ephone cable. The FDM schemes used around the world are standardised to some extent. In the United States, AT&T has designated a hierarchy of FDM schemes to accommodate transmission systems of various capacities. Unfortunately this is not identical to the standards, which are internationally adopted by the International Telecommunications Union (ITU). The FDM standards adopted are listed in Table 3.1. Multiplexing was not confined to the frequency domain only, and with the introduction of digital communication came the most successful multiplexing techniques, which today is used in most telecom- munication networks - time division multiplexing (TDM). Attention will turn to TDM after discussing another multiplexing technique, which has been the outcome of the invention of the optical fibre - wavelength division multiplexing (WDM). Simply put, it multiplexes different light wave signals by a diffraction grating and sends them on a single optical fibre to a destination where the signals are split Transmission Network Planning and Optimisation Table 3.1 United States and international FDM carrier standards Number of voice channels Bandwidth Spectrum ITU-T 60-108 Group Group 312-552 Supergroup Supergroup 1.232 812-2044 Mastergroup 564-3084 Mastergroup 3.872 8.516-12.388 Supermastergroup 1234123412 Frame Figure 3.3 Bit interleaving again. This type of multiplexing is independent of any protocol like Ethernet, etc. With the amount of bandwidth the optical fibre has to offer, this technique shows great potential for the years to come. Time Division Multiplexing Time division multiplexing (TDM) is by far the most commonly used and effective means of subdividing the capacity of the digital transmission service/channel among a number of sources/paths. Put in a different way, it can also be described as combining more than one signal such that each piece of a particular signal gets the channel only for a small amount of time, called its timeslot. This switching of different signals is done SO quickly that the service users f