Optimal Routeing for Late Call Forwarding

Description

Optimal Routeing for Late Call Forwarding (ORLCF) is a network intelligence feature defined within the 3GPP specifications for circuit-switched telephony services, primarily in GSM and UMTS networks. It addresses the specific scenario where a call cannot be completed to the intended recipient (the B-party) because the subscriber is not reachable (e.g., switched off, out of coverage), and the call is subsequently forwarded to a pre-defined forwarding number (the C-party). The core function of ORLCF is to determine and establish the most efficient signaling and bearer path for this forwarded call leg, bypassing unnecessary network nodes to reduce latency and conserve transmission resources.

Architecturally, ORLCF involves logic within the Mobile Switching Centre (MSC) or Gateway MSC (GMSC) that handles the call. When a call setup attempt fails due to the subscriber being unreachable, and Call Forwarding on Not Reachable (CFNRc) is active, the serving MSC retrieves the forwarding number from the Home Location Register (HLR). Without ORLCF, the call might be routed back through the originating GMSC or via the B-party's MSC before reaching the C-party. ORLCF enables the network to perform an optimal routeing analysis, often allowing the MSC that detected the unreachable condition to directly route the call to the C-party's network or to break out to the PSTN at the earliest possible point.

The implementation relies on signaling protocols like ISDN User Part (ISUP) or Bearer Independent Call Control (BICC). Key parameters in the call setup messages, such as the Original Called Party Number and the Redirecting Number, are used to inform downstream switches about the forwarding event, allowing them to apply optimal routing policies. This prevents tromboning, where call legs double back through the same international or inter-operator gateways. ORLCF is a cornerstone of intelligent network (IN) or Camel-based service execution, ensuring that supplementary services like call forwarding do not degrade overall network performance. Its role is critical in minimizing inter-operator transit costs and improving call setup times for forwarded calls, which is especially important for international roaming scenarios.

Purpose & Motivation

ORLCF was created to solve inefficiencies inherent in basic call forwarding implementations in early digital mobile networks. Prior to its standardization, a forwarded call, especially after a late event like 'subscriber not reachable,' could follow a suboptimal path. For example, an international call to a roaming subscriber might be routed to the visited country's MSC, only to be forwarded back to a number in the caller's home country, creating an expensive and lengthy 'trombone' route across international links twice. This wasted transmission bandwidth, increased call setup delay, and incurred unnecessary inter-operator charges.

The primary motivation was operational and economic: to reduce network load and minimize interconnection costs between operators. By enabling more direct routing for forwarded calls, ORLCF lowers the signaling traffic on core network links and reduces the occupation times of expensive international circuits. From a service perspective, it also improves the user experience by decreasing the post-dial delay for the calling party when their call is forwarded. Its introduction in Release 4 was part of a broader 3GPP effort to optimize circuit-switched core network operations and signaling efficiency as networks scaled and international roaming became more prevalent.

Key Features

• Optimizes routing for Call Forwarding on Not Reachable (CFNRc) conditions
• Reduces signaling load and transmission resource usage by avoiding tromboning
• Utilizes ISUP/BICC parameters like Redirecting Number for route analysis
• Implemented as logic in the MSC/GMSC call control function
• Lowers inter-operator transit costs for forwarded calls
• Decreases call setup time for the originating party

Evolution Across Releases

Defining Specifications