Telecommunications Industry Association
(TIA)
TR-30.3/00-01-607
Boca Raton January 12 – 14, 2000
COMMITTEE CONTRIBUTION
Technical Committee TR-30 Meetings
SOURCE:
Paradyne
CONTRIBUTOR:
Gordon Bremer
Phone:
727-530-8446
Fax:
727-532-5244
email:
gbremer@paradyne.com
TITLE:
Telephony Drop Wire Characteristics and Related xDSL Performance Issues
PROJECT:
PN-4254/PN-4255
DISTRIBUTION:
Members of TR30.3 and attendees at Boca Raton meeting.
ABSTRACT
Drop wire is the telephony cabling that connects the subscriber loop and the NID (Network Interface
Device) at residential and small business sites. Thus, a drop wire is present in the communications link
of most xDSL services, even “NID splittered” services and VDSL.
Physical inspection of common drop wire types reveals that most have little or no twist and thus should
not be expected to have the same characteristics as 25 pair telephony binders.
This contribution reports the measured characteristics of five types of common drop wire and highlights
potentially xDSL performance issues that warrant inclusion of drop wire characteristics in all xDSL
evaluations. Characteristics were measured at frequencies up to 6 MHz.
Results reported here are:

loss,

coupled interfering signals from external sources and

inter-wire-pair coupling.
Text is proposed for inclusion in TR30.3 draft standard “Network Access Transmission Model for
Evaluating xDSL Modem Performance”
Introduction
“Drop wire” (or “entrance cable” or “service wire”) refers to the telephony cabling that connects the subscriber loop binder
and the NID (Network Interface Device) at residential and small business premises. (In larger premises, the 25 pair cable
binder may directly enter the premises and connect differently to the premises wiring.)
In residences and small businesses, an xDSL device is desirably connected to a standard telephone wall jack via a ubiquitous
telephone line cord. If the premises wire pair is also line shared with POTS, additional telephone line cords are usually
connected to the wire pair at other telephone jacks as well, and each of these line cords typically has a telephone or modem
connected to it. Less desirably, the xDSL device is connected directly to the NID via special CAT3 wire. Thus, all xDSL
devices are subjected to drop wire impairments. Unless special wiring is provided from the NID, DSL devices are
additionally subjected to premises wiring and telephone line cord impairments.
A recent survey of manufacturers by this contributor has concluded that multi-line drop wire (2-line, 3-line, 5-line or 6-line)
has been installed perhaps exclusively in North America since about 1992. Typical lengths range from 75 feet to 300 feet.
Premises wiring includes both the intra-wall wiring and the telephone line cords from the telephone wall jacks. A recent
survey by this contributor in Contribution TR-30.3/98-11-128 “Residential Telephone Wiring Survey & Proposed Premises
Wiring Models” concluded that a large percentage of premises have two or three line, non-twisted, unshielded, #22 or #24
AWG intra-wall telephone wiring at a length of at least 200 – 250 feet and as much as 1000 feet. Five to 10 telephone jacks
are typical. Each telephone line cord has typical length of 6 to 25 feet. Both 1-line and 2-line telephone line cords are
common.
Drop Wire Impairments
The performance tests and claims of virtually all xDSL technology standards presently ignore drop wire and premises wiring
impairments. In contrast, the technology-independent TR30.3 draft standard “Network Access Transmission Model for
Evaluating xDSL Modem Performance” (PN 4254) identifies the drop wire and the premises wiring as contributors to xDSL
performance limitations.
Tests reported here measure:

loss,

coupled interfering signals from external sources and

inter-wire-pair coupling.
Characterization for a single data service must address signal loss and coupled interfering signals. Characterization where a
second service may exist must additionally consider inter-wire-pair crosstalk. Second service consideration includes (1)
bonded xDSL technologies, (2) another xDSL service, (3) VDSL or (4) home-LAN signals. In all these cases, inter-wire-pair
coupling must be carefully considered.
Types of Drop Wire Tested
The drop wires in Table I were tested. All are very common. Thus, it should be anticipated that DSL installation should
have a high probability of encountering the impairments reported here.
Table I
Nomenclature
General Description
Wire Description
QTDW
300 ft Quad Twist Drop Wire (2 line)
Aerial Service Wire Asw-2/22-A
2PAD
300 ft 2-pair Aerial Drop Wire (2 line)
Superior Cable T 1999 2x22 Adp-Nms (Ul) E-39169
3PBD
285 ft 3-pair Buried Drop wire (3 line)
Superior Cable T 1998 6x22 Adp-Nms (Ul) E-39169
5PBD
285 ft 5-pair Buried Drop Wire (5 line)
5x22 Bw Brf Superior Cable 03/99 23200 Feet
6PAD
285 ft 6-pair Aerial Drop Wire (6 line)
Superior Cable T 1998 6x22 Adp-Nms (Ul) E-39169
Test Descriptions
Each of the types of drop wire was tested separately in the physical configuration shown in Figure 1 below. This physical
configuration was selected to avoid potential self-interference of the drop wire. Experimental repositioning indicated that this
configuration was not introducing significant loss or crosstalk peculiar to this particular configuration. Unused wire pairs
were not terminated. Note that the configuration does not include vertical segments. The addition of these may increase RFI.
Inter-wire-pair coupling (crosstalk) was tested using a modified version of Figure 1 wherein the driven pair was terminated at
the far end instead by a 100 ohm resistor and the victim pair was terminated at both ends with either a 100 ohm resistor or the
100 ohm spectrum analyzer termination (as appropriate for either the NEXT or FEXT measurement). All victim pairs were
tested to determine variations. Again, unused wire pairs were not terminated.
To achieve accurate measurements, the equipment setup loss, the noise floor and self-interference was carefully measured
and optimized without any drop wire under test. Equipment selection and physical configuration was achieved (with
difficulty) that allowed test
Drop Lead NEXT/FEXT Wire Position Setup
results that are not affected
by the test equipment.
Supplementary tests were
made to determine the
effect of coiling (one or
three 3 ft diameter coils).
The results increased NEXT
and FEXT greatly in some
frequency bands.
Characteristics were plotted
in the ranges 0 to 6 MHz
and 0 to 1 MHz.
Figure
1
~160 ft
~30 ft
yard area (grass)
exterior wall
~2
5
ft
test
equipment
~30 ft
building area (concrete)
drop lead position for NEXT
drop lead position for FEXT
A Note About Inter-Pair Coupling: “PEXT”
Because loss on the drop wire is small, crosstalk measured at both ends of the drop wire is nearly identical, especially at
frequencies below 1 MHz. Furthermore, crosstalk within the drop wire effects DSL performance differently than classical
NEXT and FEXT: it is a direct additive coupling occurring at the premises end. For these reasons, the drop wire crosstalk is
referred to herein as PEXT: Premises End Crosstalk. For multiple services, PEXT causes direct coupling of the upstream
transmit signals and direct coupling of the downstream receive signal which establishes an SNR floor in both directions
irrespective of loop length.
Secondary Results
It is advantageous to first discuss secondary or arguable findings that may otherwise cloud the primary results.

The 6-pair aerial drop wire PEXT was measured with both the physical layouts of Figure 1 as well as with an entirely
different alternate layout that was inside a building. PEXT below 600 kHz was unaffected. PEXT in some bands above
600 kHz changed as much 8 dB from one layout to another; however, these differences were within the same amount of
variation found from pair to pair. Thus, physical layout of the drop wire is not a significant factor for PEXT.

The 6-pair aerial drop wire PEXT was measured with unused pairs both terminated and un-terminated. Results were
within ± 1 dB from 0 to 6 Mhz. Thus, the termination of unused pairs is not a factor.

The 6-pair aerial drop wire PEXT was measured with the drop wire coiled 1. There was negligible effect below about
300 kHz. However, coils increased NEXT as much as 6 dB at 400 kHz and as much as 18 dB at frequencies as low as
650 kHz. Drop wire coiling can be detrimental to PEXT, but only above about 300 kHz.

PEXT varied as much as 15 dB from victim pair to victim pair. The highest values are used in the primary findings
below.

The 3-pair buried drop wire PEXT was measured with and without the shield grounded. The difference was within ± 1
dB up to 600 kHz. Un-grounding actually improved PEXT as much as 7 dB up to 1 Mhz. Data was not taken above 1
MHz. The grounding status of a shield does not significantly affect PEXT.
1
A single coil approximately three feet in diameter was created. Then three overlaid coils were created.
Primary Results

Loss: Loss was about 2 dB per Mhz up to 6 MHz on all drop wires. Ripple was negligible up to 1 MHz, but increased
to ±2 dB above 3 MHz. A representative plot (300 ft 2-pair Aerial Drop Wire) is shown in Figure 2 below.
Figure 2

Radio Interference: AM band radio signals coupled onto both aerial drop wires at levels as high as 12.6 mV in a 3 kHz
band (-28 dBm at 100 ohms). The levels are non-stationary, dropping as much as 10 dB from the above level at times,
apparently due to the audio content of the modulating signal. An example (300 ft Quad Twist Drop Wire) encountered
during PEXT measurements is shown in Figure 3 below Thus, interference from a single AM radio station can be as
large as 12.6 mV in a 3 kHz band (-28 dBm at 100 ohms).
Figure 3

Inter-Pair Coupling: PEXT for the five types of drop wire is shown in Figure 4 with the maximum PEXT for any of
the five also shown. A comparison of the maximum drop wire PEXT to theoretical binder group 24-disturber 99%
NEXT and 99% FEXT is shown in Figure 5. From Figure 5, it is seen that measured PEXT is about 11 dB less severe
than the worst case NEXT expected on a subscriber loop. Measured PEXT can be up to 45 dB more severe than worst
case FEXT expected on a subscriber loop.
A model that approximates the maximum drop wire PEXT is:
Drop Wire PEXT = -83 + 15log10(f/10) dB
for f in kHz and 10 kHz < f < 1 MHz
Drop Wire PEXT = -53 + 30log10(f/10) dB
for f in kHz and 1 MHz < f < 6 MHz
300 ft Drop Wire PEXT (Five Types): 10 kHz to 6 MHz
0
Q TDW
-10
-20
2PAD
Figure 4
-30
dB
-40
3PBD
-50
5PBD
-60
6PAD
-70
-80
DW-MAX
-90
10
100
kHz
1000
10000
Measured Drop Wire PEXT Compared to "Theories": 0 to 6 MHz
0
-10
-20
-30
-40
-50
dB
-60
-70
-80
-90
-100
-110
-120
-130
-140
kHz
NEXT T HEORY
FEXT -18 T HEORY
FEXT -12 T HEORY
FEXT -06 T HEORY
DW-MAX
Figure 5
10
100
1000
10000
Conclusions
1.
A drop wire is present in the communications link of most xDSL services, even “NID splittered” services and VDSL,
and should be included in all xDSL evaluations. Typical lengths are 75 to 300 feet. Tested length was 300 feet.
2.
Five common drop wire types are identified and tested for loss, RFI and inter-pair coupling.
3.
Results presented here are not worst case, but represent likely impairments in practical xDSL installations.
4.
Loss slope is about 2 dB per Mhz up to 6 MHz on all drop wires. Ripple was negligible up to 1 MHz, but increased to
±2 dB above 3 MHz.
5.
Interference from a single AM radio station can be as large as 12.6 mV in a 3 kHz band (-28 dBm at 100 ohms).
6.
Crosstalk within the drop wire affects DSL performance differently than classical NEXT and FEXT. For these reasons,
the drop wire crosstalk is referred to herein as PEXT: Premises End Crosstalk.
7.
A model that approximates the maximum drop wire PEXT is:
Drop Wire PEXT = -83 + 15log10(f/10) dB
for f in kHz and 10 kHz < f < 1 MHz
Drop Wire PEXT = -53 + 30log10(f/10) dB
for f in kHz and 1 MHz < f < 6 MHz
Recommended Text
It is recommended that the following text be adopted for PN 4254, Section A.4.3, Drop Models:
D.4.3 Drop Models
The appropriate drop model differs significantly depending upon whether a second service is present or may be added to the
deployment environment. In cases where a second service may be deployed, inter-wire-pair crosstalk can be the dominant
effect. Because ‘second services’ such as home-LAN signals may become quite common, testers are cautioned against using
the Single Digital Service Model where the possibility exists for second service deployment.
D.4.3.1 Model for Single Digital Service
In an environment where a second service is not present and it can be safely assumed that a second service will not be added,
the effect of the drop can be simply modeled by 300 feet of 22 AWG wire.
D.4.3.2 Model for Second Service Environments
Here, the PEXT effect can be predominant. Model comprises 300 feet of 22 AWG wire with the following PEXT (which
approximates the maximum drop wire PEXT).
Drop Wire PEXT = -83 + 15log10(f/10) dB
for f in kHz and 10 kHz < f < 1 MHz
Drop Wire PEXT = -53 + 30log10(f/10) dB
for f in kHz and 1 MHz < f < 6 MHz