The use of the word, “skinny,” often refers to a scaled down device that functions purposefully with fewer features or functions than its “fat” version of that same device. In VoIP, the Skinny Client Control Protocol (SCCP, also known as Skinny) is a ‘lite’ proprietary protocol that Cisco uses with its ‘fat’ telephone equipment systems. Skinny reduces the processing load on its hardware.

How it Works

In this system, Cisco allows SKINNY clients to communicate with H.323 VoIP systems, as the H.323 processing capabilities are used in an intervening Call Manager device.

The SKINNY client and the Call Manager use a simple messaging set called Skinny Client Control Protocol (SCCP) to communicate with each other over TCP/IP. SKINNY systems use a proxy for the H.225 and H.245 signaling, and use RTP/UDP/IP for audio. 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. Skinny gateways are a series of digital gateways that include the DT-24+, the DT-30+, and the WS-X6608-x1 Catalyst voice module.

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 signaling.

When calling a non-Skinny client, the clients establish a connection through the Call Manager using TCP and then the two endpoints communicate using UDP. When Skinny phones connect to each other, they use RTP over UDP. Some vendors in addition to Cisco also support SCCP, and Cisco Call Manager 4.0 supports a secure version of SCCP, which uses Transport Layer Security (TLS) to encrypt communications and provide for confidentiality of voice conversations.

Basic Usage

If you already maintain a Cisco system, the changeover might prove seamless. However, the use of this system limits the use of open source systems and it locks you into proprietary software that may be subject to budget-pinching upgrades and licenses. On the other hand, the Cisco Call Manager is an H.323 proxy that communicates with Skinny clients. This may result in much less overhead than with the H.323, especially for a business that is connected to a company Local Area Network (LAN) or Wide Area Network (WAN).

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