Point to Multipoint

Description

Point to Multipoint (PTM) is a core network service architecture defined by 3GPP for delivering content from one source to multiple recipients. It operates across multiple network domains, including the Core Network (CN) and Radio Access Network (RAN). The architecture involves specific functional entities and interfaces for managing session establishment, bearer management, and data distribution. In the CN, the Broadcast-Multicast Service Center (BM-SC) acts as the entry point, handling service announcement, security, and charging. For the RAN, PTM defines specific radio bearers and transmission modes, such as Multimedia Broadcast Multicast Service (MBMS), which can use either a dedicated multicast channel or a shared broadcast channel depending on the number of users in a cell.

The operation begins with service announcement and user subscription. When a PTM session is activated, the network establishes a common bearer path for data delivery. In the RAN, this involves configuring multicast radio bearers. Data packets are replicated at optimal points in the network—often at the radio base station (NodeB/gNB)—to minimize core network traffic. Key protocols include the MBMS protocols (GTP for user plane, and specific control plane signaling) and RAN protocols for scheduling and transmission over the air interface.

PTM's role is critical for scalable group communication. It underpins services like mobile TV, public warning systems, and software updates. By utilizing shared network resources, it significantly improves spectral efficiency and reduces network load compared to establishing individual Point-to-Point (PTP) connections for each user, making it a foundational technology for broadcast and multicast services in cellular networks.

Purpose & Motivation

PTM technology was created to address the inefficiency of using multiple unicast (point-to-point) connections for delivering identical content to many users. Prior to PTM, services like video streaming to a group would consume excessive network bandwidth and core network resources, as the same data packets were sent individually to each subscriber. This approach does not scale and becomes prohibitively costly for mass-market services.

The historical motivation stemmed from the growing demand for multimedia group communication, such as mobile TV and live event broadcasting, in 3G UMTS networks. 3GPP introduced the Multimedia Broadcast Multicast Service (MBMS) framework, which relies on PTM principles, to enable efficient one-to-many data delivery. PTM solves the problem of network congestion and inefficient radio resource usage by allowing data replication at the edge of the network (the RAN) rather than at the core.

Furthermore, PTM supports essential services like emergency alerts and group calls for public safety and commercial applications. It provides a standardized mechanism for service providers to offer broadcast/multicast services, creating new revenue streams while optimizing infrastructure utilization. The evolution into 5G with further enhancements like MBS (Multicast and Broadcast Services) continues this purpose for new use cases like V2X and immersive media.