Cable Modem OverviewEEL 4930 – Computer Networks

Fall 2002

Bradley C. Spatz

Agenda for Today

• Review national/regional/HFC networks• Intro to DOCSIS• CM Operation• Protocol Layers, RF overview• Future of DOCSIS

• Questions

HFC Architecture

• Hybrid Fiber Coax

• Broadband signal is transmitted over fiber to a local neighborhood “node”

• Node connects to a small coax network• 500 or less homes passed per node

• Each node is conceptually a subnet or collision domain

Cox Hybrid-Fiber Coax (HFC)

MTC/Hub

node

node

node

node

Ring-in-ringfiber

neighborhoodcoax

What is DOCSIS?

• Data Over Cable Service Interface Spec.• Developed by cable industry, including Cox• CableLabs is R&D• Ensures vendor interoperability• Internet-track standards• Baseline Privacy Security (public key, DES)• External modem: ethernet/USB

DOCSIS PlayersBroadband Router (CMTS) Cable Modem (CM)

DOCSIS Architecture

HFCnetwork

Internet

Mgmt.network

CMTS

MTC/Hub

cable modem

transparent IP through system

CM Initialization

1. Scan downstream (DS), sync, listen for DOCSIS

2. Listen for upstream descriptor(s)

3. Upstream (US) connectivity: power ranging

4. Establish IP connectivity: DHCP

5. Determine Time of Day (RFC 868)

6. Download operational params (TFTP)

7. Register with CMTS

8. Establish Baseline Privacy (public key, DES)

9. Surf!

DOCSIS Protocol Stack

IP Forwarding

Not your traditional LAN:• DS and US are at different frequencies• Each channel is simplex• DS is a shared media• US is point-to-point• Conceptual MAC forwarder connects DS+US

Protocol Layers

• Network+: DHCP, TFTP, TOD, SNMP• Network: IPv4• Data Link: structured as IEEE 802

• Logical Link Control (LLC) sublayer• Link-Layer Security sublayer (DOCSIS)• Media Access Control (MAC) sublayer

Protocol Layers: MAC

• CMTS controls bandwidth (collision-free)• stream of mini-slots in the upstream• dynamic mix of contention/reservation• efficiency via variable length packets• extensions for ATM (Europe) or other PDU format• class of service support (e.g. VoIP)• wide range of data rates

Protocol Layers: Physical

• DS Transmission Convergence sublayer• 188-byte MPEG-2 packets (4-5 byte header)

• Physical Media Dependent (PMD) sublayer• DS: 64, 256 QAM in 6 MHz carrier (27 or 38 Mbps)• Frequency agility• Programmable Reed-Soloman FEC, interleaver• US: TDMA• US: QPSK, 16 QAM, 200-3200 kHz carrier (up to 10 Mbps)• US: multiple symbol rates

Modulation Tradeoffs

RF Spectrum Intro

analog TV digital

V

C

A

6 MHz

• Each TV channel is 6 MHz analog signal• Includes video, color, and audio sub-carriers

RF Spectrum Layout

5 MHz5 MHz 42 MHz42 MHz 54 MHz54 MHzDownstreamDownstream

750/860 MHz750/860 MHzUpstreamUpstream

TwoTwo--WayWay"Broadcast""Broadcast"

TwoTwo--WayWay

550 MHz550 MHz

VoiceVoice& &

DataDataSvcsSvcs

Digital TVDigital TVServicesServices

Analog TVAnalog TVServicesServices

650 MHz650 MHz30 MHz30 MHz

SpareSpareHDTV,HDTV,VODVOD

ReserveReserveReserveReserve

IImmppuullssee

VoiceVoice&&

DataData

DOCSIS Future

• DOCSIS 1.1• Interoperable with 1.0• QoS: data rate and latency guarantees (VoIP)• Improved security• Transmit pre-equalization

• DOCSIS 2.0• Interoperable with 1.0/1.1• Wider upstream (6.4 MHz)• New modulation formats: A-TDMA, S-CDMA:• Symmetric services (30 Mbps upstream)

DOCSIS Future

• DOCSIS in everything• Integrated into set-top boxes, home gateways• VoIP modems (“MTAs”)• DOCSIS adapters for gaming consoles

• Data Over Cable Cable Over Data

• The triumph of IP!

brad.spatz@cox.com

www.cox.com