Hi all!
I've gotten a few emails from people asking me what ADSL is, so I thought I'd
put out what info I know about this stuff. I'm still not too clear on all the
details of how it works (that's for the guys with the thick glasses and
pocket protectors, hehehe), I just build some of boards that are using this
technology.
But basically, it's a way to use existing copper phone lines to transmit
voice and digital data at high speeds. The customer who we build boards for
is a company called CopperCom ( http://www.coppercom.com ) and they're
further along solving some of the technical issues with ADSL that everybody
faces when getting into this. The one board that we build in production
(actually it's in more of a pre-production stage) is their POTS (Plain Ol'
Telephone System board), it's the one with the 144-ball microBGA DSP.
On the front page of today's business section in the San Jose Mercury news
there's a story about Cisco, Nortel, and Ericsson buying three small niche
companies on Tuesday for $3 Billion that I feel are positioning themselves
for the "convergence" that I spoke of yesterday. Below is some info I've
gathered about ADSL that can explain things a little better than I can. Some
of the info is a little dated, but does give you a basic idea of what this is
all about.
-Steve Gregory-
About ADSL
ADSL started out as the phone company's way to compete with cable TV by
delivering both TV and phone service on your plain old copper phone line. Now
it's also a good candidate for high speed Internet access.
The "A" stands for "Asymmetric", meaning the phone company can send lots of
data to you, but you can't send much to them. Originally, only a tiny uplink
of 16 or 64kbps was supported; recent flavors of ADSL support up to ten times
that much.
ADSL is one member of a continuum of last-mile transport systems called DSL,
or Digital Subscriber Line, which can carry about 1 to 6 megabits/sec over
copper lines. It does not include any way to make long distance data calls,
or even local calls. That's another matter entirely-- one which is still up
in the air. ADSL was originally designed so you still can use it as a regular
phone line when the power goes out, which would be a big improvement over BRI
ISDN.
For the moment, the only form of DSL really being deployed is HDSL, which is
more or less a direct replacement for traditional T1 service. T1 lines have
been around forever, but require technicians to tune the line to perfection;
HDSL modems can handle dreadful lines cheerfully, so should be much cheaper
to install and run.
Asymmetric Digital Subscriber Lines (ADSL) are used to deliver high-rate
digital data over existing ordinary phone-lines. A new modulation technology
called Discrete Multitone (DMT) allows the transmission of high speed data.
ADSL facilitates the simultaneous use of normal telephone services, ISDN, and
high speed data transmission, eg., video.
DMT-based ADSL can be seen as the transition from existing copper-lines to
the future fiber-cables. This makes ADSL economically interesting for the
local telephone companies. They can offer customers high speed data services
even before switching to fiber-optics.
(My note: Think about all the copper wire all over the world and how long it
would take to re-wire the world with fiber optic cables...that's why I think
ADSL will take off.)
The Technology
How does ADSL work? (Non-techie alert!!) By incorporating frequency division
multiplexing (FDM) to utilize the entire possible bandwidth (1 mHz) of the
phone cable, ADSL can create several carrier frequencies to carry different
parts of a large data transmission simultaneously, creating a "bigger pipe".
ADSL reserves part of the bandwidth (the lower frequencies) for your plain
old telephone service (POTS). There is a guardband between the data and the
phone service so conversation won't be affected even if ADSL goes down.
Sounds simple, doesn't it?
Actually, in concept it is simple. But in practice there are limitations to
overcome. The full utilization of the small POTS cable requires some very
sophisticated fine-tuning in the area of noise control. A dynamic (constantly
changing) noise cancellation scheme must be incorporated to deal with
changing noise problems as they occur. This is where Amati steps in. They
hold the patent on this scheme, approved by ANSI (American National Standards
Institute). How important is that patent? Motorola has already licensed it
from Amati for their own ADSL implementation.
Alternative Technologies
Where does ADSL fit into the communications world? There is currently a lot
of debate about what the next generation of data communication is going to
be. The main contenders are ISDN, ADSL, ATM, Frame Relay, and cable modems.
We can immediately divide these into two camps; those which use the installed
base of telco wiring, and those that do not. Only ISDN and ADSL use POTS
wiring. ATM and Frame Relay require special leased phone lines that carry a
hefty monthly charge. They often require fiber-optic cable to link them to
the phone companies leased lines in order to preserve the bandwidth. Cable
modems have a similar problem. They use star topology wiring schemes which
makes them useful only for downstream communication. Any data traveling
upstream (out of the home) will travel unmercifully slow, and have the onus
of being very public (non-secure). In fact, if you were to transmit upstream
on most TV cable presently installed, you would likely be broadcasting to
your neighbors. (POTS topology is discrete switched circuits, the most secure
you can get.) To overcome this problem, cable companies are researching
contention based systems (IEEE 802.3) much like ethernet LANs. This is
heartedly endorsed by the likes of Cisco who proclaim it most workable. I
disagree. Contention based systems are great for "bursty" traffic, as in a
business LAN, but it is not suited for steady stream data such as video on
demand, teleconferencing, or interactive games. Why would Cisco endorse it
then? Simple, the plan is extremely hardware intensive - they will make a ton
of money selling routers and switches to cable companies.
Recent Quotes In The Media
Here is a quote from the recent PCWeek magazine: "Analysts say these [cable]
modems may not impact the market for several years, with only 8 percent of
the United States having two-way cable technology in place today and only 16
percent by next year."
Concerning ADSL, the CEO of Bell Atlantic had this to say, "Some may see ADSL
as a short-term solution and I agree, short-term for the next forty years."
Trial applications of ADSL have already begun nationwide. Several RBOC's
including Bell Atlantic are testing already and are set to begin extensive
testing soon.
As one Silicon Investor aptly wrote: "When it comes to reliability, who do
you trust, your phone company or your cable TV provider? To whom have you
made the most service calls?"
Amati describes ADSL like this: With ADSL you (or your family) can have a
"Voice Call (or send/receive a fax), A selected video on demand or
interactive education program, PC access to your office LAN at LAN speeds in
one window, an Internet video source in a second window, and a
videoconference using a screen corner." All on the same phone line!!
Investment Opportunity?
The ADSL forum expects ADSL to be available to the public near the end of
1996. Amati (AMTX), the company with the ANSI standard ADSL patent, was a
recent Nasadq entry, November 29, 1995 to be exact, and as of this writing is
currently trading at a mere $7 per share. Volume is increasing daily, and I
believe it could take off soon. This kind of foothold in a new proprietary
technology is exactly what industry analysts are always on the lookout for,
look at FORE Systems and ATM as a perfect example.
Concluding Remarks
One last word. How will ADSL affect ISDN? It can be argued that ADSL could
cut into ISDN implementation, but the important thing to remember is that
ISDN is the perfect "scalable" (the ability to upgrade cost-efficiently)
solution until ADSL becomes widely used. They both incorporate the same Telco
wiring and their service will be supplied by the same providers. This choice
is highly regarded over a complete paradigm shift as cable modems will entail.
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