WDM
Last updated
Last updated
WDM (Wavelength Division Multiplexing) is used to pass multiple carrier connections onto a single fiber, by differentiating each connection with its own wavelength frequency. Because the signal is light, this wavelength is quite literally a color. For this reason, diagrams often depict each different frequency as a separate color.
In the above diagram, four separate links are multiplexed onto a single fiber. Each link has its own color, or wavelength. A multiplexer (or “MUX”) joins all signals onto the fiber, and a demulitplexer (or “DEMUX”) separates each signal and splits them into their own, separate, fiber links.
There are two main variations of WDM you see today: CWDM and DWDM.
CWDM is Coarse WDM, and typically has only 8 channels. Some equipment supports up 18 channels. These channels are separated very far apart, at 20nm. In comparison, DWDM (Dense WDM) spaces channels out at .4nm or .8nm
As seen above, channels in the range 1470-1610 are most commonly used, but 1270-1450 is also available, yeilding an additional 10 available channels for a total of 18 channels.
CWDM is limited to around 60km in distance, as the frequencies used don’t lend themselves to amplification to achieve a farther distance. We will see that DWDM can use amplification.
CWDM systems are typically less expensive than DWDM at 1Gbps capabilities. But at 10Gbps the cost difference is less, and typically you will choose DWDM for the additional capabilities.
Channel numbers for CWDM are represented as the middle two integers in the wavelength measurement. For example, 1530nm is called channel 53, and 1550nm is channel 55, etc.
When using a single fiber cable for CWDM, you are effecitively limited to N/2 duplex point-to-point connections, where N is the number of channels. This is because each point-to-point connection needs one channel for transmitting and one channel for receiving.
For example, in the diagram below an 8channel CWDM mux/demux is used. Each SFP pair transmits/receives on the opposite wavelength as the “partner” SFP on the other end. The first connection uses 1270nm to transmit at Site A, and the other transceiver receives at 1270nm. The Site B transceiver transmits at 1290nm and the transceiver at Site A receives at 1290nm. This creates multiple full duplex connections out of a single fiber.
Sixteen channels are needed to create eight duplex point-to-point connections over a single fiber.
Because the light is already filtered at the correct wavelength by the demux, a duplex SFP will typically receive on any wavelength, not a filtered wavelength. For example, a 1270nm (Channel 27) CWDM duplex SFP will transmit at 1270nm on the transmit laser, but receive on any wavelength on the receive laser. The receive wavelength will happen to be 1290nm, because each channel typically operates in a pair (27/29, 31/33, etc). The SFP is denoted by its transmit wavelength in CWDM.
For example, you would likely use these two transceivers as a pair:
Although I believe it is only convention to pair wavelengths like this. Nothing should stop you from pairing 1430 and 1570 if you wanted to. You can just plug in the correct port on the mux/demux into the correct transmit/receive ports on the transceivers.
DWDM is Dense WDM, and has up to 160 channels. Normally only 80 channels are used in the C-band, which is in the range of 1530-1565nm. A 40 channel DWDM system spaces channels at 100GHz in the C-band, or approximately .8nm apart. An 80 channel system spaces channels at 50GHz, or approximately .4nm apart. The L-Band, in the range of 1570-1610nm, is available for use as well, but it is much more common to see the C-band used.
The C-band and L-band lend themselves to amplification using EDFAs (Erbium Doped Fiber Amplifiers). This allows you to extend a signal up to 1500km with the use of amplifiers placed along the path! After 1500km you need to electrically regenerate the signal.
DWDM also allows for a channel to use 40Gbps and 100Gbps optics, while CWDM tops out at 10Gbps.
The principals of mux/demux are the same as we saw for CWDM, just operating at much more narrow band widths. DWDM equipment needs to be much more precise, to ensure that the wavelength doesn’t “drift” too much, because channels are so close together.
An OADM allows you to remove and add wavelengths in the “middle” of a fiber run. For example, imagine you have the following topology:
Instead of connecting R1 to R2, and R2 to R3, you want to connect all routers in a full mesh so that failure of R2 does not result in R1 being unable to reach R3. Using an OADM in the middle of the fiber run, you can create a full mesh.
You can use six channels to create a full mesh as follow:
The OADM in the middle adds channels 18 and 21 to the fiber path, and removes channels 17 and 22.
This ceates the following logical topology:
The transmit channel is labeled on each interface
You may also see OADMs called ROADMs, which stands for Reconfigurable OADM. This allows you to configure which channels and added and dropped.
An active mux/demux uses power to provide multiplexing/demultiplexing. These typically have a management interface and are more expensive. You will often see active technology for DWDM.
A passive mux/demux uses no power at all, which allows for a lot of flexibility. The device uses prisms to filter the wavelengths of the laser onto different paths. CWDM mux/demuxs are commonly passive.
