ETSI TR 102 861 V1.1.1 (2012-01)

Technical Report
Intelligent Transport Systems (ITS);
STDMA recommended parameters and
settings for cooperative ITS;
Access Layer Part
2 ETSI TR 102 861 V1.1.1 (2012-01)

Reference
DTR/ITS-0040020
Keywords
CSMA, ITS, MAC, MS-Aloha, STDMA
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: +33 4 92 94 42 00  Fax: +33 4 93 65 47 16

Siret N° 348 623 562 00017 - NAF 742 C
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° 7803/88

Important notice
Individual copies of the present document can be downloaded from:
http://www.etsi.org
The present document may be made available in more than one electronic version or in print. In any case of existing or
perceived difference in contents between such versions, the reference version is the Portable Document Format (PDF).
In case of dispute, the reference shall be the printing on ETSI printers of the PDF version kept on a specific network drive
within ETSI Secretariat.
Users of the present document should be aware that the document may be subject to revision or change of status.
Information on the current status of this and other ETSI documents is available at
http://portal.etsi.org/tb/status/status.asp
If you find errors in the present document, please send your comment to one of the following services:
http://portal.etsi.org/chaircor/ETSI_support.asp
Copyright Notification
No part may be reproduced except as authorized by written permission.
The copyright and the foregoing restriction extend to reproduction in all media.

© European Telecommunications Standards Institute 2012.
All rights reserved.
TM TM TM
DECT , PLUGTESTS , UMTS and the ETSI logo are Trade Marks of ETSI registered for the benefit of its Members.
TM TM
3GPP and LTE are Trade Marks of ETSI registered for the benefit of its Members and
of the 3GPP Organizational Partners.
GSM® and the GSM logo are Trade Marks registered and owned by the GSM Association.
ETSI
3 ETSI TR 102 861 V1.1.1 (2012-01)
Contents
Intellectual Property Rights . 5
Foreword . 5
Introduction . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 6
3 Definitions, symbols and abbreviations . 8
3.1 Definitions . 8
3.2 Symbols . 8
3.3 Abbreviations . 9
4 Introduction . 10
5 Simulation settings . 11
5.1 Introduction . 11
5.2 Data traffic model . 11
5.2.1 Packet structure . 12
5.2.2 Slot length, guard time and clock hold-on . 12
5.2.3 Frame length . 13
5.2.3.1 STDMA . 13
5.2.3.2 MS-Aloha . 13
5.3 Vehicle traffic model . 14
5.3.1 Highway scenario (STDMA) . 14
5.3.2 Urban scenario (MS-Aloha) . 15
5.4 Channel model . 16
5.4.1 Highway scenario . 16
5.4.2 Urban obstructed and non-obstructed scenarios . 19
5.4.2.1 Receiver model used for the urban scenarios . 21
5.5 CSMA specific parameters . 22
5.6 Performance metrics . 23
5.6.1 Introduction. 23
5.6.2 Channel access delay . 23
5.6.3 Packet reception probability . 24
6 Simulation results of STDMA . 25
6.1 Introduction . 25
6.2 Parameter settings. 25
6.3 Simulation results: highway scenario . 25
6.3.1 Packet reception probability . 25
6.3.1.1 Normal vehicle density . 25
6.3.1.2 High vehicle density . 26
6.3.2 Simultaneous transmissions . 27
6.3.3 Channel access delay . 31
6.4 Conclusions . 32
7 Simulation results of MS-Aloha . 33
7.1 Guide to the Interpretation of Results from Simulations . 33
7.1.1 Rational and Effects of Spatial Multiplexing . 33
7.1.2 Configuration Rules . 34
7.1.2.1 Framing rules . 35
7.1.2.2 Re-Use and Threshold Algorithm . 36
7.1.2.3 Pre-emption . 36
7.1.3 Hidden terminals in an urban environment . 36
7.2 Simulation Results: Urban Scenario . 38
ETSI
4 ETSI TR 102 861 V1.1.1 (2012-01)
7.2.1 Analysis of Results: Urban Obstructed . 38
7.2.2 Analysis of Results: Urban Non-Obstructed . 41
7.2.2.1 Motivation of the Analysis in Non-Obstructed Scenarios . 41
7.2.2.2 Analysis of Results . 42
7.3 Conclusions: Recommended Parameter Settings . 44
8 Executive summary . 45
Annex A: Bibliography . 47
History . 48

ETSI
5 ETSI TR 102 861 V1.1.1 (2012-01)
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
It is essential to decrease the number of fatalities on our roads, not only because it causes much grief for individuals
each year, but also because it costs enormous amounts of money for society. There are different ways of increasing the
road traffic safety, which all contribute to a better and more efficient road traffic environment. One way is to build new
highways with separated lanes as these are less prone to traffic accidents. However, this is only possible to some extent
due to space limitations. Another way is to introduce wireless communications between vehicles which enable new
applications for increasing road traffic safety such as wrong way warning, red light violation, intersection collision
warning and emergency brake warnings. This is termed cooperative intelligent transport systems (ITS).
The impact of road traffic safety applications as well as road traffic efficiency applications is likely dependent of a
considerably amount of vehicles being equipped with communication devices. The exact penetration of course depends
on the application in question, but generally the more vehicles that are equipped the better. However, it is also at this
stage the current technology chosen for cooperative ITS may encounter problems. When the number of ITS equipped
vehicles increases, the standardized technology based on CSMA will face problems with scalability. The scalability of
CSMA directly influences the reliability of the transmission, the channel access delay and thereby the fairness. When
the number of nodes increases, the number of simultaneous transmissions will increase, resulting in lower reliability and
decoding problems due to interference. One way to counteract the scalability issue of CSMA is to introduce
decentralised congestion control methods (DCC) such that the amount of data traffic transmitted is restricted and
transmit power levels adjusted. However, by decreasing the amount of data traffic transmitted the road traffic safety
applications may suffer with performance degradation as a result.
Another way to counteract the scalability issue is to investigate the performance of other medium access control (MAC)
protocols in terms of scalability, reliability, delay and fairness. Self-organizing time division multiple access (STDMA)
and mobile slotted Aloha (MS-Aloha) are two time slotted MAC approaches designed for ad hoc networking (they are
self-organizing and decentralized) and both can cope with a high and varying number of nodes without collapsing.
When the number of nodes increases within radio range and all free resources are exhausted, both algorithms still admit
transmissions through careful scheduling to maintain a high reliability for the nodes closest to the transmitter. This
implies that the channel access delay has a maximum upper limit and the resulting network is fair and predictable.
In the present document, the performance of CSMA, STDMA and MS-Aloha are investigated through simulations with
a varying number of vehicles, all equipped with cooperative ITS units. In particular, the performance measures channel
access delay and packet reception probability are evaluated as these measures captures the reliability, the delay and the
fairness of resulting system as well as how these depend on scalability.
ETSI
6 ETSI TR 102 861 V1.1.1 (2012-01)
1 Scope
The present document summarises the result from performance evaluations of CSMA and two time slotted MAC
approaches through simulations. Two different time slotted MAC approaches, self-organizing time division multiple
access (STDMA) and mobile slotted Aloha (MS-Aloha), have been considered in two different scenarios; highway and
urban. CSMA, the MAC algorithm proposed for the current generation of vehicular ad hoc networks (VANETs) has
been used as a benchmark. Packet reception probability at different distances from the transmitter together with the
channel access delay has been used as performance measures. The purpose is first and foremost to evaluate the
scalability of the resulting system, as initial results have shown that CSMA may degrade in performance when the
number of vehicles equipped with cooperative ITS units increase.
NOTE 1: Håkan Lans holds a patent on STDMA [i.25], which expires in July 2012. The patent has been
re-examined in the US cancelling all claims on March 30, 2011.
NOTE 2: A European patent procedure has been started by ISMB on MS-Aloha techniques (European patent
request filed with number 10163964.9, May 26, 2010). They have received in September 2011
Communication Under Rule 71(3) EPC of the intention to grant a patent.
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
referenced 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
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.
[i.1] ETSI TR 102 862: "Intelligent Transport Systems (ITS); Performance Evaluation of Self-
Organizing TDMA as Medium Access Control Method Applied to ITS; Access Layer Part".
[i.2] IEEE 802.11p: 2010: "IEEE Standard of Information Technology - Telecommunications and
information exchange between systems - Local and metropolitan area networks - Specific
requirements; Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications; Amendment 6: Wireless Access in Vehicular Environments".
[i.3] M. Nakagami: "The m-distribution, a general formula of intensity distribution of the rapid fading",
Oxford, England, Pergamon, 1960.
[i.4] V. Taliwal, D. Jiang, H. Mangold, C. Chen and R. Sengupta: "Empirical determination of channel
characteristics for DSRC vehicle-to-vehicle communication," in Proc. ACM Workshop on
Vehicular Ad Hoc Networks (VANET), Philadelphia, PA, USA, October 2004, pp. 88-88.
[i.5] L. Cheng, B. E. Henty, D. D. Stancil, F. Bai and P. Mudalige: "Mobile vehicle-to-vehicle narrow-
band channel measurement and characterization of the 5.9 GHz dedicated short range
communication (DSRC) frequency band," IEEE Journal on Selected Areas in Communications,
vol. 25, no. 8, pp. 1501-1516, October 2007.
ETSI
7 ETSI TR 102 861 V1.1.1 (2012-01)
[i.6] R. Scopigno and H.A. Cozzetti: "Evaluation of time-space efficiency in CSMA/CA and slotted
Vanets," in Proc of the IEEE 71st Vehicular Technology Conference (VTC Fall 2010), Ottawa,
Canada, Sept. 2010.
[i.7] H.A. Cozzetti and R. Scopigno: "Scalability and QoS in slotted VANETs: forced slot re-use vs
pre-emption," in Proc of the 14th Int. IEEE Conf. on Intelligent Transportation Systems (ITSC
2011), Washington, DC, USA, October 2011.
[i.8] ETSI ES 202 663: "Intelligent Transport Systems (ITS); European profile standard for the
physical and medium access control layer of Intelligent Transport Systems operating in the 5 GHz
frequency band".
[i.9] R. Scopigno, and H.A. Cozzetti: "Signal shadowing in simulation of urban vehicular
communications", Proc. of the 6th Int. Wireless Communications and Mobile Computing
Conference (IWCMC), Valencia, Spain, September 2010.
[i.10] L. Pilosu, F. Fileppo and R. Scopigno: "RADII: a computationally affordable method to
summarize urban ray-tracing data for VANETs" in Proc. of the 7th Int. Conf. on Wireless
Communications, Networking and Mobile Computing (IEEE WiCOM 2011), Wuhan, China,
September 2011.
[i.11] T. Jiang, H. H. Chen, H. C. Wu and Y. Yi: "Channel modeling and inter-carrier interference
analysis for V2V communication systems in frequency-dispersive channels," in The Journal
Mobile Networks and Applications, vol. 15, no. 1, pp. 4-12, 2010.
[i.12] The European Road Safety Observatory.
NOTE: http://erso.swov.nl/.
[i.13] "SUMO - Simulation of Urban MObility", developed by employees at the Institute of
Transportation Systems at the German Aerospace Center June 2010.
NOTE: http://sumo.sourceforge.net.
[i.14] C. Campolo, A. Molinaro, H.A. Cozzetti and R. Scopigno: "Roadside and moving WAVE
providers: effectiveness and potential of hybrid solutions in urban scenarios", in Proc. of the 11th
IEEE Int. Conf. on ITS Telecommunications (ITST), St. Petersburg, Russia, August 2011.
[i.15] E.G. Ström: "On medium access and physical layer standards for cooperative intelligent transport
systems in Europe", in Proceedings of the IEEE, vol. 99, no. 7, pp. 1183-1188, July 2011.
[i.16] E. Giordano, R. Frank, G. Pau and M. Gerla: "CORNER: A radio propagation model for VANETs
in urban Scenarios", in Proceedings of the IEEE, vol. 99, no. 7, pp. 1280-1294, July 2011.
[i.17] D. Jiang, Q. Shen and L. Delgrossi: "Optimal data rate selection for vehicle safety
communications," in Proc. of the 5th Int. Workshop on Vehicular Inter-Networking (VANET),
San Francisco, CA, US, September 2008.
[i.18] K. Sjöberg, E. Uhlemann and E. G. Ström: "How severe is the hidden terminal problem in
VANETs when using CSMA and STDMA?", in Proc. of the 4th IEEE Symposium on Wireless
Vehicular Communications (WiVEC), San Francisco, CA, US, September 2011.
[i.19] Q. Chen, F. Schmidt-Eisenlohr, D. Jiang, M. Torrent-Moreno, L. Delgrossi and H. Hartenstein:
"Overhaul of IEEE 802.11 modeling and simulation in NS-2," in Proc. of the 10th ACM
International Symposium on Modeling, Analysis and Simulation of Wireless and Mobile Systems
(MSWiM 2007), Chania, Crete island, Greece, October 2007, pp. 159-168.
[i.20] R. Meireles, M. Boban, P. Steenkiste, O. Tonguz and J. Barros: "Experimental study on the impact
of vehicular obstructions in VANETs," in Proc. of the 2nd IEEE Vehicular Networking
Conference (VNC 2010), Jersey City, New Jersey, USA, December 2010.
[i.21] E. Giordano, R. Frank, A. Ghosh, G. Pau and M. Gerla: "Two Ray or not Two Ray this is the price
to pay", in Proc. of the 6th IEEE International Conference on Mobile Ad Hoc and Sensor Systems
(MASS 2009), Macau SAR, China, October 2009, pp. 603-608.
ETSI
8 ETSI TR 102 861 V1.1.1 (2012-01)
[i.22] C. Sommer, D. Eckhoff, R. German and F. Dressler: "A computationally inexpensive empirical
model of IEEE 802.11p radio shadowing in urban environments", in Proc. of the 2011 8th Int.
Conf. on Wireless On-Demand Network Systems and Services (WONS 2011), Bardonecchia,
Italy, January 2011, pp.84-90.
[i.23] M. Boban, T. T. V. Vinhoza, M. Ferreira, J. Barros, and O. Tonguz: "Impact of vehicles as
obstacles in vehicular ad hoc networks" in IEEE Journal on Selected Areas in Communications,
vol. 29, no. 1, pp. 15-28, January 2011.
[i.24] ETSI TS 102 687: "Intelligent Transport Systems (ITS); Decentralized Congestion Control
Mechanisms for Intelligent Transport Systems operating in the 5 GHz range; Access layer part".
[i.25] H. Lans: "Position Indicating System," US patent 5,506,587, issued 1996.
[i.26] IEEE 802.11a-1999: "IEEE Standard for Telecommunications and Information Exchange Between
Systems - LAN/MAN Specific Requirements - Part 11: Wireless Medium Access Control (MAC)
and physical layer (PHY) specifications: High Speed Physical Layer in the 5 GHz band".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
broadcast: simplex point-to-multipoint mode of transmission
NOTE: This may contain additional information.
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
ETSI
9 ETSI TR 102 861 V1.1.1 (2012-01)
N Number of slots in a period
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
T PLCP transmit period
PLCP
T STI transmit period
STI
T FI transmit period
FI
T Time required to transmit X bytes
TX
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
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
AIFS Arbitration InterFrame Space
AIFSN Arbitration InterFrame Space Number
AP Access Point
ARQ Automatic Repeat reQuest
CAM Cooperative Awareness Message
CCA Clear Channel Assessment
CCH Control CHannel
CDF Cumulative Distribution Function
CSMA Carrier Sense Multiple Access
CW Contention Window
DCC Decentralized Congestion Control
DENM Decentralised Environmental Notification Message
EA Extra Attenuation
EPC European Patent Convention
FI Frame Indications
GNSS Global Navigation Satellite System
GPS Global Positioning System
HT Hidden Terminal
IEEE Institute of Electrical and Electronic Engineers
ISI Inter Symbol Interference
ISMB Istituto Superiore Mario Boella
ITS Intelligent Transport Systems
LOS Line of Sight
MAC Medium Access Control
MAX MAXimum
MS-Aloha Mobile Slotted Aloha
NLOS Non Line of Sight
OFDM Orthogonal Frequency Division Multiplexing
PDF Probability Density Function
PDR Packet Delivery Ratio
PHY Physical layer
PLCP Physical Layer Convergence Procedure
QoS Quality of Service
QPSK Quadrature Phase Shift Keying
RADII Ray-tracing Data Interpolation and Interfacing
RR Report Rate
RX Receiver
ETSI
10 ETSI TR 102 861 V1.1.1 (2012-01)
RX Receiver
SI Selection Interval
SINR Signal-to-Interference-plus-Noise Ratio
SNIR Signal to Noise and Interference Ratio
SNR Signal-to-Noise Ratio
STDMA Self-Organizing Time Division Multiple Access
STI Short Temporary Identifier
SUMO Simulation of Urban MObility
TCL Tool Command Language
TDMA Time Division Multiple Access
TX Transmitter
TX Transmitter
VANET Vehicular Ad Hoc Networks
XML eXtensible Markup Language
4 Introduction
Cooperative intelligent transport systems (ITS) applications are a promising approach in an effort to decrease road
traffic accidents. Road traffic safety applications will mainly use broadcast communication in a vehicular ad hoc
network (VANET), i.e. one sender and many receivers communication in a decentralized ad hoc network. All nodes
will share a common frequency channel, commonly referred to as the control channel. The ad hoc topology together
with broadcast will have a major impact on the requirements of the developed communication protocols. All
communication systems use a communication stack consisting of several layers containing protocols, which are more or
less complex depending on the developed system. The medium access control (MAC) algorithm, residing in the
sublayer MAC of the data link layer, Figure 1, is one of the cornerstones in data communication because it determines
when a node has the right to transmit.

Application
Presentation
Session
Transport
Network
Logical Link Control
Data link Medium Access Control
Physical
Figure 1: Generic protocol stack showing the
logical position of the medium access control sublayer
Three MAC methods are examined through simulations in the present document; carrier sense multiple access (CSMA),
self-organizing multiple access (STDMA) and mobile slotted Aloha (MS-Aloha). STDMA and MS-Aloha are two time
slotted MAC approaches, where the available time is divided into time slots with fixed length. One transmission fits
into one time slot and when all time slots are occupied, STDMA and MS-Aloha allow more than one transmission in
each slot through careful scheduling (i.e. simultaneous transmissions can take place to cope with high network loads).
CSMA, on the other hand, is a random access scheme, where nodes that want to transmit will start by sensing the
channel for a predetermined sensing period and if the channel is sensed free the transmission can commence. If not, a
random backoff procedure is invoked. Detailed descriptions of the MAC algorithms are found in [i.1].
In clause 4.2 in [i.1] requirements on the MAC algorithm applied in VANETs were detailed. It was concluded that road
traffic safety applications have requirements on the MAC layer in terms of upper bounded channel access delay,
reliability and fairness. The ad hoc topology calls for a decentralized, self-organizing and scalable MAC method. The
scalability property is closely coupled to the requirements of road traffic safety applications. A lightly loaded network
results in a lower channel access delay, a higher reliability and fairness. In other words it is generally no problem to
fulfil the requirements of road traffic safety applications regardless of MAC method, if the network load is light enough.
ETSI
11 ETSI TR 102 861 V1.1.1 (2012-01)
However, when the network load increases all three requirements delay, reliability and fairness, are affected more or
less severely depending on MAC method.
All three examined MAC protocols are self-organizing and do not have to rely on any access point or base station in
order to schedule transmissions, i.e. they are decentralized. STDMA and MS-Aloha are scalable and always guarantee
channel access regardless of the number of nodes within radio range and this makes them predictable as the maximum
channel access delay is known. CSMA is scalable in terms of the number of nodes but not in number of transmissions.
Table 1, also found in clause 10 in [i.1], summarizes to what extent the three different MAC methods can fulfil the
requirements of road traffic safety applications for light network load as well as heavy network load, respectively.
Table 1: An overview of the road traffic safety applications' requirements and
the MAC methods ability to fulfil those
Light network load Heavy network load
STDMA MS-Aloha CSMA STDMA MS-Aloha CSMA
Delay Predictable Predictable Random Predictable Predictable Random
Reliability High High High High High Low
Probability of
High High High High High Low
fairness
In the present document simulations have been carried out to evaluate the three MAC methods mentioned above in a
VANET setting. All transmissions are in broadcast mode excluding traditional automatic repeat request (ARQ)
mechanisms for increasing reliability. MS-Aloha has been simulated in an urban environment and STDMA has been
simulated in a highway scenario. CSMA has been simulated and used as benchmark in both scenarios. Periodic
cooperative awareness messages (CAMs) containing position, speed, heading etc., of each vehicle have been used as a
data traffic model. Packet reception probability and channel access delay are the performance measures used for
evaluating the performance. Decentralized congestion control (DCC) methods as outlined in [i.24] and required by
ITS-G5 [i.8] to combat the scalability issue of CSMA have not been implemented in the simulators nor evaluated in the
present document.
5 Simulation settings
5.1 Introduction ®
This clause describes the different settings of the simulators: STDMA simulations have been carried out using Matlab
whereas MS-Aloha simulations use the NS-2 environment with mobility traces from SUMO. The data traffic model is
the same for both simulators: time-triggered position messages, i.e. CAMs, with two different packet lengths and update
frequencies (2 Hz/800 bytes and 10 Hz/300 bytes). All MAC methods assume the same the physical (PHY) layer
derived from 802.11p [i.2]. A transfer rate of 6 Mbit/s is used and 10 MHz frequency channel is adopted. Two different
road traffic scenarios have been considered: urban and highway. STDMA has employed the latter whereas MS-Aloha
has mainly been simulated for the urban scenario. The channel model for the highway scenario is a Nakagami m model
with varying m values depending on distance between transmitter (TX) and receiver (RX). The urban channel model is
partly based on ray tracing technique in order to get as realistic radio propagation environment as possible. The selected
performance measures for evaluating the results are the same for both simulators: packet reception probability and
channel access delay.
5.2 Data traffic model
All simulations are conducted using time-triggered position messages, i.e. CAMs. Two different heartbeats (CAM
update rates) and packet lengths have been considered, based on the discussions in standardization within ETSI and
IEEE, Table 2. The packet length excludes the preamble and signal fields of the physical layer (PHY). In clause 5.2.1
the packet structure is outlined.
ETSI
12 ETSI TR 102 861 V1.1.1 (2012-01)
Table 2: Data traffic settings
Required bandwidth by
Update rate [Hz] Packet length [bytes]
each node [kbit/s]
ETSI setting
2 800 12,8
IEEE setting
10 300 24
5.2.1 Packet structure
The PHY parameters are derived from IEEE 802.11p [i.2]. All transmissions have been conducted using a transfer rate
of 6 Mbit/s. This implies the modulation scheme quadrature phase shift keying (QPSK) with a code rate of 1/2 (r = 1/2).
In Figure 2, the packet structure for the simulations is depicted. The PHY data field corresponds to the packet length
found in Table 2.
MAC
Header MAC Data Trailer
Preamble Signal PHY Data
PHY
Figure 2: The packet structure for the simulations
The preamble field consists of 12 orthogonal frequency division multiplexing (OFDM) symbols, which has a total
duration of 32 µs. The signal field is one OFDM symbol of duration 8 µs. In Table 3 the duration of a packet
transmission for each of the two different packet lengths are tabulated.
Table 3: Packet duration
Duration of packet
Packet length/ Preamble Signal Total duration of packet in the air
transmission at 6 Mbit/s
PHY data [µs] [µs] [µs]
[µs]
300 bytes 400 32 8 440
800 bytes 1 067 32 8 1 107
5.2.2 Slot length, guard time and clock hold-on
Guard times are added in time slotted MAC approaches to avoid inter-slot interference. It accounts for propagation
delay and synchronization jitter. The latter is due to drifting clocks when synchronization is lost. The synchronization
method intended for the two time slotted MAC approaches evaluated herein is GNSS, such as GPS. If GPS
synchronization is lost, the quality of the local oscillator determines how long a node can stay synchronized. This is
called the clock hold-on property. A clock hold-on of 50 µs is considered in the simulations; clause 8 in [i.1], together
with a propagation delay of 6 µs. In Table 4 the total duration of a slot for each of the two packet lengths are given.
Table 4: Slot duration
Packet length/ Total duration of packet Propagation Clock
Total duration of one slot [µs]
PHY data in the air [µs] delay [µs] hold-on [µs]
300 bytes
440 6 50 496
800 bytes
1 107 6 50 1 163
ETSI
13 ETSI TR 102 861 V1.1.1 (2012-01)
5.2.3 Frame length
5.2.3.1 STDMA
The frame length in STDMA is set to 1 s. The number of slots and the total duration of each slot for the two different
packet lengths are tabulated in Table 5.
Table 5: Number of slots in the STDMA frame
Total duration of one slot
Packet length/PHY data Number of slots in the frame
[µs]
300 bytes 496 2 016
800 bytes
1 163 859
5.2.3.2 MS-Aloha
In MS-Aloha the following entities are mutually linked and jointly contribute to define the frame settings:
• number of slots in a MS-Aloha period
• number of bits used for the STI (typically 8 bits)
• slot length: it is connected to the length of the packets being transmitted; it has to account also for the signaling
information carried within the FI (appended to the slots in the signaling frames)
• guard time
• number of signaling frames: it is possible to send the FI only in certain MS-Aloha frames (signaling frames).
For instance, it can be chosen to append FI to the slots in one frame every 2 or 10
All the parameters together determine the frame duration. Considering that the FI contains as many subfields as the
number of slots, the number of slots n in a period P can be computed as the solution of an equation of power 2, whose
slot
solution is presented in clause 7.1.2.
Obviously, the equation should be called only initially, to make decisions on the settings. More details on the settings
are discussed in clause 7.1.2. However, in Table 6 columns 2 to 5 the settings of the here presented MS-Aloha's
simulations are tabulated. In this analysis, all MS-Aloha frames will be signaling frames (i.e. FI will be appended to all
the slots), waiving the opportunity to improve the efficiency by less frequent FI transmissions. In Table 6 columns 6 to
7, also other possible settings are mentioned, for the non-continuous update of FIs (only once per second).
Table 6: Number of slots in the MS-Aloha frame for two possible setting:
k=1 (columns 2 to 5), as used in the simulations; k�1 (columns 6 to7)
(3) Number (4) Number (6) Number of
(1) Packet length (2) Slot duration
of slots in of the slots (5) Settings slots per (7) Settings
and PHY settings excluding T [µs]
g
the frame per second second
300 bytes 6 Mbit/s 707 131 1 310 K=1, P=0,1s 1 870 K=10, P=1s
800 bytes 6 Mbit/s 1 736 287 574 K=1, P=0,5s 668 K=2, P=1s
300 bytes 12 Mbit/s 444 200 2 000 K=1, P=0,1s 3 060 K=10, P=1s
800 bytes 12 Mbit/s 1 033 459 918 K=1, P=0,5s 1 104 K=2, P=1s

ETSI
14 ETSI TR 102 861 V1.1.1 (2012-01)
5.3 Vehicle traffic model
As many as 55 % of all fatal accidents occur in rural areas [i.12]. The majority of rural fatal accidents are due to head-
on collisions. In rural areas the roads usually have one lane in each direction with occasional support for two lanes and
thus the scalability of the MAC protocol is likely not to be a major issue. The lowest probability of fatal accidents has
the highway environment with 10 % and the majority of those accidents are rear-end collisions and single collisions
(only one vehicle involved). In urban environment 35 % of all fatal accidents occurs but here it is often vulnerable road
users that get killed such as pedestrians and bicyclists, which collide with vehicles. Despite the practical relevance, very
few theoretical studies address such scenarios, basically due to a lack of propagation models accounting also for the
obstruction by buildings. Very recently, some models have been proposed and validated to account for obstruction by
buildings ([i.9]). The results presented here exploit one of them ([i.9], the simplest but most general one): more details
are provided in clause 5.3.2.
Three main scenarios are mentioned in literature for the study of VANETs: highway, urban, and rural. The selected
scenarios for simulations are highway and urban as the highest vehicle densities are found here, which should stress the
MAC protocols most. As mentioned earlier the scalability of the MAC protocol is closely connected to the channel
access delay, reliability and fairness.
• In a highway scenario, the relative speed can be as high as almost 300 km/h. Although PHY layer phenomena
are supposed to be counteracted by the underlying layers, network topology sometime changes too rapidly due
to high relative speeds between the driving directions (whereas the topology is more stable within one
direction). The density of vehicles can be high, especially during rush hour and when an accident occurs. This
scenario can test how scalable a MAC protocol is.
NOTE: In IEEE 802.11p [i.2], the doubled symbol duration, with respect to IEEE 802.11a [i.26], is meant to
counteract inter-symbol interference (ISI) [i.15]; the frequency displacement by Doppler's effect,
according to Jake's model is about 2 kHz at almost 400 km/h mutual speed [i.11], which is below the
guard-band of IEEE 802.11p [i.2] PHY.
• In an urban environment, the speeds are lower resulting in slower changes of the network topology. However,
as buildings will obstruct the signal, vehicles can therefore suddenly disappear and reappear again. An
example of this is when a vehicle travels through an intersection. The urban layout contributes to many nodes
in certain areas being hidden to one another and this may have a major impact on the performance, i.e. due to
the hidden terminal problem. The density of vehicles in large cities can be rather high so scalability is an
important issue also in this scenario.
5.3.1 Highway scenario (STDMA)
Simulations of STDMA and CSMA have been carried out in a highway scenario with several lanes, where the vehicles
appear Poisson distributed and receive a speed drawn from a Gaussian distribution with different mean values
depending on lane. Once given, the speed is constant as long as the vehicle remains on the highway. No overtaking is
considered. The purpose of the selected vehicle traffic model is to capture the mobility of nodes and evaluate different
vehicle densities. In a highway scenario, the highest relative speeds are found, which results in the most rapidly
changing network topology for the VANET and thereby likely the most stressful situation for the MAC method. Two
different highway scenario settings have been used, reflecting a normal vehicle traffic density and a high vehicle traffic
density. In Table 7 the two different settings are detailed together with the approximate vehicle density.
Table 7: Two different highway scenario settings for STDMA and CSMA
Poisson - mean Vehicle density for
Highway scenario Number of lanes
inter-arrival time 100 m of highway
1 Normal vehicle density 6 (3 in each direction) 3 seconds 9 to 10 vehicles
2 High vehicle density 12 (6 in each direction) 1 second 25 to 26 vehicles

In Figure 3 the two highway scenarios with different number of lanes together with the Gaussian distributed mean
values of the speed for the different lanes are depicted. The vehicle speeds are approximately between 70 km/h to
...