October 2000

Pulling Together -- Interoperability Through Open Standards

BY GREG GALITZINE

We've taken a quick pulse of the industry and asked several vendors to comment on the importance of interoperability. Respondents include an Internet telephony service provider (ITSP), a next-gen Integrated Communications Provider (ICP), and one of the so-called "arms merchants" -- a company tasked with supplying products and solutions to the industry.

We've also taken a moment to ask the industry's leading players to describe a standard that falls within their particular area of expertise. Below you'll find descriptions and definitions of several of the more prevalent standards operating in the Internet telephony industry: SIP, H.323, MGCP, and MPLS. One thread you'll find throughout this whole section is the need for the industry to work together toward a rather elusive goal: Only by pulling together can we ever achieve the holy grail of interoperability.

[ Return To The October 2000 Table Of Contents ]

Jonathan Rosenberg, Chief Scientist, dynamicsoft, Inc.

As next-generation networks move from the design stage to deployment, a key challenge for service providers and network operators is how to ensure interoperability between different communication protocols. The Session Initiation Protocol (SIP) is an Internet standard specified by the Internet Engineering Task Force that is used to used to initiate, manage, and terminate interactive sessions between one or more users on the Internet. SIP has gained tremendous momentum among service providers because it can deliver enhanced services over next-generation networks.

However, one of SIP's most attractive features is that its simple, open design supports interworking with ISUP and H.323 -- key protocols from both the SS7 and IP environments. SIP's ability to work with these widely deployed standards will not only speed the integration of the PSTN with IP networks, but gives service providers with H.323 networks a clear migration path to offer new services that can go well beyond VoIP.

SIP And ISUP
Let's look first at how SIP works with ISUP, the protocol used in SS7 networks to set up and manage basic call connections. One common network scenario is IP trunking, also known as phone-to-phone, in which PSTN calls hop onto the Internet over a gateway and then hop back off to the PSTN. While SIP is used to route calls over the Internet, the question becomes how to preserve feature transparency. It is generally believed that the primary way to achieve feature transparency is to ensure that all the information in the incoming ISUP is carried through the SIP network to the gateway on the far end.

This can be accomplished with SIP-T (SIP for telephony), a simple extension of SIP that preserves needed ISUP information as the call is carried through the Internet. This is done by actually carrying the ISUP message, byte for byte, in the body of the SIP messages. Since SIP supports transfer of any type of content, ranging from JPEG pictures to HTML Web pages, it can easily carry ISUP messages. What's more, SIP-T is backwards compatible so that it will operate with all existing SIP phones. If a phone gets a message with ISUP, but doesn't understand ISUP, those bodies are simply ignored. SIP-T is currently a work item of the SIP working group in the IETF.

SIP And H.323
How well does SIP interwork with H.323? Since H.323 is an ITU standard, while SIP springs from the IETF, these two protocols are often portrayed as rivals. As usual, the real picture is more complicated than that, and network operators stand to benefit from closer interworking between SIP and H.323.

Architecturally, these two protocols are very different, and the way they handle many functions (such as multiparty conferencing) share few similarities and are difficult to interwork. However, many softswitches already support SIP to H.323 translations, and interworking between SIP and H.323 has been demonstrated for some time now. There is currently work going on within the IETF to develop guidelines for such interworking. It's already been shown that basic functions like call setup and teardown will work between these two protocols, but even more complex functions are being investigated.

One of the key advantages of SIP/H.323 interworking is that a service provider with deployed H.323 networks will be able to begin to gradually realize some of the benefits of SIP services. SIP is rapidly emerging as the clear choice for delivering applications; soon we'll begin to see the advent of application servers that will likely support SIP and not H.323. The International Softswitch Consortium's applications working group has defined a typical VoIP application server based on SIP.
Service providers with H.323 networks can simply begin to use SIP/H.323 conversion boxes, or gateways, that will let them leverage their existing investments in H.323 gear. They'll be able to deploy SIP application servers that can work in H.323 networks via conversion boxes. They won't have access to the powerful new features and services SIP defines, but SIP itself is powerful enough to "know" what features a H