ABSTRACT

The transmission of plesiochronous digital hierarchy (PDH) and synchronous digital hierarchy (SDH) signals is possible on any of the prevalent media, i.e., copper wire pairs, coaxial cables, radio, or optical fiber, as the multiplexing technologies and hierarchies are totally independent of the media type. However, as the data rates increase, the higher bandwidth media, i.e., optical fiber, emerges as the natural choice. In fact, SDH has been designed with optical fiber cable (OFC) in mind, and thus, OFC is naturally best suited for SDH and high-rate PDH signals. As has been seen (Section 10.1) the PDH has already become obsolete with the advent of SDH now. In the initial days, PDH was and still is being transported on all types of media because the feeder tributaries to SDH are still the low-rate PDH signals by and large. As the OFC technology matured, all the long-distance circuits of high bit rates and high densities started shifting from their then preferred media, which was radio, to the optical fiber. Shifting to OFC boosted the possibility and hence the implementation of higher data rates and, consequently, many more numbers of user circuits. However, OFC has its limitations as well. Take two adjacent hills, a kilometer or so apart and separated by a valley, as a hypothetical example. Deployment of any type of cable including OFC is very difficult in such a situation. This is the reason for the radio media still being alive-for patching gaps that cannot be covered by the OFC. A similar situation is in metro city areas where laying cables through the buildings and roads is a stupendous task. Currently, the demand of radio media is driven more by such difficulties in metro city areas, rather than in barren territories.