RTLinux, and Real-Time Linux with PREEMPT_RT patches added to the kernel. The only RTOS that I'm familiar with is VxWorks. This is the OS the Nortel Passport Multi-Service telecom switches ran on.
Coincidentally, both VxWorks and RTLinux are both developed by Wind River Systems. VxWorks was quite impressive in the detail level of configuration, performance tuning and troubleshooting. Combined with the excellent hardware engineering of the line cards, these switches where often very challenging for us lowly, knuckle dragging field engineers to provision correctly for service. "A Cat 6 cable is 5 nanoseconds per meter, 10 ns down andback. Some degradations (such as RJ45 connectors) are smaller, but can add up and limit bandwidth. The right hardware and software might help network engineers observe and understand these fast phenomena, and cure some rather subtle gigabit-rate signalling and cabling problems." One thing that I really loved about Nortel, is that IMHO, they were a hardware engineering company. I spent many an evening & weekend digging through their technical hw engineering documents. Which were just top notch. They were just so detailed and thorough about specs, limits, bottlenecks, industry standards and if they departed from them and performance optimization, for which based on their own engineering lab tests, the set all the defaults to. I'll never love nor hate a piece of hardware so much as the Nortel Passport Multi-Service Switch. RIP Nortel Networks.
