The phone will decide on which AP it will associate to based on the roaming rules.
It will monitor the beacon frames and start by looking for the strongest and try to associate to it. Then if this fails it will look for others. By the sound of it you do not have an overlapping coverage area giving the phone another valid choice other than the AP that has 2 many associations.
Have you considered using the Call adminsion control CAC feature instead max associations. This will force the phone to roam if there is not enough bandwidth to support a call, which appears to be your main goal.
Here is an extract from the design guide your account team should be able to get you the full document if needed
11.1 Call Admission Control (CAC)
CAC provides the mechanism that prevents the presence of the next voice call to interrupt or provide poor QoS for all existing voice calls. In the TDM world this is accomplished by a finite number of DS0 channels. In wired Ethernet AVVID networks, this is provided by an interaction between CallManager regions and Cisco H.323 Gatekeepers. But these mechanisms only address CAC for the initial setup of IP Telephony calls. They do not address CAC when the underlying network is changing for the IP Phone throughout the call. This would be the case when a Cisco 7920 roams between two APs.
While the Cisco APs will send out QBSS information about the Channel Utilization (CU), and the Cisco 7920 can utilize this information to determine the best AP to associate with, this does not provide a 100% guarantee that calls will retain proper QoS during a roam between APs. One example of when this degraded QoS could occur is an event that cause more than 7-8 active Cisco 7920s to roam into an area served by a single AP, thus exceeding the guidelines for CALLS per AP.
If the CU is above the CU threshold the call is not setup and the phone displays a "network busy message". If the CU of the candidate handoff AP is above the threshold, the 7920 remains associated to existing AP for as long as possible. Once the current AP is lost (no probe response/beacons received), the 7920 will handoff to the candidate AP regardless of CU. A beep is heard before this happens so the user can wonder back towards the current AP or stop and finish the conversation.
Another area of admission control that needs to be considered with the 7920 is when a user takes their 7920 from one location to another (ie. Campus to Branch). When the 7920 re-registers with the CallManager, there is no mechanism that updates which CallManager Region (or Gatekeeper Zone) that the 7920 is associated with. This means that it’s possible that calls to or from that 7920 would not properly be accounted for across a WAN link and could cause poor voice quality to phones that are registered in that region.
David
