363-206-295
Maintenance Description
9-22 Issue 1 December 1997
Path Protection Scheme 9
Path protection rings feed a SONET payload (STS) from the ring entry point,
simultaneously in both rotations of the ring, to the signal's ring exit point as shown
by traffic AC and CA in Figure 9-7(a). This duplication of the signal that enters the
ring is called a "head-end bridge." The node that terminates the signal from the
ring monitors both ring rotations and is responsible for selecting the signal that
has the highest quality based on LOS/LOF, path AIS, LOP, path unequipped and
path BER performance. This function at the ring exit point is called a "tail-end
switch." Path switching is nonrevertive. On pass-through paths, detected hard
failures (LOS, LOF, LOP, line AIS, and STS-1 path AIS) result in path AIS insertion
in the outgoing signals. This allows the terminating node to detect path failure and
switch to protection.
Under normal conditions, both incoming SONET path signals to the switch
selection point are of high quality, and the signal can be selected from either ring.
A failure or a transmission degradation on one of the rings requires that the other
ring path be selected, and requirements specify that this path selection must
occur within 60 milliseconds after a hard failure condition. Figure 9-7(b) shows
how traffic is switched when a dual-fiber cut occurs. Nonrevertive switching is
provided to minimize the impact on critical customer services by giving the service
provider control when, and if, the critical service should revert to a particular ring.
A manual path protection switching command allows switching back to the original
path for ease of ring maintenance. Refer to the switch-path commands in
Section 11, "Commands and Reports."
To Next Page
Loading...
Need help?
General
