User guide

Packet Over SONET Module Installation and User Guide 3-75

Configuring and Monitoring APS Functions

Automatic Protection Switching, or APS, is a resiliency feature specified in the SONET

standards. Of the different modes of operation defined in the APS specifications, the

BlackDiamond 6800 series switch supports the Linear 1+1 APS architecture to protect

tributary SONET lines connecting switches to Add-Drop Multiplexers (ADMs). Linear

APS can be contrasted with the Ring APS architectures, which protect the lines between

the equipment comprising a SONET ring. Figure 3-11 is an example of the Linear 1+1

architecture, where there is one protection line for each working line, and the ADM

transmits the same data to both the working line and the protection line.

Figure 3-11: Linear 1+1 APS architecture

The APS standards specify both unidirectional and bidirectional-switching modes. In

the bidirectional mode, both ends must select, or receive data from, the same line. Thus,

switching from one line to another must be coordinated. This synchronization is

achieved using APS protocols that are carried in the K1 and K2 bytes of the SONET line

overhead. The K1 and K2 bytes must be transmitted on the protection line, and may

also be transmitted on the working line; however, receivers cannot assume that the K1

and K2 bytes will be transmitted on the working line.

Bidirectional switching is advantageous for data communication applications where the

working line and the protection line are terminated in different switches, as depicted in

Figure 3-12. Because the working and protection lines form a single SONET interface

with respect to the rest of the network, it is clearly more straightforward and efficient

for one switch to handle all the payload transmission and reception responsibilities for

PoS_011

BlackDiamond

Protection line

Working line

1234AB5678

ADM PoS

device

SONET

ring