Ad Hoc Networks : Technologies and Protocols - University of ...

QoS Routing 239
where is the number of consecutive backoffs experienced for the packet
to be transmitted. To support different priorities, the backoff computation
can be changed as follows
where is the priority of node
2 DIFS: As shown in Figure 8.3, this is the minimum interval of time
required before initiating a new packet transmission after the channel has
been busy. To lower the priority of a flow we can increase the DIFS period
for packets of that flow. However, it is difficult to find an exact relation
between the DIFS period for a flow and its throughput. Figure 8.8 shows
the different DIFS values and the corresponding relative priorities. This
idea is similar to the concept of AIFS in 802.11e, as described in Section
8.4.3.
Figure 8.8. Service Differentiation using different DIFS values
3 Maximum Frame Length: Channel contention using the DCF functionality
is typically used to send a single frame. By using longer frames,
higher throughput can be provided to high-priority flows.
8.5 QoS Routing
The QoS metrics of an end-to-end route depends on the links of the computed
route. There are three main challenges in computing a route satisfying QoS
requirements. First, the QoS metric on each link must be either computed
continuously or discovered on demand, when the route request packet is being
forwarded. Second, broadcast based routing algorithms do not explore all
possible routes. Third, mechanisms to compute the available bandwidth on
a link are coarse and are based on observing other parameters such as queue
length and channel access history.