Holdoffs indicate the number of times that the radio transmitter could not transmit because the receiver detected the "carrier busy" message. Large numbers of holdoffs can occur due to many other Wireless LAN devices that transmit in the area, or due to the presence of other devices, for example, cordless telephones, that operate in the 2.4GHz range.
It indicates that a transmission was heldoff by other activity longer than the length of time it would take to transmit the longest allowed 802.11 packet. This is usually an indication of some sort of outside interference. Things that are known to cause holdoffs: 2.4Ghz cellular phones, wireless frequency hopping devices, interference form anything running in the 2.4 Ghz range, and excessive traffic.
Remove the interference source and try reducing the number of station per access point.
Radio Error Statistics
The Radio Error Statistics Display provides a detailed summary of radio receiver and transmitter errors. In order to access the Error Statistics Display, select Main > Statistics > Radio.