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Please tell me why the Access point does not prioritize the upstream traffic?

CSCO11981350
Level 1
Level 1

Hi,

I am really strougling to know why the Access Point (airenet 1260) does not prioritize the upstream traffic while it it applies the QoS on the downstream traffic only?

please help me

Thank you

Naoufl

7 Replies 7

Stephen Rodriguez
Cisco Employee
Cisco Employee

From the AP to the client, you don't need to have a priority, as the AP can only talk to one client at a time per radio.  Once the packet, with QoS, hits the radio it will be put into the appropriate egress queue.

Make sense?

Steve

HTH,
Steve

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Thank you for your reply,

yes you are right, but why the access point applies the QoS (WMM) only on downstream direction not upstream. what is the reason behind this.  i did this practiclly and i proved it.

this is according to the access point configuration guid:

http://www.cisco.com/en/US/docs/routers/access/1800/wireless/configuration/guide/s37qos.html

Thank you

Naoufl

dOh!  thinking in the wrong direction.

Ok, so you have QoS configured on the AP, and when you sniff the port the AP is connected to, you don't see the mark?

Can you share the AP config, as well as the port that it is connected to?

Steve

HTH,
Steve

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ok here is the story:

upstream = from client to AP

downstrean from AP to client

according to cisco, the AP is not designed for upstream QoS. the upstream traffic is always treated as Best-effort even if you configure the QoS on the AP. On the other hand, if you configure the QoS at the AP, the AP applies QoS only to downstream traffic (it maps the DSCP to COS/wmm).

so for example if the AP receives an IP packet marked with ef and this packet comes from the AP's gigaEthernet, then the AP match the DSCP value to COS/wmm to give it high priority, after that the AP transmits this packet out of its 5GHZ radio interface.

now the question why the AP is designed for radio downstream QoS but not for radio upstream QoS?

I hope you understand me as maybe am not good at explaination.

Thank you

naoufl

Sorry thought you were referring to how traffic was marked from the AP to the WLC and beyond when referring to upstream traffic.

For WMM markings upstream over the air, this is the client's responsibility.

WMM is for over the air.  WMM to 802.1p conversion is performed once the packet hits the AP.

To do this, first the client would need to support and negotiate WMM, then secondly it would need to differentiate and mark different traffic types accordingly.

For example, the 792x phone can transmit voice frames as DSCP = EF & WMM = UP6 and call control frames as DSCP = CS3 & WMM = UP4.

Then the AP can do a WMM to 802.1p conversion per the Wired QoS profile specified in the WLAN/SSID configuration (e.g. Voice = 6), which also maps to priority queuing inside the AP.

Other clients that may support WMM may not differentiate traffic types and may tag all traffic as UP0 (e.g. Apple iPhone).

WMM/802.11e is only for over the air (wireless) priority.

For wired QoS, you will configure the QoS Profile's 802.1p tag for each queue (e.g. Video = 6).

Then on the switch side between the AP and WLC you can trust DSCP.

See the Mobility SRND for more info.

http://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/emob41dg/emob41dg-wrapper.html

one last question please.

regarding to the Contention Window (CW).

as we know that the wireless client who wants to send data to the access point must generate a back-off time between 0 and CWmin and and waits until the RF channel is free for an interval called DIFS.

now who gives the CWmin to the client? (i think the AP, right?)

if the client's CWmin is given by the access point, how the AP gives it to the client ?

please tell me, i really could not find any explaination on the internet.

thank you

naoufl

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