H.323 Quality of Service

From ViDeNet's Videoconferencing Cookbook

...H.323 videoconferencing will fare much better on network links that are not congested or are optimized in some way to provide a predictable level of service to the video application. True QoS (Quality of Service) standards that will be integrated into existing LAN/WAN protocol suites are currently under development and not ready for wide-scale deployment. However, some level of service guarantees can still be achieved through technologies that support "packet shaping" (giving particular data packets priority over others when sending them across the network) or simply through good network management (optimizing the network to support typical traffic patterns)."

What is Quality of Service?

QoS is a measure of how smoothly your transmission reaches the other endpoint and theirs to you. WA-K20 is researching current and developing methods for providing a supported level of quality for H.323 videoconferences.

These include RSVP and IP Precedence and other methods. These methods are under development and no one method works in every network or is supported by network hardware the same. There is a lot of discussion and research within the H.323 Standards community. We will post additional information as our WA-K20 provides it.

Until then, all H.323 videoconferences are considered "best effort". With ample bandwidth for your LAN's and your WA-K20 network connection's typical data traffic levels, you should be able to make videoconference connections that are good quality most of the time:

• For 384K connections, add 20% for overhead, or about 460K of bandwidth. For 768K connections, figure on 922K with overhead.
• Your H.323 video data should be less than 30% of the available bandwidth on your LAN and to WA-K20.
• The total bandwidth, data and video, should be less than 75% of the available bandwidth.
• Check the current utilization of your WA-K20 bandwidth.

Why implement a QoS solution?

Initially, you may have enough bandwidth between your codec and a codec somewhere on WA-K20 or Internet2 to not be too concerned. However, at any time, if any router along that route becomes busy and its queue is saturated, your H.323 data is competing for the same bandwidth that all the other data traffic is competing for. This can happen when you saturate your available bandwidth, during high traffic times or during a denial of service attack.

See Polycom's paper (pdf) on QOE and QOS.

Jitter and Latency:

Jitter and Latency have to do with video quality.

Latency is the delay between the sending of information from an endpoint and the reception of that information at the far endpoint. A consistent latency will result in a delay in reception and response. While this delay does not affect the quality of the received and processed video and audio signals, it does affect the perceived quality, because it introduces a “walky-talky” or half-duplex feel to an interactive conference.

Jitter describes the variance in latency during a videoconference. In IP-based communication, packets are not guaranteed a particular transmittal order or throughput. Therefore, while one packet may be transmitted with minimal delay (latency), the following packet may be transmitted with large latency. When audio and video signals must be reconstructed from the received packets in a continuous stream, the stream becomes interrupted with each variance in latency. This results in jumpy or dropped video sequences, and scratchy or unintelligible audio.

See Polycom's paper on latency and jitter.

~Exerpts for the material on this page have been graciously contributed by Wisconsin VCS Videoconference Services.