Originating Network

Description

The Originating Network is a core network concept in 3GPP specifications, defined as the network where the calling party is located. It is the network that initiates a communication session, such as a voice call or a data session, on behalf of the calling party. The originating network is responsible for call setup, authentication, authorization, and routing the call towards the terminating network, which is where the called party is located. This concept is fundamental to understanding how calls are established and managed across different networks, including roaming scenarios and inter-network operations.

In a typical mobile communication scenario, when a user initiates a call, the request is first processed by the originating network. This network may be the user's home network or a visited network if the user is roaming. The originating network performs several key functions: it authenticates the user, checks the user's subscription and service permissions, and then sets up the call by allocating resources and establishing a bearer. It also determines the route to the terminating network, which may involve interworking with other networks or protocols. The originating network may also handle charging and policy control for the call.

The concept of the originating network is closely tied to the architecture of the Public Land Mobile Network (PLMN). Each PLMN can act as an originating network for calls initiated by its subscribers. In roaming scenarios, the visited network acts as the originating network for calls made by roaming subscribers, while the home network may be involved in authentication and authorization. The originating network must support various interfaces and protocols to communicate with other networks, such as the Nc interface for inter-network call control in the IP Multimedia Subsystem (IMS).

In IMS, the originating network includes functional entities such as the Proxy Call Session Control Function (P-CSCF) and the Serving Call Session Control Function (S-CSCF). The P-CSCF is the entry point for the user equipment (UE) and handles SIP signaling, while the S-CSCF performs session control and interacts with application servers. The originating network may also include the Media Gateway Control Function (MGCF) for interworking with circuit-switched networks. The terminating network, on the other hand, includes similar entities but for the called party.

The Originating Network concept has been stable across 3GPP releases. It was introduced in Rel-4 and has been carried forward with minimal changes. The definition "The network where the calling party is located" has remained consistent, although the formatting of the definition text has varied slightly (e.g., line breaks). This stability reflects the fundamental nature of the concept, which is essential for call control and inter-network operations. The specifications 21.905 and 22.066 provide the definition and context for this term.

Purpose & Motivation

The Originating Network concept exists to clearly delineate the responsibilities and functions of the network that initiates a communication session. In a multi-network environment, it is crucial to distinguish between the network where the calling party is located and the network where the called party is located. This distinction enables proper call routing, charging, and policy enforcement. Without a clear definition of the originating network, it would be difficult to manage calls that span multiple networks, especially in roaming and interworking scenarios.

Historically, as mobile networks evolved from circuit-switched to packet-switched and later to all-IP networks like IMS, the need for a standardized way to identify the originating network became more pronounced. In early GSM networks, call control was simpler, but with the introduction of GPRS and IMS, the complexity increased. The Originating Network concept provides a common understanding for network operators and equipment vendors, facilitating interoperability and seamless services.

The concept also supports regulatory requirements, such as emergency call handling and lawful interception, where it is important to know which network initiated the call. Additionally, it aids in troubleshooting and network management by providing a clear reference point for call-related procedures.