If one user decides to download a very large file, that bandwidth is potentially being taken away from other users. </li></ul><ul><li>The more users, the more competition for bandwidth. </li></ul><ul><li>The telephone system does not have this particular property: downloading a large file over an ADSL line does not reduce your neighbour's bandwidth. </li></ul><ul><li>On the other hand, the bandwidth of coax is much higher than that of twisted pairs. </li></ul><ul><li>The way the cable industry has tackled this problem is to split up long cables and connect each one directly to a fiber node. </li></ul>
  115. 124. Internet over Cable (2) <ul><li>The fixed telephone system. </li></ul>
  116. 125. Spectrum Allocation <ul><li>Frequency allocation in a typical cable TV system used for Internet access </li></ul>
  117. 126. <ul><li>Note that since the television signals are all downstream, </li></ul><ul><li>it is possible to use upstream amplifiers that work only in the 5–42 MHz region and downstream amplifiers that work only at 54 MHz and up, as shown in the figure. </li></ul><ul><li>Thus, we get an asymmetry in the upstream and downstream bandwidths because more spectrum is available above television than below it. </li></ul><ul><li>On the other hand, most of the traffic is likely to be downstream </li></ul><ul><li>Long coaxial cables are not any better for transmitting digital signals than are long local loops, so analog modulation is needed. </li></ul><ul><li>The usual scheme is to take each 6 MHz or 8 MHz downstream channel and modulate it with QAM-64 or, if the cable quality is exceptionally good, QAM-256. With a 6 MHz channel and QAM-64, we get about 36 Mbps. </li></ul><ul><li>For upstream, even QAM-64 does not work well. There is too much noise from terrestrial microwaves, CB radios, and other sources, so a more conservative scheme—QPSK—is used. </li></ul><ul><li>In addition to upgrading the amplifiers, the operator has to upgrade the headend, too, from a dumb amplifier to an intelligent digital computer system with a high-bandwidth fiber interface to an ISP. </li></ul><ul><ul><li>Often the name gets upgraded as well, from ' 'headend '' to CMTS (Cable Modem Termination System). In the following text, we will refrain from doing a name upgrade and stick with the traditional ''headend.'' </li></ul></ul>
  118. 127. when a cable modem is plugged in and powered up…. <ul><li>Cable Modem scans the downstream channels looking for a special packet </li></ul><ul><ul><li>periodically put out by the headend to provide system parameters to modems that have just come on-line </li></ul></ul><ul><li>Upon finding this packet, the new modem announces its presence on one of the upstream channels . </li></ul><ul><li>The headend responds by assigning the modem to its upstream and downstream channels. </li></ul><ul><ul><li>These assignments can be changed later if the headend deems it necessary to balance the load. </li></ul></ul><ul><li>The modem then determines its distance from the headend by sending it a special packet and seeing how long it takes to get the response. This process is called ranging . </li></ul><ul><li>It is important for the modem to know its distance to accommodate the way the upstream channels operate and to get the timing right. They are divided in time in minislots . </li></ul><ul><ul><li>Each upstream packet must fit in one or more consecutive minislots. </li></ul></ul><ul><ul><li>The headend announces the start of a new round of minislots periodically </li></ul></ul><ul><li>During initialization, the headend also assigns each modem to a minislot to use for requesting upstream bandwidth </li></ul>
  119. 128. <ul><li>If the request is accepted, the headend puts an acknowledgement on the downstream channel telling the modem which minislots have been reserved for its packet. </li></ul><ul><ul><li>The packet is then sent, starting in the minislot allocated to it. Additional packets can be requested using a field in the header. </li></ul></ul><ul><li>if there is contention for the request minislot, there will be no acknowledgement and the modem just waits a random time and tries again. </li></ul><ul><li>After each successive failure, the randomization time is doubled. </li></ul><ul><li>The downstream channels are managed differently from the upstream channels. </li></ul><ul><li>For one thing, there is only one sender (the headend) so there is no contention and no need for minislots, which is actually just time division statistical multiplexing. (see next slide…) </li></ul><ul><li>For another, the traffic downstream is usually much larger than upstream, so a fixed packet size of 204 bytes is used. </li></ul><ul><li>Part of that is a Reed-Solomon error-correcting code and some other overhead, leaving a user payload of 184 bytes. </li></ul><ul><li>These numbers were chosen for compatibility with digital television using MPEG-2, so the TV and downstream data channels are formatted the same way. Logically, the connections are as depicted in Fig. 2-49 . </li></ul>when a cable modem is plugged in and powered up….Cont
  120. 129. Cable Modems <ul><li>Typical details of the upstream and downstream channels in North America. </li></ul>
  121. 130. 2.7.5 Cable Vs. ADSL CABLE ADSL Cable providers do not make any claims for effective capacity because the effective capacity depends on how many people are currently active on the user's cable segment. Sometimes it may be better than ADSL and sometimes it may be worse. ADSL providers give specific statements about the bandwidth (e.g., 1 Mbps downstream, 256 kbps upstream) and generally achieve about 80% of it consistently. With cable, as more subscribers sign up for Internet service, performance for existing users will drop. Because of Shared medium. As an ADSL system acquires more users, their increasing numbers have little effect on existing users , since each user has a dedicated connection. Any cable user can easily read all the packets going down the cable . For this reason, any decent cable provider will encrypt all traffic in both directions. Being a point-to-point medium, ADSL is inherently more secure than cable With cable, if the power to any amplifier along the chain fails, all downstream users are cut off instantly. The telephone system is generally more reliable than cable . For example, it has backup power and continues to work normally even during a power outage