University of Kansas AAI Throughput Studies

The current throughput limitations on the AAI are caused by bandwidth mismatches at the ATM switches connecting the LAN and WAN environments. The effects of the mismatches are exacerbated by the small switch buffers (256 cells) on the ATM switches currently in use.

The bound on throughput is expressed by the equation

throughput = Ri * L / ( T * ( Ri - Ro ))
where Ri is the input rate to the switch, Ro is the output rate, L is the buffer capacity, and T is the round trip time. Note that this is an upper bound, and assumes perfectly bursty TCP traffic, so your results may vary.

These effects are illustrated by the graph below, where the bound expressed above is the "Mismatch Limit" curve. The other curves express the limits imposed by the TCP window size. In the case of the OC-3c/DS3 rate mismatch, the 16 kB TCP window is adequate because the larger windows cannot be exploited due to the rate mismatch problem.

It is anticipated that this effect will diminish when the small buffer problem is addressed through switch upgrades in the near future. In the longer term, rate shaping at the ATM level should alleviate the problem.

Send any comments to Joseph B. Evans, evans@ittc.ku.edu.