Wireless LAN Control Plane Protocol

Description

The Wireless LAN Control Plane Protocol (WLCP) is a key protocol defined for the S2a-based trusted WLAN access architecture in 3GPP. It operates between the User Equipment (WLAN-UE) and the Trusted WLAN Access Gateway (TWAG), which is the network entity that connects a trusted WLAN to the Evolved Packet Core (EPC). WLCP is conceptually analogous to the control plane part of the GTP protocol used over the S1-U and S5/S8 interfaces, but it is adapted for the WLAN access stratum. It uses UDP as its transport layer protocol.

The primary function of WLCP is to manage Packet Data Network (PDN) connections for a UE attached via trusted WLAN. It carries signaling messages for PDN connectivity procedures: PDN Connection Establishment, PDN Connection Modification, and PDN Connection Release. During PDN connection establishment, the UE uses WLCP to send a PDN Connectivity Request to the TWAG. The TWAG then interacts with the PGW (via GTP-C on S2a) to establish the connection and returns the assigned IP address(es) and other parameters to the UE via a WLCP response. WLCP also supports bearer-level control, allowing the establishment, modification, and deletion of dedicated bearers to provide different Quality of Service (QoS) levels for different traffic flows over the WLAN access.

WLCP messages are encapsulated within EAP over WLAN (specifically, within EAPoL-Key frames) for transmission over the IEEE 802.11 link between the UE and the WLAN Access Point (which is part of the trusted WLAN). This encapsulation ensures the signaling is carried securely over the WLAN link after the initial EAP-based authentication is complete. The protocol includes necessary elements like TEIDs (Tunnel Endpoint Identifiers) to identify specific connections and bearers, and it supports error handling and recovery procedures. By providing this standardized control plane, WLCP enables the trusted WLAN to be integrated into the EPC as a true, QoS-aware access network on par with 3GPP radio access.

Purpose & Motivation

WLCP was created to address a fundamental gap in the initial 3GPP-WLAN interworking architectures: the lack of a standardized, bearer-aware control plane for trusted WLAN access. Early interworking (e.g., via ePDG for untrusted access) focused on IPsec tunneling and did not support native, efficient QoS differentiation or seamless mobility procedures that were standard in 3GPP accesses. The motivation was to enable the "trusted WLAN" concept, where the operator has full control over the WLAN infrastructure, to offer services with the same reliability and quality as cellular.

The problem it solved was how to extend the EPC's PDN and bearer model—central to LTE's QoS architecture—over a non-3GPP IEEE 802.11 access link. Without WLCP, a trusted WLAN could only provide simple IP connectivity without the ability to establish dedicated bearers for guaranteed bit rate services or to efficiently manage multiple PDN connections (e.g., for IMS and internet simultaneously). WLCP provided the necessary signaling protocol to bring the rich connectivity and policy control of the EPC directly to the WLAN-attached UE.

Introduced in Release 12 as part of the S2a-based trusted WLAN access (TWAN) feature, WLCP was a critical enabler for deep network integration. It allowed operators to deploy carrier Wi-Fi that could not only offload data but also deliver voice over Wi-Fi (VoWiFi) with proper QoS, support seamless session continuity, and be managed as an integral part of the mobile network. This protocol laid groundwork for the later convergence of access types in 5G's Common Core.