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

MGCP Common Messages

Here are some MGCP messages that you can see in the interaction between the Call Agent and the endpoint:

  • EndpointConfiguration (EPCF) : A Call Agent sends this message to a gateway to specify signal encoding that will be received by an endpoint . A good example of these message will be the audio encoding using ulaw or alaw.
  • CreateConnection (CRCX) : This command creates a connection between 2 endpoints. the connection is created based on parameters included with the commad such as codec , allowable bandwidth, use of echo cancellation, silence suppression , gain control, and so on.
  • ModifyConnection (MDCX) : This is used to modify the parameters associated with a connection that was previously created.
  • DeleteConnection (DLCX) : The Call Agent can send this command to inform a gateway that it should terminate a connection, and a gateway can send this command to indicate that a connection can no longer be sutained. In response to a DeleteConnection, an endpoint sends statistics associated with the connection.
  • NotificationRequest (RQNT) : This is sent by the Call Agent to instruct a gateway to informit when specific events occur in an endpoint. Example : be on-hook/off-hook, tones,..
  • Notify (NTFY) : An endpoint uses this command to inform a Call Agent when requested events occur
  • AuditEndpoint (AUEP) : The Call Agent sends the AuditEndpoint command to the gateway to audit the status of an endpoint. The Call Agent can, for example, find out signal status, bearer information, …
  • AuditConnection (AUCX) : The Call Agent sends this command to the  gateway to find out the status of  a connection. Connection status information that can be retrieved using this command includes Call ID, Connection Mode, and connection parameters
  • RestartInProgress (RSIP) : The gateway sends this command to the Call Agent to inform the Call Agent that it is taking an endpoint or group of endpoints out of service or is returning an endpoint or group of endpoints to service.

Cisco Discovery Protocol – CDP : Quick Definition

CDP is a Cisco proprietary protocol that was designed for neighbor discovery. It is a Layer 2 protocol that sends multicast advertisements using SNAP encapsulation. The advertisements are sent by default every 60 seconds, and there is a default hold time of 180 seconds.

CDP advertisements can contain information such as DeviceID, Address, PortID, Capabilities, Version, Platform and native VLAN with much information in ASCII format.

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