The Physical Layer

1
The Physical Layer

• Twisted Pair (Modem, DSL, PSTN)
• Coaxial Cable (coax)
• Fiber
• Wireless

2
Coaxial Cable

• 50 ohm
• 75 ohm
• Bandwidth 1 GHz

3
Internet over Cable
100-2000 homes
DOCSIS Data Over Cable Service Interface
Specification
CMTS Cable Modem Termination System
HFC Hybrid Fiber Coax
4
Cable Architecture
New Services Opportunities
PacketCable
DOCSIS
CableHome

• HVAC control
• Fire sense control
• Security
• Air quality monitoring
• Child monitoring
• Energy management, etc.

CM
CM
CMTS
CM
MPEG Services
CM
CM
Operator Aggregation network
Operator Core Backbone
CM
CMTS
CM
CM

• Remote file sharing
• Shared calendar
• Unified messaging
• Managed services

CM
CMTS
IP Services
CM
CM
CPE
Headend
Backend
5
Cable Modems

• Typical details of the upstream and downstream
  channels in North America.

6
Cable Broadband Spectrum Allocation

• TV band 54 550 MHz, 6MHz per channel
• Downstream 550 750 MHz
• 6MHz QAM-64 36 Mbps (gross), 27 Mbps (net)
• Total effective downstream bandwidth 200 / 6 27
  891 Mbps
• Upstream 5 42 MHz
• 6MHz QPSK 12 Mbps (gross), 9 Mbps (net)
• Total effective upstream bandwidth 37 / 6 9
  54 Mbps

TV band 54 550 MHz, 6MHz per
channel Downstream 550 750 MHz 6MHz QAM-64
(6bit) 36 Mbps (gross), 27 Mbps (net) Total
effective downstream bandwidth 200 / 6 27 891
Mbps Upstream 5 42 MHz 6MHz QPSK (2bit)
12 Mbps (gross), 9 Mbps (net) Total effective
upstream bandwidth 37 / 6 9 54 Mbps
More if QAM-256
7
Cable Modem Operations

• Initialization
• New modem scans for system parameter pkt from
  CMTS and announce its presence
• CMTS assigns a downstream channel, an upstream
  channel, and a minislot assignment (for
  requesting upload bandwidth) to the modem
• Modem determines its distance (ranging) via
  response time
• Reasons for ranging
• Upstream is divided into mini-slots (8 bytes)
• Each up pkt must fit in one or more consecutive
  minislots
• CMTS announces the start of a new round of
  minislots periodically, but the starting gun is
  not heard at the same time due to variable delays
• Each modem computes the actual start time of the
  first minislot by calibration with its CMTS
  distance
• Modem gets an IP address via DHCP
• CMTS and modem establish secret key
  (Diffie-Hellman algorithm)
• Modem completes its login to CMTS
• Upload process
• Modem request bandwidth via designated minislot
  (req)
• CMTS grants request by assigning needed minislots
  to the modem (ack)
• If Modem does not receive ack, wait for random
  time to retry, double the retry time, etc.
  (binary exponential backoff)
• Download process
• No contention

8
DOCSIS Road Map
DOCSIS Key Features Benefits/Services
DOCSIS 2.0 (30 Mbps u/s)2002 Mandatory S-CDMA/ TDMA Best of DOCSIS Symmetric services Peer-to-peer Business-to-business (20 T1 capacity)
DOCSIS 1.1 (10 Mbps u/s)2001 QoS Pre-EQ Operations Security Tiered service Double u/s capacity Lower ops costs Better than competitor
DOCSIS 1.0 (5 Mbps u/s)1999 Specd for retail Standard spec High speed data Internet access
9
OSI DOCSIS DOCSIS
Application Application DOCSIS Control Message
Transport TCP/UDP DOCSIS Control Message
Network IP DOCSIS Control Message
Link IEEE 802.2 IEEE 802.2
Physical UpStream DownStream
Physical TDMA (mini-slots)5 - 42(65) MHzQPSK/16-QAM TDM (MPEG)42(65) - 850 MHz64/256-QAMITU-T J.83 Annex B(A)
Items in parenthesis refer to EuroDOCSIS, which
is a version of DOCSIS with a modified physical
layer targeted at the more DVB centric European
market.