rfc2728.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                    R. Panabaker
Request for Comments: 2728                                  Microsoft
Category: Standards Track                                  S. Wegerif
                                               Philips Semiconductors
                                                           D. Zigmond
                                                       WebTV Networks
                                                        November 1999


    The Transmission of IP Over the Vertical Blanking Interval of a
                           Television Signal


Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Abstract

   This document describes a method for broadcasting IP data in a
   unidirectional manner using the vertical blanking interval of
   television signals.  It includes a description for compressing IP
   headers on unidirectional networks, a framing protocol identical to
   SLIP, a forward error correction scheme, and the NABTS byte
   structures.

2. Introduction

   This RFC proposes several protocols to be used in the transmission of
   IP datagrams using the Vertical Blanking Interval (VBI) of a
   television signal.  The VBI is a non-viewable portion of the
   television signal that can be used to provide point-to-multipoint IP
   data services which will relieve congestion and traffic in the
   traditional Internet access networks.  Wherever possible these
   protocols make use of existing RFC standards and non-standards.

   Traditionally, point-to-point connections (TCP/IP) have been used
   even for the transmission of broadcast type data.  Distribution of
   the same content--news feeds, stock quotes, newsgroups, weather




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   reports, and the like--are typically sent repeatedly to individual
   clients rather than being broadcast to the large number of users who
   want to receive such data.

   Today, IP is quickly becoming the preferred method of distributing
   one-to-many data on intranets and the Internet. The coming
   availability of low cost PC hardware for receiving television signals
   accompanied by broadcast data streams makes a defined standard for
   the transmission of data over traditional broadcast networks
   imperative.  A lack of standards in this area as well as the expense
   of hardware has prevented traditional broadcast networks from
   becoming effective deliverers of data to the home and office.

   This document describes the transmission of IP using the North
   American Basic Teletext Standard (NABTS), a recognized and industry-
   supported method of transporting data on the VBI.  NABTS is
   traditionally used on 525-line television systems such as NTSC.
   Another byte structure, WST, is traditionally used on 625-line
   systems such as PAL and SECAM.  These generalizations have
   exceptions, and countries should be treated on an individual basis.
   These existing television system standards will enable the television
   and Internet communities to provide inexpensive broadcast data
   services.  A set of existing protocols will be layered above the
   specific FEC for NABTS including a serial stream framing protocol
   similar to SLIP (RFC 1055 [Romkey 1988]) and a compression technique
   for unidirectional UDP/IP headers.

   The protocols described in this document are intended for the
   unidirectional delivery of IP datagrams using the VBI.  That is, no
   return channel is described, or for that matter possible, in the VBI.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

3. Proposed protocol stack

   The following protocol stack demonstrates the layers used in the
   transmission of VBI data.  Each layer has no knowledge of the data it
   encapsulates, and is therefore abstracted from the other layers. At
   the link layer, the NABTS protocol defines the modulation scheme used
   to transport data on the VBI.  At the network layer, IP handles the
   movement of data to the appropriate clients.  In the transport layer,
   UDP determines the flow of data to the appropriate processes and
   applications.






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RFC 2728                         IPVBI                     November 1999


              +-------------------+
              |                   |
              |    Application    |
              |                   |
              +-------------------+
              |                   |  )
              |        UDP        |   )
              |                   |   )
              +-------------------+   (-- IP
              |                   |   )
              |        IP         |   )
              |                   |  )
              +-------------------+
              |    SLIP-style     |
              |   encapsulation   |
              |                   |
              +-------------------+
              |        FEC        |
              |-------------------|
              |       NABTS       |
              |                   |
              +---------+---------+
              |                   |
              |     NTSC/other    |
              |                   |
              +-------------------+
                        |
                        |
                        |            cable, off-air, etc.
                        +--------<----<----<--------

   These protocols can be described in a bottom up component model using
   the example of NABTS carried over NTSC modulation as follows:

   Video signal --> NABTS --> FEC --> serial data stream --> Framing
   protocol --> compressed UDP/IP headers --> application data

   This diagram can be read as follows: television signals have NABTS
   packets, which contain a Forward Error Correction (FEC) protocol,
   modulated onto them.  The data contained in these sequential, ordered
   packets form a serial data stream on which a framing protocol
   indicates the location of IP packets, with compressed headers,
   containing application data.

   The structure of these components and protocols are described in
   following subsections.





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3.1. VBI

   The characteristics and definition of the VBI is dependent on the
   television system in use, be it NTSC, PAL, SECAM or some other.  For
   more information on Television standards worldwide, see ref [12].

3.1.1. 525 line systems

   A 525-line television frame is comprised of two fields of 262.5
   horizontal scan lines each.  The first 21 lines of each field are not
   part of the visible picture and are collectively called the Vertical
   Blanking Interval (VBI).

   Of these 21 lines, the first 9 are used while repositioning the
   cathode ray to the top of the screen, but the remaining lines are
   available for data transport.

   There are 12 possible VBI lines being broadcast 60 times a second
   (each field 30 times a second).  In some countries Line 21 is
   reserved for the transport of closed captioning data (Ref.[11]).  In
   that case, there are 11 possible VBI lines, some or all of which
   could be used for IP transport.  It should be noted that some of
   these lines are sometimes used for existing, proprietary, data and
   testing services. IP delivery therefore becomes one more data service
   using a subset or all of these lines.

3.1.2. 625 Line Systems

   A 625-line television frame is comprised of two fields of 312.5
   horizontal scan lines each.  The first few lines of each field are
   used while repositioning the cathode ray to the top of the screen.
   The lines available for data insertion are 6-22 in the first field
   and 319-335 in the second field.

   There are, therefore, 17 possible VBI lines being broadcast 50 times
   a second (each field 25 times a second), some or all of which could
   be used for IP transport.  It should be noted that some of these
   lines are sometimes used for existing, proprietary, data and testing
   services. IP, therefore, becomes one more data service using a subset
   or all of these lines.

3.2. NABTS

   The North American Basic Teletext Standard is defined in the
   Electronic Industry Association's EIA-516, Ref. [2], and ITU.R
   BT.653-2, system C, Ref. [13].  It provides an industry-accepted





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   method of modulating data onto the VBI, usually of an NTSC signal.
   This section describes the NABTS packet format as it is used for the
   transport of IP.

   It should be noted that only a subset of the NABTS standard is used,
   as is common practice in NABTS implementations.  Further information
   concerning the NABTS standard and its implementation can be found in
   EIA-516.

   The NABTS packet is a 36-byte structure encoded onto one horizontal
   scan line of a television signal having the following structure:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            clock sync         |   byte sync   |  packet addr. |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  packet address (cont.)       |  cont. index  |PcktStructFlags|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      user data (26 bytes)                     |
   :                                                               :
   :                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                               |              FEC              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The two-byte Clock Synchronization and one-byte Byte Synchronization
   are located at the beginning of every scan line containing a NABTS
   packet and are used to synchronize the decoding sampling rate and
   byte timing.

   The three-byte Packet Address field is Hamming encoded (as specified
   in EIA-516), provides 4 data bits per byte, and thus provides 4096
   possible packet addresses.  These addresses are used to distinguish
   related services originating from the same source.  This is necessary
   for the receiver to determine which packets are related, and part of
   the same service.  NABTS packet addresses therefore distinguish
   different data services, possibly inserted at different points of the
   transmission system, and most likely totally unrelated.  Section 4 of
   this document discusses Packet Addresses in detail.

   The one-byte Continuity Index field is a Hamming encoded byte, which
   is incremented by one for each subsequent packet of a given Packet
   Address.  The value or number of the Continuity Index sequences from
   0 to 15. It increments by one each time a data packet is transmitted.
   This allows the decoder to determine if packets were lost during
   transmission.





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   The Packet Structure field is also a Hamming encoded byte, which
   contains information about the structure of the remaining portions of
   the packet.  The least significant bit is always "0" in this
   implementation.  The second least significant bit specifies if the
   Data Block is full--"0" indicates the data block is full of useful
   data, and "1" indicates some or all of the data is filler data.  The
   two most significant bits are used to indicate the length of the
   suffix of the Data Block--in this implementation, either 2 or 28
   bytes (10 for 2-byte FEC suffix, 11 for 28-byte FEC suffix).  This
   suffix is used for the forward error correction described in the next
   section.  The following table shows the possible values of the Packet
   Structure field:

         Data Packet, no filler                     D0
         Data Packet, with filler                   8C
         FEC Packet                                 A1

   The Data Block field is 26 bytes, zero to 26 of which is useful data
   (part of a IP packet or SLIP frame), the remainder is filler data.
   Data is byte-ordered least significant bit first. Filler data is
   indicated by an Ox15 followed by as many OxEA as are needed to fill
   the Data Block field. Sequential data blocks minus the filler data
   form an asynchronous serial stream of data.

   These NABTS packets are modulated onto the television signal
   sequentially and on any combination of lines.

3.3. FEC

   Due to the unidirectional nature of VBI data transport, Forward Error
   Correction (FEC) is needed to ensure the integrity of data at the
   receiver.  The type of FEC described here and in the appendix of this
   document for NABTS has been in use for a number of years, and has
   proven popular with the broadcast industry.  It is capable of
   correcting single-byte errors and single- and double-byte erasures in
   the data block and suffix of a NABTS packet.  In a system using
   NABTS, the FEC algorithm splits a serial stream of data into 364-byte