Page 134 - Optical Switching And Networking Handbook

06_200023_CH05/Batesx 1/17/01 8:19 AM Page 119

Wave-Division and Dense-Wave-Division Multiplexing 119

DWDM requires that the transmission lasers have a very tight
wavelength tolerance so that the signals do not interfere with each
other. After all, the systems carry 16 or 32 different wavelengths on
a single fiber. It is imperative that the tolerances be held tightly to
prevent mass destruction of the data signals being generated on the
fibers. The International Telecommunications Union (ITU) has spec-
ified 100 GHz standard spacing between the wavelengths, and most
of the vendors are now manufacturing lasers at the standards-based
wavelengths for combination into DWDM.
The most advanced today, OC-192 lasers has a tolerance of 0.1 nm,
which is small enough to preclude interference between two adjacent
wavelengths. Designing OC-192 DWDM systems is ostensibly more
difficult than the older OC-48 systems. OC-192 systems being
deployed in the networks also use both unidirectional and bidirec-
tional systems in support of the fiber and DWDM multiplexers. How-
ever, the future may change things very quickly. Where OC-192
offers a 10-Gbps operation, the future offers an OC-768, which will
create a fourfold increase in the capacity of the network. At the OC-
768 rate, a multiplexing scheme will produce 40 Gbps on a single
fiber (SONET rate), which then can use the DWDM capabilities to
create a throughput of up to four times the 320-Gbps rate currently
available.This equals terabit speeds (1.28 Tbps). However, the use of
spacing on the various wavelengths actually will allow up to 400
Gbps and offers future speeds of up to 1.68 Gbps. OC-768 will also
bring to the table a capability to concatenate the OC-192 levels.
Today’s current technology can use a concatenated OC-48 but stops
there in the bonding of higher speeds. In the future, as Internet ser-
vice providers (ISPs) and traditional telephone carriers need to
expand their capacities, the need to draw a 10-Gbps concatenated
throughput will appear. Today, this is not a big concern, but in three
to five years this will become a commonplace occurrence.Table 5-4 is
a summary of some of the other things that we are expecting to occur
over the next five to 10 years in the area of fiber transport systems.
This table takes advantage of the work being done in many of the
laboratories and research facilities around the world. The ability to
push the envelope is what drives the communications industry.