ETSI TR 102 862 V1.1.1 (2011-12)

Technical Report
Intelligent Transport Systems (ITS);
Performance Evaluation of Self-Organizing TDMA as Medium
Access Control Method Applied to ITS;
Access Layer Part
2 ETSI TR 102 862 V1.1.1 (2011-12)

Reference
DTR/ITS-0040021
Keywords
ITS, MAC, TDMA
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ETSI
3 ETSI TR 102 862 V1.1.1 (2011-12)
Contents
Intellectual Property Rights . 5
Foreword . 5
Introduction . 5
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definitions, symbols and abbreviations . 12
3.1 Definitions . 12
3.2 Symbols . 12
3.3 Abbreviations . 13
4 Introduction . 14
4.1 Medium access control in VANETs . 14
4.2 Requirements for road traffic safety applications . 15
4.3 Hidden terminal problem . 16
5 CSMA. 19
5.1 Introduction . 19
5.2 Channel access procedure and parameters . 19
5.3 Simultaneous transmissions. 21
5.4 Summary . 21
6 Motivations for time slotted MAC approaches . 21
7 Time slotted MAC approaches . 22
7.1 Introduction . 22
7.2 STDMA . 24
7.2.1 Introduction. 24
7.2.1.1 The AIS system . 24
7.2.1.2 Position reports . 25
7.2.1.3 Overhead to run the STDMA algorithm. 26
7.2.2 Parameters. 26
7.2.3 Channel access procedure . 28
7.2.3.1 Initialization . 28
7.2.3.2 Network entry . 28
7.2.3.3 First frame . 29
7.2.3.4 Continuous operation . 29
7.2.3.5 Summary . 30
7.2.4 Simultaneous transmissions . 30
7.2.5 Summary . 32
7.3 MS-Aloha . 32
7.3.1 Introduction. 32
7.3.2 Channel access procedure . 33
7.3.2.1 Memory refresh . 34
7.3.2.2 Solutions against protocol overheads . 35
7.3.3 Simultaneous transmissions . 36
7.3.3.1 Prevention of hidden terminals and unintentional slot re-use . 37
7.3.3.2 Slot reuse at four-hop distance . 37
7.3.3.3 Mechanisms for forced slot re-use . 38
7.3.3.4 Dynamic mechanisms for the forced slot re-use . 38
7.3.3.5 Pre-emption . 39
7.3.4 Parameters. 40
7.3.5 Summary . 40
7.4 Other time slotted approaches . 42
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4 ETSI TR 102 862 V1.1.1 (2011-12)
8 Time synchronization . 44
8.1 Introduction . 44
8.2 Motivation for GNSS synchronization . 44
8.3 From the accuracy of GNSS synchronization to the required Guard-Times . 45
8.4 Fallback solution in absence of GNSS . 47
9 Migration and coexistence in road traffic scenarios . 47
9.1 Introduction . 47
9.2 Backward compatibility . 47
9.3 Coexistence with CSMA . 48
10 Executive summary . 48
Annex A: Bibliography . 50
History . 51

ETSI
5 ETSI TR 102 862 V1.1.1 (2011-12)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (http://ipr.etsi.org).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Intelligent Transport System (ITS).
Introduction
By introducing wireless communications between vehicles and between vehicles and road infrastructure or other fellow
road users such as pedestrians and bicyclists, the road environment will become safer and potentially more
environmentally friendly. Many different cooperative intelligent transport systems (ITS) applications have been
suggested for the vehicular environment, both for road traffic safety and efficiency. Depending on application area, the
resulting communication requirements are quite diverse. Different wireless access technologies have different features
and different benefits and all cooperative ITS applications suggested for the vehicular environment cannot be solved
with one single technology due to resource constraints and diverse requirements. Vehicular ad hoc networks (VANETs)
based on, e.g. IEEE 802.11p [i.2], will be used for road traffic safety applications, [i.1], [i.2]. However, other wireless
carriers such as cellular technology (e.g. 3G, LTE) will also be used to support different cooperative ITS applications in
general.
The major difference between VANETs and cellular technology is that there is no central controller in the former. The
central controller usually has perfect knowledge about the nodes within range and it can distribute and optimize the
available resources. However, in cellular technology there is a central controller in the form of a base station present,
otherwise communication is not possible. VANETs do not need coverage by base stations - instead if there is someone
to communicate with, communication will take place directly in between any two nodes within range of each other. The
ad hoc structure is advantageous, since it does not require coverage by base stations, but without a central control
mechanism, problems with scalability may arise. Due to the lack of a central coordinator, all nodes typically transmit on
a common frequency channel. This frequency channel, called the control channel, is known a priori to all nodes. For
road traffic safety applications, this channel is where the most important data will be transmitted. To facilitate additional
cooperative ITS applications with higher bandwidth requirements, two or more service channels are also available.
However, the control channel is the core of a VANET.
Many emerging road traffic safety applications will be based purely on broadcast communication, [i.3], i.e. one-to-
many. Due to the broadcast communication, the assurance of sufficient reliability is limited. A sender does not know if
the transmitted data has arrived at the intended receiver because no acknowledgments of successful reception are
possible in broadcast mode (receivers cannot send an acknowledgment to the sender since the number of intended
receivers is not known and this may flood the network). One way to increase the reliability in broadcast mode is instead
to repeat the same message several times.
Ultimately, cooperative ITS applications for enhancing road traffic safety should be designed taking the characteristics
of a VANET into account. These characteristics can be summarized by: a decentralized network topology, a common
control channel and broadcast as the preferable communication mode. The utilization of the control channel should be
carefully designed so it can be used to its maximum. The medium access control (MAC) protocol schedules access to
the shared control channel. A MAC protocol suitable for road traffic safety applications in VANETs should be
decentralized such that it functions without a central controller, it should support broadcast such that channel access is
fair and predictable for all participating nodes and it should aim to minimize interference between transmitters to
maximize scalability. Further, as road traffic safety typically involves interaction with vehicles located in the vicinity of
each other, the MAC method should maximize the packet reception probability for the closest neighbouring nodes.
ETSI
6 ETSI TR 102 862 V1.1.1 (2011-12)
ETSI has standardized a VANET protocol based on a profile of IEEE 802.11p [i.2], called ITS-G5 [i.1], which uses the
MAC method carrier sense multiple access (CSMA). CSMA has some of the desired properties, i.e. it is decentralized
and aims at minimizing interference between any transmitters. However, it does not necessarily maximize the packet
reception probability for the closest neighbouring nodes or provide fair and predictable channel access for broadcast.
The present document therefore scrutinize time slotted MAC protocols, to determine if these can utilize the common
control channel more efficiently than the current proposed MAC from IEEE 802.11p [i.2].
ETSI
7 ETSI TR 102 862 V1.1.1 (2011-12)
1 Scope
The present document describes the use of time slotted MAC algorithms in VANETs. Two specific MAC methods,
self-organizing time division multiple access (STDMA) and mobile slotted Aloha (MS-Aloha), are described in detail,
not excluding other time slotted approaches. Time slotted approaches are suitable for road traffic safety applications as
the maximum delay is predictable and channel access can be made fair among all participating nodes even during
broadcast. However, time slotted approaches do require synchronization between nodes to build a common framing
structure for transmissions, something that is not needed for non-time slotted approaches, e.g. CSMA as used by
ITS G5 [i.1]. In the literature of time slotted MAC protocols for VANETs, synchronization is provided by a global
navigation satellite system (GNSS) such as the global positioning system (GPS) or Galileo. The present document also
describes the GNSS synchronization issue as well as proposals for dealing with synchronization when the GNSS signal
is absent or weak, which can occur in urban environments and tunnels. Further, time slotted approaches use fixed-length
time slots for transmissions, implying that packet lengths are fixed. However, as the physical (PHY) layer suggested for
VANETs offers several transfer rates, this means that different packet sizes can be obtained in the fixed time slots. The
analysis of the most preferable configuration in this context constitutes the second technical topic covered by the
present document. Finally the present document also deals with the coexistence between CSMA and time slotted MAC
approaches nodes. The backward compatibility and coexistence are of crucial importance since the first generation of
VANETs will use CSMA technology. This represents the third and final topic of the present document.
2 References
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
reference document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
http://docbox.etsi.org/Reference.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
2.1 Normative references
The following referenced documents are necessary for the application of the present document.
Not applicable.
2.2 Informative references
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
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Specifications; Amendment 6: Wireless Access in Vehicular Environments".
[i.3] ETSI TR 102 638: "Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of
Applications; Definitions".
ETSI
8 ETSI TR 102 862 V1.1.1 (2011-12)
[i.4] IEEE 802.11: 2007: "IEEE Standard of Information Technology - Telecommunications and
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[i.6] ETSI TS 102 637-2: "Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set
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[i.7] ETSI TR 101 683: "Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; System
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11 ETSI TR 102 862 V1.1.1 (2011-12)
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12 ETSI TR 102 862 V1.1.1 (2011-12)
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
ad hoc network: wireless networks based on self-organization without the need for a centralised coordinating
infrastructure
broadcast: simplex point-to-multipoint mode of transmission
3.2 Symbols
For the purposes of the present document, the following symbols apply:
A Symbol used to indicate a node in the examples
a Sub-period of a period c, used for asynchronous MAC
AC_BE Access Category Best Effort
AC_BK Access Category Background
AC_VI Access Category Video
AC_VO Access Category Voice
B Symbol used to indicate a node in the examples
b Sub-period of a period c, used for synchronous MAC
c Fixed period of time for the coexistence of two MAC methods
C Symbol used to indicate a node in the examples
CW Contention Window
CW Maximum possible value of CW
max
CW Minimum possible value of CW
min
D Symbol used to indicate a node in the examples
E Symbol used to indicate a node in the examples
F% Percentage of slots perceived free by a node
F Upper Threshold used by 2-SMtd to evaluate F% for the near-exhaustion condition
F Lower Threshold used by 2-SMtd to evaluate F% for the unloaded condition
FI Frame Indication
FI' Extended Frame indication, including both FI and STI
FI_j The j-th subfield of the FI field
j Index used in the examples for the indication of slot number
J The j-th slot in MS-Aloha's Frame
L1 Layer 1
L2 Layer 2
LA Set of nodes receiving from node A
MB Set of nodes receiving from node B
N Number of slots in a period
P Clock precision in ppm
ppm Parts per million
PSF Priority Status Field
SX Equivalent number of slots required to transmit X Bytes
SLOT_n Slot number n of MS-Aloha Frame structure
STATE The field of each FI_j indicating the perceived state (busy/free/collision/2-hop)
STI Short Temporary Identifier
T Arbitration interframe space period
AIFS
TX Time required to transmit X Bytes
Tg Guard Time
Thr MS-Aloha threshold used for 2SMt and 2SMtd algorithms
T Duration of a slot
slot
X Generic number of Bytes in a frame
ETSI
13 ETSI TR 102 862 V1.1.1 (2011-12)
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
2-SM 2-Hop Spatial Multiplexing
2-SMt 2-Hop Spatial Multiplexing with Threshold
2-SMtd 2-Hop Spatial Multiplexing with Dynamic Threshold
AC Access Category
ACK Acknowledgment
AIFS Arbitration InterFrame Space
AIS Automatic Identification System
AP Access Point
ARQ Automatic Repeat request
ATS Average TimeSync Protocol
BS Base Station
CAM Cooperative Awareness Message
CCA Clear Channel Assessment
CCH Control Channel
CDMA Code Division Multiple Access
CRC Cyclic Redundancy Check
CSAP Concurrent Slot Assignment Protocol
CSMA Carrier Sense Multiple Access
CTS Clear-To-Send
DCAP Decentral Channel Access Protocol
DCC Decentralised Congestion Control
DENM Decentralised Environmental Notification Message
DTDMA Decentralised TDMA
EDCA EDCF Controlled Channel Access
FCS Frame Check Sequence
FI Frame Information
GNSS Global Navigation Satellite System
GPS Global Positioning System
GPSDO Global Positioning System Disciplined Oscillator
HCCA HCF Controlled Channel Access
HPOCXO High Performance OCXO
HW Hardware
ITS Intelligent Transport Systems
LDM Local Dynamic Map
MAC Medium Access Control
MS-Aloha Mobile Slotted Aloha
NI Nominal Increment
NS Nominal Slot
NSS Nominal Start Slot
NTP Network Time Protocol
NTS Nominal Transmission Slot
PDR Packet Delivery Ratio
PHY Physical layer
PPM Part Per Million
PPS Pulse Per Second
PR-Aloha Priority R-Aloha
PSF Priority State Field
QoS Quality of Service
RAIM Receiver Autonomous Integrity Monitoring
R-Aloha Reservation Aloha
RBS Reference Broadcast Synchronization
RR Report Rate
RR-Aloha Reliable R-Aloha
RTS Request-To-Send
RX Receiver
SCH Service Channel
SDH Synchronous Digital Hierarchy
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14 ETSI TR 102 862 V1.1.1 (2011-12)
SDMA Space Division Multiple Access
SI Selection Interval
SINR Signal-to-Interference-plus-Noise Ratio
STDMA Self-Organizing Time Division Multiple Access
STI STI stands for Short Temporary Identifier, which is stated in the Symbol list. What to do?
TDMA Time Division Multiple Access
TPC Transmit Power Control
TPSN Timing-sync Protocol for Sensor Networks
TX Transmitter
TXCO Temperature-Controlled Crystal Oscillator
UTC Universal Coordinated Time
V2V Vehicle-to-Vehicle
VANET Vehicular Ad Hoc Networks
VCO Voltage Controlled Oscillator
VHF Very High Frequency band
WLAN Wireless Local Area Network
4 Introduction
4.1 Medium access control in VANETs
The MAC algorithm resides in the MAC sub-layer of the data link layer in the protocol stack of a communication
system, see figure 1. It is responsible for scheduling transmissions in e.g. time, frequency or space. The objective is
often to minimize interference and thereby increase reception probability at the receivers. Providing access to the shared
medium, while at the same time enabling the quality of service (QoS) requested by the application is the most important
but also the most challenging task of the MAC layer. There exist several different MAC methods tailored to the network
topology in question (centralized or ad hoc). In centralized networks, an access point (AP) or a Base Station (BS) is
usually responsible for scheduling the transmissions and share the resources equally among all nodes. The AP or the BS
has perfect knowledge of which nodes that are associated to them. In centralized networks, AP and BS are single point
of failures. The loss of an AP or BS, due to hardware failure or loss of power will result in outage because the nodes
cannot self-organize. In the ad hoc topology, a decentralized MAC method is needed, such that the scheduling of
transmissions is distributed among the nodes. The network self-organizes and the failure of one node does not
necessarily affect the rest of the network. The ad hoc structure is advantageous, since it does not require coverage by
BS or AP to function, but without a central control mechanism, problems with scalability may arise.

Application
Presentation
Session
Transport
Network
Logical Link Control
Data link Medium Access Control
Physical
Figure 1: Generic protocol stack showing the logical position
for the medium access control the sublayer
ETSI
15 ETSI TR 102 862 V1.1.1 (2011-12)
From ongoing standardization activities [i.1] and [i.2], it is clear that many road traffic safety applications will be based
on 802.11p, forming ad hoc networks. Further, two types of messages are envisioned; decentralized environmental
notification messages (DENM) [i.5] and cooperative awareness messages (CAM) [i.6]. DENM are event-driven
messages that are generated as a result of a hazard, whereas CAMs are time-triggered and contain position, speed,
heading, etc of each vehicle. CAMs are broadcasted regularly by every vehicle and are the foundation for the local
dynamic map (LDM) [i.3] facility. These broadcasted warning and positioning messages imply a distributed control
system, with concurrent requirements on high reliability and real-time deadlines.
To increase reliability in broadcast mode, the same message is repeated several times. This implies that the MAC
method should be able to handle temporarily high network loads that may occur as the result of a hazard. To support
real-time deadlines, the MAC method should be predictable such that the maximum delay before granting channel
access is known. Further, the ad hoc network implies that the MAC method has to be decentralized. Note that in a
VANET the number of nodes cannot be restricted and therefore the MAC method used in VANETs have to be self-
organising, fair and scalable. A self-organising MAC algorithm implies that nodes are responsible for scheduling
transmissions without the intervention of infrastructure such as AP or BS. i.e. the channel access scheduling is
distributed. The MAC algorithm has to be fair in the sense that all nodes have equal right to access the wireless channel
at least once during a limited time period. For example, in overloaded situations potential packet dro
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