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4.1.3
 
Security 
IMS provides a variety of security options. IMS is divided into two different parts, access 
security and network security.
4.1.3.1
 
Access security 
Users who want to use IMS services need to be authenticated and authorized before 
using the services. The authentication and authorization is done by using a REGISTER 
transaction. The S-CSCF downloads the information it needs from the HSS database 
while the P-CSCF and the IMS UE establish the necessary IP Security (IPSec)[39] 
security associations. The P-CSCF and the IMS UE establish two IPSec security 
associations, and to be able to establish these, the IMS UE and the P-CSCF have to agree 
on some parameters first. These IPSec security associations are used to protect the traffic 
between the IMS UE and the P-CSCF. 
The security functions are not implemented directly on the IMS UE. Instead the security 
functions are to be found in a smart card which is inserted in the UE. One of the smart 
cards that exist is the Universal Integrated Circuit Card (UICC). The smart cards contain 
different applications. Among these applications we can mention IP-Multimedia Services 
Identity Module (ISIM), UMTS Subscriber Identity Module (USIM), and Subscriber 
Identity Module (SIM). With UICC it’s preferred to use ISIM, but USIM is allowed in 
cases where the smart card is not upgraded to contain the ISIM application. The SIM 
application is not allowed when using IMS because of the poor security functions in the 
SIM application.
4.1.3.2
 
Network security 
Network security deals with security traffic between different security domains. As 
showed in the figure Figure 4-2 taken from [22] traffic going from one security domain 
to another has to go through two Security Gateways (SEG), one from each security 
domain. When sending traffic between the different SEGs the traffic is encapsulated and 
tunneled.