In other words, it's up to the DSL service provider to ensure that its service functions properly within its operating environment, part of which includes the copper plant used by the DSL product. Whether susceptibility to RFI is occurring at the modem or the plant, it is the service provider's responsibility to deal with that susceptibility, which presents a problem. Improving the plant is not an option for DSL products, because the value of the product is dependent on the reuse of the existing plant.

Another option for reducing RFI ingress is to avoid sharing spectrum with broadcast bands. At the International Switching Symposium 2000 (Birmingham, UK), Kate Harris, senior engineer, access systems design, for Nortel Networks (Brampton, ON, Canada), spoke on the impact of RFI ingress on asymmetric DSL (ADSL) performance. According to her study, measured ADSL data rates suffer as much as a 2000-b/sec drop when exposed to RFI, which can occur in the bands where DSL networks and AM broadcast radio share spectrum. In North America, this sharing occurs at the medium-wave AM broadcast band. The downstream bands of ADSL and ADSL.lite intersect with AM radio broadcasts in the 535–1104-kHz and 535–552-kHz ranges, respectively. There is no overlap between the upstream bands of ADSL and ADSL.lite and the AM bands. In fact, the narrow overlap of ADSL.lite and AM radio enables minimal capacity loss in ADSL.lite services when combined with low-pass filtering and modem RFI immunity.

In Europe, however, the upstream band of symmetric high-bit-rate DSL (SHDSL) does share spectrum with AM radio, at the long-wave AM band. And the downstream bands of ADSL and ADSL.lite overlap the long-wave AM band in addition to the medium-wave AM band. Therefore, DSL networks in Europe must contend with a broader RFI threat.

According to Eckert, regulatory agencies are more concerned about egress than ingress. "From a regulatory perspective, the burden always rests on the unintentional radiator when there's interference into a licensed or even an unlicensed legally operating intentional radiator," says Eckert.

But standards development is lacking even in this area. "On the egress side, the existing standards don't address the issue other than to specify a maximum power spectral density," says Harris. "However, that's not enough to determine whether you would have egress issues."

Like ingress, egress is the responsibility of the service provider, because regulatory bodies are concerned with protecting licensed services from unintentional radiators, which in this case are the DSL services. However, because DSL services are white and not concentrated in a carrier, interference produced by these services sounds like noise, making it difficult for AM radio users to identify the problem as interference. Complaints received by a regulatory body would not necessarily pinpoint the origin of the interference or the use of a DSL product as the culprit. So regulatory bodies have been slow to act, which has in turn slowed standards work.

"The thing that is most difficult right now is that it's not clear where the regulations will end up; it's not clear what steps need to be taken," says Harris. "There's a difference between what can happen in the modem standards and what would happen in the regulatory area, and it's hard to know exactly what would have to be done in the modem standards until it's clear where the regulations will finally sit."

This does not, however, mean that standards development in these areas is not proceeding. For instance, standards work on very-high-bit-rate DSL (VDSL) recognizes ingress as a noise contributor and considers interference from amateur radios. And the International Telecommunications Union (ITU; Geneva) has adopted a test model for ingressors in North America. This model will enable testing for impairments from ingress and will be incorporated into a new version of G.test, a collection of methods for testing various DSL modems (mostly lite and full rate) that should be standardized in February 2001. Similarly, a European ingress model is currently under revision.

Current DSL network standards—specifically those standards that specify the technical requirements for implementation of the physical layer and the layers immediately above—in the United States include T1.413 on ADSL, T1.418 on ADSL.lite, and T1.419 on high-bit-rate DSL (HDSL2). ITU standards include G.992.1 on full-rate ADSL, G.992.2 on G.lite, and G.996 on G.test. Standards still in development include high-speed DSL (HSDSL) and consumer DSL (CDSL). For further information on T1 or ITU, visit the organizations' Web sites at http://www.t1.org and http://www.itu.int, respectively.

IEEE and ETSI Team Up on Wireless Networking

Enhanced TETRA Standard Approved

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