116 QUALITY OF SERVICE IN WIRELESS NETWORKS OVER UNLICENSED SPECTRUM
, where t
is the timestamp of node b when it sends the most recent update (x
) to node a, and
is the current time at node a (node a is not marked in Figure 5.3.). It means the end-to-end delay
prediction to node b will be equal to the delay experienced by the most recent update that has arrived
from b. Note that this is feasible since we assume that clocks of mobile nodes are synchronized.
5.2.2 QoS Routing
The location-and-delay prediction is extensively used in the QoS routing decision. The presented
routing algorithm is a proactive source-driven routing algorithm, where each node a has information
about the whole topology of the group network. The reason for the predictive location-based QoS
routing protocol being a table-driven, proactive source routing is the group-based application class that
we assume (see Figure 5.1). In this class of applications, to enable seamless, fast and high quality
access and browsing of data, we need sources to predict the data locations, route fast and access
data in efﬁcient manner. Hence, we need a proactive sharing of routing information accessible to
sources. One could consider also other routing algorithms such as reactive source-driven or distrib-
uted routing algorithms, but the access time to shared data would suffer due to higher overhead in
route discovery and route selection. A proactive source routing works best if (a) nodes move slowly,
and/or (b) nodes move in coordinated fashion, and/or (c) nodes move in predictive (e.g., regular)
fashion which is the case in the group-based application.
The proactive source routing can thus compute the route from itself to any other node, using
the routing information it has, and can include this source route in the packet to be routed within
a group. The state information about the group network at a node a consists of two tables—the
update table and the route table.
The update table contains information related to every node that a receives updates from. The
update table stores the node ID of b, the time the update packet was sent, the time of update it was
received, the geometric coordinates of b, the speed of b, the resource parameters of b, and optionally, the
direction of motion of b as contained in the update packet. In the update table, the entry for some
node k also contains proximity list, which is a list of all nodes lying within a distance of 1.5 × trans-
mission range (called proximity distance) of k. The reason for choosing 1.5 × transmission range is
because we assume that a node, which is not even within the proximity distance of k at the update
reception time t, would not have moved within the transmission range at route computation time
t. Furthermore discussion follows in Section 5.2.3.
The proximity list is used at the time of QoS route computation (selection). During the route
computation, the source a is required to compute the neighbors of the intermediate node k, which
are within k’s transmission range to be used as next hops from k. However, if we only maintain a
neighbor list for k rather than proximity list, then nodes that were outside of k’s transmission range at
the time of their respective last updates, but have since moved into it, will not be considered.