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volte call flow architecture

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Voice over LTE (VoLTE) is a technology that enables voice calls over a 4G LTE network, offering better call quality, faster call setup times, and simultaneous use of voice and data services. The call flow architecture of VoLTE involves several network elements and procedures.

Here's a technical explanation of the VoLTE call flow architecture:

  1. UE (User Equipment): The User Equipment refers to the device used by the subscriber, such as a smartphone, which supports VoLTE functionality.
  2. eNodeB (Evolved NodeB): It's a key component in the LTE network that connects the UE to the core network.
  3. IMS (IP Multimedia Subsystem): The IMS is a framework used for delivering multimedia services over IP networks. It's the core component for VoLTE and consists of several functional entities.
  4. P-CSCF (Proxy Call Session Control Function): This is the first point of contact within the IMS network. It receives SIP (Session Initiation Protocol) signaling messages from the UE and acts as a proxy towards other IMS elements.
  5. S-CSCF (Serving Call Session Control Function): This is responsible for session control, routing, and policy enforcement. It determines the appropriate services for the subscriber and manages the call session.
  6. I-CSCF (Interrogating Call Session Control Function): This is the entry point to the IMS network and is responsible for selecting the appropriate S-CSCF for the user's session.

Now, let's break down the VoLTE call flow:

  1. Attach and Registration: When the UE powers on or moves into the LTE coverage area, it attaches to the network. The UE then registers with the IMS network by sending an initial SIP REGISTER message, which is forwarded by the P-CSCF to the appropriate S-CSCF.
  2. Session Setup:
    • Outgoing Call: When a user initiates an outgoing VoLTE call, the UE sends an INVITE message to the P-CSCF containing the callee's SIP address.
    • Incoming Call: For an incoming VoLTE call, the P-CSCF receives an INVITE message from the caller's IMS network.
  3. Call Establishment:
    • The P-CSCF authenticates the UE and forwards the INVITE message to the S-CSCF.
    • The S-CSCF performs services-related tasks, authenticates the user, and determines the call treatment based on the user's subscription.
    • It then interacts with other network elements (like HSS - Home Subscriber Server) to fetch subscriber information and service profiles.
  4. Media Negotiation:
    • Once the S-CSCF determines the call treatment, it initiates media negotiation (codec selection, bandwidth allocation, etc.) between the caller and callee UE.
    • This negotiation helps establish a suitable media path for the voice call.
  5. Call Control:
    • After successful negotiation, the S-CSCF instructs the P-CSCF to establish the call session.
    • The P-CSCF communicates with the eNodeB to set up the bearers for voice data.
    • The voice data is then transmitted between the caller and callee over the LTE network.
  6. Call Termination:
    • Upon call completion, a SIP BYE message is sent to terminate the call.
    • Network elements release resources and update call state information.