# Ogg logical and physical bitstream overview

## Ogg bitstreams

Ogg codecs use octet vectors of raw, compressed data (packets). 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.

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.

## Logical and physical bitstreams

Raw packets are grouped and encoded into contiguous pages of structured bitstream data called logical bitstreams. 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.

Multiple logical bitstreams can be combined (with restrictions) into a single physical bitstream. 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'.

Ogg Logical Bitstream Framing 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.

## Mapping Restrictions

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.

### additional end-to-end structure

The framing specification 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.

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.

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.

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.

### sequential multiplexing (chaining)

The simplest form of logical bitstream multiplexing is concatenation (chaining). 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.

Each chained logical bitstream must have a unique serial number within the scope of the physical bitstream.

### concurrent multiplexing (grouping)

Logical bitstreams may also be multiplexed 'in parallel' (grouped). 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.

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.

Each grouped bitstream must have a unique serial number within the scope of the physical bitstream.

### sequential and concurrent multiplexing

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.

### multiplexing example

Below, we present an example of a grouped and chained bitstream:

In this example, we see pages from five total logical bitstreams multiplexed into a physical bitstream. Note the following characteristics:

1. 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.
2. The pages of concurrently multiplexed bitstreams need not conform to a regular order; the only requirement is that page `n` of a logical bitstream follow page `n-1` 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).

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