Information Technology Reference
In-Depth Information
Analog and TDM Signaling
Cisco voice gateways can connect to the PSTN and analog devices using a variety of interface and signaling types, including the
following:
Foreign Exchange Office (FXO)
Foreign Exchange Station (FXS)
Ear & Mouth (E&M)
Direct Inward Dial (DID)
Centralized Automated Message Accounting (CAMA)
ISDN (BRI/PRI)
T1/E1 Channel Associated Signaling (CAS)
These interfaces and signaling types are discussed in the following sections.
FXO
An FXO interface is an RJ-11 connector that provides a connection to a PSTN central office (CO) or PBX interface.
An FXO interface can be configured to use either loop-start or ground-start signaling:
Loop-start signaling: This type of supervisory signaling allows the indication of on-hook and off-hook states. Loop-start
signaling is vulnerable to a condition known as glare that involves two endpoints seizing a trunk at the same time.
Ground-start signaling: This is similar to loop-start signaling in that it is a supervisory signaling type that allows indication
of on-hook and off-hook states. However, ground-start is preferred to loop-start on high-usage trunks because it uses current
detection to help prevent glare.
 
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