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May 2007
Volume 10 / Number 5
Feature Articles

SIP's Role in Enterprise IP Telephony

By Pat Rudolph, Feature Articles

Session Initiation Protocol (SIP) is an Internet Engineering Task Force (IETF) initiative for managing the handshake procedures for beginning and ending real-time communications between IP network endpoints.

SIP is a text-based protocol, similar to HTTP and SMTP, for initiating interactive communication sessions between users. This makes SIP easy to troubleshoot, enables fast application development and presents a stable framework for establishing interoperability between devices, applications, call controllers and gateways. SIP is used to enable humanto- human communications that might include voice, video, chat, interactive games and virtual reality.

Since February 1996, SIP has developed a substantial industry infrastructure and momentum to encourage and promote its use, including over 130 IETF drafted initiatives influenced by SIP and the technology-promoting SIP Forum.

SIP is designed to perform session setup independent from the communications flow. End devices speak to each other directly using whatever application they have available. This delivers a greater degree of flexibility, failure recovery and scalability since the network maintains no state information about the end devices. Let’s take a close look at some attributes behind SIP’s growing adoption.


SIP is Simple

Using the IETF fundamental of technology reuse and the proven value of simplicity, the SIP message set is a simple construction - six messages that appear in clear text to facilitate call setup. Clear text allows for easy troubleshooting and avoids complex software interactions and other processing that affects interoperability. The six messages are Invite, Trying, Ringing, OK, ACK and Bye.

SIP is both elegant and practical. Its base assumption is that all SIP endpoints and elements exist in the IP environment, an arena already equipped with standard mechanisms to handle packet transport priorities, privacy and other required services. These value-added services do not require specification in the SIP framework. Conversely, in legacy environments, such as the public telephone network, the use of HTTP is neither integral nor readily available.

This building block approach on an IP base is unique to IETF initiatives. The results are nearly trivial protocol definitions such as SIP in contrast with older session control or interface protocols such as H.323 or Q.SIG from the International Telecommunications Union (ITU).


SIP is Secure

There are tradeoffs between interoperability and cost when considering security. Simple standards make it easier to interoperate within a multi-vendor network, but they also expose the network to potential abuse. This vulnerability, however, can be effectively addressed by strong authentication and privacy services that do not interfere with the primary business benefit of standard-based solutions and also enable a long investment life through interchangeable vendors, services, applications and devices.

The IETF framework for SIP offers a rich set of standards for authentication and privacy, including secure SIP, secure RTCP, and secure RTP. These capabilities leverage IETF proposals for the use of standard implementations such as Transport Layer Security (TLS) for robust session privacy service and Secure/Multipart Internet Mail Extensions (S/MIME) for session control packet privacy.

Most vendors implementing H.323 offer no such options. Instead, they choose to implement proprietary derivatives to facilitate rudimentary forms of privacy in first-generation IP-PBX devices. This approach is typical of the PBX vendor solutions brought to market during the past two decades in which proprietary digital signaling protocols were implemented on endpoints and PBX fabric. The result of these decisions was nominally better security but considerably more vendor-specific lock-in that failed to provide many useful features - such as call forward and hold - at a standard service level.


SIP is a Standard

Despite being a standard, SIP implementations do vary. Some vendors, most notably TDM-based PBX vendors and first-generation IP telephony vendors, offer SIP interoperability as an interface into their otherwise non-SIP telephony system. This approach helps the vendor maintain account and feature control. They can charge extra for SIP support for only the most rudimentary features while offering advanced premium-priced feature options with their proprietary call control protocol and implementation.

Another approach is using standard SIP among endpoints such as IP phones, video cameras, call controllers and gateways to the public telephone network. This can enable a robust and inexpensive, vendor-neutral environment capable of rapidly delivering a portfolio of applications for presence, conferencing, contact center, messaging and mobility services.


SIP in the Enterprise

SIP has become a foundation for building innovative services aimed at enterprise. For example, SIP trunking services allow two SIP-empowered enterprises to communicate using RTP from endpoint to endpoint without the need for a gateway between them. The growth of this category of services is helping reduce costs by lowering the load on packet-circuit gateways and reducing regulatory and tax burdens.

SIP trunking also improves audio quality since bandwidth allocation can be negotiated end-to-end instead of endto- gateway-over-digital-to gateway-toend. Fewer digital hops with more bandwidth enable wideband audio quality, as well as SIP-initiated video conferencing.

As demand for IP telephony and SIP services and implementation continues to accelerate, the public telephone network faces change. A mainstay of the global economy for the past ten decades, it is losing ground to a network of worldwide IP communications integrated with simple, secure, standards-based, applications- rich implementations and services.

There can be little doubt of the important role that SIP will play in facilitating this transition and enabling powerful enterprise applications that reduce cost, improve user productivity, and strengthen customer interactions. Some vendors will be slow to appreciate and take advantage of this opportunity. They may focus their energies on attempting to retrain the transition rather than exploring its possibilities.

Organizations should look to partner with companies that embrace innovations and standards. They are a foundation on which a company builds businessenhancing solutions. The question is no longer why, but rather when SIP will be integral to every business’ future.

Pat Rudolph, is Vice President, Technology, for 3Com Corporation. (quote - news - alert) For more information, visit the company online at


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