Source: David R. Goff. Fiber Optic Video Transmission, 1st ed. Focal Press: Woburn, Massachusetts, 2003

and other private writings.

Technology Basics

Modern society is highly completely dependent on communications technology. Network failures, whether due to human error or faulty technology, can be very expensive for users and network providers alike. As a result, the subject of so-called fall-back mechanisms is one of the most talked about in the communications industry. A wide range of standardized mechanisms is incorporated into synchronous networks in order to compensate for failures in network elements.

Automatic Protection Switching (APS)

Two basic types of protection architecture are distinguished in APS. One is the linear protection mechanism used for point-to-point connections. The other basic form is the so-called ring protection mechanism which can take on many different forms. Both mechanisms use spare circuits or components to provide the back-up path.
 

Linear Protection

The simplest form of back-up is the so-called 1+1 APS. Here, each working line is protected by one protection line. If a defect occurs, the protection agent in the network elements at both ends switch the circuit over to the protection line. The switch over is triggered by a defect such as LOS. Switching at the far end is initiated by the return of an acknowledgment in the backward channel. The advantage of the 1+1 APS is that protection can be implemented at the most basic hardware level. The disadvantage of 1+1 APS is that it requires twice as much hardware as is required to carry the data.
 

1:N Protection

A more sophisticated form of protection is called 1:N diverse protection structure. In this case a group of N working lines are protected by a single protection line. The underlying assumption is that only one of the working lines will fail at any given time and that the working lines are designed so that they are independent from each other. The advantage of 1:N protection is that it requires much less hardware than the 1+1 APS scheme. The disadvantage is that the switching between working systems and the backup system must occur at a higher level in the system. In most cases, the cost advantage of 1:N protection is so significant that other considerations are overridden.

Figure 1 shows an example of 1+1 APS protection, 1:N protection, and diverse protection path. The 1:N protection scheme is shown between CO #1 and CO #2. There are N working systems (usually a pair of optical fibers, one for each direction) and a single backup system (usually a single pair of optical fibers, one for each direction). If any of the systems from 1 to N fail, the traffic that would normally be carried by that system is switched to the backup system.

Between CO #1 and CO #3, CO #3 and CO #4, and CO #4 and CO #2 are examples of 1+1 APS. In each case, there is a single working system (usually a pair of optical fibers, one for each direction) and a single backup system (usually a single pair of optical fibers, one for each direction). If the working system fails, the traffic that would normally be carried by that system is switched to the backup system.
 

One additional example of a protection strategy is shown on Figure 1. A backup system is shown between CO #3 and CO #2. The idea here is that this backup fiber runs along a different path than the normal working and protection fibers. This is a hedge against an event that may sever all fibers between two locations.

Figure 1 - Examples of SONET Protection Strategies