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SIP B2BUA-Back-to-Back User Agents – Solving Another Part of the SIP Puzzle
[August 18, 2006]

SIP B2BUA-Back-to-Back User Agents – Solving Another Part of the SIP Puzzle

SIP B2BUA-Back-to-Back User Agents – Another Part of the SIP Puzzle
The animated tutorial is available at

SIP-Session Initiation Protocol is the real-time communication protocol for VOIP-Voice over IP. SIP-Session Initiation Protocol is a signaling protocol for internet conferencing, telephony, presence, events notification (emergency calling) and instant messaging.

SIP has been expanded to support video and instant-messaging applications. SIP uses a text-based programming language designed to perform basic call-control tasks, such as session call set up and tear down and signaling for features such as call hold, caller ID, conferencing and call transferring. However with SIP, the intelligence for call setup and features resides on the SIP device or UA-User Agent, such as an IP phone or a PC with voice or instant-messaging software. In contrast, traditional telephony or H.323-based telephony uses a model of intelligent, centralized phone switches with dumb phones with SS7-Signaling System 7 in PSTN-Public Switched Telephone Network telephone switching and H.323 or Media Gateway Control Protocol in IP telephony providing call control/routing. For more go to

Proxy Server call flow is where Proxy Server sets up call. Proxy Server is an optional SIP-Session Initiation Protocol component that handles routing of SIP signaling but does not initiate SIP messages. SIP uses Proxy Server Forks (select different paths) Invite(s) to multiple User Agents (telephones). Forks can be sequential or concurrent. Session established to first UA-User Agent to respond with OK. Cancel is sent to a non-responding (no answer) User Agents. Forks are useful in a call center environment or call hunting.

Shown here is SIP call flow using a Redirect Server. A Redirect Server is an optional SIP component that does not route SIP messages. Redirect Server returns a redirect (change in routing) to UA-User Agent for direct routing (SIP is designed for end-to-end signaling without intervention by a server). Here is an example of SIP-Session Initiation Protocol used for instant messaging. The function of a Presence Server is to manage access, connections, directory (who's who), billing and tracking and other management functions. Shown here is an example of SIP-Session Initiation Protocol used for enhanced, multi-user, multi-media conferencing. Note internet access or VXML-Voice eXtensible Markup Language applications can be directed based on an Exception-triggered instant conference such as a disaster.

Some SIP servers/PBX systems may use a special type of server called a B2BUA-Back-to-Back User Agent. Like a central server or PBX acts as a central signaling connection system (or mimic-UA - to "act like" a receiver to one end and the caller on the other), B2BUA's may be able to provide gateway functions to connect calls to the PSTN-Public Switched Telephone network or convert to other protocols such as Cisco's Skinny, H.323 or proprietary call signaling.

While B2BUA is a migration strategy for some PBX systems, they are not yet today an off-the-shelf solution. For example, in many cases SIP servers based on the evolving standard may not be able to provide all the features even though comply with IETF-RFC-Request For Comments. This is especially important for a multi-vendor SIP environment. In addition, the "open" programming capability of SIP makes it very possible that the way a feature like automatic callback is programmed for one customer could mean group call in another SIP system. At the same time, B2BUA’s give an informed SIP staff the ability to create their own applications rapidly and easily. In summary, B2BUA’s give the enterprise customer another tool in building a flexible SIP network.

ABOUT produces dealer and customer training programs, online presentations including iPod and PC formats sales brochures, virtual installation manuals and animated online presentations. The company has more than 2,832+ free online presentations on data communications,, internet, wireless, VoIP-Voice over Internet Protocol, PBX Systems, central office switching, protocols, telephony, telecommunications, networking, routing, IPTV, WiMax, power systems, broadband, WiFi-wireless fidelity and other related technologies available at Thomas Cross is a magazine columnist with many key technology publications and a member of the Technical Board of Advisors for the VoIP-Security Alliance. He can be reached at 303-594-1694, or [email protected] TECHtionary also produces VoIP Dealer Sales and Business Development Training. Some of the key highlights of these courses are: Building a VoIP Business, Selling & Marketing VoIP, Customer and End User Training, VoIP Technology, Network Design, Provisioning, Customer Service, Dealer Portal, and Enhanced & Professional Services. More details on this VoIP Dealer are available by calling Tom Cross at 303-594-1694 or [email protected] This training is also available for presentation, web seminar or online tutorial.

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