ISO 22642:2005

INTERNATIONAL ISO
STANDARD 22642
First edition
2005-07-15
Space data and information transfer
systems — TC (telecommand)
synchronization and channel coding
Systèmes de transfert des données et informations spatiales —
Synchronisation TC (télécommande) et codage de canal

Reference number
©
ISO 2005
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 22642 was prepared by the Consultative Committee for Space Data Systems (CCSDS) (as CCSDS
231.0-B-1, September 2003) and was adopted (without modifications except those stated in Clause 2 of this
International Standard) by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 13, Space data and information transfer systems.

INTERNATIONAL STANDARD ISO 22642:2005(E)

Space data and information transfer systems —
TC (telecommand) synchronization and channel coding
1 Scope
This International Standard defines synchronization and channel coding schemes used with the telecommand
(TC) space data link protocol as defined in ISO 22664. These schemes are to be used over ground-to-space
or space-to-space communications links by space missions.
The scope and field of application are furthermore detailed in subclauses 1.1 and 1.2 of the enclosed CCSDS
publication.
2 Requirements
Requirements are the technical recommendations made in the following publication (reproduced on the
following pages), which is adopted as an International Standard:
CCSDS 231.0-B-1, September 2003, TC synchronization and channel coding.
For the purposes of international standardization, the modifications outlined below shall apply to the specific
clauses and paragraphs of publication CCSDS 231.0-B-1.
Pages i to v
This part is information which is relevant to the CCSDS publication only.
Page 1-5
Add the following information to the reference indicated:
[1] Document CCSDS 232.0-B-1, September 2003, is equivalent to ISO 22664:2005.
Page B-1
Add the following information to the reference indicated:
[B2] Document CCSDS 201.0-B-3, June 2000, is equivalent to ISO 12171:2002.
It has been agreed with the Consultative Committee for Space Data Systems that Subcommittee
ISO/TC 20/SC 13 will be consulted in the event of any revision or amendment of publication CCSDS 231.0-B-1.
To this end, NASA will act as a liaison body between CCSDS and ISO.

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RECOMMENDATION FOR SPACE
DATA SYSTEM STANDARDS
TC SYNCHRONIZATION
AND CHANNEL CODING
CCSDS 231.0-B-1
BLUE BOOK
September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
AUTHORITY
Issue: Blue Book, Issue 1
Date: September 2003
Location: Not Applicable
This document has been approved for publication by the Management Council of the
Consultative Committee for Space Data Systems (CCSDS) and represents the consensus
technical agreement of the participating CCSDS Member Agencies. The procedure for
review and authorization of CCSDS Recommendations is detailed in reference [B1], and the
record of Agency participation in the authorization of this document can be obtained from
the CCSDS Secretariat at the address below.

This Recommendation is published and maintained by:

CCSDS Secretariat
Office of Space Communication (Code M-3)
National Aeronautics and Space Administration
Washington, DC 20546, USA
CCSDS 231.0-B-1 Page i September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
STATEMENT OF INTENT
The Consultative Committee for Space Data Systems (CCSDS) is an organization officially
established by the management of member space Agencies. The Committee meets
periodically to address data systems problems that are common to all participants, and to
formulate sound technical solutions to these problems. Inasmuch as participation in the
CCSDS is completely voluntary, the results of Committee actions are termed
Recommendations and are not considered binding on any Agency.
This Recommendation is issued by, and represents the consensus of, the CCSDS Plenary
body. Agency endorsement of this Recommendation is entirely voluntary. Endorsement,
however, indicates the following understandings:
– Whenever an Agency establishes a CCSDS-related standard, this standard will be in
accord with the relevant Recommendation. Establishing such a standard does not
preclude other provisions which an Agency may develop.
– Whenever an Agency establishes a CCSDS-related standard, the Agency will provide
other CCSDS member Agencies with the following information:
• The standard itself.
• The anticipated date of initial operational capability.
• The anticipated duration of operational service.
– Specific service arrangements are made via memoranda of agreement. Neither this
Recommendation nor any ensuing standard is a substitute for a memorandum of
agreement.
No later than five years from its date of issuance, this Recommendation will be reviewed by
the CCSDS to determine whether it should: (1) remain in effect without change; (2) be
changed to reflect the impact of new technologies, new requirements, or new directions; or,
(3) be retired or canceled.
In those instances when a new version of a Recommendation is issued, existing CCSDS-
related Agency standards and implementations are not negated or deemed to be non-CCSDS
compatible. It is the responsibility of each Agency to determine when such standards or
implementations are to be modified. Each Agency is, however, strongly encouraged to direct
planning for its new standards and implementations towards the later version of the
Recommendation.
CCSDS 231.0-B-1 Page ii September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
FOREWORD
This document is a technical Recommendation for use in developing synchronization and
channel coding systems and has been prepared by the Consultative Committee for Space
Data Systems (CCSDS). The synchronization and channel coding concept described herein
is intended for missions that are cross-supported between Agencies of the CCSDS.
This Recommendation establishes a common framework and provides a common basis for
the synchronization and channel coding schemes to be used by space missions with the TC
Space Data Link Protocol (reference [1]) over ground-to-space and space-to-space
communications links. This Recommendation was developed from an older CCSDS
Recommendation [B2], which defines essentially the same schemes but in a slightly different
context.
This Recommendation does not change the major technical content defined in [B2], but the
presentation of the specification has been changed so that:
a) these schemes can be used to transfer any data over any space link in either direction;
b) all CCSDS space link protocols are specified in a unified manner;
c) the layered model matches the Open Systems Interconnection (OSI) Basic Reference
Model (reference [2]).
Together with the change in presentation, a few technical specifications in [B2] have been
changed in order to define all Space Data Link Protocols in a unified way. Also, some
technical terms in reference [B2] have been changed in order to unify the terminology used
in all the CCSDS Recommendations that define space link protocols and to define these
schemes as general communications schemes. These changes are listed in annex C of this
Recommendation.
Through the process of normal evolution, it is expected that expansion, deletion or
modification to this document may occur. This Recommendation is therefore subject to
CCSDS document management and change control procedures, as defined in the Procedures
Manual for the Consultative Committee for Space Data Systems. Current versions of
CCSDS documents are maintained at the CCSDS Web site:
http://www.ccsds.org/
Questions relating to the contents or status of this document should be addressed to the
CCSDS Secretariat at the address indicated on page i.
CCSDS 231.0-B-1 Page iii September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

At time of publication, the active Member and Observer Agencies of the CCSDS were:

Member Agencies
– Agenzia Spaziale Italiana (ASI)/Italy.
– British National Space Centre (BNSC)/United Kingdom.
– Canadian Space Agency (CSA)/Canada.
– Centre National d’Etudes Spatiales (CNES)/France.
– Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)/Germany.
– European Space Agency (ESA)/Europe.
– Instituto Nacional de Pesquisas Espaciais (INPE)/Brazil.
– National Aeronautics and Space Administration (NASA)/USA.
– National Space Development Agency of Japan (NASDA)/Japan.
– Russian Space Agency (RSA)/Russian Federation.

Observer Agencies
– Austrian Space Agency (ASA)/Austria.
– Central Research Institute of Machine Building (TsNIIMash)/Russian Federation.
– Centro Tecnico Aeroespacial (CTA)/Brazil.
– Chinese Academy of Space Technology (CAST)/China.
– Commonwealth Scientific and Industrial Research Organization (CSIRO)/Australia.
– Communications Research Laboratory (CRL)/Japan.
– Danish Space Research Institute (DSRI)/Denmark.
– European Organization for the Exploitation of Meteorological Satellites
(EUMETSAT)/Europe.
– European Telecommunications Satellite Organization (EUTELSAT)/Europe.
– Federal Service of Scientific, Technical & Cultural Affairs (FSST&CA)/Belgium.
– Hellenic National Space Committee (HNSC)/Greece.
– Indian Space Research Organization (ISRO)/India.
– Institute of Space and Astronautical Science (ISAS)/Japan.
– Institute of Space Research (IKI)/Russian Federation.
– KFKI Research Institute for Particle & Nuclear Physics (KFKI)/Hungary.
– MIKOMTEK: CSIR (CSIR)/Republic of South Africa.
– Korea Aerospace Research Institute (KARI)/Korea.
– Ministry of Communications (MOC)/Israel.
– National Oceanic & Atmospheric Administration (NOAA)/USA.
– National Space Program Office (NSPO)/Taipei.
– Space and Upper Atmosphere Research Commission (SUPARCO)/Pakistan.
– Swedish Space Corporation (SSC)/Sweden.
– United States Geological Survey (USGS)/USA.
CCSDS 231.0-B-1 Page iv September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
DOCUMENT CONTROL
Document Title and Issue Date Status
CCSDS TC Synchronization and Channel September Original Issue
231.0-B-1 Coding, Issue 1 2003

CCSDS 231.0-B-1 Page v September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

CONTENTS
Section Page
1 INTRODUCTION. 1-1

1.1 PURPOSE.1-1
1.2 SCOPE.1-1
1.3 APPLICABILITY.1-1
1.4 RATIONALE.1-1
1.5 DOCUMENT STRUCTURE.1-2
1.6 CONVENTIONS AND DEFINITIONS. 1-2
1.7 REFERENCES.1-4

2 OVERVIEW. 2-1

2.1 ARCHITECTURE.2-1
2.2 SUMMARY OF FUNCTIONS . 2-1
2.3 INTERNAL ORGANIZATION OF SUBLAYER. 2-3

3 BCH CODING . 3-1
3.1 INTRODUCTION.3-1
3.2 CODEBLOCK FORMAT.3-1
3.3 ENCODING PROCEDURE.3-1
3.4 FILL BITS.3-2
3.5 DECODING PROCEDURE.3-3

4 COMMUNICATIONS LINK TRANSMISSION UNIT . 4-1

4.1 INTRODUCTION.4-1
4.2 CLTU UNIT FORMAT. 4-1
4.3 CLTU RECEPTION LOGIC. 4-2

5 RANDOMIZER . 5-1

5.1 INTRODUCTION.5-1
5.2 RANDOMIZER DESCRIPTION.5-1
5.3 APPLICATION OF THE RANDOMIZER. 5-2

6 PHYSICAL LAYER OPERATIONS PROCEDURES. 6-1

6.1 INTRODUCTION.6-1
6.2 DATA FORMATS.6-1
6.3 CARRIER MODULATION MODES . 6-2
6.4 PLOP-1.6-2
CCSDS 231.0-B-1 Page vi September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

CONTENTS (continued)
Section Page
6.5 PLOP-2.6-3

7 MANAGED PARAMETERS . 7-1

7.1 OVERVIEW OF MANAGED PARAMETERS . 7-1
7.2 MANAGED PARAMETERS FOR BCH CODING . 7-1
7.3 MANAGED PARAMETERS FOR CLTU . 7-1
7.4 MANAGED PARAMETERS FOR THE RANDOMIZER . 7-2
7.5 MANAGED PARAMETERS FOR PLOPS. 7-2

ANNEX A ACRONYMS AND TERMS . A-1
ANNEX B INFORMATIVE REFERENCES.B-1
ANNEX C SERVICE DEFINITION. C-1
ANNEX D CHANGES FROM REFERENCE [B2]. D-1

Figure
1-1 Bit Numbering Convention. 1-4
2-1 Relationship with OSI Layers. 2-1
2-2 Internal Organization of the Sublayer at the Sending End . 2-3
3-1 BCH Codeblock Format . 3-1
3-2 (63,56) Modified BCH Code Generator . 3-2
4-1 Components of the CLTU. 4-1
4-2 CLTU Reception State Diagram (Receiving End) . 4-2
5-1 Bit Transition Generator Logic Diagram. 5-2
6-1 Sequence of CMMs Composing PLOP-1. 6-3
6-2 Sequence of CMMs Composing PLOP-2. 6-4

Table
4-1 CLTU Reception States (Receiving End). 4-3
4-2 CLTU Reception Events (Receiving End) . 4-4
6-1 Carrier Modulation Modes . 6-2
7-1 Managed Parameters for BCH Coding . 7-1
7-2 Managed Parameters for CLTU. 7-1
7-3 Managed Parameters for Randomizer. 7-2
7-4 Managed Parameters for PLOPs. 7-2
D-1 Terms That Have Been Changed from Reference [B2]. D-2

CCSDS 231.0-B-1 Page vii September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

1 INTRODUCTION
1.1 PURPOSE
The purpose of this Recommendation is to specify synchronization and channel coding
schemes used with the Telecommand (TC) Space Data Link Protocol [1]. These schemes are
to be used over ground-to-space or space-to-space communications links by space missions.
1.2 SCOPE
This Recommendation defines synchronization and channel coding schemes in terms of:
a) the services provided to the users of this specification;
b) data formats; and
c) the procedures performed to generate and process the data formats.
It does not specify:
a) individual implementations or products;
b) the methods or technologies required to perform the procedures; or
c) the management activities required to configure and control the system.
1.3 APPLICABILITY
This Recommendation applies to the creation of Agency standards and to the future data
communications over space links between CCSDS Agencies in cross-support situations.
This Recommendation includes comprehensive specification of the data formats and
procedures for inter-Agency cross support. It is neither a specification of, nor a design for,
real systems that may be implemented for existing or future missions.
The Recommendation specified in this document is to be invoked through the normal
standards programs of each CCSDS Agency, and is applicable to those missions for which
cross support, based on capabilities described in this Recommendation, is anticipated.
Where mandatory capabilities are clearly indicated in sections of this Recommendation, they
must be implemented when this document is used as a basis for cross support. Where
options are allowed or implied, implementation of these options is subject to specific
bilateral cross support agreements between the Agencies involved.
1.4 RATIONALE
The CCSDS believes it is important to document the rationale underlying the
recommendations chosen, so that future evaluations of proposed changes or improvements
will not lose sight of previous decisions.
CCSDS 231.0-B-1 Page 1-1 September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

1.5 DOCUMENT STRUCTURE
This document is divided into seven numbered sections and four annexes:
a) section 1 presents the purpose, scope, applicability and rationale of this
Recommendation and lists the conventions, definitions, and references used
throughout the Recommendation;
b) section 2 provides an overview of synchronization and channel coding;
c) section 3 specifies the Bose-Chaudhuri-Hocquenghem (BCH) coding;
d) section 4 specifies the Communications Link Transmission Unit (CLTU);
e) section 5 specifies the randomizer;
f) section 6 specifies the Physical Layer Operations Procedures (PLOPs);
g) section 7 lists the managed parameters associated with synchronization and channel
coding;
h) annex A lists acronyms and terms used within this document;
i) annex B provides a list of informative references;
j) annex C defines the service provided to the users;
k) annex D lists the changes from the older CCSDS Recommendation [B2].
1.6 CONVENTIONS AND DEFINITIONS
1.6.1 DEFINITIONS
1.6.1.1 Definitions from the Open Systems Interconnection (OSI) Basic Reference
Model
This Recommendation makes use of a number of terms defined in reference [2]. The use of
those terms in this Recommendation shall be understood in a generic sense; i.e., in the sense
that those terms are generally applicable to any of a variety of technologies that provide for
the exchange of information between real systems. Those terms are as follows:
a) Data Link Layer;
b) Physical Layer;
c) service;
d) service data unit.
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
1.6.1.2 Definitions from OSI Service Definition Conventions
This Recommendation makes use of a number of terms defined in reference [3]. The use of
those terms in this Recommendation shall be understood in a generic sense; i.e., in the sense
that those terms are generally applicable to any of a variety of technologies that provide for
the exchange of information between real systems. Those terms are as follows:
a) indication;
b) primitive;
c) request;
d) service provider;
e) service user.
1.6.1.3 Terms Defined in This Recommendation
For the purposes of this Recommendation, the following definitions apply. Many other terms
that pertain to specific items are defined in the appropriate sections.
asynchronous: not synchronous.
Mission Phase: a period of a mission during which specified communications
characteristics are fixed. The transition between two consecutive mission phases may cause
an interruption of the communications services.
Physical Channel: a stream of bits transferred over a space link in a single direction.
space link: a communications link between a spacecraft and its associated ground system or
between two spacecraft. A space link consists of one or more Physical Channels in one or
both directions.
synchronous: a sequence of events occurring in a fixed time relationship (within specified
tolerance) to another sequence of events.
CCSDS 231.0-B-1 Page 1-3 September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

1.6.2 NOMENCLATURE
The following conventions apply throughout this Recommendation:
a) the words ‘shall’ and ‘must’ imply a binding and verifiable specification;
b) the word ‘should’ implies an optional, but desirable, specification;
c) the word ‘may’ implies an optional specification;
d) the words ‘is’, ‘are’, and ‘will’ imply statements of fact.
1.6.3 CONVENTIONS
In this document, the following convention is used to identify each bit in an N-bit field. The
first bit in the field to be transmitted (i.e., the most left justified when drawing a figure) is
defined to be ‘Bit 0’; the following bit is defined to be ‘Bit 1’ and so on up to ‘Bit N-1’.
When the field is used to express a binary value (such as a counter), the Most Significant Bit
(MSB) shall be the first transmitted bit of the field, i.e., ‘Bit 0’ (see figure 1-1).
BIT N-1
BIT 0
N-BIT DATA FIELD
FIRST BIT TRANSMITTED = MSB
Figure 1-1: Bit Numbering Convention
In accordance with standard data-communications practice, data fields are often grouped into
8-bit ‘words’ which conform to the above convention. Throughout this Recommendation,
such an 8-bit word is called an ‘octet’. The numbering for octets within a data structure
starts with 0. By CCSDS convention, all ‘spare’ bits shall be permanently set to ‘0’.
1.7 REFERENCES
The following documents contain provisions which, through reference in this text, constitute
provisions of this Recommendation. At the time of publication, the editions indicated were
valid. All documents are subject to revision, and users of this Recommendation are
encouraged to investigate the possibility of applying the most recent editions of the
documents indicated below. The CCSDS Secretariat maintains a register of currently valid
CCSDS Recommendations.
CCSDS 231.0-B-1 Page 1-4 September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
[1] TC Space Data Link Protocol. Recommendation for Space Data Systems Standards,
CCSDS 232.0-B-1. Blue Book. Issue 1. Washington, D.C.: CCSDS, September 2003.
[2] Information Technology—Open Systems Interconnection—Basic Reference Model:
The Basic Model. International Standard, ISO/IEC 7498-1. 2nd ed. Geneva: ISO,
1994.
[3] Information Technology—Open Systems Interconnection—Basic Reference Model—
Conventions for the definition of OSI services. International Standard, ISO/IEC
10731:1994. Geneva: ISO, 1994.
[4] Radio Frequency and Modulation Systems—Part 1: Earth Stations and Spacecraft.
Recommendation for Space Data Systems Standards, CCSDS 401.0-B. Blue Book.
Washington, D.C.: CCSDS, March 2003.
NOTE – Informative references are listed in annex B.

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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
2 OVERVIEW
2.1 ARCHITECTURE
Figure 2-1 illustrates the relationship of this Recommendation to the Open Systems
Interconnection (OSI) reference model (reference [2]). Two sublayers of the Data Link
Layer are defined for CCSDS space link protocols. The TC Space Data Link Protocols
specified in reference [1] corresponds to the Data Link Protocol Sublayer, and provides
functions for transferring data using the protocol data unit called the Transfer Frame. The
Synchronization and Channel Coding Sublayer provides additional functions necessary for
transferring Transfer Frames over a space link. These functions are error-control
coding/decoding, delimiting/synchronizing codeblocks, and bit transition
generation/removal.
CCSDS
OSI LAYERS CCSDS LAYERS
PROTOCOLS
NETWORK AND NETWORK AND
UPPER LAYERS UPPER LAYERS
DATA LINK
TC SPACE DATA LINK
PROTOCOL
PROTOCOL
SUBLAYER
DATA LINK LAYER
SYNCHRONIZATION TC SYNCHRONIZATION
AND CHANNEL AND
CODING SUBLAYER CHANNEL CODING
PHYSICAL LAYER PHYSICAL LAYER
Figure 2-1: Relationship with OSI Layers
2.2 SUMMARY OF FUNCTIONS
2.2.1 FUNCTIONS
The Synchronization and Channel Coding Sublayer provides the following three functions
for transferring Transfer Frames over a space link:
a) error-control coding;
b) synchronization; and
c) pseudo-randomizing (optional).
CCSDS 231.0-B-1 Page 2-1 September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

2.2.2 ERROR-CONTROL CODING
This Recommendation specifies an error-control coding method using a modified BCH code.
This is described in section 3.
The modified BCH code specified in this Recommendation may be decoded either in an
error-detecting mode or in an error-correcting mode, depending on mission requirements. If
the modified BCH code is decoded in an error-detecting mode, the Frame Error Control Field
(FECF) defined in reference [1] may be used to reduce the probability of undetected errors.
NOTE – In this Recommendation, the characteristics of the codes are specified only to the
extent necessary to ensure interoperability and cross-support. The specification
does not attempt to quantify the relative coding gain or the merits of each
approach discussed, nor the design requirements for encoders or decoders.
2.2.3 SYNCHRONIZATION
This Recommendation specifies a method for synchronizing BCH Codeblocks using a data
unit called the Communications Link Transmission Unit (CLTU), which consists of a Start
Sequence, BCH Codeblocks, and a Tail Sequence. This is described in section 4.
The Start Sequence of the CLTU may also be used for resolution of data ambiguity (sense of
‘1’ and ‘0’) if data ambiguity is not resolved by the modulation method used in the Physical
Layer.
This Recommendation also specifies a procedure called the Physical Layer Operations
Procedure (PLOP) for activating and deactivating the physical communications channel so
that the Physical Layer of the receiving end can achieve and maintain bit synchronization.
NOTE – Although PLOP belongs to the Physical Layer, it is included in this
Recommendation because it must be used to transmit CLTUs specified in this
document. The other specifications of the Physical Layer are contained in
reference [4].
2.2.4 PSEUDO-RANDOMIZING
This Recommendation specifies an optional randomizer to improve bit transition density as
an aid to bit synchronization. This is described in section 5.
NOTE – For brevity, the word ‘random’ is used in place of ‘pseudo-random’ throughout
this document. See annex A.
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
2.3 INTERNAL ORGANIZATION OF SUBLAYER
2.3.1 SENDING END
Figure 2-2 shows the internal organization of the Synchronization and Channel Coding
Sublayer of the sending end. This figure identifies functions performed by the sublayer and
shows logical relationships among these functions. The figure is not intended to imply any
hardware or software configuration in a real system.
At the sending end, the Synchronization and Channel Coding Sublayer accepts Transfer
Frames from the Data Link Protocol Sublayer (see figure 2-1), performs functions selected
for the mission, and delivers CLTUs to the Physical Layer. If necessary, fill bits are added
either before or after randomization to complete the integral number of BCH Codeblocks.
The Physical Layer transmits CLTUs using the PLOP.
Data Link Protocol Sublayer
Transfer Frames
Random Sequence Generation (optional)
(Randomized) Transfer Frames
BCH Encoding
BCH Codeblocks
CLTU Generation
CLTUs
Physical Layer
Physical Layer Operations Procedure (PLOP)
Modulated Radio Waveforms
Figure 2-2: Internal Organization of the Sublayer at the Sending End
2.3.2 RECEIVING END
Figure 2-3 shows the internal organization of the Synchronization and Channel Coding
Sublayer of the receiving end. This figures identifies functions performed by the sublayer
and shows logical relationships among these functions. The figure is not intended to imply
any hardware or software configuration in a real system.
CCSDS 231.0-B-1 Page 2-3 September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
At the receiving end, the Synchronization and Channel Coding Sublayer accepts streams of
channel bits together with information on the state of the physical communications channel
from the Physical Layer, performs functions selected for the mission, and delivers Transfer
Frames (possibly incomplete or with fill bits) to the Data Link Protocol Sublayer.
Data Link Protocol Sublayer
Transfer Frames (+Fill)
Random Sequence Removal (optional)
(Randomized) Transfer Frames (+Fill)
BCH Decoding
BCH Codeblocks+Tail Sequence
Search for Start Sequence
Channel Bits
Physical Layer
Figure 2-3: Internal Organization of the Sublayer at the Receiving End
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

3 BCH CODING
3.1 INTRODUCTION
The Synchronization and Channel Coding Sublayer establishes the reliable, error-controlled
data channel through which user data bits may be transferred. The data are encoded to
reduce the effects of noise in the Physical Layer on the user data. A modified Bose-
Chaudhuri-Hocquenghem (BCH) code has been chosen to provide this protection.
3.2 CODEBLOCK FORMAT
3.2.1 The BCH Codeblock format is a fixed-length data entity shown in figure 3-1. The
Codeblock is formulated using a systematic coding technique which contains 56 information
bits in the leading octets, and the error control bits in the last octet. The BCH Codeblock
contains an integer number of octets with an overall length of 8 octets (64 bits).
BCH CODEBLOCK
INFORMATION ERROR CONTROL
I , I , I , …, I P’ , P’ , …, P’ F
0 1 2 55 0 1 6 0
56 INFORMATION BITS (may be 7 PARITY CHECK APPENDED
randomized) BITS FILLER BIT
Figure 3-1: BCH Codeblock Format
3.2.2 The COMPLEMENTS of the seven parity check bits, P through P , are located in
0 6
the first seven bits of the last octet of the BCH Codeblock. The complements are used to aid
in maintaining bit synchronization and detection of bit slippage. The encoding procedure for
generating these parity bits is described in 3.3.
3.2.3 The last bit of the last octet, F , is a Filler Bit appended to provide an overall
Codeblock length which is an integer number of octets. This Filler Bit shall always be a
zero.
3.3 ENCODING PROCEDURE
3.3.1 A systematic block coding procedure shall be used which always generates 7 parity
check bits per Codeblock and which shall always be computed from 56 information bits.
The parity check bits are then COMPLEMENTED and placed into the Codeblock as shown
in figure 3-1.
3.3.2 The code used is a (63,56) modified Bose-Chaudhuri-Hocquenghem (BCH) code
which uses the following generator polynomial to produce the seven parity bits:
CCSDS 231.0-B-1 Page 3-1 September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING
7 6 2
g(x) = x + x + x + 1
NOTE – The code generator implementation is shown in figure 3-2. The shift registers are
initialized to zero. The ganged switch is in position 1 while the 56 information
bits are being transmitted, in position 2 for the seven parity bits, and in position 3
for the appended Filler Bit.
INFORMATION BITS I   • • •  I
0 55
CODED
DATA
(1) (1) OUTPUT
(2) (2)
(3)
(3)
ZERO
P P PP PPP ZERO
6 43 10
5 2
0 2 3 5 6
1 4
I
X X X X X
X X
PARITY BITS
Figure 3-2: (63,56) Modified BCH Code Generator
3.4 FILL BITS
3.4.1 If the Transfer Frame(s) to be transmitted in a Communications Link Transmission
Unit (CLTU) do not fit exactly within an integral number of BCH Codeblocks, then the last
octet(s) and ONLY the last octet(s) of the information field of the last Codeblock within the
CLTU may contain ‘Fill’ bits. The pattern of the fill shall consist of a sequence of
alternating ‘ones’ and ‘zeros’, starting with a ‘zero’.
3.4.2 The Synchronization and Channel Coding Sublayer may require the introduction of
these fill bits in the encoding process; they are not removed by the decoding process.
Removal of fill is the responsibility of the sublayer above, which delimits the end of the
Transfer Frame(s) and discards extraneous bits (e.g., fill).
3.4.3 If randomization is used, the fill bits mentioned above shall be added either before or
after randomization.
NOTE – If randomization is being used, any fill octets that were added to the last
Codeblock of the CLTU will be derandomized even if they were not randomized.
CCSDS 231.0-B-1 Page 3-2 September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

3.5 DECODING PROCEDURE
Codeblocks that have been encoded using the modified BCH code described in 3.3 may be
decoded either in an error-detecting mode (Triple Error Detection, or TED) or in an error-
correcting mode (Single Error Correction, or SEC), depending on mission requirements.
When the error-detecting mode is chosen, one, two or three bits in error will be detected
within the Codeblock (not counting the appended Filler Bit); when the error-correcting mode
is chosen, one bit in error will be corrected and two bits in error will be detected.
NOTE – The decoding procedure described in 3.5 assumes the use of a hard-limiting
detector before decoding, but the use of a soft-limiting detector is not intended to
be precluded.
CCSDS 231.0-B-1 Page 3-3 September 2003
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CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

4 COMMUNICATIONS LINK TRANSMISSION UNIT
4.1 INTRODUCTION
4.1.1 Synchronization for the Codeblock and delimiting of the beginning of user data are
provided by the Communications Link Transmission Unit (CLTU) data structure.
4.1.2 Resolution of data ambiguity (sense of ‘1’ and ‘0’) when receiving the symbol stream
shall be a service of the Synchronization and Channel Coding Sublayer if it is not performed
by the Physical Layer (e.g., with a differential modulation technique). In the
Synchronization and Channel Coding Sublayer, ambiguity resolution shall use inherent
information in the CLTU Start Sequence.
4.2 CLTU UNIT FORMAT
4.2.1 STRUCTURE OF CLTU
The CLTU is the data structure which carries the data as a contiguous series of encoded BCH
Codeblocks across the Synchronization and Channel Coding Sublayer. The data contained in
the BCH Codeblocks in the CLTU consist of Transfer Frame(s) from the sublayer above
(possibly with fill bits). The CLTU has the structural components shown in figure 4-1.
COMMUNICATIONS LINK TRANSMISSION UNIT
START TAIL
ENCODED DATA
SEQUENCE SEQ UENCE
LENGTH OF
16 BITS BCH CODEBLO CKS
ONE BCH
CODEBLOCK
Figure 4-1: Components of the CLTU
4.2.2 START SEQUENCE
The CLTU Start Sequence field shall delimit the start of the encoded data within the CLTU.
It consists of a 16-bit synchronization pattern with low autocorrelation side lobes and shall
have the following pattern:
1  1  1  0  1  0  1  1  1  0  0  1  0  0  0  0
BIT 0 BIT 15
CCSDS 231.0-B-1 Page 4-1 September 2003
CCSDS RECOMMENDATION FOR TC SYNCHRONIZATION AND CHANNEL CODING

4.2.3 ENCODED DATA
The Encoded Data field shall consist of a set of BCH Codeblocks which have been encoded
in accordance with the BCH Codeblock encoding procedure. In addition to error control bits,
these Codeblocks contain the Transfer frame(s), plus any fill bits that were appended to meet
codeblock length constraints. The Transfer Frames contained in the Encoded Data field may
have been randomized before encoding, or not randomized, as selected for the mission.
4.2.4 TAIL SEQUENCE
The CLTU Tail Sequence field is a data structure which is constructed specifically to be a
noncorrectable sequence which delimits the end of a CLTU by stopping the decoding
process. The Tail Sequence shall have the same length as the BCH Codeblock that is being
used. The Tail Sequence shall consist of leading octets having the pattern 11000101,
repeated as necessary until the next-to-last octet of the Tail Sequence field is reached. The
last octet completes the Tail Sequence field, and
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