[link|http://www.linuxdevices.com/articles/AT2311433767.html|Five nines and freedom too]

Excerpt:

Linux is already becoming the embedded operating system of
choice for many high-end communications infrastructure
applications. In fact, 12 of the 15 globally recognized
telecommunications service providers are already prototyping and
building Linux-based systems. Today, embedded Linux is primarily
in demand for control plane and management controllers. But
many industry observers believe Linux will soon migrate to
network processors in the data plane and to the back-end servers
dominated by Sparc and Solaris today.

The telecommunications equipment providers, however, want a
product that is tailored to their market; they don't want to just
"stuff" a commercial Linux server product into a highly critical
piece of equipment such as a control blade for a Class 4 switch.
Some of the key characteristics required by this new class of
carrier-grade Linux are that it must be highly available and robust,
based on existing and emerging open standards, have significant
third-party software support, and be easily embedded.

Carrier-grade embedded Linux will be the first Linux platform to
offer high-level server fault resilience, with multiple solutions for
clustering and distributed computing, and technology that descends
to line-level interfaces. Systems built with carrier-grade class Linux
products should support next-generation PICMG compliant
systems with Ethernet backplanes, board-level hot-swap and
hot-insertion, multi-medium redundant networking (bonding
10/100/1G Ethernet, CPCI backplane, LANE, etc.), software and
hardware RAID, publish-and-subscribe event management,
water-marking, and other "hardened kernel" features.