In the world of digital communications and enterprise VoIP infrastructures, call management and signaling are among the most critical components of telecommunication systems. In this context, devices such as SBCs (Session Border Controllers) play a key role in securing and controlling the flow of signaling and media in VoIP networks. However, in this article, our focus is on the SCCP signaling protocol, which is specifically used in Cisco equipment.
If you have worked with Cisco networking equipment, you have likely come across the term SCCP, or Skinny Client Control Protocol. This protocol is one of Cisco’s most important proprietary technologies for managing voice calls in enterprise environments.
In this article, we will provide a comprehensive introduction to SCCP, exploring its use cases, structure, advantages, differences from other protocols, and its role in VoIP call operations.
What is SCCP?
SCCP (Skinny Client Control Protocol) is a proprietary protocol developed by Cisco Systems, designed to control voice communications between client devices (such as IP phones) and call management servers (such as CUCM, or Cisco Unified Communications Manager).
SCCP is recognized as a lightweight signaling protocol that operates over the TCP/IP protocol stack, managing the communication between IP phones and the server.
Which devices use SCCP?
- Cisco IP Phone 7900 Series
- Cisco IP Communicator Softphone Software
- Call management servers such as CUCM (Cisco Unified Communications Manager)
- Cisco Unity voicemail systems
Differences Between SCCP and Other Protocols
Upon hearing the name SCCP, you might think of another protocol with the same abbreviation: Signaling Connection Control Part, which is part of the Signaling System No. 7 (SS7) in traditional telecommunication networks. It is important to note that these two protocols are completely different.
| Feature | SCCP (Cisco) | SCCP (SS7) |
| Usage | VoIP communications over TCP/IP networks | Traditional telephony systems |
| Manufacturer | Cisco Systems | ITU-T |
| Type | Proprietary signaling protocol | Public telecommunication protocol |
The Role of SCCP in VoIP Calls
In a VoIP call, the transmission of voice is handled by a protocol called RTP (Real-Time Transport Protocol). However, it is signaling protocols like SCCP that initiate, manage, and terminate the call.
SCCP Functionality in Brief
- IP Phone Registration with the Server
- Receiving Control Commands from the Server (e.g., activating the ringer, displaying messages on the screen)
- Sending Call Status to the Server (e.g., off-hook, button presses, dialing)
- Terminating the Call and Disconnecting
Unlike many VoIP protocols such as SIP or H.323, which have more complex structures, SCCP is designed to be simple and requires minimal processing resources. This makes SCCP an optimal choice for telephony hardware with limited processing power.
Message Structure in SCCP
One of the prominent features of SCCP is the simple and understandable structure of its messages. Each message has a unique identifier (Message ID) and a defined header that indicates the event to which the message pertains.
An Example of Call Flow:
IP Phone: StationInit: handset lifted
Server: StationD: activate the ringer
Server: StationD: display the message “Please dial”
IP Phone: StationInit: first digit dialed: 4
IP Phone: StationInit: second digit dialed: 7
…
IP Phone: StationInit: call ended
Introduction to Common Message IDs in SCCP
| Message ID | Operation |
| 0x0000 | Keep Alive – Server message to maintain the connection after registration |
| 0x0001 | Station Register – Request for the phone to register with the server |
| 0x0002 | Station IP Port – Client announces the RTP port |
| 0x0006 | Station Off Hook – Handset lifted |
| 0x0082 | Start Tone – Activating the ringer or tone |
| 0x0099 | Display Text – Displaying text on the screen (e.g., “Please dial”) |
| 0x0027 | Soft Key Event – Soft key action (e.g., call start or end) |
| 0x0107 | Connection Stats Request – Request for connection statistics (alternative to RTCP) |
This design makes call tracking and network troubleshooting using SCCP logs much simpler compared to similar protocols.
How SCCP Works with RTP
SCCP serves only a control function and is not responsible for transmitting voice. Voice data is carried between the phone and the destination via RTP. Importantly, unlike many other protocols, SCCP does not use RTCP for call quality control. Instead, SCCP has dedicated messages for call monitoring and statistics, providing information such as:
- Delay
- Packet Loss
- Jitter
- Number of packets sent and received
These are provided to analyze the call status.
Support for SCCP by Other Companies
Although SCCP is a proprietary Cisco protocol, some other companies have also included support for SCCP in their products due to its widespread use, including:
- Digium – Manufacturer of Asterisk equipment
- SocketIP – Provider of SIP/SCCP solutions
- Symbol Technologies – Manufacturer of wireless equipment and IP phones
This indicates that SCCP also holds a significant position in the industry.
Advantages and Disadvantages of SCCP
Advantages:
- Simple and lightweight structure
- Low processing resource consumption
- Suitable for large internal networks
- Easy troubleshooting through logs
- Complete and accurate integration with CUCM
Disadvantages:
- Proprietary nature (dependent on Cisco)
- Incompatibility with open standards such as SIP
- Less flexibility in non-Cisco environments
Conclusion
The SCCP protocol is a key component in Cisco’s VoIP architecture. With its simple yet efficient design, it enables the establishment and management of voice calls at the enterprise level. Familiarity with its operation is essential for network engineers, especially those working with CUCM or Cisco IP phones. Additionally, knowledge of devices such as SBCs, which ensure signaling and media security at the network edge, can be highly beneficial for advanced VoIP deployments.