critical unfairness characterized

We introduce two critical unfairness cases.

In the first case, downstream TCP connections are penalized with respect to upstream ones. This is explained as follows: packets belonging

to multiple downstream TCP connections are buffered inside the Access Point wireless interface. Note that the Access Point does not enjoy a privileged access to WLAN capacity, with

respect to user terminals. Hence, a single station transmitting upstream packets will get the same priority as that of the Access Point which needs to transmit downstream packets

heading towards many stations. Thus, downstream TCP connections suffer because of the arising congestion and corresponding packet losses happening in the download buffer

at the Access Point. These losses in conjunction with TCP congestion control mechanism cause the starvation of downstream connections. This is defined as "critically" unfair.

The second case arises from the interaction of multiple TCP connections in the upstream direction. In this case, the Access Point wireless interface has to transmit TCP ACK packets

traveling downstream towards stations in the WLAN. Also, in this case, we have a bottleneck because the Access Point can not access the medium with a priority higher than other

stations. Hence, the Access Point buffer will be congested leading to severe loss of TCP ACK packets. Due to the cumulative nature of TCP ACKs, few connections will be able

to "survive" and open their window, while the majority of connections will get starved. Note that this situation is not specific of our scenario; it can happen in whatever

environment characterized by heavy losses of ACK packets. This case is also another example of "critical unfairness".