Communications Server 2007 supports three types of media gateways: Advanced, Basic, and Basic Hybrid. To review the differences between these gateway types, see Media Gateways, in Architecture. You can find a current list of qualified gateways that work with Communications Server at http://r.office.microsoft.com/r/rlidOCS?clid=1033&p1=IPpbxVend.The advantages and disadvantages of these three gateway types are summarized in the following table:

Table 4. Basic and Collocated Gateways Compared

When attempting to answer the four fundamental questions of gateway deployment listed earlier in Plan for Media Gateways in Office Communications Server, 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 15. 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 16. Datacenter Gateway 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 represent the extremes of a deployment continuum along which seemingly infinite combinations are possible. 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, and these vary from country to country. If all your gateways are located in a single country, this is not an issue, but if you locate gateways in several countries, each must be configured according to the PSTN standards of that country. Moreover, gateways that are certified for operation in, for example, 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 (1 PSTN call per hour) should allocate 1 port for every 15 users. For example, if you have 20 users, you will require a gateway with 2 ports.
  
• Moderate telephony users (2 PSTN calls per hour) should allocate 1 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 (3 or more PSTN calls per hour) should allocation 1 port for every 5 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 more, 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 organizations 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.

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.

The following table indicates minimum hardware requirements for Mediation Server based on the required transmission standard (T1 or E1) and the desired number of T1 or E1 interfaces:

Table 5. Mediation Server hardware requirements

CPU utilization is assumed to be 75 percent of the capacity, a conservative estimate that allows sufficient performance capacity for traffic and bandwidth spikes.

To help ensure the physical as well as logical separation of your Enterprise Voice infrastructure from the media gateways, you should install Mediation Server on a computer that is equipped with two network interface cards (NICs). One card faces the gateway; the second card faces the Communications Server 2007 server that acts as the Mediation Servers internal next hop.

When you install Mediation Server, the Deployment Wizard detects the presence of the two network cards and writes their IP addresses to the Communications Server listening IP address list and the Gateway listening IP address list, both on the General tab of the Mediation Server properties dialog box.

The Communications Server listening IP address is the address on an advanced media gateway that listens for call traffic from Communications Server. Until advanced media gateways are available, this address corresponds to the network card that serves as the internal edge of the Mediation Server.

Important:

The IP address that you select from the Communications Server listening IP address must match the address that is returned by a DNS query on the Mediation Servers FQDN. If the two addresses do not match, the IP address listed in DNS for your FQDN you will not be able to connect, and call traffic will be directed to an interface that is not listening for OCS traffic rather than to the one that is listening.

The Gateway listening IP address is the address on the Mediation Server that lists traffic from a basic media gateway or Basic Hybrid Media Gateway. For Office Communications Server 2007, this address corresponds to the network card that serves as the external edge of the Mediation Server.

Note:
It is possible to configure both edges on a single adapter card, but this alternative is not recommended.

For basic media gateways, the bandwidth requirement between gateway and Mediation Server is 64,000 bps (bits per second) for each concurrent call. Multiplying this number by the number of ports for each gateway is a fair estimate of the required bandwidth on the gateway side of the Mediation Server. On the Communications Server side, the bandwidth requirement is considerably lower.

When configuring Mediation Server, you are advised to accept the default media port gateway range of 60,000 to 64,000. The default range media port range enables the server to handle up to 1,000 simultaneous voice calls. Reducing the port range greatly reduces server capacity and should be undertaken only for specific reasons by an administrator who is knowledgeable about media port requirements and scenarios. For this reasons, altering the default port range is not recommended.

High-bandwidth traffic such as voice and video tends to stress poorly provisioned networks. Limiting media traffic to a known range of ports makes troubleshooting such problems easier.

Because a media gateway receives calls from the PSTN, it presents a potential security vulnerability. The recommended mititgation is to:

• Enable TLS on the link between the gateway and the Mediation Server. This will assure that signaling is encrypted end to end between the gateway and your internal users.
  
• Physically isolate the media gateway from the internal network by deploying the Mediation Server on a computer with two network interface cards: one accepting traffic only from the internal network; the second accepting traffic from a media gateway. Each card is configured with a separate listening address so that there is always clear separation between trusted traffic originating in the Communications Server network and untrusted traffic from the PSTN.
  The internal edge of a Mediation Server should be configured to correspond to a unique static route that is described by an IP address and a port number. The default port is 5061.
  The external edge of a Mediation Server should be configured as the internal next-hop proxy for the media gateway. It should be identified by a unique combination of IP address and port number. The IP address should not be the same as that of the internal edge, but the default port is 5060.
  

Encryption