# 450 SM modulation

**URL:** <https://community.cambiumnetworks.com/t/450-sm-modulation/39136>\
**Category:** PMP\
**Created:** [December 5, 2014, 3:33am UTC](https://community.cambiumnetworks.com/t/450-sm-modulation/39136 "2014-12-05T03:33:22Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Nathan\_Dothager](https://d10gw3vjmzyp8.cloudfront.net/user_avatar/community.cambiumnetworks.com/nathan_dothager/32/11743_2.png) [@Nathan\_Dothager](https://community.cambiumnetworks.com/u/Nathan_Dothager)\
**Post date:** [December 5, 2014, 3:33am UTC](https://community.cambiumnetworks.com/t/450-sm-modulation/39136/1 "2014-12-05T03:33:22Z")

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I'm seeing that on our 3.65 PMP450 SM's we cannot obtain over a 25% 256-QAM score no matter how close or clean the signal. &nbsp;Link efficiency is near 100% too. &nbsp;Is this the highest this rating can go on an SM in the "Receive Fragments &nbsp;Modulation" section under the Alignment Tool?&nbsp;

Currently both A and B paths show: QPSK: 25% , 16 QAM: 25%, 64 -QAM: 25%, 256- QAM: 25% &nbsp;(does this mean it is 100% 256-QAM?)

But...Shouldn't a link like I've described be doing something like 90% 256-QAM, 9% 64-QAM, 1% 16 -QAM??

Anyone?

Nathan

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**Author:** ![rylan](https://d10gw3vjmzyp8.cloudfront.net/letter_avatar_proxy/v4/letter/r/97f17d/32.png) [@rylan](https://community.cambiumnetworks.com/u/rylan)\
**Post date:** [December 5, 2014, 6:20pm UTC](https://community.cambiumnetworks.com/t/450-sm-modulation/39136/2 "2014-12-05T18:20:32Z")

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You have a good link. I can understand the confusion. If you have a good 256 QAM link all of the data is being sent at 256 qam and the percentages will be 25 fo all modulations.

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**Author:** ![CambiumMatt](https://d10gw3vjmzyp8.cloudfront.net/user_avatar/community.cambiumnetworks.com/cambiummatt/32/33533_2.png) [@CambiumMatt](https://community.cambiumnetworks.com/u/CambiumMatt)\
**Post date:** [December 6, 2014, 12:39pm UTC](https://community.cambiumnetworks.com/t/450-sm-modulation/39136/3 "2014-12-06T12:39:57Z")

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Yes Nathan, I will agree with Rylan... that's about the healthiest link you can have.

Unfortunately, the way the modulation breakdown is reported is a bit confusing. When a fragment is received in 256-QAM modulation, it is&nbsp;_also_ counted in the lower modulations. This has to do with how OFDM technology works, and the constellation diagram of the OFDM signal.

To give a few examples, a perfect 8x link (i.e. a link that is able to achieve and maintain 256-QAM modulation) will report:

256-QAM: 25%

64-QAM: 25%

16-QAM: 25%

QPSK: 25%&nbsp;

just as you describe.

A link that cannot achieve 8x, but is a solid 6x link will show:

256-QAM: \<1%

64-QAM: 33%

16-QAM: 33%

QPSK: 33%

In real world deployments, it would be common to see something like:

256-QAM: \<1%

64-QAM: 14%

16-QAM: 42%

QPSK: 44%

which would indicate that the majority of the time, this link transmits at 4X (16-QAM), but sometimes is able to maintain 6X modulation... hope this explanation helps.