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Chapter 19: Quality Of Service (QoS)
QoS Queues And Parameters To Coordinate Traffic Flow
190 Psion Teklogix 9160 G2 Wireless Gateway User Manual
from the AP to the client station (AP EDCA parameters).
The other phases of the traffic flow (to and from the network) are not under control of
the QoS settings on the AP.
19.1.3.2 EDCF Control Of Data Frames And Arbitration Interframe Spaces
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.
Note: A Frame is similar in concept to a Packet, the difference being that a packet operates
on the Network layer (layer 3 in the OSI model) whereas a frame operates on the
Data-Link layer (layer 2 in the OSI model).
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 9160 G2 Wireless Gateway 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.
Note: Sending data frames in AIFS allows higher priority management and control frames
to be sent in SIFs first.
The AIFS ensures that multiple access points do not try sending data at the same time but
instead wait until a channel is free.