In case of failure of fiber optic communication, how does four fiber BLSR help. Explain with neat diagram

 

The full form of BLSR is Bidirectional Line Switched Ring.
Two primary fiber loops (1p to 8p) are used for normal bidirectional communication.
And other two secondary fiber loops are stand by links for protection purposes (1s to 8s).

Lets consider, the connection between Node 1 and Node 3 then traffic from Node 1 to Node 3 flow in clockwise direction links 1p & 2p.
In return path, traffic flow counter clockwise from Node 3 to Node 1 along links 7p & 8p.
If a transmitter or receiver circuit card fails, then the affected Node detects a loss of signal condition.
And switches both primary fiber connecting Nodes to secondary protection pair.

So in that condition four fiber bidirectional line switched ring is very useful and helpful.