Active Cell Discard Mechanism in Atm Networks

When dealing with packet oriented communications over an ATM network, some problems arise. The most important one is that of having to discard an entire packet when only one ATM cell of that packet has been lost. Thus, we can see that the important parameter in this type of communications is not the Cell Loss Ratio (CLR), but the Packet Loss Ratio (PLR). This paper presents a general and efficient congestion control mechanism for packet oriented communications over ATM. The proposed mechanism is called Active Cell Discard (ACD) and it derives from the Partial Packet Discard (PPD) and Early Packet Discard (EPD). Its objective is to use a threshold in the buffer to control that there is always enough free space to queue the set of initial cells of every packet. The usefulness of this mechanism becomes clear if we think that critical information of the connection (like synchronization or control) is located in that set of initial cells. Introduction Maybe the most important characteristic of an ATM network is its flexibility, i.e. the ability of the network to accept all kinds of traffic: real-time and non-real time, interactive and non-interactive, connection-oriented or connectionless oriented ... In fact, the goal of ATM is to offer a unique bearer service able to deal with heterogeneous traffic. In a general ATM communication, one of the important parameters that controls the performance of the network is the Cell Loss Ratio (CLR). However, when packets are transmitted, the unit of information that must be preserved by the network is no longer the cell but the packet, because once one cell of the packet is lost, that packet is not useful for the end-user. But in that case, the remaining cells of the lost packets are wasting network resources unnecessarily. The objective of selective cell discarding mechanisms is either reacting or preventing congestion by making the waste of network resources as low as possible. This is accomplished by selectively eliminating those cells belonging to a lost packet. As a continuation of the work presented in (DOM 1995a) and (DOM 1995b), this article presents a congestion control mechanism which also derives from the PPD and EPD mechanisms (ROFL 1995). This mechanism is called Active Cell Discard (ACD). The main objective of this article is to show the behavior of the ACD mechanism without the influence of upper layer protocols (like TCP) and to compare it with other mechanisms in the same simulated conditions. In the next section, some of the results obtained in ROFL 1995 are stated. Then, we present the Active Cell Discard Mechanism. And the source model implemented is discussed in the following section along with the considerations made in our simulations. We also show the results of our simulations and finally we present our conclusions and the work to be done. * This work has been funded by the Spanish Ministry of Education (CICYT) under grant number TIC950982-C02-02 and by the Catalonian Research and Universities Council (CIRIT 1995SGR-00464).