Friday, 26 July 2013

QoS packets

WiFi MultiMedia (WMM)

Wireless Multimedia Extensions (WME), also known as Wi-Fi Multimedia (WMM), is a Wi-Fi Allianceinteroperability certification, based on the IEEE 802.11e standard. WMM not only prioritize traffic on the basis of the type of traffic (voice, video, best effort data, or background data) , but also taking into account network load and channel conditions.

With WMM-Admission Control, the access point (AP) in a Wi-Fi network admits only the traffic streams it can support based on the available network resources.  Users can confidently use voice applications knowing the quality of traffic stream will be consistently high and will provide the reliability needed to host real-time applications in Wi-Fi networks.

 When the network resources are not sufficient to provide this level of performance, the new traffic stream is not admitted, allowing the client device to seek association with an alternative AP that has sufficient network resources to support the traffic stream, and preserving the quality of already admitted traffic streams.


WMM replaces the traditional Wi-Fi DCF distributed coordination function for traditional CSMA/CA wireless frame transmission with EDCF.

QoS:

QoS modifies the media access rule such that Data having a higher priority is given preferential access to the medium. EDCA contention access is an extension of the legacy CSMA/CA DCF mechanism to include priorities. 

The contention window and backoff times are adjusted to change the probability of gaining medium access to favor higher priority classes. A total of eight user priority levels are available. Each priority is mapped to an Access Category (AC), which corresponds to one of four transmit queue.



A station that wins an EDCA contention is granted a TXOP—the right to use the medium for a period of time. The duration of this TXOP is specified per access category, and is contained in the TXOP limit field of the access category (AC) parameter record in the EDCA parameter set(e.g. in beacon frame as shown below). A QoS STA can use a TXOP to transmit multiple frames within an access category.

WMM/WME IE in beacon




 AC parameters format:

AP advertises ACM bit in Beacon to indicate if admission control
is mandatory for any Access Category.

Case 1: ACM bit not set



AC Index (ACI)is chosen as per the following table.

The AIFSN (arbitration interframe space number) subfield indicates the number of slots after a SIFS duration a non-AP STA should defer before either invoking a backoff or starting a transmission. The minimum value for the AIFSN subfield is 2.



ECWmin/ ECWmax is the minimum/maximum value of contention window which is expressed in exponent.
As in 802.11-2007 spec, table 7.37, CWmin = (aCWmin+1)/4 – 1 and CWmax = (aCWmin+1)/2 – 1

aCWMin is PHY dependent. For example, for OFDM PHY, aCWmin = 15.
So, CWmin = 3 and CWmax = 7.
ECWmin = 2
ECWmax = 3

The Queue Size subfield is an 8-bit field that indicates the amount of buffered traffic for a given TC or TS at the non-AP STA sending this frame.

TXOP grants a particular STA the right to use the medium at a defined point in time, for a defined maximum duration. The allowed duration of TXOPs are communicated globally in the beacon for stations using EDCA.Non-AP STAs should make sure that TXOPs should not exceed TXOP limit. 

Case 2: ACM bit is set

When ACM bit is set, STA sends ADDTS Request Action Frame to AP that includes a TSPEC. Before a client can send traffic of a certain priority type, it must have requested to do so via the TSpec mechanism. For example, a WLAN client device wanting to use the voice AC must first make a request for use of that AC.

Add Traffic Stream

The Add Traffic Stream (ADDTS) function is how a WLAN client performs an admissions request to an AP.

Association and re-association message


The association message might contain one or more TSpecs and one TSRS IE if the STA wants to establish the traffic stream as part of the association. The re-association message might contain one or more TSPECs and one TSRS IE if an STA roams to another AP. 


The ADDTS contains the TSpec element that describes the traffic request. See figure 1 and figure 2 for examples of an ADDTS request and response. Apart from key data describing the traffic requirements, such as data rates and frame sizes, the TSpec element also tells the AP the minimum physical rate that the client device will use. This allows the calculation of how much time that station can potentially consume in sending and receiving in this TSpec, and therefore allowing the AP to calculate whether it has the resources to meet the TSpec. 

TSpec admission control is used by the WLAN client (target clients are VoIP handsets) when a call is initiated and during a roam request. During a roam, the TSpec request is appended to the re-association request. 

Note that action code is 0 for ADDTS request.


                                             figure-1

Action code is 1 for ADDTS response. Status code 0 specifies that admission is accepted.



                                            figure -2



Following is the sample data packets for voice(with UP = 7) and best effort (UP = 0).

Data frame (voice):




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