Wavelength-division multiplexing
[1] This technique enables bidirectional communications over a single strand of fiber (also called wavelength-division duplexing) as well as multiplication of capacity.
In glass fiber, velocity is substantially slower - usually about 0.7 times c. The data rate in practical systems is a fraction of the carrier frequency.
This is often done by the use of optical-to-electrical-to-optical (O/E/O) translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.
[3] Most WDM systems operate on single-mode optical fiber cables which have a core diameter of 9 μm.
Coarse wavelength-division multiplexing (CWDM), in contrast to DWDM, uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs.
Originally, the term coarse wavelength-division multiplexing (CWDM) was fairly generic and described a number of different channel configurations.
In general, the choice of channel spacings and frequency in these configurations precluded the use of erbium doped fiber amplifiers (EDFAs).
Newer fibers which conform to the G.652.C and G.652.D[6] standards, such as Corning SMF-28e and Samsung Widepass, nearly eliminate the water-related attenuation peak at 1383 nm and allow for full operation of all 18 ITU CWDM channels in metropolitan networks.
The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs.
This limits the total CWDM optical span to somewhere near 60 km for a 2.5 Gbit/s signal, suitable for use in metropolitan applications.
Precision temperature control of the laser transmitter is required in DWDM systems to prevent drift off a very narrow frequency window of the order of a few GHz.
These factors of smaller volume and higher performance result in DWDM systems typically being more expensive than CWDM.
Recent innovations in DWDM transport systems include pluggable and software-tunable transceiver modules capable of operating on 40 or 80 channels.
External wavelengths in the 1,550 nm most likely need to be translated, as they almost certainly do not have the required frequency stability tolerances nor the optical power necessary for the system's EDFA.
[14][15] As mentioned above, intermediate optical amplification sites in DWDM systems may allow for the dropping and adding of certain wavelength channels.
In most systems deployed as of August 2006 this is done infrequently, because adding or dropping wavelengths requires manually inserting or replacing wavelength-selective cards.
Numerous technological approaches are utilized for various commercial ROADMs, the tradeoff being between cost, optical power, and flexibility.
Cisco's Enhanced WDM system is a network architecture that combines two different types of multiplexing technologies to transmit data over optical fibers.
In addition to this, C form-factor pluggable modules deliver 100 Gbit/s Ethernet suitable for high-speed Internet backbone connections.
