7. Flow Control in Sampled Data Systems

For telecommunication operators, the cost of running a particular algorithm is an important factor when deciding about its applicability in the supervised network. Therefore, the business point of view will generally favor such control strategies which allow for the explicit specification (or at least estimation) of the amount of the exchanged feedback information in relation to the transferred users’ data. In consequence, emitting feedback information carriers at regular time intervals (which is assumed in traditional approaches for discrete-time network modeling, e.g., [2–5]) is not cost efficient in the economical terms. Instead, one can send a feedback information carrier every N data packets and, in this way, place a direct limit on the extent of the transmitted management traffic with respect to the profit generating transmission of the users’ data. Since this method relies neither on continuous feedback information availability nor on maintaining the synchronization of constant sampling period (which is a serious challenge in multisource systems [1]), it is more scalable and requires less control effort than the classical regulation schemes presented in the literature. However, sending a control unit every N data packets, and not every T seconds, means that (after RTT) the feedback information will be available for rate adaptation at the sources at irregular time instants. Consequently, in order to maintain adequate system performance, the variable, input-dependent sampling period should be explicitly accounted for in the design of flow control algorithm. We will show, however, that the controllers developed for the system with constant discretization period in Chaps.​ 5 and 6 can be quite intuitively adapted for the case of variable sampling rate analyzed here. Moreover, we will demonstrate that provided that certain additional constraints are met, the new strategies maintain the favorable properties of constant-sampling-rate controllers. In particular, the proposed schemes will be shown to eliminate packet losses originating from unknown bandwidth variations and to ensure full bandwidth usage.