Hi Shaymaa,
The decision for which AP the wireless phone connects to is NOT determined by the user. This is transparent to the user completely.
***Roaming is a client decision (in this case the client is the IP Phone 7920 or 7921). The client is responsible for deciding it needs to roam, and then detecting, evaluating, and roaming to an alternative AP.
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Note Roaming is a client decision. WLAN standards bodies (such as the IEEE) and industry bodies (such as the Wi-Fi Alliance) do not specify when a client should roam, or how the client determines to which alternative AP it should roam. Each vendor's roaming algorithms are proprietary and are not generally published.
Voice over WLAN Roaming
http://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/vowlan/41dg/vowlan_ch5.html
Voice over WLAN Introduction
http://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/vowlan/41dg/vowlan_ch1.html
Extension Mobilty does not require any additional components;
Extension Mobility
From this doc:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803edae9.html#wp1030809
Hope this helps!
Rob