So a little more info and some pictures.
First is a block diagram of the connections between the components, illustrating how the system is "wired" up.
On the left we have our fibre pair coming in from another site. On the inwards direction this is fed into a pre-amp where the signal is amplified before being sent in to the WSS. A connection from here to the wave tracker (which is essentially a very basic OSA) allows us to see the power level of all wavelengths that are coming in to this amplifier.
On an extremely long span there may also be a Raman amplifier installed before the pre-amp. In terms of span length, 100km and longer is possible between amplifiers.
The WSS can work a lot of magic. If we want to drop a wave to terminate it at this site, it can be passed out of the DROP port at the bottom which feeds in to the ITLU (interleaver.) The ITLU splits the incoming wavelengths in to odd and even and feeds them into the appropriate splitter where a prism splits each wavelength out on to its appropriate port on the front of the panel.
If we want we can also allow the wavelength to express through this node, in which case it will pass to the THRU OUT port on the right and in to the adjacent degree and out on to the line towards the next site. The THRU ports are used specifically in a 2 degree node that may be mid span where you might need to drop some waves. In a situation where you have a multi-degree node, i.e. 3 or more degrees, you wouldn't use the THRU ports, but there are other ports that allow meshing of all of the degrees to allow waves to be switched individually as required.
In the other direction, waves are fed in to the top splitters which feed in to the WSS, which can switch them in various directions. One of which may be the SIG OUT direction, in which case the signals are sent through the post-amp and out on to the fibre to the adjacent site. We also have a feed in to the wave tracker here, so we can see the power levels of all outgoing channels. The equipment is quite good in that we can see the power levels of signals and channels in various stages through the system for troubleshooting etc.
As I understand it there is one limitation, in that you cant pick up a wave on one degree and send it through the THRU port to go out on to the line via another degree. Each degree should have its own set of splitters which will accept waves to be sent out in that direction.
With this diagram, if you combine two of these together, linked via the THRU ports, you have a 2 degree node, something that could sit mid-span on a long haul route, or in this case, in a ring.
Next is a picture of the degree wired up. I took this earlier in the week, have been busy with testing and commissioning.
A couple of notes about the picture and optics in general:
The two amps on the left have a couple of small patch leads at the top. These are actually the OSC (Optical Supervisory Channel) connections that link this node to the node in the opposing site connected via this degree. The OSC connection is an STM-1 in this case (but can also be 100BaseFX), and OSPF is enabled on. This means if there is a particular failure mode affecting management connectivity directly to this node, traffic can be routed via another node and over the OSC so you dont lose connectivity to the node.
There is also a very tiny loop on the post-amp, this is the "DCM" in this case. I am not sure of the particular reason why a "proper" DCM module is not used, the guys that designed it I guess figured that for a metro system it wasnt necessary. Im not an optical guy, so Im not even going to try and understand the mechanics behind it...
DCM is Dispersion Compensation Module, and basically it works like this. Different wavelengths travel at different speeds through fibre. If you transmit a pulse of light a the same time on 3 different wavelengths, then over distance what you would see is that one of them arrives before another, followed closely behind by the 3rd, and the pulses will have "stretched". Consider that high frequency pulses of light that start stretching, blurring, in to each other are not good for recovery of the original signal that was transmitted. Dispersion compensation corrects these effects so that by the time the signal is received its "almost brand new." Some very long fibre spans, e.g. submarine cables, use different types of fibre spliced together to counteract the effects en route, rather than using (but possibly in conjunction with) DCMs (which are basically just a long loop of fibre in a small module/box/cartridge) on either side. Simple explanation, but hopefully suffices (someone correct me if Im wrong!)
And in terms of amplification, it is not possible to simply amplify a signal infinitely. Different signals, usually governed by speed, need to be regenerated every so often. A 1Gbit signal can be amplified and transported much further than a 10Gbit signal, which will go further than 40 or 100Gbit. Each time a signal is amplified, noise is introduced which over time will begin to reduce the optical SNR, and once SNR gets too low it starts to become difficult to determine what the original signal was, and you end up with errors. Think of it like trying to listen to someone speak in a room that is gradually filling with people.
And finally, the black cable coming out of the top of the WSS unit is the inventory cable connecting to the ITLU at the bottom. An inventory cable then runs from the ITLU to the top splitter shelf, and then a 3rd from that splitter shelf to the bottom one. I havent asked exactly what the inventory connection does, or what kind of things happen over it, but I do know that it allows the system to "see" those particular devices, so I am guessing that there are some smarts involved with it.
And youre not seeing things, but none of the cards were plugged all the way in. When I took this photo (and the ones before) we were still in the early stages of initial commissioning, which involved upgrading the software on the first controller card, and then allowing the software to be upgraded on all the other shelves controller cards before we went and complicated things with lots of other modules.
Its a very precise step by step process, its not a plug and play environment!