ADSL

[David Farber <farber () central cis upenn edu>]

From: RUSSELK () ctrvax Vanderbilt Edu  (RUSSELK)
Subject:  ADSL Details
Organization: Vanderbilt University
Date: Fri, 3 Dec 1993 18:56:41 GMT




Because I have received so many requests for more information about
Northern Telecom's ADSL system, I will try to share my understanding
of how it works, from the point of origin, through the Public Switched
Network, the CO, Subscriber Loop and into the Customer's Premise.  I
must say that I have serious questions as to the affordability of
ADSL, given what I know about the way new services are being tariffed
across the country.  I am, however, intrigued with the technology.


I make no warranties as to the accuracy of this information, and would
welcome any corrections from anyone with a better understanding of the
system.




Ken Russell


      ----------- My Understanding of ADSL ------------


The loop between the Information Provider (Video Program Source or
whatevery you wish to call this provider) and the Telco Central Office
is likely to be capable of carrying multiple signals, allowing the
service provider to deliver programming to more than one customer
simultaneously.  This interface is therefore likely to be a T3 or OC-3
SONET loop.  The loop will terminate at the CO in an ADSL Interface,
which will de-multiplex the signal, separating the signals destined
for various subscribers.


The line card used to provide POTS service to any individual
subscriber will be disconnected from the subscriber loop and
terminated in a special transciever.  This transceiver will be
connected to the subscriber loop.  This procedure would, of course
require a service order from the RBOC or serving telco.  If the
subscriber wishes ISDN Basic Rate Access service to be included in the
ADSL subscription, then an ISDN CO BRI line would also be terminated
in the same transceiver.  If the customer wishes to subscribe to
Digital Wideband H0 service, the appropriate 384 Kbps H0 line would
also be terminated in the same transceiver.  If an MPEG1 1.5 Mbps
video channel is desired, then this would be provided by providing
either a single T1 circuit for termination in the same transceiver, or
most likely a special ADSL CO Line Card would be similarly terminated.


This arrangement will provide for continuous POTS service, should
there be a service disruption of any of the other services.  The H0
and ISDN channels would be similarly protected from disruption.


This special transceiver will then test the subscriber loop,
determining its actual transmission characteristics.  This testing is
repeated throughout the service period.  The available bandwidth is
then divided into 4 khz channels.  The total number of channels
available for transmission will vary on each subscriber loop, but
could be as many as 256.  Each of the 4 khz channels is then tested
for transmission characteristics.  Any channel that cannot meet specs
is shut off.


The baseband from 0 to 10 khz is filter isolated from the rest of the
bandwidth, and is used for POTS service.  Another multiplex channel is
created between 10 khz and 50 khz.  This channel is used for upstream
transmission (from the subscriber to the CO and Information Services
Provider).  NOTE: All upstream data is transmitted in this channel,
this includes the upstream portion of all full duplex services,
including ISDN and H0 upstream data, as well as all upstream network
maintenance and testing traffic -OAM&P, although not POTS (analog)
service.


All downstream data is transmitted using a wide channel above 50 khz.
The exact capacity of this channel will vary, depending on the
transmission characteristics of the individual loop.  The wire guage,
number of bridged taps, and the effects of electromagnetic
interference will all affect the achievable bandwidth.  For good
quality service, this channel will transmit all downstream data from
ISDN, H0 and OAM&P duplex services, as well as all downstream video.


The downstream video can be any combination of MPEG1 and MPEG2
circuits, up to the available bandwidth, which on a good line will be
6 Mbps or four 1.5 Mbps channels.  Four 1.5 Mbps channels, or two 3
Mbps channels, or two 1.5 Mbps channels and one 3 Mbps channel.  I am
told that the entire 6 Mbps channel can also be used to deliver a
single Video 1 - 6 Mbps real time video channel, or even a single
compressed HDTV channel.


At the Customer Premise, the subscriber would be required to provide
another special transceiver which would attach directly to the
subscriber loop network termination.  This transceiver would probably
include the MPEG1 and MPEG2 codec chipsets and a TV driver chipset
with a coaxial port for attaching a standard TV.


ISDN service may include an ISDN NT-1 with 2-RJ-45 Jacks.  POTS
service would be provided through a standard RJ-11 jack.  H0 switched
wideband access would be provided through a standard RJ-45 jack, which
would then interface with an external dataset and codec for
videoconferencing.


Video 1 - 6 Mbps real time video would probably be provided through
the same coaxial port provided for MPEG1 and MPEG2 decoded signals.
It is likely that compressed HDTV signals would require a separate
codec, and therefore would require a separate port.


What I have sketched out above is all theory, and will not be the
deployed configuration for the first two to three years of service.
As I understand the goal of the product team, ADSL I will include no
H0 Wideband channel, but will include ISDN Basic Rate Interface, and a
single 1.5 Mbps video delivery channel, while ADSL II will emerge
sometime in 1995 with full service.  The delivery of 3 Mbps and 6 Mbps
signals will also require expanded switching matrix structures at the
Central Office, although this could conceivably be provided through
multi-mode networks as the cable companies either co-locate at the
Telco CO or merge and become part of the same company.  Northern had
inferred that they may be preparing to offer these rates as part of
the broadband switching architectures they are preparing to roll out,
but this is purely speculation on my part.  I know nothing of any such
plans by any other manufacturer.


The Multicarrier technology used by Northern is supposedly defined by
ANSI T1E1.4 specs, and is known as Discrete Multi-Tone multicarrier
modulation, and uses Fast Fourier Transform and Inverse Fast Fourier
Transform coding algorithms.  This is supposedly the ANSI standard for
ADSL, and has also supposedly been adopted by the Exchange Carriers
Standards Association.  Northern explains that it actually requires
less complex circuitry than QAM or CAP encoding techniques, and
supports higher speed transmissions than other methods.


I am about ready to evaluate AT&T's specifications, and will share any
understanding I gain with the group.




Ken Russell
Access Development Corporation
5115 Maryland Way
Brentwood, Tennessee  37027
russelk () ctrvax vanderbilt edu


Volunteer Instructor, Vanderbilt Virtual School Program