Making IP Voice Calls

In a converged IP-PBX system, a call begins when the IP telephone goes off-hook and the PBX common control complex is alerted via a control signaling path between the IP telephone and the common control complex functioning as a gatekeeper. The signaling is transmitted to and from the LAN via an IP port interface card, dedicated Ethernet TCP/IP interface card, or integrated Ethernet connector. Dial tone packets or a control signal will alert the IP phone to activate a dial tone signal, an indication to the station user that the IP phone is ready for use. As the calling number digits are dialed, the common control complex will send multiple signals to the desktop, such as ring back or other call progress tones. If the call is being placed to a non-IP station port, such as an analog telephone, or a PSTN trunk circuit is required for an off-premises call, the following steps will occur:

1. A local TDM bus talk slot will be assigned

2. A gateway bearer communications channel will be assigned

3. Voice communication signals will be transmitted over the LAN to the IP port circuit card

4. The packeted voice signals will be reformatted, buffered, and transmitted across the gateway channels to the local TDM, and the call will continue until one party releases (disconnects) the connection.

For purposes of the call, the IP telephone appears to the common control complex and switching network as a PCM peripheral endpoint for voice transmission and feature activation operations. Converged IP-PBX systems typically use the same proprietary control signals for IP telephone support as those used for digital telephones.

If an IP station user is placing an intercom call to another IP telephone, the common control complex will direct the IP telephone to start sending voice packets directly to the IP address of the called IP telephone. The common control complex also will direct the called party’s IP telephone to send voice packets directly to the IP address of the calling party’s IP telephone. The direct audio communications path between the two IP telephones, using only LAN facilities, is often referred to as an IP-PBX peer-to-peer LAN switched connection or direct IP. No traditional PBX circuit switched connections are used for the call.

Direct IP connections will be established automatically between two IP endpoints if several conditions are satisfied:

• Both IP endpoints are administered to allow direct IP connection

• No TDM connections are required for either IP endpoint, and a point-to-point LAN connection is available

• The IP endpoints are in the same network region or in different network regions that are interconnected via LAN/WAN facilities

• The two IP endpoints share at least one common voice codec in their voice codec lists and the internetwork region connection management voice codec list

• The IP endpoints have at least one voice codec in common, as shown in their current codec negotiations between the endpoints of the IP-PBX

If any of these conditions are not satisfied, the call may require TDM connectivity.

A direct IP connection established for an existing call may be torn down if circuit switched TDM connectivity is required during the call. Conditions that may require TDM connectivity, based on the IP-PBX system, are:

• Additional parties are conferenced onto the call, including IP endpoints

• A PBX signaling tone or announcement needs to be inserted into the connection

• The connection is put on hold—music on hold

When the event requiring TDM connectivity is no longer in effect, a direct IP connection may again be established. The generic term for call connections that change from direct IP to TDM connectivity and back to direct IP is null capability.

The first generation of converged IP-PBXs required talk slots on the local TDM buses to support multiparty conference calls for calls among IP endpoints, even if all the parties were IP endpoints. Each conferenced party required a talk slot per TDM local bus to support the call. The manufacturers have future plans to support non-TDM conference calls among IP endpoints through enhanced versions of their current IP port circuit cards. Planned versions of IP port circuit cards will include conferencing circuits to support multiparty calls exclusively among IP endpoints, thereby eliminating the need for TDM switched connections. The IP endpoints may be internal IP telephones or IP trunk circuits connecting off-premises stations.