Once viewed as the telephone industry's secret weapon in its war with cable TV providers, asymmetrical digital subscriber line (ADSL) seems to have been sidelined by the baby Bells as they scramble to maintain their local loop monopoly. In a blow to U.S. ADSL vendors who saw their stock value tumble, Ameritech, BellSouth, Pacific Bell and SBC Communications retained France's Alcatel Telecom as their future supplier of ADSL products. At the same time, they emphasized that the deal depends on the conclusion of successful contract negotiations.
Before discussing this in more detail, it would be useful to review the history of high bit rate subscriber line. First off the line during the late 1980s came HDSL (high bit rate subscriber line). This is based on bi-polar 2B1Q bit coding, which lowers the cost of deploying T-1 access lines. It enables the telcos to send high-speed digital signals over unrepeatered, unshielded twisted pair (UTP) wire to business users.
HDSL was followed a few years later by ADSL, which uses advanced multilevel modulation techniques to send compressed video and data signals to single line residential users. ADSL was followed by VDSL (very high bit rate digital subscriber line), designed for drop wire applications, and SDSL (symmetrical digital subscriber line). These technologies have been collectively dubbed xDSL.
Bell Atlantic used ADSL to enable its subscribers to order movies and other kinds of TV programming at 1.5 Mbps. The regional Bell's original intention was to provide libraries of MPEG (Motion Pictures Experts Group) compressed movies and other kinds of television programs at a server at the telephone exchange. Using a set-top box and a handheld controller, users could enjoy on-demand programming, and the company didn't have to sink capital into a complete network rebuild.
That relatively straightforward application is now history. The latest idea is a family of high bit rate solutions up to 50 Mbps which utilize cell-based solutions such as ATM (asynchronous transfer mode) to provide interactive broadband to the home and office. The trouble is that the standards for these are not in place, and many experts feel that such full service solutions are best done in other ways.Residential ATM
Returning to the Alcatel announcement, on the surface the regional Bells appear to be supporting the introduction of end-to-end ADSL using a combination of ATM cells and modems. In fact, the news probably signals a delay in widespread ADSL deployment in the U.S. for at least 18 months. The French vendor will need at least that long to meet emerging standards for residential ATM.
"It may be technically possible to have a point-to-multipoint physical architecture [based on ATM] in the home," observes David Thorne, technical area leader, BT Labs, and chairman of the ATM Forum's Residential Broadband working group. "However, this would require a much more complex system and is not a scenario which the ADSL standard supports."
In a presentation during the Maximizing Copper conference held in London in September, Thorne noted: "Alternatively, the access network system could be terminated at a single point within the home and then fed into a separate in-home distribution system. These two scenarios have become known as passive and active network termination (NT), respectively."
Another problem is that it is not known whether unshielded twisted pair wiring (UTP) in most home and small business premises is capable of carrying 6.0 Mbps signals. Thorne believes the solution is to use an active NT. This presents difficulties in the U.S., where the telcos do not control inside wiring.
"The ADSL system is terminated at the NT, which feeds into a general purpose, symmetric in-home distribution system," notes Thorne. "This capability needs to be maintained right up (to) the customer equipment, as multiple traffic types (e.g., video, voice and data) could be delivered to a single physical item of CPE (customer premises equipment), i.e., a PC." (See Figure 1.)
The reaction of U.S. vendors to the Alcatel announcement has been muted. "Today's ADSL vendors must formulate a strategy for incorporating ATM switching as soon as possible," notes Rob Faw, president of global operations for Westell Technologies. "The deployment of extensive services configuration and management capabilities across a wide range of access elements including ADSL will be part of this process."
Faw explains that a handful of telecom vendors and specialized software-centric companies can offer these capabilities, but the process will take time. "It is not clear that the telephone operators and service providers can wait for interoperable ADSL equipment to emerge on the market during the next several years, in an environment of increasing competition and deregulated access," he concludes.
The problem for ADSL is that it is rapidly taking on the garb of a fully fledged access infrastructure. As it acquires more and more complexity, the likelihood that it will be delayed by standards and interoperability concerns increases. Peter Krawarik, manager of business development and quality, local loop transmission, with Ericsson Austria, explains: "The impact of standards on new product emergence is complex. Bodies such as the ADSL and ATM Forums promote their respective solutions and try to influence larger standards bodies such as ANSI and ETSI. In Europe, where telecommunications are operator dominated, standards processes are frequently used for exclusion, while U.S. standards bodies are vendor dominated and tend to promote technology."
This holds important implications for U.S. consumers. The cable TV industry's 65 percent share of the television audience was gained using a straightforward point-to-multipoint network that provided 60-plus channels to all households and did not require complex standardization.
Could a combination of ATM and ADSL replace this infrastructure by providing broadband bidirectional multimedia to the home? Not if it gets bogged down in the standards process, implies Krawarik. "New applications develop consistently faster than the relevant standards, and standards become more of a hindrance as they are not creating, but only responding to market applications."VOD stumbles
Globally, telcos have field trialed ADSL with varying degrees of success. Starting in the early 1990s with Bell Atlantic, British Telecom and Telstra (formerly Telecom Australia), GTE, other regional Bells, and many international PTTs have tested the new technology.
In April 1995, Bell Atlantic, long a fan of video-on-demand (VOD) over telephone lines, installed the first paying ADSL customer in Fairfax County, Va. Six months later, 1,000 customers were receiving on-demand video entertainment with VCR functionality, accessing over 600 titles including movies, sports, children's programs and special interest features. One year later, this well publicized service, called Stargazer, was abruptly cancelled.
"We found that only about half the copper loops were adequate to support 6.0 Mbps ADSL and discontinued the service," explains Larry Plumb, communications director at Bell Atlantic. "There were other problems, including set-top box design and inside wiring. In my opinion, there never will be a shrink-wrapped ADSL consumer product you can buy at your local store," says Plumb. This puts ADSL in a totally different class from dial-up modems.Enter Internet
General Telephone uses ADSL to connect small businesses, stores and libraries in Irving, Texas to a web site. "Web browsers and PCs have created demand for high-speed, low-cost Internet and LAN access," explains Bob Olshansky, manager, advanced service platforms for GTE Laboratories. Echoing the views of the ADSL Forum, Olshansky notes that 35 million U.S. homes will have PCs in 1996, and thus, UTP provides a key entry strategy for IP services.
"We implemented a 'proof-of-concept' trial involving three local exchanges and 20 ADSL lines to small businesses and GTE employees," he explains. "The objectives are to evaluate ADSL data modems, evaluate network architecture, identify deployment and operational issues, and evaluate customer response. We are testing IP (Internet protocol) dialtone router-based IP access to IP hosts, IP networks and value-added IP services."
These services require routers and Ethernet switching at the subtending exchange and small routers at the customers' premises (see Figure 2). "We use ADSL modems from Westell and Amati, Bay Networks and Cisco routers, and GTE-provided Ethernet interfaces and browsers. We also install special shielded twisted pair premises wiring," says Olshansky. The latest rumor heard on the street is that GTE is re-evaluating ADSL for this application.
Australia's regulatory framework allows Telstra the freedom to build and operate both a telecommunications and subscription broadcast pay TV network. The company has chosen to deploy a hybrid fiber/coax (HFC) network plus explore the video and data delivery capabilities of ADSL using NEC Australia as its vendor.
A decision on ADSL is not expected until next year at the earliest and is dependent on favorable cost comparisons with other architectures. "The greatest cost reduction impact is expected to come from lower equipment prices due to the development of a global ADSL market," explains Symon Rozenthal, ADSL manager for Telstra. "Current models indicate that costs for ADSL deployment are similar to HFC deployment for the penetration levels expected in Australia."Market miscues
Unrealistic marketplace expectations have dogged ADSL since its inception. Unlike HDSL, which provides telcos with immediate cost savings, ADSL offers only vague expectations of future revenue streams. Westell's Faw admits that most ADSL forecasts are based on RFPs (requests for proposal) by the telcos. Data presented in London indicates a wide discrepancy between vendor expectations and carrier forecasts for xDSL systems (see Figure 3).
"The supplier industry, in an environment of low ADSL sales, has invested an estimated $150 million in the development of signal processing technology, access and network management systems for ADSL," says Faw. "At the present moment, no one is making any money in this industry, yet huge investments will continue to be made."
The danger here is that new technology can become self-sustaining, irrespective of its potential for meeting the real needs of users. "Strong vested interests can become proprietary in nature, and we should insure that we see interoperable network managed systems at the user and network interface level if we are to succeed," Faw concludes.
Dan Arazi, vice president, marketing at Orckit Communications, believes that the best application for ADSL is VOD integrated with Internet, as this will provide benefits to both business and residential users. Internet service providers want faster access over copper, but before this can be done, some key data access questions must be answered:
- Will IP be the only common protocol for the Internet?
- Will IP be sufficient for a network running at 50-times a regular dial-up modem speed?
- Is ATM ready to replace IP?
Because of differences in local loop quality, achievable bit rates will vary widely across the access network. To accommodate this, the data rate must be able to drop back to a default speed just as it does in high-speed dial-up modems today. "Rate adaptive ADSL is probably the only way to cover a high percentage of CSA (carrier serving area) loops today, notes Arazi (see Figure 4).
Changes in the access network and the emergence of xDSL presents the industry with a dilemma. Should it promote asymmetrical solutions at speeds of 6.0 Mbps for loops less than three miles in length, or should it move to higher speeds (up to 50 Mbps) on shorter loops that access fiber nodes? The author believes that the VDSL approach makes more sense when viewed in the context of an evolving access network (see below).
This network will use many different technologies, notes Ray Albers, vice president of technology planning for Bell Atlantic, who spoke at NCF/InfoVision in October. Albers proposes a full-service digital video dialtone platform that consists of a combined central office and host digital terminal supplying customers over HFC, fiber and copper, and copper only. Multipoint microwave distribution is also used (see Figure 5).
What is ADSL's role in a distributed architecture such as this? Probably, it would become one option among the several that make up the xDSL family listed earlier. Over the next few years it is likely that the access network will accommodate:
- Medium-sized business users connected using HDSL data pipes
- Smaller businesses and work-at-home customers connected over coax or copper drop wires using VDSL
- Residential customers supplied with cable TV over HFC
- Internet connectivity at higher speeds over cable modems, ADSL or MMDS.
In the October 31 edition of USA Today, Kevin Maney reported that ADSL could play a major role in speeding access to the Internet once problems of distance, radio interference and PC compatibility are solved. High-speed access consists of many related parts, and what happens in one part can adversely impact other parts of the network. PacTel says it plans to provide ADSL service to its Silicon Valley customers late next year, but by that time, customers will have other choices, including cable modems and broadband wireless.
|About The Author|
|Alan Stewart is a freelance writer who specializes in telecom issues.|
|Some of the information in this article was obtained from the Maximizing Copper in the Access Network, IIR conference held in London on September 16–17, 1996.|