TABLE OF CONTENTS Some aspects of the PHY layer, such as rate selection to minimize FER, are implemented in conjunction with the MAC functions and even the device driver or operating firmware. They can thus can only be verified using system-level tests; that is, tests on the complete system, with all components integrated and running the expected operating firmware. This subsection therefore treats typical system-level tests.
It has been observed by every 802.11 user that the achievable effective transfer rate of an 802.11 link within a given environment depends quite Significantly on the distance between the AP and its associated client. As the distance increases, the signal strength at the receivers on each end of the link drops off; this is due both to the reduction in field strength as the distance from the transmit antenna increases, the increased number of attenuating elements (walls, furniture, etc.), and increased multipath between the two ends of the link. The consequence is that the signal-to-noise (SNR) ratio falls, and bit errors rise sharply for a given modulation type. The reduced SNR causes the system to drop its PHY bit rates (see below) to maintain efficient data transfer, and also causes an increase in retransmissions. The user-visible effect is thus a drop-off in application layer network performance, caused by the reduction in overall data transfer rate of the 802.11 link.
As this drop-off of transfer rate determines the usable coverage of the 802.11 AP, it is...
