Cable Modems:Cable Modems:Theory of OperationTheory of Operation

Gina ColangeloGina Colangelo

EE194-SDREE194-SDR

04/03/200604/03/2006

Topics CoveredTopics Covered

Cable Modems BasicsCable Modems Basics

Cable Data Network OverviewCable Data Network Overview

Cable Modem ArchitectureCable Modem Architecture

Downstream/Upstream SpecificationsDownstream/Upstream Specifications

Cable Data Network Standards/ProtocolsCable Data Network Standards/Protocols

ConclusionConclusion

Cable Modem BasicsCable Modem Basics

Cable TV (CATV) Network serves as the Internet Service Cable TV (CATV) Network serves as the Internet Service Provider (ISP)Provider (ISP)Cable Modem modulates/transmits and Cable Modem modulates/transmits and demodulates/receives to/from a CATV channeldemodulates/receives to/from a CATV channelDownstream: data received at the modem is Downstream: data received at the modem is communicated to one or more PCs on a LAN via communicated to one or more PCs on a LAN via Ethernet, USB, PCI Bus, etc.Ethernet, USB, PCI Bus, etc.Upstream: data requests from the PC are transmitted Upstream: data requests from the PC are transmitted through the modem to the CATV network via coaxial through the modem to the CATV network via coaxial cable, phone line or wireless. cable, phone line or wireless. CATV data service interfaces to the Internet via Cable CATV data service interfaces to the Internet via Cable Modem Termination System (CMTS )Modem Termination System (CMTS )

Cable Modem Network OverviewCable Modem Network OverviewHeadend: DOCSIS-certified CMTS (Cable Modem Termination System)Headend: DOCSIS-certified CMTS (Cable Modem Termination System)

One Headend supports @ 2000 Cable Modem Users on a single TV ChannelOne Headend supports @ 2000 Cable Modem Users on a single TV Channel CMTS interfaces the CATV network to the InternetCMTS interfaces the CATV network to the Internet

CMTS output channel combined with TV video signalsCMTS output channel combined with TV video signalsCATV Network to Subscriber via coaxial cableCATV Network to Subscriber via coaxial cableOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable ModemOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable ModemCable ModemCable Modem

One Modem can support up to 16 users in a local-area networkOne Modem can support up to 16 users in a local-area networkPC/Ethernet CardPC/Ethernet Card

Cable Modem connected to PC via ethernet, USB, PCI Bus, etcCable Modem connected to PC via ethernet, USB, PCI Bus, etc

Cable Modem ArchitectureCable Modem ArchitectureTransmit/UpstreamTransmit/Upstream

QPSK/QAM Modulator performs:QPSK/QAM Modulator performs:QPSK/QAM-16 modulation QPSK/QAM-16 modulation Reed-Solomon EncodingReed-Solomon EncodingD/A ConversionD/A ConversionUp-conversion to the selected Up-conversion to the selected frequency/channelfrequency/channel

Receive/DownstreamReceive/Downstream RF TunerRF Tuner

Converts TV Channel to a fixed Converts TV Channel to a fixed lower frequency (6-40MHz)lower frequency (6-40MHz)

QAM Demodulator performs:QAM Demodulator performs: A/D conversionA/D conversionQAM-64/256 demodulationQAM-64/256 demodulationMPEG frame synchronizationMPEG frame synchronizationError Correction (Reed-Error Correction (Reed-Solomon)Solomon)

MAC - Media Access Control MAC - Media Access Control Implemented partially in Implemented partially in

hardware and softwarehardware and softwareData and Control LogicData and Control Logic

Receive PathReceive PathDiplex Filter – splits/combines bands for 2-way capability on CATV systemsDiplex Filter – splits/combines bands for 2-way capability on CATV systemsTuner – isolates TV channel and mixes it down to Analog IF (6-40MHz)Tuner – isolates TV channel and mixes it down to Analog IF (6-40MHz)Analog to Digital ConversionAnalog to Digital ConversionDecimation Filters to down-sample to the symbol rate (e.g. CIC)Decimation Filters to down-sample to the symbol rate (e.g. CIC)QAM DemodulatorQAM Demodulator

MPEG Frame synchronizationMPEG Frame synchronization Automatic Gain Control (AGC)Automatic Gain Control (AGC) Equalizer – removes distortions, and cancels echoes or multi-path conditionsEqualizer – removes distortions, and cancels echoes or multi-path conditions Carrier RemovalCarrier Removal Automatic Frequency Control (AFC) Automatic Frequency Control (AFC)

QAM Demodulator

Generic Receive Path of a Cable Modem

DiplexFilter

(Internal orExternal to

Modem)

Analog RF

TUNERAnalog IF

ADC

FixedFreq

Decimate to Symbol

Rate

QAMDemodulation

Digital Ctrl Logic/Signals

LNA TunableBPF

Digital Ctrl Logic/Signals

VGA LPF

VariableFreq

Dig BB

Downstream SpecificationsDownstream SpecificationsFrequency Range: 65-850MHzFrequency Range: 65-850MHzBandwidth: 6MHz (USA) or 8MHz (EU)Bandwidth: 6MHz (USA) or 8MHz (EU)Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)Data rate: 27-56 Mbps (depends on modulation and bandwidth)Data rate: 27-56 Mbps (depends on modulation and bandwidth)

Continuous stream of dataContinuous stream of dataFraming: MPEG-2 (based on DOCSIS spec)Framing: MPEG-2 (based on DOCSIS spec)Encryption: DES (Data Encryption Standard)Encryption: DES (Data Encryption Standard)Reed-Solomon Forward Error CorrectionReed-Solomon Forward Error Correction

* Symbol rate listed under Modulation does not compensate for error correction and other overhead* Symbol rate listed under Modulation does not compensate for error correction and other overhead** Data rates listed in the table use a symbol rate of 6.9 Msym/s for 8MHz BW and 5.2Msym/s for 6MHz BW** Data rates listed in the table use a symbol rate of 6.9 Msym/s for 8MHz BW and 5.2Msym/s for 6MHz BW

64-QAM64-QAM 256-QAM256-QAM

6 MHz6 MHz 31.2 31.2 MbpsMbps

41.6 Mbps41.6 Mbps

8 MHz8 MHz 41.4 41.4 MbpsMbps

55.2 Mbps55.2 Mbps

Transmit PathTransmit PathQAM (Burst) ModulatorQAM (Burst) Modulator

Reed-Solomon Encoder (Forward Error Correction)Reed-Solomon Encoder (Forward Error Correction) Randomizer/Scrambler – whitens the dataRandomizer/Scrambler – whitens the data

Helps to avoid discrete spurs in output spectrumHelps to avoid discrete spurs in output spectrumImproves synchronization at the receiver since data is more equiprobableImproves synchronization at the receiver since data is more equiprobable

Preamble InsertionPreamble InsertionInserts a training sequence into the systemInserts a training sequence into the systemTransmitted without R-S Encoding or ScramblingTransmitted without R-S Encoding or Scrambling

Modulation Encoder (QPSK/16-QAM)Modulation Encoder (QPSK/16-QAM) Pulse Shaping (e.g. Raised Root Cosine)Pulse Shaping (e.g. Raised Root Cosine) Programmable Interpolation Filters (e.g. CIC)Programmable Interpolation Filters (e.g. CIC) Programmable NCO, digitally modulates carrier anywhere in the Nyquist BandwidthProgrammable NCO, digitally modulates carrier anywhere in the Nyquist Bandwidth Digital to Analog ConversionDigital to Analog Conversion

Variable Gain CATV Line DriverVariable Gain CATV Line Driver

Variable GainCATV Line

Driver

DigitalData In

QAM Modulator

Reed-SolomonEncoder

Randomizer

PreambleInsertion

MUX

Pulse-ShapingFIR Filter

Pulse-ShapingFIR Filter

Modulation Encoder

(QPSK/16-QAM))

ProgrammableInterpolation

Filter

ProgrammableInterpolation

Filter

Digital Control Logic/Signals

Program-mableNCO

DAC

FixedFreq

To Diplexer

Upstream SpecificationsUpstream Specifications

Frequency Range: 5-65 MHzFrequency Range: 5-65 MHz

Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz typically)typically)

Modulation: QPSK (2 bits/symbol) or 16-QAM (4 Modulation: QPSK (2 bits/symbol) or 16-QAM (4 bits/symbol)bits/symbol)

Data rate: Variable 320 Kbps to 10 MbpsData rate: Variable 320 Kbps to 10 Mbps

Transmit bursts of data in timeslots (TDM)Transmit bursts of data in timeslots (TDM)

Encryption: DESEncryption: DES

Reed Solomon Forward Error CorrectionReed Solomon Forward Error Correction

Media Access Control (MAC)Media Access Control (MAC)

MAC layer provides general requirements for MAC layer provides general requirements for many cable modems subscribers to share a many cable modems subscribers to share a single upstream data channel for transmission:single upstream data channel for transmission: Communication layer between Cable Modem and Communication layer between Cable Modem and

CMTSCMTS Allocates Time-slots for transmission (upstream BW)Allocates Time-slots for transmission (upstream BW) Assigns upstream frequency and data rateAssigns upstream frequency and data rate Defines collision detection and retransmissionDefines collision detection and retransmission Runs calibrations on TX levels and time references Runs calibrations on TX levels and time references

(compensates for cable delays and losses)(compensates for cable delays and losses) Includes ability to integrate encryption of user dataIncludes ability to integrate encryption of user data

DOCSIS StandardDOCSIS StandardSeveral CATV Standards:Several CATV Standards:

Various Proprietary protocols – 1GVarious Proprietary protocols – 1G DOCSIS/MNCS – mainly USA (2G)DOCSIS/MNCS – mainly USA (2G) DAVIC/DVB – mainly Europe (2G)DAVIC/DVB – mainly Europe (2G) IEEE 802.14 (3G??)IEEE 802.14 (3G??)

DOCSIS – Data over Cable Service DOCSIS – Data over Cable Service Interface SpecificationsInterface Specifications

Defined by the Multimedia Cable Network Defined by the Multimedia Cable Network System Partners (MCNS)System Partners (MCNS)

Set of standards for transferring data by Set of standards for transferring data by CATV and cable modemsCATV and cable modems

Enable multi-vendor interoperabilityEnable multi-vendor interoperability Defines all system layers such as the MAC Defines all system layers such as the MAC

and PHY Layers and PHY Layers Defines User Privacy StandardsDefines User Privacy Standards

Cable vs DSLCable vs DSL

Bandwidth/Data RatesBandwidth/Data Rates CableCable

Faster theoretical speeds (@ 30+ Mbps)Faster theoretical speeds (@ 30+ Mbps)Average Plan: 6 Mbps down, 384 Kbps up ($43/mo + cable plan)Average Plan: 6 Mbps down, 384 Kbps up ($43/mo + cable plan)Scales by the number of subscribers using a particular channelScales by the number of subscribers using a particular channelThis problem can by resolved by the cable company adding more channelsThis problem can by resolved by the cable company adding more channels

DSLDSLSlower data rates (< 10 Mbps, except for the unpopular VDSL)Slower data rates (< 10 Mbps, except for the unpopular VDSL)Average Plan: 1.5 Mbps down, and 128 kbps up ($35/mo + phone line)Average Plan: 1.5 Mbps down, and 128 kbps up ($35/mo + phone line)More consistent speedsMore consistent speeds

Performance/QualityPerformance/Quality Cable: designed to provide digital signals at a particular quality (variable Cable: designed to provide digital signals at a particular quality (variable

gain on upstream provides proper signal strength)gain on upstream provides proper signal strength) DSL: quality depends on distance from central officeDSL: quality depends on distance from central office

Websites for More Info:Websites for More Info:http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1http://www.iec.org/online/tutorials/acrobat/cable_mod.pdfhttp://www.iec.org/online/tutorials/acrobat/cable_mod.pdfhttp://www.cable-modems.org/tutorial/index.htm#TOChttp://www.cable-modems.org/tutorial/index.htm#TOChttp://www.broadcom.com/collateral/pb/3349-PB01-R.pdfhttp://www.broadcom.com/collateral/pb/3349-PB01-R.pdfhttp://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdhttp://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdffhttp://www.wideband.com/products/Modems/QAM20Mod/QAMmod.htmlhttp://www.wideband.com/products/Modems/QAM20Mod/QAMmod.htmlhttp://focus.ti.com/lit/an/slwa023/slwa023.pdfhttp://focus.ti.com/lit/an/slwa023/slwa023.pdfhttp://www.broadcom.com/collateral/pb/3419-PB02-R.pdfhttp://www.broadcom.com/collateral/pb/3419-PB02-R.pdfhttp://www.cableaml.com/website3/wireless_catv/descriptions.htmhttp://www.cableaml.com/website3/wireless_catv/descriptions.htmhttp://www.fcc.gov/mb/engineering/cablemqa.htmlhttp://www.fcc.gov/mb/engineering/cablemqa.htmlhttp://www.cablemodem.com/http://www.cablemodem.com/http://www.nextgendc.com/index.htmhttp://www.nextgendc.com/index.htmhttp://computer.howstuffworks.com/cable-modem.htmhttp://computer.howstuffworks.com/cable-modem.htmhttp://compnetworking.about.com/od/dslvscablemodem/a/http://compnetworking.about.com/od/dslvscablemodem/a/dslcablecompare.htmdslcablecompare.htm