[BC660K-GL] MQTT Connection Drops & Reconnection Failure During Downlink Storms (Sirens in High-Density Setup)

IoT Modules LPWA Module
1

Dear Quectel Support Team,

We are experiencing MQTT connection issues with the BC660K-GL module integrated into our smart siren devices.

Device Context & Environment:

  • Module / Firmware: Quectel BC660K-GL

  • Power Supply: Main-powered (permanently powered, no sleep/eDRX/PSM restrictions required)

  • Connectivity: Permanently connected via MQTT over NB-IoT (Signal quality is solid, typical RSRP ~ -85 dBm, SINR ~ 8)

Issue Description: When multiple sirens are deployed at the same physical location/cell and a bulk activation command is sent from the server (high-volume downlink traffic to many devices simultaneously):

  1. MQTT Connection Drops: Several devices lose their MQTT connection, triggered by network/socket resets or broker disconnects (+QMTSTAT: 0,1). The logs show a heavy flood of duplicate +QMTRECV URCs within milliseconds due to server-side retries before the modem can send back ACKs.

  2. Re-connection Failure: While most devices reconnect immediately (AT+QMTOPEN / AT+QMTCONN), a subset of modules fails to reconnect altogether and remains offline until a hardware reset or power cycle is performed.

Attached Data: We have attached a detailed modem AT log (BX35_BC660K_1313_20260924_5.txt) capturing the incoming burst, the +QMTSTAT: 0,1 event, and the subsequent behavior.

Our Questions:

  1. Is there a known issue or limit regarding MQTT buffer overflow / internal queueing in firmware version 1.0.8 under heavy downlink URC bursts?

  2. Is a newer firmware release available for the BC660K-GL that improves MQTT packet handling or fixes reconnect failures after a +QMTSTAT: 0,1 event?

  3. What is Quectel’s recommended AT configuration for optimizing MQTT buffer stability and auto-reconnect handling for permanently powered NB-IoT devices?

Thank you in advance for your support!

Best regards,
Philip Wolff

BX35_BC660K_1313_20260924_5.txt (16.6 KB)