## 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.