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LTE System Interfaces: S6b, S6c, S6d, S9, Gi, Gx, Gy, Gz, Sp, Rx+, Wm

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​In the Evolved Packet System (EPS) architecture of Long-Term Evolution (LTE) networks, various standardized interfaces facilitate seamless communication and interoperability among network elements. This page provides an overview of various LTE system interfaces, including S6b, S6c, S6d, S9, Gi, Gx, Gy, Gz, Sp, Rx+, and Wm. These interfaces are integral to the network’s functionality, enabling essential operations like subscriber authentication, policy enforcement, charging mechanisms and interworking with external networks. Understanding the roles and interactions of these interfaces is crucial for network engineers and operators to ensure efficient network performance, maintain quality of service (QoS) and deliver a seamless user experience.

LTE System Interfaces2

Figure 1: LTE System Interfaces

The diagram above illustrates the connections between different elements in an LTE system. The following table describes each of these interfaces in detail:

LTE system interfaceDescription
S6bThis interface exists between the PDN Gateway (PGW) and the 3GPP AAA Proxy/Server. It handles mobility-related authentication, transporting commands to retrieve and store mobility parameters, and manages the transport of static QoS (Quality of Service).
S6cThis reference point connects the PDN Gateway (PGW) and the 3GPP AAA server. Like S6b, it’s used for mobility-related authentications. It handles the transport of commands for retrieving and storing mobility parameters.
S6dThis LTE interface lies between the HSS (Home Subscriber Server) and the SGSN (Serving GPRS Support Node). It functions similarly to the S6a interface.
S9It resides between the H-PCRF (Home Network-PCRF) and V-PCRF (Visited network PCRF) within the EPS (Evolved Packet System) network. The PCRF is responsible for policy and charging rules.
GiThis reference point is located between the packet domain and an external packet data network (PDN). For example, it connects the GGSN (Gateway GPRS Support Node) to an external IP network. It handles the transfer of authentication and authorization information during APN (Access Point Name) provisioning, as well as accounting information during APN provisioning.
GxThis reference point lies between the PCRF (Policy and Charging Rules Function) and the PCEF (Policy and Charging Enforcement Function) in the EPS network. It enables the PCRF to dynamically control the PCC (Policy and Charging Control) behavior at the PCEF. The Gx interface facilitates signaling of PCC decisions, negotiation of IP-CAN (IP Connectivity Access Network) bearer establishment mode, and termination of Gx sessions.
GyThis is the online charging reference point. Gy lies between the PCEF (Policy Control Enforcement Function) and the OCS (Online Charging System). Its functionalities are similar to the R0 interface.
GzThis represents the offline charging reference point. Gz is situated between the PCEF and the CGF (Charging Gateway Function). Its functionalities are similar to the Rf interface in EPS.
SpIt connects the SPR (Subscription Profile Registry) and the PCRF. The SPR manages subscription information. Sp handles the transfer of subscriber information related to IP-CAN based on the subscriber ID. It also manages unsolicited notifications about subscriber information changes.
Rx+As Rx lies between AF (Application Function) and PCRF, Rx+ is the Rx reference point used for EPC. The Rx+ interface resides between the PCRF and IP services or proxies to the network. It manages the transport of application-level session information, similar to the Rx interface.
WmThis interface resides between the PDG (Packet Data Gateway) and a 3GPP AAA Proxy/Server. It is referred to as an intra-operator reference point.

Conclusion

Each of the LTE interface serves a specific function, from subscriber authentication (S6b, S6c, S6d) and policy control (Gx, Rx+) to online/offline charging (Gy, Gz) and interworking with external networks (Gi, Wm). The S9 interface ensures interoperability in multi operator environments, while Sp supports service provisioning. Together, these interfaces enable efficient network operations, enhance user experience and ensure compliance with Quality of Service (QoS) and billing requirements in LTE networks.