Cable Modems: Theory of Operation

1
Cable ModemsTheory of Operation

• Gina Colangelo
• EE194-SDR
• 04/03/2006

2
Topics Covered

• Cable Modems Basics
• Cable Data Network Overview
• Cable Modem Architecture
• Downstream/Upstream Specifications
• Cable Data Network Standards/Protocols
• Conclusion

3
Cable Modem Basics

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

4
Cable Modem Network Overview

• Headend DOCSIS-certified CMTS (Cable Modem
  Termination System)
• One Headend supports _at_ 2000 Cable Modem Users on
  a single TV Channel
• CMTS interfaces the CATV network to the Internet
• CMTS output channel combined with TV video
  signals
• CATV Network to Subscriber via coaxial cable
• One-to-Two splitter One signal to Set Top Box
  (STB), other to Cable Modem
• Cable Modem
• One Modem can support up to 16 users in a
  local-area network
• PC/Ethernet Card
• Cable Modem connected to PC via ethernet, USB,
  PCI Bus, etc

5
Cable Modem Architecture

• Transmit/Upstream
• QPSK/QAM Modulator performs
• QPSK/QAM-16 modulation
• Reed-Solomon Encoding
• D/A Conversion
• Up-conversion to the selected frequency/channel
• Receive/Downstream
• RF Tuner
• Converts TV Channel to a fixed lower frequency
  (6-40MHz)
• QAM Demodulator performs
• A/D conversion
• QAM-64/256 demodulation
• MPEG frame synchronization
• Error Correction (Reed-Solomon)
• MAC - Media Access Control
• Implemented partially in hardware and software
• Data and Control Logic

6
Receive Path

• Diplex Filter splits/combines bands for 2-way
  capability on CATV systems
• Tuner isolates TV channel and mixes it down to
  Analog IF (6-40MHz)
• Analog to Digital Conversion
• Decimation Filters to down-sample to the symbol
  rate (e.g. CIC)
• QAM Demodulator
• MPEG Frame synchronization
• Automatic Gain Control (AGC)
• Equalizer removes distortions, and cancels
  echoes or multi-path conditions
• Carrier Removal
• Automatic Frequency Control (AFC)

QAM Demodulator
Generic Receive Path of a Cable Modem
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Downstream Specifications

• Frequency Range 65-850MHz
• Bandwidth 6MHz (USA) or 8MHz (EU)
• Modulation 64-QAM (6 bits/symbol) or 256-QAM (8
  bits/symbol)
• Data rate 27-56 Mbps (depends on modulation and
  bandwidth)
• Continuous stream of data
• Framing MPEG-2 (based on DOCSIS spec)
• Encryption DES (Data Encryption Standard)
• Reed-Solomon Forward Error Correction
• 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

8
Transmit Path

• QAM (Burst) Modulator
• Reed-Solomon Encoder (Forward Error Correction)
• Randomizer/Scrambler whitens the data
• Helps to avoid discrete spurs in output spectrum
• Improves synchronization at the receiver since
  data is more equiprobable
• Preamble Insertion
• Inserts a training sequence into the system
• Transmitted without R-S Encoding or Scrambling
• Modulation Encoder (QPSK/16-QAM)
• Pulse Shaping (e.g. Raised Root Cosine)
• Programmable Interpolation Filters (e.g. CIC)
• Programmable NCO, digitally modulates carrier
  anywhere in the Nyquist Bandwidth
• Digital to Analog Conversion
• Variable Gain CATV Line Driver

QAM Modulator
To Diplexer
Preamble Insertion
Programmable Interpolation Filter
Digital Data In
Program- mable NCO
Pulse-Shaping FIR Filter
Reed- Solomon Encoder
Modulation Encoder (QPSK/16-QAM))
Randomizer
MUX
DAC
Pulse-Shaping FIR Filter
Programmable Interpolation Filter
Variable Gain CATV Line Driver
Fixed Freq
Digital Control Logic/Signals
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Upstream Specifications

• Frequency Range 5-65 MHz
• Variable Bandwidth 200 kHz to 3.2 MHz (2 MHz
  typically)
• Modulation QPSK (2 bits/symbol) or 16-QAM (4
  bits/symbol)
• Data rate Variable 320 Kbps to 10 Mbps
• Transmit bursts of data in timeslots (TDM)
• Encryption DES
• Reed Solomon Forward Error Correction

10
Media Access Control (MAC)

• MAC layer provides general requirements for many
  cable modems subscribers to share a single
  upstream data channel for transmission
• Communication layer between Cable Modem and CMTS
• Allocates Time-slots for transmission (upstream
  BW)
• Assigns upstream frequency and data rate
• Defines collision detection and retransmission
• Runs calibrations on TX levels and time
  references (compensates for cable delays and
  losses)
• Includes ability to integrate encryption of user
  data

11
DOCSIS Standard

• Several CATV Standards
• Various Proprietary protocols 1G
• DOCSIS/MNCS mainly USA (2G)
• DAVIC/DVB mainly Europe (2G)
• IEEE 802.14 (3G??)
• DOCSIS Data over Cable Service Interface
  Specifications
• Defined by the Multimedia Cable Network System
  Partners (MCNS)
• Set of standards for transferring data by CATV
  and cable modems
• Enable multi-vendor interoperability
• Defines all system layers such as the MAC and PHY
  Layers
• Defines User Privacy Standards

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Cable vs DSL

• Bandwidth/Data Rates
• Cable
• Faster theoretical speeds (_at_ 30 Mbps)
• Average Plan 6 Mbps down, 384 Kbps up (43/mo
  cable plan)
• Scales by the number of subscribers using a
  particular channel
• This problem can by resolved by the cable company
  adding more channels
• DSL
• Slower data rates (lt 10 Mbps, except for the
  unpopular VDSL)
• Average Plan 1.5 Mbps down, and 128 kbps up
  (35/mo phone line)
• More consistent speeds
• Performance/Quality
• Cable designed to provide digital signals at a
  particular quality (variable gain on upstream
  provides proper signal strength)
• DSL quality depends on distance from central
  office

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Websites for More Info

• http//www.ciscopress.com/articles/article.asp?p3
  1289seqNum2rl1
• http//www.iec.org/online/tutorials/acrobat/cable_
  mod.pdf
• http//www.cable-modems.org/tutorial/index.htmTOC
  
• http//www.broadcom.com/collateral/pb/3349-PB01-R.
  pdf
• http//www.analog.com/UploadedFiles/Data_Sheets/13
  374136AD9853_c.pdf
• http//www.wideband.com/products/Modems/QAM20Mod/Q
  AMmod.html
• http//focus.ti.com/lit/an/slwa023/slwa023.pdf
• http//www.broadcom.com/collateral/pb/3419-PB02-R.
  pdf
• http//www.cableaml.com/website3/wireless_catv/des
  criptions.htm
• http//www.fcc.gov/mb/engineering/cablemqa.html
• http//www.cablemodem.com/
• http//www.nextgendc.com/index.htm
• http//computer.howstuffworks.com/cable-modem.htm
• http//compnetworking.about.com/od/dslvscablemodem
  /a/dslcablecompare.htm