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ADSL/xDSL
Below is a term paper I wrote for a college class. Before reading it, I will warn you that
the frequency spectrum chart (table 1) is a best guess estimate that I created from
multiple sources. ANSI standard T1.143 covers ADSL, but costs in the neighborhood of $180
USD, so I do not have the information that this standard provides.
XDSL
The future, or just another step on the ladder?
Introduction
I remember my first modem, it was an acoustically coupled 300 baud speed demon in which
I could reach out and touch the world. Well, the part of the world that didn’t
require any long distance toll charges anyway. Since then, I have watched
telecommunications technology change and evolve into what we have today.
Telecommunications has always interested me, I guess that’s why I wanted to join the
Air Force as a Satellite Communications technician. Well, that and I was tired of washing
dishes at Denny’s.
What I will be discussing is a relatively new technology called Asynchronous Digital
Subscriber Line (ADSL). As always with computers, ADSL isn’t just black and white.
There are varying flavors of ADSL, and the generic term to encompass them is xDSL. Later I
will briefly touch on the capabilities of these related DSLs, but the ins and outs are
well beyond the scope of this paper, as well as excruciatingly boring. I’ve divided
this discussion into three areas.
The Basics of ADSL
Analog telecommunications technology is nearing it’s theoretical limits. This was
recognized early on and industry pundits have been testing new digital technologies to
replace it. The first technology to widely become implemented was Integrated Services
Digital Network (ISDN). This provided a small measure of relief to those requiring greater
bandwidth, but required expensive hardware and installations. ADSL is a solution to that
dilemma. However, ADSL isn’t without it’s own complications. First, let’s
look at what ADSL is.
This chart shows the frequency spectrum for ADSL. For now, just ignore the frequency
allocation for ISDN as I’ll cover that later. ADSL allows for dedicated bandwidths to
POTS, upstream, and downstream signals. What this means is that not only can you be
connected to the Internet, but that you have simultaneous capability for voice/fax as
well. In addition, ADSL was designed as an “always on” solution. Unlike current
Dial-up Networking (DUN) and ISDN which require you to log in to your ISP for a session,
ADSL will always be connected. While things like WebTV and cable modems provide a similar
solution, with ADSL you aren’t sharing the bandwidth with everyone else. Each ADSL
connection is a point to point with your ISP. That means all the data coming down the pipe
is yours and not a broadcast aggregate signal to all the WebTV and cable modem users in
your area.
The standard for ADSL is defined by American National Standards Institute (ANSI) and
European Telecommunications Standards Institute (ETSI) in T1.413 which was published in
late 1995. This standard calls for the use of Discrete MultiTone (DMT) as the line code to
be used. There is another line code called Carrierless Amplitude/Phase (CAP) which some
groups are trying to get accepted as a parallel standard for ADSL. I will cover this a
little later. For now, here is what the DMT line code consists of:
- Divides the channels into sub-channels referred to as tones. There are 256 tones for
downstream and 32 tones for upstream.
- Each tone is QAM modulated (AKA 2B1Q) on a unique carrier (FDM).
- Each tone has a bandwidth of 4.3125KHz with a guardband also of 4.3125KHz.
- Each tone has a 4 kilobaud symbol rate.
- Each tone carries 2 to 15 bits (Rate adaptive).
- Because it is rate adaptive, users can choose bandwidth requirements in 32Kbps steps
(64Kbps to 8.192Mbps downstream and 16Kbps to 768Kbps upstream), and hardware can work
around RFI.
- Active/Passive POTS splitter. Active requires an external power source, while passive
gets power from the telco. This could be important for lifeline emergencies during power
outages.
- Maximum throughput (optimal conditions) is 15Mbps. 4KHz tone * 15 bits per tone * 256
tones.
- Good interoperability between different manufacturers of ADSL modems.
- Distances of 15,000 feet (-26dB) and a maximum of 18,000 feet (-31 dB).
- Built in error correction.
- Open standard with minor proprietary implementations.
- Single wire pair (24 gauge) hardware.
Technology |
Speed |
Distance Limitation (24-gauge wire) |
Applications |
56 Kbps analog modems |
56 Kbps downstream |
None
28.8 or 33.6 Kbps upstream
|
E-mail, remote LAN access, Internet/intranet access |
ISDN |
Up to 128 Kbps (uncompressed) Full duplex |
18,000 feet (additional equipment can extend the
distance) |
Video conferencing, disaster recovery, leased line
backup, transaction processing, call center services, Internet/intranet access
|
Cable modem |
10 - 30 Mbps downstream 128 Kbps - 10 Mbps upstream
(shared, not dedicated, bandwidth) |
30 miles over coaxial (additional equipment can extend
the distance to 200 miles)
|
Internet access |
ADSL Lite |
Up to 1 Mbps downstream
Up to 512 Kbps upstream
|
18,000 feet |
Internet/intranet access, Web browsing, IP telephony,
video telephony |
ADSL/R-ADSL |
1.5 - 8 Mbps downstream Up to 1.544 Mbps upstream |
18,000 feet (12,000 feet for fastest speeds) |
Internet/intranet access, video-on-demand, remote LAN
access, VPNs, VoIP
|
IDSL |
Up to 144 Kbps full duplex |
18,000 feet (additional equipment can extend the
distance) |
Internet/intranet access, Web browsing, IP telephony,
video telephony
|
HDSL |
1.544 Mbps full duplex (T1)
2.048 Mbps full duplex (E1)
(uses 2 - 3 wire pairs)
|
12,000 - 15,000 feet |
Local, repeatered T1/E1 trunk replacement, PBX
interconnection, Frame Relay traffic aggregator, LAN interconnect |
SDSL |
1.544 Mbps full duplex (T1)
2.048 Mbps full duplex (E1)
(uses 1 wire pair)
|
10,000 feet |
Local, repeatered T1/E1 trunk replacement, collaborative
computing, LAN interconnect |
VDSL |
13 - 52 Mbps downstream
1.5 - 2.3 Mbps upstream
(up to 34 Mbps if symmetric)
|
1,000 - 4,500 feet
(depending on speed) |
Multimedia Internet access, high-definition television
program delivery |
Source: 3Com, March 1998.
Controversy Surrounding ADSL
As with all new and emerging technologies, there will be generation gaps. What I mean
by this is that with ISDN being also relatively new and that many corporate
infrastructures have already migrated to this, management will now have to decide on
whether to stay with ISDN or to eat the cost and upgrade to the better technology.
However, there is good news for them as a few manufacturers/providers are working out
technologies that combine ISDN and ADSL. There are two methods of coexisting ISDN and ADSL
called in-band and out-of-band.
- In-band allows ADSL to treat ISDN as a normal ADSL bitstream. Meaning, it will break up
the aggregate 128Kbps signal into 32 tones.
- Out-of-band is a bit more complicated as frequencies are shifted around so as to leave
no overlap. This introduces two problems. The first is that the startup and pilot
frequencies (for ADSL negotiation) must be translated into higher frequencies, and
secondly is that none of this is compliant with the T1.413 standard.
Another issue with ADSL is its “always on” state. If ADSL becomes mainstream
and everyone is always connected, then the need for more IP addresses becomes paramount.
Currently, IPv4 (32 bit) allows for 4.2 billion IP addresses. To cover this, IPv6 (128
bit) which has already been standardized as well as implemented allows for 3.4 x 1038 IP
addresses. As gee-wiz stuff, here is what the IP addresses look like:
IPv4 - xxx.xxx.xxx.xxx
i.e. 127.003.111.145
IPv6 - xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx.xxxx
i.e. FCDA.1456.3FDC.19A5.F4CD.32FA.A5EC.AD82
Now that the IP address issue is resolved, can the Internet backbone really handle
millions of users requesting downstream information at up to 15Mbps?
As I stated earlier, there is another line code called CAP which is vying for ADSL
standardization. In 1993 Bellcore, Bell Atlantic, and Nynex organized the “ADSL
Olympics” in order to evaluate DMT vs. CAP vs. straight QAM. The trial results showed
DTM to be superior to both CAP and QAM. There is still quite a bit of controversy
surrounding CAP and the following lists some of it’s shortcomings (please note that I
was unable to find any unbiased comparisons of the two, so please take these as you will):
- For equivalent rates, CAP requires more power.
- CAP provides course steps of 300Kbps as opposed to DMTs 32Kbps and no downstream rates
of <640Kbps. This means users will not be able to request exact bandwidth requirements.
- CAP is a proprietary architecture.
- No built in error correction.
- CAP does not follow the T1.413 standard by deviating in the following ways which
introduce crosstalk:
- Upstream passband is 55.4 - 171KHz.
- Upstream Power level of -41dBm at 181.2KHz which exceeds standard of -62dBm above
181KHz.
- Downstream extends to 1.5MHz which exceeds standard of 1.1MHz.
Set-up and Pricing Information
While set-up costs can initially be high ($200 - $500), the monthly charges are
expected to be competitive with Cable modems and WebTV. Because xDSL is geographically
sensitive, the largest factor in set-up involves the loop length to your house or place of
business. At its maximum distance of 18,000 feet you will never realize the full potential
(bandwidth). However, new advances are being made every day to overcome this limitation.
The only other cost consideration concerning ADSL is whether your modem will be DMT or
CAP. CAP is a much simpler technology and therefore costs less.
Conclusion
I’ve talked about what ADSL is and some of it’s close cousins, the
controversy surrounding ADSL, and basic pricing and set-up. Is ADSL the new wave of the
future in telecommunications? Yes and no. For now it’s the best you can get, but I
know as soon as I buy it, something better will come along.
Two very good sites to visit for further information are:
http://www.xdsl.com and http://www.adsl.com
Bibliography
Emery, Mark. The Evolution of
xDSL-Based Services, (Date Unknown), (Copyright Mark Emery)
OrcKit Communications. (1998) How
Does ADSL Work, (1998), (Copyright Orckit Communications 1998)
OrcKit Communications.(1998) ADSL
over ISDN White Paper, (1998), (Copyright Orckit Communications 1998)
Analog Devices Inc.(1995-99) Status and Issues
in xDSL, (1999), (Copyright Analog Devices Inc. 1999)
Analog Devices Inc.(1995-99) Summary,
(1999), (Copyright Analog Devices Inc. 1999)
Analog Devices Inc.(1995-99) Introduction,
(1999), (Copyright Analog Devices Inc. 1999)
Analog Devices Inc.(1995-99) DMT
& CAP, Explaining the Technologies, (1999), (Copyright Analog Devices Inc. 1999)
Analog Devices Inc.(1995-99) International
Standards, (1999), (Copyright Analog Devices Inc. 1999)
Analog Devices Inc.(1995-99) Performance
and Trials, (1999), (Copyright Analog Devices Inc. 1999)
Analog Devices Inc.(1995-99) Power
Spectral Density and Spectral Compatibility, (1999), (Copyright Analog Devices
Inc. 1999)
Analog Devices Inc.(1995-99) The Real
RADSL: Rate Adaptation and Serving Customers, (1999), (Copyright Analog Devices
Inc. 1999)
