Cable Modem Overview

1
Cable Modem Overview

• EEL 4930 Computer Networks
• Fall 2002
• Bradley C. Spatz

2
Agenda for Today

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

3
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

4
Cox Hybrid-Fiber Coax (HFC)
Ring-in-ring fiber
MTC/Hub
neighborhood coax
5
What is DOCSIS?

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

6
DOCSIS Players
Broadband Router (CMTS)
Cable Modem (CM)
7
DOCSIS Architecture
transparent IP through system
8
CM Initialization

• Scan downstream (DS), sync, listen for DOCSIS
• Listen for upstream descriptor(s)
• Upstream (US) connectivity power ranging
• Establish IP connectivity DHCP
• Determine Time of Day (RFC 868)
• Download operational params (TFTP)
• Register with CMTS
• Establish Baseline Privacy (public key, DES)
• Surf!

9
DOCSIS Protocol Stack
10
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 DSUS

11
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

12
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

13
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

14
Modulation Tradeoffs
15
RF Spectrum Intro

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

6 MHz
V
A
C
analog TV
digital
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RF Spectrum Layout
Downstream
Downstream
Upstream
Upstream
5 MHz
42 MHz
54 MHz
750/860 MHz
550 MHz
650 MHz
30 MHz
5 MHz
42 MHz
54 MHz
750/860 MHz
550 MHz
650 MHz
30 MHz
I
I
Voice
Voice
Voice
Voice
m
m

Digital TV
Analog TV

Digital TV
Analog TV
p
p


Reserve
Reserve
Reserve
Reserve
u
u
Data
Data
Services
Services
Services
Services
l
l
Data
Data
s
Svcs
Svcs
s
e
e
Two
-
Way
Two
-
Way
HDTV,
Two
-
Way
Two
-
Way
HDTV,
"Broadcast"
Spare
"Broadcast"
Spare
VOD
VOD
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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)

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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!

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brad.spatz_at_cox.com www.cox.com