D-Link DWS-3024 [229/266] Edcf control of data frames and aifs

Software User Manual D-Link Unified Access System

02/15/2011

Document 34CS3000-SWUM104-D10 Page 229

The other phases of the traffic flow (to and from the network) are not under control of the QoS settings on the AP.

EDCF CONTROL OF DATA FRAMES AND AIFS

Data is transmitted over 802.11 wireless networks in frames. A frame consists of a discrete portion of data along with some

descriptive meta-information packaged for transmission on a wireless network.

Each frame includes a source and destination MAC address, a control field with protocol version, frame type, frame

sequence number, frame body (with the actual information to be transmitted) and frame check sequence for error detection.

The 802.11 standard defines various frame types for management and control of the wireless infrastructure, and for data

transmission. 802.11 frame types are (1) management frames, (2) control frames, and (3) data frames. Management and

control frames (which manage and control the availability of the wireless infrastructure) automatically have higher priority for

transmission.

802.11e uses interframe spaces to regulate which frames get access to available channels and to coordinate wait times for

transmission of different types of data.

Management and control frames wait a minimum amount of time for transmission; they wait a short interframe space (SIF).

These wait times are built-in to 802.11 as infrastructure support and are not configurable.

The D-Link Unified Access System supports the Enhanced Distribution Coordination Function (EDCF) as defined by the

802.11e standard. EDCF, which is an enhancement to the DCF standard and is based on CSMA/CA protocol, defines the

interframe space (IFS) between data frames. Data frames wait for an amount of time defined as the arbitration interframe

space (AIFS) before transmitting.

This parameter is configurable.

RANDOM BACKOFF AND CONTENTION WINDOWS

If an access point detects that the medium is in use (busy), it uses the DCF random backoff timer to determine the amount

of time to wait before attempting to access a given channel again. Each access point waits some random period of time

between retries. The wait time (initially a random value within a range specified as the Minimum Contention Window)

increases exponentially up to a specified limit (Maximum Contention Window). The random delay avoids most of the

collisions that would occur if multiple APs got access to the medium at the same time and tried to transmit data

simultaneously. The more active users you have on a network, the more significant the performance gains of the backoff

timer will be in reducing the number of collisions and retransmissions.

1 5 10 15 20

Backoff time

in milliseconds

Backoff

= MinCW doubled

Initial Backoff = random number in

Backoff

= re-doubled

range of MinCW

Doubling continues on each try until MaxCW is reached

at which point this wait time is used on retries

until data is sent or until retries limit is reached