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High Speed Local Area !Vetworks (HSL.-LV-sl sre communication systems operating at data rates exceeding 100 Megabit per second. Since directly connecting each user to such a HSLAiL will be -O~J tvst/.v. 3 hierarchical structure seems CO be an appropriate solution. i I i this concept. users are attached to possibly heterogeneous 'lowspeed' local area subnetworks which are irrterconnected by a high speed backbone. In this paper we studj-,Multiple Token Rings tor mixed traffic imposed by different applications and reaching the HSLAIV r-ia Local Area .Vetworks (LAXs). Approximate analysis based on the ri'rst ta-o moments o f interarrival time and token rotation time $-ieIds results extremely close to simulation results. The Multiple Token Ring appears to be an excellent candidate for the en r-isaged scenario. However, buffer dimensioning a t the destination LA#studied by simulation onlyproves to be one o f the most critical points during the design phase o f a H S L A X .

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High Speed Local Area Networks (HSLANs) operate at channel capacities of at least 100 Mbitls. Taking advantage of the progress in optical transmission technique the first HSLAN based on a token passing ring, namely FDDI, has now become commercially available. However, several other strategies to determine the token holding duration at a station may be applied (e.g. single service or exhaustive service). Thus, in this paper we evaluate and compare the performance of these token ring protocols for a high speed backbone network interconnecting possibly heterogeneous conventional local area subnetworks (LANs).With FDDI now the frst High Speed Local Area Network on the basis of an optical token ring has become commercially available. Basically aiming at backend applications (e.g. mainframe interconnection) FDDI evolved during the standardizaton process in the 1980s towards other environments. Today, the interconnection of possibly heterogeneous LANs like Ethernet, token ring or token bus by a FDDI backbone appears to be one of the most important applications. Figure 1 shows a rather simple model of such an environment comprising media access control (MAC) and physical layer only. The transmit queue of the HSLAN station buffers packets arriving from the source LAN. The corresponding server processes the packets according to the media access protocol and transmits them on the backbone. In this paper we refrain from taking into account impacts of processing times at the gateway stations. Fig. 1: HSLAN-Station ModelTaking advantage of the special properties of optical transmission technology the hierarchical backbone structure enables data communication in a broader site like an university campus or a large indusmal site than LANs are able to. 'The media access in FDDI is controlled by a timed token protocol providing synchronous as well as asynchronous transmission. However, other procedures to determine the token holding time upon capturing a free token may be applied, e.g. single service or exhaustive service.In this paper we study and compare these token holding strategies in Multiple Token Rings, namely Single Service, adaptive token holding duration (provided by FDDI [l]) and Exhaustive Service, respectively, €or the backbone environment mentioned before. M o d --In token ring networks stations are actively coupled by unidirectional point-to-point links. Access to the common medium is controlled by a deterministic strategy: a unique bit pattern, the so called 'free token', is passed from station to station around the ring. Backlogged stations remove the free token from the ring before transmission starts. The station currently holding the token is given control over the transmission channel. It turns out, that the multiple token approach ('early token release') is the most suitably concept for high speed token rings 191. In this strategy a new free token is released immediately after the end of transmission. The token ring access mechanism may be modeled by a single cyclic server representing the rotation of the fre...