Download Longitudinal Output Voltage Limitations

Telecommunications Industry Association
TR41.9-06-11-008
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Document Title
Rationale for Longitudinal Output Voltage Limits in TIA-968-A
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Trone Bishop
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The document to which this cover statement is attached is submitted to a Formulating Group or subelement thereof of the Telecommunications Industry Association (TIA) in accordance with the provisions
of Sections 6.4.1–6.4.6 inclusive of the TIA Engineering Manual dated March 2005, all of which
provisions are hereby incorporated by reference.
This document has been prepared by Verizon to assist TIA Engineering Committee TR-41. It is proposed
to the TR41.9 subcommittee as a basis for discussion and is not to be construed as a binding proposal on
Verizon. Verizon specifically reserves the right to amend or modify the material contained.
Abstract
This contribution provides rationale for the Longitudinal Output Voltage limitations for telephone
terminal equipment in TIA-968-A and proposes that existing limitations be retained in TIA-968-A
for all classes of equipment. This parameter, if unconstrained, can result in crosstalk that can
adversely impact other services in the same cable.
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Rationale for Longitudinal Output Voltage Limits in TIA-968-A
Introduction
An Alcatel Canada contribution1 presented at the August 2006 meeting of TR41.9 asserted
that the out-of-band Longitudinal Output Voltage (LOV) limitations for DSL in TIA-968-A
were too onerous. Alcatel Canada recommended that the LOV limits for DSL be deleted
entirely from TIA-968-A or that the out-of-band Longitudinal Output Voltage (LOV)
limitations for DSL TE be changed from -80 dBV to -50 dBV.
This contribution provides rationale for LOV limitations and shows that well designed
Terminal Equipment (TE) that meets applicable signal power limitations and has good
transverse balance characteristics should have no problem meeting the LOV limits.
Discussion
At the August 2006 meeting of TR41.9, Alcatel Canada asserted in contribution TR41.9-0608-010-L2 that the out-of-band LOV requirements in TIA-968-A were too onerous.3 They
took exception to the DSL out-of-band limit of -80dBV over all 4 kHz bands extending out
to four times the upper operating frequency of the DSL interface under test. Alcatel
recommended that the LOV limits for DSL in TIA-968-A be deleted entirely or that the outof-band Longitudinal Output Voltage (LOV) limitations for DSL TE be changed from -80
dBV to -50 dBV to more closely align the limit with the CISPR conducted emissions limit
for telecommunications ports.
Alcatel Canada claims that the measured LOV levels are unrepresentative of actual
conditions. In other words, the values measured in the Alcatel Canada lab are not
representative of LOV being generated by the TE, but rather the measured LOV stems from
stray emissions coupling into their test configuration. While this is unfortunate, it is not
justification for removing or changing LOV limits.
One of Alcatel Canada’s rationale for not having a DSL LOV requirement, or having a
reduced requirement, is that CISPR 22 2005 already has a conducted emission limit for a
telecommunications port. However, CISPR standards are not intended to address crosstalk
noise. CISPR standards address radio interference. The limit in the CISPR is presumably
sufficient to prevent radio frequency emissions but the requirement is insufficient to address
crosstalk in telephone cables.
1 Alcatel Canada; TR41.9-06-08-010-L, Modification of the Requirements of TIA-968-B for xDSL
Interfaces, contact Philip Taffinder, August, 2006.
2 Ibid.
3 Specifically, clause 4.5.9.3 of TIA-968-A.
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Alcatel Canada claims that equipment being designed today can tolerate up to -50dBV of
LOV in the operating band but no evidence is offered. We disagree with Alcatel Canada on
this point. Industry crosstalk noise studies do not consider the interfering effect of
longitudinal signals at all. Such studies assume that terminal equipment is perfectly balanced.
Another rationale offered by Alcatel Canada is that the majority of international DSL
standards do not have LOV requirements. For those standards that do have such a
requirement, the limit is a maximum of -50dBV. It is unfortunate that the majority of
international DSL standards do not have LOV limitations however that does not justify
removing the requirements from the harms prevention standards currently used in North
America. One of the reasons that LOV limits are not found in industry DSL standards is
that such standards focus on what a DSL system should do, not what it should not do. In
addition, in the past proposals to add such harms prevention requirements to industry
standards in the U.S. were rejected because manufacturers argued that such requirements fell
under the purview of the FCC and therefore should be left to the FCC and not addressed in
voluntary industry standards.
The final rationale offered by Alcatel Canada is that lab locations with high ambient noise
are not able to perform the LOV test unless a shielded room with good grounding is
employed. It is recognized that stray emissions can cause LV to appear on telephone leads
especially at radio frequencies. This can make LOV measurements tricky at such locations.
A shielded room should mitigate most problems however in some cases subtracting the
background LV from the measured values may be an alternative.
Rationale for Longitudinal Output Voltage Limitations
Part 68 of the Commissions rules provides for uniform standards for the protection of the
telephone network from harms caused by the connection of terminal equipment and
associated wiring.4 The Commission defines harm as “electrical hazards to service provider
personnel, damage to the carrier equipment, malfunction of carrier billing equipment, and
degradation of service to persons other than the user of the subject terminal equipment and
his calling or called party.5 The Commission has long acknowledged that service can be
degraded by crosstalk from other services in the same cable.6
Terminal equipment can cause crosstalk in two ways:
1. By transmitting metallic and longitudinal signals having excessive power.
2. By converting metallic (transverse) signals into longitudinal (common mode) signals.
4 47 C.F.R. 68.1
5 47 C.F.R. 68.3
6 For example, see 10-1-1996 edition of 47 C.F.R. 68.
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Telephone terminal equipment can conduct unintentional longitudinal (common mode)
voltages on interfaces to wireline carrier networks when careful attention is not paid to
circuit design. Longitudinal voltages can be quite problematic because they crosstalk much
more easily into other pairs in the same cable than metallic voltages.7
The U.S. Federal Communications Commission and Industry Canada have long recognized
that longitudinal voltages transmitted into wireline carrier cables by telephone terminal
equipment can cause harm by degrading service to other customers served by the same
cable. As a result, both organizations long ago established mandatory longitudinal output
voltage limits for telephone terminal equipment.8
Industry standards development organizations have also acknowledged and addressed this
problem by adopting longitudinal output voltage restrictions for DSL systems like ISDN,
HDSL, and HDSL2.9 In addition, the Spectrum Management for Loop Transmission
Systems standard (ANSI T1.417-2003) defines parameters for several classes of DSL systems
from IDSL to VDSL that are intended to prevent crosstalk. That standard addresses
transverse balance requirements, the power and frequency distribution of conducted metallic
signals, as well as longitudinal output voltage limits.
Longitudinal Output Voltage Limitations
Longitudinal output voltage is measured over the applicable frequency range using the
procedures and measurement configuration specified in clause 6 of T1.417. The longitudinal
output voltage in all 4 kHz frequency bands averaged over 1 second shall not exceed the
values in Table A below over the indicated range of frequencies.
For this requirement, the operating band is the entire range of frequencies between the
upper and lower –30 dB points of the signal pass-band.
Table A – Maximum longitudinal output voltage limit
Applicable
Range
Maximum Longitudinal
Output Voltage (rms) in all
4 kHz Frequency Bands
averaged over 1 second
Operating band
-50 dBV
From upper –30 dB
-80 dBV
Frequency
7 Bell System Technical Journal Vol. 54, No.7, September 1975; The Effect of Longitudinal Imbalance on
Crosstalk, G. Miller; AT&T 1975.
8 See for example, 1996 edition of CFR Title 47, Part 68, Section 68.308.
9 See ANSI T1.601-1998, Telcordia TA-1210, ANSI T1.418-2000, etc.
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frequency to 4X the upper
-30 dB frequency
Longitudinal Output Voltage Testing Methodology
Compliance with the longitudinal output voltage (LOV) limitations in TIA-968-A for DSL
terminal equipment is required with a longitudinal termination having an impedance equal to
or greater than a 100 ohm resistor in series with a 0.15 uF capacitor. An illustrative test
configuration for LOV conformance testing on DSL equipment is shown in Figure 20 of
T1.601, Figure 19 of T1.417, and Figure 4.5 of TIA-968-A. For direct use of that test
configuration, the near end upstream transmitter needs to be able to generate an upstream
signal in the absence of a downstream signal from the far-end transceiver. The ground
reference for these measurements should be the building or green-wire ground of the device
under test.
Comparison of VDSL2 PSD and TB Limits with VDSL2 LOV Limits
Assuming that the TE is well balanced to ground and does not generate longitudinal signals,
it should have no problem meeting the LOV limits. A comparison of the VDSL2 LOV
limits to the VDSL2 PSD mask limits and applicable Transverse Balance limits can be made
in the following manner:
1- Take the metallic signal PSD limit in dBm/Hz at significant mask frequencies and
subtract the required amount of transverse balance at that frequency. This gives the
maximum longitudinal signal in dBm/Hz that should appear at the interface if the
equipment generates the maximum signal power and meets the minimum transverse balance
requirements while transmitting.
2- Convert the LOV dBV limits for a 4 kHz band into a dBm/Hz limit assuming a 100 ohm
termination.
Table A shows the significant frequencies for the VDSL2 EU-32 PSD mask in Column A.
Column B shows the maximum allowable level at that frequency from the same PSD mask.
Column C provides the Transverse Balance limit from TIA-968-A, Table 4.11(a) for the
particular frequency. Column D shows the level in dBm/Hz of the resultant longitudinal
signal when the signal in Column B is subjected to longitudinal conversion loss in Column
C. For comparison with the longitudinal signal levels in Column D, Column E shows the
proposed VDSL2 LOV limit converted to dBm/Hz. By comparing the values in Columns
D and E, it can clearly be seen, that well designed TE that meets the proposed signal power
limits and transverse balance requirements should have no problem meeting the LOV limits
as long as it does not intentionally or unintentionally generate unfiltered longitudinal signals.
Conclusion
This contribution provides rationale for the Longitudinal Output Voltage limitations for
telephone terminal equipment in TIA-968-A and proposes that existing limitations be
retained in TIA-968-A for all classes of equipment. This parameter, if unconstrained, can
result in crosstalk that can adversely impact other services in the same cable.
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Table A – Comparison of VDSL2 PSD Mask and Transverse Balance
Limits with VDSL2 LOV Limits
A
VDSL2
EU-32
Frequency
(kHz)
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[1]
B
VDSL2
EU-32
Limit PSD
Mask Lev.
(dBm/Hz)
[1]
C
Min
Required
Transverse
Balance
(dB)
[2]
0
4
4
25.875
138
242.92
686
1104
3575
3750
3750
5200
5200
5375
8325
8500
8500
12000
12000
12175
30000
−97.5
−97.5
−92.5
-34.5
-34.5
-93.2
−100
−100
−100
−80
−49.5
−49.5
−80
−100
−100
−80
−50.5
−50.5
−80
−100
−100
40
40
40
35
35
35
35
35
30
30
30
30
30
30
30
30
30
30
30
25
25
D
Level
Of
Resultant
Long. Sig.
(dBm/Hz)
[B
E
Max
LOV
Limit
(dBm/Hz)
-137.5
-137.5
-132.5
-69.5
-69.5
-128.5
-135
-135
-130
-110
-79.5
-79.5
-110
-130
-130
-110
-80.5
-80.5
-110
-125
-125
-73
-73
-73
-76
-76
-106
-106
-106
-106
-106
-76
-76
-106
-106
-106
-106
-76
-76
-106
-106
-106
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