Buffer Management

The token cycles on adjoining network segments are not synchronized and this could lead to longer latency because of temporal skews of the token arrival times on connected segments. Since the token rotation times (TRT) in both segments are the same, the maximum skew between them could be one TRT long. For example, in the worst case, the switch receives data from the incoming segment but just missed the token on the outgoing segment. Hence, it may have to wait as much as one TRT before forwarding the data onto the outgoing segment. In the meantime, data would start arriving on the incoming segment for the next cycle, leading to buffer overflow if there is only one buffer at the switch. To avoid this situation, we use a double buffering scheme in which there are two buffers for each real-time stream going across the switch. While one buffer is being filled by the input sub-connection on one segment, the other is emptied out by the output sub-connection when the token on the outgoing segment arrives. At the end of each token cycle, the roles of the two buffers switch.

The size of the buffer has a direct effect on the end-to-end latency experienced by the applications. This implies that the buffering delays at each hop must be small and that the number of hops that a real-time connection can cross, is bounded. In theory, the token cycle times on all network segments are supposed to be the same, and in each cycle the switch receives exactly one frame of data from the incoming segment and sends out exactly one frame of data on the outgoing segment. Therefore, the real-time connection suffers a maximum of one token cycle latency at each hop along the path, and the worst-case end-to-end latency is Number of hops * Token cycle time. The minimum latency would be the time to transmit the data from the sender to the receiver as though they were on the same segment plus the time to copy the data into memory and out at each intermediate switch. Unfortunately, in practice, neither the network segments have identical token cycle times, nor does the switches forward the data they receives immediately.

  

Figure: The network setup for multi-segment RETHER experiments. There are four segments, two of them 100-Mbps and the other two 10-Mbps Ethernets.