HP Survivable Branch Communication zl Module powered by Microsoft Lync Planning and Design Guide 2011-02

1-13
SBM Overview
SBM Role in a Microsoft UC&C Solution
3. The calling party and the called party use SIP to establish the call.
The SBM can be the proxy for the branch user during the call establish-
ment, and communications pass through the LAN (for local calls) or
through the LAN and WAN (for calls to other sites). If the called party is
a remote user (accesses the company network through the Internet), the
central office Access Edge Server is its SIP proxy, so SIP traffic would
pass over the WAN.
The figure shows the basic process:
a. The calling party sends an invite, which triggers the called party’s
phone to ring (if a softphone, the Lync client produces the ringing
signal).
b. The called party immediately sends messages indicating that the call
is being attempted and then that the called party’s phone is ringing.
c. After the latter message, the calling party’s phone begins a ring-back
tone.
d. When the called party accepts the call (the user picks up the phone
or indicates acceptance on the client), it sends an OK.
e. The calling party establishes the data path, which will travel on a
dynamically selected Real-Time Protocol (RTP) port. It also sends an
Acknowledgement Packet (ACK).