Understanding Signaling

Establishing a telephone call requires several different types of signaling, to inform network
devices that a telephone is off the hook, to supply destination information so that the call
may be routed properly, and to notify both caller and callee that a call has been placed. A
new technology in signaling, known as Signaling System 7 (SS7), is the ITU standard that
provides for signaling, call setup, and management for the PSTN calls. Typically, a separate
network is used for SS7 flows. Since the data transfer for SS7 does not occur on the same
path as the call, it is sometimes referred to as out-of-band.
Two key components make up an SS7 network. The Signal Transfer Point (STP) provides
routing through the SS7 network. You could think of these as the IP routers of the SS7
network. The Session Control Point (SCP) provides “800” number lookup and other
management features.
When a phone call is made, the signaling protocols get involved to find the route to the
callee, establish the connections between switches, and tear down these connections after the
call ends. The STPs communicate with the local and tandem switches to reserve capacity
between the switches in the path between caller and callee. After the call is completed, the
STPs communicate with the switches to release the reserved connections and make them
available for other calls.
VoIP systems have a corresponding set of rules for call signaling, which we discuss below.