Virtual Reality Modeling Language

MPEG-4 covers some other interesting areas, one of which is the longer-term standardization
of meta description using a description syntax known as Virtual Reality
Modeling Language (VRML). Meta data is usually described as information about
information. It provides us with a standardized way of describing information such
that we can archive it and find it again at some (possibly distant) time in the future
(back to our Doomsday project!).
The meta description includes the QoS requirements of the media file; that is, this is
declarative content—content that defines and describes its radio bandwidth and network
bandwidth quality requirements. The quality of service metrics include the following:
 Whether or not the packet stream needs to be isochronous. In an isochronous
packet stream, all packets arrive in the same order they were sent. In a nonisochronous
packet stream, they do not.
 The buffer and timing requirements—that is, how much buffering will be
needed by the complex media file. Table 7.3 shows the buffer size requirements
for what are called simple MPEG-4 profiles.
The buffer size expands as the frame size increases (from QCIF to CIF) and as the
frame rate increases.
 MPEG-4 also describes how elementary streams from a complex content stream
are linked to a complex transport channel. This is very fundamental. In Chapter
3 we described how the OVSF codes are structured on the radio channel
downlink and uplink—our complex radio bandwidth transport channel. We
need to take these complex composite streams (consisting of up to six elementary
streams per user) and preserve their properties, including time interdependencies,
as the streams move across the radio layer and into the core network.
This is, as we will see in later chapters, absolutely crucial to delivering consistent
end-to-end performance in a wireless IP network.
 To help maintain complex-content multiple-stream synchronization, MPEG-4
adds optional timestamping to each elementary stream. This can either allow
isochronous packet streams to be reclocked in a receiver or non-isochronous
streams to be individually reconstituted, reordered, and reclocked.
 MPEG-4 also supports the defining of buffer size to allow non-real-time data to
be sent ahead of a real time exchange—for example, the preloading of a Power-
Point presentation or financial spreadsheet.
The MPEG-4 encoder is effectively dictating how many per-user channel streams
are needed at the beginning of a session, how many per-user channel streams need to
be added or removed as the session progresses, and the data rate required on any one
of the individual per-user channel streams. This will need to be integrated either with
IP session management protocols (such as SIP, which we case study later in the book)
or with existing circuit-switched SS7-based session management signaling.
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