Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2
Topic Last Modified: 2009-11-30
In many organizations today, the ability to access e-mail and voice mail is critical to the success of daily operations. To provide continuous access to e-mail and voice mail, you must correctly plan and implement a solution for Microsoft Exchange Server 2010 that will ensure the availability of the servers that provide these services. To provide a highly scalable solution in Exchange 2010 Unified Messaging (UM), you have to understand how the UM components can be scaled to support your users.
Unified Messaging Server Scalability
Scalability is defined as the capability to increase resources to increase the capacity of a given service. There are two types of scalability that can be used to increase the capacity of UM servers in your organization: horizontal and vertical. In Unified Messaging, when you scale vertically, you add hardware resources to a single UM server or multiple UM servers, by, for example:
- Adding more hard disk space for message storage
- Increasing the speed or number of processors
- Increasing the amount or speed of RAM
- Increasing the number of network adapters or increasing the
number of local area network ports in a single network adapter
In Unified Messaging, when you scale horizontally, you install the Unified Messaging server role on new UM servers and add more UM servers to a dial plan to increase the number of incoming concurrent calls the system can accept. To scale your UM environment horizontally, you can also increase the number of IP gateways. This increases the number of ports available to be used for incoming calls.
Unified Messaging provides an efficient and simple deployment model that's highly scalable without increasing the complexity of the deployment. There are many deployment models for Unified Messaging in your organization. But the recommended deployment model for Unified Messaging is to centralize your UM servers. All the available deployment options for Unified Messaging have several steps in common that are required to create a scalable system to support large numbers of UM users. These steps are as follows:
- Provision PBX lines The first step in
building a highly scalable UM solution is to provision PBX
- Organize channels After you've
provisioned PBX-based voice channels, you can organize the channels
as hunt groups.
- Deploy IP gateways After you've
organized your voice channels as hunt groups, you end these
channels at IP gateways. IP gateways are used with a legacy PBX to
convert the circuit-switched protocols found on a telephony network
to IP-based packet-switched protocols.
- Add more Unified Messaging servers to a dial
plan If you have to increase the number of
calls that can be handled by Unified Messaging, you can install and
set up additional UM servers and add them to a dial plan. In most
cases, IP gateways will use DNS to load balance between the
existing UM servers and the additional UM servers that have been
Every incoming call received from an IP gateway will generate IP-based network traffic and consume some amount of your available network bandwidth. Before you deploy Unified Messaging, you should perform an analysis of the network traffic to determine current usage patterns and find any potential issues. On most networks, bandwidth demand isn't evenly distributed throughout business hours. Because all the IP-based calls are routed directly to your UM servers from the IP gateways on your network and this IP-based network traffic consumes some available bandwidth, you should follow these recommendations and guidelines:
- Place your PBXs physically close to your IP gateways.
- Place your IP gateways and your UM servers on the same
well-connected network or within the same physical site.
- Place your UM servers on the same well-connected network or
within the same physical site as other computers that have Exchange
2010 server roles installed, including Mailbox, Hub Transport, and
Client Access servers.
- End your Wide Area Network (WAN) connections close to where
your telephony equipment is located.
- In branch office scenarios or over WAN connections, use the
G.723.1 codec instead of the G.711u or G.711A codec to minimize the
network traffic that's passed between your IP gateways and your UM
Generally, Unified Messaging scalability is determined by the number of concurrent calls. By default, a single UM server can accept a maximum of 100 concurrent voice calls. These calls can be either incoming or outgoing, and can be generated when a user leaves a voice mail message, when an Outlook Voice Access user accesses their Exchange 2010 mailbox, or when a user uses the Play on Phone feature to listen to their voice messages. Although the number of concurrent calls is an important factor to consider when you build a scalable UM infrastructure, you also have to determine the best codec to use to encode the voice messages and the types and number of users who you have to support.
- Number of concurrent calls Although, by
default, a UM server can accept 100 concurrent voice calls, a
single UM server can be set up to accept a maximum of 200
concurrent voice messages. The more you increase the number of
concurrent connections on a single UM server, the more resources
will be required. It's especially important to decrease this
setting on low-end, slower computers on which the UM server role is
installed. Performance counters are available, and the
Get-UMActiveCalls cmdlet can also be used to monitor the
number of calls that are currently connected to a UM server.
If the number of concurrent calls required by your organization is larger than the number that's supported by a single UM server, you can scale horizontally and increase the capacity of concurrent calls by installing the Unified Messaging server role on an additional server and then adding the new UM server to the same dial plan.
- Voice mail storage codec The term
"codec" is a combination of the words "coding" and "decoding" and
relates to digital data. A codec is a computer program or software
that transforms digital data into an audio file format or streaming
In Microsoft Exchange Unified Messaging, there are two general types of codecs: the codec that's used between IP gateways and the codec that's used to encode voice messages. The MP3, Windows Media Audio (WMA), Group System Mobile (GSM) 06.10, and G.711 Pulse Code Modulation (PCM) Linear audio codecs are used to create mp3, .wma and .wav audio files for voice messages. But the file type that's used depends on the audio codec that's used to create the voice message audio file. In Microsoft Exchange Unified Messaging, the MP3 audio codec creates .mp3 audio files, the WMA audio codec creates .wma audio files, and the GSM 06.10 and G.711 PCM Linear audio codecs produce .wav audio files. Depending on the codec that's used, an audio file in .wma or .wav format is sent together with the e-mail message to the intended voice mail recipient. The size of UM voice messages depends on the size of the attachment that holds the voice data. Additionally, the size of the attachment depends on the following factors:
- The duration of the voice mail recording
- The audio codec that's used
- The audio file storage format
The sample bit rate (bit/sec) and compression properties for each audio codec that's used in Unified Messaging are as follows:
- MP3 - 16 bit - compressed file
- WMA – 16-bit – compressed file
- G.711 – 16-bit – uncompressed file
- GSM – 8-bit – compressed file
When the WMA codec is used, we estimate that each UM server can handle 60 to 75 concurrent IP-based calls. This estimate is based on the assumption that 14 percent of all the IP-based calls arrive during the single busiest hour of a day. Based on the following assumptions:
- The WMA codec is used
- 14 percent of all IP-based calls arrive during the single
busiest hour of a day
- Your users access their voice messages frequently using Outlook
Generally, you should allow for the following number of concurrent calls per UM server:
- 60, if the default dial plan codec is WMA
- 75, if the default dial plan codec is GSM
- The duration of the voice mail recording
- Types of users There are two types of
users who access the UM system and consume UM resources:
authenticated users and unauthenticated users. When you build a
scalable UM environment, you have to consider the effect these
users will have and the resources each of these users will
- Authenticated Authenticated users are
UM-enabled and can access their mailbox using Outlook Voice Access.
Authenticated users consume UM server resources in several ways,
for example, by directly calling in to a subscriber access number,
signing in to their mailboxes, accessing their messages, calendar,
contacts, or the directory, and using a UM server to play voice
messages over a telephone with the Play on Phone feature. They can
also indirectly consume resources by transferring a call, sending a
voice message, or calling a user's extension number and leaving a
- Unauthenticated Users who call in to a
UM auto attendant or call in to a subscriber access number but
don't sign in to their mailbox are unauthenticated callers. UM
resources are used to service their requests every time they call
in to a UM auto attendant or use a subscriber access number. Even
though they don't sign in to their mailbox, they still consume
resources by transferring a call, sending a voice message,
transferring to another auto attendant, transferring to another
telephone number, or listening to recorded audio prompts.
- Authenticated Authenticated users are UM-enabled and can access their mailbox using Outlook Voice Access. Authenticated users consume UM server resources in several ways, for example, by directly calling in to a subscriber access number, signing in to their mailboxes, accessing their messages, calendar, contacts, or the directory, and using a UM server to play voice messages over a telephone with the Play on Phone feature. They can also indirectly consume resources by transferring a call, sending a voice message, or calling a user's extension number and leaving a voice message.
Unified Messaging relies on the ability of the IP gateway to translate TDM or telephony circuit-switched based protocols, such as Integrated Services Digital Network (ISDN) or QSIG, from a PBX to protocols based on VoIP or IP, such as SIP, RTP, or T.38 for real-time facsimile transport.
IP gateways are available from multiple manufacturers in sizes and models that range from 4 ports to 32 ports. You can deploy as many IP gateways as necessary to provide for capacity and fault tolerance. If the number of calls or ports required is larger than the number of calls or ports supported by a single IP gateway, you can scale horizontally and increase the number of calls that can be accepted or the number of ports by installing and setting up additional IP gateways, creating the UM IP gateway object, and setting up the appropriate hunt groups to support your environment.
It's equally important to match the number of IP gateways you have in your environment to the number of UM servers that are available. For example, you shouldn't set up 10 IP gateways that are each connected with a T-1 line to a single UM server. This would mean that the UM server would have to support 240 concurrent incoming calls. You should consider this and scale your IP gateways to UM servers appropriately.