There are several options for sending and receiving faxes, including the following:

Traditional fax machines and computer-based faxing   Scanners, a fax modem in a computer, a printer with built-in faxing capabilities, or a dedicated fax machine can be used to send and receive faxes. They are used to transmit data in the form of pulses by using a telephone line to another fax device, usually another fax machine or computer that has a fax modem. The pulses are then transformed into images or used to print the image on paper.

The traditional fax method requires at least a single telephone line on the sending and receiving device, and only one fax can be sent or received at a time. A disadvantage of sending and receiving faxes by using a fax modem is that the computer must be turned on and running fax software or a fax service. This kind of computer-based faxing does not use the Internet to send or receive faxes. The following figure illustrates how traditional and computer-based faxing is used to send and receive faxes.

Fax servers or gateways and Internet fax services   There are several ways to send and receive faxes over the Internet. These include using a software application on a computer or using an e-mail client to receive faxes. In most cases, this kind of faxing involves using a fax server or fax gateway to convert between faxes and e-mail. This has become increasingly popular because it enables organizations to remove or not purchase additional fax machines. It also eliminates the need to install additional telephone lines. This kind of faxing involves creating the document, including a fax cover page with the correct identifying information, and sending the document to a traditional fax machine. For example, the user uses a software application such as Microsoft Office Word or Microsoft Office Outlook to create and send the fax to the fax server or gateway. The fax server or gateway receives the fax and then sends it by using a traditional telephone line to a fax machine or fax modem that is installed on a computer. The following figure illustrates how fax servers, gateways, and Internet fax services can be used to send and receive faxes.

Internet fax services let a user send faxes from a computer by using the Internet. A software application such as Office Word or Outlook can be used to create and send the fax to an Internet fax service. There are many companies that offer Internet faxing services on a subscription basis or by charging for each fax message that is sent. Internet fax services offer the following advantages:

• No fax machine is required
  
• No software or hardware must be installed
  
• No dedicated telephone lines are required
  
• Confidentiality
  
• Multiple faxes can be sent at the same time
  
• Faxes can be received when the computer is shut off
  

The following figure illustrates how Internet fax services can be used to send and receive faxes.

Faxing by using an e-mail client application   Faxes can be sent and received by a fax machine over the Internet and then received by an e-mail client such as Outlook.

The T.37 protocol was designed to enable a fax machine to send fax messages over the Internet to an e-mail client. The faxes are sent over the Internet as an e-mail attachment, typically as .tif or .pdf files. In this kind of faxing, a fax machine that supports iFax or T.37 is required, in addition to an e-mail address for the sending and receiving fax machines. To work with existing traditional fax machines and fax modems, all T.37 fax machines support standard faxing by using a telephone line. However, in some cases, T.37 fax machines can be used when a fax gateway is also being used. The following figure illustrates how T.37-based fax machines and e-mail clients can be used to send and receive faxes.

Faxing by using a VoIP network   VoIP is a technology that contains hardware and software that enables people to use an IP-based network as the transmission medium for telephone calls. On a VoIP network, voice and fax data is sent in packets by using IP instead of by traditional circuit transmissions or the circuit-switched telephone lines of the PSTN. An IP gateway that you connect to your IP network uses VoIP to send voice data packets between an Exchange 2007 Unified Messaging server and a Private Branch eXchange (PBX) system. Alternatively, you can use an IP PBX to perform the functions of both an IP gateway and a PBX.

There are two basic types of networks: circuit-switched and packet-switched. A circuit-switched network is a network in which there exists a dedicated connection. A dedicated connection is a circuit or channel that is set up between two nodes so that they can communicate. After a call is established between two nodes, the connection may be used only by these two nodes. When the call is ended by one of the nodes, the connection is canceled. In circuit-switched networks, such as the PSTN, multiple calls are transmitted across the same transmission medium. Frequently, the medium that is used in the PSTN is copper. However, fiber optic cable might also be used.

In packet-switched networks such as the Internet or a local area network (LAN), packets are routed to their destination through the most expedient route, but not all packets traveling between two hosts travel the same route, even those from a single message. This almost guarantees that the packets will arrive at different times and out of order. In a packet-switched network, packets (messages or fragments of messages) are individually routed between nodes over data links that may be shared by other nodes. With packet switching, unlike circuit switching, multiple connections to nodes on the network share the available bandwidth. Packet-switched networking has made it possible for the Internet to exist and, at the same time, has made data networks—especially LAN-based IP and VoIP networks—more available and widespread. The following figure illustrates how a VoIP network and Exchange Unified Messaging can be used to deliver faxes.

T.38 is a faxing standard and protocol that enables faxing over an IP-based network. An IP-based network that uses the T.38 protocol uses Simple Mail Transfer Protocol (SMTP) and MIME to send the message to a recipient's mailbox. T.38 allows for IP fax transmissions for IP-enabled fax devices and fax gateways. The devices can include IP network-based hosts such as client computers and printers. In Exchange 2007 Unified Messaging, the fax images are separate documents encoded as .tif files and attached to an e-mail message. Both the e-mail message and the .tif file attachment are sent to the recipient's Exchange 2007 UM-enabled mailbox.

Exchange 2007 Unified Messaging relies on the gateway's abilities to translate or convert Time Division Multiplex (TDM) or telephony circuit-switched based protocols like Integrated Services Digital Network (ISDN) and QSIG from a PBX to IP- or VoIP-based protocols like Session Initiation Protocol (SIP), Real-Time Transport Protocol (RTP), or T.38 for receiving fax messages. The IP gateway is integral to the functionality and operation of Unified Messaging. The IP gateway is responsible for sensing fax tones. Unified Messaging servers rely on the IP gateway to send a notification that a fax has been detected, at which point the Unified Messaging server will renegotiate the media session and use the T.38 protocol.

There are three components that must be configured correctly for users to be able to receive faxes by using Exchange 2007 Unified Messaging. Although, by default, all three of these components allow faxes to be received, you must verify that each setting has been configured correctly. To enable UM-enabled users to receive faxes, you must do the following:

• Verify that each UM dial plan allows the users who are associated with the dial plan to receive faxes. By default, all users who are associated with a dial plan can receive fax messages. To allow UM-enabled users to receive fax messages in their mailbox, each Unified Messaging server that is associated with the dial plan must be configured to accept incoming fax calls. You must also enable fax messages to be received by users who are associated with the dial plan. For more information about how to enable or disable the ability for users to receive faxes for a dial plan, see How to Enable UM-Enabled Users to Receive Faxes.
  

  Note:
  If you prevent fax messages from being received on a dial plan, all users who are associated with the dial plan will be unable to receive fax messages, even if you configure an individual user's properties to allow them to receive fax messages. Enabling or disabling faxing on a UM dial plan takes precedence over the settings for an individual UM-enabled user.

• Verify that the Unified Messaging servers that are associated with the UM dial plan are configured to allow one or more incoming fax calls to be processed. By default, when you install the Unified Messaging server role, the Unified Messaging server will accept 100 concurrent incoming fax calls. This allows UM-enabled users who are associated with a UM dial plan to receive fax messages into their mailbox. However, there may be times when these default settings have changed and UM-enabled users cannot receive fax messages. For more information about how to configure the number of incoming fax calls, see How to Modify the Number of Concurrent Fax Calls Setting.
  You can also prevent all users from receiving fax messages by setting the number of incoming fax calls to 0 on each Unified Messaging server that is associated with a dial plan. If each Unified Messaging server that is associated with a dial plan is configured to receive incoming fax calls but the dial plan is configured to disallow faxing, all users who are associated with the dial plan will be unable to receive faxes. Therefore, the fax settings that are configured on a UM dial plan will take precedence over the fax settings that are configured on a Unified Messaging server.
  
• Verify that the Exchange 2007 mailbox that is UM-enabled can receive fax messages. By default, all users who are associated with a dial plan can receive faxes. However, there may be situations when a user cannot receive faxes, because the ability to receive faxes has been disabled on their mailbox. For more information about how to enable a UM-enabled user to receive faxes, see How to Enable a Unified Messaging User to Receive Faxes.
  You can prevent a single user who is associated with a dial plan from receiving fax messages. To do this, configure the properties for the user by using the Exchange Management Console or by using the Set-UMMailbox cmdlet in the Exchange Management Shell. You can also use the Set-UMMailbox cmdlet to prevent multiple users from receiving fax messages. For more information about how to prevent a user or users from receiving fax messages, see How to Prevent a Unified Messaging User from Receiving Faxes.
  

When you enable a user for Unified Messaging by using the Enable Unified Messaging Wizard or by using the Enable-UMMailbox cmdlet, you must specify at least a single extension number for the user. This extension number is enabled on a per-user basis and must be unique within a given dial plan. This extension is used by Unified Messaging to locate the appropriate user in the Active Directory directory service and is used to deliver voice and fax messages into the user's Exchange 2007 mailbox. For more information about the Enable-UMMailbox cmdlet, see Enable-UMMailbox.

In this scenario, the user will use a single DID number for voice and fax. This configuration is easy to administer and does not waste additional DID numbers. If the user is away or on the telephone when a fax call arrives, UM answers the call, detects the fax tone, creates the fax message, and sends it to the user.

However, in this scenario, the user may receive calls from fax machines. The user can:

• Not answer the telephone when it rings, so that the fax call will be forwarded and answered by a Unified Messaging server and the fax message will be created and forwarded to the user's mailbox.
  
• Answer the fax call, and then transfer it to himself or herself, so that the call will be forwarded and answered by a Unified Messaging server and the fax message will be created and forwarded to the user's mailbox.
  
• Wait for the caller to retry sending the fax and let the fax call be transferred to a Unified Messaging server.
  

In summary, using a single DID number requires that the user performs additional actions to be able to receive fax messages.

When you enable a user for Unified Messaging, you must enter at least a single extension number for that user. However, you can also add multiple extension numbers for a UM-enabled user by using the Set-Mailbox cmdlet SecondaryAddress parameter. For more information about the Set-Mailbox cmdlet, see Set-UMMailbox.

Adding multiple extension numbers is useful when a UM-enabled user:

• Receives many faxes
  
• Does not want to be bothered with answering the telephone to receive a fax
  
• Does not want to hear a fax tone when they answer their telephone
  

Adding multiple extensions is more complex than using a single extension, and may require additional configuration settings on a PBX. To configure multiple extension numbers for a UM-enabled user, you must have DID extension numbers that are available but are not being used in your organization. Therefore, it is not a good idea to use multiple numbers for a UM-enabled user if your organization has a limited number of available DID extension numbers. For more information about the IP PBXs and PBXs that are supported by Exchange 2007 Unified Messaging, see IP PBX and PBX Support.

The benefit of using multiple DID telephone numbers is that the UM-enabled user receives voice calls on one DID extension number and fax calls on the other DID extension number. Although, this may be more complex and requires additional configuration steps, using separate DID numbers for voice mail and fax calls is easier for the user.

If you configure two DID extension numbers for a specific user, the DID extension numbers can come from separate UM dial plans. In this scenario, you can create a dial plan, add Unified Messaging servers to the dial plan, and use the Unified Messaging server as a dedicated server that will receive fax calls and forward fax messages to the users. For more information about how to create a UM dial plan, see How to Create a New Unified Messaging Dial Plan.

You have the following options for configuring multiple DID extension numbers for UM-enabled users:

• Multiple DID numbers (one for fax without Unified Messaging and one for voice)   This type of configuration is enabled on a per-user basis and is used when you have extra or unused DID extension numbers available. One DID extension number is published as the user’s voice mail number and the other DID extension number is published as the user's fax number. In this scenario, voice calls that are answered by a ring-no-answer or busy signal are forwarded to a Unified Messaging server, and a voice mail message is created and sent to the UM-enabled user's mailbox. The other extension number can be connected to a fax machine or to another computer that has a fax modem. Although this configuration is possible, it does not require that Unified Messaging servers process the fax calls and fax messages will not be sent to the UM-enabled user's mailbox.
  
• Multiple DID numbers (one for fax and one for voice)   This type of configuration is enabled on a per-user basis and can be used when your organization has many DID extension numbers available. In this scenario, both DID extension numbers that are answered by a ring-no-answer or a busy signal are forwarded to a Unified Messaging server that will create a voice or fax message depending on the DID extension number that is called. Although the user will publish one number for voice and one for fax, the Unified Messaging server detects the type of call that is being received on the DID extension number and can create a voice or fax message from calls to either of the DID extension numbers. This is very useful when a user does not have a separate fax machine or dedicated computer that has a fax modem to answer incoming fax calls.
  
• Two DID numbers (one “phantom” extension for fax and one for voice)   This type of configuration is enabled on a per-user basis. It is basically the same as the configuration that uses two DID numbers (one for fax and one for voice). However, in this configuration, the number that is published for fax calls for the UM-enabled user is configured in the PBX as a “phantom” extension. Incoming calls that are received on this "phantom" DID extension number are always forwarded to a Unified Messaging server.
  The advantage of this kind of configuration is that incoming fax calls are answered by a Unified Messaging server. When a ring no answer occurs, a fax is created and forwarded by the Unified Messaging server to the UM-enabled user's mailbox without disturbing the user. This happens because no telephone or fax device is positioned close to the user, and the user does not hear the ring of an incoming call.
  The disadvantages of this kind of configuration are that you must have additional DID extensions available and that you must configure the PBX to forward the call to a Unified Messaging server.
  

When you enable a user for Unified Messaging by using the Enable Unified Messaging Wizard or by using the Enable-UMMailbox cmdlet, you must specify at least a single extension number for the user. This kind of fax configuration is defined on each Unified Messaging dial plan.

In some organizations, especially those that receive many faxes each day, you might have to publish one fax number for the whole organization. This fax number would be used by all callers when they submit faxes to users in the organization. This kind of configuration is useful in the following situations:

• A user within the organization receives too many faxes in their mailbox to manage them effectively.
  
• A user receives too many spam faxes in their mailbox.
  
• Business logic is too complex to warrant creating a transport rule. This might be the case if your organization requires that you route certain faxes to one group and other faxes to another group. For more information about transport rules, see the following topics:
  
  • Overview of Transport Rules
    
  • Understanding How Transport Rules Are Applied in an Exchange 2007 Organization
    
• Filtering fax messages by using Outlook is not effective.
  

Publishing one fax number for the whole organization enables your organization to control the types of faxes that are received by users. The advantage of this configuration is that it requires only a single DID extension number or an external telephone number. Also, it does not require a separate DID number for faxing for each UM-enabled user. However, it does require a "fax secretary" or other person to distribute the incoming faxes to users within the organization based on information that is included on the fax cover page or in the fax message itself.