http://superuser.com/questions/122441/how-can-i-get-the-same-ssid-for-multiple-access-points

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
• Enable NAT on the primary WAP/router that connects to the Internet
  • Optionally enable the DHCP Server function as well
    • Only one DHCP server per subnet
• Don't use routing or NAT on the secondary WAP units
  • Leave the WAN port empty
  • Or use the WAN port and configure it as bridged mode
• Use the exact same settings on all wireless access points
  • SSID
  • Security (WPA2-PSK)
  • Password/Pre-Shared Key
• Set each WAP to a different channel
  • Use non-overlapping and (hopefully) unoccupied channels

In this image, #1 is the primary WAP and the #2's are secondaries: