Linux in an Embedded Communications Gateway
The gateway system was in place for more than a year with no significant downtime and no loss of sensor data. At one point, the system had 179 days of uptime—and the only reason it needed to be rebooted was as a result of troubleshooting a problem with the network bridge equipment. Linux has proved remarkably stable and effective, and human intervention has not been needed.
The system we installed in the field is a bit different than we originally envisioned. We had some trouble with the single-board computer we chose, but those problems were related to the on-board RS-485 hardware, not Linux. It looked grim for a few days as we fought communications problems and pored over the serial-driver source code to try to find a fix. Another major plus Linux has over other OS choices is the full availability of the source code. We had not expected to need it, but it certainly proved valuable. The developer of much of the Linux serial driver code, Theodore Ts'o, personally answered questions by e-mail—within hours in one case. While we certainly did not expect that response every time, we've never gotten that level of support from any commercial vendor.
While resolving the hardware problems on the single-board computer, we shifted development work to an old 386-40MHz motherboard we had lying around. We swapped in the hard drive, added a network card, a watchdog timer card, and an RS-485 port card and immediately had a working gateway computer. The single-board computer was a 486-100, but we discovered Linux to be so efficient with minimal hardware, a faster CPU was not needed. We never did resolve the strange hardware problem on the single-board computer. The 386 system worked so well that we stayed with it and actually used it for the field trials. It worked perfectly. The incredible range of hardware that Linux supports became another solid plus that helped us implement a working system.
The server code also ran reliably. We tested and demonstrated the software on Sun servers running both SunOS and Solaris; however, the workhorse for the field system was a Pentium PC running Linux. The code base for the two systems was identical—the only difference was a few lines in the project Makefile. Our objective of portability was made trivial by Linux's adherence to open standards.
The wireless sensor network was a moderate success, although some technical problems did arise. However, during the middle of the project, changes in California law introduced more competitiveness into the state's electric utility industry. The sensor project had been started by PG&E's Department of Research and Development. One immediate change in the state's laws was that R&D funds would be administered differently. As a result, the company abolished its R&D department and transferred the project to another department to finish. The initial phase of the project is now wrapping up, but what steps will be taken next is still unknown.
Linux has matured since this project began, and it's worth taking the time to examine how recent Linux developments might benefit the next stage of gateway design. I would like to emphasize that the following comments are hypothetical—if I were to attempt upgrading and modernizing the gateway system, this is the approach I would take.
The next stage would require deploying multiple radio networks and gateways to serve large geographical areas, and changes could be made to simplify and improve operations. Gateways in non-testing situations would not require as much logging of radio network traffic, so the hard drive, the only moving part in the system, could be removed.
Paul Moody has written an excellent new Mini-HOWTO on embedded Linux (http://users.bigpond.com/paulmoody/). His document makes adding a flash disk as simple as following a recipe.
For our field trial, we specifically chose to use external, discrete network bridging equipment to link to the corporate WAN. We felt it would be fast to implement and more reliable than something we wrote ourselves. It wasn't quite plug-and-play (configuring the bridge proved troublesome), but it was relatively quick to set up. However, the bridging equipment was the only source of problems the gateway experienced in nearly eighteen months of service, so I'm no longer convinced of the superiority of proprietary network equipment. In contrast, the custom gateway software performed flawlessly.
The June 1998 issue of Linux Journal had an excellent article on the use of the Sangoma WANPIPE S508 router card (http://www.sangoma.com/). In addition, Usenet reports from users include raves about the Spellcaster ISDN cards (http://www.spellcast.com/). Using those cards in a future system would integrate all the gateway functionality into one box (with the exception of a radio for a point-to-point link, if such a link was needed). The resulting system would be about half the size, approximately two-thirds cheaper, use half the power, and in all likelihood, be even more reliable than the present system.
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