Analog buoys ADSL modem

The Siemens SpeedStream asymmetric digital subscriber line (ADSL) Model 5100 modem represents a typical client-side terminus to the broadband world. But despite the "digital" nature of the transmission standard, analog technology plays a critical role in properly interfacing to the less-than-optimal twisted-pair phone line environment in which ADSL operates, where pulse shaping and signal filtering are key.

(A detailed explanation of ADSL intricacies are beyond the scope of this brief hardware teardown, but an excellent tutorial on the subject can be found at www.commsdesign.com. Search for article ID: 16502655.)

Even as digital cable and, potentially, wireless broadband compete to bring subscribers blazing bit streams, the DSL solution remains a strong contender globally, relying as it does on preexisting wiring and a certain level of proximity to a telco central office. As the name suggests, ADSL is an asymmetric data stream, with up to several megabits/second inbound and a fraction of that data rate outbound. Based on what is seen as a traditional usage model in home broadband, ADSL technology focuses on content delivery to the user, vs. content upload or two-way file sharing.

Design of the SpeedStream modem centers on a Texas Instruments Inc. chip set consisting of the TNETD5310 ADSL communication processor, TI TNETD5100 ADSL transceiver and TNETD5014A ADSL analog front end. The latter part contains codecs, filters, line drivers and line receivers to deal with the challenges of modulating high-speed data over an analog signal and (more critically for ADSL) recovering a bit stream from the modulated phone line signals. Similarly, the Ethernet interface represents a signal-filtering and -conditioning challenge, in this case supported by an LSI Logic Corp. L80227, which itself has an on-chip wave shaper, output driver, twisted-pair receiver and signal equalizer.

The estimated cost of goods sold for the 5100, associated Ethernet card and accessories kitted in a $90 package was around $65, suggesting that SBC/Yahoo (the service providers co-branding the 5100 kit) may well be underwriting the cost of retail DSL hardware placements as a means of driving subscriber revenue.

While much signal processing and number crunching happens in the core TI logic devices, without a healthy dose of assistance from the analog world to get bits in and out of the box over tricky signaling environments, nothing happens.

David Carey, president of Portelligent (Austin, Texas; www.teardown.com), which produces teardown reports and related industry research on wireless, mobile and personal electronics

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