SCCP is a Cisco-proprietary signaling protocol that is based on a master/slave model. In a Cisco Collaboration network, SCCP is used by Cisco Unified Communications Manager (CUCM), CUCM Express (CUCME), and Cisco Business Edition 6000/7000 to commu-nicate with devices such as Cisco IOS analog gateways, endpoints such as Cisco Unified IP Phones, and applications such as Cisco Unity Connection. SCCP is available both in its native nonsecure form and as Secure SCCP (SCCPS), where the latter provides TLS-based signaling. SCCP uses TCP ports 2000–2003 and SCCPS uses TCP port 2443.
CUCM uses SCCP to control analog ports on VGXXX gateways and ATA18X, FXS ports on Cisco IOS router modules, Cisco Unified IP Phones, Cisco roundtable confer-ence phones, media resources such as annunciator resources, conferencing resources, transcoding resources, Music on Hold (MoH) resources, and Media Termination Point (MTP). SCCP is also used by IOS routers running Cisco Unified Survivable Remote Site Telephony (SRST), H.323 proxy servers, or Tandberg video endpoints. SCCP sends dual-tone multifrequency (DTMF) digits out-of-band.
With SCCP as the signaling protocol, CUCM collects each digit that the user enters on the keypad of the phone via the StationInit message sent by the endpoint.
Simultaneously, digit analysis takes place on CUCM in real time. This occurs until the user dials digits or CUCM comes to the conclusion that there is only one potential match. Then the call is routed to the destination device or translation/route pattern. The call is routed immediately after the caller dials the final digit, provided the dial plan does not have any overlapping directory numbers/patterns or the call is not intended for an international route pattern. If the call is intended for an international route pattern, callers must wait for the inter-digit timeout (which can be avoided by pressing the # sign at the end of dial string).
The following are various call states that CUCM can send to SCCP endpoints such as Cisco Unified IP Phones in SCCP:
1—Off Hook 2—On Hook 3—Ring Out 4—Ring In 5—Connected 6—Busy 7—Line In Use 8—Hold 9—Call Waiting
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10—Call Transfer 11—Call Park 12—Call Proceed 13—In Use Remotely 14—Invalid Number
Figure 3-2 illustrates the SCCP call flow between a CUCM server and SCCP endpoints registered to it.
The following events occur during the SCCP call flow illustrated in Figure 3-2 : 1. IP Phone A goes off-hook and signals this event to CUCM.
2. CUCM sends messages to IP Phone A to play the dial tone, display text, and set its lamp state to on.
3. IP Phone A starts sending digits dialed by the user, with the first digit dialed and sent to CUCM leading CUCM to specify to IP Phone A to stop playing the dial tone.
4. The user continues dialing the number, and these digits are sent to CUCM. CUCM performs digit analysis and finds a match for the dialed number that corresponds to the directory number (DN) of IP Phone B.
5. CUCM indicates to IP Phone B that it should blink its lamp and ring to inform the user of an incoming call. CUCM also sends information about the calling party (IP Phone A) to IP Phone B. This information contains calling party name, calling party number, and so on.
6. CUCM sends an alerting (ringback) message to IP Phone A at the same time as it rings IP Phone B. CUCM also sends information about the called party (IP Phone B) to IP Phone A.
7. The user of IP Phone B answers the call and goes off-hook. An off-hook message is sent to CUCM.
8. CUCM instructs IP Phone B to stop blinking the lamp (set to a steady state) and to stop the ring tone. At the same time, CUCM also informs IP Phone A to stop the alerting tone.
9. CUCM requests information such as the to/from IP addresses and the UDP ports for RTP exchange between IP Phone A and IP Phone B. Both phones respond, and CUCM informs IP Phone A of IP Phone B’s RTP information and vice versa. CUCM notifies the IP Phones to open the media channel and start media transmission.
10. RTP traffic flow is set up between the two IP Phones as the users begin their conversation.
11. The user of IP Phone B decides to end the call, thereby sending an on-hook message to CUCM.
ptg13358382 Station Off Hook
IP Phone A CUCM IP Phone B
Station Call Information
Station Set Lamp (Blink)
Station Set Ringer (Ring)
Station Off Hook
Station Set Ringer (Off)
Station Set Lamp (Steady)
Station Open Receive Channel
Station Start Media Transmission
Station Open Receive Channel Ack
RTP
Station On Hook Station Set Lamp (Off)
Station Close Receive Channel
Station Stop Media Transmission Station Display Text
Station Set Lamp (Steady)
Station Start Tone (Dial Tone)
Station Keypad Button
Station Stop Tone (Dial Tone)
Station Keypad Button
Station Keypad Button
Station Keypad Button
Station Call Information
Station Start Tone (Alerting)
Station Stop Tone
Station Open Receive Channel
Station Cell Information
Station Open Receive Channel Ack
Station Start Media Transmission
RTP
Station Close Receive Channel
Station Stop Media Transmission
Station Set Lamp (Off)
Station On Hook
Figure 3-2 SCCP Call Flow Between Two IP Phones Registered to Same CUCM
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12. CUCM notifies the IP Phones to close the media channel and end media transmission.
13. CUCM informs the IP Phones to set their lamp states to off.
14. IP Phone A sends an on-hook message to CUCM as the user goes on-hook.