Telecommunications Industry Association (TIA)
Dana Point, CA
TR-30.3/99-02-023
February 8,1999
COMMITTEE CONTRIBUTION
Technical Committee TR-30.3
SOURCE:
Bellcore
CONTRIBUTOR:
Ricardo Perez
973-829-2960
973-829-5866 (Fax)
rperez@notes.cc.bellcore.com
TITLE:
Methodology Involving Loop Selection for xDSL Model
PROJECT:
PN 4254
DISTRIBUTION:
Members of TR-30.3 and meeting attendees
ABSTRACT
Attached is a contribution that was submitted by Gary Tennyson of BellSouth
Telecommunications to the Universal ADSL Technical Group. The contribution demonstrates an
analysis of non-loaded loops in BellSouth and compares the aggregate to the first three loops
from ANSI T1.601. This method could be expanded to determine a subset of loops to represent
the levels of severity for network coverage in the xDSL transmission model.
A set of frequencies needs to be agreed upon to represent the xDSL bandwidth. A computation
using the proposed loops can be made and compared to actual loop loss as seen in the attached
contribution.
Copyright Statement
The contributor grants a free, irrevocable license to the Telecommunications Industry Association (TIA) to incorporate text contained
in this contribution and any modifications thereof in the creation of a TIA standards publication; to copyright in name any standards
publication even though it may include portions of this contribution; and at TIA’s sole discretion to permit others to reproduce in
whole or in part the resulting TIA standards publication.
Universal ADSL Technical Group Contribution
Document # TG/98-057
Atlanta, GA,
March 10 & 11, 1998
SOURCE:
Gary Tennyson
BellSouth Telecommunications
Telephone
(205) 985 6087
Fax
(205) 733 5603
Email gary.j.tennyson@bridge.bst.bls.com
TITLE:
Proposed Loop Reach / Performance Requirements
ABSTRACT:
This contribution justifies the need for coverage of at least 99% of the
non-loaded loop population. Analysis is also presented that indicates that
Loops 1, 2 & 3 from T1.601 are good approximations to the 99% point.
Minimum downstream data rates at the 99%, 90% and 60% points are
also proposed.
Introduction
This contribution is provided in response to a debate at the last SIG meeting on the subject of
loop coverage. The question is essentially the following:
Over what loops, and at what data rates, should ADSL-lite operate?
The Service Model
It should be pointed out that the service model for ADSL-lite (and for conventional ADSL,
for that matter) is not that for voiceband-data modems. Voiceband-data modems are
used in end-to-end scenarios where the (telephone) service provider plays no role in the
data service (other than providing an analog channel).
In the ADSL-lite model, the (telephone) service provider provides both the analog
channel (the loop) and one of the ADSL modems. In this scenario, the provider must
provide an acceptable quality of service.
There is one other pertinent difference between voiceband-data and the proposed
ADSL-lite service. This has to do with the ubiquity of service. Voiceband-data modems
work  at some rate  on practically any loop. ADSL-lite, on the other hand, will never
work on loaded loops. In some areas, this may be a significant percentage of loops. It is
not even clear at this point that ADSL-lite will operate on longer, non-loaded loops.
For this reason, it is desirable that potential customers not purchase the ADSL-lite CPE
until the service provider indicates that ADSL-lite is available at the customer’s location.
This places demands on the service provider’s internal processes. In a relatively short
time frame, a determination as to whether ADSL-lite will work (at an acceptable quality of
service) must be made. This determination is often denoted ‘loop qualification.’
Loop Qualification
Loop qualification obviously requires some sort of examination of the loop. End-to-end
testing of the loop would be very accurate but would require a premises visit. The
resultant delays are unacceptable to the end-user.
The alternative is reliance on loop records. Records may exist in many forms. Looking
again at the time frame in which loop qualification must be done, mechanized records
are needed. In most cases, the only mechanized loop records system available for this
purpose is the system used to ‘assign’ a line to a particular cable pair and serving
terminal.
The capabilities of this mechanized assignment system vary from company to company.
In many companies, the only data available in the system to ‘qualify’ a loop is an
indication of whether a particular cable pair is loaded (or not.)
Coverage
To summarize the above, the only loop information, readily available at the time of
service negotiation, may be whether it is loaded or not. Given this, we suggest that
ADSL-lite be capable of providing some minimum grade of service (we suggest a
downstream data rate of 128 kb/s excluding ATM overhead) on practically all non-loaded
loops.
When considering the actual percentage of loops to be covered, we need to recognize
that most service providers are in the business of providing service with high availability.
In this context, they are not equipped or inclined to deal with any significant number of
customers that  once they have committed to serve them  cannot be served. For
this reason, we suggest that an ADSL-lite standard require operation, at the above data
rate, on 99% of the non-loaded loops.
Rate Adaptation
We are not suggesting that the above data rate is acceptable for all customers. In fact,
our best estimate is that at least 256 kb/s downstream (excluding ATM overhead) is
required on 90% of the customer’s loops. Finally, our estimate is that at least 1 Mb/s,
again excluding ATM overhead, is needed on 60% of the non-loaded loops. These
requirements are repeated in the following table. Assuming that ATM overhead adds
about 15%, a third column is shown that includes the ATM overhead.
Percentile
Required Downstream
Line Rate
Required Downstream
Line Rate
(excluding ATM overhead)
(including ATM overhead)
(kb/s)
(kb/s)
60
1000
1150
90
256
294
99
128
147
We are not making any proposals at this time regarding upstream performance.
Translation of Loop Coverage Goals into Specific Test Loops
These goals regarding loop coverage and data rates need to be translated into specific
performance requirements. We offer the following analysis that defines a set of loops as
a good approximation of the 99% point. Additional analysis is needed to approximate
the 60% and 90% points.
A Cumulative Distribution Function (CDF) of the loss of a large sample of non-loaded
loops in BellSouth is shown below. The specific frequencies were selected to give some
insight into the potential performance in both the upstream and downstream directions.
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percentiles. The loss (at these same frequencies) of the three test loops from T1.601
(ISDN BRA), Loops 1, 2 and 3 is also shown. It can be seen that these three test loops
provide a fair representation of something near the 99% point (at least in terms of loop
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Noise and Margin on the 99% Loops
The noise environment in which system should be required to operate on a 99% loop is not clear.
Coupling the 99% loop, the 99% crosstalk noise coupling model, and a worst-case set of
interferers yields a scenario that will not likely be encountered in the field. Regarding interferers,
our estimate is that the most likely are the following:
 FEXT from other ADSL systems (from both the CO and remote systems)
 NEXT from residential Basic Rate Access ISDN systems
Clearly, something less than 6 dB of margin is appropriate on these very long loops.
Proposed Requirements
We propose the following requirements:
The system shall be rate-adaptive. It shall provide at least 150 kb/s of payload (including ATM
overhead) in the downstream direction on T1.601, Loops 1, 2 & 3. This requirement shall be met
in the presence of both, but not simultaneous (a) FEXT from 49 ADSL disturbers, and (b) NEXT
from 49 DSL disturbers. In order to demonstrate compliance, the system shall operate at a BER
of 10-7 or better, with at least 4 dB of margin.