CS 453CS 453Computer NetworksComputer Networks
Lecture 8Lecture 8
Layer 1 – Physical LayerLayer 1 – Physical Layer
TopicsTopics
TV Cable as a network mediaTV Cable as a network media
WifiWifi
WiMaxWiMax
Physical LayerPhysical Layer
……but first lets talk about multiplexingbut first lets talk about multiplexing
Consider the following network graphConsider the following network graph
A
B
C D
E
E
Physical Layer – Layer 1Physical Layer – Layer 1
Note that A-C link and the B-C both need to use the C-D link to Note that A-C link and the B-C both need to use the C-D link to communicate with nodes D,E or Fcommunicate with nodes D,E or FGoal: to use the C-D link with no loss in data rate for A-C or B-C (or Goal: to use the C-D link with no loss in data rate for A-C or B-C (or E-D, F-D)E-D, F-D) Assume that the C-D bandwidth is more than four times that of A-C, Assume that the C-D bandwidth is more than four times that of A-C, B-C, D-E, or D-FB-C, D-E, or D-F
A
B
C D
E
F
MultiplexingMultiplexing
Combining multiple channels to use a Combining multiple channels to use a single medium is called Multiplexingsingle medium is called Multiplexing
There are two primary forms of There are two primary forms of multiplexing –multiplexing – Time Division MultiplexingTime Division Multiplexing Frequency Division MultiplexingFrequency Division Multiplexing
MultiplexingMultiplexing
Time Division Multiplexing – TDMTime Division Multiplexing – TDM Remember that the maximum data rate for the Remember that the maximum data rate for the
trunk (C-D) is many times that of the trunk (C-D) is many times that of the connected linksconnected links
Suppose then we allocate small time-slices of Suppose then we allocate small time-slices of the trunk to each connected line. i.e….the trunk to each connected line. i.e….
A-C gets the first 4 msecA-C gets the first 4 msecB-C gets the second 4 msecB-C gets the second 4 msec… … etc.etc.
This in simplified form is TDMThis in simplified form is TDM
MultiplexingMultiplexing
In TDM the idea is—In TDM the idea is— Each connected node contributes (if it has something Each connected node contributes (if it has something
to contribute) a small packet of its data to a to contribute) a small packet of its data to a multiplexor node…multiplexor node…
Where it is combined into a frame with packets from Where it is combined into a frame with packets from other communicating nodesother communicating nodes
These frames are transmitted over the high bandwidth These frames are transmitted over the high bandwidth trunktrunk
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 The respective packets are transmitted to their destinationsdestinations
Multiplexing – T1Multiplexing – T1
24 Channels24 Channels8 bits per channel8 bits per channel192 bits composite192 bits composite
193 bits per frame193 bits per frameEvery 125 Every 125 secsec1.54 Mbps1.54 Mbps
From: Tanenbaum (2003) pg. 141
SONETSONET
SONET (Synchronous Optical Network) SONET (Synchronous Optical Network) uses TDMuses TDM SONET used 810 byte framesSONET used 810 byte frames Transmits a frame every 125 Transmits a frame every 125 sec or 8000 sec or 8000
frames per secondframes per second So 810 x 8bits x 8000 per second = So 810 x 8bits x 8000 per second = 51.84 Mbps51.84 Mbps ……a SONET channel call STS-1 (SONET a SONET channel call STS-1 (SONET
Transport Signal-1)Transport Signal-1)
SONETSONET
SONET data ratesSONET data rates
From: Tanebaum (2003) pg. 146
MultiplexingMultiplexing
Time Division Statistical MultiplexingTime Division Statistical Multiplexing TDM – assigns fixed and fixed size slots to TDM – assigns fixed and fixed size slots to
incoming data channels in the outgoing incoming data channels in the outgoing composite linkcomposite link
Time Division Statistical Multiplexing – TDSMTime Division Statistical Multiplexing – TDSMDynamically allocates slots and slot sizes in Dynamically allocates slots and slot sizes in composite frame…composite frame………based on data traffic loadbased on data traffic loadMore efficient use of the trunk bandwidthMore efficient use of the trunk bandwidth……greater effective bandwidthgreater effective bandwidth
Frequency Division MultiplexingFrequency Division Multiplexing
Frequency Division Multiplexing – FDMFrequency Division Multiplexing – FDM Overall trunk bandwidth divided in to Overall trunk bandwidth divided in to
frequency bands…frequency bands… Incoming data channel is mapped to a Incoming data channel is mapped to a
fequency band for transmission across the fequency band for transmission across the trunktrunk
Although analog – this is exactly what cable Although analog – this is exactly what cable TV doesTV does
We’ll come back to this…We’ll come back to this…
MultiplexingMultiplexing
Wave Division Multiplexing - WDMWave Division Multiplexing - WDM We discussed this earlier when we talked We discussed this earlier when we talked
about fiber opticsabout fiber optics Exactly the same idea as FDM except that the Exactly the same idea as FDM except that the
frequencies are in the visible light or near frequencies are in the visible light or near visible light range of the spectrumvisible light range of the spectrum
96 waves at 10 Gbps ~ 30 MPEG2 movies/second96 waves at 10 Gbps ~ 30 MPEG2 movies/second DWDMDWDM
Large number, densely spaced wavesLarge number, densely spaced waves
Cable TV at a NetworkCable TV at a Network
Cable TV systems originated in the late Cable TV systems originated in the late 1940s1940sCATV – ?CATV – ? Community Antenna TeleVisionCommunity Antenna TeleVision First systems – simply to get broadcast TV First systems – simply to get broadcast TV
signals to homes in hard to reach placessignals to homes in hard to reach places Idea – a bit antenna on the hill, run cable to Idea – a bit antenna on the hill, run cable to
every home in the community…every home in the community… Required an amplifierRequired an amplifier
Cable TV as a NetworkCable TV as a Network
Later Cable TV companies began to originate Later Cable TV companies began to originate signals - play a tape, a local show, rebroadcast signals - play a tape, a local show, rebroadcast a show received off-air…a show received off-air…
This required …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 Combining it with other signals at different
frequenciesfrequencies .and “injecting” this combined sign into the cable .and “injecting” this combined sign into the cable
transmission mediumtransmission medium
Cable TV as a networkCable TV as a network
So, creating a So, creating a cable TV network cable TV network is relatively easy is relatively easy and relatively and relatively cheap…cheap…
……except for except for running all of that running all of that coax cablecoax 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.Cable TV Frequencies in N.A.
Frequency Band AllocationFrequency Band Allocation
Cable TV system in North America uses ..Cable TV system in North America uses .. 54-550 Mhz range54-550 Mhz range Excludes 88 – 108 Mhz (FM radio)Excludes 88 – 108 Mhz (FM radio) Multiple channels each 6 Mhz wideMultiple channels each 6 Mhz wide Most cable systems can operate up to 750 MhzMost cable systems can operate up to 750 Mhz
Cable TV as a networkCable TV as a network
So to create a “high speed network” over So to create a “high speed network” over cable…cable… Allocate part of the cable’s bandwidth for dataAllocate part of the cable’s bandwidth for data Modulate data streams to cable channels... Modulate data streams to cable channels...
typically…typically…Upstream (user to ISP) ~ 5 – 42 MhzUpstream (user to ISP) ~ 5 – 42 Mhz
TV signals – 54 – 550 MhzTV signals – 54 – 550 Mhz
Downstream (ISP to user) 550 Mhz – 750 MhzDownstream (ISP to user) 550 Mhz – 750 Mhz
Cable TV as a networkCable TV as a network
Coax cable over distance suffers Coax cable over distance suffers attenuation…attenuation…
Limits bandwidthLimits bandwidth
Typically downstream data signal modulated Typically downstream data signal modulated with QAM-64with QAM-64 Some systems use QAM-265Some systems use QAM-265
With 6 Mhz channel and QAM-64 ~ 36 MbpsWith 6 Mhz channel and QAM-64 ~ 36 Mbps
Cable TV as a networkCable TV as a network
Upstream – QAM-64 modulation does not Upstream – QAM-64 modulation does not work very wellwork very well Interference - Microwaves, citizen band Interference - Microwaves, citizen band
radios, etc.radios, etc. QPSK modulation QPSK modulation Considerable asymmetry Considerable asymmetry
upstream/downstreamupstream/downstream
Cable TV as a networkCable TV as a network
Cable TV data system – physical infrastructure –Cable TV data system – physical infrastructure – Typically Hybrid Fiber/CoaxTypically Hybrid Fiber/Coax
Fiber from headend to neighborhood nodeFiber from headend to neighborhood node
Coax from neighborhood node to home/officeCoax from neighborhood node to home/office That means that very high bandwidth fiber brings data That means that very high bandwidth fiber brings data
service (and video – audio) to the neighborhood service (and video – audio) to the neighborhood (nearby)(nearby)
Then transported on coax to homes/officeThen transported on coax to homes/office Coax in local area (down your street) is a shared Coax in local area (down your street) is a shared
mediummedium
Cable vs DSLCable vs DSL
Common choice for high speed network Common choice for high speed network service in Morgantownservice in Morgantown
So, which is better?So, which is better?
……well, it dependswell, it depends
Cable vs DSLCable vs DSL
Both use fiber backbone to local nodeBoth use fiber backbone to local node
Both use copper for local loopBoth use copper for local loop Cable uses coax for local loopCable uses coax for local loop DSL used twisted pair for local loopDSL used twisted pair for local loop Theoretically, coax and twisted pair have Theoretically, coax and twisted pair have
approximately, the same bandwidth, approximately, the same bandwidth, practically speakingpractically speaking
Cable vs DSLCable vs DSL
In practice,In practice, Higher bandwidths per connector available on Higher bandwidths per connector available on
cablecable
Local loopLocal loop DSL – point to pointDSL – point to point Cable – shared at the “neighborhood level”Cable – shared at the “neighborhood level”
Cable vs DSLCable vs DSL
At local level –At local level –
DSL DSL Lower data ratesLower data rates Committed data ratesCommitted data rates
CableCable Higher data rates, Higher data rates, But the medium/bandwidth is shared with the But the medium/bandwidth is shared with the
neighborhoodneighborhood The more people using the cable data network, the The more people using the cable data network, the
lower the effective per subscriber bandwidthlower the effective per subscriber bandwidth
Cable vs DSLCable vs DSL
Cable systems can relieve local loop Cable systems can relieve local loop network congestion…network congestion… Pushing out the fiber nodes/fiberPushing out the fiber nodes/fiber Breaking up the local loops in to smaller Breaking up the local loops in to smaller
segmentssegments Expensive, re-engineering of the systemExpensive, re-engineering of the system
Cable vs DSLCable vs DSL
What about availability?What about availability? DSLDSL CableCable
What about security?What about security?