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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Sonet protection usage
Bill Simpson writes to the "expertise list": | > This is typically 50ms, but one has to add the one-way delay to that. | > | I haven't seen anything about adding one-way delay to make it longer, | could you cite, please? My reading was that the 50 milliseconds | included delay, as 50 milliseconds would include a one-way trip | around the world, and section regenerators are much closer together | than that.... Your photons seem pretty damn fast to me. An ADM doing LSR will listen to N different copies of the same data, and buffers the first-arriving data so that it can compare at least one subsequent copy as a means of verifying data integrity. If the data checksums do not compare identically, then the ADM must make a decision about which copy to send onwards (if any). If both sets of data are sane, the frames received on the working interface are forwarded, the others are discareded, and an alarm is raised. If the working copy is not sane, then a protection switch is negotiated using the K1/K2 bytes in BLSR mode, or via other means if the receiver is not the K1/K2 master in unidirectional mode. The comparison threshold is 50ms, which gives ample wiggle room for large path-propagation-delay mismatches, while bounding the per-section delay inherent in this type of protection switching. It seems natural to me that the _fastest_ a receiving ADM can even in theory make an informed decision about whether to force a line switch is the difference in propagation delay between a working and protect pair. ADMs are not psychic! Finally, with a given comparison threshold, even a conceptually zero-length fibre, with zero propagation delay, will incur a time penalty between a physical cut and a line-switch. | Now, it appears to me that the question was about metro-area circuits | (recalling that these are usually called "intermediate" in the | specification) -- not short circuits inside a single facility, and | not transoceanic -- for "router redundancy". OK? Yes, and I answered Steve privately (because it was him). I merely intervened in this not to impart expertise upon competitors, but rather to correct your conflating ADM between a pair of routers and a single ADM with actual 1+1 protection along a multi-hop LSR-protected trail. As has been noted by other people in the thread, there are sound reasons why one might want to set up the latter, even on a circuit which is otherwise completely unprotected. Likewise, a higher-order container can be protected in the two router + one ADM scheme, even if the lower-order containers within it are otherwise wholly unprotected and only loosely related. | In giving a cogent answer, I've noted the carrier problems, and given | examples and rationale. In my experience, multiple circuits with | diverse topology is the best answer Protecting 12 DS3-mapped STS-1s inside an OC12 is cheaper and protects against a much more common failure mode than the kind of failures having 24 unprotected DS3s additionally protects against, and has the advantage of not making token-bucket-style or DSU-to-DSU style rate limiting much more difficult to implement in pay-for-available-bandwidth environments. This is independent of the length or locality of the DS3-mapped STS-1s (or, indeed, DS3-bearing VC3s (hi Neil!)). No charge for the partial implementation suggestion. Consult your vendor for further details. Sean.
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