Understanding the network.pdf - Back to Home

such as T1/E1, FDDI, and others. There is no defined VT container for T3/DS3
transport. Rather, it is directly mapped into the STS-1 frame.
Common Channeling Signaling System 7
To set up any type of call on the PSTN, there must be a signaling system. In its most
basic form, the signaling system conveys the subscriber information (source and
destination) to the PSTN equipment so the call can be established. When the call is
established, the signaling system is used to verify that the call is still established
and to relay "busy" messages to subscribers trying to establish calls with the
subscribers engaged in the call. After the call is complete, the signaling system is
used to reclaim the resources used by the call, so they can be reallocated. There are
five classes of signals used to relay circuit operation and management instructions
between subscribers'terminals and CO switching equipment:
• Address signals—These signals are used to relay call or switched circuit
destination information.
• Call progress tones—These are informational signals relayed to the user on
the status of a call (dial tone, busy signal, or ringing).
• Alerting signals—These signals are used for incoming call notification. The
telephone ringer and the call-waiting beep are examples of alert signals.
• Control signals—These are supplemental or enhanced call service signals
used to provide additional subscriber information services such as automatic
number identification (ANI), which is part of caller ID.
• Supervisory signals—These signals are sent between the CO switches to
perform call setup and teardown and other call maintenance functions.
Until the early 1970s, PSTN used in-band signaling to relay signaling information.
This system was limited in the number and types of signals that could be sent, and
it became increasingly slow as more features were added to the PSTN.
Today, out-of-band signaling systems are used. These provide the capability to
accommodate the transmission of circuit management, the subscriber and billing
database information used by interLATA and intraLATA carriers to manage circuit
connections. The out-of-band signaling system used today is called Common
Channeling Signaling System 7 (SS7). SS7 operates over a separate 56K dedicated
digital network outside the voice and data carrier lines used to transport subscriber
data over the PSTN. SS7 is an ITU-T standard (issued in 1980) and is used by all the
telephone carriers in the United States and throughout most of the world.