ISO 21459:2015

INTERNATIONAL ISO
STANDARD 21459
Second edition
2015-08-15
Space data and information
transfer systems — Proximity-1
space link protocol — Coding and
synchronization sublayer
Systèmes de transfert des informations et données spatiales —
Protocole pour liaisons spatiales de proximité 1 — Sous-couche de
codage et synchronisation
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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ii © ISO 2015 – All rights reserved

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 21459 was prepared by the Consultative Committee for Space Data Systems (CCSDS) (as
CCSDS 211.2-B-2, December 2013) 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.

This second edition of ISO 21459:2015 cancels and replaces the first edition (ISO 21459:2006), which has
been technically revised.
Recommendation for Space Data System Standards
PROXIMITY-1 SPACE LINK
PROTOCOL—CODING AND
SYNCHRONIZATION
SUBLAYER
RECOMMENDED STANDARD
CCSDS 211.2-B-2
BLUE BOOK
December 2013
CCSDS RECOMMENDED STANDARD FOR PROXIMITY-1 SPACE LINK PROTOCOL—
CODING AND SYNCHRONIZATION SUBLAYER
AUTHORITY
Issue: Recommended Standard, Issue 2
Date: December 2013
Location: Washington, DC, USA
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 documents is detailed in Organization and Processes for
the Consultative Committee for Space Data Systems (CCSDS A02.1-Y-3), and the record of
Agency participation in the authorization of this document can be obtained from the CCSDS
Secretariat at the address below.

This document is published and maintained by:

CCSDS Secretariat
Space Communications and Navigation Office, 7L70
Space Operations Mission Directorate
NASA Headquarters
Washington, DC 20546-0001, USA
CCSDS 211.2-B-2 Page i December 2013
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STATEMENT OF INTENT
The Consultative Committee for Space Data Systems (CCSDS) is an organization officially
established by the management of its members. 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 Recommended Standards and are
not considered binding on any Agency.
This Recommended Standard is issued by, and represents the consensus of, the CCSDS
members. Endorsement of this Recommendation is entirely voluntary. Endorsement,
however, indicates the following understandings:
o Whenever a member establishes a CCSDS-related standard, this standard will be in
accord with the relevant Recommended Standard. Establishing such a standard
does not preclude other provisions which a member may develop.
o Whenever a member establishes a CCSDS-related standard, that member will
provide other CCSDS members with the following information:
-- The standard itself.
-- The anticipated date of initial operational capability.
-- The anticipated duration of operational service.
o Specific service arrangements shall be made via memoranda of agreement. Neither
this Recommended Standard nor any ensuing standard is a substitute for a
memorandum of agreement.
No later than three years from its date of issuance, this Recommended Standard 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 Recommended Standard is issued, existing
CCSDS-related member standards and implementations are not negated or deemed to be
non-CCSDS compatible. It is the responsibility of each member to determine when such
standards or implementations are to be modified. Each member is, however, strongly
encouraged to direct planning for its new standards and implementations towards the later
version of the Recommended Standard.
CCSDS 211.2-B-2 Page ii December 2013
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FOREWORD
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights. CCSDS shall not be held responsible for identifying any or all such
patent rights.
Through the process of normal evolution, it is expected that expansion, deletion, or
modification of this document may occur. This Recommended Standard is therefore subject
to CCSDS document management and change control procedures, which are defined in
Organization and Processes for the Consultative Committee for Space Data Systems
(CCSDS A02.1-Y-3). 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 211.2-B-2 Page iii December 2013
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At time of publication, the active Member and Observer Agencies of the CCSDS were:
Member Agencies
– Agenzia Spaziale Italiana (ASI)/Italy.
– Canadian Space Agency (CSA)/Canada.
– Centre National d’Etudes Spatiales (CNES)/France.
– China National Space Administration (CNSA)/People’s Republic of China.
– Deutsches Zentrum für Luft- und Raumfahrt (DLR)/Germany.
– European Space Agency (ESA)/Europe.
– Federal Space Agency (FSA)/Russian Federation.
– Instituto Nacional de Pesquisas Espaciais (INPE)/Brazil.
– Japan Aerospace Exploration Agency (JAXA)/Japan.
– National Aeronautics and Space Administration (NASA)/USA.
– UK Space Agency/United Kingdom.
Observer Agencies
– Austrian Space Agency (ASA)/Austria.
– Belgian Federal Science Policy Office (BFSPO)/Belgium.
– Central Research Institute of Machine Building (TsNIIMash)/Russian Federation.
– China Satellite Launch and Tracking Control General, Beijing Institute of Tracking
and Telecommunications Technology (CLTC/BITTT)/China.
– Chinese Academy of Sciences (CAS)/China.
– Chinese Academy of Space Technology (CAST)/China.
– Commonwealth Scientific and Industrial Research Organization (CSIRO)/Australia.
– Danish National Space Center (DNSC)/Denmark.
– Departamento de Ciência e Tecnologia Aeroespacial (DCTA)/Brazil.
– European Organization for the Exploitation of Meteorological Satellites
(EUMETSAT)/Europe.
– European Telecommunications Satellite Organization (EUTELSAT)/Europe.
– Geo-Informatics and Space Technology Development Agency (GISTDA)/Thailand.
– Hellenic National Space Committee (HNSC)/Greece.
– Indian Space Research Organization (ISRO)/India.
– Institute of Space Research (IKI)/Russian Federation.
– KFKI Research Institute for Particle & Nuclear Physics (KFKI)/Hungary.
– Korea Aerospace Research Institute (KARI)/Korea.
– Ministry of Communications (MOC)/Israel.
– National Institute of Information and Communications Technology (NICT)/Japan.
– National Oceanic and Atmospheric Administration (NOAA)/USA.
– National Space Agency of the Republic of Kazakhstan (NSARK)/Kazakhstan.
– National Space Organization (NSPO)/Chinese Taipei.
– Naval Center for Space Technology (NCST)/USA.
– Scientific and Technological Research Council of Turkey (TUBITAK)/Turkey.
– South African National Space Agency (SANSA)/Republic of South Africa.
– Space and Upper Atmosphere Research Commission (SUPARCO)/Pakistan.
– Swedish Space Corporation (SSC)/Sweden.
– Swiss Space Office (SSO)/Switzerland.
– United States Geological Survey (USGS)/USA.
CCSDS 211.2-B-2 Page iv December 2013
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DOCUMENT CONTROL
Document Title Date Status
CCSDS Proximity-1 Space Link Protocol October Original issue,
211.0-B-1 2002 superseded
CCSDS Proximity-1 Space Link Protocol— April Superseded
211.2-B-1 Coding and Synchronization 2003
Sublayer
CCSDS Proximity-1 Space Link Protocol— December Current issue:
211.2-B-2 Coding and Synchronization 2013 This update includes
Sublayer, Recommended Standard, several improvements
Issue 2 and clarifications—
accomplishing better
alignment and
consistency with the
other Proximity-1 Blue
Books—and the
addition of an option
for Low-Density
Parity-Check (LDPC)
codes.
NOTE – Changes from the previous issue are too extensive to permit markup.
CCSDS 211.2-B-2 Page v December 2013
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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 . 1-2
1.5 DOCUMENT STRUCTURE . 1-2
1.6 CONVENTIONS AND DEFINITIONS. 1-3
1.7 REFERENCES . 1-6

2 OVERVIEW . 2-1

2.1 LAYERS OF THE PROTOCOL . 2-1
2.2 PHYSICAL LAYER . 2-2
2.3 DATA LINK LAYER . 2-2
2.4 CODING AND SYNCHRONIZATION SUBLAYER . 2-3

3 CODING AND SYNCHRONIZATION SUBLAYER . A-1

3.1 OVERVIEW . A-1
3.2 PROXIMITY LINK TRANSMISSION UNIT (PLTU) . A-1
3.3 IDLE DATA . A-3
3.4 CHANNEL CODING . A-5
3.5 SEND SIDE PROCEDURES IN THE C&S SUBLAYER . A-11
3.6 RECEIVE SIDE PROCEDURES IN THE C&S SUBLAYER . A-11

ANNEX A PROTOCOL IMPLEMENTATION CONFORMANCE
STATEMENT PROFORMA (NORMATIVE) . A-1
ANNEX B SERVICE (NORMATIVE) .B-1
ANNEX C CRC-32 CODING PROCEDURES (NORMATIVE) . C-1
ANNEX D SECURITY, SANA, AND PATENT CONSIDERATIONS
(INFORMATIVE) . D-1
ANNEX E INFORMATIVE REFERENCES (INFORMATIVE) .E-1
ANNEX F ABBREVIATIONS AND ACRONYMS (INFORMATIVE) . F-1
Figure
1-1 Bit Numbering Convention . 1-5
1-2 Proximity-1 Rate Terminology . 1-5
2-1 Simplified Overview of Proximity-1 Layers . 2-1
2-2 Coding & Synchronization Sublayer Send Side Interactions . 2-4
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CONTENTS (continued)
Figure Page
3-1 Proximity-1 Link Transmission Unit (PLTU) . A-2
3-2 Behavior of the C&S Sublayer . A-7
3-3 LDPC Coding Procedure . A-9
3-4 Pseudo-RandomizerLogic Diagram . A-10
C-1 A Possible Implementation of the Encoder .C-3
C-2 A Possible Implementation of the Decoder .C-5

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1 INTRODUCTION
1.1 PURPOSE
The purpose of this Recommended Standard is to specify synchronization and channel
coding schemes used with the Proximity-1 Data Link Protocol (reference [3]) and Physical
Layer (reference [4]). Proximity space links are defined to be short-range, bi-directional,
fixed or mobile radio links, generally used to communicate among probes, landers, rovers,
orbiting constellations, and orbiting relays. These links are characterized by short time
delays, moderate (not weak) signals, and short, independent sessions.
1.2 SCOPE
This Recommended Standard defines synchronization and channel coding schemes for
Proximity-1 links 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 protocol.
The Coding and Synchronization Sublayer is part of the Data Link Layer. The rest of the Data
Link Layer is defined in the separate CCSDS Recommended Standard entitled, Proximity-1
Space Link Protocol—Data Link Layer (reference [3]). The Physical Layer is defined in the
separate CCSDS Recommended Standard entitled, Proximity-1 Space Link Protocol—Physical
Layer (reference [4]).
1.3 APPLICABILITY
This Recommended Standard applies to the creation of Agency standards and to future data
communications over space links between CCSDS Agencies in cross-support situations. It
applies also to internal Agency links where no cross-support is required. It includes
specification of the services and protocols for inter-Agency cross support. It is neither a
specification of, nor a design for, systems that may be implemented for existing or future
missions.
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The Recommended Standard 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 Recommended Standard is anticipated.
Where mandatory capabilities are clearly indicated in sections of the Recommended
Standard, 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. Concept and rationale behind the decisions that
formed the basis for Proximity-1 is documented in the CCSDS Proximity-1 Space Link
Green Book (reference [E1]).
1.5 DOCUMENT STRUCTURE
This document is divided into three numbered sections and four annexes:
a) section 1 presents the purpose, scope, applicability and rationale of this
Recommended Standard and lists the conventions, definitions, and references used
throughout the document;
b) section 2 provides an overview of Proximity-1 synchronization and channel coding;
c) section 3 specifies convolutional coding;
d) annex A contains the Protocol Implementation Conformance Statement (PICS)
proforma for this specification;
e) annex B defines the service provided to the users;
f) annex C defines CRC-32 Coding Procedure;
g) annex D discusses security, SANA, and patent considerations;
h) annex E contains informative references;
i) annex F contains a list of abbreviations and acronyms.
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1.6 CONVENTIONS AND DEFINITIONS
1.6.1 DEFINITIONS
1.6.1.1 Terms from the Open Systems Interconnection (OSI) Basic Reference Model
This Recommended Standard makes use of a number of terms defined in reference [1]. In
this Recommended Standard those terms are used 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) protocol data unit;
d) real system;
e) service;
f) service data unit.
1.6.1.2 Terms Defined in This Recommended Standard
For the purposes of this Recommended Standard, the following definitions also apply. Many
other terms that pertain to specific items are defined in the appropriate sections.
forward link: That portion of a Proximity space link in which the caller transmits and the
responder receives (typically a command link).
physical channel: The RF channel upon which the stream of channel symbols is transferred
over a space link in a single direction.
PLTU: Proximity Link Transmission Unit, the data unit composed of the Attached
Synchronization Marker, the Version-3 Transfer Frame, and the attached Cyclic
Redundancy Check (CRC)-32.
Proximity link: A full-duplex, half-duplex or simplex link for the transfer of data between
Proximity-1 entities in a session.
return link: That portion of a Proximity space link in which the responder transmits and the
caller receives (typically a telemetry link).
space link: A communications link between transmitting and receiving entities, at least one
of which is in space.
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1.6.2 NOMENCLATURE
1.6.2.1 NORMATIVE TEXT
The following conventions apply for the normative specifications in this Recommended
Standard:
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.
NOTE – These conventions do not imply constraints on diction in text that is clearly
informative in nature.
1.6.2.2 INFORMATIVE TEXT
In the normative section of this document (section 3 and annexes B and C), informative text
is set off from the normative specifications either in notes or under one of the following
subsection headings:
– Overview;
– Background;
– Rationale;
– Discussion.
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’, as shown in figure 1-1.
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BIT 0 BITN-1
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
eight-bit ‘words’ that conform to the above convention. Throughout this Recommended
Standard, such an eight-bit word is called an ‘octet’. The numbering for octets within a data
structure begins with zero. Octet zero is the first octet to be transmitted.
Throughout this Recommended Standard, directive, parameter, variable, and signal names are
presented with all upper-case characters; data-field and MIB-parameter names are presented
with initial capitalization; values and state names are presented with predominantly lower-
case characters, and are italicized.
In Proximity-1, data rate (R ), coded symbol rate (R ) and channel symbol rate (R ) are
d cs chs
used to denote respectively:
– the data rate of the bitstream composed by PLTUs and Idle data measured at the
encoder input;
– the coded data rate measured at the interface between the Coding and
Synchronization Sublayer and the Physical Layer, and
– the rate measured at the output of the transmitter.
The terms are used as shown in figure 1-2.
BITSTREAM CODED CHANNEL
FOR ENCODING SYMBOLS
SYMBOLS
ENCODER RF POWER AMPLIFIER
Bi-Phase-L
(or Bypass) MODULATOR & RF CHAIN
CHANNEL
SYMBOL RATE
DATA RATE CODED SYMBOL RATE
REFERENCE
REFERENCE POINT REFERENCE POINT
POINT
(R ) (R )
d cs
(R )
chs
Figure 1-2: Proximity-1 Rate Terminology
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1.7 REFERENCES
The following publications contain provisions which, through reference in this text,
constitute provisions of this document. At the time of publication, the editions indicated
were valid. All publications are subject to revision, and users of this document are
encouraged to investigate the possibility of applying the most recent editions of the
publications indicated below. The CCSDS Secretariat maintains a register of currently valid
CCSDS publications.
[1] Information Technology—Open Systems Interconnection—Basic Reference Model: The
Basic Model. 2nd ed. International Standard, ISO/IEC 7498-1:1994. Geneva: ISO,
1994.
[2] TM Synchronization and Channel Coding. Issue 2. Recommendation for Space Data
System Standards (Blue Book), CCSDS 131.0-B-2. Washington, D.C.: CCSDS, August
2011.
[3] Proximity-1 Space Link Protocol—Data Link Layer. Issue 5. Recommendation for
Space Data System Standards (Blue Book), CCSDS 211.0-B-5. Washington, D.C.:
CCSDS, December 2013.
[4] Proximity-1 Space Link Protocol—Physical Layer. Issue 4. Recommendation for Space
Data System Standards (Blue Book), CCSDS 211.1-B-4. Washington, D.C.: CCSDS,
December 2013.
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2 OVERVIEW
2.1 LAYERS OF THE PROTOCOL
Proximity-1 is a bi-directional Space Link Layer protocol for use by space missions. It has
been designed to meet the requirements of space missions for efficient transfer of space data
over various types and characteristics of Proximity space links. The protocol consists of a
Data Link Layer (reference [3]) and a Physical Layer (reference [4]). The Coding and
Synchronization (C&S) Sublayer defined in this document is part of the Data Link Layer.
Proximity-1 activities are divided between a send side and a receive side. The send side is
concerned with the transmitted physical channel, and also with the acquisition of the received
physical channel in order to establish a Proximity-1 link. The receive side is concerned with
the reception of data on the received physical channel: the input symbol stream and the
protocol data units it contains.
Figure 2-1 gives a simplified view of the Proximity-1 layered structure.
Local S/C
INPUT of USER DATA
USER DATA
Controller
+ Routing information
Delivery
SEND     RECEIVE
I/O Sublayer
MAC
Sublayer
Data Services
(MIB)
Sublayer
Frame Sublayer
Coding & Synchronization Layer
Physical Layer
Figure 2-1: Simplified Overview of Proximity-1 Layers
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2.2 PHYSICAL LAYER
On the send side, the Physical Layer:
– accepts control variables from the Data Link Layer for control of the transceiver;
– accepts a stream of Proximity-1 coded symbols from the Data Link Layer for
modulation onto the radiated carrier.
On the receive side, the Physical Layer
– provides the serial stream output of Proximity-1 coded symbols from the receiver to
the Data Link Layer:
– provides status signals (CARRIER_ACQUIRED and SYMBOL_INLOCK_STATUS)
to the Data Link Layer.
2.3 DATA LINK LAYER
This subsection provides a brief overview of the Data Link Layer, with emphasis on the
features relevant to the C&S Sublayer. Reference [3] contains a more complete description
of the overall Proximity-1 system, of the Data Link Layer, and of its sublayers.
On the send side, the Data Link Layer is responsible for providing data to be transmitted by
the Physical Layer. On the receive side, the Data Link Layer accepts the serial coded symbol
stream output from the receiver in the Physical Layer and processes the protocol data units
contained in it.
Within the Data Link Layer, the Medium Access Control (MAC) Sublayer and the Frame
Sublayer have interfaces to the C&S Sublayer.
The Medium Access Control (MAC) Sublayer controls the establishment, maintenance and
termination of communications sessions for point-to-point communications between
Proximity entities. It controls the operational state of the Data Link and Physical Layers,
using control variables. It accepts Proximity-1 directives both from the local vehicle
controller and across the Proximity link to control its operations. The MAC Sublayer is also
responsible for the storage and distribution of the Management Information Base (MIB)
parameters.
The Frame Sublayer includes the processing associated with the fields contained in the
Frame Header of the Version-3 Transfer Frame. On the send side, the Frame Sublayer
determines the order of frame transmission and delivers frames to the C&S Sublayer. On the
receive side, the Frame Sublayer receives and validates frames from the C&S Sublayer and
delivers them to the Data Services Sublayer or to the MAC Sublayer, depending on their
contents.
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2.4 CODING AND SYNCHRONIZATION SUBLAYER
On the send side, the actions of the C&S Sublayer include:
– constructing PLTUs, where each PLTU contains a Version-3 Transfer Frame received
from the Frame Sublayer;
– generation of the bitstream for encoding, inserting Idle data as required;
– channel coding;
– provision of the coded symbols stream at a constant rate (R ), to the Physical Layer
cs
for modulation onto the radiated carrier.
On the receive side, the C&S Sublayer actions include:
– reception of the coded symbols stream from the receiver in the Physical Layer at a
constant rate (R );
cs
– channel decoding;
– delimiting of each PLTU, including validation;
– for each valid PLTU, delivering the delimited transfer frame to the Frame Sublayer.
On both the send and receive sides, the C&S Sublayer supports Proximity-1 timing services
defined in reference [3] by capturing the values of the clock, frame sequence number,
Quality of Service (QOS) Indicator, and direction (ingress or egress) associated with each
Proximity-1 transfer frame.
The (simplified) interaction of the Coding & Synchronization Sublayer with the other (sub)
layers at the transmitting (i.e., encoding) side of Proximity-1 is shown in figure 2-2.
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Frame Sublayer of the Data Link Layer
Version-3 Transfer Frames
Coding & Synchronization Sublayer of the Data Link Layer
Proximity-1 Coded Symbols
Physical Layer
Channel Symbols
Figure 2-2: Coding & Synchronization Sublayer Send Side Interactions

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3 CODING AND SYNCHRONIZATION SUBLAYER
3.1 OVERVIEW
On the send side, the C&S Sublayer generates the output coded symbols stream (including
PLTUs and Idle data) to be delivered to the Physical Layer for modulation onto the radiated
carrier. The PLTU is specified in 3.2. Each PLTU contains a Version-3 Transfer Frame
specified in reference [3] that also defines an Output Bitstream FIFO, which holds coded
symbol-stream data ready for delivery to the Physical Layer.
The PLTUs form a non-continuous serial stream, consisting of a sequence of variable-length
PLTUs, which can have a delay between the end of one PLTU and the start of the next.
While establishing a Proximity-1 session for a full- or half-duplex link, synchronization is
reacquired for each PLTU and Idle data is provided for the acquisition process. When no
PLTU is available, Idle data is transmitted to maintain synchronization. Idle data is specified
in 3.3. The procedures for the session establishment, data services, and session termination
phases of a Proximity-1 session are specified in reference [3].
The channel coding options for Proximity-1 are specified in 3.4. The send side and receive
side procedures in the C&S Sublayer are specified in 3.5 and 3.6.
3.2 PROXIMITY LINK TRANSMISSION UNIT (PLTU)
3.2.1 PLTU OVERVIEW
The C&S Sublayer handles the PLTU specified here and the Version-3 Transfer Frame
specified in reference [3]. The PLTU and the transfer frame are variable-length data
structures.
For the sending end of a link, the C&S Sublayer constructs PLTUs, where each PLTU
contains a transfer frame. For the receiving end, the C&S Sublayer processes each PLTU
and delimits the transfer frame.
3.2.2 PLTU STRUCTURE
A PLTU shall encompass the following three fields, positioned contiguously, in the
following sequence:
a) 24-bit Attached Synchronization Marker (ASM);
b) a Version-3 Transfer Frame;
c) 32-bit Cyclic Redundancy Check.
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NOTES
1 The length of a PLTU depends on the length of the transfer frame it contains. A
PLTU can have a length up to 2055 octets (3 octets ASM + 2048 octets maximum
transfer frame + 4 octets CRC).
2 The structural components of the PLTU are shown in figure 3-1.
PLTU
ASM
Version-3 Transfer Frame CRC-32
hex
FAF320
3octets up to 2048 octets 4octets

Figure 3-1: Proximity-1 Link Transmission Unit (PLTU)
3.2.3 ATTACHED SYNCHRONIZATION MARKER (ASM)
3.2.3.1 The Attached Synchronization Marker (ASM) shall occupy the first 24 bits of the
PLTU.
3.2.3.2 The ASM shall consist of the following bit pattern (in hexadecimal): FAF320.
NOTE – At the receiving end, the ASM is used to detect the start of a PLTU.
3.2.4 VERSION-3 TRANSFER FRAME
The Version-3 Transfer Frame (defined in 3.2) in a PLTU shall immediately follow the
ASM.
NOTE – Specification of the Version-3 Transfer Frame is contained in reference [3]. The
PLTU is not designed for use with any other type of transfer frame.
3.2.5 CYCLIC REDUNDANCY CHECK (CRC-32)
3.2.5.1 The Cyclic Redundancy Check (CRC-32) shall occupy the last 32 bits of the PLTU.
3.2.5.2 The CRC-32 shall immediately follow the Version-3 Transfer Frame.
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3.2.5.3 The CRC-32 shall be calculated by applying the encoding procedure specified in
annex C to the Version-3 Transfer Frame.
3.2.5.4 The ASM shall NOT be a part of the encoded data space of the CRC-32.
NOTE – As shown in figure 3-1, the CRC is part of the PLTU but it is not part of the
Version-3 Transfer Frame. In this respect, Proximity-1 differs from other
CCSDS space data link protocols.
3.3 IDLE DATA
3.3.1 OVERVIEW
Idle data are included in the bitstream for encoding:
– for the purpose of data acquisition (Acquisition sequence);
– when no PLTU is available (Idle sequence); and
– prior to terminating transmission (Tail sequence).
A Pseudo-Noise (PN) sequence defines the bit pattern used for the Idle data in each of these
sequences. The PN sequence is cyclic and is repeated as needed.
Reference [3] specifies the uses of the Acquisition sequence, Idle sequence, and Tail
sequence in the phases of a Proximity-1 session. It also specifies the MIB parameters
Acquisition_Idle_Duration and Tail_Idle_Duration, which represent the time that is used to
radiate the Acquisition sequence and the Tail sequence.
3.3.2 GENERAL
3.3.2.1 Idle data shall be included in the bitstream for encoding as follows:
a) an Acquisition sequence (defined in 3.3.3) shall be inserted when transmission
commences;
b) Idle sequence(s) (defined in 3.3.2 and 3.3.4) shall be inserted when no PLTU is
available; and
c) a Tail sequence (defined in 3.3.5) shall be inserted prior to terminating transmission.
3.3.2.2 Idle data shall consist of the PN sequence 352EF853 (in hexadecimal), repeated as
needed.
3.3.2.3 When LDPC coding is used, the Acquisition sequence shall start on the first bit of
the PN sequence.
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NOTE – The requirement to start the Acquisition sequence on the first bit of the PN
sequence applies only when LDPC coding is used.
3.3.2.4 Whenever the end of the PN sequence is reached, the sequence shall be repeated
starting from the first bit of the PN sequence.
NOTE – An Idle Data Generator can be used to generate Idle data for insertion into the
bitstream to be encoded.
3.3.3 ACQUISITION SEQUENCE
3.3.3.1 Overview
The Physical Layer provides the modulation necessary for the partners in a session to acquire
and process each other’s transmission. When transmission commences, the transmitter’s
modulation is sequenced (first carrier only followed by an Acquisition Sequence) such that
the receiving unit can acquire the signal and achieve a reliable channel symbol stream in
preparation for acceptance of the transmitted data units. If convolutional code is applied
(see 3.4) the Acquisition Sequence is also used to achieve node synchronization in the
decoder (reference [E1]). If LDPC code is used, the acquisition sequence duration is set long
enough for a valid CSM to be detected after symbol synchronization is established and
before the first PLTU begins.
3.3.3.2 Requirements
3.3.3.2.1 The Acquisition sequence shall consist of data as specified by the requirements
in 3.3.3.2 above.
3.3.3.2.2 The Acquisition sequence shall be transmitted for the duration specified by the
MIB parameter Acquisition_Idle_Duration specified in reference [3].
3.3.4 IDLE SEQUENCE
3.3.4.1 Overview
While in the data transfer phase of a Proximity-1 session, (encoded) PLTUs are transmitted
within a continuous stream of channel symbols from the transmitter to the receiver. When no
PLTU is available, the Idle sequence is injected into the bit stream to be encoded in order to
keep the channel symbols stream flowing and to enable the receiver to maintain
synchronization.
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3.3.4.2 Requirements
3.3.4.2.1 The Idle sequence shall consist of data as specified by the requirements in 3.3.3.2
above.
3.3.4.2.2 During the data services phase, if no PLTU is ready for transfer, then the Idle
sequence shall be transmitted.
3.3.5 TAIL SEQUENCE
3.3.5.1 Overview
Prior to terminating transmission (removing modulation) the transmitter transmits a series of
idle bits (Tail sequence) for a fixed period. This can help the receiving unit to maintain bit
lock and convolutional decoding while it completes the processing of the final received data
unit.
3.3.5.2 Requirements
3.3.5.2.1 The Tail sequence shall consist of data as specified by the requirements in 3.3.3.2
above.
3.3.5.2.2 The Tail sequence shall be transmitted for the duration specified by the MIB
parameter Tail_Idle_Duration, specified in reference [3].
3.4 CHANNEL CODING
3.4.1 OVERVIEW OF PROXIMITY-1 CHANNEL CODING
This document defines two channel codes for use on Proximity-1 links: an optional
convolutional code, and an optional LDPC code.
NOTE – Some transceivers implement additional channel codes to those defined in the
Proximity-1 recommended standards. The additions include a pair of Reed-
Solomon codes, which can be enabled or disabled by the SET PL EXTENSIONS
directive defined in annex A of reference [3]. The Reed-Solomon codes are not
specified in the CCSDS Proximity-1 space link Recommended Standards and
their use is not intended for cross-support applications.
3.4.2 CODING OPTIONS
3.4.2.1 The C&S Sublayer shall process the incoming Version-3 Transfer Frames and
produce a bitstream for encoding (including PLTUs and Idle Data) at data rate R .
d
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NOTE – The current data rate is configured using the SET TRANSMITTER
PARAMETERS and SET PL EXTENSIONS directives defined in annex A of
reference [3] and it is selected among the following discrete data rates, shown in
bits per second: 1000, 2000, 4000, 8000, 16000, 32000, 64000, 128000, 256000,
512000, 1024000, 2048000. When LDPC codes are used, these R values are
d
approximated and the true values can be found in annex A of reference [3].
3.4.2.2 The C&S Sublayer shall generate the output stream of Proximity-1 coded symbols
applying only one of the following coding options:
a) no coding;
b) convolutional code (see 3.4.3);
c) LDPC code (see 3.4.4).
NOTES
1 The convolutional and LDPC codes are optional. The use of the convolutional or
LDPC code by the transmitter is configured using the SET TRANSMITTER
PARAMETERS directive. The use by the receiver is configured by the SET
RECEIVER PARAMETERS directive. The directives are defined in annex A of
reference [3]. The MAC Sublayer configures the C&S Sublayer according to
received directives.
2 The directives SET TRANSMITTER PARAMETERS and SET RECEIVER
PARAMETERS include an option to concatenate the convolutional code with one of
the Reed-Solomon codes. Neither the Reed-Solomon code nor the concatenation
with the convolutional code is specified here.
Figure 3-2 below shows the principal behavior of the C&S Sublayer.
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Upper Layer generating
ChannelAccess.request (Frame)
Version-3 Frame
PLTU
PLTU generation
(add ASM+CRC)
Data Stream generation
Data Stream (PLTUs + idle data)
(add Acquisition + Idle + Tail
Data Rate
Sequences)
Measurement
LDPC Encoder Output Stream
LDPC Code
...