Optical time division
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.
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/s–100 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:
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:
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.
topologies that can be considered in OTDM are point-to-point, star,
The main elements
in OTDM (sub) system are:
2. optical multiplexer
3. optical demultiplexer.
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
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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The heart of our consulting spectrum comprises strategic,
organizational, and technology-intensive tasks that arise from the use of new
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