We regularly remind our readers who need very high-availability networks to make sure that the redundant WAN paths they think they are buying truly are diverse. You can sign up for two separate services or circuits, but if they are housed in the same fiber sheath, and the bundle suffers a cut, you're obviously out of luck.Truly diverse paths require circuits into two separate entrances to your building. And connections to two separate head-end devices at the service provider point of presence (POP) are also recommended. So it often pays to look under the hood at your network provider's own infrastructure design to uncover any potential single points of failure.We'd like to illustrate this point by briefly discussing Lightpath, a carrier serving about 1,500 buildings in New York, New Jersey and Connecticut. Owned by Cablevision, Lightpath currently runs an all-SONET network (with the SONET loopback restoral reportedly traversing a separate fiber), over which it delivers managed router, firewall and VPN services.Private-line, ATM and frame relay services also run over its infrastructure. At press time, the carrier was in the midst of rolling out a managed point-to-point Ethernet service that was to launch Nov. 1. The carrier says it plans to move to Dense Wave Division Multiplexing-based services early next year to offer switched Ethernet\/virtual LAN services.The carrier takes the following high-reliability steps, according to Brian Fabiano, senior vice president of network services at Lightpath:* Installs and owns it own conduit wherever possible.* Layers concrete and steel over the fiber for protection.* Connects users to two separate headends (meshed Cisco 12000 series routers).* Connects into two separate entrances at customer sites.* Runs dual multiplexers on customer premises.* Makes mobile generators available in the event of a power failure.Next time: What customers with no-nonsense high-availability needs have to say.