When the Cisco IP Phone has completed the boot process, it can be registered with the call-management system (CUCM or CME). Let us check that the Cisco IP Phone Boot Process has completed successfully:
- The phone has received Power over Ethernet (PoE) from the switch.
- The phone has received VLAN information from switch via CDP.
- The phone has received IP information from the DHCP server (including Option 150).
- The phone has downloaded its configuration file from the TFTP server.
The Cisco IP Phone then looks at a list of configured call processing servers that it found in the configuration file it downloaded from the TFTP server. The phone attempts to register with the primary call processing server. If that fails, it goes down the list of configured servers in the configuration file it downloaded from the TFTP server until the phone is successfully registered or the list is exhausted.
The IP phone uses the Session Initiation Protocol (SIP) or Skinny Client Control Protocol (SCCP) for registering with the call processing server.
The protocol used by the phone depends on the firmware. Most Cisco IP Phones use the Cisco proprietary SCCP protocol. However, SIP is an industry standard that provides benefits such as vendor neutrality and inter-vendor operation.
The SIP standard is becoming popular so quickly, SCCP is not be the most popular protocol for Cisco IP Telephony networks.
The IP Phone registration process is simple: The Cisco IP Phone connects with the call processing server and identifies server by its MAC address. The call processing server send the operating configuration to the phone from its database.
The operating configuration may vary from the settings found in the configuration XML file located on the TFTP server. The TFTP server configuration includes basic settings, including items such as device language, call processing server IP addresses, firmware version and port numbers. The operating configuration contains items such as ring tones, directory/line numbers, soft key layout, and so on.
SIP or SCCP protocol is then used for the major functionality of the phone. Whenever a user picks up the handset of the phone, it sends a protocol message to the server alerting an off-hook condition. The server then sends a message to play dial tone and collect digits. As the user dials, digits are transmitted to the call processing server using telephony protocol and call progress tones.
The Cisco IP Phone and call processing server have a dumb terminal and mainframe style of relationship, and the “language of love” between them is SCCP or SIP.