Generally, congestion can be defined as follows: A result of a mismatch between the network resources (buffer space, transmission capacity, and so on) and the amount of traffic admitted for transmission. Congestion, if once it occurs, manifests itself in two ways: Time delay will increase substantially in the network, and network throughput may begin decreasing with an increase of the admitted traffic.
In this dissertation, congestion control problem in communication networks is considered as a viewpoint of system-theoretic control problem. First, the state equation is established to represent operation of the target network and congestion controller is designed to prevent congestion in the network. Then, we analyze the overall system at the steady state to derive the network performance. Moreover, the stability of the overall system is investigated analytically. In most of the previous results, queuing theory is used as a basic tool in analyzing the target network. However, in most of communication networks, it is almost impossible to analyze the networks. Thus, the system-theoretic approach based on the averaging theory is an alternative to queuing approach.
Here, we study on congestion control in various types of communication networks that can be divided into two categories: single-hop networks and multi-hop networks. As typical examples of the single-hop networks, token-passing networks and multi-channel frequency-hopping code division multiple access with slotted ALOHA (FH-CDMA-S-ALOHA) random access for packet radio networks are considered in this dissertation. Token-passing networks have been used as a media access control sub-layer of the data link layer (Layer 2 among ISO seven-layer) of MAP (Manufacturing automation protocol). CDMA communication techniques have been used as attractive alternatives to conventional time and frequency division multiple access (TDMA and FDMA).
Specifically, in token-passing networks, we formulate the performance characteristics (throughput and time delay) of limited-service token-passing networks where each user has two prioritized buffers of finite capacity. Using these results, the effect of network parameters (buffer capacity, service discipline) on performance characteristics is investigated. Then, it is shown that the optimal choice of network parameters according to the design procedures can lead to a substantial improvement in the network's performance.
Subsequently, we propose a multi-channel FH-CDMA-S-ALOHA (Frequency-Hopping Code Division Multiple Access with Slotted ALOHA) random access for packet radio network, integrating the properties of both a multi-channel FH-CDMA (statistical multiplexing) and slotted-ALOHA (random access with retransmission) to achieve better performance (throughput) than each one, and present analytic formulas for performance evaluation of the proposed multi-channel FH-CDMA-S-ALOHA networks. Moreover, a retransmission control scheme for congestion prevention and performance maximization is developed and its stability is confirmed analytically.
Finally, as a main result, to prevent congestion in the packet-switching multi-hop networks that consist of switching nodes and communicating links, a rate-based feedforward-and-feedback congestion control scheme is proposed. First, assuming that only one link in a transmission path is congested (single congested-link case), we establish deterministic fluid-flow model in which packet's boundaries are ignored, and then develop a congestion controller concentrating on the robust control of an uncertain system and the pole assignment of a certain system, respectively.
Specifically, the robust congestion controller can guarantee robust stability of the overall system, if only the upper bound of the round-trip delays of all network traffics is available. By the pole assignment congestion controller, the desired performance of the overall system in terms of closed-loop poles can be achieved if all round-trip delays of the network traffics are given completely.
Additionally, we investigate on the multiple congested-link case.