CAS stands for Common Associated Signaling and the signaling operates by “robbing” the least significant bits of information from voice channels and using this to send clocking and framing information. CAS is also known as robbeb bit signaling.

At the opposite of the ISDN,  T1/E1 CAS uses in-band signaling which are:

  • Loop-start
  • Ground-start
  • E&M wink-start (Feature group B, FGB)
  • E&M wink-start (with acknowledge)
  • E&M double wink (Feature group D, FGD)
  • E&M Feature Group D Exchange NA, FGEANA
  • E&M immediate-start

E&M signaling is often the preferred optino for CAS because it avoids glare, it provides answer/disconnect supervision and it can receive Automatic Number Identification (ANI) with FGD and send ANI with FGD-EANA. In other words, you can have 1 channel-group for incoming calls and 1 channel-group for outgoing calls.


ISDN is a digital transport service that can support both Data and Voice over existing telephone wires. There are 2 types of ISDN:

  • Basic Rate ISDN – BRI : This provides 2 x 64 Kbps bearer channels and a 16 Kbps signaling channel. It represents then (2B+1D) channels , so 144Kbps
  • Primary Rate ISDN – PRI : this provides 23 or 30 x 64 Kbps beare channels depending on whether it is delivered as a T1 or an E1. It represents then (23B+1D) channels for a T1 and (30B+1D) for an E1

At Layer2, ISDN signaling is provided by the Link Access Procedure on the D channel (LAPD) which is relying on Q920/921 messages.

At Layer3, ISDN call control signaling and access to service are specified by the Q931/932 messages suchg as Setup, Alerting, Connect ,…

Now as the network is not end-to-end all the time, you can encounter that you don’t receive anymore busy tone after the destination is busy. It is normal as once the ISDN network has sent the busy tone to the IP Phone, it iniatiates a normal call clearing so a disconnect, then you can fix this on the your side by setting the following command voice call convert-discpi-to-prog to force the gateway to send the busy tone on the IP end even if the ISDN side is torn down

CAMA : Quick Definition

The Enhanced 911 (E911) system is a separate telephone system from the regular PSTN system. So CAMA is a protocol used to ensure that a call from a particular number is routed to the Public Service Access Point (PSAP) which is designated to handle calls from that number. The PSAP is alse responsible for dispatching emergency services.
CAMA has the capability to send both calling and called party in-band and the calling party is used for call routing in the E911 telephone system.

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