A14 Calculation Of Availability Of Los Radiolinks In Tandem

In this section we derive per-hop availability given a system availability consisting of n hops in tandem with independent outages on different links. Often such a system availability will derive from the ITU-R hypothetical reference circuit, which is 2500 km long. Panter (Ref. 4) assumes that such a 2500-km circuit consists of 54 hops each 30 mi (48 km) long.

We describe the procedure by an example. If a one-way circuit requires an availability of 99.95% over a 2500-km reference circuit, what is the required per-hop availability? First calculate the system unavailability, which, in this case, is 1 - 0.9995 = 0.0005. Divide this unavailability by 54 or 0.0005/54 = 0.000009259. This then is the unavailability for one hop. Its availability is 1 - 0.000009259 = 99.999074%.

ATT uses a 4000-mi reference circuit with a required availability of 99.99% or an unavailability of 0.0001 or 1 x 10-4. If we assume as above that each hop is 30 mi long, then there are 133 hops in the reference circuit. We now divide 1 x 10- 4 by 133 and the resulting unavailability per hop is 0.000075%. The equivalent availability per hop is then 99.999925%.

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