[This is pre-release documentation and subject to change in future releases. This topic's current status is: Milestone-Ready]

Topic Last Modified: 2010-07-18

IP-PSTN Gateways

IP-PSTN gateways are third-party hardware components that translate signaling and media between the Enterprise Voice infrastructure and the PSTN, either directly or through connection to SIP trunk. In either topology, the gateway terminates the PSTN. It is isolated in its own subnet and is connected to the enterprise network through the Mediation Server.

Three changes to the Microsoft Communications Server 2010 Mediation Server make gateway planning simpler and more flexible, allow greater scalability, and provide for increased reliability. First, relieving Mediation Server of having to process media as well as signaling frees bandwidth that makes it possible for a single Mediation Server to control multiple gateways. Second, the new ability for a single Mediation Server to control multiple gateways, and for one gateway to be affiliated with multiple Mediation Servers, provides greater flexibility in locating gateways and routing calls, improved scalability, and reduces costs of deployment and maintenance. Third, Mediation Server by default is now collocated on each Front End Server so that a Front End pool is also a pool of Mediation Servers. This collocation provides improved scalability, greater flexibility, and improved call handling under heavy loads.

An enterprise with multiple sites would typically deploy one or more gateways at each site. These gateways would be controlled by one or more Mediation Servers, which are collocated on Front End Servers. Branch-office sites can connect to the PSTN either through a gateway, in which case both a Registrar and Mediation Server are required on site, or through a Survivable Branch Appliance, which combines gateway and servers in a single box.

Determining the number, size, and location of IP-PSTN gateways is perhaps the most important and potentially costly decision you must make when planning your Enterprise Voice infrastructure.

The main questions to answer are:

  • How many media gateways are needed? The answer depends at least in part on the size of the gateways and where you plan to deploy them. That number, in turn, depends on several variables,

  • What size should the gateways be? The answer depends in part on how many you plan to deploy and where you plan to put them.

  • Where should the gateways be located? The answer depends in part on the topology and geographic distribution of your organization.

In other words, no one of the previous questions can be answered independently of the other two. Answers to all three depend ultimately on how much telephone traffic you anticipate and how that traffic is distributed across your organization. But that is only the beginning: the base data, so to speak. You must also consider your gateway topology options.

Multiple Gateway Support

A Communications Server 2010 Mediation Server pool can control multiple gateways, Service Boundary Contollers (SBCs) provided by telephony service providers, or some combination thereof. Additionally, multiple Mediation Servers in the pool can interact with a single gateway. When an internal user places a PSTN call, outbound routing logic on the Front End chooses which Mediation Server and gateway combination to use out of all possible combinations that may be available for routing that particular call. For more information on planning for multiple gateways, see IP-PSTN Gateways.

For information on other outbound routing enhancements, see Voice Routes.

Gateway Topologies

When attempting to answer the four fundamental questions of gateway deployment, take the following approach:

  1. Count the sites at which you want to provide PSTN connectivity using Enterprise Voice.

  2. Estimate the traffic at each site (number of users and average number of calls per hour per user).

  3. Deploy one or more gateways at each site to handle the anticipated traffic.

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

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.

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.

Gateway Location

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.

Gateway Size and Number

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 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.

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.

    Telephony central site topology
    Distributed gateway topology