erik@naggum.uu.no (Erik Naggum) (07/12/90)
Lars Poulsen writes in TELECOM Digest V10 #473: > On a tangent: I always have problems distinguishing between busy and > reorder. How did we end up with the "fast busy" and Europe with the > tone triad ? What does CCITT recommend ? CCITT recommends a few signals, for which I can dig up the precise definition in terms of frequencies and the duration of tones and pauses if somebody asks nicely. There are two tones for "busy", one for subscriber busy and one for network congestion. These differ in the duration of tone and pauses, but the frequency is similar to that of the ordinary ring signal. They may or may not (subject to national variations) have the same duration for tone and pause. (The ring signal you find in parts of the United Kingdom is not representative for CCITT or Europe.) There used to be three signals only: ring, busy and congestion. (Is the last what you call "reorder"? I'm unfamiliar with the term.) With SS7, lots of voice messages were introduced, and they all have the "special information tones" (same as the AT&T ones) at the start or end of the voice message. CCITT only recommends using the special information tones when the error is considered "permanent", i.e. which requires intervention by the telco before it gets fixed, as opposed to subscribers, which are considered much more transient beings. While we're on CCITT recommendations. I recently bought the Q.700- series (Signalling System number 7) and the I-series (ISDN). I was somewhat shocked to find that SS7 needs fully 1680 pages of specs! Granted, they had included about 300 pages of tests with forms to be filled out when testing conformance of SS7 users, but it's still a lot. A friend of mine, also a reader of TELECOM Digest, has some specific opinions on the relation between the OSI layer and the quality of the CCITT recommendations applying to it. I'll let him express them himself. (That's bait for you, Morten!) For those who have become a bit wary of CCITT recommendations, I can recommend the Blue Book (at least the I, Q, V and X series). They have done a tremendous job of writing this is an actual language, as opposed to the "blot on the history of prose" that the Red Book was. [Erik]