Internal calls

These calls are by far the easiest to route. For internal calls in the same office it is simply a case of using the LAN and we have already discussed how to ensure your LAN is up to the task in Chapter 2.

If you are making inter-office calls, then the best route depends on the quality of the company WAN, if there is one. A WAN with good bandwidth between offices can have a single central Asterisk server handling all the extensions. It's a simple setup, and all resilience efforts can be focussed on a core server. However, losing connectivity to an office will mean that all telephone services at that office are lost. This risk can be alleviated by introducing resilience into the WAN links too, a measure which increases ongoing circuit costs.

As we can see from this diagram, traffic from the branch office traverses the WAN to reach the PBX at the head office where it will be routed via the Internet if appropriate (for example if the call is external and to be routed via an ITSP).

Alternatively, a multi-server setup can be implemented where each office has a local Asterisk server, which can route all outbound calls via the Internet and/or PSTN if connectivity to the central Asterisk server is lost.

In this diagram we can see that each branch office now has a PBX. Calls can now be made internally in each office whether or not the WAN link is available. Call traffic can be routed from PBX to PBX, or as shown with the bottom-right branch, directly from their PBX to the Internet if needed or desired. For internal calls, this can be used as a failover route if the WAN circuit is lost. For the sake of clarity the link to the Internet for each branch has been omitted, but it can be assumed that each office could, and probably would, have an Internet circuit in addition to a WAN circuit. Of course, the WAN link can utilize the Internet circuit, with WAN traffic traversing a VPN tunnel.

So far we have assumed that the customer will have a robust WAN in place. A company with a poor or non-existent WAN will be best served either by routing call traffic through the Internet to a central Asterisk server, or by installing an Asterisk server at each site. A consideration as to which solution suits the company's needs best is the route that incoming calls take. If there is a central DDI for all offices then a central Asterisk server is an obvious solution. If each office has its own number(s), then there may be an argument for having servers on site. Either way, to secure traffic, a site-to-site VPN service should be considered.

Companies with a high number of internal calls across offices may also need to increase the bandwidth at each office, or install circuits dedicated solely to voice traffic. As discussed in the last chapter, this is also a good way of ensuring other Internet traffic does not steal bandwidth from the voice traffic.

The best solution really depends on the customer's needs and budget, and quite often is a mixture of both, whereby large branches may have their own PBX and small ones utilize a PBX that resides in another office.