📄 rfc2728.txt
字号:
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
Panabaker, et al. Standards Track [Page 1]
RFC 2728 IPVBI November 1999
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.
Panabaker, et al. Standards Track [Page 2]
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.
Panabaker, et al. Standards Track [Page 3]
RFC 2728 IPVBI November 1999
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
Panabaker, et al. Standards Track [Page 4]
RFC 2728 IPVBI November 1999
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.
Panabaker, et al. Standards Track [Page 5]
RFC 2728 IPVBI November 1999
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
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