Designing DSL Equipment That Meets Industry Standards and Regulations

Meeting the standards and regulations for devices that connect to phone networks is essential for digital subscriber line (DSL) designers, but in a rapidly changing industry, compliance can be a moving target.

The global telecommunications market has undergone significant evolution in the past few years, spurred by the overwhelming demand for high-speed Internet service for both businesses and residences (see Industry Outlook). To meet this demand, DSL technologies such as asymmetric DSL (ADSL) are evolving and multiplying, and industry and government organizations are working hard to keep the standards and regulations for these technologies up to date. In this changing environment, designers of DSL equipment can find it difficult to ensure that products meet appropriate standards.

Like all telecommunications products that connect to telephone networks, DSL equipment must meet regulatory requirements that depend on local telephone service and, therefore, vary among different countries.

For example, almost all countries except the United States provide some mandatory criteria for the return loss characteristics of the equipment connected to the typical loop-start telephone service. Return loss is a measure of how well the equipment transmission characteristics match the network line characteristics; the better the match, the higher the return loss. Each country usually has its own impedance termination characteristics that represent the network line impedance in that country. It is very difficult to create a single design to satisfy the return loss requirements in every country, so products usually incorporate impedance-matching circuits that can be set for different countries.

If return loss to analog equipment is a concern, for DSL equipment, the line impedance and loop lengths are of fundamental importance to transceiver performance. The design of DSL equipment must ensure that digital bit errors are minimized by reducing the interference caused by cross talk coupling from other systems, background noise, impulse noise, and POTS (plain old telephone service) signaling. These potential sources of impairment can be simulated in a laboratory environment using specified test loops and interference injection equipment.

In the United States, the Federal Communications Commission (FCC) regulates the attachment of terminal equipment to the telephone network. The regulations are found in Part 68 of Code of Federal Regulations 47. Part 68, which provides protection to the phone network, includes only a minimal set of technical requirements and does not yet adequately address DSL technology.

Because of restrictive signal power requirements above the voice band, DSL equipment will not be fully compliant with Part 68, and there have been restrictions on the sale of DSL customer premises equipment to over-the-counter retailers, who usually require that the equipment for sale in their stores be FCC certified.

However, a few manufacturers have received FCC waivers by demonstrating compliance with other industry standards. Part 68 does not currently address ADSL, splitterless ADSL, or single-carrier DSL (RADSL) devices that are designed in accordance with ANSI T1.413, G.992.2, and ATIS Technical Report 59, respectively. Several manufacturers have filed petitions for waivers of 68.308(e)(1) to certify such devices.

A waiver request must follow a prescribed format and must be supported by evidence that the equipment as designed will not cause harm to the network. The evidence can be compliance data obtained by testing the equipment to appropriate industry standards. An application form with supporting data showing compliance to the Part 68 requirements, with the exception of the rules that are requested to be waived, must be submitted. The FCC has so far granted three waiver petitions for DSL-type equipment (see Registration Procedure for ADSL Equipment).

With the granting of the Alcatel Petition for Waiver¹, the FCC has adopted a streamlined process whereby waivers, when submitted on the legal precedent of previous DSL waiver orders (Nortel, Paradyne, or Alcatel Order), will be granted without a notice-and-comment rulemaking proceeding. This streamlined process requires petitioners to certify conformance to two performance conditions:

1. Equipment must meet the transmitter spectral response in T1.413-1998.
2. Equipment must operate with an aggregate power < 12.5 dBm over the range of 25.875 to 138 kHz.

Note that only DSL equipment on the customer-premises end, or rather the ATU-R, may be considered for Part 68 certification under this process. This is markedly different than the Canadian certification standard CS-03, which allows for certification of both the ATU-C (CO equipment) and the ATU-R (CPE equipment).

Also, FCC regulations are evolving. The Telecommunications Industry Association (TIA) TR41.9 Committee is working in conjunction with the Alliance for Telecommunications Industry Solutions (ATIS) T1E1.4 Committee to formulate requirements for new technologies intended for inclusion in Part 68 in the near future.  

Part 68

To design successful DSL equipment that meets applicable requirements, engineers should be familiar with several important aspects of Part 68.

Registration envelope. The registration envelope is the part of a system that is subject to Part 68 rules. A design review should be conducted to locate this portion of the system.

For example, interface circuitry must meet the technical requirements for ac impedance, dc resistance, and isolation requirements. By ensuring that the circuitry meets these requirements, a designer can limit the portion of the system that must be evaluated.

Signal power requirements can involve several parts of the system, such as the components that determine the in-band and out-of-band signal power that will be transmitted over the network, and the parts that supply power to the equipment. The power supply must be well filtered and have proper grounding so that ground loops do not affect the out-of-band characteristics of the interface.

Industry Outlook The overall United States telecommunications market (equipment and services) grew by more than 11.4% in 1999, generating revenues of $517.6 billion. Spending on telecom equipment grew by 11.5% over 1998 to reach $135.4 billion, with some segments growing by 66.2%. Growth for transport services posted an 8.5% increase to $252 billion, and support services posted a 17.3% increase to $138 billion. Enterprise spending on services in support of voice and data communications equipment rose by 18.1% in 1999 to $116.4 billion. Spending by international markets grew by 16%. Source: Telecommunication Industry Association (TIA)

 

Environmental simulation. This simulation should be conducted to ensure that terminal equipment will comply with all technical requirements after being subjected to specified environmental conditions that simulate handling (drop shock) and lightning (surge stresses).

The FCC has recently adopted two types of surge requirements. The first, type A, consists of a metallic surge of 800 V at 100 A with a wave shape of 10 x 560 µsec and a longitudinal surge of 1500 V at 200 A with a wave shape of 10 x 160 µsec. This surge is an extremely high-energy surge that simulates the top 10% of surges encountered in the field. A product is permitted to enter into a permanent on-hook state following this surge.

The second, type B, consists of a metallic surge of 1000 V at 25 A with a wave shape of 10 x 700 µsec and a longitudinal surge of 1500 V at 37.5 A with a wave shape of 10 x 700 µsec. This surge is a low-energy surge that simulates the most common surges encountered in the field. Equipment is required to continue operation in both the on-hook and off-hook states following the type B surge.

The survival of equipment after any type of lightning surge should be considered during product design. There are surge-protection components (discrete or integrated in one package) that are designed to provide any number of survival options after type A or B. Surge protection components should be chosen with care to ensure that the components used in the interface circuit are protected from transients from the surges.

Leakage current. The leakage current requirements of Part 68 ensure that terminal equipment has adequate insulation (dielectric barrier) between the telephone network and main supply voltage sources. The purpose for this requirement is to limit leakage currents to levels that are safe for network personnel and equipment.

Hazardous voltage. The intention of this section of Part 68 is to limit voltages and currents supplied by the equipment (or arising from ports connected to equipment that is not part of the registration envelope) to levels that are safe for network personnel and equipment.

Signal power limitations. This section requires verification that analog or digital signals generated by the terminal equipment do not exceed levels that could cause distortion, overload, or crosstalk on network equipment. The voice band signals are limited to a level appropriate for each class of service (the criteria for loop start is different than that for tie trunks because of different network facilities), and the harmonics of these signals are also limited to avoid interference with other services in adjacent cable pairs. Part 68 regulates frequencies up to 6 MHz. DSL equipment does not comply with this section because of the typical signal power generated in the out-of-band region.

Transverse balance. The requirements of this section address measuring the equality of impedances from network connections to ground to ensure that permitted differential signals (metallic) are not converted to common-mode signals (longitudinal) that cause crosstalk in adjacent wire pairs within the same cable sheath. Equipment must be designed to provide adequate transverse balance characteristics over the entire bandwidth of operation. For example, voice band equipment, such as analog data modems, must meet the balance requirements from 200 to 4000 Hz; however, a T1 digital multiplexer must meet the balance requirements up to 1.544 MHz. For ADSL equipment, it may be necessary to test to 11 MHz.

On-hook impedance. This section prescribes minimum values to the dc and ac impedance of the terminal equipment so that it does not interfere with the phone company's line maintenance signals or with the normal sequence of detection of the on-hook state of the terminal equipment. Measurements obtained from the tests of this section determine the value of the ringer equivalence number (REN), which is a measure of the ac load that the equipment places on the telephone line.

DSL equipment is often connected to the same POTS line as a telephone set. Paying attention to the requirements of this section is important to avoid interference with the regular phone service to the set (ringing, dialing, etc.). DSL equipment must be designed to meet transceiver performance criteria in the presence of all POTS loop currents and differential loop voltages, as well as ringing voltages.

Registration Procedure for ADSL Equipment The ADSL equipment is tested and evaluated for compliance with all the applicable requirements of FCC Part 68. The equipment is tested and evaluated for compliance with industry standards. The testing and evaluation to the industry standards is used as part of the waiver process to show compliance with currently accepted industry standards. One of the current industry standards for ADSL equipment is the ANSI T1.413-1998 standard. The FCC Part 68 application is prepared and sent to the FCC for review by the FCC Part 68 staff. The FCC Part 68 application contains the common FCC Part 68 information, such as sample labels, quality assurance statement, and manual information. The waiver is sent separately to the chief of the FCC Common Carrier Bureau, Network Services Div. After successful review by the FCC Part 68 staff, the registration is granted.

 

Billing protection. The requirements of this section ensure that the phone company gets paid for the connection or the transmission of data over its network.

Hearing aid compatibility and volume control. This section requires measurement of the magnetic coupling from earpiece to nominal hearing aid location. The volume control measures the acoustic gain that the telephone provides for the hearing impaired. Neither of these requirements is based on network harm criteria; instead, the requirements were mandated in Part 68 by federal statutes.

Digital terminal equipment. Part 68 provides a minimum set of requirements for digital terminal equipment, such as digital subrate modems, basic rate ISDN terminal adapters, primary rate ISDN, and T1 equipment. Digital subrate modems have transmission speeds ranging from 2.4 to 64 kbps, with secondary channels from 3.2 to 72 kbps. In 1996, the FCC added public switched digital services (PSDS type I, II, and III) to the scope of Part 68; however, it is believed that PSDS services are becoming extremely uncommon.

Requirements under Part 68 for digital equipment include limitations on the transmitted pulse signal power; its shape, which must fit below a certain template; and its encoded analog signal content. Encoded analog content is a primary concern for the U.S. telephone network because of its predominantly analog central office equipment. Because a telephone call in the United States may not stay digital end-to-end, the analog content of the digital signal runs the risk of being decoded somewhere within the network. Therefore, a digital call must be limited to an appropriate level just like an analog call. Encoded analog content refers to analog signals that are either encoded into the digital stream from analog sources, such as messages recorded through a handset to be sent over a T1 interface, or generated directly in digital form with the purpose of being converted into analog format, such as beep tone over an ISDN interface intended to signal the beginning of a message to the far end caller.

Compliance Beyond Part 68

Because the current FCC requirements have not yet adequately addressed DSL technology, designers of this equipment must look to voluntary industry standards for aid in product development. These voluntary standards, such as T1.413, G992.2, ATIS Technical Report 59, and the T1E1 Draft Loop Spectrum Management Standard, address a number of issues far beyond the minimum regulatory requirements that must be met.

Designing DSL equipment for compliance requires thorough research to avoid redesign and remanufacturing costs, particularly when designing for an international market. For example, early research will enable a designer to develop a Venn diagram of country requirements, which will point out overlapping requirements and reduce the number of designs necessary. Early research is also necessary in this environment of rapid change to ensure compliance with evolving standards and regulations.

References

1. FCC, DA 00-388, Memorandum of Opinion and Order, FCC, February 28, 2000.

  William S. Hurst is vice president of Communication Certification Laboratories (Salt Lake City, UT), president of Telecommunication Information Services Inc., and a member of Compliance Engineering's editorial advisory board. He can be e-mailed at wsh@cclab.com.  

Anh T. Wride is director of Engineering Communication Certification Laboratory (Salt Lake City, UT), vice president of Telecommunication Information Services Inc., and chair of TIA TR 41.9 Committee on Technical Regulatory Considerations. She can be e-mailed at atw@cclab.com.