Page 40 - Optical Switching And Networking Handbook

Introduction to Optical Communications 25

multiplexing (WDM) suddenly became the big-ticket item. Man-
agers, designers, and engineers alike saw the benefit that multiple
wavelengths could add to the capacity of fiber-based networks. Many
colors of light increase the capacity of the installed fiber to 320 Gbps
4
5
and in the future 1.6 terabits per second (Tbps) and beyond. Theo-
retical limits of fiber today are around 30 to 40 Tbps, but with some
changes, in the future we may see 100-Tbps possibilities.
SONET and SDH standards were designed originally for the TDM
systems prevalent in the 1980s. Using TDM, a data stream at a
higher bit rate is generated directly by multiplexing lower-bit-rate
channels. High-capacity TDM systems operate at levels up to OC-
192, or 10 Gbps. The problem comes with moving to higher band-
width speeds at OC-768 and above. Current TDM equipment has
trouble operating at these higher speeds.
WDM, in contrast, can carry multiple data bit rates, enabling mul-
tiple channels to be carried on a single fiber. The technique quite lit-
erally uses different colors of light down the same fiber to carry
different channels of information, which are then separated out at
the distant end by a receiver that identifies each color. All optical
networks employing WDM with add/drop multiplexers and cross-
connects permit this. Dense WDM (DWDM) systems multiplex 32 or
more wavelengths in the 1550-nanometer range, increase capacity
on existing fiber, and are data-rate-transparent.
DWDM ring systems can be connected with Asynchronous Trans-
fer Mode (ATM) switches and Internet Protocol (IP) routers. ATM
networks are expected to use SONET/SDH physical-layer interfaces
with OC-12 add/drop multiplexers. ATM can carry voice, video,
and data communications in the same transport and switching
equipment.

4 Although I refer to many colors of light, the two primary lasers used are red and blue,
with variations on the wavelength of each.Variations can be considered like shades of
red and shades of blue.
5 Terabits per second is trillions of bits per second.