 VCs of a given type between a large number of TTFs, with very few restrictions on this connectivity. This, of course, is consistent with the normal role of a cross- connect, as a major traffic-routing node in a transport network, where electronically control- lable traffic-routing flexibility is its raison d'être. At the other end of the scale comes Line Terminating Equipment (LTE), which normally incorporates a multiplexer as well. The connectivity between tributaries and aggregate ports of such a multiplexer is normally hardwired, SO it has no flexibility whatsoever. Between these two extremes come drop-and-insert (add-drop) multiplexers, which attempt to strike a balance that leads to adequate, rather than comprehensive, routing flexibility, with an attendant reduction in equipment costs. All of the previously mentioned types of network element normally assume that any interconnection will be based on optical fibers. It is, however, also possible to use other interconnection media, in particular radio transmission. The problem in using radio inter- SDH Standards Transport Termination Function High- High- Order Section Multiplexer Multiplexer Regenerator Order Section Section Section Adaption Physical Protection Termination Termination Interface Termination Connection (MSP) (MST) (RST) (SPI) (HPT) (HPC) (e.g., attach (e.g., cross- (e.g., load VC-4 (i.e., attach (i.e., attach (e.g., convert POH to C4) connection into STM and MSOH) RSOH) logical STM to of VC-4s) attach pointer) optical signal) Fig. 10.21 Fragment of an SDH network element showing how it is broken down into atomic functions according to ITU-T G.783. connection is that, unlike optical fibers, the available spectrum is finite and in short supply. The liberal use of overhead capacity in SDH only exacerbates the already difficult problem of squeezing the traffic information into the existing arrangement of radio channel band- widths. These problems are by no means insuperable, but they do tend to restrict radio interconnection of NNI signals to the l