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3.1.2
 
Security 
As GPRS is particularly sensitive for misuse and eavesdropping, GPRS includes several 
security services such as authentication to protect the network against unauthorized use, 
access control which means that the network can support restrictions on access by two 
or more subscribers, user identity confidentiality which purpose is to provide privacy of 
identities to the subscribers who are using GPRS radio resources, and user information 
confidentiality which purpose is to provide for confidentiality for user data.
3.1.3
 
Charging 
GPRS doesn't enforce a charging scheme for operators to use. Instead it collects 
information which the operator may use to charge its customers. The information 
collected is at least: source and destination of the packet sent, usage of radio interface, 
usage of external data networks, and usage of the packet data protocol addresses, usage 
of general GPRS resources and location of the UE.

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3.2 TMMS Service based on GPRS: : Design 
Page 31 of 93 
It is the Charging Gateway Functionality (CGF) that collects charging records from the 
SGSN and GGSN. 
3.2
 
Design 
3.2.1
 
Introduction 
The GPRS based TMMS uses the SIP protocol for communication between clients and 
PRs and also for routing between PRs. The requests and responses used are described in 
the Scenario Implementation section below. 
Note that there is not any easy mechanism available for the TMMS Proxy-Relay (PR) to 
get notifications about clients becoming online and offline. One mechanism available is 
to use the Customized Applications for Mobile networks Enhanced Logic (CAMEL) [28] 
framework, but that makes the service more dependent on the network operator’s 
network, which we would like to avoid. Therefore an explicit registration and un-
registration process is needed.