Session Initiation Protocol (SIP) is a protocol that is used for initiating, modifying, and ending an interactive user session that involves multimedia elements such as video, voice, instant messaging, online games, and virtual reality. It is one of the leading signaling protocols for Voice over IP (VoIP), together with H.323. Most VoIP standards-based solutions use either H.323 or SIP. However, several proprietary designs and protocols also exist. The VoIP protocols typically support features such as call waiting, conference calling, and call transfer.

SIP clients such as IP gateways and IP Private Branch eXchanges (PBXs) can use TCP and UDP port 5060 to connect to SIP servers. SIP is used only for setting up and tearing down voice or video calls. All voice and video communications occur over Realtime Transport Protocol (RTP).

RTP defines a standard packet format for delivering audio and video over a given network, such as the Internet. RTP carries only voice/video data over the network. Call setup and tear-down are generally performed by the SIP protocol.

RTP does not require a standard or static TCP or UDP port to communicate with. RTP communications occur on an even UDP port, and the next higher odd port is used for TCP communications. Although there are no standard port range assignments, RTP is generally configured to use ports 16384 through 32767. It is difficult for RTP to traverse firewalls because it uses a dynamic port range.

T.38 is a faxing standard and protocol that enables faxing over an IP-based network. The IP-based network then 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 Tagged Image File Format (TIFF) images and attached to an e-mail message. Both the e-mail message and the TIFF attachment are sent to the recipient's Exchange 2007 UM-enabled mailbox.

The Unified Messaging Web services that are installed on a Client Access server use IP for network communication between a client, the Unified Messaging server, the Client Access server, and computers that are running other Exchange 2007 server roles. There are several Exchange 2007 Outlook Web Access and Microsoft Office Outlook 2007 client features that rely on UM Web Services to operate correctly.

The following Unified Messaging client features rely on UM Web Services:

• The voice mail options that are available with Exchange 2007 Outlook Web Access, including the Play on Phone feature and the ability to reset a PIN.
  
  
• The Play on Phone feature found in the Outlook 2007 client.
  
  

Note:
When an organization uses the Play on Phone and other client features in Exchange 2007 Unified Messaging, a computer that is running the Client Access, Hub Transport, and Mailbox server roles within the same Active Directory site is required in addition to the computer or computers that have the Unified Messaging server role installed.