In one example, a session management function entity is configured to obtain an indication of a backup resource usage quota associated with a rating group of a user session for a user equipment configured to communicate via a mobile core network. The rating group is subject to online charging. The session management function entity determines that a charging function entity configured to provide an indication of a primary resource usage quota associated with the rating group is unreachable. In response to determining that the charging function entity is unreachable, the session management function entity allocates the backup resource usage quota associated with the rating group while the charging function entity is unreachable.