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Optical time division multiplexing-OTDM

Since the speed at which electrical signal can modulate optical carriers is limited, to fully exploit the optical fiber bandwidth, another optical multiplexing technique is OTDM.

Optical TDM attempts to copy conventional TDM ideas and realize them optically, thereby achieving much higher speeds (Fig. 8). To overcome the bit-rate limitation imposed by electronics within a single wavelength bandwidth, OTDM techniques using narrow optical pulse laser sources have been developed. Since the approach uses extremely narrow optical timeslots with correspondingly large bandwidths, a natural increase in the data rates (40 Gb/s100 Gb/s or higher) at a single wavelength bandwidth, is obtained by multiplexing low bit rate tributary data streams directly in the optical domain. To further increase the capacity, a combination of OTDM with WDM have been developed as follows:

        7 wavelengths x200 Gb/s (1.4 Tb/s) transmitted through 50 km of dispersion shifted fiber;
        25 wavelengths x40 Gb/s (1 Tb/s) transmitted over 342 km;
        19 wavelengths x100 Gb/s (3 Tb/s) transmitted over 40 km of dispersion shifted fiber using supercontinuum WDM sources.

OTDM has features that make it very attractive as a future network technology:

        Simple access to high aggregate line rates (100 Gb/s and higher)
        Tributary data rates at any desired degree of granularity and compatible with existing technologies such as SDH
        Greatly simplified amplifier and dispersion management (due to single wavelength transmission)
        Network nodes that require electronics operating at the local data rate only

Physical topologies that can be considered in OTDM are point-to-point, star, and ring.

The main elements in OTDM (sub) system are:

1.   optical pulse source
2.   optical multiplexer
3.   optical demultiplexer.

Realization of OTDM ring networks requires OTDM add-drop multiplexers at each station on the ring.

Figure 8 - Point-to-Point transmission system using OTDM. MSSI (mid-span spectral inversion)

The technology involved in the implementation of OTDM is very different from that of WDM. Most of the devices required to implement OTDMA are still confined to the laboratory. In addition, OTDMA has still a number of difficult hurdles to solve such as synchronization and the immature technology of selecting one channel out of multiplexed picosecond optical pulse data streams. Hence, scope for evolving new innovative methods exists in order to determine the best trade-off regarding capacity, performance, flexibility and complexity of a variety of approaches to network provision


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