WiFi Multimedia (WMM)

WMM was introduced to address the varying requirements of the kinds of traffic in a network. WMM does not make any QoS guarantees but ensures that certain traffic like Video or Voice get a higher priority over the other kinds of traffic. WMM defines for categories of network traffic: background, best effort, video and voice, in increasing order of priority. The protocol ensures that priority is given to higher priority data.




Wireless networks use the CSMA/CA protocol for transmitting data. Any station that wishes to transmit the data senses whether the channel is idle. After the channel has become idle stations access the channel after a random backoff interval. If the channel is busy after this interval the algorithm is repeated. The random backoff ensures that chances of collision because of transmission from multiple stations simultaneously is minimized. It also ensures that all stations get a fair chance for transmitting data.

In WMM, data with a higher priority has a smaller random backoff interval than the other traffic. This ensures that higher priority data gets a better chance to be sent out over other stations that have lower priority data.

• Introduced for defining multiple priorities within the traffic.No guarantees are made.

• Introduces the method EDCA (Enhanced Distributed Channel Access) that builds on top of the DCF used earlier.

• Defines 4 access classes:

    0 - Background (AC_BK)
    1 - Best Effort (AC_BE)
    2 - Video (AC_VI)
    3 - Voice (AC_VO)

with priority increasing from 0 to 3

• A parameter AIFS (Arbitration Inter-Frame Spacing) is used to implement this differentiation.
• AIFS is the period a wireless node has to wait before it is allowed to transmit the next frame.
• The value of AIFS is shorter for ACs with higher priority and longer for those with lower priority. This implies that a message with higher priority has a greater probability to be transmitted, since the wireless node waits smaller before attempting the transmission.
• Detail: AIFS is calculated using a formula AIFS = AIFSN[AC] * slot-time + SIFS.
• The Access point defines the value of AIFSN for each of the Access Classes in the QOS Frame XXX ?
• EDCA (WMM) Parameter Element is used to specify the AIFSN for each Access Class. The EDCA Parameter Elements are:

   CWmin (Contention Window min)    CWmax (Contention Window max)
   AIFSN
   ACM (Admission Control Mandatory)
   TXOPlimit

for each Access Class. APs set these values up.

• The contention window is also smaller for ACs with higher priority. What is contention-window??

• TXOP implies Transmission Opportunity. EDCA provides contention-free access to the channel for a period called TXOP. In the TXOP interval the station can send as many frames as possible. The duration of the TXOP should not extend beyond that setup by the Access Point in the EDCA Parameter Elements. A TXOP interval of 0 indicates it is only for transmission of a single MSDU.By default, TXOP for BE and BK is 0.

• If the channel is fully saturated then Admission Control is used.

• ACM limits admission to an access class, such that wireless stations should first query the AP for the available bandwidth. Once the channel is getting over-subscribed the AP advertises the ACM bit in the beacon for any Access Class.
• Hence forth, stations have to explicitly request the AP for permission to use the channel for that AC. This is done by sending the ADDTS Action Framework. The frame should include parameters for helping the AP take a decision.
• APs respond to the frame using ADDTS response. The response contains the medium time for which stations can use the medium.Stations should stop using the channel for that AC.

Test 2

This is a test 2

Test 1

This is a test