j - AGH University of Science and Technology

Piotr Chołda, Andrzej Jajszczyk
As a numerical example, we compare three different protection topologies:
three working connections with the same availabilities are to be protected. The
following schemes are compared: the 1:3 protection; the topology in which each
working path has its own protection path, it is in fact the 1+1 protection applied
three times (six paths in total); and the 3:3 protection (three protection paths shared
by three working paths). The analytical formula for the last case is rather complex,
so we do not present it here. It is based on the 3-out-of-6 structure. The relevant
formulas and comprehensive information how to calculate them can be found in
[9]. The results are presented in Fig 5. We can see that the 1:3 protection results in
a large increase of the availability in relation to unprotected connections. The gain
is especially visible in the case when availabilities of the working paths are small
(even if availabilities of protection paths are relatively low). It is the least reliable
but the most cost efficient scheme of the three compared. Moreover, from the
reliability point of view, the most favorable is the 3:3 protection, even better than
applying the 1+1 protection for each working path separately. The reason is that in
the case of 3:3 protection the scheme is more flexible. When one of the working
paths fails, the traffic is rerouted on the protection path. And again, when this path
fails, the traffic can be rerouted again. It is impossible in the case of the 1+1
protection. However, we have to remember that the gain is minimal and practically
such a solution could not be advantageous: it is, after all, necessary to use
additional switching elements. Some analyses [10] indicate that the influence of
switching nodes cost on the total expenditure can be quite large. It seems that this
fact is often neglected and optimization based only on the link cost could be
insufficient.
5. RESTORATION
Unlike in the case of a protection, where an alternative path is established
before a failure, in the case of the restoration such a path is sought after the fault
notification. It makes the calculation of the availability more difficult. For example,
the availability is calculated not for connections, but for carried traffic. Therefore,
it is called the availability of a load ( A ). The authors of [4] propose the
following formula for calculating it in a restoration scheme:
M ⎛ CRec
⎞ 1
A = − ∑ ( ) × ⎜ ⎟
Load 1 P Sceni
⎜
1−
⎟
(6)
i= 1 ⎝ CT
⎠
where P ( Sceni
) is the probability of failure scenario i (out of M scenarios); C T is
the total capacity of traffic (sum of all working capacities); is the total
capacity of the connections recovered upon failure scenario number i (this capacity
includes also capacities of connections unaffected by a failure).
Load
C Rec
i