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I am trying to understand the cross-over point for when I would deploy active DWDM over passive DWDM (as active seems to be more expensive based on my initial research);

  • Passive DWDM takes in various colours / wavelengths and muxes them onto a single fibre pair. This seems to be the same principal as a CWDM mux/demux, however with DWDM we know, we can get a lot more channels. (Also I believe that with CWDM we are using different frequencies within the ITU grid?)

  • Active DWDM can take in 8/16/32 etc channels all on the same wavelength (say 1310nm) and them shift them all to different individual frequencies before muxing them onto a single fibre. They can be demuxed at the other end all back to the same frequency again (say 1310nm).

So am I correct that the reasons for deploying active DWDM over passive are that

  1. Active can provide more channels
  2. Active can use all the same input frequency, thereby any 1310nm device can connect to it, without the need for tuned/coloured optics?

Have I missed any pro's of active over passive? What are the con's?

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    As a side note, if it makes any difference; I am costing up active and passive DWDM units. This isn't the place do discuss price specifics and vendors etc so purely from a planning view; If the cost of some 16 channel passive DWDM units plus coloured/tuned optics (SFPs/SFP+/XFPs etc) is more than the price of an active 16 channel units plus regular optics, is there any reason not to buy the active gear? My concern with the passive is the price and availability of coloured optics is generally higher and longer lead times respectively...
    – jwbensley
    Aug 22 '13 at 13:11
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    ...I know an active unit will fail without power, passive or active though, I would be deploying two units at each end.
    – jwbensley
    Aug 22 '13 at 13:11
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    Usually you'd only go active if distances prohibit passive, if distances allow for passive it'll be order of magnitude cheaper. And yes, active can have (but it's not mandatory) transponder which will accept e.g. 1310nm in. Active also may (but not necessarily) support automatic protection in case of fibre faults.
    – ytti
    Aug 22 '13 at 13:16
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It all really depends on your requirements and current setup. That being said, here are some of the main ones.

Active

Pros:

  1. Active can fit a lot more wavelengths (colors) onto a single fiber pair. The pro being, the composite signal that is sent over a single fiber pair can carry more bandwidth than a passive of the same size could, in turn you don't need as much physical fiber between your two sites (this really only applies if you require that much bandwidth). This is advantageous when distance is a problem because it allows you to get more out of a single leased fiber pair as opposed to passive - see here.

  2. Active setups grant you more control over your optical network, you can dynamically re-tune wavelengths without dropping connections (it's transparent to whatever is riding on that wavelength).

  3. Scalability - Active can be easier to scale as your network grows (you can fit more wavelengths on the fiber, see above), but again - we're talking seriously big iron. I'll dig into it a bit more below.

Cons:

  1. EXPENSIVE - Active DWDM setups are extremely expensive in comparison to passive, so as ytti mentioned, if your distance requirements aren't unreasonable this is not the route to go.

  2. Setup/Configuration - Depending on your vendor, configuration can be a serious undertaking, and require a solid understanding of optical networks. There are many more components in active builds.

Passive

Pros:

  1. Inexpensive (in comparison to active) - As mentioned above, less components are required, and less engineering time is required.

  2. INITIAL Setup - Because of the colored optics there isn't a need to tune wavelengths for all of your connections. It's a matter of matching your colored optics and plugging it.

Cons:

  1. Scalability - Not only are you limited to colored optics, and thus less wavelengths on the transport fiber as you grow you would be require to have more passive devices. Furthermore, with the more passive devices you have the more difficult it can be to manage, you'll have to start managing the same wavelength on multiple passive devices and they could be serving different purposes on each depending on your setup.

  2. Control - If you need to change a wavelength or connection for whatever reason, you're option is limited to taking it out of service and disconnecting the physical cabling as the wavelength is tied to the optic.

Hope this helps.

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  • Great answer Jordan Head, I think all the key points have been covered.
    – jwbensley
    Nov 27 '13 at 12:37
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A big point in favour of active would be OTN (optical transport network) which gives advantages such as FEC (standard Reed Solomon FEC is the equivalent of 6dB gain, other implementations such as Nokia's EFEC2 go even further), performance management stats and additional channels in the overhead used for element management similar to SDH/SONET. This is specified in the ITU's G.709. There's a really interesting primer out there in the Google if you're so inclined.

Another advantage is carrying just two types of optics to cover most scenarios - 1310nm and a tunable DWDM which makes deployment and sparing a lot easier.

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Another advantage of active DWDM vs passive is:

  • dynamic provisioning (no need to go on-site to plug in the right optics)
  • you can do L1 encryption

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