In a cellular wireless system, power thresholds used for determining whether or not to add or drop servers held in a set of servers, such as an active set, for serving a user equipment are determined on the basis of the motion of the respective user equipment. A lower threshold may be used with respect to the adding or dropping of a given server to or from a set of servers for serving a fast moving user equipment than would be used for the same server with respect to its adding or dropping to or from a set of servers for serving a slow moving user equipment, while maintaining a given quality of service. As a result, the average number of servers held in sets is reduced in a typical network that comprises a plurality of user equipments moving at different speeds, compared to a situation in which the threshold is set irrespective of the motion of a user equipment. A reduction in the average number of servers held in sets of servers for serving the user equipment has the benefit of reducing data traffic loading in a backhaul network, since the need to send duplicated data to each member of sets of servers is reduced and/or increasing network capacity, since the radio resource is used more efficiently because the proportion of servers sending duplicate data is reduced.