Wireless Conectivity Problem with Oracle

Hi All!,
I have a wireless network connecting with a Oracle server.I have nearly 10 systems located around 500 meters within the range of the oracle server.I am using 3 Access Points(D-Link 1700) connected in three buildings in which all the systems including the server is located. All are around 500 mtrs.THE PROBLEM is, when I start pinging the oracle server from any of the system, I am getting the reply for 20 secs only after the I am getting RTO from the oracle server. When I start ping the server from other system it starts pinging only for 15 to 20 secs after RTO is coming.
I have configured one AP as Ap and other two as AP Clients.
Can any one please tell me where is the exact problem is?
Thanks,
Vivekh