First, identify the sites in your topology where you will deploy Enterprise Voice and the network regions to which those sites belong. In particular, consider how you will provide PSTN connectivity to each site. For manageability and logistical reasons, the regions to which these sites belong can be a deciding factor. Decide where gateways will be deployed locally, where Survivable Branch Appliances (SBA) will be deployed, and where you can configure SIP Trunks (either locally or at the central site) to an Internet telephony service provider (ITSP).

You also need to consider the bandwidth usage that you expect on the network links between your central site and its branch sites. If you have, or plan to deploy, resilient WAN links between sites, we recommend that you deploy a gateway at each branch site to provide local direct inward dial (DID) termination for users at those sites. If you have resilient WAN links, but the bandwidth on a WAN link is likely to be constrained, configure call admission control for that link. If you do not have resilient WAN links, host fewer than 1000 users at your branch site, and do not have local trained Microsoft Lync Server 2010 administrators available, we recommend that you deploy a Survivable Branch Appliance at the branch site. If you host between 1000 and 5000 users at your branch site, lack a resilient WAN connection, and have trained Microsoft Lync Server 2010 administrators available, we recommend that you deploy a Survivable Branch Server with a small gateway at the branch site. Consider also enabling media bypass on constrained links if you have a gateway peer that supports media bypass.

Concepts