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Wireless (Wifi) Network Roaming
http://support.apple.com/kb/HT4260
Try to use cabled Ethernet to connect the primary wireless access point (WAP) to the secondary WAP units
If you can't use cabled Ethernet, use a separate wireless network for the uplink from the secondaries to the primary
If you use a separate wireless uplink, use a different technology
802.11b/g/n for clients
802.11a for uplink