Wavelength Division and Dense Wavelength-Division Multiplexing

Wavelength-division multiplexing (WDM) was introduced in 1994. WDM is defined as
using channel spacing of 3 nanometers (375 GHz). Dense wavelength-division multiplexing
(DWDM) is defined as 1 nm or less (<125 GHz). The ITU specifies 100 and 50
GHz channel bandwidths and will likely specify 25 GHz spacing as device technologies
mature. The two principal allocations are as follows:
 C band. 1530 to 1570 nm
 L band. 1570 to 1610 nm
This translates as 80 nanometers, 200 × 0.4 nm (50 GHz) channels, and 10,000 GHz.
Table 13.3 defines the ETSI (European Telecommunications Standardization Institute)
SDH and ANSI SONET bit rates (SDH is Synchronous Digital Hierarchy; SONET
is Synchronous Optical Network). Table 13.4 defines the SONET and SDH Synchronous
Transfer Mode (STM) optical carrier (OC) bit rates for dense wavelength-division
multiplexers.

The chromatic dispersion of four wavelengths at 2.5 Gbps is 16 times less than a single
wavelength transmitting at 10 Gbps and, therefore, provides a more robust channel
(needs less regenerators). Rather like OFDM, it is better to have a larger number of
slower bit rate optical channels.
However, these are synchronous channels and the offered traffic is increasingly
asynchronous. Physical layer (Layer 1) traffic patterns are changing in the order of
nanoseconds or microseconds, whereas the optical cross-connects (the only existing
mechanisms we have to switch in additional optical channels or switch out channels)
take a millisecond to switch.
Buffer design either side of the optical domain is therefore very critical, particularly
in long-haul fiber. 330