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CS 453 Computer Networks. Lecture 8 Layer 1 – Physical Layer. Topics. TV Cable as a network media Wifi WiMax. Physical Layer. …but first lets talk about multiplexing Consider the following network graph. A. E. C. D. B. E. Physical Layer – Layer 1.

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cs 453 computer networks

CS 453Computer Networks

Lecture 8

Layer 1 – Physical Layer

  • TV Cable as a network media
  • Wifi
  • WiMax
physical layer
Physical Layer
  • …but first lets talk about multiplexing
  • Consider the following network graph







physical layer layer 1
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







  • Combining multiple channels to use a single medium is called Multiplexing
  • There are two primary forms of multiplexing –
    • Time Division Multiplexing
    • Frequency Division 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
  • 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
multiplexing t1
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

  • 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)
  • SONET data rates

From: Tanebaum (2003) pg. 146

  • 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
frequency division multiplexing
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…
  • 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
cable tv at a network
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
cable tv as a network
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
cable tv as a network1
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. …”

frequency band allocation
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
cable tv as a network2
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
cable tv as a network3
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
cable tv as a network4
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
cable tv as a network5
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
cable vs dsl
Cable vs DSL
  • Common choice for high speed network service in Morgantown
  • So, which is better?
  • …well, it depends
cable vs dsl1
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
cable vs dsl2
Cable vs DSL
  • In practice,
    • Higher bandwidths per connector available on cable
  • Local loop
    • DSL – point to point
    • Cable – shared at the “neighborhood level”
cable vs dsl3
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
cable vs dsl4
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
cable vs dsl5
Cable vs DSL
  • What about availability?
    • DSL
    • Cable
  • What about security?