Optical Communications, 2nd Edition

8.5 TIME DIVISION MULTIPLEXED NETWORKS

In TDM networks, multiple accessing is achieved by separating all source transmissions in time and using time gating at the receiver for tuning to the data of a desired source. The required timing for the transmitters and receivers must be accurately controlled by a master clock that is available to all stations. Thus TDM networks are inherently a time synchronized system.

A TDM network can be generated in one of two basic formats. Figure 8.12 shows a time-slotted format, in which a periodic time frame is partitioned into N time slots, and each of the N transmitters are assigned to one time slot in each frame. With all transmitters aligned to the same frame clock, a source transmits only during its own slot. Thus only one source is producing waveforms at any one time. A receiver synchronized to the same frame clock (allowing for transmission delay) gates only during the slot time of the desired source.

Source modulations can be coherent or noncoherent, intensity modulated or digital, but the source must burst on periodically during its slot times. Each transmitter, however, has available the entire optical bandwidth during its transmission time. A transmitter may have to buffer (store-and-forward) its data to properly align its data rate with its source transmission time. Crosstalk from other channels will only occur if timing errors cause time slot overlap between adjacent time channels. By inserting guard spaces between slot times to compensate ...