Communications Server offers three options for deploying a Mediation Server and media gateway:

• Basic. This option consists of a basic media gateway and a separate Mediation Server.
  
• Basic Hybrid. This option is a basic-hybrid gateway, in which the basic gateway and Mediation Server are collocated on a single computer.
  
• Advanced. This option is an advanced media gateway, in which the Mediation Server logic is incorporated within the gateway software itself.
  

For details, including a current list of qualified gateways that work with Communications Server, see http://go.microsoft.com/fwlink/?LinkId=125757.

Table 1. Basic and Collocated Gateways Compared

When attempting to answer the four fundamental questions of gateway deployment, the obvious approach is to:

• Count the sites at which your organization has offices.
  
• Estimate the traffic at each site.
  
• Deploy one or more gateways at each site to handle the anticipated traffic.
  

The resulting distributed gateway topology is shown in the following figure.

Figure 1. Distributed gateway topology

With this topology, calls among workers at each site and between the sites are all routed over the company intranet. Calls to the PSTN are routed over the enterprise IP network to the gateways that are closest to the location of the destination numbers.

But what if your organization supports dozens or hundreds or even thousands of sites spread across one or more continents, as many financial institutions and other large enterprises do? In such cases deploying a separate gateway at each site is impractical.

To address this problem, many large companies prefer to deploy one or a few large telephony data centers, as shown in the following figure.

Figure 2. Telephony data center topology

In this topology, several large gateways sufficient to accommodate the anticipated user load are deployed at each data center. All calls to users in the enterprise are forwarded by the company's telephone service provider to a data center. Routing logic at the data center determines whether the call should be routed over the intranet or to the PSTN.

Placing a gateway at every site on the one hand or at a single data center on the other represents the extremes of a deployment continuum. You can deploy single gateways at several sites and several gateways at a data center in nearly any possible combination. The best solution in each case depends on a variety of factors that are specific to each organization.

Gateway location may also determine the types of gateways you choose and how they are configured. There are dozens of PSTN protocols, none of which is a worldwide standard. If all your gateways are located in a single country/region, this is not an issue, but if you locate gateways in several countries/regions, each must be configured according to the PSTN standards of that country/region. Moreover, gateways that are certified for operation in, say, Canada, may not be certified in India, Brazil, or the European Union.

The media gateways that most organizations will consider deploying range in size from 2 to as many as 960 ports. (There are even larger gateways, but these are used mainly by telephone service providers.) When estimating the number of ports your organization requires, use the following guidelines:

• Light telephony users (one PSTN call per hour) should allocate one port for every 15 users. For example, if you have 20 users, you will require a gateway with two ports.
  
• Moderate telephony users (two PSTN calls per hour) should allocate one port for every 10 users. For example, if you have 100 users, you will require a total of 10 ports allocated among one or more gateways.
  
• Heavy telephony users (three or more PSTN calls per hour) should allocate one port for every five users. For example, if you have 47,000 users, you will require a total of 9,400 ports allocated among at least 10 large gateways.
  
• Additional ports can be acquired as the number of users or amount of traffic in your organization increases.
  

For any given number of users you must support, you have the choice of deploying fewer, larger gateways, or smaller ones. As a rule, a minimum of two gateways for an organization is recommended in the event one goes down. Beyond that, the number and size of gateways that an organization deploys are going to vary widely, based on a careful analysis of each organization’s volume of telephone traffic.

Each basic media gateway that you deploy must have at least one corresponding Mediation Server. It is possible, though not recommended, to point a single gateway to multiple Mediation Servers, but you cannot point a single Mediation Server to more than one media gateway.

For details, including specific hardware requirements, see Internal Office Communications Server Component Requirements and Capacity Planning.

Note:
A basic hybrid media gateway is configured to work only with the collocated Mediation Server and therefore should not be pointed to other Mediation Servers.

By default, E.164 numbers in the Request Uniform Resource Identifier (URI) for outgoing calls are prefixed with a plus sign (+). Most Private Branch eXchanges (PBXs) process such numbers without problem. Certain PBXs, however, do not accept numbers that are prefixed with a plus sign.

To ensure interoperability with these PBXs, Mediation Server has a new WMI Boolean setting called RemovePlusFromRequestURI, which has two values: TRUE and FALSE. If your PBX does not accept numbers prefixed with a plus sign, the value for the WMI setting should be set to TRUE, which causes Mediation Server to strip the plus sign from a Request URI for outbound calls. The default is FALSE, which causes Mediation Server to pass the outgoing INVITE’s Request URI, To URI, and From URI unchanged.

Mediation Server has a new WMI Boolean setting called QoSEnabled, which has two values: TRUE and FALSE. This setting enables or disables QoS marking on Mediation Server. When set to TRUE, the setting causes Mediation Server to perform Differentiated Services Code Point (DSCP) marking on voice packets. The default value is FALSE.

In a network that has been properly provisioned for voice transmission, packet prioritization is not necessary. However, if you are unsure of bandwidth capacity, this QoS setting assures good voice quality even in suboptimal environments.

By default, E.164 numbers in the Request URI of outgoing calls from Office Communications Server 2007 R2 are prefixed with a plus sign. Most PBXs process such numbers without problem. Some PBXs, however, do not accept numbers that are prefixed with a plus sign and do not route those calls correctly.

Additionally, the From headers of inbound calls from some PBXs does not conform to RFC 3966 because they are not prefixed with a plus sign. Microsoft Office Communicator cannot resolve these numbers to the correct user.

To assure interoperability with these PBXs, Office Communications Server 2007 R2 has a new Mediation Server setting for WMI called RemovePlusFromRequestURI. This setting can be set to TRUE or FALSE. The default value is FALSE.

• If a PBX downstream from the Office Communications Server 2007 R2 Mediation server does not accept numbers prefixed with a plus sign, set the value of RemovePlusFromRequestURI to TRUE. This causes Mediation Server to remove the plus signs from the Request URIs of outgoing calls. It also causes the plus signs to be removed from the To and From URIs.
  
• If the downstream PBX accepts numbers prefixed with plus signs, leave the value of RemovePlusFromRequestURI set to its default value of FALSE. This causes Office Communications Server 2007 Mediation Server to pass Request URIs, To URIs, and From URIs unchanged (that is, with plus signs).
  

Office Communications Server 2007 R2 also introduces support for private numbering plans by normalizing From headers that are not in E.164 format. If the result of this normalization is not in E.164 format, Office Communications Server 2007 R2 inserts a P-Asserted-ID header with a phone-context value of enterprise to enable user lookup in Office Communicator 2007 R2. However, if the URI already contains a phone-context value of enterprise, Office Communications Server 2007 R2 does not normalize the From header.