OpenWRT vs RM520N Architecture Differences
Engineering ledger detailing the differences between the RM551E OpenWRT build and SDXLEMUR on-modem Linux.
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OpenWRT vs RM520N Architecture Differences
QManager is ported across two distinctly different Linux environments. This document outlines the technical design divergence.
Architectural Divergence Summary#
| Subsystem | QManager (OpenWRT) | QManager-RM520N (SDXLEMUR) |
|---|---|---|
| Init / Service Manager | OpenWRT procd | Systemd (systemctl) |
| Network Daemon | netifd + OpenWRT uci | Direct iproute2 / Qualcomm proprietary daemons |
| Firewall | fw4 (nftables) or fw3 (iptables) | Native iptables / ip6tables |
| Package Management | opkg | Entware (opkg pointing to Entware armv7sf-k3.2 repo) |
| Web Server | uhttpd | lighttpd with mod_cgi |
| File Paths | Web files in /www/qmanager | Web files in /usrdata/qmanager/www |
| Serial AT Interface | /dev/ttyUSB2 or /dev/cdc-wdm0 | /dev/mhi_DUN or /dev/smd11 (internal IPC bus) |
Subsystem-by-Subsystem Comparison#
Serial Port Handling#
On the OpenWRT build, the modem communicates across a USB bus using the option or qmi_wwan kernel drivers. Latency is ~5–10ms per AT command.
On the RM520N-GL internal Linux, the application communicates with the baseband through internal Qualcomm shared memory devices (/dev/mhi_DUN or SMD channels) with near-zero (< 1ms) latency!
Sysfs vs AT Fetch Sources#
On the RM520N-GL, many network stats do not require AT commands at all. They can be read directly from Qualcomm sysfs nodes:
/sys/devices/platform/soc/...- Kernel thermal sensors in
/sys/class/thermal/thermal_zone* - RGMII PHY link state in
/sys/class/net/eth0/carrier