Cisco MC3810-APM-FXO= Datasheet download pdf (Page 37)

Configuring Voice Ports

Configuring Digital Voice Ports

VC-71

Cisco IOS Voice, Video, and Fax Configuration Guide

Most digital T1/E1 connections used for switch-to-switch (or switch-to-router) trunks are E&M

connections, but FXS and FXO connections are also supported. These are normally used to provide

emulated-OPX (Off-Premises eXtension) from a PBX to remote stations. FXO ports connect to FXS

ports. The FXO or FXS connection between the router and switch (CO or PBX) must use matching

signaling, or calls cannot connect properly. Either ground start or loop start signaling is appropriate

for these connections. Ground start provides better disconnect supervision to detect when a remote

user has hung up the telephone, but ground start is not available on all PBXs.

Digital ground start differs from digital E&M because the A and B bits do not track each other as

they do in digital E&M signaling (that is, A is not necessarily equal to B). When the CO delivers a

call, it seizes a channel (goes off-hook) by setting the A bit to 0. The CO equipment also simulates

ringing by toggling the B bit. The terminating equipment goes off-hook when it is ready to answer

the call. Digits are usually not delivered for incoming calls.

E&M connections can use one of three different signaling types to acknowledge on-hook and

off-hook states: wink start, immediate start, and delay start. E&M wink start is usually preferred,

but not all COs and PBXs can handle wink start signaling. The E&M connection between the router

and switch (CO or PBX) must match the CO or PBX E&M signaling type, or calls cannot be

connected properly.

E&M signaling is normally used for trunks. It is normally the only way that a CO switch can provide

two-way dialing with Direct Inward Dialing (DID). In all the E&M protocols, off-hook is indicated

by A=B=1 and on-hook is indicated by A=B=0 (robbed-bit signaling). If dial pulse dialing is used,

the A and B bits are pulsed to indicate the addressing digits. The are several further important

subclasses of E&M robbed-bit signaling:

–

E&M Wink Start—Feature Group B

In the original wink start handshaking protocol, the terminating side responds to an off-hook

from the originating side with a short wink (transition from on-hook to off-hook and back

again). This wink tells the originating side that the terminating side is ready to receive

addressing digits. After receiving addressing digits, the terminating side then goes off-hook for

the duration of the call. The originating endpoint maintains off-hook for the duration of the call.

–

E&M Wink Start—Feature Group D

In Feature Group D wink start with wink acknowledge handshaking protocol, the terminating

side responds to an off-hook from the originating side with a short wink (transition from

on-hook to off-hook and back again) just as in the original wink start. This wink tells the

originating side that the terminating side is ready to receive addressing digits. After receiving

addressing digits, the terminating side provides another wink (called an acknowledgment wink)

that tells the originating side that the terminating side has received the dialed digits. The

terminating side then goes off-hook to indicate connection. This last indication can be due to

the ultimate called endpoint’s having answered. The originating endpoint maintains an off-hook

condition for the duration of the call.

–

E&M Immediate Start

In the immediate-start protocol, the originating side does not wait for a wink before sending

addressing information. After receiving addressing digits, the terminating side then goes

off-hook for the duration of the call. The originating endpoint maintains off-hook for the

duration of the call.

Note Feature Group D is supported on Cisco AS5300 platforms, and on Cisco 2600, 3600, and 7200 series

with digital T1 packet voice trunk network modules. Feature Group D is not supported on E1 or

analog voice ports.

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