Specifications

Appendix B Synopsis of Basic VoIP Concepts
A Voice Primer
B-8
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
when a line has gone off-hook or on-hook. Foreign Exchange Station (FXS) and
Foreign Exchange Office (FXO) are types of access signaling. There are two
common methods of providing this basic signal:
• Loop start is the most common technique for access signaling in a standard
PSTN end-loop network. When a handset is picked up (goes off-hook), this
action closes the circuit that draws current from the telephone company’s
central office (CO), indicating a change in status. This change in status
signals the CO to provide a dial tone. An incoming call is signalled from the
CO to the handset by a standard on/off pattern signal, causing the telephone
to ring.
• Ground start is another access signaling method used to indicate
on-hook/off-hook status to the CO, but this signaling method is primarily
used on trunk lines or tie-lines between PBXs. Ground-start signaling works
through ground and current detectors, allowing the network to indicate
off-hook or seizure of an incoming call independent of the ringing signal.
Another signaling technique used mainly between PBXs or other
network-to-network telephony switches is known as Ear and Mouth (E&M).
There are five types of E&M signaling, as well as two different wiring methods.