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Core Issue

The most common forms of channel associated signaling (CAS) are loop-start, ground-start, Equal  Access North American (EANA), and E&M. CAS receives and places  calls, and also processes the receipt of Dialed Number Identification  Service (DNIS) and automatic number identification (ANI) information, which is  used to support authentication and other functions.

Cisco routers support these three types of E&M Signaling:

  • Wink-start (FGB) Notifies the remote side that it can send the    DNIS information.
  • Wink-start with wink-acknowledge or double-wink (FGD) A second wink that    is sent to acknowledge the receipt of the DNIS information.
  • Immediate start Does not send any winks.


These are the supported scenarios with E&M connections.

  • FGD can accept ANI and DNIS for inward calls (Network to CPE), but  can only provide DNIS for outward calls (CPE to Network).
  • FGD-EANA can also provide ANI and DNIS for outward calls (Network to CPE), but cannot accept ANI for inward calls (CPE to Network).

To provide and accept ANI and DNIS at the same time using CAS we can split the T1 into two different ds0 groups. One group is for inbound calls and one group is for outbound calls.

This output shows an example:

ds0-group 0 timeslots 1-11 type e&m-fgd;
ds0-group 1 timeslots 12-24 type fgd-eana

Refer to Understanding How Digital T1 CAS (Robbed Bit Signaling) Works in IOS Gateways.

Voice Gateways

IOS gateways

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