LTE Mission Critical Services Technology White Paper
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.
Summary of 3GPP Core Technologies
Long-term Evolution (LTE) and IP Multimedia Subsystem (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.
IP Multimedia core network Subsystem (IMS)
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:
session establishment, and
Figure 2 illustrates registration procedure for the IMS network.
According to this procedure
UE sends a SIP REGISTER request towards the IMS network by identifying its IP Multimedia Public Identity (IMPU) and IP Multimedia Private Identity (IMPI). The request is routed to the P-CSCF which validates the correctness of the request and further forwards to I-CSCF due to the UE’s home domain, identified in the SIP REGISTER request. The I-CSCF employs DIAMETER protocol and queries the HSS to retrieve the address of the most suitable S-CSCF for the UE IMS registration. The I-CSCF forwards the SIP REGISTER request towards the identified S-CSCF.
The S-CSCF which is the SIP registrar, may employ the HSS to calculate the authentication vector for the IMPI if it does not have the information to do such a calculation. The S-CSCF sends a 401 unauthorized response towards the UE via the I-CSCF and the P-CSCF to request the UE to authenticate itself by employing its security algorithms.
The UE calculates the authentication response and transmits it with its IMPI with another SIP REGISTER request towards the IMS network. The request is routed to the P-CSCF which validates the correctness of the request and further forwards to I-CSCF due to the UE’s home domain identified in the SIP REGISTER request. The I-CSCF employs DIAMETER protocol and queries the HSS to retrieve the address of the most suitable S-CSCF for the UE IMS registration. The I-CSCF forwards the SIP REGISTER request towards the S-CSCF.
The S-CSCF confirms the authentication and if the credentials are valid it requests the HSS to confirm the completion of the authentication procedure and it also requests the user profile. If there is no error in the authentication, the S-CSCF generates a SIP 200 OK response and transmits towards the UE via the I-CSCF and the P-CSCF. The S-CSCF also 3rd party registers to the application servers (AS) that the UE can access according to the obtained user profile from the HSS.
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:
UE 1 sends a SIP INVITE request comprising a Session Description Protocol (SDP) with an initial media description which may represent one or more media for the IMS session. The SIP INVITE request is forwarded towards AS via the P-CSCF 1 and S-CSCF to store transaction stage of the session. The AS is a back to back user agent (B2BUA) and it forwards the SIP INVITE request towards UE 2 via S-CSCF and P-CSCF 2.
UE 2 accepts the session and transmits SIP 200 OK response towards UE 1. The response uses the same route as the route used by the SIP INVITE request, to be transmitted towards UE 1.
UE 1 sends back a SIP ACK response towards UE 2 and a session is then established by employing the media described in the SDP which was sent in the SIP INVITE request by UE 1.
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.
Long-Term Evolution (LTE) and IMS
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.
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