Page 460 - From GMS to LTE

446 From GSM to LTE-Advanced Pro and 5G

Figure 6.28 QoS field in an IP packet. Source: www.wireshark.org. Reproduced by permission of
WireShark© 2010.

contained in beacon frames. In addition, these values are also included in association
and probe‐response frames.
Furthermore, it is important for the implementation of QoS that applications have an
easy way to use a certain QoS class for their data. In IP packets, for example, the
Differentiated Services Code Point (DSCP) parameter of the IP header is used. If the
application does not request a QoS to be used, the field is set to ‘default’. Figure 6.28
shows an IP header of a voice packet, where the DSCP parameter is set to ‘expedited
forwarding’. The network driver of the wireless card then maps this field to a QoS class
defined in 802.11e and hence, the data is preferred on the air interface as it is put into
the 802.11e ‘voice’ service class queue.
In most cases, the prioritization of data frames on the air interface will be sufficient to
ensure all QoS requirements. If, however, there are too many devices and applications
in the network transferring data with an elevated EDCA priority, collisions and hence,
congestion can occur just as with the simpler DCF scheme. This means that network
access times increase and datarates are reduced. This can only be prevented if devices
register their QoS requirements, such as datarate, frame size, and so on, with the AP.
The AP can then prevent other devices from using a certain QoS class once the current