In telecommunications installations it is common to find a set of electronics modules, the modules being held in a rack or similar device which provides mechanical support, a power supply and interconnection of signals between the modules.
The modules provide various functions, such as interfacing to a high-capacity optical link, a cross-connect (switching) function, or interfacing to subscriber lines. Building an installation in the form of modules allows simpler upgrades or repairs of the installation, with just those parts of the installation requiring change being affected.
A problem with telecommunications installations is that of replacing modules when they fail or when they need to be upgraded. In a large installation where there are tens or hundreds of similar modules, it can be difficult to properly identify the module which needs replacing. It is a common problem for incorrect modules to be temporarily withdrawn from their support frame in error, or for incorrect modules to be replaced. This leads to calls which are in progress at the time of the module removal being `dropped` and periods when this equipment cannot be used. Such events are often called outages. Incorrect removal of modules is the reason for a significant proportion of outages in networks. There is a need to minimise such outages, particularly as telecoms operators are bound by strict regulations which protect consumers by enforcing a maximum permissible level of outages.
Labelling of modules as an aid to identification is often ineffective. Labelling becomes dirty or damaged with age. Labels may also be incorrect, where modules have been upgraded or boards inside the module casing have been replaced. It may also be undesirable to label a module so as not to draw attention to it; for example, a module which carries calls from a particularly sensitive part of an organisation.