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II-NNI (Inter IMS network to network interface)

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The Inter IMS Network to Network Interface (II-NNI) is a standardized interface defined by the Third Generation Partnership Project (3GPP) that enables communication between two or more IP Multimedia Subsystems (IMS) from different operators or service providers. The IMS is a framework that provides multimedia services over IP networks, such as voice over IP (VoIP), video conferencing, instant messaging, and presence.

The II-NNI interface enables the exchange of signaling and media traffic between IMS networks, which allows operators to provide seamless multimedia services to their customers across different networks. This interface is particularly important for operators who want to offer international roaming services, as it allows them to interconnect their IMS networks with those of other operators around the world.

In this article, we will explain the key concepts and components of the II-NNI interface, including its architecture, signaling and media flows, and quality of service (QoS) mechanisms.

II-NNI Architecture

The II-NNI interface is a peer-to-peer interface that connects two IMS networks at the network level. The interface is divided into two main components: the II-NNI signaling plane and the II-NNI media plane.

The II-NNI signaling plane is responsible for the exchange of signaling messages between the two IMS networks. This includes the establishment, modification, and release of sessions, as well as the exchange of session control information such as QoS parameters, media capabilities, and charging information. The signaling messages are exchanged over the Session Initiation Protocol (SIP) and the Diameter protocol, which are the standard protocols used in IMS networks.

The II-NNI media plane, on the other hand, is responsible for the exchange of media traffic between the two IMS networks. This includes the transport of voice, video, and other multimedia streams over the IP network. The media flows are established and managed by the Session Description Protocol (SDP), which is a protocol used to describe multimedia sessions and their characteristics.

The II-NNI interface also includes a set of functional entities that are responsible for the implementation of the signaling and media planes. These functional entities include the Access Gateway (AGW), the Interworking Function (IWF), the Border Gateway (BGW), and the Interconnect Border Control Function (IBCF).

The Access Gateway (AGW) is responsible for connecting the IMS network to the external IP network. It provides the necessary functions to interface with the external network, including network address translation (NAT), firewall, and Quality of Service (QoS) mechanisms.

The Interworking Function (IWF) is responsible for providing protocol interworking between the two IMS networks. It translates the signaling and media protocols used by one IMS network into those used by the other IMS network.

The Border Gateway (BGW) is responsible for the transport of signaling and media traffic between the two IMS networks. It provides the necessary functions for routing, forwarding, and switching of the traffic.

The Interconnect Border Control Function (IBCF) is responsible for controlling the interconnection between the two IMS networks. It provides functions for admission control, security, and QoS enforcement.

Signaling and Media Flows

The II-NNI interface uses the SIP and Diameter protocols for signaling and the SDP protocol for media flows. The signaling messages are exchanged between the AGW and the BGW of each IMS network, while the media flows are exchanged directly between the endpoints.

The signaling flows in the II-NNI interface include the following steps:

  1. The user initiates a session by sending a SIP INVITE message to the AGW of the originating IMS network.
  2. The AGW of the originating IMS network forwards the SIP INVITE message to the IWF.
  3. The IWF translates the SIP INVITE message into the protocol used by the destination IMS network and sends it to the AGW of the destination IMS network.
  4. The AGW of the destination IMS network forwards the SIP INVITE message to the BGW of the destination IMS network.
  5. The BGW of the destination IMS network sends a SIP 180 Ringing message to the AGW of the originating IMS network to indicate that the destination endpoint is ringing.
  6. The BGW of the destination IMS network sends a SIP 200 OK message to the AGW of the originating IMS network to indicate that the call has been established.
  7. The AGW of the originating IMS network sends a SIP ACK message to the BGW of the destination IMS network to acknowledge the establishment of the call.

The media flows in the II-NNI interface are established using the SDP protocol. The SDP protocol is used to negotiate the media parameters between the two endpoints, including the codecs to be used, the transport protocols to be used, and the QoS parameters.

The media flows are established after the signaling flows have been completed. The media streams are exchanged directly between the endpoints using the negotiated transport protocol and codec. The media streams are transported over the IP network using the Real-time Transport Protocol (RTP) and the Real-time Control Protocol (RTCP).

Quality of Service Mechanisms

The II-NNI interface includes mechanisms for ensuring the quality of service (QoS) of the multimedia sessions. The QoS mechanisms are implemented at different levels of the architecture, including the AGW, the IWF, the BGW, and the IBCF.

The QoS mechanisms at the AGW include packet classification, marking, and policing. The AGW classifies the packets based on their IP header information and applies the appropriate QoS policy to each packet. The AGW also marks the packets with the appropriate DiffServ code point (DSCP) value to indicate the desired QoS level. The AGW polices the packets to ensure that they conform to the agreed-upon QoS parameters.

The QoS mechanisms at the IWF include protocol interworking and media transcoding. The IWF ensures that the signaling and media protocols used by the two IMS networks are compatible and translates them if necessary. The IWF also transcodes the media streams if the codecs used by the two endpoints are not compatible.

The QoS mechanisms at the BGW include traffic shaping, scheduling, and admission control. The BGW shapes the traffic to ensure that it conforms to the agreed-upon QoS parameters. The BGW schedules the traffic to ensure that the highest-priority traffic is transmitted first. The BGW also performs admission control to ensure that the network resources are not overloaded.

The QoS mechanisms at the IBCF include security, authentication, and authorization. The IBCF ensures that the interconnection between the two IMS networks is secure and that only authorized traffic is allowed to pass through. The IBCF also performs authentication and authorization to ensure that the users are authorized to access the multimedia services.

Conclusion

The II-NNI interface is a key component of the IMS architecture that enables the interconnection of IMS networks from different operators or service providers. The interface uses standard protocols such as SIP, Diameter, and SDP to exchange signaling and media traffic between the two IMS networks. The interface includes functional entities such as the AGW, IWF, BGW, and IBCF that are responsible for implementing the signaling and media planes. The interface also includes mechanisms for ensuring the quality of service of the multimedia sessions, including packet classification, marking, policing, traffic shaping, scheduling, admission control, security, authentication, and authorization.