The traditional circuit-switched telephone network, also known as the Public Switch Telephone Network (PSTN), is a connection-oriented communications system where dedicated "circuits" are used to transport the media between the end user and the media processing resources. The PSTN consists of a collection of switches, as well as connections between switches and end-user terminals (a.k.a. telephones).

The establishment and routing of circuits between the end-user devices and the central office switches are controlled via signaling. Traditional phone devices use in-band signaling, where the signaling information is carried over the same circuit as the media. An example familiar to everyone is the generation of DTMF tones from the touch-tone pad of a common telephone used to identify an end user by number.

Figure 2: Traditional circuit-switched network
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Signaling System 7 (SS7) [1] is an international standard signaling system that provides an efficient and protected method for routing circuits through the network. SS7 is an out-of-band signaling method that reserves dedicated circuits called signaling links for transmitting the signaling information. The Integrated Services Digital Network (ISDN) [2] also defines an out-of-band signaling protocol. In either case the control, or signaling, information is separate from the media.

Media servers can use in-band signaling to control the connections between end-user devices and media resources. However, out-of-band methods such as SS7 have become essential for many media service applications and have become the method of choice for media servers deployed today (see Figure 2).

Intel has a number of technologies that allow media servers to connect to the PSTN circuit-switch network using in-band or out-of-band signaling techniques.