Will 802.11g speed brecut down by 801.11b

Unanswered Question

Let say there are currently 5 user connected to a cisco 802.11b/g capable ap using the x.11g(54mbps) and if a user whose hardware only can support x.11b(11mbps). By that using conneting thru .11b will the rest of the 5 users whose speed is 54mbps be cut back to 11mbps.

From what i presume is no, but the last post from this thread (the link below) seem to seem to indicate that those 5 user on 54mbps will be cut down.

I have this problem too.
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john.preves Tue, 03/27/2007 - 18:43

Yes you will suffer severe throughput degredation..,

This is part of a post I saved that may help....

I used to have a white paper on what happens when a B client is introduced into a G network and how packet bursting would increase throughput for the G clients that would have explained everything clearly for you, but I can't find it so I'll have to try and explain it.

An access point will allow each node to transmit (or receive) one packet at a time and generally these packets are the same length, let's say for example 1000 bits long. At 11Mbps a B client will transmit this packet in 91 microseconds and at 54Mbps a G client will transmit the same packet in 19 usec.

In a system with a 802.11g AP and one G client and one B client alternating transmissions you would see 2000 bits transmitted every 110 usec which works out to about 18Mbps average for the system, much slower than the 54Mbps you'd see in a G only system.

Now if you allow packet bursting for the G client (send 3 packets at a time instead of 1), you'd send 4000 bits in 148usec which works out to 27Mbps average for the same system.

Keep in mind of course that each packet is only 50-60% data and the rest is overhead (header information, control bits, error correction, etc.) so the actual data throughput is lower. This is why you only see 4-6Mbps data rates for 802.11b systems and 20-30Mbps data throughput for 802.11g or a systems. Not to mention RTS/CTS frames, acknowledgement frames, etc. that lower throughput...

rob.huffman Wed, 03/28/2007 - 05:35

Hi Jeffrey,

Here is some added info to John's excellent post (5 points from me on this one John :)

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. The client arrives at this random period of time by selecting from a number of slots, each of which has a fixed duration. For 802.11b, there are 31 slots, each of which are 20 microseconds long. For 802.11a, there are 15 slots, each of which are nine microseconds long. 802.11a generally provides shorter backoff times than does 802.11b, which provides for better performance than 802.11a, particularly as the number of clients in a cell increases. When operating in mixed mode (operating with 802.11b clients associated) the 802.11g network will adopt 802.11b backoff times. When operating without 802.11b clients associated, the 802.11g network will adopt the higher-performance 802.11a backoff times.

When 802.11g-based networks are operating in the absence of legacy 802.11b clients, network throughput is similar to that of 802.11a. With its OFDM means of transmission and 802.11a backoff scheme, 802.11g can essentially be viewed as 802.11a applied to the 2.4-GHz band.

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.

Check out Table 2 in this excellent doc;


Hope this helps!



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