The following LTE Mission Critical Technology White Paper describes Long-Term Evolution (LTE) and Mission Critical Services and is part one of a 5-part Summary of our 3GPP Core Technologies series. You can also download and print them by clicking below.
Long-term Evolution (LTE) and IP Multimedia Subsystem (IMS)
1. Introduction
2. IP Multimedia core network Subsystem (IMS)
3. Registration
4. Session Establishment
5. standalone transaction
6. Long-Term Evolution (LTE) and IMS
3rd Generation Partnership Project (3GPP) defined IP Multimedia Subsyste or Internet Protocol Multimedia core network Subsystem (IMS) as a common platform to access different applications, thereby making it easier for a user to access and maintain ownership of subscribed applications when changing operator. This report is a high-level overview of IMS by studying three simplified call flows for IMS registration, session setup, and standalone transaction. The report, moreover, shows IMS architecture in combination with the architecture of Long-Term Evolution (LTE) network.
Figure 1 shows a high-level architecture of IMS.
The User Equipment (UE) gets access to the IP multimedia services supported by the application servers (AS) residing in the home network, by first sending a Session Initiation Protocol (SIP) request towards Proxy Call and Session Control Function (P-CSCF). The P-CSCF which may be provisioned in the UE or is obtained by Dynamic Host Configuration Protocol (DHCP), may reside in the visited network as shown in Figure 1 or in the home network. The SIP request is forwarded towards an Interrogating Call and Session Control Function (I-CSCF), which queries Home Subscriber Server (HSS) to identify the most relevant Serving Call and Session Control Function (S-CSCF) to process the SIP request. Both I-CSCF and S-CSCF are located in the home network. The S-CSCF identifies the user subscriptions to Application Servers (AS) by getting the user profile from the HSS.
The SIP requests are generally for the following categories:
Figure 2 illustrates registration procedure for the IMS network.
According to this procedure
Figure 3 illustrates a simplified version of session establishment in IMS. In this figure, it is assumed that UE 1 and UE 2 are connected to the IMS network via two different P-CSCF but they access the same S-CSCF.
Figure 3 shows:
A standalone transaction does not require a session setup and is employed to convey an information to the end entity. A standalone transaction may be transmitted when there is an ongoing dialog or when there is no dialog. However, the standalone transaction is always outside the possible existing SIP dialog.
Figure 4 illustrates an example for a standalone transaction. In this figure, a SIP MESSAGE request is transmitted by UE 1 towards UE 2 via P-CSCF 1, S-CSCF, and P-CSCF 2. It is assumed that UE 1 and UE 2 are connected to the IMS network via two different P-CSCF but they are connected to the same S-CSCF.
IMS is an architectural framework to access IP multimedia services. IMS was originally defined for the Universal Mobile Telecommunications System (UMTS) as a part of standardization for 3G network. However, gradually, IMS was defined for other access technologies such as Long-Term Evolution (LTE). Today, there are IMS services such as mission critical services which are defined by accessing the IP network through LTE.
Figure 5 illustrates IMS architecture in combination with LTE network where Packet Data Network Gateway (P-GW) employs SGi reference point to communicate with other Packet Data Networks (PDNs) and application servers. Serving Gateway (S-GW) in the LTE network forwards data between eNodeB to P-GW. Mobility Management Entity (MME) controls the security and mobility of sessions for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access. MME is responsible for tracking and paging of the UE in the idle mode. E-UTRAN is a combination of eNodeB and UE. The Policy and charging rules function (PCRF) is for subscriber policy control and for controlling flow based charging functionalities in Policy Control Enforcement Function (PCEF) which resides in the P-GW. The PCRF is connected to the Application Function (AF) which resides in P-CSCF by the reference point Rx.
For more information on our whitepaper topics including LTE EPC Technology, LTE V2X Technology, LTE ProSe Technology, LTE IMS Technology, and LTE GCS Technology, please visit our Technology White Paper section.