An accurate estimate of host reliability is important for correct analysis of many fault-tolerance and replication mechanisms. In a previous study, we estimated host system reliability by querying a large number of hosts to find how long they had been functioning, estimating the mean timeto-failure (MTTF) and availability from those measures, and in turn deriving an estimate of the mean time-to-repair (MTTR). Howevel; this approach had a bias towards more reliable hosts that could result in overestimating MTTR and underestimating availability. To address this bias we have conducted a second experiment, using a fault-tolerant replicated monitoring tool. This tool directly measures TTe TTR, and availability by polling many sites frequently from several locations. We find that these more accurate results generally confirm and improve our earlier estimates, particularly for TTR. We also find that failure and repair are unlikely to follow Poisson processes.
The Group Allocation Multiple Access (GAMA) protocol for scheduling real-time and datagram traffic in a singlechannel, single-hop, packet-radio network is specified and analyzed. By maintaining a dynamically-sized cycle that changes in length depending on the amount of network traffic, GAMA is able to provide bandwidth guarantees to the members of its "transmission group"; member stations are able to transmit real-time data at a specified rate. Each cycle contains a contention period and a group-transmission period; a station with data to send competes for the right. to be added to the transmission group by successfully completing an RTS/CTS message exchange during the contention period. Once a station is a member of the transmission group, it is able to transmit a collision-free data packet during each cycle. As long as a station has data to send, it maintains its position in the group. This can be viewed as either allowing stations to "share the floor" in an organized manner, or as establishing frames that are not synchronized on a slot-basis and vary their length dynamically based on demand. The throughput and average delay of GAMA are presented and analyzed.
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