This page of SDH networks describes type of SDH networks which include two fibre ring and four fibre ring. It explains unidirectional and bi-directional ring concepts used in SDH network.
As shown in the figure-1, simple SONET/SDH network is composed of sequence nodes referred as Add/Drop Multiplexers (ADMs). There will be either two fibres or four fibres connected between them. In case of four fibres one set acts as protection or back up pair.
In the case of linear network even if both sets of fibres were used between the nodes there will be possibility of both getting cut at the same time and communication services will be hampered. The most common topology is of type ring network in SDH. They are most popular as they provide alternate path to communicate between two nodes. A two fibre ring can be operated either as unidirectional ring or as bi-directional ring.
In unidirectional case, traffic is limited to one fibre and flows only in one direction. The second fibre is used as protection fibre. In uni-direction case, there will be propagation delay (PD) between two nodes. As shown in the figure, if node B sends traffic to node A the PD is one link and if node A sends to node B then it will be 3 links.
In four fibres case, one set is working fibre and the other set is protection fibre. Also data flows in both the direction and hence propagation delay between nodes will be less.