I’ve been assembling some information about Cisco’s Skinny Client Control Protocol and figured this was a good opportunity to share what I’ve found about how it works for those who may be curious, but don’t have time to research all the different resources.

Cisco introduced SCCP - Skinny Client Control Protocol (most just call it Skinny). It's a a Cisco proprietary protocol used between Cisco Call Manager and Cisco VOIP phones, but it is also supported by some other vendors. With SCCP, the vast majority of the H.323 processing power resides in the Cisco Call Manager. The phones run the Skinny Client, which consumes less processing overhead. The Client communicates with the Call Manager.

I'll share some of the resources I used along the way because I think one of the greatest values in this sort of communications is our ability to share. I'll share the resources I can remember, and try to cite those where I just can't recall the original source. Most of this is not my original work, but rather some research and information gathering I've done.

Skinny Client Control Protocol
From Wikipedia, the free encyclopedia

SCCP is a proprietary terminal control protocol originally developed by Selsius Corporation. It is now owned and defined by Cisco Systems, Inc. as a messaging set between a skinny client and the Cisco CallManager. Examples of skinny clients include the Cisco 7900 series of IP phone such as the Cisco 7960, Cisco 7940 and the 802.11b wireless Cisco 7920. Skinny is a lightweight protocol which allows for efficient communication with Cisco Call Manager which may act as a proxy for signalling of call events with other common protocols such as H.323, SIP, ISDN and/or MGCP.

A skinny client uses TCP/IP to and from one or more Call Managers in a cluster. RTP/UDP/IP is used to and from a similar skinny client or H.323 terminal for the bearer traffic (real-time audio stream). SCCP is a stimulus-based protocol and is designed as a communications protocol for hardware endpoints and other embedded systems, with significant CPU and memory constraints.

Cisco acquired SCCP technology when it acquired Selsius Corporation in the late 1990's. As a remnant of the Selsius origin of the current Cisco IP phones, the default device name format for registered Cisco phones with CallManager is SEP -- as in Selsius Ethernet Phone -- followed by the MAC address.

Other companies like Symbol Technologies and SocketIP have implemented this protocol in VoIP Terminals (phones) and Media Gateway Controllers or Softswitches.

A company named IPBlue has created a software phone (soft phone) which uses SCCP for signaling, too. This phone in fact appears to the Cisco CallManager server as a 7960 hardware phone.

In addition, Cisco has come out with its own version of a skinny softphone called Cisco Unified IP Communicator. Previously, Cisco had a JTAPI/CTI version of a softphone called Cisco IP Softphone.

About the SCCP Protocol
The SCCP protocol is a Cisco protocol standard for real-time calls and conferencing over Internet Protocol (IP). With the SCCP protocol, Cisco IP phones can co-exist in an H.323 environment.

An H.323 proxy can be used to communicate with the Skinny client using the SCCP protocol. In such a case, the telephone is a Skinny client over IP, in the context of H.323. A proxy is used for the H.225 and H.245 signaling.

With the SCCP protocol architecture, the vast majority of the H.323 processing power resides in an H.323 proxy — the Cisco CallManager. The end stations (IP phones) run what is called the Skinny client, which consumes less processing overhead. The client communicates with CallManager using connection-oriented (TCP/IP-based) communication to establish a call with another H.323-compliant end station. Once CallManager has established the call, the two H.323 end stations use connectionless (UDP/IP-based) communication for audio transmissions.

I can't recall where I found this, but it's a good useful overview -

Background
The SCCP protocol is a Cisco protocol standard for real-time calls and conferencing over IP.

An H.323 proxy can be used to communicate with the Skinny client using the SCCP protocol. In such a case, the telephone is a Skinny client over IP, in the context of H.323. A proxy is used for the H.225 and H.245 signaling.

With the SCCP protocol architecture, the majority of the H.323 processing power resides in an H.323 proxy — the Cisco CallManager. The end stations (IP phones) run the Skinny client, which consumes less processing overhead. The client communicates with CallManager using connection-oriented (TCP/IP-based) communication to establish a call with another H.323-compliant end station. Once Cisco CallManager has established the call, the two H.323 end stations use connectionless (UDP/IP-based) communication for audio transmissions.

Call States
This list defines the call states that are sent to SCCP endpoints, such as Cisco IP phones by Cisco CallManager in SCCP protocol. Cisco CallManager functions as an H.323 proxy. These are not the call states used in Cisco CallManager internally. Instead, they are the call states sent from Cisco CallManager and understood by the SCCP endpoints:

      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
      10—Call Transfer
      11—Call Park
      12—Call Proceed
      13—In Use Remotely
      14—Invalid Number

From protocols.com, we get this additional information -

Skinny Client Control Protocol (SCCP). Telephony systems are moving to a common wiring plant. The end station of a LAN or IP- based PBX must be simple to use, familiar and relatively cheap. The H.323 recommendations are quite an expensive system. An H.323 proxy can be used to communicate with the Skinny Client using the SCCP. In such a case the telephone is a skinny client over IP, in the context of H.323. A proxy is used for the H.225 and H.245 signalling.

The skinny client (i.e. an Ethernet Phone) uses TCP/IP to transmit and receive calls and RTP/UDP/IP to/from a Skinny Client or H.323 terminal for audio. Skinny messages are carried above TCP and use port 2000.

The messages consist of Station message ID messages. They can be of the following types:

	Code	Station Message ID Message
	0x0000	Keep Alive Message
	0x0001	Station Register Message
	0x0002	Station IP Port Message
	0x0003	Station Key Pad Button Message
	0x0004	Station Enbloc Call Message
	0x0005	Station Stimulus Message
	0x0006	Station Off Hook Message
	0x0007	Station On Hook Message
	0x0008	Station Hook Flash Message
	0x0009	Station Forward Status Request Message
	0x11	Station Media Port List Message
	0x000A	Station Speed Dial Status Request Message
	0x000B	Station Line Status Request Message
	0x000C	Station Configuration Status Request Message
	0x000D	Station Time Date Request Message
	0x000E	Station Button Template Request Message
	0x000F	Station Version Request Message
	0x0010	Station Capabilities Response Message
	0x0012	Station Server Request Message
	0x0020	Station Alarm Message
	0x0021	Station Multicast Media Reception Ack Message
	0x0024	Station Off Hook With Calling Party Number Message
	0x22	Station Open Receive Channel Ack Message
	0x23	Station Connection Statistics Response Message
	0x25	Station Soft Key Template Request Message
	0x26	Station Soft Key Set Request Message
	0x27	Station Soft Key Event Message
	0x28	Station Unregister Message
	0x0081	Station Keep Alive Message
	0x0082	Station Start Tone Message
	0x0083	Station Stop Tone Message
	0x0085	Station Set Ringer Message
	0x0086	Station Set Lamp Message
	0x0087	Station Set Hook Flash Detect Message
	0x0088	Station Set Speaker Mode Message
	0x0089	Station Set Microphone Mode Message
	0x008A	Station Start Media Transmission
	0x008B	Station Stop Media Transmission
	0x008F	Station Call Information Message
	0x009D	Station Register Reject Message
	0x009F	Station Reset Message
	0x0090	Station Forward Status Message
	0x0091	Station Speed Dial Status Message
	0x0092	Station Line Status Message
	0x0093	Station Configuration Status Message
	0x0094	Station Define Time & Date Message
	0x0095	Station Start Session Transmission Message
	0x0096	Station Stop Session Transmission Message
	0x0097	Station Button Template Message
	0x0098	Station Version Message
	0x0099	Station Display Text Message
	0x009A	Station Clear Display Message
	0x009B	Station Capabilities Request Message
	0x009C	Station Enunciator Command Message
	0x009E	Station Server Respond Message
	0x0101	Station Start Multicast Media Reception Message
	0x0102	Station Start Multicast Media Transmission Message
	0x0103	Station Stop Multicast Media Reception Message
	0x0104	Station Stop Multicast Media Transmission Message
	0x105	Station Open Receive Channel Message
	0x0106	Station Close Receive Channel Message
	0x107	Station Connection Statistics Request Message
	0x0108	Station Soft Key Template Respond Message
	0x109	Station Soft Key Set Respond Message
	0x0110	Station Select Soft Keys Message
	0x0111	Station Call State Message
	0x0112	Station Display Prompt Message
	0x0113	Station Clear Prompt Message
	0x0114	Station Display Notify Message
	0x0115	Station Clear Notify Message
	0x0116	Station Activate Call Plane Message
	0x0117	Station Deactivate Call Plane Message
	0x118	Station Unregister Ack Message

There's an excellent paper by Mark Miller of Diginet Corporate entitled Protocols for the Converged Network that was published some time ago by Sniffer Technologies as part of a briefing. You can probably Google for the PDF.

Miller had this to add -

With the SCCP architecture, the vast majority of the H.323 processing power resides in an H.323 proxy known as the Cisco Call Manager. The end stations (telephones) run what is called the Skinny Client, which consumes less processing overhead. The Client communicates with the Call Manager using connection-oriented (TCP/IP-based) communication to establish a call with another H.323-compliant end station. Once the Call Manager has established the call, the two H.323 end stations use connectionless (UDP/IP-based) communication for audio transmissions. Costs and overhead are thus reduced by confining the complexities of H.323 call setup to the Call Manager, and using the Skinny protocol for the actual audio communication into and out of the end stations.

He also provided a good diagram of implementation of the ITU-T and IETF protocols which I've recreated.



Summary
I wrote A Look at H.323 a while back. It's posted in the Realtime VoIP Community Reading Room if you want to dig into H.323. One of the clear visions many people have of H.323 is that it's big. It's been called a "pig pf a protocol" in the past due to the way it consumes resources. The reason is simple. It's extensive in an effort to incorporate all the traditional ITU-T protocols from the PSTN.

What Cisco has done is close one eye to the past and "skinnied" H.323 basics into a more efficient protocol for communications between their VoIP phones and their Call Manager. SCCP doesn't provide the comprehensive interoperability that H.323 might offer, but it provides a good mechanism for sharing the states of devices and calls to better manage network resources in a VoIP environment. Along the way, some other vendors have also supported SCCP in an effort to provide standardized support for what is becoming a common VoIP protocol in service today.

Technorati Tags: VoIP, protocols, Cisco, SCCP, Skinny Client Control Protocol, Skinny, H.323, ITU-T, IETF