Client Data Rate on 2.4GHz 802.11g/n (continued)

Unanswered Question
Aug 19th, 2008

Pls see


Great Post following through on a/b/g/n.

Deserving of more points (added)

Now, the degrading of thruput lets say in mixed environment (just say b/g for now), is this becuase of the change in modulation? ie, from OFDM to CCK?

I would assume that clients side by side would work like this?

B client runs CCK on channel 1

G client runs OFDM also on channel 1

Does CCK packets take longer to transmit/process than OFDM and that what decreases thruput?

Could I have a bit of a more technical awnser on what slows the channel down? If you dont mind?

Also, if you have a mixed environment, is it correct to say that you all broadcast and multcast will run at 1 Mbps which will slow thruput down, and thus you should really disable the lower rates if you can, but then this may affect the B clients?

Does everyone just disable B rates nowadays?

Many thx indeed guys for a really interesting post :)

Kind regards,


I have this problem too.
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kfarrington Wed, 08/20/2008 - 04:07

Ok guys,

Here is a really good article on this. But I have not made sense of it yet, as I "blue screened" some way through it!!

If anyone can makse sense of it? Let me know.



rob.huffman Wed, 08/20/2008 - 05:56

Hey Ken,

These discussions are always pretty interesting :) Here is a nice description of the b/g relationship;

From this doc;

"In just a few years, WLANs have evolved from proprietary systems with sub-Mbps capabilities to standardized offerings operating at as much as a combined data rate of 108 Mbps. These high data rates are available in both the 2.4 GHz band with 802.11g technology and the 5 GHz band with 802.11a technology. 802.11g offers backward compatibility with 802.11b devices, but is limited to three nonoverlapping channels in the 2.4 GHz band. 802.11a provides no backward compatibility but supports as many as 23 channels (depending upon local regulations). To provide both backward compatibility and high capacity, WLAN client vendors are migrating to dual-band 802.11a/g-capable client devices.The throughput provided by 802.11g networks is dependent upon a number of environmental and application factors, chief amongst them being whether or not the 802.11g network is supporting legacy 802.11b clients.

802.11 networks use Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), a media access method similar to that of shared Ethernet. Also, 802.11b devices, which share the same 2.4 GHz band as 802.11g, have no means of detecting OFDM transmissions. Although 802.11b devices can sense "noise" in the 2.4-GHz band via their Clear Channel Assessment (CCA) capabilities, they cannot decode any data, management, or control packets sent via OFDM. Given this, the 802.11g standard includes protection mechanisms to provide for coexistence and backward compatibility.

When 802.11b clients are associated to an 802.11g access point, the access point will turn on a protection mechanism called Request to Send/Clear to Send (RTS/CTS). Originally a mechanism for addressing the "hidden node problem" (a condition where two clients can maintain a link to an access point but, due to distance cannot hear each other), RTS/CTS adds a degree of determinism to the otherwise multiple access network. When RTS/CTS is invoked, clients must first request access to the medium from the access point with an RTS message. Until the access point replies to the client with a CTS message, the client will refrain from accessing the medium and transmitting its data packets. When received by clients other than the one that sent the original RTS, the CTS command is interpreted as a "do not send" command, causing them to refrain from accessing the medium. One can see that this mechanism will preclude 802.11b clients from transmitting simultaneously with an 802.11g client, thereby avoiding collisions that decrease throughput due to retries. One can see that this additional RTS/CTS process adds a significant amount of protocol overhead that also results in a decrease in network throughput.

In addition to RTS/CTS, the 802.11g standard adds one other significant requirement to allow for 802.11b compatibility. In the event that a collision occurs due to simultaneous transmissions (the likelihood of which is greatly reduced due to RTS/CTS), client devices "back off" the network for a random period of time before attempting to access the medium again.

Note that the throughput increase for 802.11g when in mixed-mode operation is relatively modest when compared to 802.11b, and is a fraction of the throughput provided by 802.11g when not supporting legacy clients."

It is nicely described in this great doc;

Hope this helps a little!


kfarrington Wed, 08/20/2008 - 06:08

Hi Rob,

This is excellent information mate.

So, please could you look at this post

Basically it says

The hidden node problem is if two nodes within a cell can hear the AP but not each other. OK.

But, when we talk about 802.11b and 802.11g backwards compatibility causing reduced throughtput in terms of bandwidth, it seems that this is always blamed on 802.11g stations having to use RTS/CTS.


Even in an 802.11g only cell, dont stations still have to use RTS/CTS mechanisms for the hidden node problem?

Can you exaplin to me that RTS and CTS is still used in 802.11g cells as everyone indicates that RTS and CTS is only a protection mechanism?

Many thx for the speedy response fella.




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