The mechanisms for access to the physical channel used in the data link layer are one of the most controversial issues in Fieldbusses. Two approaches are widely used, the centralised and the distributed ones, the performance of which strictly depends on the traffic features. Whereas a centralised approach is efficient for cyclic traffic, the same does not apply to asynchronous traffic. In this paper we discuss and evaluate a new protocol, called the Slot Swapping Protocol (SSP), which is suitable for ring networks and makes it possible to meet the requirements of both cyclic and asynchronous traffic. The protocol, which is fully distributed, operates on a slotted ring and uses the principle of slot swapping to exchange data travelling on the ring. The ring is covered uninterruptedly by a slot train (with slots of a fixed length) which pass sequentially through all the stations. When a station needs to transmit information it uses a free slot, if there is one, or makes a swap, forcibly freeing a slot (in the way provided for by the protocol) and storing the swapped data in its local queue. This operation makes it possible to meet the time constraints of single messages and, although the system operates in a distributed fashion, it gives each station a global view of the traffic's time constraints. SSP guarantees respect of the time constraints of cyclic traffic, even when the workload is heavy, and the transmission of asynchronous traffic in as short a time as possible. Lastly, by using priorities, it allows critical variables (e.g. alarm signals) to be delivered in a very short time.
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