In order to understand the basic principles involved during a typical LTE/EPC call, it is important to know about the functionalities of various nodes which comprise the EPC backbone, first. One’s key point to note here is, that the control and user data plane are kept separate. They follow a different path through the network. You may consider reading this amazing book EPC and 4G Packet Networks: Driving the Mobile Broadband Revolution which also talks about mobile broadband revolution.
The above diagram shows a very basic architecture of the EPS when the User
Equipment (UE) is connected to the EPC over E-UTRAN (LTE access network). The Evolved NodeB (eNodeB) is the base station for LTE radio. In this figure, the EPC is composed of at least four network elements: the Serving GateWay (SGW), the PDN GateWay (PGW), the Mobility Management Entity (MME) and the Home Subscriber Server (HSS). The EPC is connected to the external networks, which can include the IP Multimedia Core Network Subsystem (IMS). Though the PCRF (Policy and Charging Rules Function) is not mandatory, it plays a key role in service differentiation and classification. PGW also serves as the anchor point for any trusted or non-trusted non-3GPP network accesses, such as Wi-Fi, fixed line, etc.
When an LTE handset is powered on, it exchanges a number of messages with the MME over the E-UTRAN. MME sets up a dedicated connection with the handset over NAS (Non-Access Stratum) protocol. The MME registers the subscriber into the operator’s network. On successful registration, mobility and session contexts are stored in the MME and at least one default bearer is set up to carry user data to and from the network. The data follows the path: UE <—> SGW <—> PGW <—> External PDN (MME is not the part of user data plane). We will go through all the details one by one in the next article of this series. But before that, we must understand the node functionalities first.
Home Subscriber Server (HSS)
HSS is a database that contains user-related and subscription-related information. It also provides support functions in mobility management, call and session setup, user authentication and access authorization.
It is based on the pre-3GPP Release 4 – Home Location Register (HLR) and Authentication Centre (AuC). In EPC, the MME downloads users’ subscription data from the HSS, over S6a interface. The same HSS node is also used in IMS domain. S6a uses DIAMETER protocol for communicating with the MME. We will learn more about DIAMETER protocol and its benefits in future articles.
Author: Vineet Shandilya