MME serving the RN

Description

The MME serving the Relay Node (MME-RN) is a specialized logical function within a standard MME that manages the control plane for the Relay Node in an LTE-Advanced network. A Relay Node is a low-power eNodeB that extends coverage and capacity by wirelessly connecting to a donor eNodeB (DeNB) via the Un interface, instead of using a wired backhaul. The MME-RN is responsible for managing the RN as if it were a specialized type of User Equipment (UE) for its own control plane signaling, while the RN simultaneously operates as a base station for actual UEs.

Architecturally, the MME-RN resides in the core network and connects to the donor eNB via the standard S1-MME interface. However, the procedures differ. When the RN powers on, it performs an Attach procedure, but this is treated as a 'RN Attach' towards the MME-RN. The MME-RN authenticates the RN using credentials stored in the HSS, establishes an EPS security context, and manages its mobility as it potentially moves between different donor eNBs. Crucially, the MME-RN also plays a key role in setting up the data plane for the RN. It participates in the creation of one or more dedicated bearers (often GTP tunnels) between the RN and the Serving Gateway (S-GW) for the RN's own traffic (OAM) and, more importantly, for the aggregated user plane traffic of all UEs connected to that RN.

How it works involves layered connectivity. The MME-RN sees the RN as a single entity with its own International Mobile Subscriber Identity (IMSI). The user plane for UEs behind the RN is aggregated through this RN connection. The MME-RN does not manage the individual UEs connected to the RN; a separate MME (MME-UE) performs that function. The MME-RN's primary responsibilities include RN-specific authentication and authorization, handling RN handovers between donor eNBs, managing the RN's PDN connection for its backhaul, and supporting RN-specific security procedures as defined in TS 33.401. Its role is essential for integrating wireless relay nodes seamlessly into the EPC, enabling them to be self-configuring and manageable entities within the operator's network.

Purpose & Motivation

The MME-RN function was created to solve the specific control plane challenges introduced by the LTE Relay Node feature, standardized in 3GPP Release 10. Relay Nodes were proposed as a cost-effective solution for rapid coverage extension, especially in areas where deploying fiber backhaul is difficult or expensive. However, a relay is not a simple repeater; it is a full eNodeB that requires its own management, authentication, and connectivity to the core network.

The problem was that the standard MME procedures designed for handheld UEs were insufficient. An RN has dual nature: it is infrastructure from the perspective of the UEs it serves, but it is also a 'mobile terminal' from the perspective of the network it connects to (the donor eNB). The purpose of defining the MME-RN role was to adapt the core network to recognize and handle this duality. It addresses the need for secure, authenticated network entry for the infrastructure node itself, which is critical for preventing rogue base stations.

Furthermore, it enables efficient mobility and session management for the RN. As a wireless node, the RN might need to handover between different donor eNBs (e.g., in mobile relay scenarios for trains). The MME-RN handles these procedures. It also establishes the complex bearer architecture that allows the RN to have its own management traffic while also funneling the aggregated user plane traffic of all its attached UEs through a secure tunnel. Without the MME-RN, the core network would have no standardized way to manage, secure, and integrate these wireless relay points, limiting their deployment flexibility and operational robustness.

Key Features

• Handles RN-specific Attach and Detach procedures
• Manages authentication and security context establishment for the Relay Node
• Controls the setup and modification of PDN connections for RN backhaul
• Supports RN handover procedures between different donor eNBs
• Interworks with the HSS for RN subscriber data and authentication
• Distinct from the MME-UE which manages end-user equipment behind the RN

Evolution Across Releases

Defining Specifications