Page 352 - From GMS to LTE

338 From GSM to LTE-Advanced Pro and 5G

of the subscriber can be seen (5415468) together with the SIP domain (@sipgate.de).
Together they form the Universal Resource Identifier (URI). Further, the random number
(Nonce) that is sent by the network is also part of the message as well the authentication
response value (Digest Authentication Response).
Figure 5.2 also shows the status ‘200 OK’ of the Registrar server that reports the
presence of ‘1 binding’. This means that this is the only User Agent that has regis-
tered against the SIP‐ID at this point in time. SIP also allows registration of multiple
User Agents to a single Public User ID. For incoming calls, both devices are then
notified. Once the User Agent is registered, voice sessions can be established to other
subscribers and incoming sessions can be received. Figure 5.3 shows how a voice
session is established. As a user knows the SIP‐ID of the subscriber they want to
contact but not the current IP address of their device, signaling messages always
traverse the SIP proxy server in the network. The SIP proxy is first contacted with a
SIP ‘Invite’ message whose ultimate destination is the terminating device. Before the
session is established, the SIP proxy authenticates the originator by returning a 408
Authentication Request message. Only once authentication has been performed will
the SIP Invite request be forwarded to the destination. If the destination is a cus-
tomer of another network operator, the Invite message cannot be delivered to the
destination directly but is routed to the SIP proxy of the other network operator (SIP
proxy B). The other proxy then searches its database for the IP address of the termi-
nating device that has registered for the SIP‐ID received in the Invite message and
then forwards the message to the destination device. To ensure that SIP messages in
the other direction also traverse both SIP proxies, both proxies add their IP address
to each SIP message before forwarding it.

User Agent SIP Proxy A SIP Proxy B User Agent
Invite
408 Authentication Req.
Ack
Invite
Invite
100 Trying
Invite
100 Trying
100 Trying
180 Ringing
180 Ringing
180 Ringing 200 Ok
200 Ok
200 Ok
Ack
Ack Ack

Voice Call Established

Figure 5.3 SIP call establishment.