802.11g output power and range

[yantisj]

Looking at the cisco specs on the new radios, it looks like the range of 802.11g is about half that of 11b and the power output is 30mw instead of 100mw. If the radio falls back to 11b mode, will the power output and range be comparable to the 11b radios now found in 1200 APs? If not, then there is no way we can upgrade to g considering our extensive network and site survey to make sure every corner of our campus is covered. Can anyone verify this for me. The specs on the g radio can be found here: http://www.cisco.com/en/US/customer/products/hw/wireless/ps4570/products_data_sheet09186a00801d6316.html

Thanks

[btillery]

It's really not a case of whether the radio falls back to support 802.11b clients; it's an issue whether you configure the AP to support 802.11b clients in addtion to 'g' clients. This Cisco deployment white paper might help answer some additional questions.

http://www.cisco.com/warp/partner/synchronicd/cc/pd/witc/ao1200ap/prodlit/80211_wp.htm

[bunker]

Hey great link not a bad whitepaper. The paper does address the range difference between 802.11a and g. Since 802.11g uses a longer wavelength of 4.8” it is less susceptible to interference than 802.11a’s 2” wavelength. So, similar distances with 802.11b and less chance of getting nulled or corrupted. What the article did not cover was, whether in 802.11g does OFDM use the FEC(Forward Error Correction) feature that 802.11a uses? FEC in 802.11a basically sends two copies of the data in case the first one gets corrupted in transmission.

Also, the article does not explain whether 802.11g in mixed mode, uses the same set slot timer functions such as SIFS PIFS DIFS, superframes and PCF/DCF as in 802.11b or are they modified. For 802.11b DSSS has a slot time of 20us. Also, the article states incorrectly about the number of 802.11a channels that can be used. For indoor or mixed indoor/outdoor use - up to 8 channels can be used using the lower and middle UNNI bands. The last four channels is restricted to only outdoor use using the upper UNNI band.

The mention of using 30mw due to receiver sensitivity is interesting and makes one wonder if higher wattage can be used and the RSSI controlled at a receiver 802.11g card to compensate.

Also, remember that RTS/CTS is a half duplex protocol. Meaning that one a workstation b (I have not tested in g yet) can have it on while the other workstation does not. Only that workstation will send the RTS and have a nav timer set and wait for a CTS from the other node. The other node can just transmit according DCF and DIFS.

What I have seen via protocol analysis is that the RTS/CTS is a half duplex function. What I mean is that you can enable it on a single machine and the other machines will respond automatically with the CTS with no configuration involved. Thus only the machine with it configured will be able to reserve the carrier. I always thought you had to configure a pair of machines for RTS/CTS to function. Maybe for best performance for replies but that of course is determined on the application or error condition you are trying to overcome.

So, when I have RTS/CTS set on only one machine and send a ping to another in an ad hoc config. the sending(RTS/CTS configured) machine sends out the RTS and the non configured machine responds with a CTS, then the packet is sent, then the ACK.

Now when the non configured machines sends back the ping reply it uses normal CSMA/CS DCF. No RTS for the return trip obviously because that machine is not configured for RTS/CTS.

I ran this test against two workstations in ad hoc mode one using a Cisco adapter(configured for RTS/CTS) and the non configured machine is using an Oronico Gold card.

Not a bad article though.

Regards..