ISO 21217:2010

INTERNATIONAL ISO
STANDARD 21217
First edition
2010-04-15
Intelligent transport systems —
Communications access for land mobiles
(CALM) — Architecture
Systèmes intelligents de transport — Accès aux communications des
services mobiles terrestres (CALM) — Architecture

Reference number
©
ISO 2010
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 2010
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 2010 – All rights reserved

Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Conformance .1
3 Normative references.1
4 Terms, definitions and abbreviated terms.1
4.1 Terms and definitions .1
4.2 Abbreviated terms .4
5 Requirements.4
5.1 Principles for CALM-related International Standards.5
5.2 ITS viewpoint .5
5.3 Handover .6
5.4 ITS subsystems and ITS-Ss.8
5.5 Communication scenarios and classes.11
5.6 ITS-S architecture.14
5.7 CALM communications kernel.20
5.8 Services.20
5.9 Details related to the access layer.21
5.10 Details related to the networking and transport layer .23
5.11 Details related to facilities layer.24
5.12 Details related to ITS-S applications .25
5.13 Management elements.25
5.14 Security.26
6 Declaration of patents and intellectual property.26
Annex A (informative) CALM standards reference.30
Annex B (informative) Examples of details of communication scenarios .31
Annex C (informative) Implementations .34
Bibliography.38

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 21217 was prepared by Technical Committee ISO/TC 204, Intelligent transport systems.
iv © ISO 2010 – All rights reserved

Introduction
This International Standard is part of a family of International Standards based on the communications access
for land mobiles (CALM) concept. These International Standards specify a common architecture, network
protocols and communication interface definitions for wired and wireless communications using various
access technologies including cellular 2nd generation, cellular 3rd generation, satellite, infra-red, 5 GHz micro-
wave, 60 GHz millimetre-wave and mobile wireless broadband. These and other access technologies that can
be incorporated are designed to provide broadcast, unicast and multicast communications between mobile
stations, between mobile and fixed stations and between fixed stations in the intelligent transport systems
(ITS) sector.
This International Standard describes the common architectural framework around which CALM-compliant
communication entities called ITS stations (ITS-Ss) are instantiated, and provides the architectural reference
for use by the CALM family of International Standards, including the lower layer service access point
specifications described in ISO 21218, network protocol specifications described in ISO 21210 (IPv6
networking) and ISO 29281 (non-IP networking), and the ITS-S management specifications described in
ISO 24102.
The relationship between the members of the CALM family of International Standards is shown in Figure A.1.
The numbers in the boxes are references to the International Standard in which the indicated functionality is
specified.
The functional requirements for information transmission in the ITS sector over large distances using wireless
access technologies may be very different from the requirements for, for example, European dedicated short
range communication (DSRC). In ITS, large volumes of data are required for purposes such as safety, traffic
information and management, video downloads to mobile stations for tourist information and entertainment
and navigation-system-updates. In order to support such services, mobile stations need to be able to
communicate over longer ranges with fixed stations, and the system must be able to hand over sessions from
one fixed station to another. Thus, the CALM family of International Standards is explicitly designed to enable
quasi-continuous communications, communications of protracted duration, and short messages and sessions
of high priority with stringent time constraints.
CALM-complaint systems provide the ability to support handover of different types. One of the essential
features of the CALM concept is the ability to support media independent handover (MIH), also referred to as
heterogeneous handover, between the various access technologies supported by CALM, e.g. cellular, satellite,
microwave, mobile wireless broadband, infra-red, DSRC. With this flexibility, CALM-complaint systems
provide the ability to use the most appropriate access technology for message delivery. Selection rules that
are supported include user preferences and access technology capabilities in deciding which access
technology to use for a particular session, and when to handover between access technologies or between
service providers on the same access technology. It is also important to note that communication between
ITS-Ss is peer-to-peer, regardless of the networks providing the connectivity. This provides flexibility in
designing applications for the ITS sector. While this flexibility is very important in providing quasi-continuous
connectivity, applications may be restricted to specific access technologies and operational frequency bands,
if required.
A fundamental advantage of the CALM concept over traditional systems is that applications are abstracted
from the access technologies that provide the wireless connectivity and the networks that transport the
information from the source to the destination(s). With reference to Figure A.1, this means that ITS-Ss are not
limited to a single access technology and networking protocol, and can implement any of those supported; it
also means that the ITS-S management can make optimal use of all these resources. To exploit this flexibility,
CALM-complaint systems provide the ability to support handover of different types, including those involving a
change of communication interface (which may or may not involve a change of access technology, since
ITS-Ss may have multiple communication interfaces using the same access technology), those involving
reconfiguration or change of the network employed to provide connectivity, and those involving both a change
in communication interface and network reconfiguration.
The architecture specified within this International Standard makes provision for rapid session initialization,
where this is required, e.g. for road safety applications.
The architecture specified within this International Standard supports a variety of different ITS-S
implementations, ranging from “simple single-box implementations” up to “complex distributed
implementations” where the complete ITS-S functionality is distributed in several physical boxes
interconnected with wired or wireless local networks. The instantiation of access technologies used for these
local station-internal networks follows the same principles as the instantiation of access technologies used to
connect to external networks.
vi © ISO 2010 – All rights reserved

INTERNATIONAL STANDARD ISO 21217:2010(E)

Intelligent transport systems — Communications access for
land mobiles (CALM) — Architecture
1 Scope
This International Standard specifies the architectural communications framework of intelligent transport
systems (ITS) for the family of CALM-related International Standards. The architecture is described in an
abstract way with several graphical views and examples. The graphical representations partly follow the ISO
Open Systems Interconnection (OSI) principles. In addition to the requirements specified within this
International Standard, a number of notes and examples are provided to illustrate the CALM concept.
Wherever practicable, this International Standard has been developed by reference to suitable extant
International Standards, adopted by selection. The architecture provides for regional variations where
regulations differ in different countries and regions.
2 Conformance
Conformance declarations for the various parts of a CALM-compliant system shall be based on the relevant
CALM-related International Standards.
3 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 21210, Intelligent transport systems — Communications access for land mobiles (CALM) — IPv6
Networking
ISO 21218, Intelligent transport systems — Communications access for land mobiles (CALM) — Medium
service access points
ISO 24102, Intelligent transport systems — Communications access for land mobiles (CALM) — Management
4 Terms, definitions and abbreviated terms
4.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
4.1.1
access technology
technology employed in a communication interface to access a specific medium
4.1.2
CALM-aware application
ITS-S application which is capable of supporting features specific to CALM
NOTE CI selection management is an example.
4.1.3
central ITS-S
central ITS station
implementation of an ITS-S in a central ITS subsystem
4.1.4
FA interface
interface between the facilities layer and the ITS-S applications entity
NOTE The FA interface is presented in Figure 14.
4.1.5
geo-networking
geo-routing
network layer protocol using addresses in the form of geo-coordinates which identify target areas of possible
destination stations
4.1.6
heterogeneous handover
process by which a communication link is switched from one virtual communication interface to another one of
a different medium type
4.1.7
homogeneous handover
process by which a communication link is switched from one virtual communication interface to another one of
the same medium type
4.1.8
IN interface
interface between the access layer and the networking and transport layer
NOTE The IN interface is presented in Figure 14.
4.1.9
ITS service
service provided by a set of ITS-S applications
4.1.10
ITS-S
ITS station
entity in a communication network, comprised of application, facilities, networking and access layer
components specified in this International Standard that operate within a bounded secure management
domain
4.1.11
ITS-S application
functionality in an ITS-S that uses ITS-S services to connect to one or more other ITS-S application
4.1.12
ITS-S gateway
gateway functionality provided in the facilities layer of an ITS-S
2 © ISO 2010 – All rights reserved

4.1.13
ITS-S host
application and facilities functionality provided in an ITS-S together with a minimum communication
functionality to connect to the ITS-S internal network
4.1.14
ITS-S router
routing functionality provided in an ITS-S
4.1.15
ITS-S service
communication functionality offered by an ITS-S to an ITS-S application
4.1.16
MA interface
interface between the communication and station management entity and the ITS-S applications entity
NOTE The MA interface is presented in Figure 14.
4.1.17
medium
any entity upon which a signal is impressed or from which a signal is received, e.g. wireless or on a wire, radio
waves or light, low or high frequency band, modulation scheme
4.1.18
MF interface
interface between the communication and station management entity and the facilities layer
NOTE The MF interface is presented in Figure 14.
4.1.19
MI interface
interface between the communication and station management entity and the access layer
NOTE The MI interface is presented in Figure 14.
4.1.20
MN interface
interface between the communication and station management entity and the networking and transport layer
NOTE The MN interface is presented in Figure 14.
4.1.21
MS interface
interface between the communication and station management entity and the security entity
NOTE The MS interface is presented in Figure 14.
4.1.22
network-based multi-hopping
multi-hopping from ITS-S to ITS-S performed by a networking protocol
4.1.23
NF interface
interface between the networking and transport layer and the facilities layer
NOTE The NF interface is presented in Figure 14.
4.1.24
personal ITS-S
personal ITS station
implementation of an ITS-S in a personal ITS subsystem
4.1.25
roadside ITS-S
roadside ITS station
implementation of an ITS-S in a roadside ITS subsystem
4.1.26
SA interface
interface between the security entity and the ITS-S applications entity
NOTE The SA interface is presented in Figure 14.
4.1.27
SF interface
interface between the security entity and the facilities layer
NOTE The SF interface is presented in Figure 14.
4.1.28
SI interface
interface between the security entity and the access layer
NOTE The SI interface is presented in Figure 14.
4.1.29
SN interface
interface between the security entity and the networking and transport layer
NOTE The SN interface is presented in Figure 14.
4.1.30
vehicle ITS-S
vehicle ITS station
implementation of an ITS-S in a vehicular ITS subsystem
4.2 Abbreviated terms
API application programming interface
CAL communication adaptation layer
CALM communications access for land mobiles
CCK CALM communications kernel
CI communication interface
DSRC dedicated short range communication
ECU electronic control unit
HMI human-machine interface
IPv6 Internet protocol version 6
ITS intelligent transport systems
LLC logical link control
PDA personal digital assistant
SAP service access point
VMS vehicle motion sensor
4 © ISO 2010 – All rights reserved

5 Requirements
5.1 Principles for CALM-related International Standards
CALM-related International Standards shall focus on specifying open interfaces with regard to the
functionalities required for all relevant layers of the OSI reference model.
CALM-related International Standards shall not specify implementation aspects, except in situations where
such specification is deemed essential to interoperability of the interface protocol.
5.2 ITS viewpoint
5.2.1 Wireless links
Figure 1 illustrates the global ITS scope to be considered by the set of CALM-related International Standards.
It shows several types of access technologies for wireless communication links between individual ITS-Ss and
between ITS-Ss and legacy stations which can be expected to be present in ITS environments.
NOTE The CALM concept is not limited to the access technologies presented in Figure 1.

Figure 1 — Examples of wireless links employing various access technologies
5.2.2 Access technologies
The access technologies shown in Figure 1 with an asterisk are each fully specified by a CALM International
Standard, e.g. M5 (ISO 21215), IR (ISO 21214) and MM (ISO 21216). These access technologies have been
developed with a specific focus on ITS deployment.
The other access technologies shown in Figure 1 are examples of legacy access technologies. Legacy access
technologies are specified by reference to the standards according to which they operate (see, for example,
ISO 25111). For these legacy access technologies, an adaptation as specified in ISO 21218 can be required
in order to fit to the communication and station management entity, to the security entity and to the networking
and transport layer; see Figure 14.
Regionally specified DSRC systems may be supported in ITS-Ss as specified in ISO 24103 and ISO 29281.
Applications based on the DSRC standards ISO 15628:2007 or EN 12795:2003 can be supported in the ITS
environment as specified in ISO 29281.
Positioning data from satellite networks such as GPS, GALILEO or GLONASS may be received and provided
to the related applications, e.g. via the ITS station-internal network presented in Figure 2.
5.2.3 Communication interface
An implementation of an access technology is called a communication interface (CI). The concept of a CI and
its virtual communication interfaces (VCIs) are specified in ISO 21218.
5.2.4 Logical channel types
Logical communication channels are a key element of CALM's abstraction of ITS-S applications from the
physical communication channels used to transport the information. ITS-S applications communicate through
logical channels which are mapped by the ITS-S management to physical channels in CIs. Automatic mapping
of ITS-S applications on specific CIs, referred to as “CI selection management”, is specified in ISO 24102.
Definitions of logical channel types are provided in ISO 21218.
5.3 Handover
5.3.1 General
The essential feature of the CALM concept that distinguishes it from traditional communication systems is that
applications are abstracted from the access technologies that provide the wireless connectivity and the
networks that transport the information from the source to the destination(s). ITS-Ss are not limited to a single
access technology and networking protocol and can implement any of those supported, and the ITS-S
management can make optimal use of all these resources. To exploit this flexibility, CALM-compliant systems
provide the ability to support handover of different types including
⎯ those involving a change of CI (which may or may not involve a change of access technology, since
ITS-Ss may have multiple communication interfaces using the same access technology,
⎯ those involving reconfiguration or change of the network employed to provide connectivity, and
⎯ those involving both a change in communication interface and network reconfiguration.
The following examples illustrate the various types of handover that are possible.
⎯ Homogeneous handover:
Maintaining a session between an ITS-S application in a vehicular ITS subsystem and an ITS-S
application in a central ITS subsystem using subsequent roadside ITS-Ss along the road of the same
roadside subsystem, using the same access technology in the various ITS-Ss.
6 © ISO 2010 – All rights reserved

⎯ Heterogeneous handover:
Maintaining a session between an ITS-S application in a vehicular ITS subsystem and an ITS-S
application in a central ITS subsystem by switching from a dedicated CALM access technology, e.g. M5
or IR, to a public cellular network.
5.3.2 Network domains
The top-level point of view of networking supported by ITS is presented in Figure 2.

Figure 2 — Top-level networking view
Two domains are distinguished, i.e.
⎯ the ITS domain, and
⎯ the generic domain.
Possible networks in the ITS domain are
⎯ the ITS station-internal network,
⎯ the ITS ad-hoc network, and
⎯ the ITS access network.
NOTE 1 The concept of an ITS-S is presented in Figure 14.
NOTE 2 The ITS station-internal network may be realized simultaneously with different access technologies, both wired,
e.g. Ethernet, or wireless, e.g. Bluetooth. See also ISO 21210 and ISO 29281.
Possible networks in the generic domain are
⎯ the local data network (typically a private network),
⎯ the public access network,
⎯ the private access network, and
⎯ the core networks.
The boundary of an ITS-S is indicated in Figure 2. The ITS-S connects to the local proprietary data network
via an ITS-S gateway, as presented in Figure 15, i.e. the vehicle gateway in Figure 4, the roadside gateway in
Figure 5 and the central gateway in Figure 7. The ITS-S connects to the access networks via ITS-S border
routers, as presented in Figure 17, i.e. the routers presented in Figures 5 and 7.
5.4 ITS subsystems and ITS-Ss
5.4.1 Peer-to-peer communication
Figures 4, 5, 6 and 7 present four ITS subsystems, i.e. vehicular ITS subsystem, roadside ITS subsystem,
personal ITS subsystem and central ITS subsystem.
The essential element in every ITS subsystem is the ITS-S compliant with the ITS-S reference architecture
presented in Figures 13 and 14. ITS-S subsystems are introduced for the sole purpose of illustrating the
different implementation possibilities of ITS-Ss. All communications between ITS-Ss is peer-to-peer, as
illustrated in Figure 3, regardless of the ITS-S locations relative to the networks involved in the connection.

Figure 3 — ITS subsystems and peer-to-peer communications
8 © ISO 2010 – All rights reserved

Figures 4, 5, 6 and 7 distinguish ITS-S hosts and ITS-S routers in accordance with Figure 16, although this
functionality is completely covered by the concept of an ITS-S. This distinction illustrates that in an
implementation, the overall functionality of an ITS-S may be provided in several distinct physical units, where
several physical units with the same functionality may also exist.
ITS-S gateways in accordance with Figure 15 and ITS-S border routers in accordance with Figure 17 are
presented in order to show that the interconnection with proprietary and public networks is not CALM
compliant.
5.4.2 Vehicular ITS subsystem
The vehicular ITS subsystem presented in Figure 4 contains a vehicle ITS-S. The vehicle ITS-S may be
physically split into ITS-S hosts, ITS-S routers and the CALM-compliant part of the vehicle ITS-S gateway. A
passenger may use a personal ITS-S, as presented in Figure 6, which uses an HMI and forms an integral part
of the vehicular ITS-S.
The vehicle ITS-S gateway supports functionality in order to connect to the ITS-S host and the ITS-S router
via the ITS station-internal network presented in Figure 2. The part of the vehicle ITS-S gateway which
connects to the proprietary in-vehicle network and the ECUs is outside the scope of this International
Standard.
Figure 4 —Vehicular ITS subsystem
5.4.3 Roadside ITS subsystem
The roadside ITS subsystem presented in Figure 5 contains a roadside ITS-S. The roadside ITS-S may be
physically split into ITS-S hosts, ITS-S routers and the CALM-compliant parts of roadside ITS-S gateways and
ITS-S border routers. In this context, the ITS-S router is also called access router.
The roadside ITS-S gateway supports functionality in order to connect to the ITS-S hosts, the ITS-S routers
and the ITS-S border routers via the ITS station-internal network presented in Figure 2. The part of the
roadside ITS-S gateway which connects to the proprietary roadside network and the roadside components
such as VMS, inductive loops, etc., is outside the scope of this International Standard.

Figure 5 — Roadside ITS subsystem
5.4.4 Personal ITS subsystem
The personal ITS subsystem presented in Figure 6 provides the ITS-S functionality in consumer electronic
devices such as PDAs and mobile phones. It contains a personal ITS-S. Portable devices, e.g. PDAs, cellular
phones, with ingress connectivity (provided by Bluetooth, for example), in addition to connectivity to public
wireless network services, may be used as egress access technologies for ITS-Ss. Portable devices, e.g.
laptops and media players, can use the vehicle as an access point for longer range connectivity. This is
handled by the ITS-S router functionality and the use of IPv6 protocols specified in ISO 21210.
Personal area network devices, such as those using Bluetooth in accordance with IEEE 802.15.2:2003 and
IEEE 802.15.4:2006 compliant platforms, may be used to provide local connectivity for portable devices.

Figure 6 — Personal ITS-S (portable device)
5.4.5 Central ITS subsystem
The central ITS subsystem presented in Figure 7 contains a central ITS-S. The central ITS-S may be
physically split into ITS-S hosts and the CALM-compliant part of central ITS-S gateways and ITS-S border
routers.
10 © ISO 2010 – All rights reserved

The central ITS-S gateway supports functionality in order to connect to the ITS-S host and the central ITS-S
border router via the local data network presented in Figure 2. The interface to the central system is outside
the scope of this International Standard.

Figure 7 — Central ITS-S
5.5 Communication scenarios and classes
5.5.1 Communication scenarios
Four basic communication scenarios are identified, as illustrated in Figures 8, 9, 10 and 11. The distinction
between these scenarios is based on two criteria:
a) whether the CALM-compliant ITS-S is connected to the final peer station
1) with a single-hop link, or
2) via a network;
b) whether the final peer station is
1) also a CALM-compliant ITS-S, or
2) a legacy station.
NOTE 1 This classification of scenarios does not consider any details of the network between CALM-compliant ITS-S
and peer station.
NOTE 2 These communication scenarios are in no way restricted to a specific communication mode, i.e. broadcast,
multicast or unicast mode.
NOTE 3 The ITS-S connecting to the peer station may be either the original source of a data packet or may act as a
forwarding station.
Single-hop communication between two CALM-compliant ITS-Ss is presented in Figure 8. This can represent,
for example, a link between two vehicle stations, or between a vehicle station and a roadside station, or
between a personal station and a vehicle station.

Figure 8 — Scenario I — ITS-S to ITS-S via single-hop link

Communication between two CALM-compliant ITS-Ss involving network connectivity is illustrated in Figure 9.
This can represent, for example, a peer-to-peer communication involving a single-hop link from the ITS-S to a
base station of a cellular network which is connected to the Internet which also provides the connection to the
ITS-S of a central subsystem.
Figure 9 — Scenario II — ITS-S to ITS-S via network (multiple hops)
12 © ISO 2010 – All rights reserved

Single-hop communication between a CALM-compliant ITS-S and a legacy station is illustrated in Figure 10.
This can represent, for example, a link between a DSRC CI implemented in a vehicle ITS-S, as specified in
ISO 29281, and a legacy DSRC roadside station.

Figure 10 — Scenario III — ITS-S to legacy peer station via single-hop link
Communication between a CALM-compliant ITS-S and a legacy station involving network connectivity is
illustrated in Figure 11. This can represent, for example, a single-hop link from the ITS-S to a base station of a
cellular network which connects to the Internet which also provides the connectivity to the legacy station.

Figure 11 — Scenario IV — ITS-S to legacy peer station via network (multiple hops)
An ITS-S may have multiple simultaneously active sessions involving any or all of these basic communication
configurations.
A further illustration of scenarios by means of examples is provided in Annex B.
5.5.2 Communication classes
Table 1 gives a general classification of communications in the ITS environment with respect to multi-hopping,
networking, handover and internet access, and the relation to CALM-related International Standards.
An ITS-S may support several communication classes in a sequence or simultaneously.
Table 1 — Communication classes
a b c
Comm. Multi-hop Networking Handover Internet Related International Standard/comment
class support type support access
d
1 no non-IP no no ISO 29281
d
2 no non-IP yes no ISO 29281
3 no IPv6 no no ISO 21210
4 no IPv6 yes no ISO 21210
d
5 no non-IP no yes ISO 24103
d
6 no non-IP yes yes Standard not yet under development
7 no IPv6 no yes ISO 21210
8 no IPv6 yes yes ISO 21210
d
9 yes non-IP no no ISO 29281
d
10 yes non-IP yes no ISO 29281
11 yes IPv6 no no Standard not yet under development
12 yes IPv6 yes no Standard not yet under development
d
13 yes non-IP no yes Standard not yet under development
d
14 yes non-IP yes yes Standard not yet under development
15 yes IPv6 no yes Standard not yet under development
16 yes IPv6 yes yes Standard not yet under development
a
“Comm.” stands for communication.
b
Refers to multi-hop from ITS-S to ITS-S performed by the networking protocol.
c
Indicates the networking protocol used in the air-link.
d
Refers to protocols specified in ISO 29281.

5.6 ITS-S architecture
5.6.1 General
The “ISO Open Systems Interconnect Reference Model” (i.e. OSI, see ISO/IEC 7498-1:1994) is used in a
number of figures within this International Standard with reference to the communications architecture of
ITS-Ss that embody the CALM concept. Several levels of abstraction will be used to illustrate different points
of view.
5.6.2 Data flow in the OSI protocol stack
Figure 12 shows the general case of flow of data, i.e. service data units (SDU) and protocol data units (PDU),
through the OSI layers of two peer stations communicating with each other, and the grouping of protocol
layers as used in ITS:
⎯ Session, presentation and application OSI layers 5-7 comprise the “Facilities layer”.
⎯ Network and transport OSI layers 3-4 comprise the “Networking & transport layer”.
⎯ Physical interface and link control OSI layers 1-2 comprise the “Access layer”.
14 © ISO 2010 – All rights reserved

NOTE In a draft of this International Standard, the groups of OSI layers identified for ITS were given different names.
“Facilities layer” was called “CALM service layer”, “Networking & transport layer” was called “CALM networking”, and
“Access layer” was called “CALM CI layers”.
The naming and usage of service data units (SDU) and protocol data units (PDU) follows the principles
outlined in ISO/IEC 8802-2:1998.

Figure 12 — OSI data flow
Figure 13 illustrates a simplified view of the communications architecture of an ITS-S embodying the CALM
concept, where
⎯ the security functionality and,
⎯ the management functionality
are added to the OSI layers and the ITS-S applications; see also the ITS-S reference architecture in Figure 14.

Figure 13 — Simplified ITS-S reference architecture
The functionality of all blocks presented in Figure 13 shall be provided by an ITS-S that is compliant with this
International Standard.
This requirement does not imply anything about the implementation; these blocks may be spread over several
physical devices, or they may be implemented inside a single device.
5.6.3 ITS-S reference architecture
5.6.3.1 General
Figure 14 shows the general ITS-S reference architecture, including interfaces between the various blocks
with informative details. Such interfaces may be partly non-observable and thus non-testable service access
points (SAPs), or observable and testable interfaces.
16 © ISO 2010 – All rights reserved

Figure 14 — ITS-S reference architecture
Five categories of interface are described in detail in 5.6.3.2 to 5.6.3.6.
5.6.3.2 Interfaces towards the management entity
⎯ MI:
Enables the management entity to interact with the access layer (OSI layers 1 and 2/communication
interfaces).
NOTE 1 This interface was formerly known as “M-SAP”.
⎯ MN:
Enables the management entity to interact with the networking and transport layer (OSI layers 3 and 4).
NOTE 2 This interface was formerly known as “N-SAP”.
⎯ MF:
Enables the management entity to interact with the facilities layer (OSI layers 5 through to 7).
NOTE 3 This interface was formerly known as “A-SAP”.
⎯ MA:
Enables the management entity to interact directly with ITS-S applications.
5.6.3.3 Interfaces towards the security entity
⎯ SI:
Enables the security entity to interact with the access layer (OSI layers 1 and 2/communication interfaces).
⎯ SN:
Enables the security entity to interact with the networking and transport layer (OSI layers 3 and 4).
⎯ SF:
Enables the security entity to interact with the facilities layer (OSI layers 5 through 7).
⎯ SA:
Enables the security entity to interact directly with ITS-S applications.
5.6.3.4 Interface between access layer and networking and transport layer
IN:
Allows the networking and transport layer and the access layer to interact with each other. Typically
implemented as a SAP.
NOTE This interface was formerly known as “C-SAP”.
5.6.3.5 Interface between networking and transport layer and facilities layer
NF:
Allows the facilities layer and networking and transport layer to interact with each other. Typically implemented
as a SAP.
NOTE This interface was formerly known as “T-SAP”.
5.6.3.6 Interface between facilities layer and ITS-S applications
FA:
Allows the facilities layer to interact with ITS-S applications. Typically implemented as an API.
NOTE A valid implementation option is to merge the IN, MI and SI interfaces into a plug-and-play interface, e.g.
according to a system specification.
5.6.4 Host, router and gateway architecture
Following the principles of the ITS-S reference architecture presented in Figures 13 and 14, the functionality of
its subsets is identified as follows.
⎯ An ITS-S host provides the functionality, as presented in Figure 13. It provides, as a minimum, the ITS-S
applications and the means to connect to the ITS-S-internal network.
⎯ An ITS-S gateway provides the functionality, as presented in Figure 15. It interconnects two different OSI
protocol stacks at layers 5 through to 7. It shall be capable of converting protocols.
⎯ An ITS-S router provides the functionality, as presented in Figure 16. It interconnects two different CALM
protocol stacks at layer 3. It may be capable of converting protocols.
NOTE A router implementation can contain more functionality than presented in Figure 16.
18 © ISO 2010 – All rights reserved

Figure 15 — ITS-S gateway
Figure 16 — ITS-S router
An ITS-S border router, as presented in Figure 17, basically provides the same functionality as the ITS-S
router presented in Figure 16. The difference is that the protocol stack related to the external network might
not follow the management and security principles of CALM.

Figure 17 — ITS-S border router

5.7 CALM communications kernel
Figure 18 illustrates the concept of a CALM communications kernel (CCK), as specified in ISO 24102.
NOTE 1 The term CCK was introduced at an early stage of CALM standardization. Following the latest terminology
applied in this International Standard, the CCK provides the functionality of an ITS-S router. An instance of the ITS-S
router functionality is referred to as a CCK, which can be identified by means of a CCK-ID, unique in an ITS-S.
NOTE 2 From an IP point of view, a CCK acts as a “node”. However, the IP term “node” does not sufficiently specify
the functionality of a CCK.
NOTE The elements shown in Figure 18 inside the blocks “Facilities layer”, “Networking & transport layer” and
“Access layer” are considered to be examples of optional elements.
Figure 18 — CALM communications kernel (CCK) as an instance of an ITS-S router
5.8 Services
5.8.1 ITS service
The term “ITS service” in the context of CALM refers to a service provided by an ITS application to a user of
an ITS-S. The ITS application itself consists of two or more complementary ITS-S applications. Pairs of ITS-S
applications may be classified as, for example, server/client applications.
A client station can identify available user services in the two following ways.
⎯ User service discovery.
A client station actively tries to discover user services.
20 © ISO 2010 – All rights reserved

⎯ User service notification.
A server station is actively broadcasting service notification messages in service announcements. These
service announcements are managed through various processes, including application registration and
announcement requests, and construction of such announcement messages is to be transmitted over the
air with an appropriately chosen access technology.
Details may depend on networking protocols used.
FAST communications, as specified in ISO 29281, are based on service notification. The details of these
procedures are specified in ISO 24102. A client station receiving an announcement message can either
⎯ use this announcement message, as it already contains the complete service information, e.g. traffic
situation alert message,
⎯ reply to the notification with a privately addressed frame containing service context information, upon
which the server shall run the service transaction in the correct context, if applicable, or
⎯ run the service transaction.
NOTE ITS-Ss communicate in a peer-to-peer mode where, once the application association has been made, data
exchanges between applications occur until such time as the session is complete or the link between the applications is
broken.
ITS-S applications use ITS-S services in order to connect to one or more other ITS-S application. In
implementations with more than one wireless communication interface, quasi-simultaneous provision of ITS-S
services with data streams via different CIs is supported.
5.8.2 ITS-S service
The term “ITS-S service” in the context of CALM refers to a communication functionality offered by an ITS-S to
an ITS-S application. An ITS-S service provides the complete communication link through all the O
...