* Overview of looking at PSTN signaling in a security context
This week, we’ll take at look at the signaling system used by the PSTN as we begin to highlight some of the security concerns about networks moving toward a converged infrastructure. Today, we’ll cover a few basics about how PSTN signaling works.
PSTN signaling – including call setup, management and teardown – is controlled by the common channel Signaling System 7 (SS7). The standards allow for several variants that are deployed across the world. SS7 protocols are used for wireless and wireline networks, and they facilitate user services like number portability, toll-free calling, and enhanced features like Caller ID.
SS7 messages are exchanged over out-of-band (OOB) signaling links, so the voice carrier channel is isolated from the signaling channel. Note that an out-of-band signaling channel is inherently more secure and reliable than a channel that contains the voice (or data) and the signal. One reason the OOB method was chosen was because the signaling channel uses a packet-based protocol, so the signaling can gain the benefits of statistical multiplexing. Another reason was that OOB signaling improves control over fraudulent network use.
The SS7 network has three basic elements: the service switching point (SSP), the signal transfer point (STP), and the service control point (SCP). SSPs are switches that originate, terminate, or tandem calls. An SSP sends signaling messages to other SSPs to manage calls. An SSP may query the SCP centralized database to determine how to route calls. The STP is a packet switch that routes network messages between signaling points.
