A. UAV-to-UAV (U2U) Communication Architectures
Since in-flight UAVs are highly mobile networks, UAVs
can communicate with each other using mobile ad hoc
networking (MANET) networking protocols. Each UAV
constitutes a mobile node in a MANET. Research in this
challenging area of networking and communication has been
very active at each of Open System Interconnection (OSI)
model. This includes the physical, data link, networking,
transport and application layers (this layer typically includes
the session and presentation layers). The physical and data
link layers are considered the underlying network, which
can use the popular IEEE 802.11 protocol. This protocol
has a reasonable communication range of several hundred
meters in the line-of-sight communication. Recent extensions
of the IEEE 802.11 such as IEEE 802.11n have been developped
with longer communication ranges and relatively
high data rates [13]. Such version would can be used with
U2U communication in applications where the UAVs might
have longer distances between each other. The IEEE 802.11
protocol supports carrier sense multiple access with collision
avoidance (CSMA/CA) at the data link layer which can
also be appropriate for U2U communication. It can provide
support for best effort (BE) data traffic using the distributed
coordination function (DCF) and quality of service (QoS)
data traffic such as multimedia and real-time using the
Point Coordination Function (PCF). The latter employs a
guarantee-based approach for data exchange within a super
frame.
At the networking layer, which typically handles endto-
end routing of data packets, a wide variety of routing
protocols have been proposed. The lack of a fixed topology
and central control in MANETs poses a great challenge
to the routing process in this environment [14][15][16].
Routing protocols designed for ad hoc networks such as the
Dynamic Source Routing protocol (DSR) [17], Ad hoc On-
Demand Distance Vector (AODV) protocol [18], Temporally
Ordered Routing Algorithm (TORA) [19], and many others
[20][21][22][23][24][25][26] work very well under certain
conditions where nodes are reliable, they all behave correctly
and there are no misbehaving nodes in the network. However,
in reality different nodes exhibit different measures of reliability
and ability to effectively and correctly participate in
the routing and data transmission process. Routing protocols
were investigated and modified and new protocols were
introduced to enhance the routing process in mobile ad hoc
networks (MANETs).