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

http://support.apple.com/kb/HT4260

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

1. Try to use cabled Ethernet to connect the primary wireless access point (WAP) to the secondary WAP units
2. If you can't use cabled Ethernet, use a separate wireless network for the uplink from the secondaries to the primary
3. If you use a separate wireless uplink, use a different technology
   • 802.11b/g/n for clients
   • 802.11a for uplink
4. 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
5. 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
6. Use the exact same settings on all wireless access points
   • SSID
   • Security (WPA2-PSK)
   • Password/Pre-Shared Key
7. Set each WAP to a different channel
   • Use non-overlapping and (hopefully) unoccupied channels

1. Use the latest APs that support simultaneous dual-band 802.11n technology
   • Supports older 2.4GHz clients
   • And supports newer 5GHz 802.11n clients
2. For older 2.4GHz 802.11n radios, you might configure it for 20MHz (HT20) channels
   • This limits your 802.11n transmission rates in 2.4GHz to ~130mbps instead of 300mbps
   • But leaves some of the band free for things like Bluetooth
3. For newer 5GHz 802.11n radios, you might configure it for wider 40MHz (HT40) channels
   • For maximum throughput
4. If you don't need to support any older 2.4GHz-only devices, use the 5GHz band
   • It is generally less busy
   • And you can use HT40 without starving Bluetooth and other uses
5. When working with 2.4GHz radios:
   • If you don't have a lot of interference, try using channels 1, 6 and 11
     • They don't overlap at all
     • Choose channels in increments of three to the next closest access point or interference