up occurring in 1998 and beyond. Road warriors may have a longer wait for the same technology to hit hotel rooms, however.
The most important new remote-access technology is Digital Subscriber Line (DSL), but it's not alone. Local Multipoint Distribution Service (LMDS), cable modems, digital satellite broadcasting, and other contenders are all still in the race. Telephone companies will deliver DSL services starting late this year; meanwhile, cable companies are continuing to roll out trials.
DSL Diaries
DSL comes to you over standard phone cable -- that four-conductor, twisted-pair copper wire that's installed almost everywhere. It carries both an analog signal for audio (a 4-kHz chunk often referred to as plain old telephone service [POTS]) and a digital signal for data. DSLs run from a telephone company's central office (CO) into a customer's building, where they're eventually connected to one or more telephones, fax machines, or modems.
Asymmetric Digital
Subscriber Line (ADSL) is a specific kind of DSL developed to send video signals over existing POTS lines without needing to add to the existing copper infrastructure (see "Break the Bandwidth Barrier," September 1996 BYTE). ADSL delivers more data downstream (i.e., from the phone-company switch) to the subscriber than it receives upstream. Delivery of digital video was once thought to require a downstream bandwidth of as much as 1.5 Mbps, although an upstream rate as low as 64 Kbps was more than enough for VCR-like control signals coming from the viewer.
Upstream data rates actually range from 16 to 640 Kbps, depending on the downstream rate, which is itself a factor of the distance from the telephone company's CO. As a result, these rates are related to the length of the copper line.
ADSL, like the other flavors of DSL, is subject to a number of limiting factors, including the distance of the user's phone from the CO (see the figure
"Farther Equals Slower"
). As the distance
from the CO increases, the strength of the signal drops, reducing the amount of data that can be reliably received. Further obstacles include crosstalk between adjacent digital lines, line splices between the CO and the user site, loading coils that trap the signal above 4 kHz, random line noise, and breaks in the loop caused by phone jacks that aren't connected to a telephone.
According to a variety of sources, between 70 percent and 80 percent of the wired locations in the U.S. are located within the 18,000-foot ADSL transmission limit. A repeater, which amplifies the line signal, can overcome these distance limitations, making possible the delivery of ADSL to many locations beyond the 18,000-foot limit.
Delivering ADSL involves several steps. At the CO, a modem modulates and encodes signals from either the digital data provider (an ISP) or the phone company's Internet service connection, or data from a connection to a corporate network, into an ADSL signal. The modem combines the 4-kHz POTS si
gnal with the DSL signal before sending it to the consumer over the existing phone wiring. Downstream, at the consumer's PC, a splitter separates the POTS signal from the digital signal. The digital signal is then demodulated, decoded, and passed to the PC.
Transferring data from the PC to the CO works in reverse -- the modem modulates and encodes the upstream digital signal and combines it with the 4-kHz POTS signal. At the CO, the POTS signal is again separated from the ADSL digital, and the upstream signal is demodulated, decoded, and sent to the digital data provider.
Since it's a full-time digital connection, ADSL is always active. Although it uses telephone-company wiring, the connection is actually a link to a network. When it's installed, the connection can be made to an ISP, to a company's high-speed network, or through the CO to an Internet connection that the CO provides. There's no dial tone, and your connection to an ISP or corporate network is hard-wired, so you won't be able to chan
ge service providers without having changes made at the phone company's CO.
The POTS signal, which is combined with the ADSL signal on one wire, is powered by the phone company. It retains power even if the ADSL line goes down or your computer is turned off. Once it enters the user's location and is split from the ADSL signal, the POTS line is a standard phone line. This issue may prove to be a challenge to some ADSL providers that have not developed the infrastructure necessary to address the high security requirements of some users.
ADSL signal-modulation methods have been a major area of dispute among ADSL hardware developers (see the table
"Rival ADSL Technologies"
). Carrierless amplitude and phase modulation (CAP) was the first method applied to ADSL. CAP combines the upstream and downstream data signals, separating them at the receiving modem using echo cancellation. This method has been used successfully in V.32 and V.34 modems. "CAP is what developers of ADSL started
with," says Joseph Mouhanna, manager of a research group that's evaluating broadband technologies at Microsoft. "Most of the equipment today remains CAP, but in the future, most equipment will be DMT."
DMT, short for discrete multitone, separates upstream data from downstream data. It splits the signal into separate 43-kHz carrier bands. DMT has been adopted by ANSI (ANSI T1.413) as a standard method for modulating ADSL, and the technology could be used with other flavors of xDSL as well.
CAP and DMT ADSL modems are incompatible, but until ADSL modems begin to be unbundled from services -- which will occur sometime late next year -- interoperability won't become a critical issue. By that time, many expect DMT to overcome CAP's early lead.
While the industry has not yet chosen a standard modulation method, the clear message is that it shouldn't matter to users. "Users should never be exposed to that stuff," Mouhanna says. "They don't see CAP, they don't see DMT -- all they should see is what
comes out the other end." As long as the PC and the modem at the CO use the same modulation method, xDSL should work. And since ADSL modems won't appear in stores for two years, according to TeleChoice analyst Kieran Taylor, there's time for the standards to sort themselves out.
Other DSL variants are also being developed and/or tested. Symmetric Digital Subscriber Line (SDSL) provides upstream and downstream signals of equal size. Although SDSL's speed may not be as well suited to speedy downloads, it works well for such bidirectional applications as videoconferencing and real-time editing of code or documents.
However, it's possible for phone companies and ISPs leasing copper wires to configure their switches to make ADSL behave symmetrically, although the downstream rate would drop. For instance, Pacific Bell is now talking about providing symmetrical DSL, but initially only at 384 Kbps. The rate, and the symmetrical transmission, will permit "full VHS-quality videoconferencing" for $50 to $80
per month, according to David Dorman, president and CEO of Pacific Bell.
Phone companies and ISPs are also studying other flavors of DSL. The capabilities and distance restrictions of the versions of xDSL now being developed are shown in the table
"Comparing xDSL Technologies"
.
Connecting an ADSL line to a PC is a challenge being addressed largely with standard 10Base-T Ethernet or universal serial bus (USB) connections. ADSL's high data rates preclude the use of a standard serial port. ADSL modems that are installed as internal devices handle the interfacing to the computer.
Bell Atlantic, GTE, Pacific Bell, and many small ISPs have successfully tested ADSL. The service has been deployed by a number of firms, including Signet Partners, an ISP in Austin, Texas, and Network Access Solutions, a local-exchange carrier that licenses copper lines from a regional Bell operating company (RBOC). Pacific Bell planned to deliver ADSL service to the Silicon Valley, Los Angeles, an
d the San Francisco Bay area by September and offer regional coverage by the end of 1998. Pricing for ASDL services ranges from $50 to $150 per month.
The cost of configuration is currently in the $500-to-$1000 range, which includes a modem at the CO and at the end user's PC. Initially, the modem is supplied to the user as part of the monthly service contract. Industry observers expect a wide-scale rollout of ADSL at the end of the year, with extremely rapid growth beginning in 1998.
A Look at LMDS
LMDS is a recently developed technology that uses radio frequencies in the 28-GHz band. Although it's small now, LMDS seems to have the potential to quickly grow into a powerful beast.
An LMDS service provider attaches an antenna that's roughly the size of a Ping-Pong paddle to a window or a wall. This antenna is then connected to an LDMS receiver, and the digital signal flows to an interface card installed in the computer.
The extremely high frequency of LMDS limits the transmissio
n signal to a radius of about 25 square miles. This short range may be one of its most attractive features. Because transmission distances are so limited, signals from antennas placed 10 or more miles apart can use identical frequencies without the risk of crosstalk or other interference problems that are common with radio frequencies that have a longer reach.
The FCC has allocated LMDS a huge frequency bandwidth: 1300 MHz. By comparison, broadcast TV uses 6 MHz of bandwidth, while cell phones use 25 MHz and broadband radios use only 30 MHz.
CellularVision America, a New York-based company that was involved in developing LMDS technology, launched its broadband data-transmission service in June. "This is not a test," insists Bruce Judson, CellularVision America's executive vice president.
The service, which was originally offered to subscribers in Manhattan and parts of Brooklyn, delivers a 500-Kbps signal downstream. The current implementation uses a dial-in modem for upstream communications
. Business users pay $79.95 per month, with a one-time installation fee of $225. Home users pay $49.95 per month, with a one-time installation fee of $199. Currently, the company's system supports only Windows 95.
CellularVision will introduce a higher-speed system in January; later, the company plans to offer two-way transmissions over the 28-GHz radio frequency. "We have the equipment to go two-way," says Judson. Before offering the service, the company will wait until the demand for two-way transmissions develops. Judson expects two-way to become economically viable in late 1998.
With all the services it plans to offer, CellularVision America will be able to serve a virtually unlimited number of customers. "Bandwidth is not a problem; we can serve the city," Judson says. "If we need additional capacity, we can dedicate additional channels, and we can also decrease the distance between cells."
The FCC plans to auction LMDS frequency for other parts of the country later this year. Meanwhile
, Motorola and Texas Instruments are developing LMDS products. "LMDS could be a dark horse," says Marshall Taplinsky, vice president of marketing at Hayes Microcomputer Products. "It's elegantly simple for the consumer to hook up, and pipes will be available for everybody, so the system won't get overloaded."
Cable Modems
The cable modem faces an uncertain future. Although technically it may satisfy the needs of many users, it may be too big a risk for many cable providers to offer this type of service.
The basic idea behind the cable modem is simple: A portion of the cable bandwidth carries data, and the cable modem extracts the data signal from the cable. Although this idea is elegant in theory, cable operators face many challenges. Cable TV's generally poor financial performance, aggravated by the loss of market share to digital satellite broadcasts (e.g., DirecTV), has forced many operators to be especially conservative about new investments.
Most cable is unidirectional; that i
s, it's designed to carry a video signal from a cable company's CO to subscribers' residences. A large percentage of installed cable supports downstream only. The cost of upgrading a system to bidirectional will probably delay any improvements until larger cable companies can demonstrate an acceptable return on investment.
Even if the cable companies successfully deploy digital data services over their cables, their very success might eventually prove to be their downfall. Cable transmission requires an inverted tree topology: A large trunk carries the signal from the cable company. Branches (i.e., cables) are split off, and additional branches are further split and brought into subscribers' homes.
All users on a branch share the cable's bandwidth. If the cable can deliver 6 Mbps of data, a solitary user on a branch enjoys more capacity than he or she can use. But when you add 50 or 100 or more users on the same branch, a 6-Mbps downstream signal, divided by the number of people vying for bandwidt
h, may deliver data to each user only at speeds comparable to those provided by an analog modem. Additional channels might have to be add-ed, and additional cable may have to be pulled, to deliver high bandwidth.
Microsoft recently invested $1 billion in Comcast, a major cable-service provider. Mouhanna describes the investment as "part of an effort to jump-start broadband over the public network. The cable industry needed a little boost to make it happen."
Microsoft's involvement may go a step further. Although its acquisition of WebTV was just approved in August, there was speculation that special versions of the WebTV box with an integral cable modem could be in Microsoft's product plans, which could boost the data transfer business for cable operators.
Digital Satellite Broadcasting
DirecPC, a product from Hughes Communications, is an asymmetric system that delivers 400-Kbps downstream data from a satellite to a home or office dish. DirecPC relies on a telephone connection for u
pstream communication.
While DirecPC uses a satellite dish similar to the one used by DirecTV, separate dishes are required for the two systems. Hughes will someday offer a method for using one dish for both DirecTV and DirecPC, although no target date has been announced. Hughes has also announced a PC card that will let a PC user view DirecTV signals on a monitor. Various pricing plans range from $9.95 per month, with a charge of 60 cents to 80 cents per megabyte downloaded, to $129.95 per month for unlimited access. Service charges do not include ISP fees.
Another service, DirecPC/EE (DirecPC Enterprise Edition) offers transfers of up to 24 Mbps of shared or dedicated bandwidth. This service, which is available to corporate customers, can be useful for transmitting large amounts of data to field locations or other sites that are equipped with very small aperture terminal (VSAT) receivers.
Motorola, Teledesic, and a growing number of other companies have announced plans for the placement of
satellites around the globe to provide point-to-point communications, data access, telephone service, video, and other services. Teledesic's plan, which is backed by Bill Gates and Craig McCaw, calls for the deployment of 288 satellites. The employment of satellites for data transfer will increase significantly when the first satellites are successfully launched and become fully operational, beginning around the year 2000.
Obtainable Today
Technologies delivering high bandwidth are here today. Within the next 18 months, availability of one or more high-speed options to homes and offices should be almost ubiquitous.
The situation for road warriors looking to obtain high-speed remote access, however, currently remains unclear. The Marriott in Washington, D.C., is installing in its guest rooms OverVoice, a system that provides connection to an ISP at 1.5 Mbps; as yet, the hotel chain hasn't said what this service will cost. According to Marriott, if the test is successful, the chain will co
nsider expanding the service to its other hotels.
As for the phone companies, the short-term opportunity lies in allowing consumers to access corporate resources from home, says Kamran Sistanizadeh, director of network-systems engineering at Pacific Bell. "Later phases of the program on a larger scale will address small- and large-business market segments," he adds. With luck, that will put high-speed access everywhere anyone needs it.
Where to Find
ADSL Forum
Fremont, CA
Phone: 510-608-5905
Internet: http://www.adsl.com
Batter Up for Broadband
