Originating Local Exchange

Description

The Originating Local Exchange (OLE) is a network function defined within the 3GPP IMS Centralized Services (ICS) architecture, introduced to enable a consistent service experience for users regardless of whether they are connected via a circuit-switched (CS) radio access network (like GERAN/UTRAN) or a packet-switched (PS) access like LTE. The OLE is essentially a Mobile Switching Centre (MSC) or MSC Server that has been enhanced with ICS capabilities. Its primary role is to handle the CS access leg for a User Equipment (UE) that wishes to use IMS-based services (like voice, video, supplementary services) while camped on a 2G or 3G network. When a UE initiates a call, the OLE intercepts the CS call setup. Instead of providing services locally from the CS domain, the OLE establishes a bearer path to the IMS network and relays the call control signaling (using SIP over an Iq interface) to the user's IMS core, specifically to the S-CSCF (Serving Call Session Control Function). The OLE acts as a SIP User Agent on behalf of the UE for the CS access leg, translating between CS signaling (e.g., ISUP, BICC) and SIP. This allows the IMS core, containing the user's service profile, to control the call entirely—applying services like number translation, call forwarding, and multimedia features. The OLE manages the media bearer, connecting the CS radio bearer to an IP-based bearer (e.g., over Mb interface) that leads into the IMS media path. It works in tandem with the Terminating Local Exchange (TLE) for the called party's side. Key protocols involved include SIP (between OLE and IMS), ISUP/BICC (for interconnection), and RTP/RTCP for media transport once converted to IP.

Purpose & Motivation

The OLE was created to solve the problem of service fragmentation during the transition from circuit-switched to all-IP IMS networks. Before ICS, a user on a 2G/3G CS network would receive services from the legacy CS network (MSC/HLR), which were often different and less rich than the services available in IMS. This created a inconsistent user experience. ICS, and the OLE as a key component, was motivated by the desire to 'centralize' service control in the IMS core regardless of access type. It addresses the limitation of having duplicate service logic in both CS and PS domains, simplifying network architecture and service deployment for operators. The OLE enables operators to deploy IMS-based services (like VoLTE/RCS) and have them also used by subscribers on legacy CS networks, protecting investments and ensuring a smooth migration path. It solves the specific problem of maintaining advanced supplementary services (like call waiting, hold, forwarding) consistently when a user moves between LTE (VoIP) and 2G/3G (circuit-switched voice) coverage.