DOCSIS Cable Modem Connection Process

1
DOCSIS Cable Modem Connection Process
2
Objectives

• Examine a DOCSIS system
• Define the DOCSIS modes
• RF Return
• Telco Return
• Learn the DOCSIS Downstream and Upstream
  Parameters
• Define the DOCSIS modem registration process

3
DOCSIS Block Diagram

• Principal Function of the DOCSIS Cable Modem
  System Is to Transmit Internet Protocol (IP)
  Packets Transparently Between the Head end and
  the Subscriber Location.
• The DOCSIS System Consists of
• Cable Modem Termination System (CMTS) located at
  the headed
• Cable Network
• Cable Modem (CM) located at the Customer Premise

4
DOCSIS Support Devices
HFC

Cable Modem
COAX
Splitter
PC or MAC
Television
Home Subscriber
TOD Time of Day TFTP Trivial File Transfer
Protocol DHCP Dynamic Host Configuration
Protocol
5
DOCSIS DHCP Server

• DHCP Server
• Assigns IP addresses to client computers
• addresses are leased to clients (Cable Modems
  or CPEs) for a period of time
• IP addresses can be reserved for specific clients
  or assigned from pools
• clients may be authenticated based on their MAC
  address
• address may be assigned from different pools
  based on extended options

6
DHCP Process

• The following parameters will be requested by the
  Cable Modem (CM) from the DHCP server
• IP address of the CM
• IP address of the TFTP Server (for DOCSIS
  Configuration file)
• IP address of the DHCP Relay Agent (if the DCHP
  server resides on a different network than the
  CM)
• TFTP/DOCSIS Configuration file name
• Subnet Mask to be used by the CM
• Time offset of the CM from Universal Coordinated
  Time (UTC)
• Default IP Gateway
• Time of Day Server IP address
• SYSLOG Server IP address

7
DOCSIS ToD Server

• ToD Server
• Internet Time Protocol (ITP)
• RFC 868
• UDP and TCP requests honored on port 37
• 32-bit value defining the number of seconds
  since 0000 (midnight January 1, 1900 GMT)

8
DOCSIS TFTP Server

• TFTP Server
• Trivial File Transfer Protocol
• (RFC 1350)
• UDP port 69
• Small and easy to implement
• Read and write to and from remote servers

9
TFTP Process

• The following settings MUST be included in the
  configuration file
• Network Access Configuration Setting
• Class of Service Configuration Setting
• The following settings are optional
• Downstream Frequency
• Upstream Channel ID
• Vendor ID
• Baseline Privacy
• Software Upgrade filename
• SNMP Write-Access Control
• SNMP MIB Object
• Software Server IP Address
• CPE Ethernet MAC Address
• Maximum Number of CPEs (32 Max)
• SNMP IP Address (if applicable)
• Telephone Settings (if applicable)
• Vendor-Specific Configuration (if applicable)

10
Cable Modem ArchitecturesRF Return

• RF-Return
• Suited for CATV networks that have been fully
  upgraded for two-way communications
• Delivers high-speed data downstream and upstream
  over broadband network
• DOCSIS establishes standard specification for
  data communications over HFC network

11
Cable Modem ArchitecturesTelco Return

• Telco-Return Suited for CATV networks without
  two-way capability
• Delivers high-speed data downstream over
  broadband network
• Relies on dial-up networking technology for
  return data
• Does not require HFC plant upgrade to two-way RF
• DOCSIS also specifies data communications using a
  telephone-return architecture
• Support for MMDS Wireless systems, DOCSIS does
  not support MMDS 2-Way

12
DOCSIS Protocol Signaling

• Frames and Timing
• MPEG Frames
• 188 Bytes, 4 Byte header
• Synchronous Transmission
• Clock Synch messages from head end (613 per
  second)
• One source per downstream
• Multiple sources per upstream requiring time
  sharing
• Cable Modems identified by 16 bit Service ID
  (SID)

13
DOCSIS Protocol and Signaling contd.

• Frames and Timing
• Upstream Time Sharing (TDMA)
• Time allocation MAP from head end (every 4 ms)
• Upstream time allocated for Cable Modems in mini
  slots
• (Mini-slot 8 ticks, Tick 6.25 usec)
• Shared time slots for Maintenance Requests
  (e.g. for new modems with no SID to come online)

14
DOCSIS Downstream Architecture

• RF Channel Spacing
• 88 - 860 MHz
• 6 MHz
• 64 QAM-Occupied bandwidth 5.057 MHz plus guard
  band
• 256 QAM- Occupied bandwidth 5.4 MHz plus guard
  band

15
DOCSIS Downstream Architecture

• RF performance requirements
• CNR -- 23.5dB as measured for analog video
  performance. (assumes DOCSIS carrier at
  analog level and 64 QAM downstream.)
• Amplitude ripple (response) -- 0.5 dB
• Group delay -- 75ns
• Power levels -15 dBmV to 15 dBmV

16
DOCSIS Downstream Architecture

• The DOCSIS Specification Uses a Modulation and
  Coding Scheme Defined by ITU J.83 Annex-b, for
  the Downstream
• Modulation Type 64-QAM or 256-QAM
• Maximum Data Rate 27 Mbps at 64-QAM, 38 Mbps
  at 256-QAM
• Bandwidth 6 MHz channel
• Frequency Range 88 - 860 MHz
• Transport Protocol MPEG-2
• Forward Error Correction (FEC) encoding outer
  Reed-Solomon and inner Trellis code
• 1E-8 BER with a carrier to noise ratio (Es/No)
  of
• 23.5 dB for 64-QAM
• 30 dB for 256-QAM

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DOCSIS Upstream Architecture

• Variable RF bandwidth and modulation.
• 200 kHz,400 kHz, 800 kHz, 1600 kHz, and 3200 kHz
• QPSK ( Quadrature Phase Shift Key) or 16 QAM
  (Quadrature Amplitude Modulation)
• Frequency Range
• 5 to 42 MHz (Edge to Edge)
• RF Performance requirements
• CNR -- Not less than 25 dB

18
DOCSIS Upstream Architecture

• Motorola (GI) Developed and Designed the Flexible
  F/TDMA Upstream Approach to the Physical Layer in
  the DOCSIS Specification
• Modulation Type 16-QAM or QPSK
• Data Rates 320Kbps - 10 Mbps
• Symbol Rates 160, 320, 640, 1280 and 2560 ksym/s
• Bandwidth 200, 400, 800, 1600 and 3200 kHz
• Frequency Range 5 - 42 MHz (edge to edge)
• Range of available data rates and bandwidth used

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CMTS and Cable Modem Startup

• Provision modem in the Cable Router (operator
  configured or automatically provisioned)
• Install modem at subscriber premise (cable and
  power)

CMTS
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Downstream Channel Search

• CM searches for a downstream data channel
• Synchronize with QAM
• Synchronize with FEC and MPEG

HFC
MODEM
CMTS
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Monitor for SYNC Message

• Periodically transmitted by CMTS
• SYNC message contains a time stamp that exactly
  identifies when the CMTS transmitted the message
• CM to synchronize its time-based reference clock
  so that its transmission on the upstream will
  fall into the correct mini-slots

CMTS
22
Obtain Upstream Parameters

• Monitor for UCD message
• periodically transmitted by CMTS
• UCDs define characteristics of the upstream
  channel such as
• mini-slot size
• upstream channel ID
• downstream channel ID
• burst descriptors

CMTS
UCD Upstream Channel Descriptor
23
Initial Ranging

• CMTS periodically transmits MAP messages
• Upstream Bandwidth Allocation Map (MAP) includes
• Initial Maintenance Interval (broadcast interval)
  with start and end of connection opportunity
• CM responds with Ranging Request (RNG-REQ)

MAP Message
CMTS
RNG-REQ
MAP Media Access Protocol
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Auto Adjustments

• CMTS receives initial Ranging Request from CM
• CMTS responds with Ranging Response (unicast)
• assigns a SID and allocates bandwidth to this SID
• adjust power level, timing offset, and frequency
  adjustment
• Sets downstream and upstream channels
• CMTS starts Admission Control

RNG-RSP
CMTS
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Admission Control

• CMTS allocates a Temporary SID for the CM and
  puts the CM in the Forwarding Tables
• CMTS sends MAP with Station Maintenance
  opportunity for that SID
• CM ranges with new settings
• CMTS sends RNG-RSP to indicate success or failure
  of Admission

MAP Message
CMTS
RNG-REQ
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Bandwidth Requests

• Uses special MAC frame (REQ - 6 bytes only)
• Can also piggyback request on data frame
• Uses a 4-byte Extended Header TLV
• Request contains SID and number of minislots
  needed
• Includes all FEC other PHY overhead
• Requests may be sent in Request, Request/Data, or
  Data transmit intervals
• The MAP has a special code to signal a request
  has been received although no grant is in the
  current MAP

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MAPS

• The upstream time is allocated to modems in the
  MAP message
• MAP is variable length, typically 5-15 ms
• CMTS sends separate MAP messages for each
  upstream channel
• Set of all MAPs for a channel covers all
  minislots
• For each BW grant, containsSID, Burst type, and
  Grant length
• MAP contains US Channel ID and configuration
  count
• Allows dynamic UCD changes

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MAP Example
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IP Connectivity

• CM sends a broadcast DHCP request via the CMTS to
  the DHCP Server
• DHCP server returns
• IP address and Subnet Mask
• CM configuration file name and IP address of TFTP
  server
• UTC time offset to establish local time
• TOD Server IP address

Server
CMTS
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Time of Day

• CM sends a request to the ToD Server
• ToD Server responds GMT

Server
CMTS
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Transfer Operational Parameters

• After DHCP operation, CM must download the
  configuration file from the TFTP server
• Server address is specified in the siaddr field
  of the DHCP response

Server
CMTS
32
Registration

• CM generates a Registration Request (REG-REQ)
• Includes configuration parameters received from
  TFTP configuration file
• Downstream frequency, Upstream channel ID
• Network access configuration settings
• Class of Service
• Modem Capabilities
• Modem IP address

REG-REQ
CMTS
33
Registration

• CMTS
• checks CMs MAC address and authentication
  signature on the parameters
• assigns a SID
• provides bandwidth for CM requested Class of
  Service
• modifies forwarding table to allow full user data
  if the modem requested Network Access
• sends REG-RSP to CM (CM can pass unencrypted
  data)

REG-RSP
CMTS
34
Baseline Privacy

• Follows modem registration
• Provides user data privacy by encrypting traffic
  flows, upstream and downstream
• Provides cable operators basic protection from
  theft of service
• Mechanisms for
• authentication CM to CMTS and CMTS to CM
• key distribution traffic keys and lifetimes
• data encryption applied to Sid's
• 56 bit DES Encryption

35
Security Association

• If CM is configured for Baseline Privacy in the
  modem TFTP configuration file
• CM sends Authorization Request
• Public key, MAC address, and SIDs
• CMTS responds with an Authorization Response
• Authorization Key (encrypted KEK)
• Key Sequence number and Lifetimes
• List of SIDs (for each requested Class of
  Service)

CMTS
36
Security Association

• CM requests Key Request for each SID
• CMTS responds with DES encrypted TEK for each SID
• CM can now pass encrypted data

CMTS
37
DOCSIS Today

• DOCSIS 1.0
• Product Interoperability across available CMTSs
• 64 and 256 QAM modulation (downstream) formats
• 6-MHz occupied spectrum coexists with all other
  signals on the cable plant
• Variable-depth interleaver supports both
  latency-sensitive and -insensitive data.
• The features in the upstream direction are as
  follows
• Flexible and programmable CM under control of the
  CMTS
• Frequency agility
• Time division multiple access
• QPSK and 16 QAM modulation formats
• Support of both fixed-frame and variable-length
  PDU formats
• Multiple symbol rates
• Programmable Reed-Solomon block coding
• Programmable preambles

38
DOCSIS 1.1 Enhancements

• Telephony support a major driver for 1.1
• QoS
• Multiple (dynamic) Service Flows and classifiers
• More upstream scheduling types (polling, periodic
  grants)
• Fragmentation
• Concatenation, PHS
• Efficient use of upstream channels

39
DOCSIS 1.1 Enhancements

• BPI
• Authentication of CMs with digital certificates
• Longer keys and some new algorithms
• Secure code download
• Uses PKCS certificates and code image signing
• OSS enhancements
• SNMPv3
• Full set of standard events and messages are
  specified

40
DOCSIS 1.1 Enhancements

• DOCSIS 1.1
• Packet Classification, based on fields in the
  Ethernet, IP, and UDP/TCP headers, into a Service
  Flow
• Service Flow association with a DOCSIS Service
  Identifier
• QoS MIBs
• Fragmentation
• Concatenation
• Payload Header Suppression (for increased
  bandwidth efficiency, particularly in the case of
  relatively small Voice-over-IP VoIP packets)
• Priority Queuing (e.g. Weighted Fair Queuing) at
  the CMTS
• BPI (Base Line Privacy - Plus)
• IGMP (Internet Group Management Protocol)
  Management

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DOCSIS 1.0 and 1.1 Interoperability

• Can DOCSIS 1.0 and 1.1 Modems Can Be Used in the
  Same System?
• DOCSIS 1.1 is backward compatible with DOCSIS 1.0
• DOCSIS 1.1 CMTSs are required to to support both
  DOCSIS 1.0 and 1.1 cable modems
• DOCSIS 1.1 modems must be able to register as a
  DOCSIS 1.0 modem with a CMTS that only supports
  DOCSIS 1.0
• Can DOCSIS 1.0 and 1.1 Modems Used on the Same
  Upstream Channel?
• Yes.
• Managing 1.0 and 1.1 modems on the same upstream
  channel is a more complex task for the CMTS
• If QoS commitments cause conflicts, the CMTS can
  easily move a CM from one upstream channel to
  another

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DOCSIS 1.1 Overview

• Quality of Service (QoS)
• Baseline Privacy Plus (BPI)
• Multicast
• Secure code download
• Dynamic channel change
• SNMPv3
• Standardized event logging

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Quality of Service
E-mail
HFC
HFC
Voice
CM
CM
file
In DOCSIS 1.0, all services compete for upstream
bandwidth on a best effort basis.
In DOCSIS 1.1, each service can get performance
assurances based on QoS parameters (e.g.
bandwidth, jitter)
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Packet Processing
Upstream Scheduler
Service Queues
Classifier
Data Packet
45
Service Flow Types

• Static
• Provisioned when the CM registers
• Defined in a CMs config file
• Dynamic
• Created as needed, based on demand
• Dynamic service flow messages
• Dynamic Service Add (DSA)
• Dynamic Service Change (DSC)
• Dynamic Service Delete (DSD)
• Either CM or CMTS can create

46
Service Flow States

• Provisioned
• The CMTS has not yet reserved the resources in
  its MAC scheduler
• Admitted
• The resources are reserved, but the flow is not
  active
• Active
• The resources are in use, data is actively being
  transmitted on the flow

47
Dynamic Service Flow Example Two Phase
Activation

• When a voice call is originated
• Service flow created via DSA
• Resources are admitted (phase 1)
• When the far end answers
• DSC used to activate the resources (phase 2)
• Call in progress
• When call ends, service flow is terminated via DSD

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Fragmentation
49
Concatenation

• Transmission from single CM limited by the
  REQ/Grant handshake
• Nominal latency for REQ/Grant sequence in idle
  network is 2.5 msec, or 400 Grants/sec for a
  single CM
• Operationally, 150 grants/sec is typical
• Thus, transmission limited to 150 bursts/sec
• Concatenation allows multiple packets per burst
• Improved upstream performance and efficiency

50
Payload Header Suppression

• Allows repetitive portion of packet to be
  suppressed over the HFC link
• A set of PHS rules defines the portion of the
  packet to suppress
• Set up during DSA or DSC signaling
• Improves bandwidth efficiency

51
PHS Example
52
BPI Enhances BPI Capability

• Stronger crypto mechanisms
• Support of future upgrade of crypto capabilities
• Strong authentication
• Dynamic security associations

53
Strong Authentication

• DOCSIS 1.0 does not have a secure mechanism to
  authenticate the CM
• DOCSIS 1.1 adds strong authentication of the CM
  through the use of X.509 digital certificates
• Each CM issued a unique digital certificate that
  is verified through the DOCSIS root certificate
  authority

54
DOCSIS Trust Hierarchy
55
CM Authorization
Auth Request (CM-ID, CM-Certificate,
Security-Capability, primary SAID)
CM
CMTS
Auth Reply (Auth-key, Key-Lifetime,
Key-Sequence_Number, one or more SA-Descriptors)
CM-ID serial number, manufacturer ID, MAC addr,
RSA public key CM Certificate X.509
certificate Security-Capability crypto
capability, BPI version Primary SAID CMs
primary SID Auth-Key Authorization key
encrypted with CMs public key Key-Lifetime
remaining time that key is valid in
secs Key-Sequence-Number Sequence number of
Auth key SA-Descriptors Properties of the
security association, including SAID, SA-type,
cyrpto-suite
56
Basic Authentication (1)

• CM sends CM cert, manufacturer cert
• CMTS verifies CM cert
• MAC addr, serial , CM public key are correct
• Expiration okay
• CM cert issuer name matches manuf cert subject
  name
• CM cert signature is valid, using manuf cert
  public key
• CMTS verifies manufacturer cert
• Expiration okay
• Manuf cert issuer name is DOCSIS
• Manuf cert signature is valid, using DOCSIS root
  public key
• Success proves CM cert is valid, but still need
  to determine that CM is rightful owner

57
Basic Authentication (2)

• CMTS RSA-encrypts authorization key using CMs
  public key in CM certificate
• CM uses HMAC key (derived from authorization key)
  to generate HMAC on Key Request message
• CMTS verifies the HMAC
• Success proves CM knows the private key that
  matches public key in CM cert, hence CM is
  rightful owner

58
Dynamic Security Associations

• Useful for encrypting traffic flows that are
  dynamic or temporal (e.g. multicast)
• SA-MAP mechanism allows CM to learn of encrypted
  traffic flows and its security association.
• Currently applied to multicast downstream flow
• Inter-operate with DOCSIS 1.1. IGMP management
  mechanism which triggers the establishment of
  dynamic SAs.

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IGMP/SA-MAP Example
CM
CMTS
CPE
IGMP MR (Join)
IGMP MR (Join)
Set Multicast MAC Filter
SA-MAP Request
Determine SAID
SA-MAP Reply
Key Req/Reply
Start TEK FSM
Encrypted Multicast Data
Decrypt Multicast
Encrypt Multicast
Multicast Data
Multicast Data
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Secure Code Download

• DOCSIS provides a method to remotely download
  firmware updates to the CM
• DOCSIS 1.1 adds a digital signature to the code
  file to verify the source and integrity of the
  downloaded code
• Allows for both the manufacturer and the MSO to
  digitally sign the code file.

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Code Download Process

• DOCSIS Root CA
• Issues Manufacturer CVC
• Manufacturer
• Signs code file
• Send code file w/ CVC to MSO
• MSO
• Verifies code file
• Optionally, adds MSO co-signature and MSO CVC to
  code file
• Send code file to CM on request
• Cable Modem
• Download code file
• Verify manufacturers signature
• Verify MSO signature, if present
• If verified, install code image

62
Dynamic Channel Change

• Enables CMTS to dynamically direct the CM to
  change its downstream and/or upstream channel
• Near seamless change with minimum interruption of
  service
• Useful for traffic balancing, noise avoidance,

63
SNMPv3

• Enhances the SNMP v1/v2 framework to support
• Privacy authentication
• Authorization
• SNMPv3 defines a modular architecture within
  which network management capabilities can evolve
• SNMPv3 defines no new protocols
• Documented in RFC 2571-2576

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SNMPv3 Architecture
65
Standardized Event Logging

• DOCSIS 1.1 defines a set of standardized event
  message formats and priorities.
• 250 standard event messages
• 16 DOCSIS-specific trap types
• Eases network management operations
• Common event message across CM products
• Facilitates automated event processing

66
References

• Specifications are publically available at
  www.cablemodem.com/specifications.html
• IEEE Communications, March 2001, p. 202
• Good overview article, available as PDF file
• CableLabs training on 1.0 MAC (VGs)
• CableLabs training on 1.1 (VGs and video)
• Video is of a presentation of the VG
• Clive Holborow and Greg Nakanishi
• BCS/IPNS, San Diego

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