I want to tune my cells to gain higher performance. we have a dense AP environment and want to switch off the lower data rates. However, there is much confusion what are the best settings on a WLC. (find questions further down 1 - 3)
I found this in the newest update on the WLAN mobility design guide 7.3:
Mandatory Mode
Mandatory mode allows transmission for all packets, both unicast and multicast. The data rate on at least one of the APs must be set to Mandatory, and all clients that associate to the AP must be able to physically support this data rate on their radio to use the network. Additionally, for the wireless clients to associate to the AP they must be able to currently receive packets at the lowest mandatory rate and their radios must physically support the highest mandatory data rate. If more than one data rate is set to mandatory, multicast and broadcast frames are sent at the highest common mandatory transmission rate of all associated clients (the lowest mandatory receive rate of all of the clients). This allows all clients to receive broadcast packets. The lowest mandatory rate is normally set at 1 Mbps.
Supported Mode
Supported mode allows transmission for unicast packets only. The AP transmits only unicast packets at this rate; multicast and broadcast packets are transmitted at one of the data rates set to Mandatory. The wireless clients always attempt to transmit and receive at the highest possible data rate. They negotiate with the AP for the highest data rate set to supported or mandatory to transmit and receive unicast packets. The wireless client devices are able to receive broadcast or multicast packets at any mandatory rate at or below the negotiated rate.
but also this in a white paper:
http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps10981/design_guide_c07-693245_ps11686_Products_White_Paper.html#wp9001240
Wireless=>802.11a/802.11b=>Network
Network - Data Rates
• Mandatory = Client must support in order to associate
• Supported = Optional: client may rate shift to this rate if desired
• Disabled = Not supported on the radio
Recommendations: Set the minimum acceptable data rate for the cell as supported. Choose a higher data rate as the target client speed and set as mandatory. The first or lowest mandatory rate is the speed at which beacons will be sent. The highest mandatory rate is the speed at which multicast will be broadcast. The design for a high-density WLAN should use a minimum mandatory data rate of 18 or 24 Mbps.
1) According to above you should configure something like this:
6m disabled
9m disabled
12m supported
18m mandatory
24m and higher = supported
Does anybody know the reason why you would use 12m=supported and not as mandatory? It does not make sense to me as only clients at 18m can associate, so the 12m is useless anyway.
2) Furthermore, referring to the first stated document, broadcast and multicast can be sent flexible if more than one mandatory rate is defined. the highest mandatory rate is chosen that still is able to serve the client with currently the lowest data rate.
So I would logically choose this setting for a cell with min. data rate of 18m:
6m - 12m = disabled
18m - 54m = mandatory
In this case the broadcast/multicast rate can fluctuate depending on the slowest client and does not need to stick at 18m all the time. Is there anything that speaks against using my settings? Drawbacks of using several mandatory rates?
3) How does this work with 11n-rates? There is no distinction between supported and mandatory. Is it possible at all to disable all 11a-rates i.e.? What rates would the broadcasts and beacons be sent if there were only 11n-rates enabled?
4) and last question: is there a protection mechanism used in mixed client environment with 11a/g and 11n as it is used with 11b and 11g? And the same question applies for 11n 20MHz clients in a 40MHz cell. And how would that be even with 11n and 11ac?
Regards,
Fredy
