| The impression I get is that the protection circuit is intended to | cover failures that take out a single circuit. When talking about "protection circuits", bear in mind that this is all legacy PDH stuff which hopefully soon will be dead. An STM-1, for example, can be made fault-resistant by virtue of SDH's ring architechture. Any PDH payload encapsulated in the STM-1 consequently will be immunized against backhoe fade, failed optics and the like. Obviously a failure that SDH's mechanisms can't prevent will require any encapsulated traffic to be rerouted through non-SDH means. With IP, this is simple: you see a circuit failure, and change your routing tables in the normal way. With many VC-based networks you tear down all sorts of VCs and try to build them over other paths, updating your switching tables in the normal way. Finally, with things that are reliant on PDH protection, you wait for them to fall over onto protection circuits, which are laid out in different STM-1s, ideally along completely different physical paths. In other words, in the event of a failure that SDH cannot cope with, you either overscribe other in-use capacity or you make use of stand-by capacity that is otherwise completely unused. Completely unused capacity is expensive, and you already have quite a bit of it if you build SDH rings (half, in fact). Adding more unused capacity to support unlikely failures is unattractive. However, most failures that SDH cannot cope with tend to be very rare. I would be surprised, for example, if the fibre cuts in the U.S. affected SONET-ring-carried traffic, and I would be surprised if a PDH protection circuit scheme would provide much faster recovery in the cases where SDH/SONET failed. Of course, if the lines that failed were, in fact, not PDH, I'd appreciate a private word about this. :) | For example, if you | had 10 fibers in a system and a repeater or laser died, then only | one of your fibers stops working. Fixing that with a protection | circuit adds 1/10 to the system cost. Um, huh? If you have a fibre stop working then all the data traversing that fibre stops. If you have an SDH ring, you don't really care, because along the segment between the two MUXes at either eind, you have a second fibre that likely remains working. If you have protection circuits you have to deal with DAXCs and the like and you have to have the protection circuits provisioned along another fibre and standing idle. PDH protection switching is slow and ugly, but it allows fall-back connectivity to be oversold or not provisioned in the first place. That is, you can sell people "cheapo" circuits which have no backup, and even more, circuits which will fail when an unrelated circuit used by a customer buying an expensive circuit fails. | (Maybe less if you have spare | fibers in the ground.) Fixing it with SONET ring technology roughly | doubles the system cost. If you have spare fibres in the ground, light them up to protect your primary SDH paths. Even if you use another pair of fibres runnning through the same conduit, you do protect against failed optical and electrical equipment (repeaters, lasers, fibre tips, people unplugging the wrong thing, and so on). You don't need an additional MUX, you don't need extra DAXCs, and you don't need to scurry around reprogramming lots of equipment when someone wrecks your fibre. Sean.