VoLTE
EPC
The EPC has three main functional elements
1. Mobility Management Entity (MME). MME is the single most control point in the EPC and responsible for most of the control plane functions.
2. Serving Gateway (S-GW). All IP packets in uplink and downlink flow through S-GW. S-GW is also responsible for handling handovers.
3. P-GW allocates IP addresses to UEs. It also provides interfaces towards internet and IMS.
UE
An IMS powered UE has two main components
1. Universal Integrated Circuit Card (UICC), 有identity information
Each UE must contain one UICC and each UICC may have one or more of the following modules.
1.1 Subscriber Identity Module (SIM): SIM identity information used by a GSM network.
1.2 UMTS Subscriber Identity Module (USIM): USIM information used by a UMTS or LTE network.
1.3 CDMA Subscriber Identity Module (CSIM) or Re-Useable Identification Module (R-UIM). CDMA network uses this identity information
1.4 IP Multimedia Services Identity Module (ISIM): IMS subsystem uses IMSI identity information.
UE wants to use IMS resources
ISIM contains the following:
a. IP Multimedia Private Identity (IMPI): IMPI is a global identity allocated by the home network. IMPI contains home operator’s domain information.
b. Home operator’s domain name
c. IP Multimedia Public Identity (IMPU): IMPU acts like a telephone number. It can either be a SIP URI (sip:@:) or a tel URI. Definition of tel URI from RFC 39664
d. Secret Key: This long secret key is used for user authentication and SIP registration.
2. SIP User Agent (SIP-UA) send and receive SIP messages.
2.1 User Agent Client (UAC): As a client to send SIP request
2.2 User Agent Server (UAS): As a server to receive requests and send response
Evolved Packet Core (EPC)
1. Public Data Network Gateway (PDN-GW)
allocates IP addresses to UEs
the point of communication between EUTRA and non-3GPP services like the internet
When IMS is available, there can be more that one PDN-GW in the EPC, one for internet and one for IMS.
2. Policy and Charging Rule Function (PCRF)
determines the allowed traffic types in real-time
account for the traffic for billing purpose
UE starts a VoLTE call -> PCRF checks UE’s subscription -> if pass, setup dedicated bearer for IMS services.
IMS Core: handles session management and media control
- Call Session Control Function (CSCF)
establishes monitors, supports, and releases multimedia sessions
1. Proxy CSCF: P-CSCF acts as the initial point of contact from any SIP User Agent. It handles all requests from the UE
2. Serving CSCF:
has knowledge about the user and what applications are available to the user,
decide whether the user’s SIP messages will be forwarded to the application servers
3. Interrogating CSCF
initiates the assignment of a user to an S-CSCF (by querying the HSS) during registration
- Home Subscriber Server (HSS)
1. a database that maintains user profiles and location information
2. It handles name and address resolution
3. HSS is also responsible for authentication and authorization.
- Subscriber Location Function (SLF): assigns HSS to a user in the home network
- Media Gateways: the interface between SIP-based IMS network and traditional PSTN network.
- Media Gateway Control Function(MGCF)
1. controls media gateways and converts codecs
2. serve as a breakout to a circuit-switched network
EPS Attach for VoIP and Default Bearer Setup:
- PDN Connectivity: RRC Connection
- Authentication
1. Authentication
2. Security Mode Command
3. ESM(EPS session management) information
- Bearer Setup and EPS Attach: Radio Bearer Reconfiguration
- P-SCCF Discovery: the process of identifying (by address)
SIP IMS Call Flow
