Cable Modems: Theory of Operation
Gina Colangelo EE194-SDR 04/03/2006
Topics Covered
Cable Modems Basics Cable Data Network Overview Cable Modem Architecture Downstream/Upstream Specifications Cable Data Network Standards/Protocols Conclusion
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 )
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 Cable Modem connected to PC via ethernet, USB, PCI Bus, etc
PC/Ethernet Card
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 (ReedSolomon)
MAC - Media Access Control
Implemented partially in hardware and software
Data and Control Logic
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)
TUNER
Diplex Filter (Internal or External to Modem)
Analog RF Analog IF
QAM Demodulator
VGA
LNA
Tunable BPF
Variable Freq
LPF
ADC
Fixed Freq
Decimate to Symbol Rate
QAM Demodulation
Dig BB
Digital Ctrl Logic/Signals
Digital Ctrl Logic/Signals
Generic Receive Path of a Cable Modem
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)
64-QAM
256-QAM
6 MHz 31.2 Mbps 41.6 Mbps 8 MHz 41.4 Mbps 55.2 Mbps
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
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
To Diplexer
QAM Modulator
Programmable Interpolation Filter
Programmable Interpolation Filter
Preamble Insertion
Pulse-Shaping FIR Filter Pulse-Shaping FIR Filter Modulation Encoder (QPSK/16-QAM)) ReedSolomon Encoder Digital Data In
DAC
Programmable NCO
MUX
Randomizer
Variable Gain CATV Line Driver
Fixed Freq
Digital Control Logic/Signals
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
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
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
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
Websites for More Info:
http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1 http://www.iec.org/online/tutorials/acrobat/cable_mod.pdf http://www.cable-modems.org/tutorial/index.htm#TOC http://www.broadcom.com/collateral/pb/3349-PB01-R.pdf http://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pd f http://www.wideband.com/products/Modems/QAM20Mod/QAMmod.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/descriptions.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