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:

1. Provision PBX lines   The first step in building a highly scalable UM solution is to provision PBX lines.
   
   
2. Organize channels   After you've provisioned PBX-based voice channels, you can organize the channels as hunt groups.
   
   
3. 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.
   
   
4. 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 installed.
   
   

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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 servers.
  
  

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 audio format.
  
  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 following figure shows the four audio codecs you can use in Unified Messaging and how the size of the audio file depends on the duration of the voice mail recording.
  
  
  
  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
    
    
  By default, the MP3 format is selected. The MP3 format is a common audio file format that's used to greatly reduce the size of the audio file and is most commonly used by personal audio devices or MP3 players. MP3 is a cross-platform type of audio codec and is used for compatibility with many mobile phones and devices and different computer operating systems.
  
  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 Voice Access
    
    
  we estimate that each UM server can support between 2,000 and 10,000 users.
  
  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
    
    
• 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 consume.
  
  
  • 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.
    
    
  • 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.