Precondition in IMS (IP Multimedia Subsystem) involves a specific signaling flow designed to ensure that network resources can meet the Quality of Service (QoS) requirements before a call or multimedia session is fully established. This process is integral to maintaining high-quality communication services, such as VoIP (Voice over IP) or video calls.
The precondition mechanism in IMS adds a layer of reliability and quality assurance to multimedia communications by verifying network capabilities and reserving necessary resources before fully establishing a session. This process helps in minimizing poor quality communications, thereby enhancing user satisfaction.
Here's a high-level overview of how precondition works in terms of IMS signaling flow:
- Session Initiation: The process starts with the initiation of a session establishment using SIP (Session Initiation Protocol). The initiating party sends a SIP INVITE message to the receiving party, which includes the session description protocol (SDP) payload. This SDP payload contains the media descriptions and the desired QoS parameters for the session.
- QoS Negotiation: The SDP offer-answer model is used for negotiating the QoS parameters. The receiving party reviews the proposed QoS parameters (e.g., bandwidth, delay, jitter) and responds with a SIP response message. This message also contains an SDP payload, indicating whether the receiving party can meet the requested QoS requirements.
- Resource Reservation: Concurrently with the QoS negotiation, resource reservation protocols such as RSVP (Resource Reservation Protocol) are used for reserving the necessary network resources to meet the agreed QoS parameters. This step involves signaling through the network elements (like routers and gateways) that form the path between the communicating parties.
- Precondition Evaluation: Both parties involved in the session continuously evaluate the precondition status, which is the status of resource reservation and QoS assurance. SIP messages (such as UPDATE or re-INVITE) with specific precondition status information are exchanged between the parties. These messages indicate whether the precondition criteria have been met.
- Confirmation of Resources: Once the network has successfully reserved the necessary resources and both parties have confirmed that the precondition criteria are met, the session proceeds to the next step. If the resources cannot be reserved, or the QoS parameters cannot be met, the session establishment can be delayed or terminated.
- Session Establishment Completion: With the preconditions satisfied, the initiating party sends an ACK (Acknowledgment) message to confirm the session establishment. At this point, the media flow begins, adhering to the negotiated QoS parameters.
- Media Communication: The session is now fully established, and media communication starts with the assurance that the network can support the desired QoS. This ensures a high-quality experience for the users throughout the duration of the call or multimedia session.