TD 92326GB
2009-03-10 / Ver. E
Installation and Operation Manual
VoIP Gateway
133
19.1 Understanding the VoIP Gateway's Gatekeeper
What is a Gatekeeper in general?
– A Gatekeeper/Registrar takes registrations from Endpoints (Gateways, Phones)
– Thus the Gatekeeper is possible to route Calls to Endpoints without configuring their
IP address.
– Can determine what Endpoints are not available / not registered and so re-route
without trying first to give the call to the not available / not registered Endpoint.
There are basically two tasks that the gatekeeper has to carry out:
• Management of the terminal equipment (device management)
• Switching of voice calls (call switching)
Both functions are features of the VoIP Gateway, although device management is
optional.
Figure 122. Call sequence with a gatekeeper and RAS
a scenario with a VoIP client, an ISDN gateway and a gatekeeper is shown.
The gatekeeper can be another VoIP Gateway or alternatively, it can be the gatekeeper
incorporated in every VoIP Gateway. For a clearer understanding though, the gatekeeper
and ISDN gateway are shown separately.
The individual steps of a call which are relevant in this context are as follows. In reality, the
procedures can be far more complex.
• Both the VoIP client (1.) and the ISDN gateway (2.) register with the gatekeeper's
device management. They submit their identity and their current IP address in the
process. This step requires the RAS protocol and therefore doesn't apply when
operating without the RAS protocol.
• The VoIP client initiates a call (3.) and sets up a signalling connection to the gatekeeper.
• The gatekeeper determines the call destination and sets up a signalling connection to
the destination (4.). The VoIP client and the gateway exchange their IP addresses.
Further signalling between the two of them goes via the gatekeeper.
• The VoIP client and ISDN gateway directly set up the two voice channels (5. and 6.)
between one another.
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