![](bg6.png)
lay at FACH and DCH requests at the same time. The
increase in C at this case is a result of lowering the
probability of L state which is in the denominator of
function C.
In general, the cost function for changing the inac-
tivity timer of FACH state under FACH attack or the
inactivity timer of DCH state under DCH attack has a
minimum at t
L
= 0 and t
H
= 0 respectively. This re-
sult is correct by means of lowering the probability of
attack state. Anyway, selecting small timers in both
cases means larger number of transitions, i.e. larger
number of attacks. In case of FACH attack, setting
the inactivity timer of DCH state to higher values is
a good choice. Similarly, selecting higher values for
the inactivity timer in FACH state slightly improves
the security of the system under DCH attack. Adding
delay in setup transitions for FACH state in a system
under FACH attack, or setup transitions for DCH state
in a system under DCH attack provides good results
by lowering the probability of attack states. The other
cases of adding delay in setup transitions have nega-
tive influence.
7 CONCLUSION
The increasing use of smartphone applications has
created new security issues for mobile cellular net-
works. In order to provide better user experience and
real-time services, mobile applications are usually de-
veloped assuming they have an “always on” connec-
tivity to the Internet. However mobile networks such
as UMTS are originally designed for voice calls and
browsing type of data traffic, and do not provide a
continuous access to the network. Connections are
created and teared down dynamically by demand.
This “creation and tear down” characteristic, which is
also meant to save bandwidthcapacity for the network
as a whole, introduces some interesting liabilities that
can be easily exploited for signaling attacks.
In this paper we have analysed the influence of pa-
rameters such as the inactivity timeouts and call setup
delay on the impact of signaling attacks in UMTS
networks. We proposed a model of UMTS Radio
Resource Control mechanism under attack and de-
fined a cost function based on the probability of at-
tack states in the model. Results show that in system
under FACH/DCH attack it is a good choice to extend
the duration of inactivity timers and to add delay in
requests for the corresponding states.
Future work may include optimising the system in
terms of transitions’ probability, analysis in a simu-
lation environment as well as obtaining new mecha-
nisms for mitigation of attacks. It would also be of
interest to evaluate how such signaling attacks may
actually affect a realistic security setting, for instance
when spectators at a sports or cultural venue have
to be evacuated rapidly with the help of instructions
distributed through smartphones because of an emer-
gency (Filippoupolitis and Gelenbe, 2009; Dimakis
et al., 2010; G¨orbil and Gelenbe, 2013) and a signal-
ing attack is simultaneously launched by malicious in-
dividuals who wish to further disrupt the emergency
situation.
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