Position-Based Routing Protocols for Ad Hoc Networks - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 65

than that of GLS, and that SGLS's routing overhead is much higher than that of
GLS in terms of bytes which in some cases is more than double. That is, of course,
due to the increase in size of routing control messages with digital signatures and
MACs in SGLS.
Generally, systems using reputation system along with the a cryptography scheme
in order to defend against both compromised and malicious nodes do not scale well
since they have to track the reputation of all nodes, which might require huge tables
of information that are difficult to manage and to keep up-to-date [ 31 ]. Moreover,
SGF assumes the existence of pair-wise shared secret keys between the nodes,
which is difficult to implement in large area networks. Another drawback is that
SGF assumes all nodes have tightly synchronized clocks which is somewhat
impractical for Ad Hoc networks. Finally, it uses the greedy forwarding which is
not guaranteed to find the optimal path.

4.4 Comparison of Selected Protocols

Table 4.1 summarizes the discussed protocols together with the evaluation criteria
used. This summary is a high-level qualitative comparison of the protocols rather
than a precise quantitative performance evaluation. The following is an explanation
of the criteria used for comparison:

• Location service type: indicates the type of the location service used with the

given protocol, i.e., shows how many nodes participate in providing location
information and for how many other nodes each of these nodes maintains loca-
tion information.
Location service robustness: it is considered to be low, medium, or high depending •

on whether the position of a given node will be inaccessible upon the failure of
a single node, the failure of a small subset of the nodes or the failure of all nodes,
respectively.
Forwarding strategy type: describes the fundamental strategy used for packet •

forwarding.
Forwarding strategy tolerable position inaccuracy: forwarding strategies tolerate

•

different degrees of inaccuracy of the position of the destination. This is reflected
by the tolerable position inaccuracy criterion.
Forwarding strategy robustness: the robustness of an approach is considered to •

be high if the failure (or absence due to mobility) of a single intermediate node
does not prevent the packet from reaching its destination. It is medium if the
failure of a single intermediate node might lead to the loss of the packet but does
not require the setup of a new route. Finally, the robustness is low if the failure
of an individual node might result in packet loss and the setting up of a new
route. According to this definition, the routing protocols that begin data trans-
mission immediately without the need for routing setup have at least medium
robustness.