WDM is WDM is WDM

There is much confusion about WDM (Wave Division Multiplexing). Questions arise constantly about CWDM vs DWDM and Active vs Passive WDM. The subject is often further confused by many systems adding functionality such as Amplification, Optical Switching, Muxponding and a whole host of other transport needs that are not strictly WDM.

This document will try to explain the main discussion points, but WDM is WDM with wavelengths (lambdas) defined a long time ago by the ITU.

There are 18 CWDM lambdas & 100 in C & L band DWDM lambdas.

As all these lambdas are clearly defined and used by ALL WDM vendors, they should all work, irrespective of the WDM vendor chosen. The speed and distance are a function of the transceiver (EDFA) used NOT the WDM system.

So when you ask for a WDM system, which one do you need for your access network?

There is a growing move toward unbundling traditional metro transport. Why pay for complex functions such as Transponders, ROADM, Amplification, Muxponding, complex FEC algorithms, when a simple WDM system may meet your needs?

Whether the metro network is used to backhaul mobile (LTE, 5G), residential broadband connections, higher speed DSL/PON traffic, transporting enterprise customer's traffic or a mixture of all, the bottleneck in the metro access backhaul is an increasing challenge for any service provider or neutral host.

Regardless of data rate and protocol, the transport of this data over "typical" metro distances, which can be maybe ~40km (can be longer), can be done by either an Active or a Passive WDM solution.

Active WDM

With active WDM the signal generation (e.g. Ethernet Switch, MSAN or cell base station) and signal transport are physically separated via a transponder. The data signal will then be sent via a client transceiver to the client-side system. A corresponding transceiver (in terms of protocol, data rate and media type) is used to communicate to the end equipment.

The transport system is based on transponder cards hosting the client transceivers from which the data signal will be O-E-O (Optic-Electronic-Optic) concerted to line side transceivers. If more than one line signal is generated the line side transceiver signals are then multiplexed via an Optical CWDM or DWDM mux to the transport fibre.

The transponder cards and transceivers will need to be hosted and powered a chassis, often with redundant power supplies. As this is active equipment it must be managed remotely, requiring the additional overhead of a Management Card. Further, this management feature may need to be integrated into your umbrella network management system.

If you change the data rate needed, eg from 1G to 10G or 10G to 100G, then you will have to change the corresponding transponder.

Passive WDM

A much simpler and therefore lower cost method of optical transport is passive WDM. A passive WDM system uses the same lambdas as an active system, but any transceiver is plugged directly into the end equipment, eg switch/router, and does not perform any conversion of the signal. This is not always possible, especially if you do not own the switch/router.

However, a passive system does not need expensive transponder cards, power supplies, management cards etc.

Amplification, dispersion compensation or optical redundancy can still be added as external devices if needed. If monitoring is needed, then simple monitor ports can be added to any passive WDM device.

In addition to the power, cost and complexity savings a passive WDM solution provides, it also significantly reduces latency in your network.

A passive WDM system is agnostic to the speed of the transceiver. This means that you can use the same passive WDM system for 1G, 10G, 100G! All dependant on the transceiver used.

Example

There are many pluses and minuses for either Passive or Active WDM systems, but you can mix Passive and Active WDM systems. For example, you could use a good quality 40ch Mux/Demux, run say 20 x 10G AND also 2 x 100G services. Due to the limitations of 100G DWDM transceivers, you may have to add amplification and/or Dispersion Compensation. The 10G services would be plugged directly into the switch/router without needing a transponder.

Conclusion

There a many reasons where you might wish to use an Active or Passive WDM system. 

If you need to offer a clear demarcation point on your network, or if the end equipment does not support the transceiver needed, then an active WDM makes perfect sense.

A Passive WDM system might be deployed in remote or harsh environments, often requiring high density, so can be much more suitable than an active system.

This post does not answer all the questions regarding WDM. There is no right or wrong answer. Passive or Active WDM? It all depends on your needs.

PS - Passive WDM often being used in #5G #fronthaul

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