Understanding SONET BLSRs

A ring is defined as a set of nodes interconnected to form a closed loop, where fiber cables serve as links. There are two major types of SONET rings: path-switched and line-switched SONET rings. Line-switched rings use the SONET line level indications to initiate protection switching. Line layer indications include line layer failures and APS signaling messages that are received from other nodes. A request for switching may also be initiated via an operations interface. In a UPSR the traffic between two nodes is provisioned to travel either clockwise or counterclockwise under normal conditions. A connection/circuit on a UPSR uses capacity on the entire ring. If both directions of transmission use the same set of nodes and links, the transmission is said to be bidirectional. A connection/circuit on a BLSR uses capacity ONLY between the nodes where the traffic is added and where it is dropped, see figure 1.