Description
Media Orientation Negotiation Acceleration (MONA) is a protocol mechanism standardized by 3GPP, primarily documented in TS 29.163 (Interworking between the IMS and CS networks) and TS 29.863 (Protocols for IMS interworking). It is designed to optimize the media negotiation phase during the establishment of an IMS-based session, such as a VoLTE or ViLTE call. The standard session setup using SIP and the Session Description Protocol (SDP) involves an offer/answer model where the calling party (UE-A) sends an SDP offer in an SIP INVITE, listing its supported media codecs and parameters. The called party (UE-B) responds with an SDP answer in a 183 Session Progress or 200 OK message, selecting the agreed codec. This exchange can traverse multiple network nodes and may involve multiple round-trips, introducing delay. MONA works by allowing the network, specifically an intermediary node like a Media Gateway Control Function (MGCF) or an IMS Media Gateway (IM-MGW) in interworking scenarios, to generate and send a provisional, accelerated SDP answer on behalf of the terminating side before the final answer from the remote endpoint is received. This provisional answer allows the originating side to start reserving resources and, critically, to start sending media (like voice packets) earlier in the call flow. The network node generating the accelerated answer uses pre-configured knowledge or policies about the expected media capabilities. Later, when the genuine SDP answer from the terminating UE arrives, the network reconciles any differences and may trigger an update if necessary. This process significantly cuts the post-dial delay perceived by the user. Architecturally, MONA involves enhancements to SIP/SDP handling logic in IMS application servers, MGCFs, and sometimes the P-CSCF.
Purpose & Motivation
MONA was created to address a key user experience issue in IP-based telephony: slow call setup times, especially noticeable in inter-network or IMS-to-Circuit Switched (CS) call scenarios. In traditional SIP call setup, media path establishment cannot begin until the complete SDP answer is received from the far end, which may involve multiple signaling hops and processing delays. This results in a longer time between the user pressing the call button and hearing the ringback tone or the called party's voice. MONA solves this by enabling an intermediate network node to provide an early, predicted SDP answer, allowing media plane resources to be allocated and media flow to potentially start in parallel with the ongoing signaling negotiation. The primary motivation was to make IMS-based voice and video services (VoLTE) at least as fast as, if not faster than, traditional circuit-switched calls. It specifically addresses limitations in interworking scenarios where signaling paths are longer, such as calls from an IMS network to a legacy CS network or between different IMS operator networks. By reducing call setup delay, MONA improves user satisfaction and is a critical performance optimization for the commercial success of VoLTE and VoNR services.
Key Features
- Enables generation of provisional SDP answers by network intermediaries (e.g., MGCF)
- Accelerates media resource reservation and early media flow establishment
- Reduces post-dial delay and time-to-media for IMS sessions
- Particularly effective for IMS-to-CS network interworking calls
- Maintains backward compatibility; final SDP answer from the endpoint is still processed
- Can be applied to both voice and video session establishment
Evolution Across Releases
Initially introduced as part of the IMS Centralized Services (ICS) and IMS-CS interworking work. Defined the basic framework for accelerated media negotiation, primarily focusing on the role of the MGCF in generating early SDP answers during interworking scenarios to reduce call setup time.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.163 | 3GPP TS 29.163 |
| TS 29.863 | 3GPP TS 29.863 |