Technical White Paper

Published: July 19, 2007

Executive Summary

The technological advances from the 1970s to the 1990s enabled the birth of new messaging systems such as voice mail, e-mail, and fax, in addition to new methods for voice transmission, such as Voice over IP (VoIP). Traditionally, voice communication involved analog or digital transmission of data over distances by using physical wire through the plain old telephone service (POTS). Voice mail and fax communication occurred through POTS. The advent of the Internet and popularization of IP packet-switched networks gave rise first to e-mail, and then to VoIP communication. All these developments have gradually led to a convergence of disparate communication systems toward a common, unified infrastructure.

Microsoft incorporates emerging communication systems and technologies into its corporate environment according to business needs. By deploying Microsoft® Exchange Server 2007, Microsoft benefited from new unified messaging (UM) capabilities that combine voice mail, e-mail, and fax messages into a single Inbox for users. Technologically, Exchange Server 2007 accomplishes this through a new UM server role, which accepts traditional voice data from private branch exchanges (PBXs) through VoIP gateways or directly through IP PBXs. This results in considerable cost savings and flexibility for the Microsoft Information Technology (Microsoft IT) group, which is responsible for designing and implementing Exchange Server 2007 UM servers at Microsoft. With Exchange Server 2007 and VoIP technology, Microsoft IT eliminated separate physical telephony links between PBX switches and enterprise devices such as UM servers. Instead, VoIP gateways enable Microsoft IT to integrate traditional PBX switches into the unified IP-based communications infrastructure.

This technical white paper discusses how Microsoft IT designed and deployed an Exchange Server 2007-based unified messaging solution to support an increasingly mobile workforce with flexible and convenient access to voice mail, fax messages, calendar items, tasks, contact information, and e-mail messages in a single repository—the user's mailbox. Starting from an overview of VoIP technologies, this white paper covers the unified messaging environment at Microsoft before Exchange Server 2007. It then explains the design and deployment decisions that Microsoft IT made to transition from the third-party unified messaging environment to Exchange Server 2007-based unified messaging.

This paper contains information for business and technical decision makers who are considering deployment of the unified messaging server role in an Exchange Server 2007 organization. This paper assumes that the audience is already familiar with the concepts of TCP/IP networks, Windows Server® 2003, and the Active Directory® directory service. A high-level understanding of the features and technologies included in Exchange Server 2007 is also helpful. Detailed product information is available in the Microsoft Exchange Server 2007 TechNet Library at http://www.microsoft.com/technet/prodtechnol/exchange/2007/library/default.mspx.

Introduction

Unified messaging is the convergence of different forms of messaging, specifically voice, e-mail, and fax, in a single, integrated system. Of these messaging forms, voice mail was the first to achieve widespread use. Voice mail, invented in the early 1970s, first achieved commercial success in the mid-1980s. At that time, the rapidly declining price of semiconductors yielded processors fast enough to handle analog-to-digital conversion (that is, voice to digital signals), and innovations in disk technology resulted in more storage space for less cost. These factors, combined with the emergent PBX, enabled companies to adopt and implement voice mail systems on a large scale because voice mail became more affordable.

VoIP began to emerge in the mid-1990s with telephony applications streaming voice content over the Internet. Although the quality suffered from delays, jitter, and frequent disconnections, the IT industry quickly embraced the technology because of its compelling advantages over traditional telephone systems. IP networks are more cost-efficient to operate and provide greater flexibility than traditional time-division multiplexing (TDM)-based circuit-switched links. With increasing network bandwidths, efficient data-compression algorithms, and support for Quality of Service (QoS) levels in the physical network infrastructure, IP telephony has matured.

VoIP Trunking and Direct IP Communication

Public switched telephone network (PSTN) service providers and private companies with multiple locations use trunking to establish backbones that connect PSTNs or PBX systems in different geographical locations with each other. Traditionally, a trunk is a connection that consists of multiple individual TDM-based links, aggregated to increase the overall bandwidth. However, with the arrival of VoIP technology, TDM-based trunks are on the retreat while more cost-efficient IP-based WAN connections advance to take their place. For example, it is commonplace now for long-distance and international carriers to use VoIP trunking. TDM-to-VoIP gateways, commonly called VoIP gateways, connect local PSTNs to an IP-based backbone, such as the Internet. From the telephone to the local PSTN, the voice traffic still travels across TDM-based links, but the communication between the PSTNs now relies on VoIP.

As indicated in Figure 1, there are two options for delivering voice traffic from the PSTN to the corporate IP network: using a VoIP gateway or a dedicated IP PBX. IP PBX systems do not require a VoIP gateway to communicate over an IP-based network. All IP devices, including IP phones and IP-based equipment for video conferencing, can exchange data directly with each other over the computer network. The IP PBX switch does not need to be involved in the actual data transfer, which helps to improve the scalability of VoIP. In a pure IP PBX system, the IP PBX switch is responsible only for establishing sessions between the communication partners, thereby acting as a communication controller. A popular protocol to establish sessions is the Session Initiation Protocol (SIP). After a session between the IP phones is established, the devices use the Real-Time Transport Protocol (RTP) to transfer voice content.

Figure 1. VoIP in the corporate network

Microsoft IT uses VoIP trunking to connect older PBX switches to the corporate network through VoIP gateways. A VoIP gateway provides the necessary connectivity between the otherwise incompatible circuit-switched and packet-switched network architectures. VoIP provides Microsoft IT with the following advantages:

• The IP-based corporate network can replace legacy TDM-based trunks. The wide area network (WAN) operates at either 1.544 megabits per second (Mbps) (T1) or 2.048 Mbps (E1) speed.
• Communication within the company based on VoIP does not need to pass through public PSTN providers. This is also true for partner communication over extranet connections. Bypassing public PSTN providers helps to save costs.
• The IP-based computer network can route incoming phone calls to the user's IP phone regardless of the user's network location. For example, mobile users can receive incoming calls wherever they connect to the corporate network.
• It is possible to consolidate PSTN connections and provide telephony services to small branch offices over the IP network by using pure IP PBX systems or single IP phones.
• IP-based PBX systems facilitate remote administration because IP PBX switches provide Web-based configuration interfaces or support directory integration based on Lightweight Directory Access Protocol (LDAP).
• The IP-based corporate environment can support unified communication services, including Exchange Server 2007 for unified messaging and Microsoft Office Live Communications Server 2005 for IP telephony, call control, and instant messaging.

Unified Messaging Prior to Exchange Server 2007

From the year 2000 until Exchange Server 2007, Microsoft IT used a combination of traditional voice mail systems and a third-party UM solution to provide employees with voice mail capabilities. Microsoft IT supported 46,000 users in the unified messaging environment, 79 percent of whom accessed the UM servers from the headquarters in Redmond. Microsoft IT maintained 25 servers (including six tracing servers) for UM in eight sites located in North America, Asia, and South America. These servers answered approximately 280,000 calls per week, with 40 percent of the calls resulting in voice mail messages. The remaining 60 percent of calls resulted in answered calls or callers not leaving voice mail messages.

With the third-party UM system, Microsoft IT maintained a database for user administration of UM-enabled users. This database existed separately from the Active Directory user database. Therefore, UM user management for Microsoft IT entailed additional tasks outside managing users and user attributes through Active Directory.

Telephony and Network Infrastructure

Microsoft IT's UM environment before Exchange Server 2007 included third-party UM servers, e-mail servers, PBXs, and telephony and IP connectivity. The combination and setup of these components varied to support the user capacity needs of each UM server location. Although Microsoft IT supports over 500 office locations worldwide, only eight sites (Redmond, Silicon Valley, Shinjuku, Mexico City, Bangalore, Singapore, Sao Paulo, and Austin) housed third-party UM servers. Most of these sites were small, requiring only 16 digital phone lines each to provide UM services to regional users. The sites with more users, such as Redmond and Silicon Valley, relied on T1 connectivity.

Table 1 shows the locations that housed the third-party UM servers. The UM servers required connectivity to Microsoft Exchange Server 2003 Mailbox servers and the PBX system. Whereas Exchange Mailbox servers communicated with UM servers through the IP network, the PBXs communicated with UM servers through telephony connections, such as T1 or digital set emulation. Telephony connection runs are expensive to implement over long distances. As a result, Microsoft IT deployed the third-party UM servers in the same physical site as the PBX.

Table 1. Telephony Site Summary

* The Redmond site includes multiple PBXs and forests

Some of these sites have multiple forests and multiple PBXs. For instance, the Redmond site has one main logical layout to support the entire Redmond area. Yet, there are five Active Directory forests within Redmond for different purposes, such as legacy product support and future product testing. Multiple PBXs service these forests.

Connectivity

Connectivity in a UM system includes the telephony TDM connections between PSTNs and PBXs, the IP-based SIP/T.38 connections between VoIP gateways and UM servers, and IP network connections such as MAPI and LDAP between UM servers and the rest of the network. For its third-party UM system, Microsoft IT relied on the following types of connectivity:

• Telephony   The type of telephony connection that Microsoft IT used varied depending on the capacity needs of the area and the connectivity available. Sites that supported a lower call volume relied on a digital set emulation connection between the PBX and third-party UM servers. Larger sites required a T1 connection between the PBX and third-party UM servers to support a higher call volume. Microsoft IT used T1 Channel Associated Signaling (CAS) for connectivity. For T1 CAS SMDI call integration, Microsoft IT used RS232 integration links.
• Local area network (LAN)   Before Exchange Server 2007 UM, Microsoft IT placed third-party UM servers on the LAN that is located in the same geographical site as the PBX. Microsoft IT made this decision based on costs: Running long-distance PBX telephony connections is expensive.
• WAN   As explained in the "E-Mail Messaging Infrastructure" section later in this document, Microsoft IT consolidated mailbox servers to just four sites by using Exchange Server 2003. The third-party UM servers communicated with the Exchange Mailbox servers in these sites through WAN connections. The sites that housed UM servers had low-latency connections to the locations that housed Exchange Server 2003 Mailbox servers.

In this environment, the telephony connectivity type and number of ports, which are based on the number of UM users and call load, determine the PBX and number of third-party UM servers necessary for a particular location. Microsoft IT used the following site models:

• Small site   Most of the UM sites provided services to fewer than 2,000 users. In these sites, eight phone lines per UM server supported incoming calls. Each of these small sites contained two third-party UM servers. The third-party UM servers accepted voice mail messages received by the PBX and transferred messages to the Exchange Mailbox servers.
• Medium site   Shinjuku in Tokyo was the only medium site in the third-party UM solution, with 32 digital set emulation ports dedicated to voice mail to support a higher call volume. Shinjuku used two third-party UM servers with 16 digital lines per server, instead of the eight lines per server used in smaller configurations.
• Large site   Redmond and Silicon Valley were the largest sites, designed to support a high volume of calls. They used T1 CAS connections between the PBXs and UM servers. Each T1 line carried 24 channels of voice, and a single RS232 integration link carried the SMDI call integration information.

Figure 2 illustrates these site models.

Figure 2. UM infrastructure prior to Exchange Server 2007

PBXs

PBX systems enable companies such as Microsoft to manage incoming calls by sharing a small number of outside phone lines among many internal extensions. PBXs have evolved to meet the growing needs of the virtual office, and today they are robust systems that offer voice mail, fax, Auto Attendant, and other advanced features in call routing. Microsoft IT uses the following types of PBXs to accommodate the various usage and capacity requirements in its sites:

• Intecom   Although PBXs are not specific to an Active Directory forest, the Redmond site uses Intecom PBXs for the main corporate forest and other forests.
• Nortel SL100   The Silicon Valley site relies on the Nortel SL100 PBX to provide voice mail, Direct Inward Dialing (DID), and other features.
• Nortel Meridian   Microsoft IT uses this PBX's digital set emulation connection at its smaller sites. In a digital emulation setup, the gateway emulates a multiple digital phone set that the PBX supports.

PBX Interfaces

Both the Nortel and Intecom PBXs that Microsoft IT use support various protocols, interfaces, and switching methods. However, for Microsoft IT, the following aspects of PBX interfaces are most relevant with the third-party UM environment:

• PBX-to-trunk-line connections   For connecting PBX systems to the telephone company, the Nortel and Intecom PBXs support Integrated Services Digital Network (ISDN). ISDN is a long-established connection option that takes the form of either a Basic Rate Interface (BRI) capacity of two circuits or a Primary Rate Interface (PRI) capacity of 24 circuits in North America. Intecom and Nortel SL100 PBXs use a T1 PRI, whereas Nortel Meridian uses BRI lines through digital set emulation. Although some PBXs and telephone companies support Internet Protocols for trunk connectivity, such as H.323, SIP, Media Gateway Control Protocol (MGCP), and Inter-Asterisk Exchange (IAX), The Nortel and Intecom PBXs that Microsoft IT used did not support IP trunk connectivity options.
• PBX-to-PBX connections   Intecom and Nortel PBXs have the capacity to share signaling data. Both Nortel SL100 and Intecom PBXs support T1 CAS. Additionally, Nortel PBXs supported T1 Q signaling (Q.SIG). However, when Microsoft IT deployed the third-party UM-based environment, the third-party servers did not support T1 Q.SIG.
• Third-party UM to PBX connections   At the time of installation, the third-party UM servers supported analog, T1 CAS, and digital set emulation. The cards that connected to the PBX used the specific cable that worked for the interface. For example, the T1 CAS connection used a standard RJ 48C connector.
• Data collection and SMDI   For data gathering options, PBXs include call record log files or a network interface. For SMDI integration, PBXs use a DB-9 RS232 serial interface.

PBX Features

At a basic level, PBXs are responsible for connecting an incoming call to an internal user's extension, maintaining the established connections, and keeping a log of the information associated with the call. Microsoft IT uses Nortel and Intecom PBXs to provide other common features and capabilities, such as Auto Attendant, DID, and voice mail. For a more detailed list of features available, see Table 13 in Appendix B.

E-Mail Messaging Infrastructure

Microsoft IT consolidated its messaging environment during the Exchange Server 2003 time frame. Because of the consolidation, mailbox servers now reside in only four data centers, from which they support the other office locations according to geographical boundaries. Table 2 lists the data centers and the regions for which they are responsible.

Table 2. Microsoft Data Centers and Regions of Responsibility

After consolidation, multiple locations with third-party UM servers accessed the same Exchange Server 2003 Mailbox servers in the four data centers. For example, Singapore and Shinjuku shared the servers in the Singapore data center, whereas Mexico City, Austin, Redmond, and Silicon Valley shared servers in the Redmond data center.

Reasons for Microsoft IT to Deploy Unified Messaging

For Microsoft IT, the unified messaging capabilities of Exchange Server 2007 presented an opportunity to centralize administration and monitoring of voice mail and fax, provide users with self-service capability, and consolidate sites and servers. By integrating voice mail, fax, and e-mail in a unified messaging environment, Exchange Server 2007 provides users with convenient and flexible access to messaging information. Employees can access e-mail, voice mail, calendar items, fax messages, and contacts from one mailbox. Exchange Server 2007 enables employees to access their mailboxes from a telephone by using Outlook® Voice Access, from a mobile device, or from notebook and desktop computers by using Microsoft Office Outlook. Furthermore, the unified messaging server role includes text-to-speech features and English voice recognition for access to mailbox and directory information.

Microsoft IT recognized the following improvement opportunities with Exchange Server 2007:

• Reduced costs   With UM and voice mail systems before Exchange Server 2007, expanding service to new sites was costly. It required UM server deployment and expensive voice cards, or expensive traditional voice mail system upgrades. With Exchange Server 2007, Microsoft IT could deploy gateways at regional sites and locate UM servers in four major data centers that also house Exchange Mailbox servers.
• Site and server consolidation   The Exchange Server 2007 UM role relies on VoIP gateways or direct SIP interoperability with IP PBX systems to receive information from the PBX. This capability enables Microsoft IT to expand services easily to new sites by deploying VoIP gateways at local sites and configuring shared UM servers in major data centers.
• User self-service   By combining self-service features such as personal identification number (PIN) reset with a single mailbox for multiple message types, Exchange UM puts users in control, reducing Helpdesk calls and support costs. Additionally, users can access e-mail and voice mail through a variety of methods, including the full Office Outlook client, Microsoft Office Outlook Web Access, a mobile device, or a standard telephone.
• Active Directory integration   Whereas the third-party UM system used a separate user database, Exchange Server 2007 UM natively integrates with Active Directory. UM objects, such as dial plans, VoIP gateways, and hunt groups, have a logical representation as Active Directory objects for easier administration because all the data is stored in Active Directory. Additionally, Exchange Server 2007 UM uses the Active Directory user database for a single repository of user data.
• Next generation networking (NGN)   Unified messaging and VoIP represent a general trend toward convergence of messaging systems into a single network type: the packet-switched IP network. This network transports services and data for voice, data, video, and other media by encapsulating the streams into data packets. NGN requires essential building blocks, such as an IP network, site connectivity, and existing routing topologies for the various traffic types. As part of an effort to move to NGN, Exchange Server 2007 offers Microsoft IT UM and convergence of traditional PBX systems and the IP network.
• Administration, operation, and training   With one user database (Windows® Active Directory), one message store (Exchange Server), and one messaging infrastructure to maintain for additions, moves, changes, and backups, Microsoft IT can realize savings in administering and maintaining the voice mail, fax, and e-mail messaging systems. Using one system consistently across the enterprise network also enables Microsoft IT to reduce the time spent training users and administrators.
• Employee productivity   The features that Exchange UM provides to access voice mail, fax, and e-mail messages from one mailbox, coupled with the ability to access the mailbox by phone, mobile device, or computer, creates great flexibility for Microsoft employees. Outlook Voice Access makes it possible to adjust calendar items, check and write messages, and retrieve directory information while away from the office.

Exchange Server 2007 Unified Messaging Design

When designing an environment based on Exchange Server 2007, Microsoft IT considered not only the many components in the existing environment, but also the required components in the new Exchange UM-based environment. The transition between UM solutions and the requirement to have minimal service interruptions as well as multiple sites made it necessary for Microsoft IT to carefully evaluate the components in the UM environment and design an Exchange Server 2007-based solution.

To ensure a smooth transition at all sites, Microsoft IT designed the Exchange Server 2007 UM-based environment in stages, making early design decisions for foundational aspects, and later building on early design decisions to account for all aspects of the environment. Microsoft IT conducted the following design phases:

1.     Infrastructure inventory   In this phase, Microsoft IT outlined a high-level overview of the existing environment, including sites, Exchange and Active Directory forests, telephony, and IP connectivity. This process included analyzing the number of users and capacity needs at each site, and special configurations, such as the multiple-forest Redmond site. For more details about the data gathered in this stage, see the earlier section "Unified Messaging Prior to Exchange Server 2007."

2.     Hardware selection   The major goals in this phase included selecting which hardware components to keep, remove, or modify during the migration, in addition to selecting the best new components for the Exchange Server 2007 UM-based environment. Retaining existing PBX hardware, Microsoft IT installed new VoIP gateways and Exchange Server 2007 UM servers. For more details about these decisions, see the later sections "UM Server Design," "PBX Integration," and "VoIP Gateway Selection."

3.     Topology design and selection   In this phase, Microsoft IT designed model deployments for all locations based on the information gathered in the previous phases. The design reflected anticipated site usage, Exchange and Active Directory dependencies, and available connectivity. For more information about the specific decision factors, see the later section "Connectivity Scenarios and Model Configurations."

4.     UM features   After the hardware and topology design phases, Microsoft IT focused on Exchange Server 2007 UM features to select the best options relevant to the production environment. For more information about which features Microsoft IT implemented, see the later section "UM Features Available."

5.     Administrative and operations design   To complete the design phases and prepare for rollout, Microsoft IT planned for user provisioning, administration, and system monitoring. Microsoft IT's administrative design focused on user self-service so that common requests, such as PIN reset, would require no administrative intervention. For more information about administration and monitoring, see the later sections "System Monitoring" and "Security."

Design Components

When designing the Exchange Server 2007-based UM environment, Microsoft IT worked within the specifications of the Exchange product group to choose the various telephony and IP components that Exchange Server 2007 UM requires. The Exchange product group developed Exchange Server 2007 UM with the following dependencies:

• Telephony environment connected to PBX   As callers make voice or fax calls to Microsoft numbers, the telephony infrastructure that connects the PSTN to the PBX enables the PBX to receive calls by using TDM.
• IP PBX or PBX with VoIP gateway   For Exchange Server 2007 UM servers to process the incoming calls, the PBX must direct calls to an Exchange UM server. Microsoft IT uses VoIP gateways to connect PBXs to Exchange UM servers. Exchange UM servers use T.38 for faxes and RTP for voice calls after initiating the session by using SIP.
• IP environment After the VoIP gateways direct incoming calls to an Exchange UM server, the UM server processes calls and offers features such as directory services, Outlook Voice Access, and Interactive Voice Response (IVR). To accomplish this, Exchange UM servers communicate with the Active Directory and Exchange environment by using a variety of protocols, including Simple Mail Transfer Protocol (SMTP), MAPI, and LDAP.

Figure 3 shows the typical UM design components.

Figure 3. Typical UM design components

Component Selection and Decisions

When selecting the UM components, choosing site topologies, and determining bandwidth requirements, Microsoft IT followed the decision process shown in Figure 4. There were several decisions to make that affected the entire environment, in addition to specific design and selection processes for each site.

Figure 4. UM design process flow

1.     Microsoft IT started the design process decisions by determining whether the environment required one location or multiple locations. A location refers to a geographic area with either a single PBX or multiple PBXs that share a single voice mail system. As mentioned earlier in the "Unified Messaging Prior to Exchange Server 2007" section, Microsoft IT supports eight locations.

2.     Because Microsoft requires multiple Exchange forests, Microsoft IT did not consolidate forests or locations when designing Exchange Server 2007 UM. However, as mentioned earlier, Microsoft IT previously consolidated Exchange mailbox data centers to four locations during the Exchange Server 2003 period. Because this was already in place, Microsoft IT took advantage of earlier consolidation efforts to collocate UM servers in the four major data centers.

3.     After taking an inventory of the locations, Microsoft IT considered technical and business needs for each location. For example, Microsoft tracked the typical number of calls and users in the UM environment to serve as a baseline for designing UM servers and provisioning the necessary connectivity. Microsoft IT kept existing PBXs and configured them to work with Exchange UM servers.

4.     For each location, based on the connection and PBX, Microsoft IT chose the appropriate VoIP gateway. For more information about the decision factors and available gateways, see the later "VoIP Gateway Selection" section.

5.     Microsoft IT implemented all UM features, configured them for the environment, and verified functionality through testing.

6.     After installing, configuring, and testing the new UM voice mail infrastructure, Microsoft IT migrated users from the third-party UM system to Exchange Server 2007 UM.

UM Server Design

Microsoft IT made server design decisions based on testing results during product development. Microsoft has a policy of testing software builds during product development in a process called "dogfooding," during which Microsoft IT installs a test environment in the Exchange forest in addition to a test lab for design and implementation engineers to evaluates stability and functionality. During the initial design, Microsoft IT used a dual-core AMD Opteron 2.2-gigahertz (GHz) processor, as shown in Table 3. It is important to note that the Microsoft IT made server sizing and design decisions during the beta testing period and kept the server hardware configuration for the production environment.

Table 3. Server Hardware per Server Role

In past circuit-switched PBX UM implementations, the number of simultaneous callers was limited by the physically available ports. With Exchange Server 2007 UM, the IP-switched network is limited not by the ports, but by the network bandwidth, processing time, and virtual memory. However, Exchange Server 2007 UM-based environments can be modeled in a similar way to circuit-switched PBX environments because the connection between the PBX and telecommunications provider has a specific number of ports. For example, a T1 PRI connection carries 24 channels for a possible 24 simultaneous connections. Based on performance monitoring of the number of calls during a peak period, Microsoft IT provisioned the appropriate connectivity for each site. Regardless of the type of connectivity, Microsoft IT connects at least two VoIP gateways at each site, which are configured to at least two UM server partners. If the expected call volume and user loads exceed the capacity of a single server, the Exchange Server 2007 architecture enables Microsoft IT to increase capacity by adding UM servers to the dial plan.

Server Load Considerations

UM operations from both authenticated and unauthenticated users place a load on server resources. The load varies with the time of day and the number of features used. For example, in the beginning of a workday and after lunch, there is a high load when users access voice mail and the system processes internal and incoming calls. The load depends on the demands placed on the UM server, as explained in Table 4.

Table 4. Demands Placed on UM Servers

Microsoft IT set the Maximum Concurrent Call setting of 100 on Exchange UM servers based on testing and production environment metrics. For example, with third-party UM servers, Microsoft IT measured calls by using the Current Calls counter. This value, combined with the number of ports provisioned for voice mail, yielded the recommended number of calls per server. As part of preliminary analysis for the capacity of a UM environment, including voice mail ports, Microsoft IT used Erlang analysis, as explained in Appendix A.

Message Size

Voice mail and fax messages compose the two major message types that use a significant portion of the available processing power, memory, and hard disk storage on a UM server. Voice mail and fax messages are not permanently stored on an Exchange UM server, but in the user's mailbox on an Exchange Mailbox server. For long-term storage, knowing the size of these messages is important because it affects mailbox quotas and mailbox server sizing. For immediate processing of incoming calls, message size is also relevant because a UM server converts a message into a file that is attached to an e-mail message in the user's mailbox. Therefore, as part of determining the server requirements, determining typical message size for users was important for Microsoft IT.

Incoming calls result in answered calls, hanging up, or a voice mail or fax message. The attachment size stored in the user's mailbox varies with the following factors:

• Recording duration   Typical message length is 30 seconds or less at Microsoft.
• Audio codec used for communication between the VoIP gateway and UM server   When accepting voice mail data from VoIP gateways, Exchange Server 2007 UM servers can accept the data encoded in G.711 A-Law or G.711 mu-Law pulse-code modulation (PCM) methods (uncompressed), in addition to G.723.1 (compressed). Although G.711 generally yields better quality voice data, it requires more bandwidth because it is uncompressed. Running G.723.1 in a production requirement requires configuring QoS measures, such as higher-priority packets across the firewall, because loss of compressed G.723.1-encoded data results in a lower voice quality than if the same data is lost in G.711 encoding. The Microsoft IT environment supported the higher bandwidth requirements of G.711. Therefore, to ensure the highest voice quality, Microsoft IT uses G.711 encoding for voice mail transmission between the VoIP gateway and UM server.
• Audio storage format   After accepting voice mail data encoded through G.711 or G.723.1 from VoIP gateways, Exchange Server 2007 UM servers can create either Windows Media® Audio (WMA) or Wave (WAV) files from voice mail messages to store as attachments in a user's mailbox, using one of the three codecs shown in Table 5. WMA is the most highly compressed codec (about 11,000 bytes for each 10 seconds). GSM 06.10 is also compressed (about 16,000 bytes for each 10 seconds). G.711 is uncompressed. Of the three formats, WMA produces the smallest file sizes for recordings with a duration of about 15 seconds and longer. Because the average voice mail message is approximately 30 seconds long, Microsoft IT chose WMA as the default setting.

The server load in terms of processing power and memory requirements varies depending on message duration, communication codec, and storage file type. For example, although UM servers use processing power and memory when creating files to be delivered to users, UM servers also use processing power and memory with G.711 and G.723.1 audio transport codecs when accepting voice mail data from VoIP gateways. Because G.723.1 uses compression, using G.723.1 requires more resources than using G.711. Additionally, when users access UM servers, Outlook Voice Access communication uses fewer resources than incoming calls that result in voice mail messages. Correspondingly, the true server load varies with usage details and VoIP gateway configuration. Microsoft IT designs production Exchange UM servers to handle between 60 and 100 simultaneous calls while meeting service levels. This guidance comes from product group recommendations.

Table 5. Unified Messaging Audio Codecs for Voice Mail Storage

Connectivity Scenarios and Model Configurations

Microsoft decided to standardize the Exchange Server 2007 UM environment connectivity based on site size and gateway type, in addition to the connectivity for the site and gateway. The end goal entailed a series of model, baseline combinations of PBXs, gateways, UM servers, and associated connectivity between these components.

After analyzing the connectivity requirements and chosen components, Microsoft IT created the following model configurations for use in all locations:

• T1 CAS-based environment   Sites with thousands of users, such as Redmond and Silicon Valley, require T1 PRI connections to support the traffic. As explained in the "VoIP Gateway Selection" section later in this document, the combination of PBX and gateway choices meant that the gateway supported either one T1 or dual T1 lines. Correspondingly, the Redmond location used dual T1s for each gateway, and the Silicon Valley location used one T1 for each gateway, as shown in Figure 5.
• T1 Q.SIG-based environment   In addition to using T1 CAS connections, Microsoft IT used T1 Q.SIG in the Sao Paulo location. The chief difference between T1 CAS and T1 Q.SIG is how each handles integration information. Whereas T1 CAS requires an SMDI link, T1 Q.SIG carries integration information on the twenty-fourth channel in the PRI.
• Digital set emulation-based environment   For other sites worldwide, Microsoft IT kept the Nortel PBXs, used with third-party UM servers in combination with the Intel PIMG VoIP gateways. These components used digital set emulation for telecommunications provider connectivity.

Figure 5. Connectivity scenarios for sites

UM Features Available

For Microsoft IT, designing the Exchange Server 2007 UM-based environment entailed taking into account the available features with Exchange Server 2007 UM. The following features highlight key Exchange Server 2007 functionality for users and administrators:

• Outlook Voice Access   With Exchange Server 2007 unified messaging, UM-enabled users or subscribers can access their e-mail, contacts, and calendar information by using a standard analog, digital, or cellular telephone. When a UM-enabled user dials the designated access number, an Exchange UM server prompts the user for action through the telephone user interface (TUI). This TUI enables users to access and manipulate Exchange items by either speaking English commands or using the telephone keypad (with prompts available in many languages). The voice menu is Outlook Voice Access, with which UM-enabled users can perform the following tasks:
• E-mail and voice mail   Users can listen to new and saved e-mail and voice mail messages, and forward, reply, save, and delete e-mail and voice mail messages.
• Calendar   Users can interact with their calendar, including listening to daily calendar appointments and meeting details, accepting or declining e-mail and meeting requests, sending an "I'll be late" message to meeting participants, replying to a meeting request by using voice inputs to send a message to meeting participants, and canceling meetings.
• Directory and personal contacts   Users can interact with global address list (GAL) and personal contacts. These interactions can include locating a person in the GAL or personal contacts, playing the person's contact details, calling the person's office phone or mobile phone, and sending the person a voice message.
• Auto Attendant   The UM Auto Attendant is a set of voice prompts or WAV files that are played to callers in place of a human operator or receptionist. When external or anonymous callers access the UM system, they can use telephone keypad or speech inputs to locate a user or place a call.
• User self-service   For security-enhanced access to voice mail, users have PINs. The PIN is separate from the user's Active Directory account password and is stored as an encrypted attribute of the user's Active Directory account object
• Voice mail form   The Office Outlook 2007 and Office Outlook Web Access voice mail form resembles the default e-mail form, but it gives users an interface for performing actions such as playing, stopping, or pausing voice messages, playing voice messages on a telephone, and adding and editing notes. If users are not using Office Outlook 2007 or Office Outlook Web Access as their e-mail client, the voice mail form is not available, and they receive voice messages only as attachments. The voice mail form includes the embedded Windows Media Player and a notes field. The following three options are available on the voice mail form:
• Play   Users can play and listen to voice messages by using computer speakers or headphones.
• Play On Phone   The Exchange Server 2007 Unified Messaging Play on Phone feature enables UM-enabled users to access a voice mail message. However, instead of playing the media file over their computer speakers, they can listen to the message on the user's phone or at another telephone number specified by the user.
• Edit Notes   Users can use this option to add or edit notes associated with the voice mail message.
• Active Directory representation   With Exchange Server 2007 and Active Directory, UM physical objects such as servers and gateways, and logical objects such as dial plans, have logical representations in Active Directory. This enables Microsoft IT to easily keep records of UM data and conveniently manage components. Keeping a single directory of all users in Active Directory eliminates the need for a separate, voice-mail-only directory.

PBX Integration

To integrate PBXs with the rest of the UM environment, Microsoft IT considered the following deployment prerequisites:

• Line provisioning   For PBXs to accept calls, they require a connection to the telephone company. The connection type for each location varies with the port capacity. For example, at Microsoft, Intecom PBXs require a T1 PRI CAS trunk, whereas Nortel PBXs require either a T1 PRI or phone lines that the PBX emulates as a digital set.
• Signaling integration   For each PBX, Microsoft IT decides the signaling integration configuration. For example, Microsoft IT asks whether the PBX uses SMDI, Q.SIG, or digital set emulation. The answer may change technical requirements because Q.SIG and digital set standards have built-in signaling integration, whereas T1 CAS requires a separate SMDI link.
• Line call plan   For each location and for PBXs associated with the location, Microsoft IT considers what call plan the PBX supports. A call plan can be configured to support calls worldwide, within the country, or within the locality, or to support only internal calls. Microsoft IT configures the call plan for each site to support calling numbers within the country of the site.
• Hunt group/pilot number   Because Microsoft IT migrated to an Exchange Server 2007 UM environment, Microsoft IT considered whether to reuse voice mail numbers or create new ones. Microsoft IT makes this decision on a case-by-case basis, and the decision varies with the location-specific business requirements.

VoIP Gateway Selection

Microsoft IT focused heavily on gateway selection when designing the UM environment because of the numerous gateway selection criteria. For redundancy, Microsoft IT decided that each location should have at least two gateways. Each gateway communicates to multiple UM servers by using round robin. Microsoft IT considered the factors shown in Table 6.

Table 6. Gateway Selection Considerations

Table 7 shows the VoIP gateways that are supported for Exchange Server 2007 unified messaging servers as of release to manufacturing (RTM). It is important to note that these gateway options were established during beta testing for Exchange Server 2007 and served as a starting point for Microsoft IT.

Table 7. VoIP Gateway Options for Exchange Server 2007 UM

For the latest information about supported VoIP gateways, refer to http://technet.microsoft.com/en-us/library/bb123948.aspx.

Gateway Selection Criteria

In deciding the appropriate gateway to use with Exchange Server 2007 UM servers for each site, Microsoft IT followed several constraints and requirements. For example, the gateways must support signaling integration and must connect to the PBX. Microsoft IT systematically picked the gateway for each model configuration, making the following decisions to arrive at the final gateway for each location:

• Analog/digital choice   Microsoft IT considered whether the incoming connection was digital or analog. Analog connections are typically used for locations with few users. Correspondingly, VoIP IP gateways that support analog connections support only a small number of connections. For example, the Intel PIMGG80LS and MediaPack 114/8 Foreign Exchange Office (FXO) emulate an analog phone set, which takes in-band tone or out-of-band SMDI integration for voice mail operation. These gateways support a maximum of eight phone connections, which is too small for all Microsoft IT model configurations and locations. Therefore, Microsoft IT uses digital connections in all locations. With digital connections, Microsoft IT decided between using digital set emulation or T1 PRI trunks. This decision depends on the user load and available connectivity at each location. The Nortel Meridian PBX does not support SMDI or in-band tone integration and uses a digital set emulation or T1 Q.SIG connection. Sites other than Redmond and Silicon Valley can use Nortel Meridian PBX with the Intel PIMG80PBXDNI gateway, which supports digital set emulation. This gateway emulates eight Nortel digital phone sets, so the PBX views the gateway as eight different phone sets on the same hunt group and handles the connections and requests (transfer/receive call) as a phone set.
• T1 CAS/T1 Q.SIG choice   The Redmond site (Intecom PBX) and Silicon Valley site (Nortel SL100 PBX) use T1 CAS with SMDI for voice mail. In a T1 CAS with SMDI integration setup, in addition to the T1 lines connected to the gateway, there is a single RS232 serial feed for SMDI integration for all the T1 lines on the same trunk group. Under this connection, the possible gateways available to Microsoft IT include Intel TIMG300/600DTI and Mediant 2000. Another option for a T1-based site is T1 Q.SIG. Whereas T1 CAS provides 24 channels, T1 Q.SIG provides 23 channels for voice and uses the last channel for integration information. Both Intel TIMG300/600DTI and Mediant 2000 can be configured to support T1 Q.SIG; however, Microsoft IT decided to continue using the connectivity types from the third-party UM environment and chose T1 CAS.

After Microsoft IT selected the Intel PIMG80PBXDNI gateway for sites that use Nortel Meridian PBX with digital set emulation, two options remained for the Redmond and Silicon Valley sites: Mediant 2000 and TIMG300/600DTI. Microsoft IT considered three aspects when choosing between Mediant 2000 and TIMG300/600DTI:

• Supported port density   This refers to the number of T1 trunks that a gateway supports. Intel TIMG300DTI supports a single T1 with an RS232 DB9 connection for SMDI, TIMG600DTI supports two T1s with an RS232 DB9 connection for SMDI, and Mediant 2000 supports one to eight T1s. However, for the SMDI implementation, in order to add the DB9 connection, the Mediant 2000 gateway supports only four T1s. From a port density perspective, the Mediant 2000 gateway supports more ports than the Intel TIMG600DTI gateway.
• SMDI integration   The UM environment at Microsoft uses a T1 CAS connection with SMDI integration for Redmond and Silicon Valley locations, with each site using a single number for voice mail (same trunk group). In this configuration, the environment must share a single SMDI integration. The Intel gateway includes an option to connect the RS232 serial SMDI feed to a gateway, and then use the IP network to share SMDI information with other gateways on the same trunk group. For AudioCodes Mediant 2000, there is no feature to share SMDI integration between gateways by using the IP network. For SMDI integration to reach multiple gateways, the Mediant 2000 gateway requires the use of a split RS232 cable to connect the single SMDI link/source to multiple Mediant 2000 gateways. This is not an officially supported configuration.

  There are several issues with using a split RS232 link, including the following:

• Limitation on secondary connection   When two or more gateways are connected to the PBX, only one of the gateways can send information back to the PBX. All the other secondary connectors will have the pin number 2 (for transmitting data) disconnected. This transmission back to the PBX enables the Message Waiting Indicator (MWI) feature. Also, with a split link, caller ID is limited to 10 digits.
• Limitation on length   Microsoft IT uses a baud rate of 9600 on the RS 232 DB9 port. For this configuration, the suggested cable length is a maximum of 50 feet. Microsoft stays within these limits, even with multiple splits on one cable, by physically placing the PBX and gateway in close proximity.

  Even though Intel gateways support SMDI integration through the IP network, for redundancy Microsoft IT decided to use two gateways and split the RS232 link. In this configuration, if one gateway fails, the other gateway connected with the RS232 cable continues to receive SMDI integration data. By doing this, Microsoft IT can switch over from one gateway to another, enabling gateway firmware updates with no service interruption.

• Management features   The Mediant 2000 gateway supports Syslog, which sends tracing information via User Datagram Protocol (UDP) to a remote computer. Intel gateways accomplish reporting through the RS232 connection on the back of the gateway. The Mediant 2000 gateway also includes a Web page interface for monitoring the ports' connection status. Intel gateways require extra steps to determine which port on which T1 trunk is currently connected.

After considering the connectivity, signaling integration, and PBX requirements for each site and the gateway options based on these requirements, Microsoft IT settled on the VoIP gateways shown in Table 8 for the model deployments.

Table 8. Summary Gateway Choices

Server Consolidation

As explained in the earlier section "Unified Messaging Prior to Exchange Server 2007," Microsoft IT managed three site types: large, medium, and small, depending on the connectivity requirements. The user load and connectivity requirements fit well with the creation of model site designs. For future capacity expansion, Microsoft IT can easily add more circuits, gateway servers, and UM servers to existing sites.

In addition to easy capacity expansion, Exchange Server 2007 UM offers Microsoft IT two key opportunities for consolidation:

• IP-based UM server   By using VoIP gateways in combination with telephony-based PBX servers and IP-based Exchange UM servers, Microsoft IT can locate UM servers anywhere on the IP network, including in dedicated data centers. This makes it possible to reduce the number of sites that have UM servers, providing services to multiple sites from the same set of servers.
• Exchange Server 2007 UM features   In addition to taking advantage of general Exchange Server 2007 benefits such as 64-bit architecture, Microsoft IT can use the UM features of Exchange Server 2007, such as tighter integration with Office Outlook and Active Directory, user self-service, and voice features, to offer a richer experience for users. In addition to requiring fewer servers, the new environment offers greater capacity per server, easier administration, and less administrative overhead. For example, for the more than 30,000 UM users in North America, Microsoft uses just five UM servers.

Fax Integration

Fax transmissions occur through the T.38 protocol, which specifies how to send an audio packet through a network, similarly to G.711. Communication that uses the T.38 protocol occurs over analog connections from the telephone company to the PBX and as encoded voice between the PBX and VoIP gateway. The VoIP gateway sends a fax transmission to the associated UM server, similarly to a voice mail message. The UM server attaches the fax transmission to an e-mail message for placement in the user's mailbox associated with the number after converting it into a TIFF file.

For Microsoft IT, fax integration entails fewer design challenges than voice mail because fax communication is only one way—incoming—through the T.38 protocol. With fax communication, delay is acceptable; out-of-sequence packets are also acceptable as long as the receiver can reconstruct the final fax. If a line is not available or if the connection fails, the sender can renegotiate a connection to resubmit the fax. All UM-enabled extensions at Microsoft can now also accept fax transmissions that are delivered to the Inbox.

System Monitoring

As part of designing a monitoring solution for the Exchange Server 2007 UM environment, Microsoft IT considered which components to monitor and the technology to use in monitoring. Microsoft IT uses Microsoft Operations Manager to monitor its Exchange organization. Microsoft Operations Manager includes the ability to monitor server status and generate reports about Exchange-specific information such as message queues. The following monitoring options are available:

• PBX   All PBXs that Microsoft IT uses support Simple Network Management Protocol (SNMP). Microsoft Operations Manager can retrieve data from SNMP devices and monitor status.
• VoIP gateway   Intel PIMG and TIMG VoIP gateways support e-mail alerts. Microsoft Operations Manager can use these alerts to notify administrators of failed gateways or other issues. In addition, Intel and AudioCodes gateways support SNMP.
• UM server   Microsoft IT uses Microsoft Operations Manager to check both general UM server status (by checking the event log) and performance monitor data, in addition to UM-specific information such as voice mail queues. Microsoft Operations Manager also includes the ability to check specific UM services and UM connectivity.

Security

There are many security concerns associated with a UM environment. For example, SIP proxy impersonation, network sniffing, session hijacking, and even unauthorized phone calls can compromise network security. Microsoft IT can choose from several methods to help secure the UM environment, especially UM servers and traffic between VoIP gateways and UM servers.

• Security-enhanced protocols   In the UM environment, Mutual Transport Layer Security (MTLS) can provide security for all traffic that uses SIP. This includes the traffic between VoIP gateway and the unified messaging servers.
• Trusted LANs   To prevent network sniffing and reduce overall security risks, Microsoft IT places VoIP gateways on a Virtual LAN (VLAN) separate from the corporate production environment. This makes traffic access possible only for authorized individuals with physical access to VoIP gateways. Moreover, UM servers communicate only with gateways explicitly listed in the dial plan.
• IP security (IPsec)   The Microsoft corporate network uses IPsec for all IP communication within the network.. To ensure optimal performance, Microsoft IT created an exception for gateway-UM server traffic.

In addition to these security measures, Microsoft IT enforces general security practices such as using strong authentication methods and strong passwords.

Unified Messaging Implementation

Microsoft IT approaches the implementation phase of a project by considering the components and requirements gathered during the design phase, as mentioned earlier in the "Exchange Server 2007 Unified Messaging Design" section, and systematically deploying each component or meeting each requirement.

In general, UM implementations concern the best order of deploying voice mail, e-mail, and fax components, in addition to the configuration settings for the components. For Microsoft IT specifically, the Exchange Server 2007 UM implementation entailed a period of coexistence with Exchange Server 2003 and the third-party UM system. Microsoft IT considered the following high-level implementation challenges:

• Migration of existing voice mail   In most situations, where migration takes place from a traditional voice mail system to UM, existing voice mail messages generally are not moved to the UM environment. However, because Microsoft IT had previously deployed a third-party UM system, users were able to access existing voice mail messages via their Exchange mailboxes, and there was no need to migrate these messages.
• Voice mail system coexistence   Microsoft IT decided to implement Exchange Server 2007 UM without immediately removing the previous voice mail and UM systems. In general, when there is a period of coexistence, there are several considerations. One consideration is whether the previous UM environment supports coexistence with the new environment. For Microsoft IT, the third-party UM servers used Exchange Server 2003, whereas the Exchange Server 2007 UM environment used Exchange Server 2007 UM and mailbox servers. Effectively, Microsoft IT maintained two UM systems for a short period while migrating users from the third-party environment to Exchange UM.
• Communication between new and existing voice mail systems   Microsoft IT considered the need for the third-party UM system and the Exchange Server 2007 UM system to communicate during migration and determined that Microsoft does not have this requirement. However, in some environments, the prior and current voice mail systems require connectivity during migration.
• Telecommunications requirements for running new and existing voice mail systems in parallel   For Microsoft IT, a serious consideration when choosing whether to run two voice mail systems in parallel is telecommunications requirements. For example, there are many possible limitations on the PBX, such as running out of SMDI integration ports or insufficient T1 connection ports. These are not easy problems to fix in the telecommunications world because some are inherent system design limitations, or require major upgrades.

Migration Approach

As part of the migration and coexistence considerations, Microsoft IT looked into two migration strategies available for the Microsoft UM environment: overnight migration and staged migration. Overnight migration refers to a strategy that seeks to deploy the Exchange Server 2007-based UM environment and transition all users in a short time frame during non-business hours. This strategy requires substantial preparation before deployment to ensure that all components are available during migration. Staged migration refers to a migration strategy that seeks to deploy, configure, verify, and test one aspect of the environment at a time. Table 9 shows possibilities with each migration strategy.

Table 9. Migration Strategies