When a call is made between two phones registered on a Cisco Unified
Communications Manager Express (CME), the signaling process involves several
steps that allow the devices to communicate with each other and establish
the call. In this article, we will discuss the signaling process for a call
between two phones registered on a CME, where all three devices are
connected on the same switch.
Phone A initiates the call: The signaling process starts when Phone A
initiates the call by lifting the handset or pressing the speakerphone
button. The device sends a signaling message to the CME indicating that a
call is being initiated.
CME routes the call: The CME receives the signaling message from Phone A
and routes the call to Phone B, which is registered on the same CME. The CME
uses the Dial-peer to route the call based on the destination phone
number.
CME sends an INVITE message to Phone B: The CME sends an INVITE message to
Phone B, indicating that a call is being requested. The INVITE message
contains information about the caller (Phone A) and the call type (voice,
video, etc.).
Phone B sends a RINGING message: Upon receiving the INVITE message, Phone B
sends a RINGING message back to the CME. The RINGING message indicates that
the device is ringing and the call is in progress.
Phone B answers the call: When Phone B answers the call, it sends an OK
message to the CME, indicating that the call has been answered.
CME sends a CONNECT message to Phone A: The CME sends a CONNECT message to
Phone A, indicating that the call has been connected to Phone B.
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Phones A and B exchange voice data: With the call connected, Phones A and B
exchange voice data over the network using the Real-time Transport Protocol
(RTP).
Call termination: When the call is terminated, either Phone A or Phone B
will initiate the call termination process. This process involves sending a
signaling message to the CME indicating that the call should be
terminated.
In this scenario, all three devices (Phone A, Phone B, and the CME) are
connected to the same switch. This means that the signaling messages between
the devices will be sent over the same network switch, rather than over a
WAN link or the internet. This can help reduce network latency and improve
call quality.
In conclusion, the signaling process for a call between two phones
registered on a CME involves several steps that allow the devices to
communicate with each other and establish the call. By understanding the
signaling process, network administrators can troubleshoot call quality
issues and ensure that their network is properly configured to support voice
traffic.