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MODEM-DATA-PATH.md

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MODEM-DATA-PATH.md

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Modem data path: RNDIS / QMI / PPP

The LTE Modem page (/ltemodem) can carry IP data over three paths, chosen with the Data path mode selector. AT status polling (signal, registration, operator, IP) is the same in every mode; only the data call differs.

ModemManager is still never used in any mode. It auto-probes serial ports and can seize the flight-controller UART, so QMI uses libqmi and PPP uses pppd directly. flightController.js still hard-errors if ModemManager is installed.

Mode Interface How the data call is made Packages
RNDIS (default) usb0 AT$QCRMCALL=1,1 over the AT port none (built-in)
QMI wwan0 qmicli --wds-start-network + DHCP (udhcpc) libqmi-utils
PPP ppp0 pppd dials *99# via a chat script ppp

Set Data network interface to match the mode (usb0 / wwan0 / ppp0) so usage accounting and the connection test watch the right interface.

Connect / Disconnect

The Status section has Connect (uses the configured mode) and Disconnect:

  • RNDIS: AT$QCRMCALL=1,1 / AT$QCRMCALL=0,1
  • QMI: qmicli --device-open-proxy --wds-start-network (handle/CID captured) / --wds-stop-network. The --device-open-proxy flag routes every qmicli call through the qmi-proxy so it shares the cdc-wdm device with the kernel (and other calls) — without it a second opener gets a CID-allocation timeout, and the kept --client-no-release-cid only survives between the start and stop invocations when both go through the proxy.
  • PPP: pppd … connect "chat … ATD*99#" / poff

Auto-reconnect (registered but no IP) uses the same mode-aware path.

PPP and the flight-controller UART

PPP dials on PPP port (or the AT port if blank). The server refuses to dial the flight-controller's serial port (checked against flightcontroller.activeDevice), so a PPP data call can never disrupt the FC link.

API

Endpoint Method Notes
/api/ltemodemmodify POST now also takes dataPathMode (rndis/qmi/ppp), qmiDevice, pppPort, pppBaud
/api/ltemodemconnect POST bring the data call up using the configured mode
/api/ltemodemdisconnect POST bring the data call down

server/ltemodem.js routes every QMI/PPP shell-out through one _exec seam, so the wrappers are unit-tested via fake binaries with no hardware. The real qmicli/udhcpc/pppd round-trips are verified on-device.

Privileges

/dev/cdc-wdm0 (QMI) is root-only and pppd is setuid-root/dip-group, so the QMI/PPP helpers run their commands with sudo (sudo qmicli, sudo udhcpc, sudo ip link set, sudo pppd, sudo poff). The .deb installs a sudoers drop-in (/etc/sudoers.d/allow-vpn-control) granting the rpanion service user passwordless access to exactly those commands — nothing else. RNDIS needs no sudo (the service user opens the AT serial port directly via the dialout group).

QMI vs RNDIS — which one your modem exposes

This is a modem USB-composition setting, not a hardware limitation. SIM7600 (and similar) modules can present their data interface as either QMI (qmi_wwan → cdc-wdm0 + wwan0) or RNDIS/ECM (usb0), selected by the USB PID. A modem on PID 1e0e:9001 enumerates as QMI (no usb0). QMI is generally the more robust choice, so prefer it unless you have a reason not to.

You can switch composition from the LTE Modem page (USB composition → choose QMI/RNDIS → Switch USB mode & reboot modem), which sends AT+CUSBPIDSWITCH=<pid>,1,1 (QMI 9001, RNDIS 9011) and reboots the modem (~30 s; interfaces re-enumerate). Afterwards set the Data path mode to match, re-scan and reconnect.

On-device prerequisites (now mostly automatic)

  • QMI: libqmi-utils (qmicli) and udhcpc are .deb dependencies. The .deb installs a udev rule (77-rpanion-qmi-rawip.rules) that sets raw-IP (qmi/raw_ip=Y) automatically when the qmi_wwan interface appears, and a NetworkManager drop-in that marks wwan0 unmanaged (so NM doesn't fight the manual data call). No manual sysfs step needed.
  • PPP: ppp (pppd/poff) — already a dependency.
  • A SIM must be inserted and the carrier APN set on the LTE Modem page; then pick QMI mode + wwan0 and hit Connect.