@Aayush,thanks but I figured it out before your post.
I have created a 2Mbps 802.11n wireless network with TC

My iMac recogonises the network on all the radio frequencies except 802.11n only 5GHz thing.What's the prob?
Also the iPhone 2.0 firmware is behaving sluggishly.
I have set the multirate thing to 5.5Mbps in Wireless options.Is it Okay?Earlier it was set on 2Mbps and I am getting speeds more than that.So,I changed it to 5.5Mbps.
Which one is better,802.11n(b/g compatible) or 2.4 GHz one??Will the backup speeds decrease in 2.4 Ghz 802.11n network?
I have 2 WiFi networks in my house.One is 802.11g(Netgear+access points) and Time capsule.Is 802.11n 2.4GHz advisable?