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

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.

Digit Manipulation

Digit Manipulation is the process of modifying the called or calling party number. It can be configured in a number of different ways:

  • Route Pattern
  • Route Group Level
  • Translation Pattern
  • Translation Rules in VGW

The specific types of manipulatios are :

  • Calling Party’s Extenal Phone Mask
  • Calling Party Transform Mask
  • Prefix Digits
  • Discard Digits Instructions(DDI)
  • Called Party Transform Mask

Although Digit Manipulation canbe done within a Route Group, it is applied when the the Route Groups are added to the Route List.

Recall also as seen in a previous post, that when digit manipulations are configured at both the Route Pattern and Route Group level, the manipulation configured at the Route Group level has priority over that configured at the Route Pattern level because it is processed later.

Translation patterns can be used also to manipulate the calling or called party numbers prior to call routing. It can be also used to modify elements such as CSS.

Gatekeeper Basic Configuration

Here is a basic definition for the gatekeeper config:


zone local oks ksiazek.be 1719

zone prefix oks 1234*

gw-type-prefix 1#* default-technology

bandwidth total zone oks 1024

bandwidth session zone oks 128

no shutdown


The zone local defines the Gatekeeper’s name , the domain to which it belongs and its IP Address and the TCP port used.

The zone prefix specifies the Gatekeeper’s name and the prefix associated within the zone list.

The gw-type-prefix will help to route calls if the called number doesn’t correspond to an E.164 number.

The bandwidth total zone defines the maximum aggregate bandwidth that you can use within this specific zone.

The bandwidth session zone defines the maximum bandwidth available for a session in the zone ( so this is the bandwidth for a call)

No shut activates the Gatekeeper’ configuration.

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