Configuring Voice over IP
Call Flow
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Cisco VGD 1T3 Voice Gateway Software Configuration Guide
Legs connecting a local device (typically a phone, fax machine, or PBX) to a gateway are called POTS
(plain old telephone service) legs. Legs connecting a gateway to the IP network are called VoIP legs. A
POTS or VoIP leg is either inbound or outbound, from the perspective of the associated gateway.
Table 1
describes the different types of call legs.
A gateway uses two call legs—an inbound POTS with an outbound VoIP or an inbound VoIP with an
outbound POTS—to create an end-to-end call through the gateway. A call that passes through both an
originating gateway and a destination gateway has four call legs.
Call Flow
Table 2
and
Table 3
detail the general call flow from the perspective of an originating and destination
gateway respectively.
Table 1
Call Legs
Call Leg Source
Call Leg Destination
Call Leg Type
Originating device
Originating gateway
Inbound POTS
Originating gateway
IP network
Outbound VoIP
IP network
Destination gateway
Inbound VoIP
Destination gateway
Destination device
Outbound POTS
Table 2
VoIP Call Flow, Originating Gateway View
Event
Leg Type
User sends dialed digits using public switched telephone network to gateway.
Inbound POTS
Gateway does the following:
•
Processes information (maps dialed digits, according to information stored
in dial-peer configuration tables, either to an IP host that connects directly
to the destination gateway or to a PBX at the destination that can complete
the call).
•
Initiates H.323 session across network.
•
Processes voice signals and sends packets over network. As appropriate,
sends call-progress and other in-band signals.
•
Ends session.
Outbound VoIP
