802.11b and 802.11g

[zywvip]

These two standards are said to be compatible.Though may have performance issues. Can anybody tell me exactly what the performance issues are.

And how about romaing with 802.11b and 802.11g deployed in the same area ? Can 802.11g client roam to 802.11b AP only ? Any problems ?

Tks!

[thisisshanky]

Read this thread.

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=General&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd747dc/2#selected_message

802.11g client cannot roam to a 802.11b onlyAP. 802.11g is backwards compatible with b. not vice versa.

[carrascox]

The 802.11g include the 802.11b modulation types. So they are compatibles in both ways!!!

Please Look this extract from the IEEE 802.11g-2003 spec.

19. Extended Rate PHY specification

19.1 Overview

This clause specifies further rate extension of the PHY for the Direct Sequence Spread Spectrum (DSSS)

system of Clause 15 and the extensions of Clause 18. Hereinafter the PHY defined in this clause will be

known as the Extended Rate PHY (ERP). This PHY operates in the 2.4 GHz ISM band.

19.1.1 Introduction

The ERP builds on the payload data rates of 1 and 2 Mbit/s, as described in Clause 15, that use DSSS modulation and builds on the payload data rates of 1, 2, 5.5, and 11 Mbit/s, as described in Clause 18, that use DSSS, CCK, and optional PBCC modulations. The ERP draws from Clause 17 to provide additional

payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. Of these rates, transmission and reception

capability for 1, 2, 5.5, 11, 6, 12, and 24 Mbit/s data rates is mandatory.

Two additional optional ERP-PBCC modulation modes with payload data rates of 22 and 33 Mbit/s are

defined. An ERP-PBCC station may implement 22 Mbit/s alone or 22 and 33 Mbit/s. An optional

modulation mode known as DSSS-OFDM is also incorporated with payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s.

Enjoy,

Chris

[beckerola]

Hi

We are suffering some issues in ou network with 802.11b/g Ap's in the same area.

When a 802.11g client has to choose between a b and g AP, it always goes to the 802.11g AP, even if it has a worst signal.

I'm reading in some documents out there that there is a protection mechanism that obliges tha client to select the g protocol because it understands that 54 MB is always better that 11 MB. Not sure though.

[scottmac]

The protection mechanism exists because 802.11b and 802.11g use different modulation types, and cannot "see" each other without it.

Actually, they can never see each other; the protection mechanism is initiated by the AP when it sees the mixed client types.

The AP is the only device in the system that can see both client types.

Basically, the system produces management and control traffic to the clients saying "Do not talk unless I give you permission, and I will give you permission during a specific poll slot".

The polling cycle, and the control and management traffic are one reason that having even one associated 802.11b client in a 802.11G system can cause the entire system to slow somewhat, generally ~20% or so, I believe.

If you are seeing significant slowdowns, it's a better chance that you have some other interference, or the APs are interfering with each other .... are the APs set to chnnels 1, 6, and 11?

How many APs are in the area?

Have you tried dropping the power level on some of the APs to reduce the chance of interference?

Have you done an area scan with something like NetStumbler (www.netstumbler.com - it's free) to see if you have neighbors that are using the same channels?

802.11g has a better modulation / encoding scheme, a weaker 802.11g may be a better quality link than the stronger 802.11b signal.

Do a good scan of the area and let us know what you find.

Good Luck

Scott

[beckerola]

The APs are well configured with channels 1, 6 and 11. They are indeed overlapping but i think they are doing it in the expected way to allow a good roaming of people with laptops.

Unfortunely i'm not actually in the building where the AP's are. I'll try to drop there and make some tests.

In the worst case, what could be a good solution. I thought that maybe we could separate both AP's (b and g) from each other. Another way could be upgrade all devices to 802.11g.

Thanks for the response.