R&S M3SR 4100 Series, Operating Manual

Page: 669 / 706

Operating Manual 6172.5808.02 – 01 5.123
M3SR Series 4100 SIP – Useful Information for System Integration
The VoIP gateway radio may receive a “100 Trying“ and a “180 Ringing” response, sent by
the next user agent. If the call is accepted, the VoIP gateway radio receives a “200 OK” re-
sponse and then sends an “ACK” message back to the next user agent. A two-way RTP ses-
sion has now been established between the VoIP gateway radio and the target VoIP user.
VoIP Gateway Radio Receives a Call from an External VoIP User
In Figure 5.35 and Figure 5.36 the transactions of a SIP session establishment addressed to
the VoIP gateway radio are shown. In Figure 5.35, the VoIP gateway radio answers the in-
coming call and in Figure 5.36 a radio answers the incoming call.
If the VoIP gateway radio receives an “INVITE” request over the IP network from an external
VoIP user, it acts as a UAS and may automatically respond to the request with a
“100 Trying” response. An automatic call accept delay parameter (auto-answer) is imple-
mented in the radios to define how long the radio should wait to answer an incoming call au-
tomatically. This parameter is useful for endpoint radios in a group. A value of "0" means that
there will be no call accept delay and the radio auto-answers immediately. A large value e.g.
"65535" basically avoids the auto-answer of a radio.
Since the VoIP gateway radio is considered to work unattended as gateway for a group, the
activation of the auto-answer is not recommended here. The gateway radio will behave as
follows if despite this consideration the parameter is set to zero or to a very short time:
The gateway radio accepts the call immediately and sends a “200 OK” response directly after
the “100 Trying” (and no “180 Ringing” needs to be sent).
If the automatic call accept delay is greater than zero, the VoIP gateway radio sends a
“180 Ringing” after the “100 Trying” and waits for the period of time defined by this parameter
for a radio to accept the incoming call. If a radio accepts the call before this time has expired,
the VoIP gateway radio sends a “200 OK”, interrupting the automatic call accept delay count-
down (see Figure 5.36). Otherwise, if no radio has accepted the call during this time, the
VoIP gateway radio is able to accept the call automatically by sending a “200 OK” (see
Figure 5.35).
5.5.3.3 Proprietary Signalling Inside the Radio Network
Session signalling between the VoIP gateway radio and the radios inside the radio network
is performed by proprietary signals. These signals are divided in two types: event signalling
and state signalling.
An event signal is sent from a radio in the radio network to the VoIP gateway radio. It signal-
izes an action performed by this radio regarding a call session, i.e., when a radio calls out of
the radio network (“Dialling”), answers an incoming call to the radio network (“Accept”) or ter-
minates an existing call session (“Hang Up”).
On the other hand, the state signals are sent from the VoIP gateway radio to all addressed
units in the radio network with the purpose of signalling the current state of the session es-
tablishment or termination. To signalize that there is an incoming call to the radio network,
the VoIP gateway radio sends an “INCOMING” message. The SIP provisional messages
“100 Trying” and “180 Ringing” have the corresponding proprietary state signalling “TRYING”