Passive Wave Primer

Dear Network Gurus,

Looking for a tutorial on passive waves. How it works. Pros and cons. .




Essentially, you're providing a channel off of your DWDM filters for someone else to pass light.

Commonly in the market, a "wavelength" product generally isn't a true wavelength, especially on long-haul segments.
The 'wavelength' market really is an evolution of the old SONET market in some ways -- carriers will typically light a channel (either in fixed grid filter or flex grid) and that single channel is usually an X-gigabaud (e.g. 35-95Gbd) that uses coherent modulation on line side for say 200-800Gbps and multiplexing for tributary channels (such as TDM) on client side ports to break away a 100GE circuit for the customer end-user.

As far as technicalities are concerned, most 'wavelength' products that behave as described above, ought to be called "dedicated circuits" or "circuit-switched transport" if we're anal about its operating principles.

As for 'true' wavelength service, that brings us to your question:

When you're talking about passive wave or 'alien wave', what you're doing is you're providing a wavelength frequency assignment on your photonic filter system (a channel on your 100 Ghz fixed grid DWDM filter, or bandwidth assignment window on your flex grid ROADM) to the customer, which would typically be another network provider, or a very clued enterprise customer that wants to run his own optical transport but can't justify the economics of full dark fiber over the said span, and doesn't need more than <=95Gbd max of modulation bandwidth.

The customer would pass traffic similarly to how you yourself would light a channel, installing a coherent transponder for 200-800Gbps wave facing the line side, and breaking it out to Nx100GE for end-user traffic.


What is the difference between a normal wave and a alien wave?

An Alien wave comes in from an external source, for an example a customer has WDM optics in their kit. A normal wave the “customer” connects with a normal 10GE/100GE (or whatever is appropriate) and a line card on the OTN platform “grooms” that to the appropriate WDM channel.

If you're talking about what I think you are, the term the folks who make the transport gear seem to use is "spectrum" as in you (as service provider) sell your customer some portion of the WDM transport spectrum. It typically comes in 50GHz increments or sometimes 100GHz corresponding to the standard ITU channel system but not always.

To the service provider, this is then an "alien wave" in that they have essentially no control or visibility into it other than light shows up, and they're responsible for getting it to the other end of the path with acceptable path characteristics.

I have yet to find a service provider that is actually willing to sell this even when they have it in their service offering catalog. The difficulties of coordinating everything with the customer are so extreme that it seems to usually make sense to either lease the customer dark fiber or capitalize the transponders needed to carry it as a managed wave on the provider's transport system. Might make sense if you literally want half the spectrum on a long-haul span or something.

We sell some wavelengths on passive CWDM/DWDM path's between Datacentres
(less than 80Km) to customers to spread the cost of leasing the dark fibre.
But yes, as far as long distance (apart from bespoke offerings) I'm yet to
see a productised alien wave service. If you are spending all that money on
OTN kit the extra cost of the transponders is not really significant I

I know there are some European carriers that offer this as a fully productized service, Colt and euNetworks come to mind.

Best regards,

It seems incredibly simple to do, depending on the capabilities of your platform.

What am I missing?

It's in some providers' catalogs (amazingly) but they seem loathe to actually sell it when asked. Even then, you're spot on with the description of "bespoke". I saw Zayo's product description sheet for it, and, while it did seem to check the boxes I'd expect, it was also quite expectedly very complex (they also wouldn't actually sell it to me).

At less than 80km, you don't have to worry so much about balancing light levels, etc. as you can usually just throw things straight into a mux/demux on each end and rely on the power budget of the transceiver itself, so that makes sense for cheap DCI.

Thanks for the explanation, I always thought ‘waves’ were ‘alien waves’ I guess, I thought you had to coordinate the channel and you used wdm optics, I didn’t realize they normally are provisioned with ethernet to a OTN then get channelized, good info.

If the span between the mux/demux pair is entirely passive, it's fairly straightforward. That's going to limit distances to around 80km or so with conventional systems or maybe 120km with systems designed entirely around modern coherent optics.

If there are photonic devices in the span, you now have customer-supplied light being part of the rainbow that those photonics have to handle. Balancing things at amplifiers requires careful coordination with the customer (or adding a separate managed/monitored VOA for each alien wave which somewhat defeats the point). You end up with a scenario where a customer can do something screwy and potentially affect other waves on potentially multiple spans which your big-name carriers are obviously completely freaked out by.

It's obviously possible, but the operational headache seems large enough that the major mid-haul and long-haul carriers I've talked to (all North America and all midwest, for that matter), don't seem to want to sell it despite all the major optical transport platform vendors not just supporting it by heavily pushing it.

I really do hope it becomes a real product that I (as a smaller, local island operator) can buy, but it just doesn't seem to be there yet at least in my region.

From the perspective of a large carrier, spectrum is an operational nightmare. At a former $dayjob it was an “offering” in the sense that we had deployed it, told customers we offered it but wouldn’t actually deploy it anymore.

Logistically there are a lot of potential points of failure once you got beyond the distance threshold where mid-line amps would be required, and as alluded to here, no big carrier would want to take on risk to the network that they’re not in control of. There’s not much sense in doing it in shorter-distance scenarios when most folks needing enough bandwidth to even have the conversation are going to be able to run their own optical systems across dark fiber at that distance anyway. The customers that we talked to about it were almost exclusively other carriers that wanted to use the muxes they had in inventory (aka not the same platform as the photonic layer) without the burden of deploying/managing a lot of amps but had no issue taking OTN or even LAN PHY in bulk when push came to shove.

FWIW, and I understand these terms have become fungible over time, the scenario above is spectrum from the perspective of the photonic layer owner and alien wave from the perspective of the customer. As the photonic layer owner, alien wave would generally be thought of as 1) accepting a handoff as a WDM-specific channel of light, whether on fixed or tunable optics, not a standard 1310/1550, and, typically but not necessarily, 2) accepting a signal that is framed as OTN, not LAN PHY or WAN PHY.

For the record, the OP’s query about passive wave would suggest PON/GPON or similar low-power CWDM for short-haul use, and not spectrum or alien wave, both of which are decidedly non-passive.

Dave Cohen

+1 on this – we use it internally to manage two optical platforms (international vs local) and even that has caused some challenges!