ISO 22666:2007

INTERNATIONAL ISO
STANDARD 22666
Second edition
2007-09-01
Space data and information transfer
systems — AOS (advanced orbiting
systems) space data link protocol
Systèmes de transfert des données et informations spatiales —
Protocole de liaison pour données spatiales AOS (systèmes
perfectionnés sur orbite)
Reference number
©
ISO 2007
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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 22666 was prepared by the Consultative Committee for Space Data Systems (CCSDS) (as
CCSDS 732.0-B-2, July 2006) 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 cancels and replaces the first edition (ISO 22666:2005), which has been technically
revised.
INTERNATIONAL STANDARD ISO 22666:2007(E)

Space data and information transfer systems — AOS (advanced
orbiting systems) space data link protocol
1 Scope
This International Standard specifies the advanced orbiting systems (AOS) space data link protocol, a data
link layer protocol as defined in ISO/IEC 7498-1, that is to be used over space-to-ground, ground-to-space, or
space-to-space communications links by space missions.
The scope and field of application are furthermore detailed in subclauses 1.2 and 1.3 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 732.0-B-2, July 2006, AOS space data link protocol
For the purposes of international standardization, the modifications outlined below shall apply to the specific
clauses and paragraphs of the publication CCSDS 732.0-B-2.
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 references indicated:
[3] Document CCSDS 131.0-B-1, September 2003, is equivalent to ISO 22641:2005.
[6] Document CCSDS 133.0-B-1, September 2003, is equivalent to ISO 22646:2005.
Page B-1
Add the following information to the references indicated:
[B4] Document CCSDS 910.4-B-1, May 1996, is equivalent to ISO 15396:1998.
[B5] Document CCSDS 232.0-B-1, September 2003, is equivalent to ISO 22664:2005.
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 732.0-B-2. To this end, NASA will act as a liaison body between CCSDS and ISO.
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Recommendation for Space Data System Standards
AOS SPACE DATA
LINK PROTOCOL
RECOMMENDED STANDARD
CCSDS 732.0-B-2
BLUE BOOK
July 2006
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
AUTHORITY
Issue: Recommended Standard, Issue 2
Date: July 2006
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 Recommendations is detailed in the Procedures Manual
for the Consultative Committee for Space Data Systems, 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
Office of Space Communication (Code M-3)
National Aeronautics and Space Administration
Washington, DC 20546, USA
CCSDS 732.0-B-2 Page i July 2006
CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
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 five 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 732.0-B-2 Page ii July 2006
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
FOREWORD
This document is a technical Recommendation for use in developing flight and ground
systems for space missions and has been prepared by the Consultative Committee for Space
Data Systems (CCSDS). The Advanced Orbiting Systems (AOS) Space Data Link Protocol
described herein is intended for missions that are cross-supported between Agencies of the
CCSDS.
This Recommendation specifies a communications protocol to be used by space missions to
transfer space application data over ground-to-space or space-to-space communications links.
This Recommendation is developed from the specifications of the Data Link Layer portion of
an older CCSDS Recommendation (reference [B2]), which defines essentially the same
protocol and services but in a slightly different context.
This Recommendation does not change the major technical contents defined in reference [B2],
but the presentation of the specification has been changed so that:
a) this protocol 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 specification matches the OSI Basic Reference Model (references [1] and [2]).
Together with the change in presentation, a few technical specifications in reference [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. 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 reference [B1].
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 732.0-B-2 Page iii July 2006
CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
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.
– Federal Space Agency (Roskosmos)/Russian Federation.
– Instituto Nacional de Pesquisas Espaciais (INPE)/Brazil.
– Japan Aerospace Exploration Agency (JAXA)/Japan.
– National Aeronautics and Space Administration (NASA)/USA.

Observer Agencies
– Austrian Space Agency (ASA)/Austria.
– Belgian Federal Science Policy Office (BFSPO)/Belgium.
– 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.
– Danish Space Research Institute (DSRI)/Denmark.
– European Organization for the Exploitation of Meteorological Satellites
(EUMETSAT)/Europe.
– European Telecommunications Satellite Organization (EUTELSAT)/Europe.
– 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.
– MIKOMTEK: CSIR (CSIR)/Republic of South Africa.
– Ministry of Communications (MOC)/Israel.
– National Institute of Information and Communications Technology (NICT)/Japan.
– National Oceanic & Atmospheric Administration (NOAA)/USA.
– National Space Organization (NSPO)/Taipei.
– Space and Upper Atmosphere Research Commission (SUPARCO)/Pakistan.
– Swedish Space Corporation (SSC)/Sweden.
– United States Geological Survey (USGS)/USA.
CCSDS 732.0-B-2 Page iv July 2006
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
DOCUMENT CONTROL
Document Title and Issue Date Status
CCSDS AOS Space Data Link Protocol, September Original issue, superseded
732.0-B-1 Issue 1 2003
CCSDS AOS Space Data Link Protocol, July 2006 Current issue:
732.0-B-2 Recommended Standard, Issue 2
The Transfer Frame Primary
Header Signaling Field is
modified as follows:
- number of reserved spare bits
reduced from 7 to 2;
- two new subfields defined: a
Virtual Channel (VC) Frame
Count Cycle Use Flag, and a
Virtual Channel Frame Count
Cycle subfield.
CCSDS 732.0-B-2 Page v July 2006
CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
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.1-2
1.7 REFERENCES.1-5

2 OVERVIEW.2-1

2.1 CONCEPT OF AOS SPACE DATA LINK PROTOCOL.2-1
2.2 OVERVIEW OF SERVICES .2-3
2.3 OVERVIEW OF FUNCTIONS.2-9
2.4 SERVICES ASSUMED FROM LOWER LAYERS .2-12

3 SERVICE DEFINITION.3-1

3.1 OVERVIEW.3-1
3.2 SOURCE DATA.3-1
3.3 PACKET SERVICE.3-3
3.4 BITSTREAM SERVICE.3-6
3.5 VIRTUAL CHANNEL ACCESS (VCA) SERVICE.3-9
3.6 VIRTUAL CHANNEL OPERATIONAL CONTROL FIELD (VC_OCF)
SERVICE.3-12
3.7 VIRTUAL CHANNEL FRAME (VCF) SERVICE.3-15
3.8 MASTER CHANNEL FRAME (MCF) SERVICE .3-18
3.9 INSERT SERVICE.3-21

4 PROTOCOL SPECIFICATION.4-1

4.1 PROTOCOL DATA UNIT.4-1
4.2 PROTOCOL PROCEDURES AT THE SENDING END .4-17
4.3 PROTOCOL PROCEDURES AT THE RECEIVING END .4-23

5 MANAGED PARAMETERS .5-1

5.1 OVERVIEW OF MANAGED PARAMETERS .5-1
5.2 MANAGED PARAMETERS FOR A PHYSICAL CHANNEL .5-1
5.3 MANAGED PARAMETERS FOR A MASTER CHANNEL.5-2
5.4 MANAGED PARAMETERS FOR A VIRTUAL CHANNEL .5-2
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
CONTENTS (continued)
Section Page
5.5 MANAGED PARAMETERS FOR PACKET TRANSFER.5-3

ANNEX A ACRONYMS. A-1
ANNEX B INFORMATIVE REFERENCES . B-1
ANNEX C CHANGES FROM REFERENCE [B2] . C-1

Figure
1-1 Bit Numbering Convention. 1-4
2-1 Relationship with OSI Layers. 2-1
2-2 Relationships between Channels . 2-2
2-3 Asynchronous Service Model. 2-4
2-4 Synchronous Service Model . 2-5
2-5 Internal Organization of Protocol Entity (Sending End) . 2-10
2-6 Internal Organization of Protocol Entity (Receiving End) . 2-10
2-7 AOS Space Data Link Protocol Channel Tree. 2-11
4-1 AOS Transfer Frame Structural Components. 4-2
4-2 Transfer Frame Primary Header . 4-2
4-3 Multiplexing Protocol Data Unit (M_PDU) . 4-10
4-4 Bitstream Protocol Data Unit (B_PDU) . 4-13
4-5 Internal Organization of Protocol Entity (Sending End) . 4-17
4-6 Abstract Model of Packet Processing Function. 4-18
4-7 Abstract Model of Bitstream Processing Function. 4-19
4-8 Abstract Model of Virtual Channel Generation Function . 4-20
4-9 Abstract Model of Virtual Channel Multiplexing Function . 4-21
4-10 Abstract Model of Master Channel Multiplexing Function. 4-22
4-11 Abstract Model of All Frames Generation Function . 4-23
4-12 Internal Organization of Protocol Entity (Receiving End) . 4-24
4-13 Abstract Model of Packet Extraction Function. 4-25
4-14 Abstract Model of Bitstream Reception Function . 4-26
4-15 Abstract Model of Virtual Channel Reception Function. 4-27
4-16 Abstract Model of Virtual Channel Demultiplexing Function . 4-28
4-17 Abstract Model of Master Channel Demultiplexing Function . 4-29
4-18 Abstract Model of All Frames Reception Function. 4-30
Table
2-1 Summary of Services Provided by AOS Space Data Link Protocol . 2-6
5-1 Managed Parameters for a Physical Channel . 5-1
5-2 Managed Parameters for a Master Channel. 5-2
5-3 Managed Parameters for a Virtual Channel. 5-2
5-4 Managed Parameters for Packet Transfer. 5-3
C-1 Mapping of Terms That Have Been Redefined. C-3
CCSDS 732.0-B-2 Page vii July 2006
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
1 INTRODUCTION
1.1 PURPOSE
The purpose of this Recommendation is to specify the Advanced Orbiting Systems (AOS)
Space Data Link Protocol. This protocol is a Data Link Layer protocol (see reference [1]) to
be used over space-to-ground, ground-to-space, or space-to-space communications links by
space missions.
1.2 SCOPE
This Recommendation defines the AOS Space Data Link Protocol in terms of:
a) the services provided to the users of this protocol;
b) the protocol data units employed by the protocol; and
c) the procedures performed by the protocol.
It does not specify:
a) individual implementations or products;
b) the implementation of service interfaces within real systems;
c) the methods or technologies required to perform the procedures; or
d) the management activities required to configure and control the protocol.
1.3 APPLICABILITY
This Recommendation applies to the creation of Agency standards and to future data
communications over space links between Consultative Committee for Space Data Systems
(CCSDS) Agencies in cross-support situations. The Recommendation includes
comprehensive specification of the services and protocol 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 the 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.
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
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.
1.5 DOCUMENT STRUCTURE
This document is divided into five numbered sections and three 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 the AOS Space Data Link Protocol;
c) section 3 defines the services provided by the protocol entity;
d) section 4 specifies the protocol data units and procedures employed by the protocol
entity;
e) section 5 specifies the managed parameters used by the protocol entity;
f) annex A lists all acronyms used within this document;
g) annex B provides a list of informative references;
h) annex C lists the changes from the older CCSDS Recommendation (reference [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 [1]. 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:
a) blocking;
b) connection;
c) Data Link Layer;
d) entity;
e) flow control;
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
f) Network Layer;
g) peer entities;
h) Physical Layer;
i) protocol control information;
j) protocol data unit;
k) real system;
l) segmenting;
m) service;
n) Service Access Point (SAP);
o) SAP address;
p) service data unit.
1.6.1.2 Definitions from OSI Service Definition Conventions
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:
a) confirmation;
b) indication;
c) primitive;
d) request;
e) response;
f) service provider;
g) service user.
1.6.1.3 Terms Defined in this Recommendation
For the purposes of this Recommendation, the following definitions also apply. Many other
terms that pertain to specific items are defined in the appropriate sections.
aperiodic: not periodic (see below).
asynchronous: not synchronous (see below).
CCSDS 732.0-B-2 Page 1-3 July 2006
CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
delimited: having a known (and finite) length; applies to data in the context of data handling.
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.
periodic: of or pertaining to a sequence of events in which each event occurs at a fixed time
interval (within specified tolerance) after the previous event in the sequence.
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: of or pertaining to a sequence of events occurring in a fixed time relationship
(within specified tolerance) to another sequence of events. Note that ‘synchronous’ does not
necessarily imply ‘periodic’ or ‘constant rate’.
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 0 BITN–1
N-BIT DATA FIELD
FIRST BIT TRANSFERRED = MSB
Figure 1-1: Bit Numbering Convention
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
In accordance with standard data-communications practice, data fields are often grouped into
eight-bit ‘words’ which conform to the above convention. Throughout this
Recommendation, such an eight-bit word is called an ‘octet’. The numbering for octets
within a data structure starts with zero. 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.
[1] Information Technology—Open Systems Interconnection—Basic Reference Model:
The Basic Model. International Standard, ISO/IEC 7498-1. 2nd ed. Geneva: ISO,
1994.
[2] Information Technology—Open Systems Interconnection—Basic Reference Model—
Conventions for the Definition of OSI Services. International Standard, ISO/IEC
10731:1994. Geneva: ISO, 1994.
[3] TM Synchronization and Channel Coding. Recommendation for Space Data System
Standards, CCSDS 131.0-B-1. Blue Book. Issue 1. Washington, D.C.: CCSDS,
September 2003.
[4] Space Link Identifiers. Recommendation for Space Data System Standards, CCSDS
135.0-B-2. Blue Book. Issue 2. Washington, D.C.: CCSDS, November 2005.
[5] CCSDS Global Spacecraft Identification Field Code Assignment Control Procedures.
Recommendation for Space Data System Standards, CCSDS 320.0-B-3. Blue Book.
Issue 3. Washington, D.C.: CCSDS, April 2003.
[6] Space Packet Protocol. Recommendation for Space Data System Standards, CCSDS
133.0-B-1. Blue Book. Issue 1. Washington, D.C.: CCSDS, September 2003.
NOTE – Informative references are listed in annex B.
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
2 OVERVIEW
2.1 CONCEPT OF AOS SPACE DATA LINK PROTOCOL
2.1.1 ARCHITECTURE
The AOS Space Data Link Protocol is a Data Link Layer protocol (see reference [1]) to be
used by space missions. This protocol has been designed to meet the requirements of space
missions for efficient transfer of space application data of various types and characteristics
over space-to-ground, ground-to-space, or space-to-space communications links (hereafter
called space links).
Figure 2-1 illustrates the relationship of this protocol to the reference model of Open Systems
Interconnection (reference [1]). Two sublayers of the Data Link Layer are defined for
CCSDS space link protocols as shown in reference [B3]. The AOS Space Data Link
Protocol corresponds to the Data Link Protocol Sublayer, and provides functions of
transferring various data using a fixed-length protocol data unit called the Transfer Frame.
The Synchronization and Channel Coding Sublayer provides some additional functions
necessary for transferring Transfer Frames over a space link. These functions are
delimiting/synchronizing Transfer Frames, error-correction coding/decoding (optional), and
bit transition generation/removal (optional). For the Synchronization and Channel Coding
Sublayer, the TM Synchronization and Channel Coding Recommendation (reference [3])
must be used with the AOS Space Data Link Protocol. How the AOS Space Data Link
Protocol is used in overall space data systems is shown in reference [B3].
OSI LAYERS CCSDS LAYERS
CCSDS
PROTOCOLS
NETWORK AND NETWORK AND
UPPER LAYERS UPPER LAYERS
DATA LINK
TM SPACE DATA LINK
PROTOCOL
PROTOCOL
SUBLAYER
DATA LINK LAYER
SYNCHRONIZATION TM SYNCHRONIZATION
AND CHANNEL AND
CODING SUBLAYER CHANNEL CODING
PHYSICAL LAYER PHYSICAL LAYER
Figure 2-1: Relationship with OSI Layers
2.1.2 PROTOCOL FEATURES
The AOS Space Data Link Protocol provides the users with several services to transfer service
data units over a space link. To facilitate simple, reliable, and robust synchronization
procedures, fixed-length protocol data units are used to transfer data through the weak-signal,
noisy space links: their length is established for a particular Physical Channel (a single stream
of bits transferred over a space link in a single direction) during a particular Mission Phase by
management. These protocol data units are known as AOS Transfer Frames (unless otherwise
CCSDS 732.0-B-2 Page 2-1 July 2006
CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
stated, the terms ‘Transfer Frame’ and ‘Frame’ in this document refer to the AOS Transfer
Frame). Each Transfer Frame contains a header which provides protocol control information
and a fixed-length data field within which higher-layer service data units are carried.
A key feature of the AOS Space Data Link Protocol is the concept of ‘Virtual Channels’
(VC). The Virtual Channel facility allows one Physical Channel to be shared among
multiple higher-layer data streams, each of which may have different service requirements.
A single Physical Channel may therefore be divided into several separate logical data
channels, each known as a ‘Virtual Channel’. Each Transfer Frame transferred over a
Physical Channel belongs to one of the Virtual Channels of the Physical Channel.
2.1.3 ADDRESSING
There are three identifier fields in the header of Transfer Frames: Transfer Frame Version
Number (TFVN), Spacecraft Identifier (SCID), and Virtual Channel Identifier (VCID). The
concatenation of a TFVN and a SCID is known as a Master Channel Identifier (MCID), and
the concatenation of an MCID and a VCID is called a Global Virtual Channel Identifier
(GVCID). Therefore,
MCID = TFVN + SCID;
GVCID = MCID + VCID = TFVN + SCID + VCID.
Each Virtual Channel in a Physical Channel is identified by a GVCID. Therefore, a Virtual
Channel consists of Transfer Frames with the same GVCID.
All Transfer Frames with the same MCID on a Physical Channel constitute a Master Channel
(MC). A Master Channel consists of one or more Virtual Channels. In most cases, a Physical
Channel carries only Transfer Frames of a single MCID, and the Master Channel will be
identical with the Physical Channel. However, a Physical Channel may carry Transfer Frames
with multiple MCIDs (with the same TFVN). In such a case, the Physical Channel consists of
multiple Master Channels. A Physical Channel is identified with a Physical Channel Name,
which is set by management and not included in the header of Transfer Frames.
The relationships between these Channels are shown in figure 2-2.
Virtual Channel:
Identified by GVCID
Master Channel:
Identified by MCID
Physical Channel:
Identified by Physical
Channel Name
Figure 2-2: Relationships between Channels
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
2.1.4 PROTOCOL DESCRIPTION
The AOS Space Data Link Protocol is described in terms of:
a) the services provided to the users;
b) the protocol data units; and
c) the procedures performed by the protocol.
The service definitions are given in the form of primitives, which present an abstract model of the
logical exchange of data and control information between the protocol entity and the service user.
The definitions of primitives are independent of specific implementation approaches.
The procedure specifications define the procedures performed by protocol entities for the
transfer of information between peer entities. The definitions of procedures are independent
of specific implementation methods or technologies.
This protocol specification also specifies the requirements for the underlying services
provided by the Channel Coding Sublayer and the Physical Layer.
2.2 OVERVIEW OF SERVICES
2.2.1 COMMON FEATURES OF SERVICES
The AOS Space Data Link Protocol provides users with data transfer services. The point at
which a service is provided by a protocol entity to a user is called a Service Access Point
(SAP) (see reference [1]). Each service user is identified by a SAP address.
Service data units submitted to a SAP are processed in the order of submission. No
processing order is maintained for service data units submitted to different SAPs.
NOTE – Implementations may be required to perform flow control at an SAP between the
service user and the service provider. However, CCSDS does not recommend a
scheme for flow control between the user and the provider.
The followings are features common to all the services defined by this Recommendation:
a) unidirectional (one way) services: one end of a connection can send, but not receive,
data through the space link, while the other end can receive, but not send.
b) unconfirmed services: the sending user does not receive confirmation from the
receiving end that data has been received.
c) incomplete services: the services do not guarantee completeness, but some services
may signal gaps in the sequence of service data units delivered to the receiving user.
d) sequence-preserving services: the sequence of service data units supplied by the
sending user is preserved through the transfer over the space link, although there may
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
be gaps and duplications in the sequence of service data units delivered to the
receiving user.
NOTE – This Recommendation assumes that these services are provided at the end points of a
space link. However, this Recommendation makes no assumptions concerning how
these end points are composed or configured either on-board a spacecraft or in a
ground system. In a ground system, the services defined by this Recommendation
may be extended or enhanced with Space Link Extension Services (reference [B4]).
2.2.2 SERVICE TYPES
2.2.2.1 Overview
The AOS Space Data Link Protocol provides three service types (asynchronous,
synchronous, and periodic) that determine how service data units supplied by the user are
transferred in protocol data units over a space link.
The models shown below are intended only to illustrate the characteristics of services. They
are not intended to guide or restrict design of on-board or ground systems.
2.2.2.2 Asynchronous Service
In asynchronous service, there are no timing relationships between the transfer of service
data units supplied by the service user and the transmission of Transfer Frames generated by
the service provider. The user may request data transfer at any time it desires, but there may
be restrictions imposed by the provider on the data generation rate. In this service (figure
2-3), each service data unit from a sending user is placed in a queue, the contents of which
are sent to a receiving user in the order in which they were presented. Although transmission
errors may prevent delivery of some data units, the service provider attempts to transfer all
data units provided by the user exactly once. The timing of data transfer is determined by the
provider in accordance with mission-specific rules, and may depend on the traffic at the time
of transfer. The key feature of this service is that all of the service data units from the
sending user are transferred, and transferred only once.
Sending Receiving
User User
Provider
Transfer from sending end to
receiving end
Figure 2-3: Asynchronous Service Model
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CCSDS RECOMMENDED STANDARD FOR AOS SPACE DATA LINK PROTOCOL
2.2.2.3 Synchronous Service
In synchronous service, the transfer of service data units is synchronized with the release of either
(1) Transfer Frames of a Virtual Channel, (2) Transfer Frames of a Master Channel, or (3) all
Transfer Frames of a Physical Channel. The transfer timing may be periodic or aperiodic.
In this service (figure 2-4), each service data unit from a sending user is placed in a buffer
that can hold only one service data unit; the content of the buffer is sent to a receiving user at
the time when a Transfer Frame is transmitted. The transmission timing of Transfer Frames
is determined by the service provider according to mission-specific rules (usually known to
the user). The key feature of this service, which is essentially time-division multiplexing, is
that the timing of data transfer is driven by the transfer mechanism, not by individual service
requests from the user. Thus a particular service data unit from a user might be sent once,
several times (if the ‘new’ value is not placed in the buffer soon enough), or not at all (if one
value is replaced by a second before the service provider can send it).
Sending Receiving
User User
Buffer
Provider
Buffer content is transferred
once per Transfer Frame
Transfer from buffer at sending end
to receiving end
Figure 2-4: Synchronous Service Model
2.2.2.4 Periodic Service
Periodic service is a special case of synchronous service in which service data units are
transferred at a constant rate. Periodic transfer from service interface to service interface is
provided with a specified maximum delay and a specified maximum jitter at the service
interface. There are three cases in which a synchronous service is periodic:
a) If the service is associated with a Virtual Channel (or a Master Channel) and that
Virtual (or Master) Channel produces Transfer Frames at a constant rate, then the
service is periodic.
b) If the service is associated with a Master Channel and there is only one Master
Channel in the Physical Channel, then the service is periodic.
For periodic services, all service data units are sent only once if the user supplies service data
units
...