Page 356 - From GMS to LTE

342 From GSM to LTE-Advanced Pro and 5G

server that implements the functionality of the MMTEL (Multimedia Telephony)
specification in 3GPP TS 22.173 [7] is used for typical supplementary services such as
call forwarding, conference calls, call‐hold, suppression of the telephone number of the
calling party, etc.

5.3.2 Registration
Similar to a simple SIP device described above, an IMS VoLTE device uses SIP to register
with the IMS system when it is switched on. While it would be possible to use an estab-
lished Internet connection over LTE for this purpose, this is not done in practice.
Instead either the network assigns or the mobile device requests a separate LTE default
bearer to be established for IMS VoLTE services when the device is switched on and
connects to the LTE network, as shown in Figure 5.6. This separate bearer has its own
IPv4 and/or IPv6 address(es) and the network also informs the device of the IP
address(es) of the P‐CSCF to use in the Activate Default EPS Bearer Context Request
message (see Chapter 4). The standardized APN name that devices and networks use
for the VoLTE bearer is ‘IMS’. An LTE bearer can be thought of as a virtual network
interface and several virtual network interfaces can exchange data over the same radio
link. As the IMS bearer has its own IP addresses the device is able to route all IMS‐
related traffic to the virtual network interface connected to this bearer while all traffic
to and from the Internet is sent and received from another virtual interface on the
device. Mobile apps only have access to the virtual network interface that connects the
device to the Internet.

UE LTE Network IMS HSS
Establish Bearer for
Internet Traffic
Establish Bearer for
IMS Traffic
SIP Register
Get Authentication Information
401 Unauthorized
SIP Register over IPSec Get Subscription Information

200 OK
TAS
SIP Register
200 OK
SIP Subscribe (Registration State)
200 OK
Notify (Registration State)

200 OK

Figure 5.6 The IMS registration procedure.