I would like to clarify something regarding WDM/DWDM.
Normal switches won’t multiplex signals at L1. However, you can actually buy a switch and SFPs that can perform WDM and increase your throughput that way, right? In this case, do both the switch and the SFPs need to support it?
In that case, I am wondering, are the wavelengths tuned manually or automatically? Like how do you tell the transmitter at which wavelength it should transmit data at?
And one final thing, why is wavelength used to differentiate between fiber optic signals and not frequency? Ever since I studied Wireless, it was always the frequency that allowed you to differentiate between 2.4GHz, 5GHz and so on.
As you correctly stated, WDM/DWDM is a Layer 1 methodology, functioning at the physical layer. The operation of WDM/DWDM depends entirely on the SFP being used. With the use of an SFP, the physical layer technologies have been completely decoupled from the switch operations. Unlike a copper RJ-45 Ethernet connection, where the switch itself performs the transmission of electrical signals, SFP modules have completely separated the physical layer operation from higher-layer functions.
So the switch itself doesn’t have to support WDM, it just has to support the SFP module being used. In this way, the switch is completely oblivious to the use of any WDM/DWDM on the fiber link. Not only that, but there is no way to affect the physical layer operation of the SFP from any configuration applied on the switch!
Does the use of WDM/DWM affect the throughput on the switch? Well, because switches use standardized Ethernet with fixed speeds of 100/1000 Mbps and 2.5/5/10/25/40/100 Gbps (and so on), the use of WDM/DWDM doesn’t directly affect the resulting throughput of the switch. 10Gbps will still be 10 Gbps whether using WDM or another technology. But the designers of SFPs may use WDM/DWDM to achieve the Ethernet speeds for which the SFPs have been designed.
This depends on the type of optics used by the SFP.
Fixed-wavelength optics are the most common and the cheapest. These are manufactured to transmit a specific wavelength that cannot be changed.
Tunable DWDM optics can be used that can transmit at different channels within a range. On more advanced platforms, this is one of the only Layer 1 characteristics that can be directly configured using the CLI. More info can be found here.
Some modern optics support auto-negotiation, which is typically found in carrier-grade DWDM systems
Ah, we’re getting down to the physics of the issue. Remember that wavelength and frequency are directly related using the formula c = λ × f where
λ = wavelength
f = frequency
c = speed of light
So values in wavelength have equivalents in frequency. For example:
1550nm wavelength → 193.5 THz frequency
1310nm wavelength → 228.8 THz frequency
Why are wavelengths used for fiber? There are several reasons. One is that the numbers in nanometers are easier to work with than huge THz frequency values. Another reason is that the physics of fiber optics typically uses wavelength in its calculations. Attenuation and dispersion, which are characteristics of fiber, are typically plotted versus wavelength. So in that sense, it’s a matter of convention.
I always say “one last question” and then end up thinking of a lot more, so here is one final question
SPCOR happens to mention DWDM, so I have to also understand how DWDM works. From my understanding, the wavelength is split into different channels (I often compare this to WiFi channels if that’s right). These channels basically contain a range of wavelengths that the transmitter can transmit at.
From my understanding, the channels in DWDM are shorter and less spaced? In other words, one channel doesn’t take up as much wavelength as in WDM.
So you end up with more channels
A transmitter transmitting at, say DWDM Channel 1 won’t take up as much wavelength as someone transmitting at WDM Channel 1, since the channels are shorter
This effectively means that shorter channels = more channels that don’t interfere with eachother and more channels = more unique wavelengths to transmit at (without overlap) = more data can be carried over the fiber link but at the cost of even more complex and expensive electronics.
And by the way, sorry if I end up making an error somewhere, I literally see the website like this for whatever reason
For all the real telecom and networking engineers out there, there is no such thing as “one last question” Keep them coming…
Yes. From a physics point of view, remember that the full spectrum of EM radiation includes radio waves, which are the part of the spectrum that Wi-Fi uses, as well as visible (or near visible) light, which is what we see, and what is actually sent through fiber cables. So the principle is similar to Wi-Fi. The only difference is that EM waves at the frequencies sent over fiber cables require a conduit through which to travel. They cannot travel very far in free space.
Yes, that is correct. DWDM has closer spacing and more channels, resulting in higher throughput. Channel spacing for WDM is around 20 nm, allowing for between 8 and 18 channels, while DWDM channels have a spacing between 0.4 and 0.8 nm with up to 120+ channels! WDM is mostly passive, with mux/demux filters and basic optical transceivers. DWDM is more sophisticated with active components and even software-configurable components. It includes tunable lasers, amplifiers, dispersion compensators, and other mechanisms that are used to make it work.
As for the way you see the website, I’ve informed @ReneMolenaar and he’ll take a look…
Keep them coming… 