Open Group

Description

An open group is a concept in 3GPP group communication services, defined as a group that does not have a pre-defined set of members. Any user may participate in an open group, meaning there is no subscription or authorization required to join. This is in contrast to a closed group, where membership is restricted to a specific set of users. Open groups are used in services such as group calls, push-to-talk, and other multicast/broadcast applications where open participation is desired.

The architecture for open groups involves group communication service enablers, such as the Group Communication System Enabler (GCSE) or Mission Critical Push To Talk (MCPTT). These enablers manage group sessions, including the creation, modification, and deletion of groups. For an open group, the group management function does not maintain a member list; instead, any user can join the group by simply requesting participation. The group identity is used to identify the group, and the network routes messages to all participants. The lack of a pre-defined member list simplifies group management and reduces signaling overhead for group creation.

Key components include the group management server, which handles group definitions and policies, and the group communication server, which manages the actual communication sessions. In an open group, the group management server may still define policies such as maximum number of participants, quality of service (QoS) parameters, and security settings. However, it does not enforce a member list. The group communication server uses multicast or unicast bearers to deliver data to participants, depending on the service and network capabilities. For example, in LTE, the Multimedia Broadcast Multicast Service (MBMS) can be used for efficient group communication.

The operation of an open group involves a user initiating a request to join the group, which is granted automatically if the group is open. The network may perform authentication and authorization based on the user's subscription, but not based on group membership. Once joined, the user can send and receive group communications. The group may be identified by a Group ID, which is used in session setup and control signaling. Open groups are particularly useful for public safety, where ad-hoc groups may need to be formed quickly without pre-provisioning members, or for commercial services like social networking or live events.

In summary, open groups provide a flexible and scalable way to manage group communications without the need for predefined membership. They are defined in 3GPP specifications such as 21.905 and 22.060, and have been supported since Release 4. The definition has remained consistent across releases, with only minor formatting changes.

Purpose & Motivation

Open groups were introduced to support group communication services where membership is not restricted, enabling ad-hoc and dynamic participation. This addresses scenarios where pre-defining a set of members is impractical or undesirable, such as in public safety incidents, live events, or social applications. Before open groups, group communication typically required a closed group with a known member list, which limited flexibility and increased management overhead.

The motivation for open groups comes from the need for scalable and efficient group communication in various contexts. For example, in mission-critical push-to-talk (MCPTT), first responders may need to communicate with colleagues from different agencies who are not pre-provisioned in a closed group. An open group allows any authorized user to join the communication, facilitating interoperability and rapid response. Similarly, in commercial services, open groups enable users to participate in group chats or broadcasts without prior invitation.

Open groups solve the problem of rigid membership management by eliminating the need for a pre-defined member list. This reduces the complexity of group creation and maintenance, as the network does not need to store and update member lists for each group. It also enhances scalability, as the group can accommodate an arbitrary number of participants, subject to network capacity. The concept has been standardized since Release 4 and remains relevant in modern group communication enablers, ensuring backward compatibility and widespread applicability.