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CS 453 Computer Networks

CS 453 Computer Networks

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CS 453 Computer Networks

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  1. CS 453Computer Networks Lecture 8 Layer 1 – Physical Layer

  2. Topics • TV Cable as a network media • Wifi • WiMax

  3. Physical Layer • …but first lets talk about multiplexing • Consider the following network graph A E C D B E

  4. Physical Layer – Layer 1 • Note that A-C link and the B-C both need to use the C-D link to communicate with nodes D,E or F • Goal: to use the C-D link with no loss in data rate for A-C or B-C (or E-D, F-D) • Assume that the C-D bandwidth is more than four times that of A-C, B-C, D-E, or D-F A E C D B F

  5. Multiplexing • Combining multiple channels to use a single medium is called Multiplexing • There are two primary forms of multiplexing – • Time Division Multiplexing • Frequency Division Multiplexing

  6. Multiplexing • Time Division Multiplexing – TDM • Remember that the maximum data rate for the trunk (C-D) is many times that of the connected links • Suppose then we allocate small time-slices of the trunk to each connected line. i.e…. • A-C gets the first 4 msec • B-C gets the second 4 msec • … etc. • This in simplified form is TDM

  7. Multiplexing • In TDM the idea is— • Each connected node contributes (if it has something to contribute) a small packet of its data to a multiplexor node… • Where it is combined into a frame with packets from other communicating nodes • These frames are transmitted over the high bandwidth trunk • At the other end these frames are demultiplexed and … • The respective packets are transmitted to their destinations

  8. 24 Channels 8 bits per channel 192 bits composite 193 bits per frame Every 125 sec 1.54 Mbps Multiplexing – T1 From: Tanenbaum (2003) pg. 141

  9. SONET • SONET (Synchronous Optical Network) uses TDM • SONET used 810 byte frames • Transmits a frame every 125 sec or 8000 frames per second • So 810 x 8bits x 8000 per second = • 51.84 Mbps • …a SONET channel call STS-1 (SONET Transport Signal-1)

  10. SONET • SONET data rates From: Tanebaum (2003) pg. 146

  11. Multiplexing • Time Division Statistical Multiplexing • TDM – assigns fixed and fixed size slots to incoming data channels in the outgoing composite link • Time Division Statistical Multiplexing – TDSM • Dynamically allocates slots and slot sizes in composite frame… • …based on data traffic load • More efficient use of the trunk bandwidth • …greater effective bandwidth

  12. Frequency Division Multiplexing • Frequency Division Multiplexing – FDM • Overall trunk bandwidth divided in to frequency bands… • Incoming data channel is mapped to a fequency band for transmission across the trunk • Although analog – this is exactly what cable TV does • We’ll come back to this…

  13. Multiplexing • Wave Division Multiplexing - WDM • We discussed this earlier when we talked about fiber optics • Exactly the same idea as FDM except that the frequencies are in the visible light or near visible light range of the spectrum • 96 waves at 10 Gbps ~ 30 MPEG2 movies/second • DWDM • Large number, densely spaced waves

  14. Cable TV at a Network • Cable TV systems originated in the late 1940s • CATV – ? • Community Antenna TeleVision • First systems – simply to get broadcast TV signals to homes in hard to reach places • Idea – a bit antenna on the hill, run cable to every home in the community… • Required an amplifier

  15. Cable TV as a Network • Later Cable TV companies began to originate signals - play a tape, a local show, rebroadcast a show received off-air… • This required … • Taking an original signal (say the output of a VCR) • Modulating this signal to a frequency band… • Combining it with other signals at different frequencies • .and “injecting” this combined sign into the cable transmission medium

  16. Cable TV as a network • So, creating a cable TV network is relatively easy and relatively cheap… • …except for running all of that coax cable “…Connect any video source to the RCA-type input, audio connects via a 3.5mm stereo mini-plug port. After your A/V source is connected to the MM70 set the digitally tuned channel (UHF 14-69, CATV 70-94 & 100-125) and combine the MM70's amplified output with your Antenna (UHF) or Cable (CATV) system. …”

  17. Cable TV Frequencies in N.A.

  18. Frequency Band Allocation • Cable TV system in North America uses .. • 54-550 Mhz range • Excludes 88 – 108 Mhz (FM radio) • Multiple channels each 6 Mhz wide • Most cable systems can operate up to 750 Mhz

  19. Cable TV as a network • So to create a “high speed network” over cable… • Allocate part of the cable’s bandwidth for data • Modulate data streams to cable channels... typically… • Upstream (user to ISP) ~ 5 – 42 Mhz • TV signals – 54 – 550 Mhz • Downstream (ISP to user) 550 Mhz – 750 Mhz

  20. Cable TV as a network • Coax cable over distance suffers attenuation… • Limits bandwidth • Typically downstream data signal modulated with QAM-64 • Some systems use QAM-265 • With 6 Mhz channel and QAM-64 ~ 36 Mbps

  21. Cable TV as a network • Upstream – QAM-64 modulation does not work very well • Interference - Microwaves, citizen band radios, etc. • QPSK modulation • Considerable asymmetry upstream/downstream

  22. Cable TV as a network • Cable TV data system – physical infrastructure – • Typically Hybrid Fiber/Coax • Fiber from headend to neighborhood node • Coax from neighborhood node to home/office • That means that very high bandwidth fiber brings data service (and video – audio) to the neighborhood (nearby) • Then transported on coax to homes/office • Coax in local area (down your street) is a shared medium

  23. Cable vs DSL • Common choice for high speed network service in Morgantown • So, which is better? • …well, it depends

  24. Cable vs DSL • Both use fiber backbone to local node • Both use copper for local loop • Cable uses coax for local loop • DSL used twisted pair for local loop • Theoretically, coax and twisted pair have approximately, the same bandwidth, practically speaking

  25. Cable vs DSL • In practice, • Higher bandwidths per connector available on cable • Local loop • DSL – point to point • Cable – shared at the “neighborhood level”

  26. Cable vs DSL • At local level – • DSL • Lower data rates • Committed data rates • Cable • Higher data rates, • But the medium/bandwidth is shared with the neighborhood • The more people using the cable data network, the lower the effective per subscriber bandwidth

  27. Cable vs DSL • Cable systems can relieve local loop network congestion… • Pushing out the fiber nodes/fiber • Breaking up the local loops in to smaller segments • Expensive, re-engineering of the system

  28. Cable vs DSL • What about availability? • DSL • Cable • What about security?