All of the Exchange implementations will have a VoIP component because of the SIP signaling used for the Exchange voice ports. As a result we rely heavily on the T.38 protocol.
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 2010 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 2010 UM-enabled mailbox.
Exchange 2010 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-based 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. Then the Unified Messaging server will renegotiate the media session and use the T.38 protocol.
When a call is received into a PBX, the PBX forwards the call to the appropriate extension. If a ring-no-answer occurs at the user's extension number, the PBX forwards the call to an IP gateway, and the IP gateway forwards the fax call to the appropriate Unified Messaging server. When the call is received by the Unified Messaging server, the Unified Messaging server must decide whether it's a voice call or a fax call. When the SIP protocol is used, the Unified Messaging server processes the call as a voice message. However, if the T.38 protocol is used from the IP gateway, the Unified Messaging server recognizes that the call is for a fax and processes the call. Exchange 2010 Unified Messaging forwards incoming fax calls to a dedicated fax partner server, which then establishes the fax call with the fax sender and receives the fax on behalf of the UM-enabled user. The fax partner's server then sends the fax included as a .tif attachment in the SMTP message to the recipient's mailbox.
When an incoming T.38 fax signal is sent from the IP gateway to the Exchange 2010 UM server, the UM server queries Active Directory using LDAP to determine if the intended recipient of the incoming fax call is allowed to receive incoming fax messages and to determine the SIP address of the fax partner server. After these have been checked, the UM server sends a fax call referral request to the IP gateway or SIP peer, which then forwards the fax request on to the fax partner server. After the fax call has been successfully established, the fax sender sends the fax media and data to a fax partner server. After the fax media and data has been received by the fax partner server, the fax partner server sends an e-mail message to a Hub Transport server using SMTP that contains a .tif image of the fax message and special X-headers to the intended fax recipient.
After the fax message is authenticated and is sent from a valid fax partner server, the UM mailbox assistant on the Mailbox server issues an RPC call to the UM server. By doing this, the UM server ensures that the fax message properties match those of fax messages that are created by an Exchange 2010 Unified Messaging server. Finally, the UM server again submits the final fax message, which includes the e-mail message and .tif attachment for the incoming fax, to a Hub Transport server. Then, using MAPI RPC, the completed and formatted version of the fax message is delivered to the intended recipient. The following figure shows the steps that are involved when an incoming fax is received.
What would a tech blog be without the protocol exchange:
An incoming call to a UM pilot number is directed to UM as an INVITE with a voice (RTP/audio) SDP profile (1). UM accepts the invitation and media streams are established. After the call has been established, fax transmission is initiated by the caller (2). The SIP peer detects the calling fax tone (CNG). The SIP peer issues a re-INVITE to the UM server, this time specifying a fax (T.38 or G.711) profile in the SDP (3). UM responds to the invitation with a 200 OK that places the SIP peer “on hold” (4). UM issues a REFER, referring the SIP (CNG) peer to a fax partner solution end point, obtained from its configuration data (5). The SIP peer sends the fax session INVITE to the fax partner solution (6). The fax partner solution accepts the invitation, and a media session is established with the SIP peer.
Configuring FAXING:
By default, when you install the Unified Messaging server role, the server isn't configured to allow incoming fax calls to be processed or delivered to a UM-enabled user. To configure Exchange 2010 UM with a fax partner server, you must configure the UM mailbox policy and configure authentication between the UM server and the fax partner server. For more information, see Deploy and Configure Incoming Faxing.
Single DID:
When you enable a user for Unified Messaging by using the Enable Unified Messaging wizard or 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. The extension is used by Unified Messaging to locate the correct user in Active Directory and to deliver voice and fax messages to the user's Exchange 2010 mailbox. For more information, 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 doesn't waste additional DID numbers. If the user is away or on the phone 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:
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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.
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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.
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Wait for the caller to retry sending the fax and let the fax call be transferred to a Unified Messaging server.
Multiple DID:
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 SecondaryAddress parameter with the Set-Mailbox cmdlet. For more information, see Set-UMMailbox.
Adding multiple extension numbers is useful when a UM-enabled user:
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Receives many faxes
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Doesn't want to be bothered with answering the phone to receive a fax
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Doesn't want to hear a fax tone when they answer their phone
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 a Unified Messaging server as a dedicated server that will receive fax calls and forward fax messages to the users. For more information, see Create a UM Dial Plan.
You have the following options for configuring multiple DID extension numbers for UM-enabled users:
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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 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 doesn't require that Unified Messaging servers process the fax calls,
and fax messages won't be sent to the UM-enabled user's mailbox.
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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's called. Although the user will publish one
number for voice and one for fax, the Unified Messaging server detects
the type of call that's 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 doesn't have a
separate fax machine or dedicated computer that has a fax modem to
answer incoming fax calls.
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Two DID numbers (one “phantom” extension for fax and one for voice) This
type of configuration is enabled on a per-user basis. It's 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's
published for fax calls for the UM-enabled user is configured on 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 doesn't hear the ring of an incoming call.
The disadvantages of this kind of configuration are that you must have additional DID extensions available and you must configure the PBX to forward the call to a Unified Messaging server.
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