Test Requirements for North American Telecom Networks

Larry Stillings

Understanding FCC Part 68 and IC CS 03 test requirements is crucial to developing compliant network products.

Telephone test stand for volume-control measurements.

Two standards in North America regulate connection to the telephone network. In the United States, the Federal Communications Commission (FCC) Part 68 regulates this connection, and in Canada, Industry Canada (IC) CS 03, Issue 8, addresses these requirements.^1,2 This article examines the scope of these two regulations and the tests required in order to design compliant products. Mexico will not be discussed because it was not involved in the harmonization project. This article also looks at how these regulations came about and discusses recent changes to them (see the sidebar "Telecom History").

Regulated Digital Interfaces

A number of different types of interfaces are currently regulated by FCC and IC. Six digital telecommunication interfaces are regulated by the agencies. These include several types of T1/DS-1/1.544-Mb/sec: full T1; fractional T1; and fractional T1 with signaling bits. Other interfaces include integrated services digital network (ISDN) primary rate interfaces (PRI); ISDN basic rate interfaces (BRI); and digital data services (DDS) 2.4 64Kb/sec interfaces.

Full T1. Data are transmitted at 1.536 Mb/sec, and an 8-kHz framing bit is used.

Fractional T1. The interface is divided into 24–64-kb/sec channels using channel 24 as the information or call-handling channel. The 8-kHz framing bit is used.

Fractional T1. with Signaling Bits. This interface contains the 24-, 56-, or 64-Kb/sec channels and uses channel 24 for information or call handling. Two bits (A, B) or four bits (A, B, C, D) are used to tell whether a given channel is on-hook, off-hook, or ringing. Signaling bits are used when analog data are encoded into one or more of the 64-Kb/sec channels (this is used for encoded analog content).

ISDN PRI. This interface uses layered software operated over a fractional T1 interface with signaling bits. Adding ISDN PRI to a T1 interface gives it enhanced capability to handle call information for voice, data, and video transmission.

ISDN BRI. This is a 128-Kb/sec interface that contains two 56-Kb/sec channels and a 16-Kb/sec signaling channel. The channels can be used to transmit voice, video, and data. Depending on how the interface is connected to the network, reference points define whether the interface is connected directly (U reference point) to the network or indirectly (S and T reference points) to the network through a network termination 1 (NT-1) device.

DDS 2.4 64-Kb/sec Interface. This interface provides a digital data connection to the network that can transmit at rates of 2.4, 4.8, 9.6, 19.2, 38.4, 56, and 64 Kb/sec. Three types of public switched digital services (PSDS) interfaces are defined in FCC Part 68 rules. These include PSDS Type I, Type II, and Type III. PSDS Type I is a four-wire, 56-Kb/sec data interface using alternate mark inversion (AMI) line coding based on a 135-W impedance. Type II is a two-wire, 56-Kb/sec full duplex interface that uses time compression multiplexing plus dual-tone multifrequency (DTMF) signaling and AMI line coding based on a 120-W impedance. Type III is a two-wire, 64-Kb/sec full duplex interface that uses time compression multiplexing plus an 8-kHz signaling channel and AMI line coding based on a 135-W impedance.

Lightning surge simulation and dielectric strength test rack.

Digital subscriber line (DSL) interfaces are also regulated by FCC and IC. Asymmetric DSL (ADSL), rate-adaptive DSL (RADSL), and splitterless ADSL (G.Lite) interfaces are currently regulated by FCC by means of a streamlined waiver process that became effective February 28, 2000, and July 26, 2000, respectively. ADSL and high-data-rate DSL (HDSL) interfaces are currently regulated by IC Part VIII of the CS 03 rules. Single-line DSL (SDSL) interfaces are regulated by the IC PROV-SDSL standard.³

ADSL. This interface uses transmission rates of up to 8 Mb/sec downstream (toward the customer) and 640 Kb/sec upstream (toward the telephone company) over a single pair of lines. A voice call could be placed while an ADSL modem is in operation. This simultaneous operation is achieved by the use of a low-pass filter located at the customer’s site allowing regular telephone devices to be connected. Software variations include carrierless amplitude phase modulation (CAP), discrete multitone technology (DMT), and quadrature amplitude modulation technology (QAM).

RADSL. This technology is similar to ADSL; however, the interface monitors the telephone line’s condition and changes speed (rate) as appropriate.

G.Lite (SADSL). Based on ADSL technology, this interface is limited to 1.5 Mb/sec downstream. However, no external splitter is located at the customer’s site. Microfilters are used on telephones and other devices connected to the same line.

HDSL. Transmission for this interface is limited to 1.544 Mb/sec or 2.048 Mb/sec with a maximum distance of 12,000 ft. The interface is an extension of T1/E1 using bandwidths of 80–240 kHz. Several line coding techniques of HDSL are available, including CAP and two-binary, one-quaternary (2B1Q).

SDSL. This multirate interface operates from 384 to 2320 Kb/sec with a maximum distance of 10,000 ft. It is typically used for business installations.

Regulated Analog Interfaces

This section describes four analog telecommunication interfaces regulated by FCC and IC. These are the loop start; ground start; E and M two- and four-wire-lead; and direct inward dial (DID) reverse-battery interfaces.

Loop Start. For loop start, a call is begun by picking up the line or taking the line off the hook to initiate a dial tone. Devices include telephones, modems, fax machines, caller identification boxes, and foreign exchange office or service (FXO or FXS) cards used by analog private branch exchanges (PBX). This interface is typically the service provided by the telephone companies to homes.

Ground Start. Ground start indicates that the ring conductor of a tip-ring connection is momentarily grounded to get a dial tone from the telephone company. Such an interface is preferred for a PBX because of its more reliable operation.

E&M Two- and Four-Wire. A two-wire E&M interface contains four wires: tip, ring, E lead, and M lead. The E and M leads provide dc signaling from the central office to the customer premises to control on-hook and off-hook states. A four-wire E&M interface contains eight wires: tip, ring, tip1, ring1, E lead, M lead, signal-battery lead, and signal-ground lead. It also uses dc signaling.

DID/Reverse Battery. Using this interface, an outside caller can call an extension within a company directly without using an operator or voice attendant. Typically, a block of 100 telephone numbers is purchased in the format of 555-12xx, where xx is 00 to 99.

Telephone Handsets: Special Requirements

Music on Hold. For products such as a PBX that allows external audio to be placed on the line, a special test must be performed on the level of the audio signal. A 1-kHz tone is injected into the audio input port, and the device is configured to place the audio input onto the telephone line. The input is increased until the interface starts to clamp, or clip the signal. The amplitude is then reduced 1 dB. At this point, the input is increased 10 dB from the reference level, and the output on the central office port cannot exceed –9 dBm (600 W). This test typically requires that the external audio contain automatic gain control and a low-pass filter for out-of-band signals.

Hearing-Aid Compatibility. On telephone handsets, the magnetic field of the earpiece must be measured for frequency response. Axial and radial measurements must be made and normalized based on the 1000-Hz reading.

Volume Control. Telephones with handsets must have a minimum gain of 12 dB from the lowest to the highest volume setting. Phones that have a gain of more than 18 dB must automatically reset to a nominal setting after each call is completed.

Environmental Test Requirements

Environmental test requirements are specified in FCC Part 68.302b and IC CS 03 Section 2.4. These tests are written to protect users, service personnel, and installation personnel from dangerous voltage levels caused by lighting and power crosses.

Voltage (Lightning) Surge (FCC Part 68.302b Type A and IC CS 03). For this test, an 800-V, 100-A 10 x 560-microsecond surge is applied from tip to ring once in each polarity. The surge is also applied for four-wire interfaces from tip-ring (transmit pair) to tip-ring (receive pair) once in each polarity. In addition, a 1500-V, 200-A 10 x 160-µS surge is applied from tip-ring to ground once in each polarity. It is important to note that the 800- and 1500-V surges are applied with the equipment under test (EUT) in the on-hook and off-hook states.

Voltage (Lightning) Power Line Surge (FCC Part 68.302d and IC CS 03 Section 2.5). A 2500-V, 1000-A 2 x 10-microsecond surge must be applied from mains power phase to neutral three times in each polarity. A mains power supply can sustain damage after this surge provided it does not cause a short on tip-to-ring connections.

Type B Surge (FCC Part 68.302c). This test, required by FCC only, is to ensure that a product’s interface cannot become open or shorted in such a way that it would sustain damage. In addition, the interface must not be damaged in such a way that it affects compliance with other parts of the rules. A 1000-V, 25-A 9 x 720-microsecond (voltage), 5 x 320-microsecond (current) surge is applied from tip to ring once in each polarity. The surge is also applied to four-wire interfaces from tip-ring (transmit) to tip-ring (receive) once in each polarity. A 1500-V, 37.5-A 9 x 720-microsecond (voltage), 5 x 320-microsecond(current) surge is applied from tip-ring to ground once in each polarity. The 800- and 1500-V surges are applied with the EUT in the on-hook and off-hook states (see Figure 1).

Mechanical Shock (Drop) Testing (FCC Part 68.302a and IC CS 03 Section 2.1). This test requires that handheld equipment be dropped 18 times from 1.5 m onto a concrete surface covered with a 3-mm tile or similar surface. Table or desktop equipment weighing between 0 and 5 kg must be dropped six times from 750 mm onto the same type of surface. This test is performed so that one of a product’s major surfaces impacts parallel to the tiled concrete surface every sixth drop.

Dielectric Strength (Hipot) Testing (FCC Part 68.304 and IC CS 03 Section 2.2). This test is performed on tip-ring combinations. A 1000-V ac (50–60 Hz) is applied from tip-ring to ground with a 30-second ramp of the voltage and then 60 seconds of steady-state application. The leakage cannot exceed 10 mA at any point during the 90-second interval. Components that provide an intentional dc path to ground can be excluded from this test; however, these components must meet the hazardous voltage limitation requirements. It is important to note that if surge protectors or radio-frequency (RF) filters to ground are used, a dc voltage equivalent to the ac peak voltage can be used.

Pass-Fail Design Considerations

Telephone isolation transformers should have at least 1000 V ac rms of dielectric isolation from primary windings to secondary windings. For best results, 1500 V ac rms isolation is suggested. Electromagnetic interference (EMI) filter capacitors should have a minimum working voltage of 500 V dc; however, 2–3 kV is recommended to minimize any ac current flow to ground, which could damage a data access arrangement or other circuitry. Optocouplers and relays (coil to contact) should have at least 1000 V ac of dielectric isolation.

On two-sided circuit boards, ground and power traces should be kept a minimum of 25 mil from the telephone traces up to the isolation transformer. Boards with four or more layers should have the power and ground layers voided under the telephone interface components from the telephone connector to the isolation barrier (transformer).

Figure 1. Type B generator and waveform requirements per FCC Part 68.302c(1) and (2): (a) simplified surge generator, (b) open-circuit current waveshape (Tr x Td), (c) short-circuit current waveshape (Tr x Td).

Hazardous Voltage Limitations (FCC Part 68.306b and IC CS 03 Section 2.3). This regulation requires a measurement of the ac voltage present on tip-ring leads. The ac voltage cannot exceed 70 V peak for more than 1 second from tip to ring, tip to ground, and ring to ground.

Intentional Operational Paths to Ground (FCC Part 68.306e and IC CS 03 Section 2.3.9). Components with a dc path to ground (removed for the leakage current tests) must be replaced. For this test, a dc current source is gradually increased from 0 to 1 A and then applied for 1 minute. During the 90-second interval, the voltage must not exceed 0.1 V dc peak. This test should be used for interfaces such as the ground start interface and in cases in which a relay or other device is used to provide a ground to a telephone lead. The ground point of the device can beused for the dc current test.

Intentional Protective Paths to Ground (FCC Part 68.306e and IC CS 03 Section 2.3.9). The ac current from tip-ring leads to ground must be measured with the intentional dc paths to ground installed (previously removed for the ac dielectric strength test). A dielectric strength test must be performed using 120 V ac as the test voltage. The voltage is ramped up from 0 to 120 V ac over a 30-second interval and then maintained for another 60 seconds. The current cannot exceed 10 mA peak past the 90-second interval.

Electrical Requirements for Analog Interfaces

Metallic Transmitted Signal Power (FCC Part 68.308b and IC CS 03 Section 3.4). Network signals such as dual-tone multifrequency (DTMF) signals must not exceed 0 dBm into a 600-W termination when averaged over any 3-second interval. Non-network signals such as modem carriers must not exceed –9 dBm into a 600-W termination when averaged over a 3-second interval. For two- and four-wire lossless trunk interfaces, non-network signals such as modem carriers must not exceed –15 dBm into a 600-W termination when averaged over a 3-second interval.

Metallic (Tip-to-Ring) Out-of-Band Ac Signals (FCC Part 68.308c and IC CS 03 Section 3.4.6). Signal power in the band from 3995 to 4005 Hz must not exceed –27 dBV over any 3-second interval into a 600-W impedance termination. Signal power in the band from 4 to 12 kHz must not exceed –14 to –20 dBV falling logarithmically over any 0.1-second interval into a 300-W impedance termination. In the band from 12 to 90 kHz, signal power must not exceed –20 to –55 dBV falling logarithmically over any 0.1-second interval into a 135-W impedance termination. From 90 to 270 kHz, the signal power must not exceed –55 dBV over any 0.1-second interval into a 135-W impedance termination. The signal power in the band from 0.270 to 6 MHz must not exceed –15 dBV averaged over a 0.2-microsecond interval into a 135-W impedance termination (see Figure 2).

Longitudinal (Tip/Ring-to-Ground) Out-of-Band AC Signals (FCC Part 68.308d and IC CS 03 Section 3.3). In the off-hook transmitting state, signals in the 100–4000-Hz band must not exceed –30 dBV into a 500-W impedance termination when averaged over a 100-millisecond interval. In the band of 8–12 kHz in the off-hook transmitted state, signals must not exceed –30 to –40 dBV into a 500-W impedance termination. For 12–42 kHz, they must not exceed –40 to –62 dBV into a 90-W impedance termination; for 42–266 kHz, –62 dBV into 90 W ; and for 0.266–6 MHz, signals must not exceed –30 dBV into a 90-W impedance termination (see Figure 2).

Figure 2. Out-of-band ac signals.

Transverse Balance Requirements (FCC Part 68.310 and CS IC 03 Section 3.6). Transverse balance is the measure of the tip and ring leads of a telephone interface with respect to ground, (earth or chassis). A signal is applied from tip to ring, and the resulting reflection from the interface to ground is measured. The greater the difference between tip to ground and ring to ground, the lower the balance. Both voice and data interfaces must have a balance of 60 dB from 200 to 1000 Hz and a balance of 40 dB from 1000 to 4000 Hz in the on-hook state. Data interfaces must have a balance of 40 dB in the off-hook state from 200 to 4000 Hz. The purpose of this test is to minimize crosstalk.

On-Hook Dc Impedance Requirements (FCC Part 68.312b and IC CS 03 Section 3.7). The impedance from tip to ring, tip to ground, and ring to ground must be greater than 5 MW from 0 to 100 V dc in the on-hook state. The impedance must be greater than 30 kW at 150 and 200 V dc in both polarities. The dc current of the interface during ringing must not exceed 3.0 mA in the presence of 56.5 V dc. For a B-type ringer, 15.3–68 Hz is applied from 40 to 150 V ac. This ac signal is then superimposed on the 56.5 V dc.

Digital telecommunications test stand.

On-Hook Ac Impedance Requirements (FCC Part 68.312b and IC CS 03 Section 3.7). The ac current of the interface during ringing must provide an impedance of less than 1600 W. The telephone company will provide up to 8000 W of ac impedance per line (8000 W is equal to a ringer equivalence number [REN] of 5). In addition, there should be 100 kW of ac impedance from tip to ground and ring to ground.

Billing Protection Requirements (FCC Part 68.314 and IC CS 03 Sections 3.3.1 and 3.5). In the on-hook state, signals in the 200–3995-Hz band must not exceed –55 dBm into a 600-W termination when averaged over any 3-second interval. The current must not decrease by more than 25% from the maximum in the first 5 seconds of the transition from the on-hook to the off-hook state. The power transmitted in the frequency of 2450–2750 Hz must be less than or equal to the energy in the 800–2450-Hz band for the first 2 seconds after going from the on-hook to the off-hook state. This test is performed to ensure that older billing equipment using these frequency bands is not affected.

Electrical Requirements for Digital Interfaces

A critical requirement for digital interfaces is the transverse balance, formerly known as longitudinal balance. This test must be performed for the following interfaces: T1, DS-1, ISDN PRI, ISDN BRI, DDS, and xDSL. It measures the balance of the tip and ring leads with respect to earth from 200 Hz (10 kHz) to 192 kHz (1.544 MHz). The balance must be at least 40 dB from 200 Hz to 12 kHz and 35 dB from 12 to 192 kHz. For T1, DS-1, ISDN PRI, and ADSL interfaces, the termination impedance is 100 W. For ISDN BRI, DDS , HDSL, and SDSL interfaces, the termination impedance is 135 W.

The Telecommunications Industry Association (TIA) is proposing to expand the range of balance for ADSL interfaces from a maximum of 1.544 MHz up to 10 MHz. Other tests required for digital interfaces are as follows:

Analog telecommunications test stand.

Pulse Repetition Rate (DS-1, T1, and ISDN PRI Interfaces). This test measures the data rate of the telephone interface. The interface's data rate must be ±50 bits/sec (±32 ppm).

Pulse Shape (DS-1, T1, ISDN PRI, and Subrate DDS Interfaces). An isolated pulse must meet the mask requirements as defined in Part II of IC CS 03 or in FCC Part 68 (TIA/EIA TSB-31B standard).⁴ FCC also requires measurements at –7.5 and –15.0 dB (line build-out). Subrate DDS interfaces must also be measured against pulse masks as defined in TIA/EIA TSB-31B and CS 03 Part VII.

Output Power (DS-1, T1, and ISDN PRI Interfaces). The power of the digital interface in an all-ones state must not exceed 19 dBm at 772 kHz. The level at 1.544 MHz must be reduced by at least 25 dB from the measurement at 772 kHz.

Answer Supervision (DS-1, T1, and ISDN PRI Interfaces). This test measures the A and B bits in an interface when the product goes into the off-hook state. The bitsmust remain representing the off-hook state for at least 5 seconds unless the interface is returned to the on-hook state.

Transmission Paths (DS-1, T1, ISDN PRI, ISDN BRI, and DDS Interfaces). Connection of one type of network interface to another type of network interface typically should not add gain to the signals in the interface. A chart showing the allowable gain can be found in IC CS 03 Part II. The following tests apply to interfaces that internally generate analog signals that are intended for eventual conversion to an analog telephone network:

Metallic Ac Energy (DS-1, T1, ISDN PRI, ISDN BRI, and DDS Interfaces). A decoded digital time slot (b-channel) measured against a 600-W balanced reference must not exceed –55 dBm in the band of 200–4000 Hz in the on-hook state.

Encoded Analog Content (DS-1, T1, ISDN PRI, ISDN BRI, and DDS Interfaces). Decoded analog signals in the voice band (200–4000 Hz) must not exceed –12 dBm for non-network tones and –3 dBm for network signals.

Signaling Interference (DS-1, T1, ISDN PRI, ISDN BRI, and DDS Interfaces). This test entails measurement of the signal power in the single-frequency (SF) signaling bands. The equivalent decoded analog content energy in the signaling band (2450–2750 Hz) must be less than or equal to the energy in the guard band (800–2450 Hz). The following tests pertain only to digital connections. Analog tests for telecom interfaces (e.g., POTS splitter) are not addressed.

Transmitted Spectral Response (ADSL Interfaces). Measurement of the signal’s spectral density must be limited so that it does not cause interference to the analog part of the network. The peak output (upstream signal) of an ADSL transmission unit–remote (ATU-R) must not exceed –34.5 dBm/Hz from 25.875 to 138 kHz. The peak output (downstream signal) of an ADSL transmission unit–central (ATU-C) must not exceed –36.5 dBm/Hz from 25.875 to 1104 kHz. It is interesting to note that in the provisional standard a lower average limit with a peak ripple was specified.

Transmitted Spectral Response (HDSL 2B1Q–392K-Baud Interfaces). For these interfaces, the signal’s spectral density is measured to ensure that there is no interference to the analog part of the network. The average power spectral density of the transmitted signal must not exceed –37 dBm/Hz from 0 to 196 kHz. The signal must not fall 80 dB/decade from 196 to 1960 kHz or be less than –117 dBm/Hz above 1.96 MHz.

Total Signal Power Requirements

For an ADSL ATU-R interface, the average signal power cannot exceed 12.5 dBm across 100 W. For an ADSL ATU-C interface, the average signal power cannot exceed 20.4 dBm across 100 W. The average signal power for an HDSL (2B1Q) interface two-pair system cannot exceed 14 dBm in high-power mode or 8 dBm in low-power mode across 135 W. The average signal power cannot exceed 16 dBm in high-power mode or 10 dBm in low-power mode across 135 W for an HDSL (2B1Q) interface one-pair system.

Conclusion

It is crucial to understand the definitions of the different telephone interfaces covered by FCC Part 68 and IC CS 03. Depending on the type of interface used, both FCC and IC require a variety of environmental and electrical tests for analog and digital telephone interfaces. Many changes are coming in the next year with the implementation of telecommunications certification bodies. The Alliance for Telecommunications Solutions and TIA are addressing this issue. It will be interesting to see whether the ATIS/TIA Administrative Council for Terminal Attachment adopts all of the current Part 68 rules and how quickly it develops a set of requirements for the wide variety of xDSL interfaces available.

References

1. Code of Federal Regulations, 47 CFR 68.

2. Industry Canada, IC CS 03, Issue 8.

3. Industry Canada, IC PROV-SDSL.

4. "Measurement Guidelines and Rational for Telecommunications Terminal Equipment," TIA/EIA TSB-31B, Telecommunications Industry Association/Electronic Industry Association, Arlington, VA,1998.

Larry Stillings is president and founder of Compliance Worldwide Inc. (Sandown, NH), which specializes in telecommunications testing. He can be contacted at lkstillings@complianceworldwide.com or at 603/887-3903.

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