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Understanding Cable Architecture and DOCSIS Protocol

Learn about cable vs. DSL, cable modems, CableLabs, and the DOCSIS system for high-speed internet. Explore architecture, implementation, and how data is transmitted through cable television networks.

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Understanding Cable Architecture and DOCSIS Protocol

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  1. Data Over Cable & DOCSIS Tom Bahnck • Cable Architecture • Cable Implementation • Cable vs. DSL • Cable Modems • Cable Television Laboratories • DOCSIS System Scope • DOCSIS/CableLabs Certified Cable Modems protocol 5/5/2005

  2. Cable Architecture • Cable modem at client, cable modem termination system (CMTS) at central office. • Downstream data sent to all users, like Ethernet, but encrypted (DOCSIS). MAC absorbs appropriate packets. • Upstream seen only by CMTS. • Downstream FDMA, upstream TDMA. Reflects volume of data passing in each direction. • TDMA - better for bursty data (requests, commands, DNS). FDMA - better for statistical multiplexing. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  3. Cable Implementation • TV channels transmitted in 6 MHz bands, in 65 MHz - 850 MHz range. • Data allocated two channels, an upstream and downstream. Upstream bandwidth only 2 MHz, range = 5 - 42 MHz. Downstream range= 42-850 MHz. • Downstream 6 MHz capacity = ~1,000 users, data rate = 30 - 40 Mbps. • Users allocated bandwidth as proportional to total concurrent users. 1 user -> 100% BW, 10 users -> 10%. • Cable providers increase capacity by adding data (6 MHz) channels, segment users to new channels. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  4. Cable vs. DSL • Cable: No performance loss with increased distance from client to central office. • DSL: Permanently slower transfer rates with increased distance from central office. • Quality of data reception similar to quality of TV channel: Central office designed to boost signal enough at origin in order to reach client intact. • Strength of data transmission set by client modem modulator programmed with distance to central office, adjusts signal accordingly. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  5. Cable vs. DSL • Peak downstream data rates for basic service (2004): Comcast Cable 3 Mbps, Verizon DSL 1.5 Mbps. • Price (2004): Comcast Cable ~$45, Verizon DSL ~$35. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  6. Cable Modems • Tuner: Listens for downstream channel, passes data to demodulator. • Demodulator: 64 or 256 QAM. Frames structured in MPEG format for digital set top MPEG stream modulation compatibility. • Modulator: Inverse of demodulator in the upstream. • MAC: Works with CPU to absorb correct packets. • CPU: Motorola PowerPC. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  7. Cable Television Laboratories • Short CableLabs, R&D consortium for cable telecommunications technology. • Members include Comcast, Time Warner, Cablevision. • Research and document new broadband technology. • Certify data over cable products. • Author specifications. • Own specifications through standardization in ITU in the U.S. • Supported through subscription and certification fees. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  8. Cable Television Laboratories • Promote cable over ADSL as better broadband, because idle BW is recoverable (statistical multiplexing). • Publish DOCSIS specifications, adopted by most member companies. Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  9. DOCSIS System Scope Active Directory Kerberos Access Token Descriptors EFS Registry CTMS routes client traffic to ISP servers, operates DOCSIS/CableLabs Certified Cable Modems protocol. Source: DOCSIS Set-top Gateway (DSG) Interface Specification Bahnck 5/5/2005

  10. DOCSIS Protocol • DOCSIS: Data Over Cable Service Interface Specification. Also known as CableLabs Certified Cable Modems protocol. • Defines interface for cable modems operating over cable television system networks. • Most cable modems certified in DOCSIS – sell to any cable provider. • DOCSIS 1.0: 1995, widely deployed • DOCSIS 1.1: 2003, security, QoS for VoIP, interactive gaming, etc. • DOCSIS 2.0: 2005, upstream for symmetric services increased- 30 Mbps. [end] Architecture Implementation Cable vs. DSL I Cable vs. DSL II Cable Modems CableLabs I CableLabs II DOCSIS Bahnck 5/5/2005

  11. Sources • Cable Television Laboratories, Inc. Cable Modem/DOCSIS. http://www.cablemodem.com/, various sources. Cable Television Laboratories, Inc., 2000 – 2005. • Cable Television Laboratories, Inc. DOCSIS Set-Top Gateway (DSG) Interface Specification. Cable Television Laboratories, Inc., 2001 – 2005. • Franklin, Curt. How Cable Modems Work. http://computer.howstuffworks.com/cable-modem.htm. HowStuffWorks, Inc., 1998 – 2005. • Lake, Matt. The Promise of More DSL Speed. http://reviews.cnet.com/4520-6028_7-5144462-1.html. CNET Networks, Inc., 2004. Bahnck 5/5/2005

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