Point Of Interconnection

Description

The Point Of Interconnection (POI) is a critical network interface defined in 3GPP specifications that serves as the demarcation point between a mobile network operator's core network and external networks, primarily the Public Switched Telephone Network (PSTN). It encompasses both physical connectivity, such as cabling and ports, and logical protocols for signaling and media transmission. In architectural terms, the POI is typically located at the boundary of the core network, involving network elements like the Mobile Switching Center (MSC) in 2G/3G or the Media Gateway (MGW) and IMS components in 4G/5G. The POI facilitates the exchange of voice calls, SMS, and other circuit-switched services between cellular users and PSTN subscribers, ensuring interoperability across different network technologies. Key components include signaling interfaces like SS7 (Signaling System No. 7) for call setup and tear-down, and media gateways that convert between packet-switched (e.g., IP in LTE/5G) and circuit-switched (e.g., TDM in PSTN) formats. The POI operates by routing incoming and outgoing traffic based on numbering plans (e.g., E.164), with signaling protocols handling address translation, billing, and service features. In 3GPP networks, the POI evolves with technology generations: in 2G/3G, it involves direct MSC-to-PSTN links; in 4G, it integrates with IMS through elements like the Breakout Gateway Control Function (BGCF) and MGW for VoLTE; and in 5G, it supports interconnection via the 5G Core and PSTN/ISDN simulation services. The POI plays a vital role in ensuring end-to-end service continuity, enabling mobile users to communicate with landline phones and vice versa. It also supports regulatory requirements for interconnection, such as number portability and emergency services access. Technically, the POI must handle high reliability and low latency to maintain call quality, with redundancy mechanisms to prevent service disruptions. Standards like 3GPP TS 23.271 specify POI requirements for location services, while others cover security and protocol details, ensuring consistent implementation across operators.

Purpose & Motivation

The POI exists to solve the fundamental problem of interconnecting disparate telecommunication networks, enabling seamless communication between mobile cellular systems and the global PSTN infrastructure. Historically, as mobile networks emerged (starting with GSM in 2G), there was a need to interface with existing fixed-line networks to allow subscribers to call any phone worldwide. Without a standardized POI, operators would face interoperability challenges, leading to service fragmentation and increased costs. The motivation for defining POI in 3GPP standards, beginning in Release 99, was to establish uniform technical and operational requirements for interconnection, ensuring reliable call routing, signaling, and billing across network boundaries. It addresses limitations of proprietary interfaces by providing a common framework that supports voice, SMS, and supplementary services, facilitating competition and regulatory compliance. As networks evolved to packet-based technologies like LTE and 5G, the POI adapted to handle IP-based interconnection while maintaining backward compatibility with legacy PSTN, solving the transition challenge from circuit-switched to all-IP networks. The POI also enables critical services like emergency calls (e.g., 911, 112) by providing a gateway to public safety answering points, and supports lawful interception and security functions as specified in 3GPP security specs. By standardizing the POI, 3GPP promotes global roaming and service interoperability, which is essential for the telecommunications ecosystem's growth and user experience.