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Telecommunications Industry Association TR41.9-06-11-008 Document Cover Sheet Project Number Document Title Rationale for Longitudinal Output Voltage Limits in TIA-968-A Source Verizon Contact Trone Bishop Verizon 13100 Columbia Pike 6E Silver Spring, MD 20904 Distribution TR-41.9 Intended Purpose of Document (Select one) X Phone: 301-236-3754 Fax: 301-989-6505 Email: [email protected] For Incorporation Into TIA Publication For Information Other (describe) Notices 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. v1.0 – 20050426 1 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. 2 Telecommunications Industry Association 008 TR41.9-06-11- 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. v1.0 – 20050426 3 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. 4 Telecommunications Industry Association 008 TR41.9-06-11- 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. v1.0 – 20050426 5 6 Telecommunications Industry Association 008 TR41.9-06-11- Table A – Comparison of VDSL2 PSD Mask and Transverse Balance Limits with VDSL2 LOV Limits A VDSL2 EU-32 Frequency (kHz) v1.0 – 20050426 [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 7