PDF of this tutorialA cable data system is comprised of many different technologies and standards. To develop a mass market for cable modems, products from different vendors must be interoperable.
To accomplish the task of interoperable systems, the North American cable television operators formed a limited partnership, Multimedia Cable Network System (MCNS), and developed an initial set of cable modem requirements (DOCSIS). MCNS was initially formed by Comcast, Cox, TCI, Time Warner, Continental (now MediaOne), Rogers Cable, and CableLabs. The DOCSIS requirements are now managed by CableLabs. Vendor equipment compliance to the DOCSIS requirements and interoperability tests are administered by a CableLabs certification program.
For further details see http://www.cablemodem.com.
Some of the details of cable modem requirements are listed below.
Physical Layer
Downstream Data Channel
At the cable modem physical layer, downstream data channel is based on North American digital video specifications (i.e., International Telecommunications Union [ITU]–T Recommendation J.83 Annex B) and includes the following features:
- 64 and 256 QAM
- 6 MHz–occupied spectrum that coexists with other signals in cable plant
- concatenation of Reed-Solomon block code and Trellis code, supports operation in a higher percentage of the North American cable plants
- variable length interleaving supports, both latency-sensitive and latency-insensitive data services
- contiguous serial bit-stream with no implied framing, provides complete physical (PHY) and MAC layer decoupling
Upstream Data Channel
The upstream data channel is a shared channel featuring the following:
- QPSK and 16 QAM formats
- multiple symbol rates
- data rates from 320 kbps to 10 Mbps
- flexible and programmable cable modem under control of CMTS
- frequency agility
- time-division multiple access
- support of both fixed-frame and variable-length protocol data units
- programmable Reed-Solomon block coding
- programmable preambles
MAC Layer
The MAC layer provides the general requirements for many cable modem subscribers to share a single upstream data channel for transmission to the network. These requirements include collision detection and retransmission. The large geographic reach of a cable data network poses special problems as a result of the transmission delay between users close to headend versus users at a distance from cable headend. To compensate for cable losses and delay as a result of distance, the MAC layer performs ranging, by which each cable modem can assess time delay in transmitting to the headend. The MAC layer supports timing and synchronization, bandwidth allocation to cable modems at the control of CMTS, error detection, handling and error recovery, and procedures for registering new cable modems.
Privacy
Privacy of user data is achieved by encrypting link-layer data between cable modems and CMTS. Cable modems and CMTS headend controller encrypt the payload data of link-layer frames transmitted on the cable network. A set of security parameters including keying data is assigned to a cable modem by the Security Association (SA). All of the upstream transmissions from a cable modem travel across a single upstream data channel and are received by the CMTS. In the downstream data channel a CMTS must select appropriate SA based on the destination address of the target cable modem. Baseline privacy employs the data encryption standard (DES) block cipher for encryption of user data. The encryption can be integrated directly within the MAC hardware and software interface.
Network Layer
Cable data networks use IP for communication from the cable modem to the network. The Internet Engineering Task Force (IETF) DHCP forms the basis for all IP address assignment and administration in the cable network. A network address translation (NAT) system may be used to map multiple computers that use a single high-speed access via cable modem.
Transport Layer
Cable data networks support both transmission control protocol (TCP) and user datagram protocol (UDP) at the transport layer.
Application Layer
All of the Internet-related applications are supported here. These applications include e-mail, ftp, tftp, http, news, chat, and signaling network management protocol (SNMP). The use of SNMP provides for management of the CMTS and cable data networks.
Operations System
The operations support system interface (OSSI) requirements of DOCSIS specify how a cable data network is managed. To date, the requirements specify an RF MIB. This enables system vendors to develop an EMS to support spectrum management, subscriber management, billing, and other operations.
