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Voice over VPN for call terminations

A SearchEnterpriseVoice.com member wants to setup voice over VPN for several sites for call terminations. Expert Patrick Ferriter explains hardware requirements and implementation options here.

We want to set up Voice over VPN for three different sites worldwide for call terminations. We will be using H323/SIP 729/723 for this. We are now connected on Internet through Global Public IP's at each site. We use a combination of Cisco & Quintum devices and we have Cisco AS5300. How can we interconnect the offices through VPN? What is the required bandwidth for each location and what are the hardware requirements at each site?
If you have legacy PBX systems at any of the three sites you'll need to connect a VoIP to PSTN gateway to each of legacy PBX systems. The AS5300 can be used for this purpose. For example:

Old PBX<--E1-->AS5300<--SIP-->TO OTHER SITES

The Quintum gateways can also be used for this purpose. Once all three PBX systems have the capability of routing calls over IP, you'll need to establish the VPN between the offices so calls can be routed over the private network.

You first need a device at each site capable of setting up a VPN connection. One of the most straight forward approaches is to have the two smaller sites terminate their VPN tunnels to the headquarters site. The hardware requirements at each site are really minimal for this particular purpose since you really just need to have between 1 and 3 VPN tunnels. You can use a SonicWALL appliance (e.g. TZ150) to accomplish this.

The number of calls you can route between sites depends on the speed of the connection. If you are going to use G.711 uncompressed speech, then you need about 87 kbps per call with overhead. If you are going to use G.729 compressed voice, then you need about 24 kbps per call with overhead.

This was last published in November 2005

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