Wavelength-division multiplexing (WDM) has become increasingly popular in fiber-optics communications over the years. The key advantage of this technology is that it supports the transmission of multiple signals on a single fiber allowing networks to expand their capacity without laying more fiber.
The earliest WDM systems were referred to as normal WDM or BWDM (Bi-directional Wavelength Division Multiplexing) as they used two standard wavelengths of 1310nm and 1550nm on a single fiber, one for transmitting and one for receiving. Modern WDM systems operating in the single-mode wavelength range can be divided into three types– CWDM, LWDM and DWDM. However, there are also a few systems which deploy WDM technology for multi-mode wavelengths around 850nm.
CWDM (Coarse Wavelength Division Multiplexing), LWDM (LAN Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing) are all based on the same concept but differ in certain areas such as wavelength spacing, number of channels, transmission distance, power requirements, and of course cost.
What is CWDM?
The acronym, CWDM stands for Coarse Wavelength Division Multiplexing. It uses 18 channels from 1271nm to 1611nm with a channel spacing of 20nm as defined by ITU G.694.2. In typical CWDM systems, transmissions are centered around 8 channels, 1470 /1490 /1510 /1530 /1550 /1570 /1590 /1610. The introduction of single-mode fibers based on G.652.C and G.652.D standards made 10 more channels possible centered on 1270 /1290 /1310 /1330 /1350 /1370 /1390 /1410 /1430 /1450.
Typically, Ethernet transceivers based on CWDM technology have 4 channels in the O-band region and are multiplexed together to be transmitted through a single SMF (single mode fiber) cable.
What is DWDM?
DWDM or Dense Wavelength Division Multiplexing is commonly used in long-haul communication systems (>80km) and backbone networks. With its tight wavelength spacing of 0.8/0.4nm, far more channels (up to 160) can be packed into one fiber. DWDM systems primarily operate in C and L-bands. They require cooled precision lasers so that wavelengths remain stabilized over a wide temperature range.
What is LWDM?
LWDM stands for LAN Wavelength Division Multiplexing. It is basically a subset of the CWDM technology except for the fact that the wavelengths are more tightly spaced and centered around zero-dispersion wavelength of the fiber. This allows transmission for longer distances compared to CWDM as the signal is not severely degraded by chromatic dispersion.
Modern Ethernet transceivers based on LWDM technology have 4 channels in the O-band centered roughly around 1310 nm. Standard LWDM transceivers operate around 4 wavelengths – 1295.56/1300.05/1304.58/1309.14.
CWDM versus DWDM versus LWDM
1.Wavelength Spacing and Number of Channels
One of the key differences of these methodologies is the spacing that each uses to separate the signals they carry. CWDM supports up to 18 channels with different wavelengths, each channel spaced exactly 20nm apart while LWDM supports 4 channels with a ~6nm spacing. DWDM technology is capable of 40, 80, or up to 160 wavelength channels with a spacing of 0.8/0.4nm.
DWDM technology is capable of transmitting data over long distances by keeping wavelengths tightly packed. CWDM systems on the other hand have a shorter reach as the dispersion and attenuation losses are very high at longer distances. One reason for CWDM’s distance limitation is that the wavelengths it carries are not amplified. DWDM connections however can be amplified enabling it to span much longer distances. LWDM systems transmit data up to 10km.
CWDM systems use an uncooled distributed-feedback laser while DWDM systems use a cooled distributed-feedback laser. The difference between having a cooled versus uncooled laser is that DWDM consumes much more power than its uncooled counterpart (CWDM). Although it requires more power, a cooled laser that adapts to temperature variations aids in ensuring better performance, better safety, and a longer lifespan for DWDM systems.
Overall, CWDM systems are less expensive than DWDM systems. One reason for that is the transmitting lasers in CWDM systems need not be that precise and they can allow some drift in the wavelengths. Due to this tolerance, uncooled laser diodes which consume much less power and are relatively inexpensive can be used for CWDM transmission. Since CWDM systems do not require the use of amplifiers (due to shorter reaches), the overall deployment is less complicated and considerably less expensive.
The higher cost of DWDM can be attributed to the fact that it requires cooled temperature-controlled lasers and cooling systems. Further, the necessity for amplification and dispersion compensation for extremely long reach applications also drives up costs.
The choice of CWDM or DWDM or LWDM networks ultimately depends on user requirements and budget. CWDM and LWDM systems are well suited for short reach applications and lower bandwidths. WDM architecture plays an important role in data transport today. DWDM systems enabled by EDFAs can extend data transmission up to 100s of miles.