Studies on Congestion Control Schemes for Best-Effort Traffic in ATM Networks

An ATM (Asynchronous Transfer Mode) is a promising technology for realizing B-ISDN (Broadband Integrated Services Digital Network) that can transfer many types of multimedia information. In ATM-based networks, all information is transferred by dividing into fixed-size packets (called as cells). The ATM network is a connection-oriented network by virtue of a notion of virtual circuits called VPs (Virtual Paths) and VCs (Virtual Channels). The switch capability can be implemented in hardware because all information is transferred as fixed-size cells, and because cell routing at the switch is rather simple due to connection-oriented communication. For these reasons, the ATM technology is able to realize gigabit-class networks, which had been considered to be difficult with conventional packet-switching technology. To handle multimedia information effectively, five types of service classes are defined by the ATM standard bodies according to QoS (Quality of Services) requirements of various kinds of applications. At a connection setup, an application has to declare its traffic parameters and negotiates its QoS requirements with the network. However, since most of the existing applications such as data communications generate best-effort traffic, it is usually difficult for applications to predict their traffic patters. Thus, it has been an important issue how to accomodate best-effort traffic into ATM networks. Since best-effort traffic manages to utilize available resources in the network, sophisticated congestion control is required to resolve congestion in the network. In the thesis, we therefore focus on congestion control schemes particularly for best-effort traffic. Congestion control schemes for ATM networks can be classified into two categories: internal congestion control that controls short-time congestion in a switch, and global congestion control that regulates cell flow from sources into the network. In the thesis, we first evaluate the performance of an input-output buffered type ATM switch with back-pressure function as an internal congestion control scheme. The back-pressure function prohibits cell transmission from input buffers to the corresponding output buffer to avoid buffer overflow due to temporary congestion in the switch. By using an analytic method, we derive the maximum throughput, the packet delay distribution, and the approximate packet loss probability of such an ATM switch with bursty traffic. In addition to a balanced traffic condition, conditions of unbalanced traffic and mixture of bursty and stream traffic are also analyzed. Through numerical examples, we show the effect of the average packet length and the output buffer size on the performance. We next focus on a rate-based congestion control algorithm that controls cell emission rates of sources according to feedback information from the network. The rate-based congestion control algorithm is applied to the ABR (Available Bit Rate) service class, and has been extensively developed and standardized by many researchers. Although behavior of source and destination are standardized, operation algorithms of ATM switches are left to manufacturers. Several switch algorithms have been proposed in the ATM Forum. While many studies have been devoted for these switch algorithms in the past, only the ABR service class is taken into account; that is, the effect of VBR and CBR service classes, in which multimedia traffic is accommodated, are not considered. In this thesis, we evaluate the performance of the rate-based