TABLE OF CONTENTS The operation of most FDM monitoring systems is fundamentally very similar. In the following text, two specific applications are described briefly: a microwave network-monitoring system and an undersea cable-transmission testing system.
Many communications companies using microwave transmission as the backbone of their networks use an FDM-baseband monitoring system as one of the automated systems deemed necessary for network surveillance. The main purpose of such a monitoring system is to provide a means of detecting and controlling high levels in the transmission network.
Before describing how such a system might function, it is important to note one significant difference about the baseband access that may be required when monitoring a microwave network. Unlike a conventional cable transmission system, where access can be made at baseband or lower levels of the multiplex hierarchy, a microwave network may require access to be provided at repeater stations where there is no direct baseband access. At such stations, the information signal is normally brought down to IF (e.g., 70 MHz) before being modulated back up to RF for transmission over the next microwave hop (see Fig. 8.40). In such cases, the FDM baseband is not directly accessible and hence an IF demodulator unit is required. If the microwave system carries several channels, then an equivalent number of demodulators would normally be required to demodulate the individual signals down to baseband before connecting them to a baseband-access switch. An alternative method using an access switching arrangement having a frequency...
