Cable Industry Issues Specification for High-Speed Data Delivery
Anaheim, California, December 11, 1996—The Data Over Cable System Interface Specification working group has developed a set of specifications aimed at gaining interoperable high-speed cable modems.
"This is a major accomplishment for the cable television industry," said Dr. John C. Malone, chairman of CableLabs' Board of Directors and TCI Chairman, President and CEO. "We said last year at the Western Show we would do this as quickly as we could and here we are one year later at the Western Show announcing our success," Malone said.
"In the meantime, we continue to deploy early versions of these modems and our customers are delighted at the high speed and convenience these modems afford by not requiring customers to tie up a phone line each time they wish to connect to the Internet," Malone added.
The set of specifications includes a radiofrequency (RF) interface specification which was released December 5 to the 95 vendors that have signed the Data Over Cable System Interface Specification Access Agreement. This document is under review by the vendor community. The network security specification is being released this week and the operations support system interface will be released by year end.
These documents are considered complete. Modems compliant with the specification will be capable of delivering data to users at a minimum rate of 27 million bits per second (Mbps). So far, suppliers Hewlett-Packard, Bay Networks' LANcity Cable Modem Division and Com21 have indicated an interest in building interoperable modems that comply with this specification.
This unprecedented effort by the cable industry was headed by the Data Over Cable System Interface Specification working group, which is comprised of cable operators Comcast Cable Communications, Cox Communications, Tele-Communications Inc. (TCI), Time Warner Cable, Continental Cablevision, Rogers Cablesystems Limited and the research and development consortium, Cable Television Laboratories Inc. (CableLabs).
The interface specifications benefit consumers and cable operators by providing multiple sources of interoperable modems, thereby encouraging marketplace competition and enabling economies of scale.
Multiple suppliers building to the industry specification, but adding unique capabilities, will give consumers a wide selection of products from which to choose. Manufacturers will benefit by the reduction of development risk afforded by building to an industry supported, well-known specification.
The set of documents put together by the Data Over Cable Service Interface Specification working group, describe internal and external cable network interfaces. "The process has defined a system that allows for transparent, two-way transfer of data using Internet Protocols between cable headends and cable customers with telephone return or RF return on the cable system," said Stephen Dukes, vice president technology, TCI Technology Ventures.
Alex Best, senior vice president of engineering at Cox Communications, Inc., said: "This interface specification represents a major victory for all parties involved. This includes cable operators, modem suppliers, and most of all our customers. The on-ramp to the information highway just took a giant step forward... and at high speeds."
Because "time to market" was a primary driver, cable operators selected technology that was based on existing hardware and software. "Care was taken to ensure that the technology choices would anticipate the evolving market needs over the next three to five years," said James A. Chiddix, Time Warner Cable chief technical officer.
"I want to commend the team of people who worked tirelessly to arrive at a specification that offers a reasonable balance between different technical solutions while meeting our needs to get compatible modems to market quickly," said Mark Coblitz, vice president of strategic planning for Comcast.
In addition, the specification includes ways to incorporate new technologies as they become available, said Michelle Kuska, data modem specification project manager and a director of network technology with TCI Technology Ventures. Kuska managed the project on behalf of the Multimedia Cable Network System (MCNS), which is made up of TCI, Time Warner Cable, Cox and Comcast.
MCNS plans to grant non-exclusive licenses to vendors wanting to manufacture to these specifications. The MCNS license grant is conditioned upon a manufacturer's agreement to contribute freely, or on a reciprocal, no-cost basis, any crucial intellectual property required to implement a compliant modem.
Vendors unwilling to contribute critical technology to the royalty free intellectual property pool associated with the specification would be able to obtain required intellectual property through conventional licensing arrangements made directly with the property holders. Manufacturers with no crucial intellectual property would also be eligible for an MCNS royalty free license to obtain technology required to produce compliant cable modems.
Included below is the executive summary of the Radio Frequency Interface Specification that was distributed to vendors. This summary provides an overview of the RF specification.
Executive Summary—Radio Frequency Interface Specification
High speed delivery of data is now available in select cable systems using first generation hardware from several manufacturers. Early experience reveals two important facts: 1) The technology works very well, and 2) Customers are surprised and delighted by the high speed and the convenience of connectionless service.
While short-term deployment will continue using the proprietary and non-interoperable modems available today, the timely availability of product conforming to widely adopted industry specifications for interoperability will provide powerful advantages for cable operators, consumers, and manufacturers. The advantages include:
- Operators may purchase interoperable equipment from multiple manufacturers, thus promoting competition and subsequent price reductions that benefit both the operator and consumer. The interoperability specification provides a basic transport scheme that will allow early modems developed to the specification to continue to work on cable systems even though more advanced service capabilities may be added to future modems. As a result, investment in the early interoperable modems will not be stranded when more advanced modems are brought to the marketplace.
- Cable modems that will work on a data enabled cable system may be purchased by the consumer from a retail outlet.
- Economies of scale advantages will be realized for key components of the cable modem system.
- Manufacturers can reduce risk by developing to a well known specification.
This draft specification defines the radio frequency interface and is part of the set of data-over-cable interface specification documents being developed by the Multimedia Cable Network System (MCNS) consortium. MCNS is comprised of Comcast Cable Communications, Inc., Cox Communications, Tele-Communications, Inc., and Time Warner Cable. MCNS along with Continental Cablevision, Inc., Rogers Cablesystems Limited, and Cable Television Laboratories, Inc. (on behalf of the CableLabs member companies) make up the MCNS consortium. This draft specification is being issued to solicit comments and submissions from manufacturers on the functionality, clarity, and precision of the described interfaces.
Goals of the Specification
The data-over-cable service interface documents describe the internal and external network interfaces for a system that allows transparent bi-directional transfer of Internet Protocol (IP) traffic, between the cable system headend and customer premises, over a cable television system.
Because low risk and rapid time-to-market were critical drivers, this specification development effort incorporated contributions from manufacturers that were based on working hardware. Care was taken to insure that the technology choices meet the market needs for the first 3-5 years of service. In addition, because the technology is evolving, hooks were incorporated to facilitate the introduction of new technology as it becomes available. However, it should be noted that it was not a goal of this specification to incur excessive risk or delays by pushing the limits of technology and performance. Instead, this specification development effort involved a deliberate series of prudent tradeoffs. In the order of most important first, the selection criteria that was used is listed below:
- Meets basic performance, feature and cost needs for the first three to five years of service--These requirements require Internet Protocol transparency and support for multiple grades of service. It is expected that first-generation product will retail for under $400, with second generation product retailing below $250.
- Offered for use without intellectual property rights encumbrances; minimizes intellectual property issues and cost, thereby facilitating manufacture of compatible equipment by multiple vendors on a non-discriminatory royalty free basis.
- Technology has been implemented and tested--Timely availability of both prototypes for testing as well as volume field deployable equipment was considered in all technology choices. The desired schedule is to have hardware conforming to this specification available from at least one vendor as early as possible in 1997, and from multiple manufacturers by year-end 1997.
- Support for the evolutionary aspects of the architecture--The protocols are layered so as to be decoupled and include the ability to support future upgrades and changes by negotiation of the physical and higher-layer protocols at session establishment.
- Provides technology based performance or feature benefits--Support is provided for additional vendor-specific features that add value.
The system consists of the Cable Modem Termination System (CMTS), cable network, and Cable Modems (CMs). The CMTS is located at the headend and the cable modems are located at customer premises. This document defines both the characteristics of the radio frequency interface on the cable system and the message sets and signaling sequences between the headend and modems that are necessary to achieve interoperability.
To enable transparent transfer of Internet Protocol messages across a cable system the Network Layer, Data Link Layer and Physical Layer protocols and sublayers are defined. These are briefly summarized below.
The Network Layer is IP.
The Data Link Layer is comprised of three sublayers:
- A Logical Link Control (LLC) Sublayer, which conforms to Ethernet standards.
- A link-security sublayer that supports the basic needs of privacy, authorization, and authentication.
- A Media Access Control (MAC) Sublayer, suitable for cable system operation, that supports variable-length protocol data units (PDU). The main features of the Media Access Control protocol defined in this document are:
- Headend (CMTS) controlled mix of contention and reservation transmission opportunities
- A stream of mini-slots in the return, or upstream communications channel
- Bandwidth efficiency through support of variable-length packets
- Extensions provided for future support of Asynchronous Transfer Mode (ATM) or other Protocol Data Units
- Support for multiple grades of service
- Support for a wide range of data rates
In the downstream direction, the Physical (PHY) layer is based on North American digital video transmission specifications (i.e. ITU-T Recommendation J.83 Annex B) and includes these features:
- 64 level and 256 level Quadrature Amplitude Modulation (64-QAM and 256-QAM) modulation formats
- Concatenation of Reed-Solomon and Trellis forward error correcting codes supports operation in a higher percentage of North American cable plants
- Variable depth interleaving supports both latency-sensitive and latency-insensitive data
- Contiguous serial bit-stream (input and output) with no implied framing provides complete physical layer and Media Access Control sublayer decoupling
The cable modem's upstream transmission Physical layer characteristics, under headend control, include:
- Quadrature Phase Shift Keying (QPSK) and 16 level Quadrature Amplitude Modulation (16-QAM) modulation formats
- Multiple symbol rates
- Frequency agility
- Time-division multiple access
- Support of both fixed-frame and variable-length PDU formats
- Programmable Reed-Solomon block coding
- Programmable preambles
- Minimal coupling between physical and higher layers accommodates future physical layer technologies.
The specification identifies means by which cable modems can "self-discover" the appropriate cable system frequencies for reception and transmission, bit rates, modulation formats, error correction and power levels. To protect service to other users, and to ensure network reliability, cable modems are allowed to transmit after downstream synchronization and receipt of the appropriate type of transmission grants. Additional channels in both the upstream and downstream direction can be provisioned as necessary to optimize traffic loading based on cable system topology, service penetration and usage levels.
Support of Future New Cable Modem Capabilities
The specification strives to facilitate the introduction of modems in the future that will leverage technology advances and enable additional services that are not well defined or obvious today. These enhancements may include new physical layer modulation techniques, or different data transport structures optimized for differing traffic flows and classes of services ( e.g., for synchronous transfer mode telephony).
Moderate "future proofing" is provided by the protocols defined in the specification. Standard headends will be able to activate new modems for enhanced communications options at the time of set-up. In addition, the means for updating modem software via the cable network have been included in the specification.
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