Non-GPRS Alert Flag

Description

The Non-GPRS Alert Flag (NGAF) is a legacy signaling parameter defined within the 3GPP core network specifications, specifically for GSM and UMTS networks. It is an information element used in the Mobile Application Part (MAP) protocol, primarily in messages like Send Routing Info (SRI) and Provide Subscriber Info (PSI), exchanged between network entities such as the Home Location Register (HLR), Visitor Location Register (VLR), and Gateway MSC (GMSC). Its primary function is to convey the packet-switched (PS) attachment status of a mobile subscriber from the HLR to the circuit-switched (CS) domain entities.

NGAF works by being set in the subscriber's profile within the HLR. When a CS domain entity (like a GMSC trying to route a voice call) queries the HLR for routing information, the HLR includes the NGAF in its response if the subscriber is currently attached to the GPRS (packet-switched) network. This flag acts as a simple binary indicator (alert or no alert) to inform the CS network that the subscriber has an active PS context. This information can then be used by network logic to influence call handling procedures, particularly for implementing services that require awareness of the subscriber's dual-domain status.

Architecturally, NGAF is a component of the MAP protocol suite, which operates over the SS7 signaling network. It is a key element in the coordination mechanism between the CS core network (MSC/VLR) and the PS core network (SGSN/GGSN). Its role was crucial in early 2G/3G networks to support basic simultaneous voice and data awareness, allowing features like network-initiated detach procedures or optimized routing decisions based on the subscriber's attachment state, although it provided only a simple alert rather than detailed session information.

Purpose & Motivation

NGAF was created in the early phases of GPRS introduction (3GPP Release 4) to address a fundamental coordination problem between the newly introduced packet-switched domain (GPRS) and the existing circuit-switched domain. Prior to GPRS, the core network only handled voice and SMS via the CS domain. The introduction of data services created a scenario where a subscriber could be simultaneously attached to both domains, but the CS network entities had no inherent way of knowing this. This lack of awareness could lead to suboptimal network behavior, such as unnecessary paging attempts in both domains.

It solved the problem of basic inter-domain awareness. The flag allowed the HLR, as the central subscriber database, to notify CS entities (like the MSC) about the subscriber's GPRS attachment status. This enabled early implementations of features like 'GSM Suspend' during a circuit-switched call, where the packet session could be temporarily suspended, and optimized paging strategies. Its creation was motivated by the need for a simple, backward-compatible signaling mechanism to facilitate the coexistence and rudimentary interaction of CS and PS services without requiring a complete overhaul of the existing MAP protocols.

Key Features

• Binary indicator of subscriber's GPRS attachment status
• Transported within MAP signaling messages (e.g., SRI, PSI)
• Set by the HLR based on subscriber's PS registration state
• Used by CS network entities for call handling decisions
• Enables basic coordination between circuit-switched and packet-switched domains
• Part of legacy GSM/UMTS core network signaling

Evolution Across Releases

Defining Specifications