Answer Message

Description

The Answer Message (ANM) is a specific message within the Integrated Services Digital Network User Part (ISUP) protocol, which is a key component of the Signaling System No. 7 (SS7) suite used in traditional Public Switched Telephone Networks (PSTN) and early cellular networks like GSM. ISUP is responsible for the setup, management, and teardown of voice calls over circuit-switched connections. The ANM is sent in the backward direction, from the destination exchange (or mobile switching center) back towards the originating exchange, following the successful delivery of an Initial Address Message (IAM) and an Address Complete Message (ACM).

Architecturally, the ANM is generated by the exchange that serves the called party once the called device goes off-hook (in a fixed network) or the mobile subscriber answers the call. This generation is triggered by an event from the access network, such as a CONNECT message in GSM. The message traverses the signaling path established during call setup, informing each intermediate exchange along the route that the call has been answered. This allows these exchanges to update their internal call state and, crucially, to connect the voice path through the circuit-switched network, enabling two-way speech.

The message itself contains parameters that identify the specific call using a Circuit Identification Code (CIC) and other routing labels. Its reception at the originating exchange is the definitive trigger to start call timing for billing purposes. In the context of 3GPP specifications, ANM is referenced in the context of interworking between GSM/UMTS core networks and external PSTN/ISDN networks, as well as in CAMEL (Customised Applications for Mobile network Enhanced Logic) for prepaid billing control. While its use is primarily in legacy circuit-switched domains, understanding ANM is essential for comprehending the signaling flows that modern IP-based systems like VoLTE and VoNR evolved from or interwork with.

Purpose & Motivation

The ANM exists to provide a reliable, in-band signaling mechanism to confirm that a called party has answered a telephone call within a circuit-switched network. Prior to standardized signaling systems like SS7, call supervision (detecting answer, hang-up) was often done using in-band tones or DC loop supervision on physical trunks, which were less reliable and more limited in functionality. The ISUP protocol, and the ANM message specifically, solved the problem of efficient, out-of-band call control across complex, multi-vendor telecommunications networks.

Its creation was motivated by the need for automated billing and efficient resource utilization. Without an explicit answer signal, networks could not accurately determine when a billable conversation began, leading to potential revenue loss or customer disputes. Furthermore, the ANM allows network switches to commit transmission resources (the timeslot or circuit) for the voice bearer only after the call is confirmed as answered, preventing wasteful reservation of resources for calls that are never picked up. It is a fundamental building block for creating a predictable, billable, and reliable telephony service on a global scale.