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

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CS 453 Computer Networks Lecture 8 Layer 1 – Physical Layer

Topics TV Cable as a network media WifiWiMax

Physical Layer …but first lets talk about multiplexing Consider the following network graph A B CD E E

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 Assume that the C-D bandwidth is more than four times that of A-C, B-C, D-E, or D-F A B CD E F

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

Multiplexing Time Division Multiplexing – TDM Remember that the maximum data rate for the trunk (C-D) is many times that of the connected links 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…. 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 This in simplified form is TDM

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… 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 Where it is combined into a frame with packets from other communicating nodes These frames are transmitted over the high bandwidth trunk These frames are transmitted over the high bandwidth trunk At the other end these frames are demultiplexed and … At the other end these frames are demultiplexed and … The respective packets are transmitted to their destinations 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 From: Tanenbaum (2003) pg. 141

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

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

Multiplexing Time Division Statistical Multiplexing TDM – assigns fixed and fixed size slots to incoming data channels in the outgoing composite link TDM – assigns fixed and fixed size slots to incoming data channels in the outgoing composite link Time Division Statistical Multiplexing – TDSM 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 – FDM Overall trunk bandwidth divided in to frequency bands… Overall trunk bandwidth divided in to frequency bands… Incoming data channel is mapped to a fequency band for transmission across the trunk Incoming data channel is mapped to a fequency band for transmission across the trunk Although analog – this is exactly what cable TV does Although analog – this is exactly what cable TV does Well come back to this…

Multiplexing Wave Division Multiplexing - WDM We discussed this earlier when we talked about fiber optics 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 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 DWDM Large number, densely spaced waves

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

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) Taking an original signal (say the output of a VCR) Modulating this signal to a frequency band… Modulating this signal to a frequency band… Combining it with other signals at different frequencies Combining it with other signals at different frequencies.and injecting this combined sign into the cable transmission medium.and injecting this combined sign into the cable transmission medium

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. …

Cable TV Frequencies in N.A.

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

Cable TV as a network So to create a high speed network over cable… Allocate part of the cables bandwidth for data Allocate part of the cables bandwidth for data Modulate data streams to cable channels... typically… 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 network Coax cable over distance suffers attenuation… Limits bandwidth Typically downstream data signal modulated with QAM-64 Some systems use QAM-265 Some systems use QAM-265 With 6 Mhz channel and QAM-64 ~ 36 Mbps

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

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

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

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

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

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

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

Cable vs DSL What about availability? DSL DSL Cable Cable What about security?

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