Your experience leasing lines from RBOCs other than BellSouth and Ameritech must be vastly better?

Again, when ADMs and routers are in the same facility the local loop diversity issues are a non-issue. As Sean points out though, intra-facility connections fail, be it a result of human intervention or divine intervention. Also, maintenance affects this as well.

Now, in my experience, when routers _do_ fail, usually soon after upgrading the software, APS won't help at all. The card is down, too. IP has to route around the problem.

Maybe this is some new use of the term "router"?

Funny. You're clearly missing the point of the application. Consider the following: +-----+ | A3 | | | +-----+ / \ +-----+ +-----+ | A1 | | A2 | | | | | +-----+ +-----+ \w p/ +---------------+ | ADM-A | | | +---------------+ | | +---------------+ | ADM-B | | | +---------------+ /w p\ +-----+ +-----+ | B1 | | B2 | | | | | +-----+ +-----+ \ / +-----+ | B3 | | | +-----+ You're a provider with two locations (A & B), you own the ADMs (not the transmission facilities) and have only a single connection between the sites, say an OC12c that you lease from an RBOC. Now, you don't have another $100k (monthly) to give to the RBOC for a redundant, presumably diverse secondary OC12 connection, though you would like to be able to load new software on a router or perform maintenance such as hardware upgrades without losing connectivity between the sites. Considering you average ~300 Mbs on the OC12 buying a lower speed connection for redundancy isn't a viable option. So, you buy some ADMs (a capital expense which is likely less than one month of the recurring OC12 fees) and do inter-router APS at the site. Since APS isn't required on both sides (or in the network), you could even do one side at a time. The OC12 connection from the telco may or may not be protected. Under normal conditions the ADMs would transmit frames ingress from the network to both tributary ports (routers) and the inactive routers (A2 & B2, both inactive protect circuits at this point) are responsible for discarding the frames. The ADM would only transmit the ingress signal from the active tributary port to the network. The ADM and routers communicate via the K1 & K2 SONET bytes. Now, you want to do maintenance on B1 so you simply trigger a force switch to the protect circuit from the protect router interface. This in essence forces the ADM to use B2 as the active circuit. B1 is notified via the ADM (K1/K2) and becomes inactive, while B2 frames will be transmitted to the network. This is an overly simplistic model, though I hope the application has been better demonstrated. And again, it becomes even more interesting when the OC12 is trans-oceanic, or its an OC48 and the only viable completely diverse option requires 3 times the circuit route miles (which you pay for) and a few million dollars to provision a diverse fiber path into the facility, etc.. I assure you that it has it's uses and is employed by both network service providers (which in _this definition I refer to as owning the transmission facilities), as well as ISPs. There are lots of higher layer issues such as interaction with the IGP that impact convergence times considerably (e.g. the fact that when B2 becomes active above a new adjacency must be formed with A1, the remote router, or SPF run timers throttling rerun when the new adjacency is reported because of a recent run triggered by the report of the failure itself), but it's still far, far better than doing nothing at all.

Virtually every router I've installed, even "ancient" NetBlazers and Ascends sitting out at the customer sites, has been stable for years without interruption. OTOH, links fail often, in ice storms, in rain, on windy days, even for no known reason at all!

As for this, let me let you in on a little secret: There's always a reason the links fail. (Just kidding! ;-). HTH, -danny