Today, low memory prices and embedded processors with memory-management units make Linux viable for many devices, transforming ADSL modems from humble "bit pumps" into fully featured gateways. The same software selection criteria apply:

- Performance: ADSL provides Internet access at CD-ROM speeds (8 Mbits/second today, moving to 16 Mbits/s with the ADSL2+ standard).

- Reliability: ADSL modems are always on; continuous, reliable operation is mandatory.

- Availability of software components: Modern ADSL modems require a comprehensive set of networking protocols including DHCP, DNS, RIP, etc. Additionally, stateful packet-inspection firewalls with built-in network address translation are becoming baseline functionality, and modem manufacturers must find differentiation in software to generate a competitive edge.

- Price: The bill of materials for an ADSL customer-premises modem is below $30 and dropping; any software royalties are immediately significant.

LSI Logic looked at these issues in developing its ARM9-based HomeBase ADSL modem chip set before selecting Linux as the reference software for its turnkey modem reference design. The availability of source code meant the kernel could be rapidly modified to take advantage of the on-chip hardware acceleration, ensuring that performance goals were met.

Linux itself is CPU-efficient, not carrying the performance penalty that proprietary-OS writers would suggest. LSI Logic downloaded the protocols, applications and tools it needed to build its product quickly.

ADSL modems, for cost reasons, are typically constrained to 2 Mbytes of flash ROM and 8 Mbytes of DRAM. Accordingly, many embedded OSes such as Vx-Works, PSOS and Nucleus were specifically designed with a small memory footprint in mind. LSI Logic found that, when properly configured, the Linux kernel is also small.

Stephen Ellwood is director of product marketing for broadband access products at LSI Logic Corp. (Milpitas, Calif.).