Data Rates

[Wes Schochet]

I am trying to get a better handle on data rate settings.  Looking at the guide (https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-2/config-guide/b_cg82/b_cg82_chapter_010111.html) I can understand the categories as described:

• disabled —Clients specify the data rates used for communication.

• mandatory —Clients support this data rate in order to associate to an access point on the controller.

• supported —Any associated clients that support this data rate may communicate with the access point using that rate. However, the clients are not required to be able to use this rate in order to associate.

Here is where it becomes less clear to me:

802.11a Operational Rates
    802.11a 6M Rate.............................. Mandatory
    802.11a 9M Rate.............................. Supported
    802.11a 12M Rate............................. Mandatory
    802.11a 18M Rate............................. Supported
    802.11a 24M Rate............................. Mandatory
    802.11a 36M Rate............................. Supported
    802.11a 48M Rate............................. Supported

There are three "Mandatory" data rates.  What does Mandatory mean in this context?  In the above example, I read this to mean that a client has to have at least a 6M connection to associate.  But what does the Mandatory designation for the 12M and 24M rates mean?

Thanks

[jeffkushner]

Hi Wes,

Here is a quick explanation, the mandatory data rates mean that the client has to connect at that rate as a minimum and can go to the supported if it can. Here is the best example I can give. In the 2.4Ghz range there is the B and G standard. B was only capable of a 10meg max, where G could do 54meg. In the early days, a B client would pull the AP down to only 10meg, limiting all the G clients. So a way to stop B clients was to set the mandatory at above 10 and support all above the mandatory. And set anything 10 or lower as not supported. It can also be used as a basic form of security, an AP inside a building will have a level of signal strength, outside the building it will be attenuated and weaker, so if you set a higher mandatory, you may stop people from outside from being able to connect, a trick from the old days. The use of a higher mandatory is also part of manipulating cell size in a high density design environment. 

That is a brief overview. There is a lot more to it that is covered in design guides and the SRNDs.

Jeff

[Wes Schochet]

Thanks, that makes sense.  What I am having a hard time with is the multiple mandatory data rates.  It seems like you should only have one that is marked mandatory?

[Scott Fella]

Those are default. You also have to understand other things like multicast. That is sent on the highest mandatory rate and will negotiate lower if needed. Clients have to meet one of the mandatory rates, so different clients might have different results. It’s really experience and seeing what works and doesn’t and learning from this.

[Rasika Nayanajith]

In WiFi cell operation, there are few general rules

- Management frames (eg-beacon frames) use the lowest mandatory rate

- Multicast frames use the highest mandatory rate

- ACK frames use the highest mandatory rate  below the data frame speed

In that context having a couple of multiple basic/mandatory rates helps (ACK to be transmit higher rates, multicast scenario,etc)

HTH

Rasika

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[Christian Faessler]

Hi Rasika

I think I once also learned that multicast is transmitted at the highest mandatory data rate but current documentation says otherwise like the config example (https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/81671-multicast-wlc-lap.html), which states that 'APs transmit multicast packets at one of the configured mandatory data rates.' and the Cisco Wireless LAN Controller (WLC) Configuration Best Practices (https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-6/b_Cisco_Wireless_LAN_Controller_Configuration_Best_Practices.html#concept_A00DD1DF19234C8EAD756137594CC2AA) mentions that 'Multicast is sent on the range between lowest and highest priority, depending on associated clients'

I would be interested to understand how this is implemented in the different software versions available today.

[Scott Fella]

I think folks keep what they have implemented when they decide to upgrade.  Like what Raski mentioned, multicast is sent at the highest mandatory rate, but can also drop to a lower mandatory or supported rate if needed.  Many will focus on what should be their lowest supported or mandatory rate, along with what they feel they should set as the highest mandatory rate.  I always have two defined mandatory rates and I also keep the data rate below my lowest mandatory rate as supported.