Specifications

Cisco 1751 Router Software Configuration Guide

OL-1070-01

In Cisco’s voice implementations, echo cancellers are enabled using the echo-cancel enable command.

The echo trails are configured using the echo-cancel-coverage command. VoIP has configurable echo

trails of 8, 16, 24, and 32 milliseconds.

Signaling

Although there are various types of signaling used in telecommunications today, this document

describes only those with direct applicability to Cisco’s voice implementations. The first one involves

access signaling, which determines when a line has gone off-hook or on-hook (in other words, dial

tone). FXS and 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 sending a signal in a standard on/off pattern, which causes 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 by using ground and current detectors. This allows the network to indicate off-hook

or seizure of an incoming call independent of the ringing signal.

In Cisco’s voice implementations, access signaling is configured using the signal command.

Another signaling technique used mainly between PBXs or other network-to-network telephony

switches is known as E&M. There are five types of E&M signaling, as well as two different wiring

methods. Cisco’s voice implementation supports E&M types I, II, III, and V, using both two-wire and

four-wire implementations. In Cisco’s voice implementations, E&M signal types are configured using

the type command.