Cable modem theory


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Cable modem theory

  1. 1. Cable Modems: Theory of Operation Gina Colangelo EE194-SDR 04/03/2006
  2. 2. Topics Covered <ul><li>Cable Modems Basics </li></ul><ul><li>Cable Data Network Overview </li></ul><ul><li>Cable Modem Architecture </li></ul><ul><li>Downstream/Upstream Specifications </li></ul><ul><li>Cable Data Network Standards/Protocols </li></ul><ul><li>Conclusion </li></ul>
  3. 3. Cable Modem Basics <ul><li>Cable TV (CATV) Network serves as the Internet Service Provider (ISP) </li></ul><ul><li>Cable Modem modulates/transmits and demodulates/receives to/from a CATV channel </li></ul><ul><li>Downstream: data received at the modem is communicated to one or more PCs on a LAN via Ethernet, USB, PCI Bus, etc. </li></ul><ul><li>Upstream: data requests from the PC are transmitted through the modem to the CATV network via coaxial cable, phone line or wireless. </li></ul><ul><li>CATV data service interfaces to the Internet via Cable Modem Termination System (CMTS ) </li></ul>
  4. 4. Cable Modem Network Overview <ul><li>Headend: DOCSIS-certified CMTS (Cable Modem Termination System) </li></ul><ul><ul><li>One Headend supports @ 2000 Cable Modem Users on a single TV Channel </li></ul></ul><ul><ul><li>CMTS interfaces the CATV network to the Internet </li></ul></ul><ul><li>CMTS output channel combined with TV video signals </li></ul><ul><li>CATV Network to Subscriber via coaxial cable </li></ul><ul><li>One-to-Two splitter: One signal to Set Top Box (STB), other to Cable Modem </li></ul><ul><li>Cable Modem </li></ul><ul><ul><li>One Modem can support up to 16 users in a local-area network </li></ul></ul><ul><li>PC/Ethernet Card </li></ul><ul><ul><li>Cable Modem connected to PC via ethernet, USB, PCI Bus, etc </li></ul></ul>
  5. 5. Cable Modem Architecture <ul><li>Transmit/Upstream </li></ul><ul><ul><li>QPSK/QAM Modulator performs: </li></ul></ul><ul><ul><ul><li>QPSK/QAM-16 modulation </li></ul></ul></ul><ul><ul><ul><li>Reed-Solomon Encoding </li></ul></ul></ul><ul><ul><ul><li>D/A Conversion </li></ul></ul></ul><ul><ul><ul><li>Up-conversion to the selected frequency/channel </li></ul></ul></ul><ul><li>Receive/Downstream </li></ul><ul><ul><li>RF Tuner </li></ul></ul><ul><ul><ul><li>Converts TV Channel to a fixed lower frequency (6-40MHz) </li></ul></ul></ul><ul><ul><li>QAM Demodulator performs: </li></ul></ul><ul><ul><ul><li>A/D conversion </li></ul></ul></ul><ul><ul><ul><li>QAM-64/256 demodulation </li></ul></ul></ul><ul><ul><ul><li>MPEG frame synchronization </li></ul></ul></ul><ul><ul><ul><li>Error Correction (Reed-Solomon) </li></ul></ul></ul><ul><li>MAC - Media Access Control </li></ul><ul><ul><li>Implemented partially in hardware and software </li></ul></ul><ul><li>Data and Control Logic </li></ul>
  6. 6. Receive Path <ul><li>Diplex Filter – splits/combines bands for 2-way capability on CATV systems </li></ul><ul><li>Tuner – isolates TV channel and mixes it down to Analog IF (6-40MHz) </li></ul><ul><li>Analog to Digital Conversion </li></ul><ul><li>Decimation Filters to down-sample to the symbol rate (e.g. CIC) </li></ul><ul><li>QAM Demodulator </li></ul><ul><ul><li>MPEG Frame synchronization </li></ul></ul><ul><ul><li>Automatic Gain Control (AGC) </li></ul></ul><ul><ul><li>Equalizer – removes distortions, and cancels echoes or multi-path conditions </li></ul></ul><ul><ul><li>Carrier Removal </li></ul></ul><ul><ul><li>Automatic Frequency Control (AFC) </li></ul></ul>QAM Demodulator Generic Receive Path of a Cable Modem Diplex Filter (Internal or External to Modem) Analog RF TUNER Analog IF ADC Fixed Freq Decimate to Symbol Rate QAM Demodulation Digital Ctrl Logic/Signals LNA Tunable BPF Digital Ctrl Logic/Signals VGA LPF Variable Freq Dig BB
  7. 7. Downstream Specifications <ul><li>Frequency Range: 65-850MHz </li></ul><ul><li>Bandwidth: 6MHz (USA) or 8MHz (EU) </li></ul><ul><li>Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol) </li></ul><ul><li>Data rate: 27-56 Mbps (depends on modulation and bandwidth) </li></ul><ul><li>Continuous stream of data </li></ul><ul><li>Framing: MPEG-2 (based on DOCSIS spec) </li></ul><ul><li>Encryption: DES (Data Encryption Standard) </li></ul><ul><li>Reed-Solomon Forward Error Correction </li></ul><ul><li>* Symbol rate listed under Modulation does not compensate for error correction and other overhead </li></ul><ul><li>** 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 </li></ul>55.2 Mbps 41.4 Mbps 8 MHz 41.6 Mbps 31.2 Mbps 6 MHz 256-QAM 64-QAM
  8. 8. Transmit Path <ul><li>QAM (Burst) Modulator </li></ul><ul><ul><li>Reed-Solomon Encoder (Forward Error Correction) </li></ul></ul><ul><ul><li>Randomizer/Scrambler – whitens the data </li></ul></ul><ul><ul><ul><li>Helps to avoid discrete spurs in output spectrum </li></ul></ul></ul><ul><ul><ul><li>Improves synchronization at the receiver since data is more equiprobable </li></ul></ul></ul><ul><ul><li>Preamble Insertion </li></ul></ul><ul><ul><ul><li>Inserts a training sequence into the system </li></ul></ul></ul><ul><ul><ul><li>Transmitted without R-S Encoding or Scrambling </li></ul></ul></ul><ul><ul><li>Modulation Encoder (QPSK/16-QAM) </li></ul></ul><ul><ul><li>Pulse Shaping (e.g. Raised Root Cosine) </li></ul></ul><ul><ul><li>Programmable Interpolation Filters (e.g. CIC) </li></ul></ul><ul><ul><li>Programmable NCO, digitally modulates carrier anywhere in the Nyquist Bandwidth </li></ul></ul><ul><ul><li>Digital to Analog Conversion </li></ul></ul><ul><li>Variable Gain CATV Line Driver </li></ul>Variable Gain CATV Line Driver Digital Data In QAM Modulator Reed- Solomon Encoder Randomizer Preamble Insertion MUX Pulse-Shaping FIR Filter Pulse-Shaping FIR Filter Modulation Encoder (QPSK/16-QAM)) Programmable Interpolation Filter Programmable Interpolation Filter Digital Control Logic/Signals Program- mable NCO DAC Fixed Freq To Diplexer
  9. 9. Upstream Specifications <ul><li>Frequency Range: 5-65 MHz </li></ul><ul><li>Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz typically) </li></ul><ul><li>Modulation: QPSK (2 bits/symbol) or 16-QAM (4 bits/symbol) </li></ul><ul><li>Data rate: Variable 320 Kbps to 10 Mbps </li></ul><ul><li>Transmit bursts of data in timeslots (TDM) </li></ul><ul><li>Encryption: DES </li></ul><ul><li>Reed Solomon Forward Error Correction </li></ul>
  10. 10. Media Access Control (MAC) <ul><li>MAC layer provides general requirements for many cable modems subscribers to share a single upstream data channel for transmission: </li></ul><ul><ul><li>Communication layer between Cable Modem and CMTS </li></ul></ul><ul><ul><li>Allocates Time-slots for transmission (upstream BW) </li></ul></ul><ul><ul><li>Assigns upstream frequency and data rate </li></ul></ul><ul><ul><li>Defines collision detection and retransmission </li></ul></ul><ul><ul><li>Runs calibrations on TX levels and time references (compensates for cable delays and losses) </li></ul></ul><ul><ul><li>Includes ability to integrate encryption of user data </li></ul></ul>
  11. 11. DOCSIS Standard <ul><li>Several CATV Standards: </li></ul><ul><ul><li>Various Proprietary protocols – 1G </li></ul></ul><ul><ul><li>DOCSIS/MNCS – mainly USA (2G) </li></ul></ul><ul><ul><li>DAVIC/DVB – mainly Europe (2G) </li></ul></ul><ul><ul><li>IEEE 802.14 (3G??) </li></ul></ul><ul><li>DOCSIS – Data over Cable Service Interface Specifications </li></ul><ul><ul><li>Defined by the Multimedia Cable Network System Partners (MCNS) </li></ul></ul><ul><ul><li>Set of standards for transferring data by CATV and cable modems </li></ul></ul><ul><ul><li>Enable multi-vendor interoperability </li></ul></ul><ul><ul><li>Defines all system layers such as the MAC and PHY Layers </li></ul></ul><ul><ul><li>Defines User Privacy Standards </li></ul></ul>
  12. 12. Cable vs DSL <ul><li>Bandwidth/Data Rates </li></ul><ul><ul><li>Cable </li></ul></ul><ul><ul><ul><li>Faster theoretical speeds (@ 30+ Mbps) </li></ul></ul></ul><ul><ul><ul><li>Average Plan: 6 Mbps down, 384 Kbps up ($43/mo + cable plan) </li></ul></ul></ul><ul><ul><ul><li>Scales by the number of subscribers using a particular channel </li></ul></ul></ul><ul><ul><ul><li>This problem can by resolved by the cable company adding more channels </li></ul></ul></ul><ul><ul><li>DSL </li></ul></ul><ul><ul><ul><li>Slower data rates (< 10 Mbps, except for the unpopular VDSL) </li></ul></ul></ul><ul><ul><ul><li>Average Plan: 1.5 Mbps down, and 128 kbps up ($35/mo + phone line) </li></ul></ul></ul><ul><ul><ul><li>More consistent speeds </li></ul></ul></ul><ul><li>Performance/Quality </li></ul><ul><ul><li>Cable: designed to provide digital signals at a particular quality (variable gain on upstream provides proper signal strength) </li></ul></ul><ul><ul><li>DSL: quality depends on distance from central office </li></ul></ul>
  13. 13. Websites for More Info: <ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul>