Registration is something what was missing in the times of wires. Ok, not completely but it was done more or less once when an MSISDN was assigned to some line.
In contrast to the previous types of the networks the 4G network is ‘user-centric’. It means the user can use multiple devices and identities and we have to deal with it. The main purpose of the registration is to create a binding between user (her public identity) and IP address of the device, so we know where we can send the data to. That’s why there is a Contact header in the SIP REGISTER message. The Contact header contains an address which identifies the current location of the user (Point-of-Presence – e.g. IP/FQDN of a particular client). During the registration is the Contact Address is linked to a Public Identity (IMPU, AOR in SIP terminology). The IMPU is an equivalent to MSISDN in GSM and has to be present in the To header of the SIP REGISTER. One identity can by used by more terminals and as each terminal can have different capabilities this has to be also taken into account. This information is part of the Contact header.
Contact: <sip:firstname.lastname@example.org:49635;transport=tcp>; expires=3200; +g.oma.sip-im; language="en,fr"; +u.asmc.apn="6a6044869e2aba3d23d4cfc0ef1384d00da28854c2408ddbbf"; +sip.instance="<urn:uuid:D4196919-ED8A-4E00-9436-B9CDD1E76813>"