802.11 B&G

If i have an accesspoint 80211 G, but have multiple clients using B or G, will the clients using G have slower rates?

Kind regards,

Marcel

It really depends on the hardware, and perhaps more-so, on the manufacturer.

Remember that each manufacturer usually have some proprietary "turbo rates" in addition to "standard rates".

I can only comment on how D-Link handles different rates and schemes, as I'm only familiar with D-Link -- but would assume other manafucturers would be similiar.

With my D-Link stuff (couple routers and many AP's, repeaters, ect -- mix of

802.11a/b/g):

• On an 802.11g SSID, if all the clients connected support D-Link's "turbo rate", they will get 108Mbs. (Well, not really, 108Mbs is the theoretical limit. In a clean room, maybe, but in real world obviously its much lower.)

• While all those 802.11g "turbo" clients are connected (108Mbs), if an additional 802.11g client connects that can't support "turbo" then everything falls back to standard 54Mbs.

or

• While all those 802.11g "turbo" clients are connected (108Mbs), if an

802.11b (standard 11Mbs or 22Mbs "turbo") connects to that SSID, then all the 802.11g clients fall back to 54Mbs and the 802.11b will see 11Mbs or 22Mbs (if it supports 802.11b "turbo").

-

My 802.11a's work the exact same way, with the first and second statements above -- just subsitute 802.11a for 802.11g. Obviously, there wouldn't be the third statement for 802.11a

So -- to answer your question, at least for my D-Link stuff here, an 802.11b client connecting to an 802.11g SSID would not effect "standard rates", but it would disable "turbo rates".

Some other things that my be useful or not:

For the AP's that I want SSID's to be able to use "turbo", they must be on channel 6 for 802.11g and (I forget offhand) also the center channel for

802.11a. (With the proprietary "turbo", its actually using all of channels for bandwidth.)

With my tri-band AP's (802.11a/g/b) capable of being used as repeaters, I can repeat one band and maintain "turbo", but if I want to repeat two bands then "turbo" is disabled and they will be at standard vanilla rates.

Hopefully that wasn't clear as mud... :^)

Cheers Eric

On a slightly different vein, if you were to connect a dlink turbo b card to a dlink non turbo g router do you get 11Mbps for the b card?

F. Plant

You'll see a slowdown in 802.11g even if there's no traffic to the

802.11b clients.

In general, the max thruput for 802.11g gets cut in half in the presence of 802.11b. What happens is that the 802.11g access point has to occasionally switch to 802.11b in order to detect 802.11b transmissions. That's because *ALL* management packets for 802.11b are at the slowest speed of 1Mbit/sec. If the access point doesn't hear anything on 802.11b, it just goes back to 802.11g mode after about 200msec. However, if it hears just one 802.11b broadcast or packet, it will sit there waiting for 802.11b data for perhaps 3-5 seconds. That's a huge amount of time to not move any 802.11g data. If you're transferring data at full 802.11g speeds, and just one

802.11b packet drifts by, you stop for a long time. There's also the possibility of packet collisions if the 802.11b transmissions do not coincide with when the access point is listening for 802.11b packets. Obviously, the 802.11b client radio cannot hear 802.11g transmissions.

This is stolen from an Atheros PDF at:

some additions and corrections by me.

Non-overlapping Modulation Max Max Max Channels ------- | Link TCP UDP | | | | |

802.11b 3 CCK 11 5.9 7.1 802.11g (with 802.11b) 3 OFDM/CCK 54 14.4 19.5 802.11g only 3 OFDM 54 24.4 30.5 802.11g turbo 1 OFDM 108 42.9 54.8 802.11a 13 OFDM 54 24.4 30.5 802.11a turbo 6 OFDM 108 42.9 54.8

The paper claims that encryption is enabled for these calculations, but my numbers seem to indicate that these number are for encryption disabled. Dunno for sure. The Max TCP and Max UDP are the theoretical maximum thruput rates.

There are various chipsets that claim to have "solved" the compatibility issue. At best, they tinker with the timing and b/g selection algorithms. I've seen improvements, but not solutions. Meru claims they have solved the problem by "masking" the two protocols from each other. I can't find any details on their web pile:

The important thing to remember is that even with an 802.11b induced slowdown, the 802.11g units operate much faster than the best that can be done with just 802.11b. Also, most 802.11g users cannot maintain a maximum speed connection and will be running at one of the lower speeds. Therefore, you might not even notice a slowdown.

Hi Eric,

It is very clear, but in contradiction to another poster in this group. All my stuff is Senao, not in turbo mode. I gues i have to give it a try. I replace my B accesspoint to a G (only B clients until now), and i will connect a G client also to it. Simple as it is.

Thanks for your comment,

Marcel

Hi,

I stand corrected on my previous post. Overlooked the switching and extra overhead... Makes perfect sense.

Thanks for keeping me honest.

Eric "...but the little icons still say they are connected at 108Mbs" :^)