July 2004
SIP And Converged Communications
BY TONY RYBCZYNSKI & JOHN YOAKUM
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.
Summary
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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