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4.1.1.7
 
Home and visited Networks 
A home network is the network where the infrastructure is provided by the network 
operator the subscriber is using. When the subscriber goes outside its home network it 
comes to a network with an infrastructure which is different from its home network 
infrastructure. This is the visited network. In order to use the visited network, the home 
network and the visited network need to have an agreement. This agreement may involve 
charging, and how to exchange accounting records. 
4.1.2
 
IMS Identifiers 
IMS identifies its subscribers with allocating two different Public User Identities. This is 
the home networks responsibility. These identities are used by any user for requesting 
communication with other users. The Public User Identities can either be a SIP URI or a 
TEL URL. A subscriber gets two different because it is not possible to register a TEL 
URL in SIP. A TEL URL is also needed to make a call from an IMS UE to a PSTN 
phone, because PSTN numbers are represented only by digits. 
sip:
[e-mail];user=phone
tel:[phone] 
An IMS subscriber is assigned a Private User Identity in addition to the Public User 
Identities. The Private User Identity is not on the same format as the Public User 
Identities. Instead it takes the format of a Network Access Identifier (NAI).
[e-mail] 
These identities are assigned by the home network, and are used for subscription 
identification, registration, authentication, authorization, administration and accounting 
purposes [38]. [xxx] It can be compared to the International Mobile Subscriber Identifier 
(IMSI) in GSM. In 3GPP release 6 a subscriber is assigned not only one Private User 

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4.1 TMMS Service based on IMS: : Overview 
Page 47 of 93 
Identity but with several. In 3GPP Release 6, the concept of Public Service Identities 
(PSI) is introduced. This is an identity located to a service hosted in an Application 
Server. PSI does not have an associative Private User Identity. This is because Private 
User Identity is used for authentication purposes and PSIs are not applicable to users.