Cisco ASR 900 Series Configuration Guide

Scenario for Unidirectional APS 1+1
Figure 5: Unidirectional APS 1+1
In the above figure, two devices are connected to provide APS 1+1 unidirectional protection. The figure shows
a working line and a protection line. The traffic is transmitted on both working and protection line and received
only on one line.
In a scenario where you encounter a fiber cut,
1
Device 1 receives a LOS on RX working line.
2
Device 2 detects LOS and starts receiving the data from the protection line. The protection line now
becomes the active line.
3
Device 1 receives the K2 byte and knows about switching event on device 2.
UPSR Path Protection
A Unidirectional Path Switching Ring (UPSR) is a unidirectional network with two rings, one ring used as
the working ring and the other as the protection ring. The same signal flows through both rings, one clockwise
and the other counterclockwise. It is called UPSR because monitoring is done at the path layer. A node receives
two copies of the electrical signals at the path layer, compares them, and chooses the one with the better
quality. If part of a ring between two ADMs fails, the other ring still can guarantee the continuation of data
flow. UPSR, like the one-plus-one scheme, has fast failure recovery.
UPSR Path Protection is supported at VT level and STS level.
1-Port OC-192 or 8-Port Low Rate CEM Interface Module Configuration Guide, Cisco IOS XE Everest 16.7.x (Cisco
ASR 900 Series)
31
Configuring SONET
1+1 APS

Cisco ASR 900 Series Specifications

General

Brand	Cisco
Model	ASR 900 Series
Category	Network Router
Language	English