ISO 22671:2005

INTERNATIONAL ISO
STANDARD 22671
First edition
2005-06-15
Space data and information transfer
systems — Space link extension (SLE) —
Forward command link transmission unit
(CLTU)
Systèmes de transfert des données et informations spatiales —
Extension de liaisons spatiales (SLE) — Unité de transmission pour la
liaison d'envoi de télécommande (CLTU)

Reference number
©
ISO 2005
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2005
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2005 – 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 22671 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 13, Space data and information transfer systems.
International Standard ISO 22671 was prepared by the Consultative Committee for Space Data Systems
(CCSDS) (as CCSDS 912.1-B-1, April 2002) 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 22671:2005(E)

Space data and information transfer systems — Space link
extension (SLE) — Forward command link transmission
unit (CLTU)
1 Scope
This International Standard specifies the forward command link transmission unit (CLTU) that may be
provided by a space link extension (SLE) system for inter-agency cross support. The CLTU service is an SLE
transfer service that enables a mission to send CLTUs to a spacecraft.
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 912.1-B-1, April 2002, Space link extension — Forward CLTU service specification.
For the purposes of international standardization, the modifications outlined below shall apply to the specific
clauses and paragraphs of publication CCSDS 912.1-B-1.
Pages i to v
This part is information which is relevant to the CCSDS publication only.
Page 1-12
Add the following information to the references indicated:
[1] Document CCSDS 910.4-B-1, May 1996, is equivalent to ISO 15396:1998.
[2] Document CCSDS 201.0-B-3, June 2000, is equivalent to ISO 12171:2002.
[3] Document CCSDS 202.0-B-3, June 2001, is equivalent to ISO 12172:2003.
[4] Document CCSDS 202.1-B-2, June 2001, is equivalent to ISO 12173:2003.
[5] Document CCSDS 301.0-B-3, January 2001, is equivalent to ISO 11104:2003.
[6] ISO/IEC 8824:1990 has been cancelled and replaced by ISO/IEC 8824-1:2002.
[8] ISO/IEC 9594-2:1998 has been cancelled and replaced by ISO/IEC 9594-2:2001.
3 Revision of publication CCSDS 912.1-B-1
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 912.1-B-1. To
this end, NASA will act as a liaison body between CCSDS and ISO.

2 © ISO 2005 – All rights reserved

RECOMMENDATION FOR SPACE
DATA SYSTEM STANDARDS
SPACE LINK EXTENSION—
FORWARD CLTU
SERVICE SPECIFICATION
CCSDS 912.1-B-1
BLUE BOOK
April 2002
(Blank page)
4 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
AUTHORITY
Issue: Blue Book, Issue 1
Date: April 2002
Location: Oberpfaffenhofen, Germany

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 [E1], 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
Program Integration Division (Code M-3)
National Aeronautics and Space Administration
Washington, DC 20546, USA
CCSDS 912.1-B-1 Page i April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
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:

o 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.

o 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.

o Specific service arrangements shall be 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 912.1-B-1 Page ii April 2002
6 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
FOREWORD
This document is a technical Recommendation for use in developing ground systems for
space missions and has been prepared by the Consultative Committee for Space Data
Systems (CCSDS). The Space Link Extension (SLE) Forward Command Link Transmission
Unit (CLTU) Service described herein is intended for missions that are cross supported
between Agencies of the CCSDS.

This Recommendation specifies a data service that extends certain of the space-to-ground
communications services previously defined by CCSDS (references [2] and [3]) within the
framework established by the CCSDS SLE Reference Model (reference [1]). It allows
implementing organizations within each Agency to proceed with the development of
compatible derived Standards for the ground systems that are within their cognizance. Derived
Agency Standards may implement only a subset of the optional features allowed by the
Recommendation and may incorporate features not addressed by the 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 [E1].
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 912.1-B-1 Page iii April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
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 Centre (CRC)/Canada.
– 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 912.1-B-1 Page iv April 2002
8 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
DOCUMENT CONTROL
Document Title Date Status and Substantive Changes

CCSDS Space Link Extension— April Original Issue
912.1-B-1 Forward CLTU Service 2002
Specification
CCSDS 912.1-B-1 Page v April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
CONTENTS
Section Page
1 INTRODUCTION . 1-1

1.1 PURPOSE OF THIS RECOMMENDATION. 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 DEFINITIONS, NOMENCLATURE, AND CONVENTIONS. 1-4
1.7 REFERENCES.1-12

2 DESCRIPTION OF THE FORWARD CLTU SERVICE . 2-1

2.1 OVERVIEW.2-1
2.2 SPACE LINK EXTENSION REFERENCE MODEL. 2-2
2.3 SERVICE MANAGEMENT.2-3
2.4 ARCHITECTURE MODEL – FUNCTIONAL VIEW. 2-4
2.5 ARCHITECTURE MODEL – CROSS-SUPPORT VIEW. 2-6
2.6 FUNCTIONAL DESCRIPTION.2-8
2.7 OPERATIONAL SCENARIO.2-15

3 OPERATIONS AND THEIR PARAMETERS . 3-1

3.1 GENERAL CONSIDERATIONS.3-1
3.2 CLTU-BIND.3-7
3.3 CLTU-UNBIND.3-14
3.4 CLTU-START.3-17
3.5 CLTU-STOP.3-21
3.6 CLTU-TRANSFER-DATA.3-24
3.7 CLTU-ASYNC-NOTIFY.3-30
3.8 CLTU-SCHEDULE-STATUS-REPORT.3-36
3.9 CLTU-STATUS-REPORT.3-40
3.10 CLTU-GET-PARAMETER.3-45
3.11 CLTU-THROW-EVENT.3-49
3.12 CLTU-PEER-ABORT.3-53

4 CLTU PROTOCOL . 4-1

4.1 GENERIC PROTOCOL CHARACTERISTICS . 4-1
4.2 CLTU SERVICE PROVIDER BEHAVIOR . 4-4

CCSDS 912.1-B-1 Page vi April 2002
10 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
CONTENTS (continued)
Section Page
ANNEX A DATA TYPE DEFINITIONS . A-1
ANNEX B INDEX TO DEFINITIONS. B-1
ANNEX C ACRONYMS. C-1
ANNEX D CONFORMANCE OPTIONS MATRIX . D-1
ANNEX E INFORMATIVE REFERENCES .E-1
ANNEX F THROW EVENT DEFINITIONS.F-1
ANNEX G PRODUCTION STATUS . G-1

Figure
1-1 Space Link Extension (SLE) Services Documentation . 1-4
2-1 Forward TC Space Link Processing SLE-FG. 2-4
2-2 Forward CLTU Service Production and Provision. 2-6
2-3 Example of Management and Provision of Forward CLTU Service. 2-7
2-4 Forward CLTU Service Provider State Transition Diagram. 2-10
2-5 Communications Realization of Forward CLTU Service. 2-13
G-1 CLTU Production Status Transitions. G-1

Table
2-1 Forward CLTU Service Operations . 2-9
3-1 Setting of Forward CLTU Service Configuration Parameters. 3-7
3-2 CLTU-BIND Parameters. 3-8
3-3 CLTU-UNBIND Parameters. 3-15
3-4 CLTU-START Parameters. 3-17
3-5 CLTU-STOP Parameters . 3-21
3-6 CLTU-TRANSFER-DATA Parameters. 3-24
3-7 CLTU-ASYNC-NOTIFY Parameters. 3-30
3-8 CLTU-SCHEDULE-STATUS-REPORT Parameters. 3-36
3-9 CLTU-STATUS-REPORT Parameters . 3-40
3-10 CLTU-GET-PARAMETER Parameters. 3-45
3-11 Forward CLTU Parameters . 3-47
3-12 CLTU-THROW-EVENT Parameters. 3-50
3-13 CLTU-PEER-ABORT Parameters . 3-53
4-1 Behavior of Provider . 4-7
4-2 Event Description References. 4-10
4-3 Predicate Definitions . 4-10
4-4 Boolean Flags . 4-11
4-5 Compound Action Definitions . 4-11
CCSDS 912.1-B-1 Page vii April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
CONTENTS (continued)
Table Page
D-1 Conformance Matrix for CLTU Service (Operations).D-1
D-2 Conformance Matrix for CLTU Service (Other Requirements).D-2
F-1 Throw Event Examples. F-1
G-1 Production Status Changes and Notifications .G-2
G-2 Effect of Production Status on Operations.G-3

CCSDS 912.1-B-1 Page viii April 2002
12 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
1 INTRODUCTION
1.1 PURPOSE OF THIS RECOMMENDATION
This Recommendation defines the Command Link Transmission Unit (CLTU) service in
conformance with the transfer services specified in reference [1], Cross Support Reference
Model—Part 1: SLE Services. The Forward CLTU service is a Space Link Extension (SLE)
transfer service that enables a mission to send Command Link Transmission Units (CLTUs) to
a spacecraft.
1.2 SCOPE
1.2.1 This Recommendation defines, in an abstract manner, the Forward CLTU service in
terms of:
a) the operations necessary to provide the transfer service;
b) the parameter data associated with each operation;
c) the behaviors that result from the invocation of each operation; and
d) the relationship between, and the valid sequence of, the operations and resulting
behaviors.
1.2.2 It does not specify:
a) individual implementations or products;
b) the implementation of entities or interfaces within real systems;
c) the methods or technologies required to radiate data to a spacecraft and to acquire
telemetry frames from the signals received from the spacecraft for extraction of the
Operational Control Field;
d) the methods or technologies required for communications;
e) the management activities necessary to schedule, configure, and control the Forward
CLTU service.
1.3 APPLICABILITY
1.3.1 APPLICABILITY OF THIS RECOMMENDATION
This Recommendation provides a basis for the development of real systems that implement the
Forward CLTU service. Implementation of the Forward CLTU service in a real system
additionally requires the availability of a communications service to convey invocations and
returns of Forward CLTU service operations between service users and providers. This
CCSDS 912.1-B-1 Page 1-1 April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
Recommendation requires that such a communications service ensure that invocations and
returns of operations are transferred:
a) in sequence;
b) completely and with integrity;
c) without duplication;
d) with flow control that notifies the application layer in the event of congestion; and
e) with notification to the application layer in the event that communications between the
Forward CLTU service user and the Forward CLTU service provider are disrupted,
possibly resulting in a loss of data.
It is the specific intent of this Recommendation to define the Forward CLTU service in a
manner that is independent of any particular communications services, protocols, or
technologies.
1.3.2 LIMITS OF APPLICABILITY
This Recommendation specifies the Forward CLTU service that may be provided by an SLE
System for inter-Agency cross support. It is neither a specification of, nor a design for, real
systems that may be implemented for the control and monitoring of existing or future
missions.
1.4 RATIONALE
The goal of this Recommendation is to create a standard for interoperability between the
tracking stations or ground data handling systems of various agencies and the users of forward
services.
1.5 DOCUMENT STRUCTURE
1.5.1 DOCUMENT ORGANIZATION
This Recommendation is organized as follows:
a) section 1 provides purpose, scope, applicability, and rationale of this Recommendation
and lists definitions, nomenclature, conventions, and references used throughout the
Recommendation;
b) section 2 presents an overview of the Forward CLTU service including a functional
description, the service management context, and protocol considerations;
c) section 3 specifies the operations of the Forward CLTU service;
CCSDS 912.1-B-1 Page 1-2 April 2002
14 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
d) section 4 specifies the dynamic behavior of the Forward CLTU service in terms of the
state transitions of the Forward CLTU service provider;
e) annex A is a formal specification of Forward CLTU service data types, using the
Abstract Syntax Notation One (ASN.1);
f) annex B lists all terms used in this document and identifies where they are defined;
g) annex C lists all acronyms used within this document;
h) annex D provides a conformance matrix that defines what capabilities must be
provided for an implementation to be considered compliant with this
Recommendation;
i) annex E contains a list of informative references;
j) annex F contains examples of usage of the CLTU-THROW-EVENT operation;
k) annex G explains the relationship of the Forward CLTU service to the status of the
forward space link channel.
1.5.2 SLE SERVICES DOCUMENTATION TREE
This Recommendation is based on the architectural model for cross support defined in the
SLE Reference Model (reference [1]). It expands upon the concept of an SLE transfer service
as interactions between SLE Mission User Entities (MUEs) and an SLE transfer service
provider for the purpose of providing the Forward CLTU transfer service.
This Recommendation is part of a suite of documents specifying the SLE Services. The SLE
Services constitute one of the three types of Cross Support Services:
a) Part 1: SLE Services;
b) Part 2: Ground Communications Services;
c) Part 3: Ground Domain Services.
The basic organization of the SLE services documentation is shown in figure 1-1. The
documents are described in the following paragraphs.
CCSDS 912.1-B-1 Page 1-3 April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
Standard Terminology
Conventions, and
Cross Support
Methodology for
Concept
Defining Data Services
Part 1:
SLE Services
Cross Support
Reference Model
Part 1:
SLE Services
SLE Transfer Services
SLE Service Forward Return
Management SLE SLE
Specification Suite Service Specifications Service Specifications

Figure 1-1: Space Link Extension (SLE) Services Documentation
a) Standard Terminology, Conventions and Methodology (TCM) for Defining Data
Services (reference [E3]): a Report identifying selected international standards relevant
for the definition of data services;
b) Cross Support Concept – Part 1: Space Link Extension Services (reference [E4]): a
Report introducing the concepts of cross support and SLE services;
c) Cross Support Reference Model—Part 1: Space Link Extension Services (reference
[1]): a Recommendation that defines the framework and terminology for the
specification of SLE services;
d) SLE Service Management Specification Suite; a set of Recommendations that
establish the basis for SLE service management (reference [E5] is the primary
Recommendation in that set);
e) Forward SLE Transfer Service Specifications; a set of Recommendations that will
provide specification of all forward link SLE transfer services (this Recommendation is
one of the specifications in that set);
f) Return SLE Transfer Service Specifications; a set of Recommendations that will
provide specification of all return link SLE transfer services.
1.6 DEFINITIONS, NOMENCLATURE, AND CONVENTIONS
1.6.1 DEFINITIONS
1.6.1.1 Definitions from OSI Basic Reference Model
This Recommendation makes use of a number of terms defined in reference [7]. The use of
those terms in this Recommendation shall be understood in a generic sense; i.e., in the sense
CCSDS 912.1-B-1 Page 1-4 April 2002
16 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
that those terms are generally applicable to technologies that provide for the exchange of
information between real systems. Those terms are:
a) abstract syntax;
b) application entity;
c) application layer;
d) flow control;
e) Open System Interconnection (OSI);
f) real system;
g) service access point (SAP).
1.6.1.2 Definitions from OSI Abstract Syntax Notation One
This Recommendation makes use of the following terms defined in reference [6]:
a) Abstract Syntax Notation One (ASN.1);
b) object identifier;
c) (data) type;
d) (data) value.
NOTE − In annex A of this Recommendation, ASN.1 is used for specifying the abstract
syntax of the invocations and returns of the operations of the Forward CLTU
service. The use of ASN.1 as a descriptive language is intended to support the
specification of the abstract Forward CLTU service; it is not intended to constrain
implementations. In particular, there is no requirement for implementations to
employ ASN.1 encoding rules. ASN.1 is simply a convenient tool for formally
describing the abstract syntax of the invocations and returns of the Forward
CLTU service.
1.6.1.3 Definitions from OSI Directory – Part 2: Models
This Recommendation makes use of the following terms defined in reference [8]:
a) distinguished name (DN);
b) relative distinguished name (RDN).
CCSDS 912.1-B-1 Page 1-5 April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
1.6.1.4 Definitions from CCSDS Telecommand Channel Service
This Recommendation makes use of the following terms defined in reference [2]:
a) acquisition sequence;
b) command link transmission unit (CLTU);
c) carrier modulation mode (CMM);
d) idle sequence;
e) physical layer operations procedure (PLOP).
1.6.1.5 Definitions from CCSDS Telecommand Data Routing Service
This Recommendation makes use of the following term defined in reference [3]:
command link control word (CLCW).
1.6.1.6 Definitions from CCSDS SLE Reference Model
This Recommendation makes use of the following terms defined in reference [1]:
a) abstract binding;
b) abstract object;
c) abstract port;
d) abstract service;
e) CLTU channel;
f) Forward CLTU service;
g) invoker;
h) Mission Data Operation System (MDOS);
i) Mission User Entity (MUE);
j) offline delivery mode;
k) online delivery mode;
l) operation;
m) performer;
n) physical channel;
CCSDS 912.1-B-1 Page 1-6 April 2002
18 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
o) service agreement;
p) service provider (provider);
q) service user (user);
r) SLE Complex;
s) SLE Complex Management;
t) SLE data channel;
u) SLE functional group (SLE-FG);
v) SLE protocol data unit (SLE-PDU);
w) SLE service data unit (SLE-SDU);
x) SLE service package;
y) SLE System;
z) SLE transfer service instance;
aa) SLE transfer service production;
bb) SLE transfer service provision;
cc) SLE Utilization Management;
dd) space link;
ee) space link data channel;
ff) space link data unit (SL-DU);
gg) space link session.
1.6.1.7 Additional Definitions
1.6.1.7.1 General
For the purposes of this Recommendation, the following definitions also apply.
1.6.1.7.2 Association
An association is a cooperative relationship between an SLE service-providing application
entity and an SLE service-using application entity. An association is formed by the exchange
of SLE protocol data units through use of an underlying communications service.
CCSDS 912.1-B-1 Page 1-7 April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
1.6.1.7.3 Communications Service
A communications service is a capability that enables an SLE service-providing application
entity and an SLE service-using application entity to exchange information.
NOTE – If an SLE service user and an SLE service provider are implemented using
different communications services, then interoperability between them is possible
only by means of a suitable gateway. Adherence to this Recommendation ensures,
at least in principle, that it is possible to construct such a gateway.
1.6.1.7.4 Confirmed Operation
A confirmed operation is an operation that requires the performer to return a report of its
outcome to the invoker.
1.6.1.7.5 Initiator
The initiator is the object that issues the request to bind to another object (the responder).
1.6.1.7.6 Invocation
The invocation of an operation is the making of a request by an object (the invoker) to
another object (the performer) to carry out the operation.
1.6.1.7.7 Parameter
A parameter of an operation is data that may accompany the operation’s invocation or return.
NOTE – The term parameter is also used to refer to mission-dependent configuration
information used in production or provision of the service.
1.6.1.7.8 Performance
The performance of an operation is the carrying out of the operation by an object (the
performer).
1.6.1.7.9 Port Identifier
A port identifier identifies a source or a destination in a communications system.
NOTE – See 2.6.4.6 for more information.
CCSDS 912.1-B-1 Page 1-8 April 2002
20 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
1.6.1.7.10 Responder
The responder is the object that receives a request to bind and completes the binding (if
possible) with the initiator in order for a service association to exist between the two objects.
1.6.1.7.11 Return
The return of an operation is a report, from the performer to the invoker, of the outcome of
the performance of the operation.
1.6.1.7.12 Service Instance Provision Period
The service instance provision period is the time during which a service instance (i.e., the
capability to transfer one or more SLE data channels of a given type) is scheduled to be
provided.
1.6.1.7.13 Unconfirmed Operation
An unconfirmed operation is an operation that does not require a report of its outcome to be
returned to the invoker by the performer.
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
1.6.3.1 Specification of Operations
1.6.3.1.1 General
Section 3 of this Recommendation specifies the operations that constitute the Forward CLTU
service. The specification of each operation is divided into subsections as follows:
CCSDS 912.1-B-1 Page 1-9 April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
1.6.3.1.2 Purpose Subsection
The Purpose subsection briefly describes the purpose and functioning of the operation.
Additionally, it indicates whether the operation may be invoked by the user, provider, or both;
whether the operation is confirmed or unconfirmed; and whether there are any constraints on
when the operation may be invoked.
1.6.3.1.3 Invocation, Return, and Parameters Subsection
The Invocation, Return, and Parameters subsection describes the parameters associated with
each operation, including their semantics. A table accompanying the description of each
operation lists all parameters associated with the operation and, for both the invocation and
return, whether the parameter is always present, always absent, or conditionally present.
For parameters that are conditionally present, the parameter description specifies the
conditions for the presence or absence of the parameter. The condition is generally based on
the value of another parameter in the same invocation or return; for example, in return of an
operation, the parameter diagnostic is present if and only if the value of the parameter
result is ‘negative result’. For a conditional parameter in a return, the condition may be
based on the value of a parameter in the corresponding invocation.
In the table, the following convention is used to indicate whether a parameter is always
present, always absent, or conditionally present:
M Always present (Mandatory)
C Conditionally present
Blank Always absent
NOTE – Even though a parameter may be characterized as always present, its description
may specify that its value is permitted to be ‘null’ or ‘unused’ or the like.
1.6.3.1.4 Effects Subsection
The Effects subsection describes the effects an operation has on the invoker, the performer,
the association between them, or any combination thereof. The details of how those effects
occur or the mechanisms used are outside the scope of this Recommendation.
1.6.3.2 Typographic Conventions
1.6.3.2.1 Operation Names
Names of Forward CLTU service operations always appear in uppercase and begin with the
characters ‘CLTU-’ (e.g., CLTU-TRANSFER-DATA).
CCSDS 912.1-B-1 Page 1-10 April 2002
22 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
1.6.3.2.2 Parameter Names
In the main text, names of parameters of Forward CLTU service operations generally appear
in lowercase and are always typeset in a fixed-width font (e.g., responder-port-
identifier). In annex A, the corresponding name is generally formed by omitting any
hyphens contained in the name and using mixed-case (e.g.,
responderPortIdentifier).
1.6.3.2.3 Value Names
The values of many parameters discussed in this Recommendation are represented by names.
In the main text, these names are shown in single quotation marks (e.g., ‘no such service
instance’). The corresponding name in annex A is generally formed by omitting any hyphens
or white space contained in the name and using mixed-case (e.g.,
noSuchServiceInstance). The actual value associated with the name is constrained by
the type of the parameter taking on this value. Parameter types are specified in annex A of this
Recommendation.
NOTE – The name of a value does not imply anything about type. For example, the value
‘no such service instance’ has the appearance of a character string but might be
assigned to a parameter whose type is ‘integer’.
1.6.3.2.4 State Names
This Recommendation specifies the states of Forward CLTU service providers. States may be
referred to by number (e.g., state 3) or by name. State names are always shown in single
quotation marks (e.g., ‘active’).
1.6.3.2.5 SLE-PDU Names
The names of SLE-PDUs appear in mixed-case (e.g., cltuBindInvocation).
1.6.3.2.6 Data Type Definitions
Data type definitions for the Forward CLTU service are presented in annex A in the form of a
set of ASN.1 modules. Regardless of the conventions used elsewhere in this
Recommendation, the text of the ASN.1 modules is typeset entirely in a fixed-width font.
1.6.3.3 Other Conventions
This Recommendation uses the conventions specified in reference [1].
CCSDS 912.1-B-1 Page 1-11 April 2002
CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
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.
NOTE – A list of informative references forms annex E of this Recommendation.
[1] Cross Support Reference Model — Part 1: Space Link Extension Services.
Recommendation for Space Data System Standards, CCSDS 910.4-B-1. Blue Book.
Issue 1. Washington, D.C.: CCSDS, May 1996.
[2] Telecommand Part 1—Channel Service. Recommendation for Space Data System
Standards, CCSDS 201.0-B-3. Blue Book. Issue 3. Washington, D.C.: CCSDS, June
2000.
[3] Telecommand Part 2—Data Routing Service. Recommendation for Space Data System
Standards, CCSDS 202.0-B-3. Blue Book. Issue 3. Washington, D.C.: CCSDS, June
2001.
[4] Telecommand Part 2.1—Command Operation Procedures. Recommendation for Space
Data System Standards, CCSDS 202.1-B-2. Blue Book. Issue 2. Washington, D.C.:
CCSDS, June 2001.
[5] Time Code Formats. Recommendation for Space Data System Standards, CCSDS
301.0-B-3. Blue Book. Issue 3. Washington, D.C.: CCSDS, January 2002.
[6] Information Technology—Open Systems Interconnection—Specification of Abstract
Syntax Notation One (ASN.1). International Standard, ISO/IEC 8824:1990. 2nd ed.
Geneva: ISO, 1990.
[7] Information Technology—Open Systems Interconnection—Basic Reference Model:
The Basic Model. International Standard, ISO/IEC 7498-1. 2nd ed. Geneva: ISO,
1994.
[8] Information Technology—Open Systems Interconnection—The Directory: Models.
International Standard, ISO/IEC 9594-2:1998. 3rd ed. Geneva: ISO, 1998.
CCSDS 912.1-B-1 Page 1-12 April 2002
24 © ISO 2005 – All rights reserved

CCSDS RECOMMENDATION FOR SLE FORWARD CLTU SERVICE
2 DESCRIPTION OF THE FORWARD CLTU SERVICE
2.1 OVERVIEW
The Forward CLTU service enables the user of the service to send Command Link
Transmission Units (CLTUs) to a spacecraft via an established forward space link channel. A
forward space link channel is a physical channel carrying an asynchronous stream of CLTUs
(reference [1]).
The service user submits CLTUs, encapsulated in Space Link Extension (SLE) Service Data
Units (SLE-SDUs), by means of the CLTU-TRANSFER-DATA operation. Production of the
Forward CLTU service by the provider entails processing the CLTUs transferred by the user
through the necessary transformations to modulate the Radio Frequency (RF) carrier channel
providing uplink communications with the spacecraft.
The Forward CLTU service transmits the CL
...