VoLTE Call Model
This topic describes an example VoLTE service provider call model for conversational voice services. Network deployments choose to differentiate Internet and IMS services from the initial attach of the subscriber to the network. During such initial attach, the device registers with a specific APN (Access Point Name) for each type of service. That is, the subscriber connects to an APN for Internet access and to a different APN (collocated or not) for voice services.
The following topics illustrate the flow of events taking place when a UE makes a VoLTE call:
Accessing Network for VoLTE
The following illustrates the processes performed to access the network for VoLTE: The UE attaches to the network, authenticates and then the first default bearer is created for the internet APN.
UE Initial Attach and Authentication procedures.
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APN-Internet Default Bearer Creation. The UE has internet access after Default Bearer Creation.
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UE gets connected and the internet access is enabled. The UE then connects to the PDN that provides voice services (IMS)and creates a second default bearer.
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The following illustrates APN-IMS Default Bearer Creation.
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The following illustrates Subscriber with access to an IMS subsystem. After creating a second default bearer, the subscriber uses the default bearer to send IMS signaling information and registers to the IMS network.
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The Subscriber uses SIP-REGISTER Message to Register with the IMS and Authenticates. The UE initiates the IMS registration process and authentication (if necessary). By the end of such process, the UE has:
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Establishing a Voice Call
For example, to establish a call, when you click a call contact, the UE sends a SIP INVITE message to the IMS network with the called party information (user identity and domain). This SIP INVITE message contains all the necessary session descriptors in order to agree to a voice channel configuration.
Establish a call by clicking a contact and the subscriber attempts a VoLTE call by issuing a SIP INVITE message.
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IMS requests resources for establishing voice call. The SIP signaling exchange to establish the call takes place and the IMS communicates to the PCRF that there is a call taking place.
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PCRF triggers Network Initiated Dedicated Bearer for the voice call. The PCRF then instructs the PGW to create a dedicated bearer to be used for the voice call.
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RTP Voice using Dedicated Bearer and SIP signaling using Default Bearer is is established. The voice carried in RTP streams will use the dedicated bearer until the end of the call or until there are changing conditions in the network.
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Releasing a Voice Call
A call all is released when you hang up, and triggers a SIP BYE message on the default (SIP signaling) bearer towards the IMS.
UE issues a SIP BYE message on the Default Bearer as a result of the subscriber ending the call The IMS indicates to the PCRF that the voice conversation is over. The PCRF then deletes the Dedicated Bearer and releases the resources. The parties complete the SIP message flow to release a session.
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IMS and PCRF remove the dedicated resources and the Default Bearers remain active.
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After the vocie call is terminated, the IMS and Internet Default Bearers remain active.
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VoLTE Priority Call
A VoLTE priority Call is similar to a VoLTE normal call except it contains additional information for showing a priority of such call. As a result, a network may route priority calls different than others such as place them in a higher priority queue for faster service or preempt normal calls. Because of this special treatment, priority calls often go through a permitting process where a network determines whether subscribers have permission to make such calls. The following diagram briefly shows the topology and steps. Per 3GPP TS 23.040 [1].
The following terms are used in the setup of a VoLTE Priority Call.
- MPS Priority: MPS Priority is a 32-bit unsigned value used for indicating whether a subscriber has subscribed to (purchased) MPS option. MPS Priority is part of user-profile.
- Resource Priority: Resource priority is a string of characters used for indicating requested resource priority of a call (i.e. dsn.immediate). Resource priority are used in SIP (Gm interface)
- Reservation Priority: Reservation-priority is a number between 0-7 for indicating lowest to highest reservation priority. Reservation-priority is used between P-CSCF and PCRF for indicating reservation priority of a call. When a priority call is permitted, P-CSCF chooses a reservation-priority for a requested resource-priority and sends to PCRF. The decision of which resource will get which priority is local/operator dependent.