The idea of IMS is old but after it’s deployment with LTE users and operators can harness the true power of IMS. IMS (IP Multimedia Subsystem) is a standalone system which resides out of the LTE network and connected to PDN Gateway through SGi interface.
The EPC basically contains three functional elements. The first one is Mobility Management Entity (MME). MME is the single most control point in the EPC and responsible for most of the control plane functions. The second in the list of node is Serving Gateway (S-GW). All IP packets in uplink and downlink flow through S-GW. S-GW is also responsible for handling handovers. The last in the list is P-GW or PDN Gateway. P-GW allocates IP addresses to UEs and also provides interfaces towards internet and IMS.
Below is a simple network architecture diagram which demonstrate how IMS is connected to EPC (Evolved Packet Core).
User Equipment (UE)
User Equipment (UE) is the mobile terminal which may be a smartphone or tablet or any communication device which is authorized to be used in the network. An IMS powered UE has two main components.
- Universal Integrated Circuit Card (UICC)
- Session Initiation Protocol User Agent (SIP UA)
Universal Integrated Circuit (UICC)
Each UE must contain one UICC and each UICC may have one or more of the following modules.
- Subscriber Identity Module (SIM): SIM identity information used by a GSM network.
- UMTS Subscriber Identity Module (USIM): USIM information used by a UMTS or LTE network.
- CDMA Subscriber Identity Module (CSIM) or Re-Useable Identification Module (R-UIM): identity information used by a CDMA network.
- IP Multimedia Services Identity Module (ISIM): ISIM identity information used by the IMS subsystem.
Let’s look into ISIM which is important when UE wants to use IMS resources in the network. ISIM contains the following:
- IP Multimedia Private Identity (IMPI): IMPI is a global identity allocated by home network. IMPI contains home operator’s domain information.
- Home operator’s domain name
- IP Multimedia Public Identity (IMPU): IMPU acts like a telephone number which can either be a SIP URI (sip:<username>@<host>:<port>) or a tel URI as defined in RFC 39664 (tel:<country_code><national_destination_code><subscriber_number>).
- Secret Key: This long secret key is used for user authentication and SIP registration.
SIP User Agent (SIP-UA)
SIP User Agent resides in the UE to transmit and receive SIP messages. SIP-UA provides basic telephony functionality. It can act in two different roles:
- User Agent Client (UAC): As a client to send SIP request
- User Agent Server (UAS): As a server to receive requests and send response
Evolved Packet Core (EPC)
For VoLTE and IMS prospective two nodes are important in the Evolved Packet Core.
Public Data Network Gateway (PDN-GW)
PDN Gateway is responsible for allocating IP addresses to UEs. PDN-GW is also the point of communication between EUTRA and non-3GPP services like internet.
When IMS is used there can be more that one PDN-GW in the EPC one for internet and one for IMS.
Policy and Charging Rule Function (PCRF)
The PCRF provides real-time determination of what types of traffic are allowed under what conditions, and also determines how to account for this traffic (for billing purposes). Based on requests for IMS services, the PCRF also initiates the appropriate bearers.
When a user initiate a VoLTE call PCRF checks if that user is allowed to start a VoLTE call or not and if it is allowed PCRF set up dedicated bearer.
IMS core is responsible for session management and media control.
IMS core has the following important nodes.
Call Session Control Function (CSCF)
CSCF is responsible for establishing, monitoring, supporting and releasing multimedia sessions. It has three different functional elements which may or may not be separate physical entities.
- Proxy CSCF: P-CSCF is seen as the initial point of contact from any SIP User Agent. It handles all requests from the UE and is, from the UE’s point of view, the “SIP proxy” to the entire subsystem.
- Serving CSCF: S-CSCF has knowledge about the user and what applications are available to the user. It acts as a decision point and it’s main job is to decide whether or not the user’s SIP messages will be forwarded to the application servers.
- Interrogating CSCF: I-CSCF is the entity that initiates the assignment of a user to an S-CSCF (by querying the HSS) during registration.
Home Subscriber Server (HSS)
HSS is a database that maintains user profile and location information and is responsible for name/address resolution. HSS is also responsible for authentication and authorization.
Subscriber Location Function (SLF)
SLF is responsible for assigning HSS to user in home network. To achieve this function SLF keeps track of all HSSes.
Media Gateway resides at the inteface between SIP based IMS network and traditional PSTN network. More details are found in RFC 3372 (Session Initiation Protocol for Telephones (SIP-T): Context and Architectures)
Media Gateway Control Function(MGCF)
Media Gateway Control Function controls media gateways, converts codecs where necessary and may serve as a breakout to a circuit-switched network.
In the case when MGCF works as a breakout to CS network it is also responsible for managing the conversion of signaling messages, converting SIP messaging to the Bearer Independent Call Control (BICC) and ISDN User Part (ISUP) protocols used in legacy systems.
Breakout Gateway Control Function (BGCF)
When Media Gateway Control Function does not include breakout to circuit-switched network, BGCF takes care of this functionality.