Service selection gateway (SSG) supporting tariff changes for traffic volume

A service selection gateway (SSG) which receives data indicating tariff switching points, and sends accounting records enabling accurate computation of traffic volumes in each switching duration is described. In one embodiment, a counter value indicating the traffic volume for each tariff duration is sent. In an alternative embodiment, at least one accounting record containing two counters is sent in each duration, with one counter indicating an aggregate count of traffic volume (e.g., since the subscriber started a session) and another counter containing a marginal (or incremental) value since a switching point. Such an accounting record can be sent at any time point in a tariff duration, and the traffic volume in each tariff duration can be computed accurately.