Everything On The Network
Like it or not, AV and IT increasingly share the same network. For AV pros, that's both a problem and an opportunity.
“For AV pros, the convergence trend means that they need to become experts in networking technologies,” Stoner says. “Running real-time content across an IP network creates unique challenges and may uncover network design and configuration issues that would otherwise be undetected.”Quality is priority one
A major concern in any network is quality of service (QoS). For example, suppose that the majority of traffic running over an office's LAN is email, file transfers, and documents headed to the floor's laser printer. Although that traffic is important, it can be categorized as a low priority in the sense that a few seconds of extra time getting across the network won't be noticeable to the sender or recipient.
Audio and video, on the other hand, aren't as tolerant of latency. For them, QoS means ensuring that enough network bandwidth is set aside for their needs. When it's in short supply, the AV traffic takes priority. So even if several other users on the LAN start sending 10-MB PowerPoint files to one another, the network still will provide enough bandwidth for the audio and video traffic — even if it means that PowerPoint files get delayed because there's only so much bandwidth to go around.
QoS can be maintained via QoS profiles, where each traffic type is tagged with a priority level that network nodes such as servers read in order to know how to handle it. “It can be assigned on a time-of-day basis, by LAN segment, or by user basis,” says Jim Smith, a systems engineer with Polycom, a Pleasanton, CA-based maker of conferencing equipment. “Most network hardware is capable of supporting QoS.”Bandwidth bottlenecks
The big catch is that depending on what the network is carrying, it may be difficult to reserve enough bandwidth for AV traffic or ensure that it always gets priority. For example, if the network also carries voice calls — known as voice over IP (VoIP) — then they also must be given priority. An increasing number of enterprises now use VoIP, especially between offices, because it's cheaper than regular telephony. So if a company's wide-area network (WAN) routinely carries VoIP calls between multiple offices, and a videoconferencing session needs to share those links, it becomes tricky to ensure that both types of traffic aren't delayed by each other or by other uses.
“You want to maintain your streaming traffic below about 30 percent of network capacity,” Smith says. “A good rule of thumb is that you don't want to exceed 70 percent of your network's rated capacity for total traffic burden.”
That leaves breathing room in case traffic unexpectedly spikes, or if part of the network goes down, and the other links have to shoulder the whole traffic load. These types of issues highlight the importance of being able to work with the IT staff or even configure parts of the network yourself. “You can manage — or have the network operations center manage it for you — the QoS and provision network switches to be video friendly for distribution across the LAN,” says Cornell's Pfleiderer. “However, going off campus, there's much less control, and it's much more difficult to manage the bandwidth needed for video distribution.”
When the network includes links that are owned and operated by a third party, such as the phone company, it may be difficult to maintain the control that some AV applications require. Many telecom providers offer service level agreements (SLAs), where the contract specifies minimum benchmarks, such as how many megabits of bandwidth will always be available to the customer and its applications. If an SLA isn't available, or if it's too expensive to justify, one option is to package the AV traffic in a way that makes it more likely to zip through the WAN with no delays.
“Sometimes you have to seek technologies that work better under unmanaged circumstances,” Pfleiderer says. “For example, the use of MPEG-2 on the LAN can be managed, but in a WAN environment it might be better to employ MPEG-4 to reduce the amount of bandwidth needed to transmit the video.”
If you're working with a network or combination of networks where you don't have a guaranteed amount of bandwidth, it's worth running a few tests to see what they can support. These “Internet speedometers” aren't 100 percent accurate, but they can provide a helpful ballpark estimate.