Signaling Between the Telephone and PBX
A telephone can be in one of two states: off-hook or on-hook. A line is seized when the phone goes off-hook.
Off-hook—A telephone is off-hook when the telephone handset is lifted from its cradle. When you lift the handset, the hook switch is moved by a spring and alerts the PBX that the user wants to receive an incoming call or dial an outgoing call. A dial tone indicates “Give me an order.”
On-hook—A telephone is on-hook when its handset is resting in the cradle and the phone is not connected to a line. Only the bell is active, that is, it will ring if a call comes in.
The phone company can provision a Private Line, Automatic Ringdown (PLAR) between two devices. A PLAR is a leased voice circuit that connects two single instruments. When either handset is lifted, the other instrument automatically rings. Typical PLAR applications include a telephone at a bank ATM, phones at an airport that ring a selected hotel, and emergency phones.
Signaling Between the PBX and CO
A telephone system “starts” (seizes) a trunk, or the CO seizes a trunk by giving it a supervisory signal. There are three ways to seize a trunk:
– Loop start—A signaling method in which a line is seized by bridging through a resistance at the tip and ring (both wires) of a telephone line.
– Ground start—A signaling method in which one side of the two-wire line (typically the “ring” conductor of the tip and ring) is momentarily grounded to get dial tone.
– Wink—A wink signal is sent between two telecommunications devices as part of a handshaking protocol. It is a momentary interruption in the single frequency tone indicating that one device is ready to receive the digits that have just been dialed.
With a DID trunk, a wink signal from the CO indicates that additional digits will be sent. After the PBX acknowledges the wink, the DID digits are sent by the CO.
PBXs work best on ground start trunks, though many will work on both loop start and ground start. Normal single-line phones and key systems typically work on loop start trunks.
Signaling Between Switches
Common channel signaling (CCS) is a form of signaling where a group of circuits share a signaling channel.
Signaling system 7 (SS7) provides three basic functions:
- – Supervisory signaling
- – Alerting
- – Addressing
SS7 is an ITU-T standard adopted in 1987. It is required by telecommunications administrations worldwide for their networks. The major parts of SS7 are the Message Transfer Part (MTP) and the Signaling Connection Control Part (SCCP). SCCP works out-of-band, thereby providing a lower incidence of errors and fraud, and faster call setup and take-down.
SS7 provides two major capabilities:
– Fast call setup via high-speed circuit-switched connections.
– Transaction capabilities that deal with remote data-base interactions. SS7 information can tell the called party who’s calling and, more important, tell the called party’s computer.
SS7 is an integral part of ISDN. It enables companies to extend full PBX and Centrex-based services—such as call forwarding, call waiting, call screening, call transfer, and so on—outside the switch to the full international network.