IPVBI R. Panabaker, Microsoft
Internet Draft S. Wegerif, Philips Semiconductors
D. Zigmond, WebTV Networks
Document: <draft-ietf-ipvbi-nabts-02.txt> February 1999
The Transmission of IP Over the Vertical Blanking Interval
of a Television Signal
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
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Internet-Drafts are draft documents valid for a maximum of six
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A revised version of this draft document will be submitted to the
RFC editor as a Proposed Standard for the Internet Community.
Discussion and suggestions for improvement are requested. This
document will expire before August 1999. Distribution of this draft
is unlimited.
1. Abstract
This document describes a method for broadcasting IP data 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
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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
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.
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.
+-------------------+
| |
| Application |
| |
+-------------------+
| | )
| UDP | )
| | )
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+-------------------+ (-- 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.
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
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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
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:
|Clock|Byte | Packet |Cont.|Packet | User Data |FEC |
|Sync |Sync | Address |Index|Structure| | |
| | | | |Flags | | |
| 2B | 1B | 3B | 1B | 1B | 26B | 2B |
|_____|_____|_____________|_____|_________|___________________|____|
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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 index number is determined by position of the packet
in the FEC bundle mentioned in the FEC section of this document.
The first packet in the bundle has count 0, and the two FEC only
packets at the end have counts 14 and 15 respectively. This allows
the decoder to determine if packets were lost during transmission.
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-bit 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:
Type of Packet PS field value
-------------------------------------------------------
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 a 0x15 followed by as many 0xEA as are needed to fill
the packet. 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.
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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 "bundles". These are arranged as 14 packets of
26 bytes each. A function is applied to the 26 bytes of each packet
to determine the two suffix bytes, which (with the addition of a
header) complete the NABTS packet (8+26+2).
For every 14 packets in the bundle, two additional packets are
appended which contain only FEC data. That is, they contain 28
bytes of FEC data. This data is obtained by first writing the
packets into a table of 16 rows and 28 columns. The same expression
as above can be used on the columns of the table with the suffix now
represented by the bytes at the base of the columns in rows 15 and
16. With NABTS headers on each of these rows, we now have a bundle
of 16 NABTS packets ready for sequential modulation onto lines of
the television signal.
At the receiver, these formulae can be used to verify the validity
of the data with very high accuracy. If single bit errors, double
bit errors, single-byte errors or single- and double-byte erasures
are found in any row or column (including an entire packet lost)
they can be corrected using the algorithms found in the appendix.
The success at correcting errors will depend on the particular
implementation used on the receiver.
3.4. Framing
A framing protocol identical to SLIP is proposed for encapsulating
the packets described in the following section, thus abstracting
this data from the lower protocol layers. This protocol uses two
special characters: END (0xc0) and ESC (0xdb). To send a packet,
the host will send the packet followed by the END character. If a
data byte in the packet is the same code as END character, a two-
byte sequence of ESC (0xdb) and 0xdc is sent instead. If a data
byte is the same code as ESC character, a two-byte sequence of ESC
(0xdb) and 0xdd is sent instead. SLIP implementations are widely
available, see RFC 1055 [Romkey 1988] for more detail.
+--------------+--+------------+--+--+---------+--+
| packet |c0| packet |db|dd| |c0|
+--------------+--+------------+--+--+---------+--+
END ESC END
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The packet framed in this manner shall be formatted according to its
schema type identified by the schema field, which shall start every
packet:
+-----------+---------------------------------------------+
| schema | packet (formatted according to schema) |
| 1B | ??B (schema dependant length) |
+-----------+---------------------------------------------+
In the case where the most significant bit in this field
is set to "1", the length of the field extends to two bytes,
allowing for 32768 additional schemas:
+-----------+---------------------------------------------+
| extended | packet (formatted according to schema) |
| schema | ??B (schema dependant length) |
| 2B | |
+-----------+---------------------------------------------+
In the following section, one such schema for sending IP is
described. This is the only schema specified by this document.
Additional schemas may be proposed for other packet types or other
compression schemes as described in section 7.
3.5. IP Header Compression Scheme
The one-byte scheme defines the format for encoding the IP packet
itself. Due to the value of bandwidth, it may be desirable to
introduce as much efficiency as possible in this encoding. One such
efficiency is the optional compression of the UDP/IP header using a
method similar to the TCP/IP header compression as described by Van
Jacobson (RFC 1144). UDP/IP header compression is not identical due
to the limitation of unidirectional transmission. One such scheme
is proposed in this document for the compression of UDP/IP version
4. It is assigned a value of 0x00. All future encapsulation
schemes should use a unique value in this field.
Only schema 0x00 is defined in this document; this schema must be
supported by all receivers. In schema 0x00, the format of the IP
packet itself takes one of two forms. Packets can be sent with
full, uncompressed headers as follows:
+----+-------+----------------+---------------+-----------+--------+
| 0 | Group | IP Header | UDP Header | Payload | CRC |
| 1b | 7b | 20B | 8B | <1472B | 4B |
| | | (uncompressed) | (uncompressed)| | |
+----+-------+----------------+---------------+-----------+--------+
The first byte in the 0x00 scheme is the Compression Key. It is a
one-byte value: the first bit indicates if the header has been
compressed, and the remaining seven bits indicate the compression
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group it belongs to. If the high bit of the Compression Key is set
to "0", no compression is performed and the full header is sent, as
shown above. The client VBI interface should store the uncompressed
header for future potential use in rebuilding subsequent compressed
headers. Packets with identical group bits are assumed to have
identical UDP/IP headers, with the exception of the "IP
identification" and "UDP checksum" fields.
If the high bit of the Compression Key is set to 1, a compressed
version of the UDP/IP header is sent. The client VBI interface must
then combine the compressed header with the stored uncompressed
header of the same group and recreate a full packet. For compressed
packets, the only portions of the UDP/IP header sent are the "IP
identification" and "UDP checksum" fields:
+----+-------+----------------+---------------+-----------+--------+
| 1 | Group | IP | UDP checksum | Payload | CRC |
| 1b | 7b | identification | 4B | <1472B | 4B |
| | | 4B | | | |
+----+-------+----------------+---------------+-----------+--------+
All datagrams belonging to a multi fragment IP packet shall be sent
with full headers, in the uncompressed header format. Therefore,
only packets that have not been fragmented can be sent with the most
significant bit of the compression key set to "1".
A 32-bit CRC has also been added to the end of every packet in this
scheme to ensure data integrity. It is performed over the entire
packet including schema byte, compression key, and either compressed
or uncompressed headers. It uses the same algorithm as the MPEG-2
transport stream (ISO/IEC 13818-1). The generator polynomial is:
1 + D + D2 + D4 + D5 + D7 + D8 + D10 + D11 + D12 + D16 + D22 + D23 +
D26 + D32
As in the ISO/IEC 13818-1 specification, the initial state of the
sum is 0xFFFFFFFF. This is not the same algorithm used by Ethernet.
This CRC provides a final check for damaged datagrams that spanned
FEC bundles or were not properly corrected by FEC.
4. Addressing Considerations
The addressing of IP packets should adhere to existing standards in
this area. The inclusion of an appropriate source address is needed
to ensure the receiving client can distinguish between sources and
thus services if multiple hosts are sharing an insertion point and
NABTS packet address.
NABTS packet addressing is not regulated or standardized and
requires care to ensure that unrelated services on the same channel
are not broadcasting with the same packet address. This could occur
due to multiple possible NABTS insertion sites, including show
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production, network redistribution, regional operator, and local
operator. Traditionally, the marketplace has recognized this
concern and made amicable arrangements for the distribution of these
addresses for each channel.
5. IANA Considerations
IANA will register new schemes for header compression on a "First
Come First Served" basis [RFC 2434]. Anyone can register a scheme,
so long as they provide a point of contact and a brief description.
The scheme number will be assigned by IANA. Registrants are
encouraged to publish complete specifications for new schemes
(preferably as standards-track RFCs), but this is not required.
6. Security considerations
As with any broadcast network, there are security issues due to the
accessibility of data. It is assumed that the responsibility for
securing data lies in the application layer protocol, which is
beyond the scope of this document.
7. Conclusions
This document provides a method for broadcasting Internet data over
a television signal for reception by client devices. With an
appropriate broadcast content model, this will become an attractive
method of providing data services to end users. By using existing
standards and a layered protocol approach, this document has also
provided a model for data transmission on unidirectional and
broadcast networks.
8. Acknowledgements
The description of the FEC in Appendix A is taken from a document
prepared by Trevor Dee of Norpak Corporation.
9. References
[1] Deering, S. E. 1989. "Host Extensions for IP Multicasting," RFC
1112, 17 pages (Aug.).
[2] EIA-516, "Joint EIA/CVCC Recommended Practice for Teletext:
North American Basic Teletext Specification (NABTS)" Washington:
Electronic Industries Association, c1988
[3] Jack, Keith. "Video Demystified: A Handbook for the Digital
Engineer, Second Edition," San Diego: HighText Pub. c1996.
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[4] Jacobson, V. 1990a. "Compressing TCP/IP Headers for Low-Speed
Serial Links," RFC 1144, 43 pages (Feb.).
[5] Narten, T. Alvestrand, H. 1998. "Guidelines for Writing an IANA
Considerations Section in RFCs," RFC 2434, (Oct.).
[6] Norpak Corporation "TTX71x Programming Reference Manual", c1996,
Kanata, Ontario, Canada
[7] Norpak Corporation, "TES3 EIA-516 NABTS Data Broadcast Encoder
Software User's Manual." c1996, Kanata, Ontario, Canada
[8] Norpak Corporation, "TES3/GES3 Hardware Manual" c1996, Kanata,
Ontario, Canada
[9] Romkey, J. L. 1988. "A Nonstandard for Transmission of IP
Datagrams Over Serial Lines: SLIP," RFC 1055, 6 pages (June).
[10] Stevens, W. Richard. "TCP/IP Illustrated, Volume 1,: The
Protocols" Reading: Addison-Wesley Publishing Company, c1994.
[11] Recommended Practice for Line 21 Data Service (ANSI/EIA-608-94)
(Sept., 1994)
[12] International Telecommunications Union Recommendation.
ITU-R BT.470-5 (02/98) "Conventional TV Systems"
[13] International Telecommunications Union Recommendation.
ITU.R BT.653-2, system C
10. Acronyms
VBI - Vertical Blanking Interval
FEC - Forward Error Correction
NABTS - North American Basic Teletext Standard
NTSC - National Television Standards Committee
PAL - Phase Alternation Line
SECAM - Sequentiel Couleur Avec Memoire (sequential color
with memory)
NTSC-J - NTSC-Japanese
RFC - Request for Comments
IP - Internet Protocol
UDP - User Datagram Protocol
TCP - Transmission Control Protocol
SLIP - Serial Line Internet Protocol
WST - World System Teletext
11. Editors' address and contacts
Ruston Panabaker, co-editor
Microsoft
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One Microsoft Way
Redmond, WA 98052
i-rustop@microsoft.com
Simon Wegerif, co-editor
Philips Semiconductors
811 E. Arques Avenue
M/S 52, P.O. Box 3409
Sunnyvale, CA 94088-3409
Simon.Wegerif@sv.sc.philips.com
Dan Zigmond, WG Chair
WebTV Networks
One Microsoft Way
Redmond, WA 98052
djz@corp.webtv.net
12. Appendix A: Forward Error Correction Specification
This FEC is optimized for data carried in the VBI of a television
signal. Teletext has been in use for many years and data
transmission errors have been categorized in to three main types:
1) Randomly distributed single bit errors
2) Loss of lines of NABTS data
3) Burst Errors
The quantity and distribution of these errors is highly variable and
dependent on many factors. The FEC is designed to fix all these
types of errors.
12.1. Mathematics used in the FEC
Galois fields form the basis for the FEC algorithm presented here.
Rather then explain these fields in general, a specific explanation
is given of the Galois field used in the FEC algorithm.
The Galois field used is GF(2^8) with a primitive element alpha of
00011101. This is a set of 256 elements, along with the operations
of "addition", "subtraction", "division", and "multiplication" on
these elements. An 8-bit binary number represents each element.
The operations of "addition" and "subtraction" are the same for this
Galois field. Both operations are the XOR of two elements. Thus,
the "sum" of 00011011 and 00000111 is 00011100.
Division of two elements is done using long division with
subtraction operations replaced by XOR. For multiplication,
standard long multiplication is used but with the final addition
stage replaced with XOR.
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All arithmetic in the following FEC is done modulo 100011101; for
instance, after you multiply two numbers, you replace the result
with its remainder when divided by 100011101. There are 256 values
in this field (256 possible remainders), the 8-bit numbers. It is
very important to remember that when we write A*B = C, we more
accurately imply modulo(A*B) = C.
It is obvious from the above description that multiplication and
division is time consuming to perform. Elements of the Galois Field
share two important properties with real numbers.
A*B = POWERalpha(LOGalpha(A) + LOGalpha(B))
A/B = POWERalpha(LOGalpha(A) - LOGalpha(B))
The Galois Field is limited to 256 entries so the power and log
tables are limited to 256 entries. The addition and subtraction
shown is standard so the result must be modulo alpha. Written as a
"C" expression:
A*B = apower[alog[A] + alog[B]]
A/B = apower[255 + alog[A] - alog[B]]
You may note that alog[A] + alog[B] can be greater than 255 and
therefore a modulo operation should be performed. This is not
necessary if the power table is extended to 512 entries by repeating
the table. The same logic is true for division as shown. The power
and log tables are calculated once using the long multiplication
shown above.
12.2. Calculating FEC bytes
The FEC algorithm splits a serial stream of data into "bundles".
These are arranged as 16 packets of 28 bytes when the correction
bytes are included. The bundle therefore has 16 horizontal
codewords interleaved with 28 vertical codewords. Two sums are
calculated for a codeword, S0 and S1. S0 is the sum of all bytes of
the codeword each multiplied by alpha to the power of its index in
the codeword. S1 is the sum of all bytes of the codeword each
multiplied by alpha to the power of three times its index in the
codeword. In "C" the sum calculations would look like:
Sum0 = 0;
Sum1 = 0;
For(i = 0;i < m;i++)
{
if(codeword[i] != 0)
{
Sum0 = sum0 ^ power[i + alog[codeword[i]]];
Sum1 = sum1 ^ power[3*i + alog[codeword[i]]];
}
}
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Note that the codeword order is different from the packet order.
Codeword positions 0 and 1 are the suffix bytes at the end of a
packet horizontally or at the end of a column vertically.
When calculating the two FEC bytes, the summation above must produce
two sums of zero. All codewords except 0 and 1 are know so the sums
for the known codewords can be calculated. Let's call these values
tot1 and tot2.
Sum0=0=tot0^power[0+alog[codeword[0]]]^power[1+alog[codeword[1]]]
sum1=0=tot1^power[0+alog[codeword[0]]]^power[3+alog[codeword[1]]]
This gives us two equations with the two unknowns that we can solve:
codeword[1]=power[255+alog[tot0^tot1]-alog[power[1]^power[3]]]
codeword[0]=tot0^power[alog[codeword[1]]+alog[power[1]]]
12.3. Correcting Errors
This section describes the procedure for detecting and correcting
errors using the FEC data calculated above. Upon reception, we
begin by rebuilding the bundle. This is perhaps the most important
part of the procedure because if the bundle is not built correctly
it cannot possibly correct any errors. The continuity index is used
to determine the order of the packets and if any packets are missing
(not captured by the hardware). The recommendation, when building
the bundle, is to convert the bundle from packet order to codeword
order. This conversion will simplify the codeword calculations.
This is done by taking the last byte of a packet and making it the
second byte of the codeword and taking the second last byte of a
packet and making it the first byte of a codeword. Also the packet
with continuity index 15 becomes codeword position one and the
packet with continuity index 14 becomes codeword position zero.
The same FEC is used regardless of the number of bytes in the
codeword. So let's think of the codewords as an array
codeword[vert][hor] where vert is 16 packets and hor is 28. Each
byte in the array is protected by both a horizontal and a vertical
codeword. For each of the codewords, the sums must be calculated.
If both the sums for a codeword are zero then no errors have been
detected for that codeword. Otherwise an error has been detected
and further steps need to be taken to see if the error can be
corrected. In "C" the horizontal summation would look like:
for(i = 0;i < 16;i++)
{
Sum0 = 0;
Sum1 = 0;
for(j = 0;j < hor;j++)
{
If(codeword[i][j] != 0)
{
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Sum0 = sum0 ^ power[j + alog[codeword[i][j]];
Sum1 = sum1 ^ power[3*j + alog[codeword[i][j]];
}
}
if((sum0 != 0) || (sum1 != 0))
{
Try Correcting Packet
}
}
Similarly, vertical looks like:
for(j = 0;i < hor;i++)
{
Sum0 = 0;
Sum1 = 0;
for(i = 0;i < 16;i++)
{
If(codeword[i][j] != 0)
{
Sum0 = sum0 ^ power[i + alog[codeword[i][j]];
Sum1 = sum1 ^ power[3*i + alog[codeword[i][j]];
}
}
if((sum0 != 0) || (sum1 != 0))
{
Try Correcting Column
}
}
12.4. Correction Schemes
This FEC provides four possible corrections:
1) Single bit correction in codeword. All single bit errors.
2) Double bit correction in a codeword. Most two-bit errors.
3) Single byte correction in a codeword. All single-byte errors.
4) Packet replacement. One or two missing packets from a bundle.
12.4.1. Single Bit Correction
When correcting a single-bit in a codeword, the byte and bit
position must be calculated. The equations are:
Byte = 1/2LOGalpha(S1/S0)
Bit = 8LOGalpha(S0/POWERalpha(Byte))
In "C" this is written:
Byte = alog[power[255 + alog[sum1] - alog[sum0]]];
if(Byte & 1)
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Byte = Byte + 255;
Byte = Byte >> 1;
Bit = alog[power[255 + alog[sum0] - Byte]] << 3;
while(Bit > 255)
Bit = Bit - 255;
The error is correctable if Byte is less than the number of bytes in
the codeword and Bit is less than eight. For this math to be valid
both sum0 and sum1 must be non-zero. The codeword is corrected by:
codeword[Byte] = codeword[Byte] ^ (1 << Bit);
12.4.2. Double Bit Correction
Double bit correction is much more complex than single bit
correction for two reasons. First, not all double bit errors are
deterministic. That is two different bit patterns can generate the
same sums. Second, the solution is iterative. To find two bit
errors you assume one bit in error and then solve for the second
error as a single bit error.
The procedure is to iteratively move through the bits of the
codeword changing each bit's state. The new sums are calculated for
the modified codeword. Then the single bit calculation above
determines if this is the correct solution. If not, the bit is
restored and the next bit is tried.
For a long codeword, this can involve many calculations. However,
tricks can speed the process. For example, the vertical sums give a
strong indication of which bytes are in error horizontally. Bits in
other bytes need not be tried.
12.4.3. Single Byte Correction
For single byte correction, the byte position and bits to correct
must be calculated. The equations are:
Byte = 1/2*LOGalpha(S1/S0)
Mask = S0/POWERalpha[Byte]
Notice that the byte position is the same calculation as for single
bit correction. The mask will allow more than one bit in the byte
to be corrected. In "C" the mask calculation looks like:
Mask = power[255 + alog[sum0] - Byte]
Both sum0 and sum1 must be non-zero for the calculations to be
valid. The Byte value must be less than the codeword length but
Mask can be any value. This corrects the byte in the codeword by:
Codeword[Byte] = Codeword[Byte] ^ Mask
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12.4.4. Packet Replacement
If a packet is missing, as determined by the continuity index, then
its byte position is known and does not need to be calculated. The
formula for single packet replacement is therefore the same as for
the Mask calculation of single byte correction. Instead of XORing
an existing byte with the Mask, the Mask replaces the missing
codeword position:
Codeword[Byte] = Mask
When two packets are missing, both the codeword positions are known
by the continuity index. This again gives two equations with two
unknowns, which is solved to give the following equations.
Mask2 = POWERalpha(2*Byte1)*S0+S1
-------------------------------
POWERalpha(2*Byte1+Byte2) +POWERalpha(3*BYTE2)
Mask1 = S0 + Mask2*POWERalpha(Byte2)/POWERalpha(BYTE1)
In "C" these equations are written:
if(sum0 == 0)
{
if(sum1 == 0)
mask2 = 0;
else
mask2=power[255+alog[sum1]-alog[power[byte2+2*byte1]^
power[3*Byte2]]];
}
else
{
if((a=sum1^power[alog[sum0]+2*byte1]) == 0)
mask2 = 0;
else
mask2 = power[255+alog[a]-alog[power[byte2+2*byte1]^
power[3*byte2]]];
}
if(mask2 = 0)
{
if(sum0 == 0)
mask1 = 0;
else
mask1 = power[255+alog[sum0]-byte1];
}
else
{
if((a=sum0^power[alog[mask2] + byte2]) == 0)
mask1 = 0;
else
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mask1 = power[255+alog[a]-byte1];
}
Notice that, in the code above, care is taken to check for zero
values. The missing codeword position can be fixed by:
codeword[byte1] = mask1;
codeword[byte2] = mask2;
12.5. FEC Performance Considerations
The section above shows how to correct the different types of
errors. It does not suggest how these corrections may be used in an
algorithm to correct a bundle. There are many possible algorithms
and the one chosen depends on many variables. These include:
. The amount of processing power available.
. The number of packets per VBI to process.
. The type of hardware capturing the data.
. The delivery path of the VBI.
. How the code is implemented.
As a minimum, it is recommended that the algorithm use single bit or
single byte correction for one pass in each direction followed by
packet replacement if appropriate. It is possible to do more than
one pass of error correction in each direction. The theory is that
errors not corrected in the first pass may be corrected in the
second pass because error correction in the other direction has
removed some errors.
In making choices, it is important to remember that the code has
several possible states:
1)Shows codeword as correct and it is.
2)Shows codeword as correct and it is not (detection failure).
3)Shows codeword as incorrect but cannot correct (detection).
4)Shows codeword as incorrect and corrects it correctly
(correction).
5)Shows codeword as incorrect but corrects wrong bits (false
correction).
There is actually overlap among the different types of errors. For
example, a pair of sums may indicate both a double bit error and a
byte error. It is not possible to know at the code level which is
correct and which is a false correction. In fact, neither might be
correct if both are false corrections.
If you know something about the types of errors in the delivery
channel, you can greatly improve efficiency. If you know that
errors are randomly distributed (as in a weak terrestrial broadcast)
then single and double bit correction are more powerful than single
byte.
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12.6. Appendix References
[1] Norpak Corporation "TTX71x Programming Reference Manual", c1996,
Kanata, Ontario, Canada
[2] Norpak Corporation, "TES3 EIA-516 NABTS Data Broadcast Encoder
Software User's Manual." c1996, Kanata, Ontario, Canada
[3] Norpak Corporation, "TES3/GES3 Hardware Manual" c1996, Kanata,
Ontario, Canada
[4] Pretzel, Oliver. "Correcting Codes and Finite Fields: Student
Edition" OUP, c1996
[5] Rorabaugh, C. Britton. "Error Coding Cookbook" McGraw Hill,
c1996
[6] Mortimer, Brian. "An Error-correction system for the Teletext
Transmission in the Case of Transparent Data" c1989 Department of
Mathematics and Statistics, Carleton University, Ottawa Canada
13. Appendix B: Architecture
The architecture that this document is addressing can be broken down
into three areas: insertion, distribution network, and receiving
client.
The insertion of IP data onto the television signal can occur at any
part of the delivery system. A VBI encoder typically accepts a
video signal and an asynchronous serial stream of framed IP as
inputs and packetizes the data onto a selected set of lines using
NABTS and an FEC. This composite signal is then modulated with
other channels before being broadcast onto the distribution network.
Operators further down the distribution chain could then add their
own data, to other unused lines, as well.
The distribution networks include coax plant, off-air, and analog
satellite systems and are primarily unidirectional broadcast
networks. They must provide a signal to noise ratio, which is
sufficient for FEC to recover any lost data for the broadcast of
data to be effective.
The receiving client must be capable of tuning, NABTS waveform
sampling as appropriate, filtering on NABTS addresses as
appropriate, forward error correction, unframing, verification of
the CRC and decompressing the UDP/IP header if they are compressed.
All of the above functions can be carried out in PC software and
inexpensive off-the-shelf hardware.
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14. Appendix C: Scope of proposed protocols
The protocols described in this document are for transmitting IP
packets. However, their scope may be extensible to other
applications outside this area. Many of the protocols in this
document could be implemented on any unidirectional network.
The unidirectional framing protocol provides encapsulation of IP
datagrams on the serial stream, and the compression of the UDP/IP
headers reduces the overhead on transmission, thus conserving
bandwidth. These two protocols could be widely used on different
unidirectional broadcast networks or modulation schemes to
efficiently transport any type of packet data. In particular, new
versions of Internet protocols can be supported to provide a
standardized method of data transport.
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Datatracker
draft-ietf-ipvbi-nabts-02
Internet-Draft
