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Telecommunications Industry Association
TR41.9-07-02-016
Document Cover Sheet
Project Number
Document Title
Changes to Onhook DC Resistance Requirement and Test Procedure
Source
Joe Randolph, Randolph Telecom, Inc.
Contact
Name: Joe Randolph
Complete Address: 325 Highland
Avenue, Winchester, MA 01890
Distribution
TR-41.9
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of Document
(Select one)
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Phone: 781-721-2848
Fax: 781-721-0582
Email: [email protected]
For Incorporation Into TIA Publication TIA-968-A
For Information
Other (describe) -
The document to which this cover statement is attached is submitted to a Formulating Group or
sub-element 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.
Abstract
Modifications are proposed to TIA-968 for reducing the required minimum onhook metallic DC
resistance from 5 megohms to 1 megohm, and for increasing the minimum test voltage from 1 volt to 25
volts. The change to the minimum test voltage affects TSB-31 as well.
Note that the proposed change only relates to the metallic DC resistance between tip and ring, and does
not include any change to the minimum DC resistance between tip and ground or between ring and
ground.
It should be noted that the present requirements in TIA-968 for onhook metallic DC resistance are the
most restrictive requirements in the world. The vast majority of countries worldwide have requirements
comparable to the proposed revisions, and their national networks do not appear to be adversely affected.
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Telecommunications Industry Association
TR41.9-07-02-016
MINIMUM ONHOOK METALLIC DC RESISTANCE
PROPOSED CHANGES
1)
In the present version of TIA-968, the on-hook DC resistance between tip and ring is required to be
at least 5 megohms for loop start interfaces. It is proposed that this value be changed to 1 megohm.
2)
The requirement in TIA-968 states that the minimum DC resistance of 5 megohms applies for
"voltages up to 100 VDC." In TSB-31, the test procedure calls for measuring the effective resistance
at voltages down to 1 VDC. It is proposed that the minimum test voltage be changed to 25 VDC.
BACKGROUND
The present 5 megohm onhook DC resistance requirement is believed to date back to the original draft of
FCC Part 68. The requirement applies for voltages “up to 100 VDC.” When formal test procedures were
developed for inclusion in TSB-31, test voltages ranging from 1 volt to 100 volts were called out.
At the time, most switchhook functions were implemented with mechanical contacts or mechanical relays
that physically broke the connection between tip and ring in the onhook state. When the switchhook was
a mechanical contact, the on-hook DC resistance was essentially infinite, so the minimum allowable
resistance could be almost any specified value and the equipment would still pass. Similarly, the
requirement could be tested at any reasonable voltage and it would still pass.
As technology has evolved toward the use of solid state circuits for line interface circuitry, the existing
requirements have become a burden for designers and have limited the opportunities for technical
innovation. While certain forms of solid state interface circuits can be designed to meet the existing
requirements, a much broader range of possible implementations and features could be considered if the
requirements were modified as proposed.
Most solid state circuits require a tiny amount of current in order to keep certain portions properly biased
and ready to respond to various state changes. The proposed modifications to the existing requirements
would allow greater technical innovation for the design of various types of solid state interface circuitry in
terminal equipment, including telephones, modems, caller ID devices, and protection devices.
It should be noted that the present TIA-968 requirements for onhook metallic DC resistance are the most
restrictive in the world. The majority of countries worldwide specify a minimum on-hook DC resistance
of 1 megohm. Examples include Europe (under the now voluntary TBR-21 specification), Australia,
Japan, and China. Other countries, such as India and New Zealand, specify even lower DC resistances.
There are a few countries, such as South Korea and Taiwan, whose requirements are partially copied from
FCC Part 68 and therefore duplicate the 5 megohm requirement, but the 5 megohm requirement appears
in these specifications simply because FCC Part 68 was used as a source in drafting these standards.
Even so, Taiwan specifies a test voltage of 100 VDC, while South Korea’s standard duplicates the “up to
100 VDC” phrase in the specification but is silent on the recommended test procedure.
For testing onhook metallic DC resistance, most other countries use a test voltage of 25 VDC or higher,
while TSB-31 requires tests at voltages down to 1 VDC. It seems unusual to require tests at voltages
down to 1 VDC, since the typical on-hook voltage is about 48 VDC. There does not appear to be any
network function whose proper performance is sensitive to the leakage current when only 1 VDC is
applied across tip/ring.
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Telecommunications Industry Association
TR41.9-07-02-016
RATIONALE FOR PROPOSED CHANGES
These changes are being proposed to adapt the on-hook DC resistance requirements in TIA-968 and TSB31 to better reflect the needs of modern electronic line interface circuitry. It is believed that these changes
would provide greater opportunities for technical innovation in the design of terminal equipment.
It seems likely that adopting the proposed changes would not adversely affect the USA network, since the
public networks in many other countries worldwide use the proposed values and suffer no apparent
adverse effects.
In the past, the unusually restrictive requirements for onhook metallic DC resistance in TIA-968 did not
present much hardship for designers of terminal equipment, since the switchhook function was often a
mechanical contact and there was little or no solid state circuitry in the line interface.
With the advent modern line interfaces that use transistors for the switchhook function and solid state
circuitry for other line interface functions, the situation has changed. Silicon devices typically exhibit a
small but finite amount of leakage current, and sometimes this leakage current is relatively independent of
the applied voltage. For example, a transistor switchhook that has a constant leakage current of 1
microamp would have an effective DC resistance of 50 megohms at a test voltage of 50 volts, but only 1
megohm at a test voltage of 1 volt.
In line powered terminal equipment, certain newer features such as wake-on-ring, caller ID, and line
status sensing are more easily implemented with solid state circuits that remain in a state of readiness for
possible state changes. The restrictive DC resistance requirement in TIA-968 limits the opportunities for
design innovation in such circuitry.
CONCLUSION
Modifying the on-hook DC resistance requirements as proposed would make them more compatible with
the technical realities of modern electronic circuitry, and would make it easier for manufacturers to use
more innovative designs for their line interface circuitry. The modifications would also make the USA
requirements more consistent with those of most other countries worldwide.
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