ABSTRACT
A very strong set of operations and maintenance features is one of the biggest advantages of the SDH over its predecessor technologies, second only to the huge capacity that it provides.
One of the most beautiful features of the digital technology is the ease of signaling. If we recall the kind of signaling associated with the analog technologies, the signaling functions were, by and large, restricted to the call setup and release, such as detecting the “on hook” and “off hook” conditions, extending the dial tone, seizure of trunk line, call terminations, etc. Adding any extra features to signaling called for increased complexities and, of course, increased costs. In sharp contrast to this, digital technology provides for a very large number of signaling functions. In fact, it will not be an exaggeration if we call it an unlimited number of functions. The secret of this quantum jump lies in the fact that the digital signal does not differentiate between signaling and speech when it comes to carrying it across the distances. Signaling information is just a few extra bits added to the speech data. Since these additional bits are not speech data, or payload as it is called, it is called the “overhead.” However, the overhead is considerably insignificant in comparison to the total payload data, but what gives it a real outstanding status to digital systems is that the provision of this overhead is extremely easy and cost effective as compared with that of analog technology signaling. In fact, it was unimaginable in the analog era to provide for so many O&M functions in the system as in digital technology systems. The functionalities of the overhead bytes are defined in ITU-T recommendation G.707/Y1322 (2007), amendment I.
