The main topology change in LTE vs. UMTS is the segregation of access control and mobility management in the MME, allowing the SGW and PGW to be dedicated to user traffic routing, management, and charging.
- Mobility Management Entity (MME) – network access control, subscriber authentication and UE validation, gateway selection, MME selection, UE paging;
- Serving Gateway (SGW) –terminates EPC interface towards the E-UTRAN, user packet routing/forwarding and buffering (downlink traffic is buffered when UE is idle and during mobility), transport-level packet marking (DSCP), accounting per UE and QoS Class Identifier (QCI) for offline charging, mobility anchor for UMTS/LTE mobility, local mobility anchor for inter-E-UTRAN mobility;
- Packet Data Network (PDN) Gateway (PGW) – terminates EPC interface towards the PDN, UE IP address anchor, address allocation, user packet routing, packet filtering via DPI, packet screening, DSCP, service-level gating control/rate and policy enforcement, service-level online charging.
Other EPC management elements provide and collect information regarding subscribers, subscriber activity, and network policies:
- Home Subscriber Server (HSS) – subscriber profile repository (IDs, subscriptions, authentication information), subscriber location tracking;
- Policy and Charging Rule Function (PCRF) – service-level QoS policy and charging rule repository, authorizes QoS rules for user bearers at activation, controls aggregate subscriber QoS across all bearers, proactively activate or deactivate rules to accomplish priority or conflict resolution;
- Offline Charging System (OFCS) – gateways collect network resource usage by subscriber, concurrent with that usage, and transfer usage reports to the OFCS where they are used for subscriber billing, inter-operator accounting (roaming agreements), and statistics;
- Online Charging System (OCS) – usage collection similar to OFCS but usage is tied to service that must first be authorized (usage granted by OCS prior to access, may be drawing on a pre-paid credit balance), usage may be limited by volume or duration and service may be terminated when limit is reached (user may be prompted for immediate payment in order to continue).
Management interfaces are very functional in nature (subscriber feature-specific behavior, accumulation and validation of accounting, interaction with previous controlling node) and server emulators are required to initiate control procedures, such as push policies, simulate failures or restarts, and rate changes.
These interfaces are performance intensive, not throughput intensive. They need to support millions of subscribers, and 5-50K calls/sec with limited messaging (4-10 message/call) but potentially highly CPU-intensive, such as subscriber identification, multiple authentication vectors, and location update/cancellation.
Traffic Flow Templates (TFT) are used to filter the traffic into dedicated bearer sessions. Tunnel Endpoint IDs (TEID), exchanged during bearer/context setup, identify bearer sessions. TFTs may originate from the PCRF and may be dynamically compiled based on policy rules or network conditions or be fixed in the PGW/SGW, and are always exchanged during dedicated bearer/context setup, and may be exchanged during bearer modification. QoS is applied to bearer sessions (per TEID), which may be made up of one or many L4-L7 flows.
Networks must utilize a PCRF in order to implement dynamic and/or subscription based policy and charging.
Performance testing in the EPC can be segregated:
- MME testing – involves the S1-MME, S6a, SLs, SLg, and S11 interfaces – all of which deal exclusively with the control plane. In order to fully test an MME’s access control and mobility management functions, the S6a interface must be included;
- Gateway testing (SGW and PGW) testing – involves control plane interfaces, user plane interfaces (S1-U and SGi), and the S5 interface which includes both control and user planes. With the increased emphasis in dynamic policy management (QoS) and charging/credit control, a full test of the data capabilities of the gateways must include Gx, Rx, Gy, and Gz interface testing concurrent with S11, S1-U, S5, and SGi testing.
- Scalability of test configuration should support a single test verifying MME capacity and performance;
- Focus of testing should be on access control, session management, gateway selection, location and inter-eNodeB mobility;
- Control plane traffic.
- Scalability to support single test verifying SGW capacity and performance;
- Control signaling and user traffic;
- Focus on bearer and traffic management, inter-MME and inter-network (3G, non-3GPP) mobility, and offline charging.
- Scalability to support single test verifying PGW capacity and performance;
- Control signaling and user traffic;
- Focus is on bearer and traffic management, policy enforcement, and online/offline charging.
In this Use Case (download PDF file), the following EPC Testing points are presented (with message flow diagrams):
- UE attempts to register with network – subscriber must be authenticated, equipment ID may be validated;
- MME entirely responsible with support from HSS/EIR;
- Default bearer activation – MME initiates, PGW responsible;
- PGW may get policy and charging rules from PCRF, may get subscriber’s available credit from OCS;
- User plane is established;
- Tracking Area Update procedure is performed;
- Detach procedure is performed.
For each of these procedures, the Use Case shows details on:
- MME Testing
- SGW Testing
- PGW Testing