Why 11n wireless throughput is so slow?

I have Fortinet FAP220B access point supports 802.11n.
I have 3 laptops in testing.
pc1 is connected to FAP through wirless at 100Mbps.
pc2 and pc3 are connected to FAP wirlesslessly using 11n 5GHz. Channel bonding is enabled on both FAP and pc2 and pc3. The Windows show the wireless connection speed is 270Mbps on pc2 and pc3.
There is no radio interference in the test environment.
I use iperf to test the throughput.

pc1: run iperf -s
pc2: iperf -c <pc1>
The throughput is only 20-30Mbps.
Q#1. Why throughput is 20Mbps not somewhere around 100Mbps?

pc2: run iperf -s
pc3: iperf -c <pc2>
The throughput is usually around 15Mbps.
Q#2. Why the throughput is not around 150Mbps?

I researched the Internet and found out someone can get around 100Mbps wireless throughput using 11n.
Please comment on my test results and share your experience on 11n performance.

Thanks a lot.
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How far away are your test computers and is there anything blocking the way?
Aaron TomoskySD-WAN SimplifiedCommented:
I'll gve you my experience with g as it still applies to n. 54 is a bi directional lie so /2= 27. Packet overhead and things take you to about 24. Wpa2  pulls it down to about 20.

Same thing on n. However your 270 to 15 sounds a bit odd. Are you confusing bits and bytes? 8 megabits is one megabyte.
richtreeAuthor Commented:
The clients are very close to AP. Signal is very strong.
I do use WPA2 AES enterprise as security.
All the numbers are in Mega bit per second (Mbps), not byte.
Do you mean 20Mbps is normal for 11n? The vendor claims up tp 300Mbps throughput. What's your experience on 11n? How to get the best throughput?
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did you check adapter power management? were the clients connected to power? Is the client running vista or windows 7 SO?
richtreeAuthor Commented:
Laptops are Windows 7 Pro, running on AC adapter. Wireless NIC settings are ok, e.g. Full transmit power.
Craig BeckCommented:
The vendor always claims the radio throughput, and not the actual data throughput, so 300Mbps may be possible over the air, but not to shift data.

Radio throughput is what you see when you check the NIC status, but data throughput is theoretically only ever able to achieve half of that.  If you add encryption you lose even more throughput, so if your link is 144Mbps, you can technically achieve 72Mbps data throughput with no encryption.  It's more like 65Mbps with WPA.

I would usually assume that in a clean RF environment and close to the AP, with a 275Mbps radio link you should be able to achieve 100Mbps data throughput quite easily.

You must also consider the hardware and software you're running whilst your're performing the tests.  If your CPU is busy, or you're shifting data in another program at the same time obviously this will affect your test result.

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Aaron TomoskySD-WAN SimplifiedCommented:
What is the antenna configuration on the client adapter?
richtreeAuthor Commented:
The test machine is doing nothing but one thread of iperf.
I also tried different wireless NIC and never see throughput over 30Mbps.
I never get any results near 65Mbps. I'm also puzzled by the following result:
When only 1 pc connected wireless to AP, iperf shows about 25Mbps throughput; When 2 pc connected wireless to AP, running iperf at the same time on both pcs shows the aggragated throughput is around 25Mbps. Is it normal?
Craig BeckCommented:
This might sound like a silly question, but is the AP connected to the network via Gigabit Ethernet or Fast Ethernet?
richtreeAuthor Commented:
The AP wired port is 100Mbps. But single wireless client gets only 25Mbps, far less than 100Mbps. Thus so far, the issue should NOT be related to the 100Mbps AP wired port yet.
richtreeAuthor Commented:
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