Released on = February 5, 2007, 1:38 am

Press Release Author = Adi Paz / Radvision

Industry = Telecommunications

Press Release Summary = A description of the key elements comprising IMS SIP
technology, including its architecture, network components and signaling protocols.

Press Release Body = Today's telecommunication arena is rapidly moving towards
next-generation networks (NGNs) that offer ubiquitous, converged services over
converged voice, video, data and mobile networks. Until very recently, the
prevailing telecommunications paradigm was based on multiple types of networks, each
serving different types of applications.

While IP-related technology holds the most promise for meeting the requirements of
NGNs, the use of the global Internet is quite limiting. The IP Multimedia Subsystem
(IMS) is an architecture that allows delivery of identical services to fixed and
mobile customers - regardless of whether they are connected through the
packet-switched (PS) or circuit-switched (CS) network. IMS-based services enable
communication in a variety of modes - including voice, text, location, presence,
messaging, pictures and video, or any combination of these.

In addition to service creation and delivery, IMS handles call control issues, and
can easily be adapted to serve roaming subscribers. The IMS architecture is
inherently capable of bridging between separate networks, and will ultimately be
used for all types of networks, such as wireline Voice-over-IP (VoIP) networks,
WiMax wireless networks and packet cable networks.

The expansion and acceptance of IMS is an indication of just how important this
technology is to the future of unified communications. Many organizations which are
responsible for network standardization are currently adopting IMS technology. The
ramifications for the industry are significant, and the entire telecommunications
industry is gearing up for the imminent widespread implementation of IMS.

Based on a Strong Signaling Foundation: Session Initiation Protocol (SIP)
IMS technology was originally developed for the cellular arena to define how to set
up advanced services for 3G cellular networks and grew out of a group of standards
created by the 3rd Generation Partnership Project (3GPP).

IMS is a Media-over-IP network and uses the Session Initiation Protocol (SIP),
originally standardized by the IETF, as its base signaling protocol. The 3GPP chose
SIP as its base protocol because previous telecom signaling protocols failed to
comply with all IMS requirements. Because SIP is an Internet protocol, it can
accommodate convergence, and has the potential to meet all the needs of the IMS
architecture. For instance, SIP can signal between different network entities,
including endpoints and servers. In IMS, each network server has its own role, in
contrast to traditional networks where a central office switch does it all,
including call control and service control. In addition, SIP uses Internet
extensibility mechanisms. A service provider with IMS networks initially may only
have a small number of subscribers. As the subscriber base grows, IMS networks must
be easily scalable to add more subscribers. SIP is also very flexible, and uses
standard extensions. SIP's flexibility enables IMS networks to adapt and change
signaling protocols to meet dynamic market needs. Finally, SIP provides adequate
security, with both internal and external security mechanisms.


IMS SIP: A Complex Challenge
While offering the right foundation, SIP in its IMS form has proven to be quite
complex and presented many technological challenges. There were many gaps between
the SIP initially defined by the IETF, and the features required for full IMS
support. To solve this problem the 3GPP defined dozens of SIP extensions - additions
that are specific to IMS networks. Collectively, these extensions comprise the IMS
SIP protocol, which is defined in the 3GPP TS.24.229 standard. These extensions,
such as extended call control, presence and instant messaging, extend the
functionality of SIP on IMS networks. This new IMS SIP usage profile is perhaps the
most important in the telecommunications industry, and is uniquely the most
appropriate for NGN networks.

To illustrate the inherent complexity of IMS SIP and all its extensions, we will
review the major extensions below:

SigComp (RFC 3320)
The SigComp extension defines how to compress SIP textual signaling data, which can
be very large and problematic to transmit, causing delay. SigComp solves the
challenges of roundtrip delays, as well as mobile user equipment battery life
P-headers (RFCs 3455 and 3325)
(P- Private) In addition to standard headers, the 3GPP defined additional headers
targeted at solving specific IMS network problems, such as obtaining information
about the access network (cell ID) and the visited network (roamed network), and
determining caller identity.
Security Agreement (RFC 3329)
This IMS SIP extension specifies how to negotiate security capabilities for multiple
types of endpoints.
AKA-MD5 (RFC 3310)
This IMS SIP extension determines how terminals and networks are authenticated using
already defined mechanism (e.g. ISIM), as well as specific key exchange.
IPSec
IPSec is used on various IMS interfaces and between different IMS networks. IMS uses
IPSec in the transport mode, as opposed to the standard used in VPN services.
Media Authorization (RFC 3313)
Ensures that only authorized media resources are used.
Mobile Registration (RFCs 3327 and 3608)
On IMS networks, the terminal registration process is more complicated, as it
includes various security extensions and must deal with registration from a visited
network. RFC 3608 and RFC 3327 define the syntax and SIP entity usage of the
Service-route and Path headers.
Reg-event Package (RFC 3680)
Used by the terminal and the P-CSCF to know the terminal registration status on the
network.
IPv6
IMS prefers IPv6 networks, which offers distinct advantages. It permits a larger set
of addresses and contains embedded IPSec functionality that may eliminate the need
for entities like NATs and firewalls.
Preconditions (RFC 4032)
Specifies method for negotiating QoS, security and other required call behavior
between two terminals.
IMS Resource Reservation (RFC 3312)
Defines how to make resource reservations for phone calls or sessions.
Session Description Protocol (SDP)
SDP defines the basic negotiation process for the media streams, and includes the
bit rate and codec to be used, as well as other media attributes. IMS extends SDP
with even more extensions, such as grouping of media lines, QoS and preconditions
attributes, supplemental codec support, and bandwidth modifiers.
XML Usage
IMS SIP signaling uses XML protocols extensively, including XCAP, to implement
various kinds of SIP message contents, and to allow full function interfaces between
IMS entities.
IMS Simple Extensions
The SIMPLE group is an IETF working group that defines presence and instant
messaging signaling requirements. Basic SIMPLE definitions were inadequate for IMS
applications because they were not efficient enough for use on the air link. IMS SIP
extended this standard with the following:
. Partial Notifications / Publications
. Notifications filtering
. Resource list / SIP exploders
. Message Session Relay Protocol (MSRP)

IMS SIP Expertise: A Prerequisite for Success
The use of SIP in IMS networks requires a great deal of adaptation and extension of
the original signaling protocol. Given the breadth, variety and complexity of IMS
SIP, it is indeed an arduous task to develop new services and applications from
scratch. A more reasonable approach is to use prepared toolkits and infrastructure
products that encompass all the nuances of IMS SIP, and where much of the
development effort and interoperability testing (IOT) have already been completed.

In order to execute IMS roadmaps and ensure on-time deployment, developers need
solutions that are finely tuned to the unique characteristics of IMS SIP and that
provide the extended SIP signaling infrastructure needed for IMS applications.

A longer 25 page version of this white paper is available at
http://www.radvision.com/Resources/WhitePapers/ims_sip.htm
Adi Paz is a Senior Product Marketing Director at RADVISION, a leading provider of
video network infrastructure and developer tools for unified visual communications
over IP, 3G, and emerging next-generation networks.

Web Site = http://www.radvision.com

Contact Details = RADVISION Ltd.
24 Raoul Wallenberg St.
Tel Aviv, 69719 Israel
Tel: +972-3-767-9300