Performance degradation with WEP enabled

[Jihong Kim]

** note - what I mean by host-based enc/decryption is WEP
encryption/decryption performed solely on the host-CPU without using WEP
enc/decryption feature on hardware or PRISM chip.

The NIC that I used to build the host AP for this performance test is
SMC2602W (PLX card that uses SMC2632W PCMCIA card.)  I believe this card
uses PRISM 2 chipset.

The host CPU that was used to build a host AP (using the SMC2602W) is
Pentium 4, 1.4GHz.  The OS for host AP is Redhat Linux v7.2.

I used the host-based encryption/decryption (which I believe is the
default, when you enable WEP with 'iwconfig wlan0 key xxxx-xxxx-xx'
command.). 

In the same test environment, I switched the host AP with Linksys AP
(WAP 11 version 2.2), and I did the performance test again (Open system,
WEP 64, WEP 128).  In this test, there was no difference in throughput
rate between different security modes.  The rate was around 4.9 Mbps for
open, WEP64, and WEP 128.  I performed this test to see if anything
other than host AP could have contributed to the performance gap between
open mode and WEP 64 in the host AP performance test.  However, with the
findings from the Linksys AP performance test, it seems most likely that
the host-based encryption/decryption (as opposed to hardware
enc/decryption) would have contributed to the performance loss in host
AP performance test.

Question 1> Any opinion on this issue?

Question 2> Jouni, you said you've seen near 6 Mbps throughput rate with
host-based enc/decryption.  Can you tell me how you've got 6 Mbps?  What
was its environment (host CPU, NIC used to build host AP, etc.)?  

I'm interested in how to boost performance with host-based
enc/decryption, which, in my opinion, is likely to hurt the throughput
rate.

Thanks in advance.

- Jihong


-----Original Message-----
From: hostap-admin at shmoo.com [mailto:hostap-admin at shmoo.com] On Behalf
Of Jouni Malinen
Sent: Tuesday, January 28, 2003 11:11 PM
To: hostap at shmoo.com
Subject: Re: Performance degradation with WEP enabled

On Tue, Jan 28, 2003 at 07:54:17PM -0500, Jihong Kim wrote:

> With open system (no encryption), I got about 4.7 Mbps (GUI,
> daemons/processes are turned off).  When they were on, the throughput
> was about 4 Mbps.
>  
> With WEP (encryption) enabled, the highest I got was about 2.7 Mbps,
> when GUI and other daemons/processes were turned off.  (With GUI and
> other daemons/processes running in the background, the throughput was
> about 600 Kbps.)

Few questions.. ;-)

Which Prism2/2.5/3 and which station firmware version are you using?
What about host CPU? Did you use host-based encryption and/or
decryption? Which operation mode (Master?) were you using? Which card
was used as the other part of the test and which driver? Have you seen
the other card with good performance when using WEP?

> I believe this is due to the fact that WEP encryption implemented in
the
> device driver is used instead of the WEP encryption in PRISM MAC chip.

This can be the case, if the host system has very low-performance CPU.
However, WEP is not very processor intensive at throughputs provided by
IEEE 802.11b. You should also note that the Host AP driver defaults to
hardware-based decryption (and also encryption, if STA firmware is
version 1.4.9 or newer), so hardware WEP performance might also be an
issue. At least some of the older Prism2 cards used to have poor WEP
performance (I have seen results in 2-3 Mbps range when using hardware
WEP).

> Is there a way to improve the throughput of WLAN with WEP enabled?
Do
> you think using a more powerful processor (CPU) will reduce the
> performance gap between open system and WEP?

I have seen near 6 Mbps throughput with WEP, both using hardware
encrypt/decrypt and doing encrypt/decrypt fully in the driver code. So
yes, the throughput you are seeing can certainly be improved. The first
thing would be to find out which component is the real bottleneck. If it
is indeed host CPU, there are two options that could be used to improve
situation. You could either use faster host CPU or try using hardware
WEP by upgrading the wlan card firmware to at least STA f/w 1.4.9.

-- 
Jouni Malinen                                            PGP id EFC895FA
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