ISO 21217:2014

INTERNATIONAL ISO
STANDARD 21217
Second edition
2014-04-01
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 2014
© ISO 2014
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ii © ISO 2014 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 5
5 Requirements . 7
6 Overview of ITS communications . 7
6.1 ITS services and applications . 7
6.2 ITS communication means . 7
6.3 ITS communication characteristics . 8
6.4 ITS communication networks . 9
6.5 ITS station interconnection scenarios .10
6.6 ITS concept of paths and flows .11
7 ITS station overview .13
7.1 ITS station concept .13
7.2 ITS-S architecture .14
8 Details of elements of ITS-S reference architecture.20
8.1 ITS-S interfaces .20
8.2 ITS-S access layer .21
8.3 ITS-S networking and transport layer .24
8.4 ITS-S facilities layer .26
8.5 ITS-S management entity .29
8.6 ITS-S security entity .31
8.7 ITS-S applications .32
9 Typical implementations of ITS station units .34
Annex A (informative) Illustration of typical ITS-SU implementations .36
Annex B (informative) ITS-S configurations .40
Bibliography .45
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
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 204, Intelligent transport systems.
This second edition cancels and replaces the first edition (ISO 21217:2010) which has been technically
revised.
iv © ISO 2014 – All rights reserved

Introduction
“Communications Access for Land Mobile” (CALM) is the acronym used to refer to ISO TC204 WG16
work items. This acronym is used in the titles of the set of International Standards on communication
for “Intelligent Transport Systems” (ITS). These International Standards focus on specifying open
interfaces with regard to the functionalities required for all relevant layers and entities of the ITS station
reference architecture specified in this International Standard. Note that these International Standards
may also specify implementation details in situations where such specifications are deemed essential to
interoperability of interface protocols.
The set of CALM International Standards is designed to allow interoperable instantiations of ITS
stations which are based on the concept of abstracting applications and services from the underlying
communication layers of the ITS station. This abstraction and the functionalities and services that
can be easily implemented make the ITS station architecture described herein also well-suited to the
development and deployment of ITS applications and services that share information amongst each
other to improve the safety, sustainability and efficiency of transport systems.
The set of CALM International Standards include specifications for
— ITS station management,
— ITS communications security,
— ITS station facilities layer protocols,
— ITS station networking and transport layer protocols,
— communication interfaces (CIs) designed specifically for ITS applications and services such as those
designed specifically for safety of life and property,
— interfacing existing access technologies into ITS stations,
— distributed implementations of ITS stations, and
— interfacing ITS stations to existing communication networks and communicating with nodes
thereon.
This International Standard describes the common architectural framework around which ITS stations
are instantiated and provides references to relevant International Standards, including access technology
support standards, various networking and transport protocol standards, facilities standards, and ITS
station management and security standards. It also describes the general architecture of peer-to-peer
communications over various communication networks between ITS communication nodes. These
nodes may be ITS stations as described in this International Standard or any other reachable nodes.
The set of CALM International Standards is complemented by ITS communication International
Standards from other International Standards development organizations which together form the
basis for implementation of ITS communications networks around the world.
INTERNATIONAL STANDARD ISO 21217:2014(E)
Intelligent transport systems — Communications access
for land mobiles (CALM) — Architecture
1 Scope
This International Standard describes the communications reference architecture of nodes called “ITS
station units” designed for deployment in intelligent transport systems (ITS) communication networks.
The ITS station reference architecture is described in an abstract way. While this International Standard
describes a number of ITS station elements, whether or not a particular element is implemented in an
ITS station unit depends on the specific communication requirements of the implementation.
This International Standard also describes the various communication modes for peer-to-peer
communications over various networks between ITS communication nodes. These nodes may be ITS
station units as described in this International Standard or any other reachable nodes.
This International standard specifies the minimum set of normative requirements for a physical
instantiation of the ITS station based on the principles of a bounded secured managed domain.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
None.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
access technology
technology employed in a communication interface to access a specific medium
3.2
application data unit
data unit exchanged between ITS-S application processes
3.3
communication adaptation layer
set of protocols and functions to adapt access technologies to the ITS-S networking and transport layer
3.4
communication interface
instantiation of a specific access technology and ITS-S access layer protocol
3.5
communication path
directed sequence of nodes connected by links, starting at a source node and ending at one or more
destination nodes
3.6
FA interface
interface between the ITS-S facilities layer and the ITS-S applications entity
3.7
IN interface
interface between the ITS-S access layer and the ITS-S networking and transport layer
3.8
in-vehicle network
generic term for a network in a vehicle which is not an ITS station-internal network
3.9
ITS application
instantiation of an ITS service that involves an association of two or more complementary ITS-S
application processes
Note 1 to entry: Fragments of an application may also reside in nodes that are not ITS stations.
3.10
ITS message set
set of messages designed for an ITS-related purpose
3.11
ITS service
functionality provided to users of intelligent transport systems designed e.g. to increase safety,
sustainability, efficiency, or comfort
3.12
ITS station
functional entity comprised of an ITS-S facilities layer, ITS-S networking and transport layer, ITS-S
access layer, ITS-S management entity, ITS-S security entity, and ITS-S applications entity providing ITS
services
Note 1 to entry: From an abstract point of view, the term “ITS station” refers to a set of functionalities. The
term is often used to refer to an instantiation of these functionalities in a physical unit. Often, the appropriate
interpretation is obvious from the context. The proper name of the physical instantiation of an ITS-S is ITS station
unit (ITS-SU).
3.13
ITS-S access layer
protocol layer in the ITS-S reference architecture containing the OSI physical and data link layer
protocols for ITS communications
3.14
ITS-S access layer protocol data unit
protocol data unit exchanged between peer ITS-S access layers
3.15
ITS-S access layer service data unit
service data unit exchanged between ITS-S access layer and ITS-S networking and transport layer
3.16
ITS-S access router
ITS-S border router with additional functionality that provides other ITS communication nodes a point
of attachment to an external network
3.17
ITS-S access technology
access technology dedicated to operation in an ITS-S
3.18
ITS-S application
ITS-S application process residing in the ITS-S application entity
2 © ISO 2014 – All rights reserved

3.19
ITS-S application process
element in an ITS station that performs information processing for a particular application and uses
ITS-S services to transmit and receive information
3.20
ITS-S border router
ITS-S router with additional functionality that provides connectivity to other ITS communication nodes
over external networks
3.21
ITS-S communication unit
physical unit in an ITS-SU containing a part or all of the functionality of an ITS-S
Note 1 to entry: In case an ITS-SU consists of a single physical unit, the ITS-SU and the ITS-SCU are identical. In
case an ITS-SU consists of more than one ITS-SCU, then these ITS-SCUs are interconnected via the ITS station-
internal network of the ITS-SU.
3.22
ITS-S facilities layer
layer in the ITS-S reference architecture containing OSI layers 5, 6, and 7 that connects applications to
the ITS-S networking and transport layer
3.23
ITS-S facilities layer protocol data unit
protocol data unit exchanged between peer ITS-S facility layers
3.24
ITS-S facilities layer service data unit
service data unit exchanged between ITS-S facilities layer and ITS-S application entity
3.25
ITS-S facility application
ITS-S application process residing in the ITS-S facilities layer
3.26
ITS-S gateway
ITS-S node used to interconnect two different OSI protocol stacks at layers 5 through 7
Note 1 to entry: An ITS-S gateway may convert between different protocols.
3.27
ITS-S host
ITS-S node comprised of ITS-S functionalities other than the functionalities of an ITS-S router, ITS-S
border router, ITS-S mobile router, or an ITS-S gateway
3.28
ITS-S internal router
ITS-S router that connects two or more ITS station-internal networks
3.29
ITS-S management application
ITS-S application process residing in the ITS-S management entity
3.30
ITS-S mobile router
ITS-S border router with additional functionality that allows a change of point of attachment to an
external network while maintaining session continuity
3.31
ITS-S networking and transport layer protocol data unit
protocol data unit exchanged between peer ITS-S networking and transport layers
3.32
ITS-S networking and transport layer service data unit
service data unit exchanged between ITS-S networking and transport layer and ITS-S facilities layer
3.33
ITS-S networking and transport layer
layer in the ITS-S reference architecture containing OSI layers 3 and 4 that connects the ITS-S facilities
layer to the ITS-S access layer
3.34
ITS-S node
node comprised of a set of functionalities in an ITS station unit that is connected to the ITS station-
internal network or comprises an entire ITS station unit
3.35
ITS-S router
ITS-S node comprised of routing functionalities of an ITS station unit used to connect two networks and
to forward packets not explicitly addressed to itself
3.36
ITS-S security application
ITS-S application process residing in the ITS-S security entity
3.37
ITS-S service
communication functionality of an ITS-S that provides the capability to connect to other nodes
3.38
ITS station unit
implementation of an ITS-S
3.39
MA interface
interface between the ITS-S management entity and ITS-S applications
3.40
medium
physical entity that supports the transmission of signals carrying information between ITS
communication nodes, e.g. a set of wires supporting Ethernet signals or the space between two antennas
that supports electromagnetic, optical, or acoustical transmissions
3.41
MF interface
interface between the ITS-S management entity and the ITS-S facilities layer
3.42
MI interface
interface between the ITS-S management entity and the ITS-S access layer
3.43
MN interface
interface between the ITS-S management entity and the ITS-S networking and transport layer
3.44
MS interface
interface between the ITS-S management entity and the ITS-S security entity
3.45
NF interface
interface between the ITS-S networking and transport layer and the ITS-S facilities layer
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3.46
SA interface
interface between the ITS-S security entity and ITS-S applications
3.47
SF interface
interface between the ITS-S security entity and the ITS-S facilities layer
3.48
SI interface
interface between the ITS-S security entity and the ITS-S access layer
3.49
SN interface
interface between the ITS-S security entity and the ITS-S networking and transport layer
4 Symbols and abbreviated terms
ADU Application Data Unit
API Application Programming Interface
BSMD Bounded Secured Managed Domain
BSME Bounded Secured Managed Entity
CAL Communication Adaptation Layer
CALM Communications Access for Land Mobiles
CI Communication Interface
C-ITS Cooperative ITS
DSRC Dedicated Short-Range Communication
ETSI European Telecommunications Standards Institute
FA name of interface between ITS-S facilities layer and ITS-S application entity
IN name of interface between ITS-S access layer and ITS-S networking and transport layer
IP Internet Protocol
IPv6 IP version 6
IR Infra-Red
ISO International Standards Organization
ITS Intelligent Transport Systems
ITS-APDU ITS Station Access layer Protocol Data Unit
ITS-ASDU ITS Station Access layer Service Data Unit
ITS-FPDU ITS Station Facility layer Protocol Data Unit
ITS-FSDU ITS Station Facility layer Service Data Unit
ITS-NTPDU ITS Station Networking and Transport layer Protocol Data Unit
NOTE  The deprecated term ITS-NPDU is in use in published standards with the same mean-
ing as ITS-NTPDU.
ITS-NTSDU ITS Station Networking and Transport layer Service Data Unit
ITS-S ITS Station
ITS-SCU ITS-S Communication Unit
ITS-SU ITS-S Unit
IVN In-Vehicle Network
LCH Logical Channel
LTE Long Term Evolution
MA name of the interface between the ITS-S management entity and ITS-S applications
MAE Management Adaptation Entity
MAP name of an ITS message set used to carry information on digital maps covering the area of
intersections
MF name of the interface between the ITS-S management entity and the ITS-S facilities layer
MI name of the interface between the ITS-S management entity and the ITS-S access layer
MIB Management Information Base
MN name of the interface between the ITS-S management entity and the ITS-S networking and
transport layer
MS name of the interface between the ITS-S management entity and the ITS-S security entity
NF name of the interface between the ITS-S networking and transport layer and the ITS-S facili-
ties layer
PCH Physical Channel
PDM Probe Data Management; name of an ITS message set
PDU Protocol Data Unit
POI Point of Interest
PVD Probe Vehicle Data; name of an ITS message set
SA name of the interface between the ITS-S security entity and ITS-S applications
SAP Service Access Point
SDU Service Data Unit
SF name of the interface between the ITS-S security entity and the ITS-S facilities layer
SI name of the interface between the ITS-S security entity and the ITS-S access layer
SMIB Security Management Information Base
SN name of the interface between the ITS-S security entity and the ITS-S networking and trans-
port layer
6 © ISO 2014 – All rights reserved

SOA Service Oriented Architecture
SPaT Signal Phase and Timing; name of an ITS message set
SRM Signal Request Message; name of an ITS message set
SSM Signal Status Message; name of an ITS message set
TOPO name of an ITS message set used to carry information on digital maps covering the area of
intersections
UMTS Universal Mobile Telecommunication System
5 Requirements
A physical instantiation of an ITS-S shall provide as a minimum
— the functionality of an ITS-S host specified in 7.2.2, i.e. acting as a terminal only or
— the functionality of an ITS-S host and ITS-S router specified in 7.2.2.
This includes a minimum set of related security procedures and principles that can be verified by an
appropriate ITS-related authority described in Reference [54]. These security procedures and principles
are used to allow the BSME to assert a level of trust to other BSMEs in the communication network.
6 Overview of ITS communications
6.1 ITS services and applications
The wide variety of services and applications to be deployed in the ITS sector and the global time-varying
nature of transportation itself lead to challenges in the design of communication systems to support
these services and applications. One of the challenges is to support widely disparate communication
requirements with respect to reliability, security, latency, and other performance parameters.
Furthermore, the possibility of having multiple applications in an ITS station unit (ITS-SU) simultaneously
competing for communication resources leads to the need for a controlled access to these resources.
Useful means for addressing this issue are e.g. application and message prioritization and logical
channels.
6.2 ITS communication means
ITS communications involves communications between a wide variety of ITS communication nodes on
different platforms, e.g. vehicles, roadside equipment, portable devices, control centres, using various
means and methods as illustrated in Figure 1. The various access and networking technologies illustrated
are used to interconnect stations on a peer-to-peer basis serving a range of ITS service domains. For
example, any of the vehicles in Figure 1 connected to an RSE via 5 GHz or IR could communicate with the
vehicle connected to the wireless LAN hotspot.
Figure 1 — Examples of ITS communications
6.3 ITS communication characteristics
Characteristics of ITS communications are presented in the following list:
— station mobility leads to complex time-varying networking topologies and time-varying properties
of wireless communication channels (fading, hidden-nodes, etc.);
— variety of stations connected via various networking and access technologies including the Internet,
various public and private networks, Bluetooth and WiFi, dedicated technologies, such as 5,8 GHz
DSRC for road tolling:
— a station with multiple access and networking technologies can maintain session continuity
through a change of either or both;
— two stations with different access technologies can establish end-to-end connectivity
— variety of communication requirements resulting from different ITS applications with different
priorities, e.g. road safety, traffic efficiency, mobility and infotainment, e.g. with respect to
communications capacity (data rate), communications reliability, communications availability;
— variety of communication requirements resulting from user needs, e.g. with respect to
communications cost (in terms of money), communications privacy, communications security;
— variety of communication requirements resulting from regional regulations and policies;
8 © ISO 2014 – All rights reserved

— global applicability, where intended.
6.4 ITS communication networks
An illustration of the various networks used in ITS communications is presented in Figure 2.
Figure 2 — Networking view of ITS communications
Figure 2 illustrates the following networks:
— an ITS infrastructure network comprised of ITS-SUs with a (quasi-) static topology, e.g. a collection
of roadside stations connected via a fibre backbone;
— an ITS ad hoc network comprised of ITS-SUs in which the topology may change rapidly, e.g. a mesh
network of (vehicle) stations connected via microwave technologies;
— a local data network, e.g. a proprietary in-vehicle network based on CAN bus technology or a
6LoWPAN wireless sensor network;
— a public access network, e.g. WiFi hotspot or cellular networks;
— a private access network, e.g. a proprietary road operator network;
— a core network, e.g. the Internet, a virtual private network.
NOTE An ITS station-internal network is not presented in Figure 2; however, it is necessary in implementations
illustrated in Figures 14 and 15.
ITS infrastructure and ITS ad hoc networks are networks specifically designed to accommodate and
implement ITS services and applications. They are connected to each other and to public access, private
access, and local data networks through an ITS-SU as shown in Figure 2. The concept of an ITS-SU is
described in Clause 7.
6.5 ITS station interconnection scenarios
Four basic ITS station unit interconnection scenarios are identified as illustrated in Figures 3, 4, 5, and
6. The distinction between these scenarios is based on two criteria:
— whether ITS station units connect to peer stations with or without a network;
— whether a peer station unit is a BSME presented in 7.1 or not.
This classification of scenarios does not consider any details of the network(s) between the peer station
units.
Single-hop communication between two BSMEs is illustrated in Figure 3. This can represent, for example,
a link between two vehicle BSMEs, or between a vehicle BSME and a roadside BSME, or between a
personal BSME and a vehicle BSME.
Figure 3 — BSME to BSME communication without an external network (single-hop)
Communication between two BSMEs over a network is illustrated in Figure 4. This can represent, for
example, a peer-to-peer communication involving a single-hop link from a BSME to a base station of
a cellular network which is connected to the Internet through which connection to a central BSME is
established.
Figure 4 — BSME to BSME communication over an external network (multiple hops)
Single-hop communication between a BSME and an ITS station unit not implementing the principles of a
BSMD is illustrated in Figure 5. This can represent, for example, a link between a 5,8 GHz DSRC on-board
unit implemented in a vehicle BSME, as specified in Reference [43], and a 5,8 GHz DSRC roadside unit.
10 © ISO 2014 – All rights reserved

Figure 5 — BSME to ITS station unit (no BSME) communication without an external network
(single-hop)
Communication between a BSME and an ITS station unit (no BSME) involving network connectivity is
illustrated in Figure 6. This can represent, for example, a single-hop link from a BSME to a base station
of a cellular network which connects to the Internet through which connection to an ITS station unit is
established.
Figure 6 — BSME to ITS station unit (no BSME) communication over an external network
(multiple hops)
An ITS-SU (with or without implementing the principles of a BSMD) may have multiple simultaneously
active sessions involving any or all of these basic communication scenarios.
6.6 ITS concept of paths and flows
The concept of paths and flows in ITS is very beneficial in describing the abstraction of ITS-S application
[55]
processes from the communications services available in an ITS-S. This concept is based on similar
[103]
concepts in IPv6 networking. Procedures for ascertaining available communication paths and
for mapping flows to those paths are divided into distinct functions within the ITS-S management as
specified in Reference [35].
A communication path is defined as a directed sequence of nodes connected by links, starting at a
source node (VCI which connects to the next hop node) and ending at one or more destination nodes.
Note that for bidirectional communications, two such paths exist, i.e. one at each peer station. Note
further that there could be multiple paths between a source and its destination.
A flow type is a set of communication requirements and characteristics associated with a specific flow.
A flow is an identifiable sequence of packets of a given flow type to be transmitted to one or more
entities over a communication path. Each flow is identified by a FlowID which is unique in an ITS-SU and
is mapped to a given communication path or a set of available communication paths.
Categories of communication requirements and objectives requested by an ITS-S application process
to select an appropriate communication profile and communication path include e.g. operational,
[55]
destination type, performance, financial, security, and protocol requirements.
Note that, in general, it cannot be ensured that the communication requirements will be met all along a
communication path as there may be no knowledge of the capabilities of all the nodes along a particular
path.
Figure 7 illustrates the architectural components (building blocks and management data flows)
of the ITS-S which are involved in the path selection process. The same architecture applies to the
[55]
communication profile selection process introduced in 7.1.
Figure 7 — Architecture of communication profile and path selection
“Communication protocols status” contains continuously updated properties and status of
— the various CIs and VCIs in the ITS-S access layer,
— protocols and parameters in the ITS-S networking and transport layer, and
— protocols, functions, and parameters in the ITS-S facilities layer.
It is updated via MI-SAP, MN-SAP, and MF-SAP.
Requirements and objectives obtained from ITS-S application processes, e.g. from ITS-S applications
[55]
via the API or from ITS-S facility applications via the MF-SAP, are maintained in “ITS-S application
requirements and objectives”.
[35]
NOTE The path selection process requires maintenance of further tables not illustrated in Figure 7.
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7 ITS station overview
7.1 ITS station concept
The ITS station concept is based on the abstraction of ITS application processes from communication
protocols serving these ITS application processes along with the ability to securely manage those
application processes and communications. It is embodied in the abstract definition of an ITS station
(ITS-S) as a “Bounded Secured Managed Domain” (BSMD), i.e. a trusted ITS-S described in this
International Standard. An instantiation of a trusted ITS-S is referred to as a “Bounded Secured Managed
Entity” (BSME) if the trust nature of the implementation is relevant. In general, an instantiation of an
ITS-S is referred to as an “ITS station unit” (ITS-SU).
NOTE In this International Standard, the acronym ITS-S is used to indicate an ITS station based on the
principles of the BSMD. A general classification of stations used in ITS is outside the scope of this International
standard. A general high-level description of communications in cooperative ITS is presented in Reference [14].
The salient feature of the ITS-S concept that distinguishes it from the concept behind traditional
communication systems is that application processes are abstracted from both 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 either a single access technology or to a specific
networking and transport protocol. ITS-SUs can implement any of those technologies that are supported
through appropriate adaptation specifications.
While the aforementioned abstraction is generally useful for most application processes, this abstraction
does not prevent application processes from requesting a specific communication profile to be
[55]
considered in the communication profile selection process or specifying communication parameters
[25][42][95]
on a packet-per-packet basis.
The flexibility that ITS-S management has to make optimal use of all available ITS-S resources
(communication media and higher layer protocols) is one of the key enabling features of ITS
communications and applications. The means for (dynamically) assigning ITS-S application processes
to communication media and networking and transport layer protocols is specified in References [25],
[35], [54], and [55]. To exploit this flexibility, BSMD-compliant systems provide the ability to support
handover of different types including
— those involving a change of “Communication Interface” (CI) (which may or may not involve a change
of access technology) because ITS-SUs 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 handover architecture is specified in Reference [8].
Finally, to be able to meet the stringent security requirements of ITS application processes related
to safety of life and property, the ITS-S concept provides for secure peer-to-peer communications
between entities that are themselves capable of being secured and remotely managed. While this is an
abstract definition, it has very specific physical consequences. The bounded nature is derived from the
requirement for ITS-Ss to be able to communicate amongst themselves, i.e. peer-to-peer, as well as with
devices that are not secured. Realizing that to achieve this in a secure manner often requires distribution
and storage of security-related material that must be protected within the boundaries of the ITS-S leads
to the secured nature of the entity. As there is great flexibility to achieve desired communication goals,
there is a requirement that this flexibility be managed. Thus, ITS-Ss are referred to as bounded secured
managed domains (BSMD).
7.2 ITS-S architecture
7.2.1 Generalized OSI model
[47]
The “ISO Open Systems Interconnect Reference Model” is used in a number of figures within this
International Standard with reference to the ITS-S communications architecture that embody the ITS
station concept. Several levels of abstraction will be used to illustrate different points of view.
Figure 8 shows the general ITS-S reference architecture, including interfaces (IN, NF, FA, ;I, MN, MF,
MA, SI, SN, SF, SA, MS, API) 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 (e.g. plug-and-play), or application programming interfaces (APIs).
Figure 8 — ITS-S reference architecture
A simplified presentation of the ITS-S reference architecture is shown in Figure 9.
14 © ISO 2014 – All rights reserved

Figure 9 — Simplified ITS-S reference architecture
NOTE The interfaces MA, FA, and SA are not shown explicitly in Figure 9, as the functionality of these
interfaces is provided in the API.
The blocks in Figures 8 and 9 contain the following functionality:
— ITS-S access layer, referred to as “Access”, comprised of OSI layers 1 (Physical) and 2 (Data Link) of
the OSI communication protocol stack;
— ITS-S networking and transport layer, referred to as “Networking & Transport”, comprised of OSI
layers 3 (Network) and 4 (Transport) of the OSI communication protocol stack;
— ITS-S facilities layer, referred to as “Facilities”, comprised of OSI layers 5 (Session), 6 (Presentation),
and 7 (Application) of the OSI communication protocol stack;
— ITS-S management entity, referred to as “Management”, containing station management
functionalities;
— ITS-S security entity, referred to as “Security”, comprised of security services provided to the OSI
communication protocol stack and to the ITS-S management entity;
— ITS-S application entity, referred to as “Applications”, which make use of the OSI communication
protocol stack.
The functional blocks presented in Figures 8 and 9 are interconnected either via observable interfaces
or via service access points (SAPs) as specified in e.g. References [25], [32], and [42] or via an API. The
identifiers of these interfaces are shown in Figures 8 and 9.
Implementations of ITS stations, referred to as ITS station units (ITS-SU), constitute “endpoints” of a
communication path. An ITS-SU designed and configured to provide one or several specific ITS services
to its user is expected to provide those functionalities of the blocks in Figures 8 and 9 necessary for these
ITS services . Some of the functionalities in the various blocks may not be applicable and therefore do
not need to be implemented. For example, some ITS-S application processes might not require specific
support from the ITS-S facilities layer or from the ITS-S security entity. The requirement to instantiate
certain functionalities does not imply anything about the actual implementation. These functionalities
(blocks) may be spread over several physical devices or they may be implemented inside a single device,
as illustrated in this International Standard.
7.2.2 ITS station nodes
An ITS-SU comprises ITS-SCUs connected via an ITS station-internal network as illustrated in 7.2.4. The
functionality contained in an ITS-SCU may be expressed by the functionalities of one or several ITS-S
nodes as illustrated in Annex B. The following ITS-S nodes are identified:
a) ITS-S router
1) ITS-S border router
— Access router
— Mobile router
2) ITS-S internal router
b) ITS-S host
c) ITS-S gateway
The following definitions apply.
— An ITS-S router is an ITS-S node comprised of routing functionalities of an ITS-S used to connect
two networks and to forward packets not explicitly addressed to itself as illustrated in Figure 10
with the example of an ITS station-internal network and an external network B.
Figure 10 — ITS-S router
— An ITS-S border router is an ITS-S router with additional functionality that provides connectivity
to other ITS communication nodes over external networks (network B in Figure 10).
— An ITS-S access router is an ITS-S border router with additional functionality that provides other
ITS communication nodes a point of attachment to an external network.
— An ITS-S mobile router is an ITS-S border router with additional functionality that allows a change
of point of attachment to an external network while maintaining session continuity.
— An ITS-S internal router is an ITS-S router that connects two or more ITS station-internal networks.
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— An ITS-S host is an ITS-S node comprised of ITS-S functionalities other than the functionalities
of an ITS-S router, ITS-S border router, ITS-S mobile router, or an ITS-S gateway, i.e. not capable to
forward packets not explicitly addressed to itself.
NOTE Being an ITS-S node, an ITS-S host obviously contains the communication functionality to connect to
at least one network, although routing functionality is not part of the ITS-S host functionality.
— An ITS-S gateway interconnects an “external protocol stack” to the ITS-S management entity, or
to the ITS-S facilities layer, or to the ITS-S networking and transport layer, and thus supports also
direct routing on a default path to Internet which enables end-to-end communications. An ITS-S
gateway may convert between different protocols. The protocol stack on the right hand side in
Figure 11 is connected to the ITS station-internal network. The protocol stack on the left-hand side
in Figure 11 is connected to an external network.
NOTE The external network may be a proprietary network, i.e. its technical specification may not be publicly
available.
Figure 11 — Example of an ITS-S gateway at the ITS-S facilities layer
7.2.3 Protocol and service data units in the ITS-S protocol stack
Figure 12 shows the data unit transfer, i.e. service data units (SDU) and protocol data units (PDU),
through the ITS-S communication stack of two peer ITS stations communicating with each other and
the grouping of protocol layers as used in ITS:
— Session, presentation, and application OSI layers 5 through 7 comprise the “ITS-S facilities layer”.
— Network and transport OSI layers 3 and 4 comprise
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