Symmetrical high-speed digital subscriber lines (DSL) are the fastest-growing segments in the DSL family of high-speed Internet access methods. Adopted in February 2001 by the International Telecommunication Union (ITU) (G.991.2), G.SHDSL is a two-wire "leased line" interface that speeds data with the same upload speed as the download. It is not installed over POTS lines and, therefore, lends itself to upgrades of older digital services. The maximum speed of 2.312 Mb/sec is sufficient to handle T1 or E1 traffic. Single-pair high-speed digital subscriber line (SHDSL) technology is attractive to small businesses that want to establish an Internet presence and sustain e-commerce.

In the world of regulatory testing, we engineers like to hang our hats on stable, predictable, repeatable, and understandable things. Although a tremendous amount of work has been completed to standardize the new wave of high-speed Internet access technologies, products and demands are on a swiftly moving path and struggling to gain market share in an otherwise depressed economic sector. The challenge for interoperability and regulatory test labs is to remain on top of the standards and be ready with a test bed and plans.

SHDSL offers some advantages over other DSL family offerings. Having a ratified standard is the first obvious advantage. The speed and loop-reach improvements for a two-wire connection give it an obvious advantage over the older SDSL and HDSL products. With easy installation and integrated T1 and E1 interfaces, it is sure to become popular with many businesses.

Payload Data Rate,R (Kb/sec) KSHDSL Order N ƒsym(ksymbol/sec) ƒ3 dB PSHDSL (dBm) R < 1536 7.86 6 1 (R + 8)/3 1.0 X ƒsym/2 P1(R) ¾ PSHDSL ¾ 13.5 1536 or 1544 8.32 6 1 (R + 8)/3 0.9 X ƒsym/2 13.5 R > 1544 7.86 6 1 (R + 8)/3 1.0 X ƒsym/2 13.5

Table I. Symmetric PSD parameters.

This article describes the requirements of testing a G.SHDSL modem to the current mandatory telecom requirements for the United States, Canada, and Australia. Although the regulatory filing requirements are different, a single design should meet each country's requirements globally.

The telecom requirements for Europe are not deemed "essential" for the CE mark and are regional and carrier specific. Asia-Pacific and South America also follow the generic requirements.

The Standards

The technical core of all the standards is ITU-T Recommendation G.991.2-2001 SHDSL Transceivers. This standard gives the design requirements for interoperability and interfacing. This article does not address the technical requirements of interoperability because the mandatory telecom requirements generally deal with network harm rather than performance.

In the United States, one always begins with IS-968 (Telecommunications—Telephone Terminal Equipment—Technical Requirements for Connection of Terminal Equipment to the Telephone Network), the grandfather of telecom standards. Within ANSI/TIA-968-A-2002 (October 29, 2002) the following test cases apply: 

These test cases ensure that the product does not harm basic telephone services. The next standard is T1.TRQ.6-2001 (SHDSL, HDSL2, HDSL4 Digital Subscriber Line Terminal Equipment to Prevent Harm to the Telephone Network). This is the TIA/EIA standard that gives the requirements for G.SHDSL. (This is only a reiteration of the basic requirements from the ITU-T G.991.2 standard.) For G.SHDSL, go to Section 4, Technical Criteria. The following test cases apply:

In Canada, the following standard applies: CS03 Issue 8 Part VIII (Requirements and Test Methods for Digital Subscriber Line (XDSL) Terminal Equipment). The following sections apply:

For Australia, the following three standards: AS/ACIF S043.1:2001 (Requirements for Customer Equipment for Connection to a Metallic Local Loop Interface of a Telecommunications Network—Part 1: General); AS/ACIF S043.2:2001 (Requirements for Customer Equipment for Connection to a Metallic Local Loop Interface of a Telecommunications Network—Part 2: Broadband); and AS/ACIFS043.3:2001 (Requirements for Customer Equipment for Connection to a Metallic Local Loop Interface of a Telecommunications Network—Part 3: DC, low-frequency AC, and voice band).

For Part 1, the applicable sections are noted below:

For Part 2, the applicable sections are as follows:

For Part 3, the pertinent sections include:

Sequence of Testing

Generally, the order of testing follows this sequence: connecting arrangements, operational check, dielectric strength, hazardous-voltage limitations, network-harm test cases, mechanical shock, surge voltage, Section 2.5 (Part I) power line surge, Section 1.5 operational check, dielectric strength, hazardous-voltage limitations, and network-harm test cases.

Figure 1. PSD masks for 0 dB power backoff.

Lab Difficulties

Operational States. There are three basic states that the modem has to attain for assessment. Chipset manufacturers normally have these states built in. However, when the end integrator has finished the whole product design, these states are sometimes overlooked because they are not required for the end-user. The three states include:

Two-Wire Versus Four-Wire Mode. There is often a reference to an optional four-wire mode. Engineers must be careful with this distinction because it overlaps with other telecom references. Four-wire mode means that there are two different circuits working together to give the appearance of a doubled bandwidth. It is not a separation of a single service into transmit and receive pairs. This is important to distinguish because the submissions are multiline, not single line. Currently, there are no formally recognized jacks or plugs for multiline SHDSL connectors.

Figure 2. Nominal PSDs for 0 dB power backoff.

Balun Requirements (Down to DC). Unlike ADSL, SHDSL transmits through the voice band. To make accurate measurements of the PSD, a balun (or more likely more than one) is needed that covers the whole transmit band.

Calculation of the PSD Masks. The calculation of the PSD mask tends to be a little tricky. The test method must be considered before embarking on the calculations for limits. If the masks will be downloaded to a spectrum analyzer, it is critical to take care to mate the limits to the transmission speed. It is a good idea to consider an external program to control the test interface, collect the data, draw the graph, test for pass/fail, and then grab the graph for a report. Also, the spectrum under test may have to be moved to specific windows so that true dBm/Hz values are measured.

For automated measurements, some monitoring method is needed to ensure that the modem is operating at a specific speed and that it has not dropped the transmission. The standard ITU-T G.991.2 (Single-Pair High-Speed Digital Subscriber Line [SHDSL] Transceivers) provides the steps through the calculation.

ITU-T G.991.2

PSD Masks. For all data rates, the measured transmit PSD of each SHDLS tranceiver unit (STU) shall not exceed the PSD masks specified in this section of the standard (PSDMASK_SHDSL(f)), and the measured total power into 135 W must fall within the range specified in this section (PSHDSL ± 0.5 dB).

Support for the symmetric PSDs specified in section 4.2.1 is mandatory for all supported data rates. Support for the asymmetric PSDs, specified in section 4.2.2, is optional.

Symmetric PSD Masks (4.2.1). For all values of framed data rate available in the STU, the following set of PSD masks (PSDMASK_SHDSL(f)) shall be selectable:

where MaskOffsetdB(f) is defined as

f_int is the frequency where the two functions governing PSDMASK_SHDSL(f) intersect in the range 0 to f_sym. PBO is the power backoff value in dB. K_SHDSL, Order, N, f_sym, f_3dB, and P_SHDSL are defined in Table I. P_SHDSL is the range of power in the transmit PSD with 0 dB power backoff. R is the payload data rate.

P1(R) is defined as follows:

P1 (R) = 0.3486log₂ (R X 1000 + 8000) + 6.06 dBm.

For 0 dB power backoff, the measured transmit power into 135 W shall fall within the range P_SHDSL ±0.5 dB. For power backoff values other than 0 dB, the measured transmit power into 135 W shall fall within the range P_SHDSL ±0.5 dB minus the power backoff value in dB. The measured transmit PSD into 135 W must remain below PSDMASK_SHDSL(f).

Figure 1 shows the PSD masks with 0 dB power backoff for payload data rates of 256, 512, 768, 1536, 2048, and 2304 Kb/sec.

The equation for the nominal PSD measured at the terminals is

where f_c is the transformer cutoff frequency, assumed to be 5 kHz. Figure 2 shows the nominal transmit PSDs with 13.5 dBm power for payload data rates of 256, 512, 768, 1536, 2048, and 2304 Kb/sec. Note: The nominal PSD is intended to be informative in nature; however, it is used for purposes of crosstalk calculations (see sections A.3.3.5 and A.3.3.6 of the standard) as representative of typical implementations.

Asymmetric 1.536 or 1.544 PSD Mask (4.2.2). For 1.536 and 1.544 Mb/sec payload data rates (1.544 and 1.552 Mb/sec framed data rates) in North America, the asymmetric PSD mask set specified is supported optionally. The PSD masks are described for the 0 dB power backoff case. For other values of power backoff, the passband PSD masks shift, but the out-of-band mask remains constant. Power and power spectral density are measured into a load impedance of 135 ‡.

Conclusion

Symmetrical high-speed digital subscriber lines are the fastest-growing segments in the DSL family of high-speed Internet access methods. Although a tremendous amount of work has been completed to standardize these high-speed Internet access technologies, the challenge for interoperability and regulatory test labs is to remain on top of the standards and be ready with a test bed and plans.

Independent test labs allow for the concentration on repeatable test results. With experience in testing, they incorporate the rigors of external quality audits and documented test plans. To propagate the market, manufacturers of service providers' equipment, end terminals, and ancillary equipment will have to work toward a free exchange of standards and test methods. This cooperation should help to eliminate the bottleneck of noncompliance.

David Kay is senior telecom test engineer for TÜV Rheinland (Newtown, CT). He can be reached at dkay@us.tuv.com.

Recommended Web Sites   http://www.nwfusion.com/news/tech/2002/1111techupdate.html http://www.dslforum.org/SHDSL_wp.pdf http://www-comm.itsi.disa.mil:8006/itu/r_g0900.html http://www.adsl.com http://www.part68.org