Multimedia vs. your network

By Tom Henderson, ITworld.com |  Operating Systems

Last week, I covered one of the distinct advantages of Windows 2000 -- its support for new
network cards that are able to function autonomously and thus decrease CPU and bus
utilization. But the keys to Windows 2000 network performance amount to more than
network card and bus efficiency. While getting data in and out of a server is
important, you should also consider several other factors when striving to improve the
overall performance of networks running Windows 2000 Server editions.

These factors include, for example, the time-sensitivity of your network traffic,
appropriate compression methods, and transmission prioritization. Quality of service
(QoS), which Windows 2000 supports to some extent, encompasses all these factors and
more.

Isochronicity defined

Consider that there are two types of data: the kind that needs no special timing and
the kind that's time-sensitive. When you load an application or data into a client or
server, you won't often care about how fast it actually gets there so long as it's a
reasonably short amount of time. But there are several classes of applications for
which the timing of data arrival is especially important.

Generally, applications that must convert data from analog to digital formats are bound
to preserve the timing associated with the method that was used to digitize the analog
data. To ensure correct reassembly, these applications record the timing values of an
analog stream during its conversion to digital. Sampling occurs at specific intervals
of speed, to correlate the resulting digital stream to the original analog signal.
Transmission flow rates must also be kept within specified bounds so that receiving
devices can then reassemble the analog signal correctly on playback.

In other words, while nonanalog media just simply arrive, the timing requirements of
voice, video, and multimedia fall outside the methods employed by normal
get-there-when-you-can delivery mechanisms, and must be delivered within a time domain.
Such data is termed isochronous, which means "timed separately."

Different isochronous data streams have different sampling, linking, and compression
requirements. As an example, voice streams can be sampled satisfactorily at a low
analog-to-digital data rate and can use one of two linking methods, either half duplex
(one channel sent one direction at a time) or full duplex (two channels established
bidirectionally).

Compression and latency

Codecs (short for compressor/decompressors) are used for voice and many other types of
analog media, because raw, uncompressed data can require enormous transfer rates.

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