6.1 MS shared protection rings MS shared protection rings can be categorized into two types: two-fibre and four-fibre. The ring APS protocol accommodates both types. For MS shared protection rings, the working channels carry the normal traffic signals to be protected while the protection channels are reserved for protection of this service. Protection channels may be used to carry extra traffic when not being used for protection of normal traffic. Normal traffic signals are transported bidirectionally over spans: an incoming tri butary travels in one direction of the working channels while its associated outgoing tributary travels in the opposite direction but over the same spans. The pair of tributaries (incoming and outgoing) only uses capacity along the spans between the nodes where the pair is added and dropped. Thus, as illustrated in Figure 6-1, the pattern that these pairs of tributaries are placed on the ring impacts the maximum load that can be placed on MS shared protection rings. The sum of the tributaries that traverse a span cannot exceed the maximum capacity of that particular span.