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March 1997
By Robyn Aber, 3Com Corp.
WAN Services
xDSL Supercharges Copper
DSL schemes promise multimegabit rates over local phone lines--and carriers and vendors are nearly ready to deliver
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This article is the first in an occasional series on emerging WAN access technologies. Future topics include cable modems, ISDN, and 56-kbit/s modems.
The local
loop is hot again. After years of speculation that the
phone network would never deliver data faster than 33.6 kbit/s (or
28.8 kbit/s, or 14.4 kbit/s, and so on down the line), it will soon be
possible to send digital data at multimegabit rates over conventional
telephone lines.
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MORE INFO
Getting More Out of Copper
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The enabling technology is digital subscriber line (xDSL), a scheme
that allows mixing data, voice, and video over phone lines. That's
good news for net managers, but it's complicated by the fact that
there are different types of DSL to choose from--hence "xDSL"--each
suited to different applications. What's more, most DSL services
haven't actually arrived yet. Carriers are holding trials, but it will
likely take a year or two before widespread commercial deployment
beg
ins. Further, the more exotic strains are still in the early
development stage.
THE DEAL FOR xDSL
Actually, DSL isn't a new idea. Bell Communications Research Inc.
(Bellcore, Piscataway, N.J.) developed the first DSL back in 1987 to
deliver video on demand and interactive TV over copper wires. That
effort stalled, however, when RBOC and cable company alliances
soured.
Interest in DSL regained momentum after it became clear that
installing fiber-based broadband loops was too costly and
time-consuming. Another major boost came with the passage of the
Telecommunications Reform Act of 1996 (see
"U.S. Deregulation: Beyond
the Letter of the Law,"
September 21, 1996). This legislation ended
local service monopolies and allowed competition among local phone
companies, long-distance carriers, cable companies, radio and TV
broadcasters, and Internet service providers (ISPs). Suddenly, local
exchange carriers needed a broadband service for the local loop
to
combat cable companies' plans to offer cable modem and telephony
services and interexchange carriers' attempts to gain access to the
local carriers' former monopoly.
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BUILD YOUR OWN CUSTOM TABLE
xDSL Examined
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All DSL technologies run on existing copper phone lines and use
modulation to boost transmission rates (see
"Getting More Out of
Copper"
). But the different approaches--ADSL, RADSL, HDSL, SDSL, and
VDSL--are best suited to different applications (
see Table 1
). In
considering each, the key points to keep in mind are the trade-offs
between signal distance and speed and the differences in symmetry of
upstream and downstream traffic.
STARTING W
ITH 'A'
Asymmetric digital subscriber line (ADSL) is the most widely
publicized of the xDSL schemes and is commonly touted as an ideal
transport for linking branch offices and telecommuters in need of
high-speed intranet/Internet access. The "asymmetric" refers to the
fact that it allows more bandwidth downstream (from the carrier's
central office to the customer's site) than upstream (from the
subscriber). Downstream, ADSL supports speeds of 1.5 to 8 Mbit/s,
depending on line quality, distance, and wire gauge. Upstream rates
range between 16 and 640 kbit/s, again depending on line quality,
distance, and wire gauge. For up to 18,000 feet, ADSL can move data at
T1 (1.544 Mbit/s) using standard 24-gauge wire. At distances of 12,000
feet or less, the maximum speed is 8 Mbit/s.
ADSL delivers a couple of other key benefits. First, ADSL equipment
being installed at carriers' central offices offloads overburdened
voice switches by moving data traffic off the public switched
telephone network and ont
o data networks--a critical problem resulting
from the huge amount of Internet traffic tying up voice networks.
Second, the power for ADSL is sent by the carrier over the copper
wire; as with conventional phone service, the line works even when
local power fails. That's an advantage over ISDN, which requires a
local power supply and thus a separate phone line for comparable
service guarantees. Third, and another benefit over ISDN, ADSL
furnishes three information channels--two for data and one for voice.
Thus data performance is not hampered by voice calls.
ADSL has already been standardized by both ANSI (American National
Standards Institute), which is now working on a revised version, and
ETSI (European Telecommunications Standards Institute). The rollout of
ADSL services will begin this year (a few services have already been
deployed), and widespread availability will likely occur during 1998
and 1999.
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Rate-adaptive digital subscriber line (RADSL) has the same
transmission limits as ADSL. But as its name suggests, it adjusts
transmission speed according to the length and quality of the local
line. Connection speed is established when the line syncs up or is set
by a signal from the central office. (RADSL devices poll the line
before transmitting; standards bodies are deciding if products will
constantly monitor the line speed. Standards activity is just now
beginning, and prestandard equipment may appear by year's end. RADSL
applications are the same as those for ADSL: Internet/intranet access,
video on-demand, database access, remote LAN access, and lifeline
phone service.)
High-bit-rate digital subscriber line (HDSL) technology is
symm
etric, meaning that it furnishes the same amount of bandwidth both
upstream and downstream. The most mature of the xDSL approaches, HDSL
has already been implemented in the telco feeder plant (the lines that
extend from central offices to remote nodes) and also in campus
environments. Because of its speed--T1 over two twisted pairs of
wiring, and E1 (2.048 Mbit/s) over three--telcos commonly deploy HDSL
as an alternative to T1/E1 with repeaters. At 15,000 feet, HDSL's
operating distance is shorter than ADSL's, but carriers can install
signal repeaters to extend its useful range (typically by 3,000 to
4,000 feet). HDSL's reliance on two or three wire-pairs makes it ideal
for connecting PBXs, interexchange carrier POPs (points of presence),
Internet servers, and campus networks. In addition, carriers are
starting to offer HDSL to carry digital traffic in the local loop,
between telco central offices and customer premises. HDSL's symmetry
makes this an attractive option for high-bandwidth services like
multimedi
a, but availability is still very limited.
Single-line digital subscriber line (SDSL) is essentially the same
as HDSL with two notable exceptions: It uses a single wire pair and
has a maximum operating range of 10,000 feet. Since it's symmetric and
needs only one twisted pair SDSL is suitable for applications like
videoconferencing or collaborative computing with identical downstream
and upstream speeds. Standards for SDSL are still under development,
though, and won't be firmed up until mid-1998 at the earliest.
Very high bit rate digital subscriber line (VDSL) is the fastest
DSL technology. It delivers downstream rates of 13 to 52 Mbit/s and
upstream rates of 1.5 to 2.3 Mbit/s over a single wire pair. But the
maximum operating distance is only 1,000 to 4,500 feet. In addition to
supporting the same applications as ADSL, VDSL, with its additional
bandwidth, could potentially enable carriers to deliver
high-definition television (HDTV). VDSL is still in the definition
stage, and a standard isn
't expected before late 1998.
GETTING STARTED WITH ADSL
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MORE INFO
ADSL In The Field
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ADSL is the service that's most likely to interest corporate
networkers--understandably so, since it looks like it will let them
supercharge copper connections sometime soon. But it's important to
remember that ADSL is still an emerging technology and is not
generally available. Numerous trials are taking place all over the
world (see
"ADSL in the Field"
), and though there have been a spate of
recent announcements about ADSL products and services, most won't be
available until the latter half of the year.
Before services really take off, though, carriers and equipment
makers must reduce their costs and develop scalable offerings.
For net managers
, that means asking carriers a lot of questions. In
particular, prospective enterprise users of ADSL will need to ask
whether they'll be offered a turnkey solution. Most carriers are
considering bundling ADSL modems with their services. It will be
important for enterprise users to ask if this is the case with their
carrier, for two reasons. First, it's unlikely that ADSL modems will
be available at retail outlets this year. Second, net managers will
want to ensure that their ADSL modems work with those at the carrier's
central office.
Given that ADSL modems are crucial to this scheme, it's worth
taking a closer look at how this gear gets the job done. To create
upstream and downstream channels, ADSL modems divide the phone line's
available bandwidth using one of two methods: frequency-division
multiplexing (FDM) or echo cancellation.
FDM assigns one band for upstream data and another band for
downstream data. The downstream path is further divided by
time-division multiplexing (TDM) into one o
r more high-speed channels
for data and one or more low-speed channels, one of which is for
voice. The upstream path is multiplexed into several low-speed
channels.
Echo cancellation assigns the upstream band to overlap the
downstream band and separates the two by means of local echo
cancellation--the same technique used by V.32 and V.34 modems. Echo
cancellation uses bandwidth more efficiently, but at the expense of
complexity and cost.
With both FDM and echo cancellation, a splitter front-ends an ADSL
modem to allocate a 4-kHz channel for voice, giving users both
conventional phone service and digital data over the same wire
pair.
ADSL ACCESS
ADSL requirements will vary according to applications and
configurations. For data-only traffic, one ADSL modem per LAN is
enough. For telecommuters (or small businesses), one modem is required
per subscriber line.
Deploying ADSL is relatively simple. Essentially, all it means is
adding an ADSL interface to the LAN. ADSL modems
sit on the carrier
side of the access termination at the subscriber's premises. Routers
are on the enterprise side of the connection and view an ADSL modem as
just another router interface. There's no need to reconfigure routing
tables just because a remote user has switched from leased lines to
ADSL access.
Design issues are a bit more complicated, however. Corporate
networkers will have to deal with asymmetric bandwidth. Further, flow
control becomes critical when high-speed LAN traffic hits a
lower-speed ADSL link.
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ADSL Mixes It Up
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Carriers also will need ADSL modems and line splitters on their end
(see
Figure 1
). These will slot into digital subscriber line access
multiplexers (DSLAMs) that terminate and aggregate incoming ADSL lines
for transmission onto voice
and data networks. (The splitters divide
the ADSL stream and shunt voice onto the public switched network.)
Service providers also must implement billing systems, and develop
testing and network management capabilities to offer ADSL service on a
large scale.
STUMBLING BLOCKS
A number of critical issues must be resolved before DSL
technologies achieve widespread commercial deployment. For starters,
standards are still under development. During 1996, both ANSI and ETSI
split on the choice between carrierless amplitude phase (CAP) and
discrete multitone (DMT) modulation for ADSL. At press time there
still is no agreement on the type of modulation, which is the key to
how ADSL works.
Some of the other outstanding issues are interoperability,
security, eliminating interference with ham radio signals, and
lowering power system requirements from the present 8 to 12 watts down
to 2 or 3. A nontechnical but critical factor will be how well
carriers can translate the successes they've realized in
their xDSL
technology trials to market trials and then to commercial
introduction.
Assuming those issues get ironed out, xDSL technology could
potentially be used within 5 to 10 years to deliver ATM to branch
offices and homes over a combination of optical fiber and copper
lines. Efforts to define the necessary standards are now under way in
ANSI, ETSI, the ADSL Forum, the ATM Forum, and the Digital
Audio-Visual Council (DAVIC).
What remains to be seen is how xDSL standards and technologies will
evolve; how carriers will handle the transition from trials to
low-cost services; which companies will emerge as the leading systems
vendors; which xDSL services will take off in which parts of the
world; and, of course, how big the demand for xDSL will be compared
with the demand for other types of local access services.
Robyn Aber is director of WAN business development at 3Com Corp. (Santa Clara, Calif.).
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