I'm assuming you're asking - "why don't we use Dispersion Compensator Modules at In-line Amplifier sites?"
Off hand - I can't think of a reason why you couldn't, but honestly I can't recall a system that didn't also have some kind of O-E-O regeneration at the same sites that also had DCMs.
I'm not sure what product you're using, but the Ciena 6500s show Pads as DCMs in the GUI also, and those of course can be used at any site regardless of whether it's an ILA or ROADM or terminating site.
As always - I'd let the Ciena Engineering team make the final recommendation on whether a particular span requires DCMs or not.
First, I will add some context for those not familiar with dispersion compensation. When a wavelength travels through a fiber, it not only experiences loss in intensity (optical power) but also may experience other transmission impairments such a chromatic dispersion (CD), polarization mode dispersion (PMD) and or self phase modulation (SPM). (When travelling with other wavelngths, it may experience 4-wave mxing (4WM) and cross-phase modulation (XPM) For short distances, (e.g. a 10G wavelength travelling less than 300 km - distance varies with fiber type), it is possible the CD does not require compensation (i.e the accumulated impairment is tolerable.) When the impairment is too great then the periodic use of DCMs (e.g. every 5 amplifier sites) can be employed to overcome this impairment. However, DCMs are costly and need to be matched to the CD experienced. (however, it does not need to match perfectly but it does need to be close) (An alternate solution would be to take a page from the submarine operators. Older cables system were built by alternating fibers with different dispersion slopes (positive and negative) effectively cancelling out the CD impairment.)
In order to simplify the deployment of longer reaches, Ciena created its WaveLogic brand of optical modems. The modems no longer depend upon the need to deploy DCMs to overcome CD. Ciena's first WaveLogic modem operated at 10G. It was not coherent but had the ability to pre-compensate for CD the wavelength in the transmission fiber. The 40G and greater capacity modems that Ciena sells uses coherent transmission technology. It not only compensates for CD but also PMD. (WaveLogic 2, 3 and Ai) I suggest you read the answers about coherent technology in this post. What is coherent technology?
It you are not using coherent modems or Ciena WaveLogic modems for longer amplified reaches (e.g. XFP @ 10G), then you may need to compensate for CD. Ciena's One Planner Optical Modelling tool can help you determine the ideal placement of DCMs. Most Ciena customers no longer deploy DCMs as it is easier to deploy WaveLogic modems to deliver the maximum capacity and reach without the need to engineer and deploy DCMs.
Thank you for information.
Yes, thank you Edward and Gary!!
Gary nailed it. As an SE, we place DCMs in the "most proper" location. Sometimes that IS at ILAs, but it may be at terminal nodes instead / as well. It all depends on link specs, amplification requirements, and additional co-propagating signals. For example, while many 10G only networks try to zero compensate all spans, many coherent signals prefer a single large DCMs over numerous smaller valued DCMs within individual spans.
In summary, there is no single answer to you question, rather designs are done on a case-by-case basis.
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