| 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 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 |
