Calculating Throughput

Several factors conspire to reduce the bandwidth that a CD-ROM server can deliver. The data transfer from the server must deal with the bandwidth and latency imposed by its CD-ROM drive, SCSI bus, and network interface card (NIC). On the receiving end, similar inefficiencies apply.

A detailed analysis of transmission protocols, packet sizes, overhead, and other variables might be interesting in the abstract. But for evaluating these servers, it's simpl er to do a back-of-the-napkin calculation based on the full 10-Mbps Ethernet bandwidth.

All the systems we tested use 8X or 12X CD-ROM drives, providing 1200- and 1800-Kbps transfer rates, respective ly. All drives were connected to a fast SCSI or equivalent, providing approximately 10-Mbps bandwidth. We measured the performance of the servers directly in bytes per second delivered to the client.

The graph shows the data delivered by each server in each of the three tests as a percentage of Ethernet bandwidth. The highest scores range from 81 percent to 83 percent bandwidth. This is consistent with a ballpark estimate of 15 percent to 20 percent overhead on the network, and it represents saturation. Servers scoring high in this test are in a good position to fill requests quickly.

Lower scores indicate that the server was unable to collect and transmit data fast enough to fill the network pipe. If you assume that other network traffic would likely reduce the available bandwidth by half, percentages as low as 40 percent still represent respectable throughput. Scores below 40 percent are indicative of unresolved contentions within the server itself.

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CD-to-Client Throughput

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The ability of these servers to fill the network pipeline varied from excellent to unacceptable.