Mixed 802.1 g/b network performance?

I understand that b links use DSSS/CCK modulation with a theoretical maximum transmission rate of 11 Mb/s, while 802.11g uses OFDM, with a maximum rate of 54 Mb/s. I also understand that in a "real" WLAN, the actual transmission rates are much less due to limitations of the Ethernet interfaces, etc.

But what about a mixed network? Are the links betweena b/g router and a g adapter/client PC using OFDM at its higher speed, while a b adapter/client PC linked to the same router does with DSSS/CCK transmission at its lower rate?

I recently upgraded my home WLAN PARTIALLY from b to g. I replaced the Microsoft MN500 base station with a Linksys WRT54G router, and my laptop D-Link b adapter with a Linksys WPC54G adapter. But my wife's downstairs computer (which is "off" most of the time) is still equipped with its old Microsoft MN510 USB adapter.

The router is set to a "Mixed" network. The downstairs computer connects to the WLAN just fine with its b adapter.

But, even if it is only "on" part of the time, is the downstairs computer's adapter causing the transmission speeds between my WPC54G and the WRT54G to be less than they would be in a "pure" 802.11g WLAN?

In other words, would there be any benefit (in terms of transmission speeds on the overall network) to my upgrading the downstairs PC's adapter to a WUSB54G, and setting the router to "G-only"?

- Jeff

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"jschall" hath wroth:

Worse. It switched or "scans" between "b" modes and "g" resulting in a substantial performance penalty. See the FAQ table at:

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a mixed system, if you have any 802.11b hardware, the maximum

802.11g performance will drop from 24.4Mbits/sec thruput to about 14.4Mbits/sec thruput, even if the 802.11b devices are not moving any data. The time is lost while the access point "scans" for "b" devices as it can't hear them when in "g" mode.

The way it works is that if the WRT54G is in mixed mode, it will occasionally listen for "b" packets. The time slice is rather small, about 10% or so (not sure). However, if it successfully hears just one "b" packet, it will switch to listening longer for "b" packets, perhaps about 33%. The packet obviously will not come from your wife's MN510 adapter if it's off. However, it could easily come from the neighbors or any other "b" device.

Yep. However, only in peak performance. If you're downloading mostly from the internet and are limited by the speed of your broadband connection, then dropping from 24 to 14.4Mbits/sec maximum thruput isn't going to be noticed. Only if you're moving data around your LAN is there going to be an obvious performance loss.

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Jeff Liebermann

Thanks for a VERY complete answer, Jeff!

- Jeff

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