5 Summaries of Broadband Access Technologies

5.1 Digital Subscriber Line
The varieties of broadband access via Digital Subscriber Line (ADSL, SDSL, G.lite ADSL, etc.) all technically support close to if not the ideal open access system. Today's DSL deployments are more or less following the specifications set forth by the ADSL Forum; that is, they are following a standard. The basis for DSL open access support is Asynchronous Transfer Mode (ATM) net-working between the subscriber premises equipment and the service provider. ATM virtual circuits are straightforwardly provisioned from the ADSL modem via the broadband access network provider, through the central office and the access network and backend networks, to the service provider. The architecture of DSL and developed technology supports open access from initial deployment.

5.2 Fiber to the Curb
Over the past decade there has been significant development of Fiber-To-The-Curb (FTTC) technologies for the delivery of voice, video, and data services to subscribers. Deployment of this broadband access technology has slowed from initial predictions. It is likely that efforts will renew after deployment of DSL has reached sufficient penetration. Motivation for moving to FTTC solutions include the timely replacement of aging twister pair copper plant, and the need to push more last-mile access bandwidth closer to the subscriber. FTTC architectures are capable of sup-port ideal open access provisioning.

5.3 Metropolitan Area Wireless
The deployment of high-speed data for Internet access over wireless systems (e.g. MMDS) has been undergoing a substantial amount of churn in the past several years. At this time, both the market demands, service providers, and the technology appear to be better aligned for the next attempt to grow the market. The high-speed data over broadband wireless access network environment has no standard at this point in time. There are rumors that several companies will enter the market in the near future with adapted systems using the DOCSIS RFI Media Access Control (MAC) protocol. In contrast, Com21 is investigating entering the market with a wireless version of its CommUNITY ATM based system. Hybrid Networks has been in the market for a several years, has ridden the ups and downs, but does not appear to have an ideal open access solution. In the absence of public or defacto standards and policy, the politics may select a DOCSIS based system thereby inheriting the same lack of support for open access provisioning as the cable television specification. There is short and shrinking window in which to persuade the wireless market to adopt a more ideal open access posture.

5.4 Broadband Satellite
The deployment of high-speed Internet services via satellite transmission systems has some active deployments to date. It is expected that there will be an increasing number of satellite based deployments over the next several years, including changes in deployment architectures. DirectPC is an example of a satellite-based Internet service for personal computers. Initial satellite deployments provide chiefly and high-speed one-way downstream service (provider to subscriber), using a terrestrial return path. Newer deployments will make use of two-way transmissions. Due to propagation delays and bandwidth per transponder, there will be varying degrees of scale. It may be possible for satellite systems to support ideal open access provisioning, however more study is needed in this area.

5.5 Cable Television
Today's DOCSIS specifications come from a project which was initiated from a group called MCNS: Multimedia Cable Network System Partners Limited: TCI, Time Warner, Cox, Comcast, with addition partners: Continental, Rogers, and CableLabs helping out. They set forth to rapidly develop the Data Over Cable Service Interface Specification: (DOCSIS) project, on behalf of the North American cable industry, the necessary set of communications and operations support inter-face specifications for cable modems and associated equipment. The specifications are intended to be non-vendor-specific, allowing cross-manufacturer compatibility for high-speed data communications services over two-way hybrid fiber-coax (HFC) cable television systems.

MCNS/DOCSIS was triggered by John Malone in December, 1995 in response to broadband access competition, vendor postures, and lack of progress in public standards process taking place in the IEEE 802 LAN/MAN committee. The DOCSIS specification was born out of an initially very closed development effort by the MCNS six cable companies and selected vendors (BayNet-works/ LANCity - now Nortel, GI, Broadcom) with CableLabs helping in the process management. Many vendors are participating now in a Non-Disclosure Agreement (NDA) fashion. Once completed, DOCSIS specifications are made publicly available via www.cablemodem.com.

CableLabs maintains a strict revision and control process for updates to the specifications. The DOCSIS project is actually a family of coordinated specifications dealing with many aspects of a cable modem access system. The most well known specification is the Radio Frequency Inter-face (RFI) specification. The RFI specification is usually referred to as the DOCSIS Specification, DOCSIS Version 1.0 or DOCSIS Version 1.1. DOCSIS RFI Version 1.0 was adopted by the Society of Cable Telecommunications Engineers (SCTE) Data Standards Subcommittee (DSS) as their standard in July 1997. Subsequently, the SCTE selection was adopted as the U.S. position into the ITU J.112 recommendation starting in the fall of 1997. The DOCSIS RFI specification is based technically on an evolved LANCity based protocol with the target having the qualities of: residential, low-cost, off the shelf, Internet access, interoperable (base functions) with vendor differentiation. The architecture of the DOCSIS system is a single large Ethernet-based bridged LAN. DOCSIS has a single ISP service provider architecture. Version 1.0 is primarily a best effort Internet access system. Version 1.1 adds protocol support and sufficient operation detail to provide dynamic Quality of Service facilities for packet voice services in addition to packet data services. There are other enhancements such as base line privacy, multicast support, etc. In addition, Version 1.1 has packet recognition support for IEEE 802.1p tagged Ethernet frames. The tagging supports both Priority tagging as well as Virtual LAN (VLAN) tagging.

Today, CableLabs is running an impressive vendor certification process for cable modems. The current focus is on DOCSIS V1.0. Several vendors have just been recently certified. The acceptance of DOCSIS in the cable operator community is predicated on sufficient vendors being certified and product being available. DOCSIS V1.0 was originally predicted to begin large roll out in Fall 1998, however delays with vendors and the certification process itself has pushed that out at least nine months. Summer 1999 should see the beginning of wide spread roll out in North America of Version 1.0 modems. The first release of the DOCSIS Version 1.1 specification became available in March 1999. It is expected that vendors will move rapidly to DOCSIS Version 1.1 capable hardware and software. DOCSIS Version 1.1 certification is dependent on CableLabs's efforts. Note that DOCSIS Version 1.1 certification requires DOCSIS Version 1.0 certification;

Version 1.1 is in addition to Version 1.0 operation, not a replacement for Version 1.0 operation. Version 1.1 is fully backward compatible to Version 1.0. Subscribers who purchase Version 1.1 cable modems can enjoy DOCSIS service anywhere provided, with Version 1.1 services sup-ported only on those systems which are enable for Version 1.1.

There is another DOCSIS RFI project effort underway called Version 1.2, which adds the support for high performance upstream physical RF channels. Version 1.2 is being developed in conjunction with the efforts of the IEEE 802.14 Hi Performance Physical-working group. Version 1.2 cable modems will need to be separately certified for Version 1.0, Version 1.1 and Version 1.2 operation. As Version 1.2 is only a physical RF channel improvement addition to Version 1.1; all issues relating to open access provisioning for Version 1.1 will apply equally to Version 1.2.

NOTE: prior to DOCSIS being widely available, there are a number of proprietary cable modem systems being deployed in the United States. The vendors include 3Com, ADC, Com21, Hybrid Networks, LANCity (became Bay Networks, now Nortel), Motorola, Phasecom, Terayon, Zenith, and others. Of these, Motorola, LANCity (now Nortel), Com21, and Terayon are the most deployed. The number of cable modems deployed in North America is on the order of several hundred thousand. Of these vendors, the Com21 system is the only one that directly supports open access provisioning by use of direct Layer 2 Virtual LAN support and ATM networking (similar to DSL), which more or less gets close to providing the ideal open access system. At this time, there are cable overbuilders (e.g. Knology, south east U.S.) who are using Layer 3 approaches (enumerated later in this paper) to provide less than ideal, but workable, open access provisioning.

Back to index