<title>Transfer delays in a SIMA network</title>

Abstract

Differentiated Services approach has been recently proposed for providing Quality of Service in Internet. Simple Integrated Media Access is a network service concept that use this approach by utilizing priority bits in packet headers. Three drop preference bits are used for making the decision about packets to be discarded when congestion arises in a core network node. A single delay indication bit marks the packet either as belonging to a realtime or non real time flow. The acceptance of packets is not directly connected with real time properties of a packet. The real time support of SIMA is based on two features. First, real time packets are placed to a short real time buffer inside a core network node. Secondly, the real time buffer is always emptied before the longer, non real time buffer. This means a shorter queuing delay for real time packets. For the successful operation of the network it is crucial to have small enough transfer delay for real time packets. On the other hand, the difference in transfer delay must be significant enough between real time and non real time service in order to justify their existence. Finally, there is also the question, whether real time packets can block non real time flows since the real time packets are always transmitted first in a SIMA node. To answer these questions this paper addresses the issues related with transfer delay of packets over a SIMA network. We present extensive simulation results about the transfer delay distributions in order to provide insight into SIMA and transfer delays.