Interfaces¶

The AP/Modem Split¶

In almost every cellular device — a USB dongle, an embedded module soldered onto a carrier board, or the baseband chip inside a phone — the part that actually speaks NR/LTE to the network is a separate processor from the one running your applications or OS. That link between them (commonly USB, sometimes PCIe) needs a standardized way to carry two different kinds of traffic:

• Control — commands and status: register on the network, activate a data session, check signal strength, manage the SIM
• Data — the actual IP packets flowing in and out of that data session

Every protocol on this page is really just a different answer to "how do I carry those two things over this physical link, in a way the host's existing OS drivers already understand?"

The Interfaces at a Glance¶

ECM (Ethernet Control Model)¶

CDC-ECM is one of the original USB-IF Communications Device Class subclasses — it makes the modem present itself as a generic Ethernet network adapter. It's simple and widely supported, but it has two real limitations for cellular use: it carries no cellular-specific signalling of its own (network registration, session activation, and so on all have to happen out-of-band, typically over AT commands), and it has known latency issues at higher throughput.²

RNDIS¶

RNDIS (Remote NDIS) is Microsoft's protocol for making a USB device look like a network adapter to Windows' NDIS (Network Driver Interface Specification) stack — the same interface real Ethernet/Wi-Fi cards use internally. Because it's native to Windows, a lot of consumer USB dongles default to it for the "it just works, no driver install" experience. The tradeoff: it's tightly coupled to Microsoft's own driver model, and Microsoft's own specification documentation has historically been incomplete, with real Windows implementations issuing requests outside what's documented — which makes RNDIS a genuinely harder protocol to implement correctly on non-Windows systems, even though Linux, FreeBSD, NetBSD, and OpenBSD all support it.²

NCM (Network Control Model)¶

CDC-NCM is USB-IF's follow-up to ECM, designed specifically to fix ECM's throughput and latency problems — it can pack multiple network packets into a single USB transfer instead of one-packet-per-transfer, which matters a lot once data rates climb into LTE/5G territory. Like ECM, though, it's still not cellular-specific on its own — no built-in session/PDN management, no SIM or signal status. That's exactly the gap MBIM fills.

MBIM (Mobile Broadband Interface Model)¶

MBIM is best understood as NCM's efficient data path, plus a proper cellular control channel bolted on.³ It was introduced alongside Windows 8 in 2012, jointly with USB-IF, specifically to give mobile broadband devices a standardized, vendor-agnostic model — one Windows (and other OSes) could drive with a single generic driver, instead of every vendor needing a custom one. Beyond IP data, MBIM's control channel also covers SMS, USSD, SIM toolkit interactions, and — critically — multiple simultaneous sessions, so one physical modem can expose more than one active PDN/PDU session at once over a single USB connection.¹

Qualcomm's Stack: QMI + RmNet¶

On Qualcomm-chipset devices — which is most of the cellular module market — the dominant pairing is QMI for control and RmNet for data:⁴

• QMI (Qualcomm MSM Interface) exposes a control character device (/dev/cdc-wdm0 on Linux) that userspace tools like libqmi/mmcli talk to — activating sessions, checking registration status, managing the SIM — without needing AT commands at all.
• RmNet is the actual high-throughput data path underneath, using a framing scheme called MAP (Multiplexing and Aggregation Protocol): each PDN gets a multiplexer ID, multiple PDNs' traffic can be aggregated together for throughput, and the rmnet driver de-aggregates incoming MAP frames and routes each one to the right PDN based on that ID.⁴

This combination is why qmi_wwan — the Linux driver implementing this — has become one of the most common WWAN drivers in existence: it's genuinely built for exactly this job, at exactly the throughput cellular data now demands.

Real-World Usage¶

On real commercial modules, these usually aren't fixed — they're selectable USB modes. A Quectel RM510Q-GL 5G module, for example, ships with four selectable modes: RmNet/QMI, ECM, MBIM, and RNDIS, chosen based on what the host OS needs.⁵

Useful Resources¶

• 524wifi — Network Drivers for Cellular Modules – ECM, NDIS, RNDIS, MBIM, RMNET, QMI
• Poulain, L. (Linaro) — Modern WWAN Modem Support in Linux (PDF)
• ModemManager — WWAN Device Types
• Quectel — LTE & 5G Linux USB Driver User Guide (PDF)