Interfaces and Reference Points

As already explained, the E-UTRAN is an all-IP network. Figure 1 shows the network elements that are typically involved in the signaling procedures and routing of payload data from the UE to the PDN and vice versa. The figure also shows the reference points for inter-RAT handover (and inter-RAT packet routing) between E-UTRAN, UTRAN, and GERAN.

The pipeline symbols in the figure illustrate the different signaling connections and tunnels for IP payload transport established and maintained during the connection. The signaling on Gx and Rx used to negotiate specific QoS policies is ignored for reasons of better understandability. Besides, the existence of the PCRF is optional. Due to the fact that the MME and the S-GW may also be combined into a single physical entity, the S11 interface is also optional. The lab test scenarios existing at the time of writing (spring 2010) all have separated physical entities for the MME and S-GW.

The signaling connection across the LTE-Uu interface is the RRC signaling connection, represented by a set of Signaling Radio Bearers (SRBs). The user plane tunnel across LTE-Uu is the radio bearer. The other user plane tunnels are named after the appropriate reference points: namely, S1 bearer and S5 bearer. After the PDN-GW the connection is carried by the external bearer on SGi. S1AP signaling between the E-UTRAN and MME will be used to establish the tunnel on S1-U and GTP-C signaling will be used to create the tunnel on S5. On SGi we can see already plain IP traffic – pure payload, so to say.

The reference points can be briefly described as follows:

• S1-MME: Reference point for the control plane protocol between the E-UTRAN and MME. This control plane protocol is the S1AP, which is quite similar to UTRAN RANAP. Indeed, in early drafts of LTE specs this protocol was called "E-RANAP."
• S1-U: Reference point between the E-UTRAN and S-GW for the per bearer user plane tunneling and inter-eNB path switching during handover. The protocol used at this reference point is the GPRS Tunneling Protocol for the User Plane (GTP-U).
• S3: This is the reference point between the MME and SGSN. The SGSN may serve UTRAN, GERAN, or both. On S3 we can see plain control plane information for user and bearer information exchange for inter-3GPP access network mobility (inter-RAT handover) in the idle and/or active state. If the connection was set up originally in the E-UTRAN and is handed over to UTRAN/GERAN the appropriate user plane streams are routed across the S4 reference point. What happens in the case of UTRAN/GERAN to E-UTRAN handover depends on whether S-GW also acts as the anchor for UTRAN/GERAN traffic. If this is true the user plane tunnel can be switched smoothly between S4 and S1-U during the handover. The protocol used at the S3 reference point is the GTP-C.
• S4: The S4 reference point provides related control and mobility support between the GPRS core and the 3GPP anchor function of the S-GW using GTP-C. In addition, if a direct tunnel across S12 is not established, it provides user plane tunneling using GTP-U.
• S5: The S5 reference point provides user plane tunneling and tunnel management between the S-GW and PDN-GW. It is used in case of S-GW relocation due to UE mobility and if the S-GW needs to connect to a non-collocated PDN-GW for the required PDN connectivity. The protocol used at this reference point is GTP for both the control plane and user plane.
• S6a: The S6a reference point enables the transfer of subscription and authentication data for authorizing user access to the network. The reference point can be also described as the AAA interface between the MME and HSS. Compared to the legacy core network of 2G/3G standards, the functionality provided by S6a is similar to the one on the Gr interface, but due to the all-IP concept of EPC the protocol used at this reference point is the DIAMETER protocol. In the IP world DIAMETER is known as the successor of RADIUS, a protocol for granting access and authentication. However, the DIAMETER used on S6a does not have much in common with what is found in the IP world. The protocol header is based on IP standards, but the messages and parameters on the application layer are defined in a 3GPP-specific DIAMETER standard that has no meaning in the IP world.
• Gx: This point provides transfer of (QoS) policy and charging rules from the PCRF to the Policy and Charging Enforcement Function (PCEF) in the PDN-GW. This means that a set of rules for charging the transmission of a particular user data stream (called service flow) will be requested by the PDN-GW upon bearer establishment and the PCRF will provide the required parameters for the charging process. Especially, it will signal which of the following charging models will apply:
  • – Volume-based charging.
  • – Time-based charging.
  • – Volume-and time-based charging.
  • – Event-based charging.
  • – No charging (if the user pays at a monthly flat rate).
  • Also, information about prepaid limits and other thresholds can be included.
• S8: The S8 reference point is used by roaming subscribers only. It is the inter-PLMN reference point providing the user plane and control plane between the S-GW in the Visited PLMN (VPLMN) and the PDN-GW in the Home PLMN (HPLMN). S8 is the inter-PLMN variant of S5, based on GTP as well, and can be compared to the Gp interface defined for GERAN GPRS.
• S8: The S8 reference point is also used by roaming subscribers only. It provides transfer of (QoS) policy and charging control information between the home PCRF and the visited PCRF in order to support the local breakout function. For example, imagine a prepaid limit that can only be known by the home PCRF and must be provided to the visited PCRF to allow roaming services for this user.
• S10: This is the reference point between MMEs for MME relocation and MME-to-MME information transfer. This reference point provides mobility functions for intra-E-UTRAN handover/relocation. In other words, signaling procedures on this interface are triggered by UE mobility. It should be noted that this kind of MME relocation in 3GPP 23.401 is called S1 handover. Hence, S10 is seen as special kind of S1 interface and the S1AP is used at this reference point.
• S11: This is the reference point between the MME and S-GW. The protocol used here is the GTP-C. The appropriate user plane is routed across S1-U.
• S12: The S12 reference point is located between the RNC in the 3G UTRAN and the S-GW for user plane tunneling when a "direct tunnel" is established. It is based on the Iu user plane and Gn user plane reference points using the GTP-U as defined between the SGSN and RNC, or between the SGSN and GGSN in the 3G core network. Use of the S12 reference point is an operator configuration option. On S12 only GTP-U traffic can be monitored, as on S1-U.
• S13: This point enables a UE identity check procedure between the MME and EIR (Equipment Identity Register). Typically there is no EIR installed in public networks due to the high administrative efforts, but this network element is found in some private networks. For instance, the GSM-based mobile network of the railway company Deutsche Bahn is equipped with an EIR. The purpose is to ensure that only staff of Deutsche Bahn can use the company's PLMN, but no private persons and staff of other European railway companies such as France's SNCF that also runs trains through Germany.
• SGi: This is the reference point between the PDN-GW and the packet data network. This network may be an operator external public or private packet data network or an intra-operator packet data network, for example for the provision of IP Multimedia Subsystem (IMS) services. To simplify the definition, it can be said that for many user plane connections SGi is the interface to the public Internet. This reference point corresponds to Gi for 3GPP access. Typically the complete TCP/IP suite can be monitored at this point.
• Rx: The Rx reference point resides between the Application Function (AF) and the PCRF defined in 3GPP 23.203. It is for instance mandatory if real-time communication services such as Voice over IP (VoIP) are to be charged differently than common PS data transfer.
• SBc: The SBc reference point lies between the Cell Broadcast Center (CBC) and MME for warning message delivery and control functions. This interface is used to broadcast warning messages to subscribers (not to send warning messages about network element status to the operation and maintenance center). A typical example of such warning messages could be the broadcast of bush fire or tsunami alarms.
  
  

 

Figure 1: Connection via E-UTRAN non-roaming architecture

 

Figure 2: Connection after inter-RAT handover from E-UTRAN to UTRAN/GERAN

 

Figure 3: Connection via E-UTRAN with roaming in EPC

The special anchor function of the S-GW can be illustrated when looking at a connection that was handed over from the E-UTRAN to UTRAN or GERAN as shown in Figure 1.12. In this case the connections on S5 and SGi remain the same, but the payload is now routed through a tunnel across S4 or S12 while the signaling necessary to execute the inter-RAT mobility will be sent across S3. The old bearers and signaling connections on S1 and LTE-Uu will be deleted after successful handover of the connection.

Figure 1.13 illustrates the basic connection of a roaming subscriber. Signaling and payload take the same route as in Figure 1.12, but the HSS and PDN-GW and, thus, the connection to the public packet network are located in a foreign network. The IP tunneling from the S-GW to PDN-GW and vice versa is realized through the S8 interface, which has identical protocol structure and functions to S5. The only difference is that S8 must fulfill higher requirements in terms of inter-operability, because equipment from different manufacturers must be interconnected through this reference point.