Under the Hood of Siemens' ADSL modem: Broadband from A-to-D (and back)

Siemens SpeedStream Asymmetric Digital Subscriber Line (ADSL) Model 5100 modem represents a typical client-side terminus to the broadband world. But make no mistake, 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 the ADSL intricacies are beyond the scope of this brief hardware teardown, but an excellent tutorial on the subject can be found at commsdesign.com.

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

Design of the SpeedStream modem centers around a Texas Instruments chipset consisting of the TNETD5310 ADSL Communication Processor, TI TNETD5100 ADSL Transceiver, and TNETD5014A ADSL analog front end (AFE). The latter part contains CODECs, filters, line drivers and line receivers to deal with challenges of modulating high speed data over an analog signal—and more critically for ADSL—recovering a bitstream 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 L80227 which itself has on-chip wave shaping, output driver, twisted pair receiver, and signal equalizer.

A $90 retail kit comprised of the the SpeedStream modem, Ethernet card, and accessories had an estimated Cost-of-Goods-Sold near $65, suggesting that SBC/Yahoo (the service providers co-branding the kit) may well be underwriting the cost of retail DSL hardware placements to drive subscriber revenue.

While much signal processing and number-crunching happens in the core TI logic devices, and business models aside, 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 is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)