Understanding SIP

Initiation is SIP's middle name ? it's also what AV pros can expect to go through over the next few years. 12/25/2006 7:33 AM Eastern

Understanding SIP

Initiation is SIP's middle name ? it's also what AV pros can expect to go through over the next few years.

Rob Smith and his colleagues had a clean slate and some simple goals: Design a state-of-the-art AV system that would allow entrepreneurs at a Canadian innovation center to collaborate effortlessly with everything from videoconferencing to virtual whiteboards. For the foundation, they chose Session Initiation Protocol (SIP), a communication standard that's increasingly common in the telecom world but a newcomer in pro AV.

“Our whole telecom infrastructure is also SIP-compliant,” says Smith, technology consultant for the MaRS Discovery District, whose Collaboration Centre opened in downtown Toronto in May 2005. “So now we're able to connect what would be the traditional AV world, which usually sits on one side of the fence, and telecom, which sits on the other side.”

For example, MaRS' offices are equipped with a variety of SIP-equipped endpoints, including desk phones and PCs. These endpoints interact with a variety of MaRS infrastructure, including presence servers — which track a person's whereabouts and the capabilities of the office they're in at that particular moment — and its Private Branch Exchange (PBX), which routes phone calls.

“When your phone rings, a little window will pop up on your screen,” Smith says. “If that person happens to be ‘on network,' you now have the opportunity to do instant collaboration using tools such as whiteboarding or desktop sharing. You can do instant videoconferencing.”

Plays well with others

SIP started showing up in videoconferencing endpoints more than a year ago but currently isn't as widely used as incumbent protocols such as H.323, which is the most common IP-videoconferencing standard. But based on AV vendors' product roadmaps and on the positive experiences of early adopters such as MaRS, SIP is a protocol that most AV pros will have to master sooner or later.

Robert Smith, technology consultant for MaRS Discovery District, Toronto, sets up a multipoint videoconference using SIP endpoints in conjunction with the MaRS Discovery District's Tandberg MPS 800 Multipoint Control Unit (MCU).Photo by Christie Spicoluk

Developed by the Internet Engineering Task Force (IETF), SIP's primary function is to broker communications between devices, including instant messaging (IM), voice over IP (VoIP) phone calls, and videoconferences. Those communications can be between just two devices or between multiple ones, such as a media server feeding video to a mall's digital signage.

“It's very computer-network-focused as opposed to telecom-network-focused,” says Jim Smith, a systems engineer with Polycom, a Pleasanton, CA-based maker of conferencing equipment. “Its goal is to allow smart devices to talk to each other rather than trying to network together dumb devices.”

For integrators, one benefit is that SIP allows videoconferencing to interact with firewalls in a way that's less likely to violate a company's IT-security policies. Trying to accommodate those policies with older protocols can affect the user experience.

“H.323 requires certain firewall ports to be open and says that traffic has to pass a particular way: Voice has to go on one port, data on another, and video on another,” says Smith, whose company has been selling SIP-equipped videoconferencing gear for more than a year. “You end up with all these strange interactions that can make for an unpleasant conferencing experience.”

Another benefit is that SIP can reduce videoconferencing's impact on an enterprise's telephone system, particularly those that use VoIP.

1 2 3 4 5 Next

Understanding SIP

Initiation is SIP's middle name ? it's also what AV pros can expect to go through over the next few years.

“Right now, when a videoconferencing system goes onto a VoIP network, they have to partition the network so that part of it is running H.323,” Smith says. “Then they lose all of the features and functionality they've come to enjoy with their phone system.”

But with SIP, there's no need for network partitioning. “All of the neat features in their SIP-based call manager environment can migrate into the videoconferencing world very nicely,” Smith says.

SIP also integrates more easily with IP-based PBXs. (Older PBXs that use Time Division Multiplexing, or TDM, can work with SIP with the addition of a gateway.)

“Often video systems are set up to be independent of the phone system,” says Cullen Jennings, a Cisco distinguished engineer at San Jose, CA-based networking equipment provider Cisco Systems. “But because the SIP signaling is like a normal phone call, it doesn't matter if it's an audio or video call through the PBX. There's a higher degree of being able to take advantage of the phone system.”

Keeping the legacy alive

Although SIP may be the future, it still has to respect the past. One reason is because it's not yet the dominant protocol. “SIP and H.323 started around the same time, but today 323 has a much larger installed base, at least in video,” says Sean Lessman, senior director of advanced technologies at Tandberg, a visual communications solutions provider based in New York and Oslo, Norway. “But things are moving toward SIP. We believe they'll co-exist. So all of our products will have dual stacks to share the same feature sets over both protocols.”

Gateways are one option for adding SIP and its enhanced features to an AV system where there's no business case for replacing functional equipment that uses older protocols. The gateway sits somewhere in the network or next to the legacy device and translates the SIP-based traffic into a protocol that the older device can handle. For example, MaRS uses bridges to enable collaboration with people who use H.323 equipment. “They create the value of being able to aggregate a variety of different technologies and connect people,” Smith says.

A second option is to add SIP support to the legacy device via a software upgrade. That's not always possible, however, unless the vendor designed the product to be flexible enough to handle those upgrades. In those cases, the product is essentially acting as its own gateway to handle protocol conversion.

“Occasionally you'll get a vendor that adds SIP to a product as an afterthought,” says Alan Hawrylyshen, director of VoIP Protocols at Ditech Networks, a Mountain View, CA-based manufacturer of networking products. “Underneath the skin, they're still doing things the old way. We see that all the time.”

AV vendors such as Tandberg say that they recognize the importance of such interoperability.

“The idea this year is to be as interoperable as possible with everything that's on the market,” Lessman says. “Next year, our focus will be on parity: A lot of the features we have in H.320 and H.323, we want to make sure that they're extended to the SIP side of the product, as well. The customer shouldn't care about the protocol they're running. If the feature is there in one protocol, it should be available with the others.”

Understanding SIP

Initiation is SIP's middle name ? it's also what AV pros can expect to go through over the next few years.

Open to debate

Like other open protocols, SIP is a set of guidelines that all vendors must follow in order for their equipment to work with other manufacturers' gear. That interoperability is a plus because it frees AV pros to mix vendors. But there's a catch: Like other open protocols, SIP allows vendors to layer additional features — often referred to as “extensions” — on top of the base standard as way to make their products stand out from the pack or justify a price premium.

“SIP is a very flexible, open standard,” says Tandberg's Lessman. “That means there's a lot of room for people to go their own way with it.”

These additional features don't always work in a multivendor environment, even when all of the equipment is SIP-based.

“Right now, every vendor has a slightly flavored version of SIP,” says Polycom's Smith. “The integrator needs to understand that infrastructure's and endpoint's capabilities may not exactly match. So they need to research this in advance before they promise certain features and capabilities.”

That situation isn't new to early SIP adopters such as MaRS. “SIP is easy to understand,” says Robert Smith. “As you get more granular with the deployment, there are non-standard things happening between vendors.”

Things can get even trickier when the SIP-based AV gear has to work with SIP-based telecom gear, such as a PBX. Even though they all use SIP, AV and telecom vendors often have different priorities, which are reflected in their support — or lack thereof — for value-added features that other manufacturers include. When it was evaluating SIP equipment, MaRS found that one vendor's PBX worked fine with another vendor's videoconferencing endpoint.

“But once you put it in a live production environment, you find some of the little things,” Smith says. “You start to realize that there are non-standard things.”

The good news is that those interoperability issues should become less common over the years. That's because although vendors will always provide additional features beyond the base SIP standard, there will be more and more real-world deployments to study as a way of identifying conflicts such as unsupported features.

“My advice to anyone who's looking to deploy SIP today is to do your homework, and make sure you test in the scenarios you want it to perform in,” says Tandberg's Lessman.

AV meets IT — again

SIP is part of a larger trend where AV, IT, and telecom are slowly converging. So working with SIP means understanding the basics of IT and telecom.

Understanding SIP

Initiation is SIP's middle name ? it's also what AV pros can expect to go through over the next few years.

For example, SIP AV equipment can work with infrastructure such as Microsoft's Office Live Communications Server 2005, which registers endpoints and enables features such as presence. (Tandberg's MXP video endpoint is one example of a SIP-based AV product that works with Microsoft's server.) So during a conference, an attendee could open a document and highlight the author's name to pull up her presence information, such as whether she's on the network.

If she is, the attendee then could click on her name to establish a connection with her. How that connection is established depends on, for example, whether she logged on that morning and indicated that she's available for videoconferences or just phone calls. The upshot is that the conference attendee doesn't have to wonder whether she's around or wasting time figuring out the best way to contact her. Instead, the network makes those decisions automatically, behind the scenes.

“It puts the recipient in charge of the types of communications they get,” says Ditech's Hawrylyshen. “I could say, ‘I don't want videoconferences with Bob because we don't get along very well or because he's too far down the org chart.'”

Those features were a large part of SIP's appeal for MaRS. “Our Holy Grail in all of this was SIP,” Smith says. “Our primary focus was to build an infrastructure that was about understanding who's on the network and how to connect to those people easily.”

SIP also can simplify things for both users and integrators. For example, suppose that there's a videoconference involving multiple displays in each room. “In H.323, it's nearly impossible to get that all set up right,” says Cisco's Jennings. “With SIP, it's a lot easier. SIP is set up to deal with things such as lots of screens across a single connection.”

SIP also can accommodate different user capabilities. “If you're calling a room that has only one screen, the SIP system could negotiate and use only the center screen,” Jennings says. “The user wouldn't configure that manually.”

Another reason why it's important to understand the myriad of ways that SIP-based AV equipment can tie into telecom and IT infrastructure: It's an opportunity to provide additional features and services, which in turn add value in the client's eyes.

“They can say: ‘You want to do these five different things. I can make them all work together,'” says Tandberg's Lessman.

One aspect that doesn't require much additional knowledge is the network. That's because SIP doesn't require a particular network technology, such as asynchronous transfer mode (ATM) or frame relay. So network issues such as bandwidth, latency, and jitter depend on the needs of the AV application rather than on SIP.

However, SIP does impact the network in one key way: It's more efficient than many legacy protocols, which is a plus because bandwidth isn't free. “An H.323 call can have up to 20 percent overhead,” says Polycom's Smith.

Understanding SIP

Initiation is SIP's middle name ? it's also what AV pros can expect to go through over the next few years.

That's yet another reason why in pro AV, SIP is a matter of when, not if.

Says Tandberg's Lessman, “SIP is moving out of the early adopter stage.”


For more information about SIP, check out:

  • For more details about MaRS' Collaboration Centre, visit Click on the Explore MaRS tab and then on the MaRS Collaboration Centre link.
  • For an overview of SIP technology, as well as news about SIP products, visit the SIP Center website at
  • A detailed overview of the SIP standard, also known as RFC 3261, is available from the IETF at
  • Tim Kridel is a freelance writer and analyst who covers telecom and technology. He's based in Kansas City and can be reached at

    Previous 1 2 3 4 5
    Want to read more stories like this?
    Get our Free Newsletter Here!
    Past Issues
    October 2015

    September 2015

    August 2015

    July 2015

    June 2015

    May 2015

    April 2015

    March 2015