Passive Optical Networks

6.3.1.2 Ring Topology

Conventionally, network protection schemes for ring-type PONs are similar to

the SONET self-healing rings (SHRs) [20], in which duplicated protection fibers

are employed to provide redundant paths, and line or path protection switching

is incorporated at both the OLT and the access nodes. There are three common

approaches [21–25] for optical ring protection, namely two-fiber unidirectional

path-switched ring (UPSR), four-fiber bidirectional line-switched ring (BLSR/4),

and two-fiber bidirectional line-switched ring (BLSR/2). UPSR is essentially a 1 þ 1

dedicated protection ring; while BLSR/2 and BLSR/4 are 1:1 shared protec-

tion rings. Figure 6.6 illustrates these protection switching operations of shared

protection rings [25] ...