E&M : Quick Definition

E&M stands for a lot of Acronym like

  • Ear an Mouth
  • Earth and Magneto
  • Receive and Transmit

E&m uses an RJ48 interface and is often used to provide tie-line connectivity to PBX. there are 5 types of E&M Interfaces ( I, II , III , IV, V) which are all supported by Cisco except the Type IV.

On this line , we can find the following signaling :

  • Wink-start
  • delay -start
  • immediate-start
  • tone-start

Need to more explanations about the different modes !

FXO : Quick Definition

FXO is an RJ11 interface which is providing a connection to a PSTN Central Office (CO) or to a PBX interface.

A FXO is considered as a telephony line. We can then find 2 types of signaling :

  • Loop-start signaling : this is a type of supervisory signaling that allows indication of on-hook and off-hook states. But this signaling is vulnerable to a condition knows as glare that involves 2 endpoints seizing a trunk a the same time.
  • Ground-start signaling : Same as loop-start signaling but this time we suppress the glare effect on these lines.

Ringback issues

When you are working with H323 configurations, you can experience teh following ringtones issues , let’s see these issues and how to fix it quickly:

  • No Ringback tone on an IP Phone when calling a destination in the PSTN : This comes from a mistmatch between the Call Manager and the PSTN. It can be fixed with the following command progress_ind alert enable 8 under the POTS dial-peer.
  • No Ringback tone on a phone in the PSTN when calling an IP Phone: The problem is more similar with the previous one but it differs because it is coming from the other side as this time , the setup message received from the PSTN did not include a progress indicator (PI=0). Indeed, the gateway assumes the network is an ISDN end-to-end network so normally the tone must be handled by the PSTN but it is not an end-to-end call , this is why you have to update again the progress-indicator with the command progress_ind setup enable 3 which is setting the tone in-band.
  • No Ringback tone from the PSTN Phone which is forwarded from an IP Phone to another IP Phone : in this case, the issue is caused by the fact that the voice gateway is tearing down logical channel when you are transferring the call , so Audio is not sent. To fix this , you need then to change a parameter under the Call Manager setting ToSendH225UserInfoMsg and put it to True

H225 RAS Messages

Here are the different messages used by the H225 RAS signaling which can occur between endpoints and GK.

  • Gatekeeper Discovery : used by endpoints to register
    • GRQ : Gatekeeper Request
    • GCF : Gatekeeper Confirm
    • GRJ: Gatekeeper Reject
  • Registration/Unregistration : used by endpoints to register or unregister their address and their zone withtin the GK.
    • RRQ : Registration Request
    • RCF : Registration Confirm
    • RRJ : Registration Reject

    • URQ : Unregistration Request
    • UCF : Unregistration Confirm
    • URJ : Unregistration Reject
  • Admission Control : These messages are used for all admission control and bandwidth management
    • ARQ : Admission Request
    • ACF : Admission Confirm
    • ARJ : Admission Reject
  • Bandwidth Control : these messages are used if you want to change the amount of bandwidth during a call
    • BRQ : Bandwidth Request
    • BCF : Bandwidth Confirm
    • BRJ : Bandwidth Reject
  • Endpoint Location : These messages are used to retrieve contact information ( also with Directory GK)
    • LRQ : Location Request
    • LCF : Location Confirm
    • LRJ : Location Reject
  • Status Information : They are used to retrieve a status information
    • IRQ : Information Request
    • IRR : Information Response Request

H225 call control signaling is on TCP/1720 and uses Q931 messages to establish, maintain and tear down calls. these messages are

  • Setup
  • Setup Acknowledge
  • Call Proceeding
  • Progress Alerting
  • Connect
  • User Information
  • Release Complete
  • Status Inquiry
  • Status
  • Information
  • Notify

So in general , here is the following reminder: While h225 call control signaling can be used for call setup, H245 must be used to determine a master/slave relationship, exchange terminal capabilities, logical channel signaling and DTMF relay.

H323 Stack

We have already spoken about Gatekeeper signaling in this previous post but let’ be more general and see what are the common elements of H.323 stack.

We have H225 RAS ( Registration Admission and Status) which is used between the GK and H323 endpoints.

But under normal operation , we have also H225 which is the call control and signaling part of the H323 stack. So we use it to establish connections.

We have also H245 which is the negotiation part of the H323 stack. So it is during this stage that you can exchange capabilities/codecs, establish logical channels. It is in fact all settings for the transmission of the media.


  • H225 RAS  => UDP/1719
  • GK Discovery => UDP/1718

The Multicast GK discovery uses IP Address

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