Communications Server 2010 provides improved flexibility in deploying IP-PSTN gateways. A given gateway can be assigned to multiple call routes and associated with one or more Mediation Servers. Multiple gateways can be associated with either a single Mediation Server or a pool of multiple Mediation Servers. When defining a call route, you specify the gateways associated with that route, but you do not specify which Mediation Servers are associated with the route. Instead, you use Topology Builder to associate gateways, and therefore routes, with Mediation Servers.

The ability to specify the PSTN gateways to which various numbers are routed enables you to determine which routes incur the lowest costs and implement them accordingly. The rule of thumb in selecting gateways is to choose the one closest to the location of the destination number in order to minimize long-distance charges. For example, if you are in New York and calling a number in Rome, you would carry the call over the IP network to the gateway in your Rome office, thereby incurring a charge only for a local call.

Communications Server 2010, like its immediate predecessors, requires all dial strings to be normalized to E.164 format for the purpose of performing reverse number lookup (RNL). Downstream components such as gateways, PBXs, or SIP trunks, however, may require numbers in local dialing formats. As a result, in Office Communications Server 2007 R2, it was sometimes necessary to individually configure downstream components or even reroute calls in order to accept the E.164 dial strings. Unlike its predecessor, however, Communications Server 2010 allows you to create one or more rules that assist in manipulating the Request URI prior to routing it to the gateway. For example, you could write a rule to remove +44 from the head of a dial string and replace it with 0144.

In planning which and how many gateways to associate with a given Mediation Server cluster, it may prove useful to group gateways with similar local dialing requirements so as to reduce the number of required translation rules and the time it takes to write them.

Communications Server 2010 provides a way to manipulate the caller ID for outbound calls. For example, if an organization wants to mask employees’ direct-dial extensions and replace them with the generic corporate or departmental number, an administrator can now do that by using the Communications Server Control Panel to suppress the caller ID and replace it with a specified alternative caller ID. In planning your routing logic, you will at least need to consider whether caller ID replaced is desirable for certain individuals, groups, or sites or, perhaps even, for all employees.

In creating outbound call routes, you should be aware of the following factors affecting routing logic:

• If the domain portion of the Request URI does not contain a supported domain for the enterprise, the outbound routing component on the server does not process the call. For example, in certain scenarios where a call is established over a federated boundary, the domain portion of the URI is used to route the call over to the enterprise that is responsible for applying the outbound routing logic.
  
  
• If a user is not enabled for Enterprise Voice, the server applies other routing logic as appropriate.
  
  
• If a call is routed to a gateway that is fully occupied (all trunk lines are busy), the gateway rejects the call and the outbound routing logic redirects the call to the next-least-cost route. Careful consideration should be given to this because a gateway sized for a small office overseas (for example, Zurich), may actually carry a significant amount of nonlocal traffic for international calls to Switzerland. If the gateway is not correctly sized for this additional traffic, calls to Switzerland may be routed by way of a gateway in Germany, resulting in larger toll charges.