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?