# PMP 450m / 450b on CBRS: Repeated SM Disconnects with 16 Registered Subs on One AP
**URL:** https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478
**Category:** PMP
**Created:** [June 4, 2026, 9:20pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478 "2026-06-04T21:20:45Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@john\_hargrove](https://community.cambiumnetworks.com/u/john_hargrove)
#### Post date: [June 4, 2026, 9:20pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478/1 "2026-06-04T21:20:45Z")
I’m troubleshooting a **PMP 450m AP (3 GHz CBRS)** with **16 registered SMs**. The AP is running **24.1.1** , and the subscribers are **PMP 450b High Gain SMs** on **20.3.1.2**.
Sector details:
- AP model: PMP 450m AP
- Band: 3 GHz CBRS
- Firmware: 24.1.1
- Registered SMs: 16
- CBRS enabled / authorized
Observed behavior:
- Multiple customer disconnects on the sector
- AP events show repeated SM drop/register activity across multiple subscribers
- We have also seen repeated CBRS EIRP change events
- Nearby affected subs are very close to the AP, so this does not appear to be a simple edge-of-cell issue
RF examples from two nearby 450b subs:
- Jennings: DL RSSI mid -30s dBm, UL around -60 dBm, SNR generally in the 30 dB range, DL RSSI imbalance around 2–5 dB
- Arterberry: usable RSSI, but more RF variation and DL RSSI imbalance around 7–10 dB
I reviewed Cambium material and found:
- 450m on 3 GHz supports up to 4 subscribers simultaneously in MU-MIMO / 8 streams
- high frame utilization on 450m does not necessarily mean overload
- overload should be judged from overload statistics, not frame utilization alone
- I also found discussion of the 5 ms / 30–40 MHz deep VC queue scaling issue, but that appears related to frame period/channel width rather than subscriber count
Main question:
**Are there any known stability, scheduler, airtime, registration, or performance concerns specifically when a 3 GHz PMP 450m sector has more than 15 total registered 450b SMs, even if only a subset are active simultaneously?**
Also, if not subscriber-count related, which counters would you consider most authoritative for proving sector stress on 450m:
- Overload stats
- Frame utilization
- Scheduler stats
- Session stats
- Registration failures
- Spatial utilization / sounding-related stats
Thanks.
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### Author:  [@Eric\_Ozrelic](https://community.cambiumnetworks.com/u/Eric_Ozrelic)
#### Post date: [June 4, 2026, 9:52pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478/2 "2026-06-04T21:52:51Z")
Upgrade everything to 25.0.1. Make sure you’re using a valid NTP server on the AP and that the time is correct. Enable LTE coexistence mode on the AP, start with 5ms 75% downlink data, LTE frame config 2, LTE S frame conf 7, LTE cyclic prefix normal.
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### Author:  [@john\_hargrove](https://community.cambiumnetworks.com/u/john_hargrove)
#### Post date: [June 4, 2026, 10:09pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478/3 "2026-06-04T22:09:43Z")
Thanks
any thoughts on growing more subs once we firmware upgrade and tweak the other items?
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### Author:  [@Eric\_Ozrelic](https://community.cambiumnetworks.com/u/Eric_Ozrelic)
#### Post date: [June 4, 2026, 10:21pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478/4 "2026-06-04T22:21:33Z")
Assuming that there’s spectrum available, if you can move up to 30 or 40MHz channel width, that will allow you to load up quite a few more SM’s depending on what speed tiers you’re trying to deliver.
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### Author:  [@john\_hargrove](https://community.cambiumnetworks.com/u/john_hargrove)
#### Post date: [June 5, 2026, 2:00pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478/5 "2026-06-05T14:00:23Z")
We are at 40 Mhz channels now, looks like you are at 106 SMs so that is reassuring
On the Firmware upgrade, do the AP first then the SMs?
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### Author:  [@Eric\_Ozrelic](https://community.cambiumnetworks.com/u/Eric_Ozrelic)
#### Post date: [June 5, 2026, 6:36pm UTC](https://community.cambiumnetworks.com/t/pmp-450m-450b-on-cbrs-repeated-sm-disconnects-with-16-registered-subs-on-one-ap/108478/6 "2026-06-05T18:36:42Z")
Typically with pretty much any radio manufacture’s firmware updates, you update the far radio first, so the SM’s in the case, and then the AP last.