Cisco Catalyst 4224 Especificaciones descargar pdf (Pagina 284)

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.

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Cisco Catalyst 4224 Especificaciones Pagina 284