Hi Max,

> I am new to iperf. I have configured an accesspoint (asus wireless AP
> WL-330gE) in gateway mode. It has 3 clients now.
> 1 is connected via RJ45 ethernet (100Mbps), 2 and 3 via wifi to AP.
> The wifi based clients have Managed mode and use IEEE 802.11abg.
>
> The access point is working on IEEE 802.11g mode (Bandwidth upto 54Mbps).
> Now, I want to calculate the bandwidth associated in the links
>
> I found the following results:
>
> The bandwidth between 1---AP----2 was 27.5 Mbps
> The bandwidth between 1---AP----3 was 29.6 Mbps
> The bandwidth between 2---AP----3 was 13.4 Mbps
>
> I don't actually get the pattern of bandwidth drop in all these
> communication links. Could anyone explain me why is the bandwidth for
> each case so low?

These numbers are actually as expected. The maximum capacity in the cell 
(54 Mbps in your case) is half duplex, "the air" it uses is a shared 
medium and the 802.11 protocol has a lot of overhead and delays (for 
good reason imho). Only one packet can be transmitted in any direction 
at any time, otherwise it will be corrupted or lost.

For cases 1<->2 and 1<->3 the bandwidth is roughly equal, this is about 
the max you'll even see on a 54 Mbps wireless connection. You won't get 
more than 50-55% of the "advertised" bandwidth; the rest is mostly lost 
in protocol overhead [1] and a bit is lost because the receiver will 
have to acknowledge the TCP segments it receives. Sending back the ACKs 
also takes up some airtime.

Between two wireless stations, the througput halves again: each packet 
travels over the air twice. Once from the sending station to the AP, 
once from the AP to the receiving station.


Regards,
Jeroen van Ingen


[1] 
http://wirelesslanprofessionals.com/why-is-the-wireless-network-slow-overhead-issues/
 
(mentions 802.11b but is equally valid for 802.11a and 802.11g; 802.11n 
has a couple of optimizations iirc)



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