Cable modem theory

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  • 1. Cable Modems: Theory 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 @ 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 Diplex Filter (Internal or External toModem) Analog RF TUNER Analog IF ADC Fixed Freq Decimateto Symbol Rate QAM Demodulation Digital Ctrl Logic/Signals LNA Tunable BPF Digital Ctrl Logic/Signals VGA LPF Variable Freq Dig BB 7. 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

55.2 Mbps 41.4 Mbps 8 MHz 41.6 Mbps 31.2 Mbps 6 MHz 256-QAM 64-QAM 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

Variable Gain CATV LineDriver Digital Data In QAM Modulator Reed- Solomon Encoder Randomizer Preamble Insertion MUX Pulse-Shaping FIR Filter Pulse-Shaping FIR Filter ModulationEncoder(QPSK/16-QAM)) Programmable InterpolationFilter Programmable InterpolationFilter Digital Control Logic/Signals Program- mable NCO DAC Fixed Freq To Diplexer 9. 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

12. Cable vs DSL

  • Bandwidth/Data Rates
    • Cable
      • Faster theoretical speeds (@ 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 (< 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

13. Websites for More Info: