At the receiving end of the high-capacity channel the bit stream is readily demultiplexed, with the demultiplexer detecting the framing pattern from which it determines the beginning of each frame, and hence each slot. A given “logical” channel therefore occupies every Nth slot, giving us N “logical” channels in which to place the N incoming messages. For example, the first “logical” channel occupies slots 1, N + 1, 2 N + 1, … the second occupies slots 2, N + 2, 2 N + 2, … the third slots 3, N + 3, 2 N + 3, … and so forth. 36 A “logical” channel occupies a single slot in every frame. Time in each frame is further subdivided into N fixed-length intervals usually referred to as slots: slot 1, slot 2, …, slot N. The procedure is as follows: time on the high-capacity channel is divided into fixed length intervals called frames. In TDM the high-capacity channel is divided into N “logical” channels and data in each of the N incoming voice channels are placed in a designated “logical” channel.
N voice 35 channels are placed sequentially on a high-capacity channel and again separated at the receiving end by the demultiplexer. 9.27a shows a multiplexer and demultiplexer, represented by a mechanically driven switch. If the sampling is faster than the inverse bandwidth of the signal, no information is lost by the discrete sampling. It is analogous to time sampling, such as in a telephone or compact disk player. Then if the grating arms occupy less than the angle λ /nb, the connecting waveguides do not undersample the spectrum. Suppose the connecting waveguides have a pitch of b. The field along one edge of the star coupler is approximately the spatial Fourier transform of the field along the other edge. This is when the connecting waveguides perfectly sample the spectra from the waveguide gratings. The sampling then meets the Nyquist criterion. This is the “perfect sampling” condition. At some point, the ripple will become zero and the passband will become perfectly smooth. The individual passbands will begin to merge into one wide passband, with multiple zero-loss peaks. Assume that all the connecting waveguides have the same length. Now suppose one decreases the spacing between the connecting waveguides at their connections to the star coupler.
Demultiplex osculator series#
The transmissivity from input to output will look like a series of well-separated passbands, shown as “undersampling” in Fig. In this case, the connecting waveguides are spaced far apart from each other at their connections to the star couplers. One way to create a multi-zero-loss peak pass-band is to connect two waveguide gratings with multiple waveguides.
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