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+
+<h1>Ogg logical and physical bitstream overview</h1>
+
+<h2>Ogg bitstreams</h2>
+
+<p>Ogg codecs use octet vectors of raw, compressed data
+(<em>packets</em>). These compressed packets do not have any
+high-level structure or boundary information; strung together, they
+appear to be streams of random bytes with no landmarks.</p>
+
+<p>Raw packets may be used directly by transport mechanisms that provide
+their own framing and packet-separation mechanisms (such as UDP
+datagrams). For stream based storage (such as files) and transport
+(such as TCP streams or pipes), Vorbis and other future Ogg codecs use
+the Ogg bitstream format to provide framing/sync, sync recapture
+after error, landmarks during seeking, and enough information to
+properly separate data back into packets at the original packet
+boundaries without relying on decoding to find packet boundaries.</p>
+
+<h2>Logical and physical bitstreams</h2>
+
+<p>Raw packets are grouped and encoded into contiguous pages of
+structured bitstream data called <em>logical bitstreams</em>. A
+logical bitstream consists of pages, in order, belonging to a single
+codec instance. Each page is a self contained entity (although it is
+possible that a packet may be split and encoded across one or more
+pages); that is, the page decode mechanism is designed to recognize,
+verify and handle single pages at a time from the overall bitstream.</p>
+
+<p>Multiple logical bitstreams can be combined (with restrictions) into a
+single <em>physical bitstream</em>. A physical bitstream consists of
+multiple logical bitstreams multiplexed at the page level and may
+include a 'meta-header' at the beginning of the multiplexed logical
+stream that serves as identification magic. Whole pages are taken in
+order from multiple logical bitstreams and combined into a single
+physical stream of pages. The decoder reconstructs the original
+logical bitstreams from the physical bitstream by taking the pages in
+order from the physical bitstream and redirecting them into the
+appropriate logical decoding entity. The simplest physical bitstream
+is a single, unmultiplexed logical bitstream with no meta-header; this
+is referred to as a 'degenerate stream'.</p>
+
+<p><a href="framing.html">Ogg Logical Bitstream Framing</a> discusses
+the page format of an Ogg bitstream, the packet coding process
+and logical bitstreams in detail. The remainder of this document
+specifies requirements for constructing finished, physical Ogg
+bitstreams.</p>
+
+<h2>Mapping Restrictions</h2>
+
+<p>Logical bitstreams may not be mapped/multiplexed into physical
+bitstreams without restriction. Here we discuss design restrictions
+on Ogg physical bitstreams in general, mostly to introduce
+design rationale. Each 'media' format defines its own (generally more
+restrictive) mapping. An 'Ogg Vorbis Audio Bitstream', for example, has a
+specific physical bitstream structure.
+An 'Ogg A/V' bitstream (not currently specified) will also mandate a
+specific, restricted physical bitstream format.</p>
+
+<h3>additional end-to-end structure</h3>
+
+<p>The <a href="framing.html">framing specification</a> defines
+'beginning of stream' and 'end of stream' page markers via a header
+flag (it is possible for a stream to consist of a single page). A
+stream always consists of an integer number of pages, an easy
+requirement given the variable size nature of pages.</p>
+
+<p>In addition to the header flag marking the first and last pages of a
+logical bitstream, the first page of an Ogg bitstream obeys
+additional restrictions. Each individual media mapping specifies its
+own implementation details regarding these restrictions.</p>
+
+<p>The first page of a logical Ogg bitstream consists of a single,
+small 'initial header' packet that includes sufficient information to
+identify the exact CODEC type and media requirements of the logical
+bitstream. The intent of this restriction is to simplify identifying
+the bitstream type and content; for a given media type (or across all
+Ogg media types) we can know that we only need a small, fixed
+amount of data to uniquely identify the bitstream type.</p>
+
+<p>As an example, Ogg Vorbis places the name and revision of the Vorbis
+CODEC, the audio rate and the audio quality into this initial header,
+thus simplifying vastly the certain identification of an Ogg Vorbis
+audio bitstream.</p>
+
+<h3>sequential multiplexing (chaining)</h3>
+
+<p>The simplest form of logical bitstream multiplexing is concatenation
+(<em>chaining</em>). Complete logical bitstreams are strung
+one-after-another in order. The bitstreams do not overlap; the final
+page of a given logical bitstream is immediately followed by the
+initial page of the next. Chaining is the only logical->physical
+mapping allowed by Ogg Vorbis.</p>
+
+<p>Each chained logical bitstream must have a unique serial number within
+the scope of the physical bitstream.</p>
+
+<h3>concurrent multiplexing (grouping)</h3>
+
+<p>Logical bitstreams may also be multiplexed 'in parallel'
+(<em>grouped</em>). An example of grouping would be to allow
+streaming of separate audio and video streams, using different codecs
+and different logical bitstreams, in the same physical bitstream.
+Whole pages from multiple logical bitstreams are mixed together.</p>
+
+<p>The initial pages of each logical bitstream must appear first; the
+media mapping specifies the order of the initial pages. For example,
+Ogg A/V will eventually specify an Ogg video bitstream with
+audio. The mapping may specify that the physical bitstream must begin
+with the initial page of a logical video bitstream, followed by the
+initial page of an audio stream. Unlike initial pages, terminal pages
+for the logical bitstreams need not all occur contiguously (although a
+specific media mapping may require this; it is not mandated by the
+generic Ogg stream spec). Terminal pages may be 'nil' pages,
+that is, pages containing no content but simply a page header with
+position information and the 'last page of bitstream' flag set in the
+page header.</p>
+
+<p>Each grouped bitstream must have a unique serial number within the
+scope of the physical bitstream.</p>
+
+<h3>sequential and concurrent multiplexing</h3>
+
+<p>Groups of concurrently multiplexed bitstreams may be chained
+consecutively. Such a physical bitstream obeys all the rules of both
+grouped and chained multiplexed streams; the groups, when unchained ,
+must stand on their own as a valid concurrently multiplexed
+bitstream.</p>
+
+<h3>multiplexing example</h3>
+
+<p>Below, we present an example of a grouped and chained bitstream:</p>
+
+<p><img src="stream.png" alt="stream"/></p>
+
+<p>In this example, we see pages from five total logical bitstreams
+multiplexed into a physical bitstream. Note the following
+characteristics:</p>
+
+<ol>
+<li>Grouped bitstreams begin together; all of the initial pages
+must appear before any data pages. When concurrently multiplexed
+groups are chained, the new group does not begin until all the
+bitstreams in the previous group have terminated.</li>
+
+<li>The pages of concurrently multiplexed bitstreams need not conform
+to a regular order; the only requirement is that page <tt>n</tt> of a
+logical bitstream follow page <tt>n-1</tt> in the physical bitstream.
+There are no restrictions on intervening pages belonging to other
+logical bitstreams. (Tying page appearance to bitrate demands is one
+logical strategy, ie, the page appears at the chronological point
+where decode requires more information).</li>
+</ol>
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