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Feature Article
July 2004

SIP And Converged Communications


If you�re looking at instant messaging and presence-enabled collaborative solutions and the vendors don�t have a commitment to SIP, stay away. Why? Because SIP, the Session Initiation Protocol, forms the basis of the industry standard IP-centric converged communications architecture. It does for real-time inter-human communications what HTML did for browsing.


How did SIP earn such an eminent position? It was developed to serve as a signaling mechanism to establish a wide variety of sessions. In this context, a session is any interactive communication that takes place between two or more entities over an IP network, from a simple two-way telephone call or an instant message exchange, to a collaborative multimedia conferencing session. SIP does not dictate the details within a session but instead negotiates interaction based on the capabilities of participants. This simplicity means that SIP is scalable, extensible, and fits comfortably into different architectural and deployment scenarios.

SIP emerged in the mid-1990s from research conducted at Columbia University in an effort to standardize a method for inviting participants from other universities to large-scale multimedia conferences. As it was developed, it became apparent that the protocol was much more flexible than anyone expected. The IETF � the body responsible for administering and developing Internet protocols � adopted SIP as the standard protocol for establishing and terminating multimedia sessions in 1999. It is similar to the two major Internet protocols � HTTP (World Wide Web) and SMTP (e-mail) � in that it uses symbolic addresses (e.g., [email protected]) to represent people who wish to communicate. By using SIP, users can locate and contact one another � regardless of location, device, speed of access, media content, and number of participants.

Because SIP can be used to enable IP telephony, there are some who mistakenly consider it to be nothing but another way of setting up a voice call. It is true that SIP can function as a voice protocol quite well. However, SIP is not simply an IP telephony protocol. It is truly much, much bigger than that. SIP is a whole new communications model. Features and applications are integrated at the session and service layers, independent of access constraints and the processes of message transport. In SIP networks, voice is just another media, albeit a very powerful one.

Evolution of Communications
As communications have evolved, there has been a fundamental change in the way information is distributed. To truly appreciate the communication implications of the SIP protocol, it helps to consider its functionality within the context of network architecture.

A useful conceptual model divides communications networks into a set of three functional planes: the bottom access layer, the middle session layer, and the top service layer. Traditional communication methods fit within these layers. For example, early departmental LANs provided simple connectivity between PCs and various forms of servers with only rudimentary peer-to-peer session management. Wireless telephony on the other hand, includes many specific session control functions, necessitated by mobility, with various databases of real-time and customer information maintained by session logic. Today�s Internet includes content switching intelligence, which resides in the service layer and directs traffic to various similar servers based primarily on the real-time availability of servers, their proximity to clients, and support for things such as the appropriate language for the person requesting information.

All the pieces come together as optical and access bandwidth continues to expand and become more economical, enabling a dramatic increase in the media content available. Both physically attached and mobile smart clients are actively involved in session management, while proxy servers and content switches help establish sessions dynamically and redirect sessions to the most available appropriate content sources. Signaling gateways, softswitches, and media gateways of many types work together to provide interaction with other network technologies.
It is apparent that the communications dynamic is evolving as the interaction between network layers increases and session management becomes highly distributed. Just as the computer has always played a significant role in session initiation over the Internet, edge devices will play a significant role in session initiation and control in next-generation networks. Session management can reside completely with smart clients or may be distributed among a few or many cooperating network elements, including smart clients.

SIP is a control protocol that initiates, modifies, and terminates communication sessions with one or more participants. The protocol enables participants to agree on a set of compatible media types and supports user mobility by proxying and redirecting requests to any user�s current location. SIP enables name translations by storing information mapping device addresses on a SIP registrar to a person�s name instead of a complex number scheme. A person will simply register one or more devices with the network and become reachable, wherever he or she may be, independent of the details of the networks and devices involved. SIP enables location independence, by allowing people to find each other without knowing the details of each others� device addresses or physical locations. It enables media independence by allowing all participants in a session to agree on common media and the technology details involved � including voice, video, audio, instant messaging, applications data exchange, or any combination thereof. It provides session participant management by allowing the addition, dropping, or transferring of participants in a session. It provides session feature changes, allowing for changing the media used in a session while the session is in progress.
By combining this basic functionality in a number of ways, vendors, system integrators and enterprise can create a number of productivity-enhancing services. For example:

SIP-Based Collaboration
Because SIP is inherently multimedia, it supports a wide variety of the media people use to collaborate interactively, from audio and video to instant messaging. SIP also supports sessions involving interactive applications such as games. Using SIP, it is possible to establish multimedia collaborative sessions between people dispersed all over the globe, relatively independent of differing client devices. Within a single session, some participants may be on a cellular phone, while others use PC interfaces, and some may even be using a PDA or a set-top-enabled television. Different PC functionality is supported as well, as some may be set up for video conferencing, some may use IP telephony, and some may only have instant messaging capabilities. With the proper applications, participants in this session could co-browse the Web, share interactive white boards, and transfer files instantly. SIP enables the creation of an environment that truly removes distance as a barrier to collaboration. A user already in a session who receives or initiates an additional session invite (a �call� in telephony terms) can easily combine multiple sessions into an ad hoc multi-party conference. Users have their own private meet-me audio conferencing resources available for meetings at any time. Participants dial in and are put on hold until the conference is activated by the arrival of the chairperson. Instant messages that announce when anyone joins or leaves the conference greatly enhance conference management. Users with PC clients can utilize the following as part of an active call or as a standalone session:
� Clipboard: Users send and receive the contents of a PC clipboard.
� File exchange: Session participants send and receive files directly with each other.
� Web push: Users send a URL that opens a Web browser on the recipient�s PC.
� White board: Participants enter and exchange text and graphics in a shared workspace.

SIP-Based Mobility and Personalized Communications
As instant messaging has become more and more popular, the concept of �presence� has entered into our personal communications. People are accustomed to checking their buddy lists to see if someone is available to chat. SIP pushes the concept into other media such as voice and video, allowing users to view and act upon real-time information about other users� status in the network before attempting to contact them. People can even be available for one media such as instant messaging while engaged in a session involving another media such as voice, a common occurrence when someone is on a conference call. SIP inherently enables mobility and location independence. SIP routes session initiation requests around the network based on dynamically updated information about the availability of a user�s registered devices. So session requests aren�t placed to a device, in the hopes of reaching a person. Requests are placed to people and the network locates them. It�s a truly personalized communications model.

Using SIP, it is possible to create a network-based agent to act on the behalf of a person 24 hours a day, 365 days a year. Users simply set personal configurations to control how, when, and by whom they are contacted, using a combination of automatic �find-me, follow-me� and call screening to control their time while assuring availability to important callers. Internet clients, wireless, and landline phones can all be easily included in a custom communications mix. The implications of this capability to the enterprise are considerable. A business with a mobile workforce can allow every employee to customize his or her communication preferences, easily updating them via the Web as often as necessary. Location ceases to matter, as telecommunications follow a mobile workforce everywhere and are filtered to minimize interruptions while keeping people in touch with those who matter most. In a personal communications context, similar benefits exist for the soccer mom or the active teenager.

SIP-Based Productivity & Information Interactions
A SIP-centric system enables numerous network capabilities, with the potential to affect both efficiency and productivity. For example, a personal address book can provide a network-based directory of addresses that a user can edit and access from any client. A change, addition, or deletion of an address book entry or buddy list made with one client will automatically update information in all other clients. So if a user updates a contact�s information on a PDA, that change would be reflected on the user�s PC as well as all other clients. In addition, SIP-centric services can be integrated with existing office productivity applications, allowing contacts and other common information to be shared. Data such as stock quotes can be sent at regular intervals, directly to the display of various clients. Web services are easily integrated with SIP to deliver real-time information updates on virtually any subject. While traditional telephony services such as hold, call forward, call waiting, and caller ID are readily enabled by the SIP protocol and inherent in any well implemented SIP system, many multimedia enhancements are possible such as calls with e-mail-style subject lines, call rejection with context-sensitive reasons for declining the call, and video or web-based announcements.

SIP-Enabled Innovation
The power of SIP goes beyond these definite examples into the realm of being an enabler of innovative capabilities. There are inherent advantages in the way the key elements of SIP are distributed. An end user with the right smart client can implement services from that end point, without the need for a centralized server. Therefore, the new service development process is fast, safe, flexible, and scalable. It�s an Internet service delivery model where it�s easy to introduce a new service, and it�s easy to grow it. In addition to the flexibility of endpoint service nodes, network-based SIP elements enable services to be readily available at all times, from any location. Services can start out highly focused on a specific problem for a limited set of individuals and very easily expand to serve a much larger community with similar needs. We can envisage a little league baseball coach creating a simple SIP-based application that allows him or her to handle schedule changes more efficiencies. Then, if other coaches in the league became interested in the application, it could be migrated to a hosted service and expanded for more wide-scale usage, enabling the coaches to create information specific to their teams, without having to know how to create the actual application. The service could be further enhanced to allow requests to come in via phone calls and responses played as audio announcements, all driven by the simple file each coach creates with the updated info.

The SIP control paradigm provides the key functions that will enable an entirely new network communications model, changing the way people communicate forever. SIP represents exciting possibilities for personal and enterprise communications. As more users adopt IP-compatible smart clients (phones, PCs, PDAs, mobile handsets), SIP-enabled sessions utilizing IP telephony, rich-media conferencing, push-to-talk, and location-based services will become more and more prevalent. In this brave new world, users will be able to locate and contact one another with little regard to physical location, media content, or the number of participants in a session. These users will enjoy a wealth of new services as well, as service providers and enterprise IT departments will be able to dramatically lower the cost of designing and deploying innovative new IP-centric services for their customers.

SIP is being embraced today by all major communications equipment manufacturers and many software companies. Because SIP is an Internet Engineering Task Force standard, it is inherently an open architecture, which serves to quicken its acceptance. The protocol readily enables voice and data convergence and is quickly becoming the backbone protocol for interpersonal interactive communications.

Tony Rybczynski is Director of Strategic Enterprise Technologies in Nortel Networks. He has over 30 years experience in the application of packet network technology. He writes a monthly �Inside Networking� column in Internet Telephony magazine.

John Yoakum is a Director of Business Development at Nortel Networks, where he builds collaborative relationships with customers and partners that capitalize on emerging opportunities and disruptive technologies.

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