ETSI TS 102 687 V1.1.1 (2011-07)

Intelligent Transport Systems (ITS); Decentralized Congestion Control Mechanisms for Intelligent Transport Systems operating in the 5 GHz range; Access layer part

DTS/ITS-0040014

General Information

• Status: Published
• Publication Date: 30-Jun-2011
• Technical Committee: ITS WG4 - Media and Medium related
• Current Stage: 12 - Completion
• Due Date: 13-Jul-2011
• Completion Date: 01-Jul-2011
• Mandate: M/453

Standards Content (Sample)

Technical Specification
Intelligent Transport Systems (ITS);
Decentralized Congestion Control Mechanisms for
Intelligent Transport Systems operating in the 5 GHz range;
Access layer part
2 ETSI TS 102 687 V1.1.1 (2011-07)

Reference
DTS/ITS-0040014
Keywords
ITS, radio, transmission
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3 ETSI TS 102 687 V1.1.1 (2011-07)
Contents
Intellectual Property Rights . 5
Foreword . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 7
3 Definitions, symbols and abbreviations . 7
3.1 Definitions . 7
3.2 Symbols . 7
3.3 Abbreviations . 8
4 Decentralized congestion control overview . 8
4.1 DCC operational requirements . 8
4.2 DCC architecture . 9
4.3 Network design limits (NDL) . 9
4.4 DCC_access functional view . 10
5 DCC access mechanisms . 11
5.1 Transmit power control . 11
5.1.1 TPC parameters . 11
5.1.2 TPC Operation . 12
5.2 Transmit rate control . 12
5.2.1 TRC parameters . 12
5.2.2 TRC Operation . 13
5.3 Transmit datarate control . 13
5.3.1 TDC parameters . 13
5.3.2 TDC Operation . 14
5.4 DCC Sensitivity control . 14
5.4.1 DSC parameters . 14
5.4.2 DSC operation . 15
5.5 Transmit access control . 15
5.5.1 TAC parameters . 15
5.5.2 TAC operation . 15
5.6 DCC Transmit model . 16
5.7 DCC receive model . 16
5.7.1 Receive model parameters . 16
5.7.2 Channel model . 17
5.7.3 Demodulation model. 17
5.7.4 Communication ranges . 18
5.7.5 Example receive model . 18
6 DCC_access components . 19
6.1 DCC transmit queueing . 19
6.2 Channel probing . 19
6.3 Transmit packet statistics . 20
6.4 DCC access control loop . 20
6.4.1 State machine . 20
6.4.2 State transitions . 21
6.4.3 State configuration . 22
6.4.4 State processing . 23
7 DCC access interfaces . 25
7.1 Interface 1: IN-UNITDATA service . 25
7.2 Interface 2: MI-SET and MI-GET services . 25
Annex A (normative): DCC parameter . . 26
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4 ETSI TS 102 687 V1.1.1 (2011-07)
A.1 Channel load measures . 26
A.1.1 Parameters . 26
A.1.2 Channel load . 26
A.1.3 Load arrival rate . 26
A.1.4 Load average duration . 27
A.1.5 Receive packet arrival rate . 27
A.1.6 Receive packet average duration . 27
A.1.7 Packet Occupancy . 28
A.1.8 Channel busy time . 28
A.2 Transmit packet statistics . 28
A.2.1 Transmit packet rate . 28
A.2.2 Transmit packet average duration. 29
A.2.3 Average transmit signal power . 29
A.2.4 Transmit channel use . 29
A.3 Communication Ranges . 30
A.3.1 Carrier Sense Range . 30
A.3.2 Estimated communication range . 30
A.3.3 Estimated communication range under interference . 30
A.4 Network Design Limits . 31
A.4.1 NDL types and formats . 31
A.4.2 NDL Input parameter and constants . 31
A.4.3 NDL database . 32
A.4.4 NDL output parameters . 33
A.4.5 Channel dependent parameter . 33
A.4.6 Priority dependent parameters . 34
A.4.7 Basic control loop NDL database . 34
A.4.8 Enhanced DCC algorithm NDL database . 35
Annex B (informative): Transmit power calibration . 36
B.1 Common principles of transmit power calibration . 36
B.2 Transmit power certification process for non modular ITS stations . 36
B.2.1 Applicability and overview for non modular ITS stations . 36
B.2.2 Determination of antenna pattern for non modular ITS stations . 36
B.2.3 Measurement of e.i.r.p. for non modular ITS stations . 37
B.3 Transmit power certification process for modular ITS stations . 37
B.3.1 Applicability and overview for modular ITS stations . 37
B.3.2 Determination of antenna pattern for modular ITS stations . 37
B.3.3 Determination of cable loss for modular ITS stations . 37
B.3.4 Determination of output power level for modular ITS stations . 37
B.4 Specified e.i.r.p. levels and step requirements to be calibrated . 38
Annex C (informative): Example of a DCC_Net algorithm. 39
C.1 Enhanced DCC algorithm . 39
C.2 Remote information . 40
Annex D (informative): Validation . 42
D.1 Validation scenarios . 42
D.2 Validation performance criteria . 43
Annex E (informative): Bibliography . 44
History . 45

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5 ETSI TS 102 687 V1.1.1 (2011-07)
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 Specification (TS) has been produced by ETSI Technical Committee Intelligent Transport System (ITS).
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6 ETSI TS 102 687 V1.1.1 (2011-07)
1 Scope
ITS road safety and traffic efficiency systems include both vehicle to vehicle communications and related vehicle to
roadside communication in highly dynamic vehicular ad hoc networks. These systems (ITS stations) are based on a set
of protocols and parameters called ITS-G5 as specified in the European profile standard on the physical and medium
access layer of 5 GHz ITS [2].
Many applications and services in ITS rely on the cooperative behavior of the vehicles and roadsides units which form a
vehicular ad hoc network (VANET). The VANET enable the time critical road safety applications where fast
information exchange is necessary to timely warn and support the driver. Special care should be taken to ensure proper
functioning of the VANET and this includes decentralized congestion control (DCC) for the channels of ITS-G5.
DCC is a cross layer function, i.e. it has functions located on several layers of the ITS station reference architecture.
Therefore the present document defines which DCC components are located on which layer of the ITS station
communication architecture [5]. Furthermore the present document specifies the DCC mechanisms on the access layer
(DCC_access) including transmit power control (TPC) per packet, transmit rate control (TRC) and transmit datarate
control (TDC). The latter two control functions modify the average transmit power by modifying the duty cycle of the
ITS station, i.e. the fraction of time that the ITS station is in "transmit" state. Additionally, DCC sensitivity control
(DSC) adapts the clear channel assessment to resolve local channel congestion. Packets with higher priority are handled
less restrictive introducing a transmit queueing concept and transmit access control (TAC).
The DCC mechanisms rely on knowledge about the channel. The channel state information is gained using channel
probing. Channel probing measures are defined that enable the DCC methods TPC, TRC and TDC. The measures are
on receive signal level thresholds or preamble information of detected packets.
The present document does not define the mechanisms at other layers than the access layer nor defines management
aspects. These other mechanisms and the management aspects are necessary in order to make DCC work properly. The
present document is primarily intended for trial use and may need to be updated after validation in field trials and/or
other projects.
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.
[1] IEEE 802.11-2007: "IEEE Standard for 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".
[2] 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".
[3] ETSI EN 302 571: "Intelligent Transport Systems (ITS); Radiocommunications equipment
operating in the 5 855 MHz to 5 925 MHz frequency band; Harmonized EN covering the essential
requirements of article 3.2 of the R&TTE Directive".
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7 ETSI TS 102 687 V1.1.1 (2011-07)
[4] ETSI TS 102 868-1: "Intelligent Transport Systems (ITS); Testing; Conformance test specification
for Co-operative Awareness Messages (CAM); Part 1: Test requirements and Protocol
Implementation Conformance Statement (PICS) proforma".
[5] ETSI EN 302 665: "Intelligent Transport Systems (ITS); Communications Architecture".
[6] IEEE 802.2-1998: "Standard for Information technology -- Telecommunications and information
exchange between systems -- Local and metropolitan area networks -- Specific requirements --
Part 2: Logical Link Control", (ISO/IEC 8802-2:1998).
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 TS 102 724: "Intelligent Transport Systems (ITS); Harmonized Channel Specifications for
Intelligent Transport Systems operating in the 5 GHz frequency band".
[i.2] ETSI TS 102 723-3: "Intelligent Transport Systems; OSI cross-layer topics; Part 3: Interface
between management entity and access layer".
[i.3] ETSI TS 102 723-10: "Intelligent Transport Systems; OSI cross-layer topics; Part 10: Interface
between access layer and network and transport layers".
[i.4] ETSI TS 102 723-1: "Intelligent Transport Systems; OSI cross-layer topics; Part 1: Architecture
and addressing schemes".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in [1], [2], [3], [5] and the following apply:
DCC component: part of the DCC, located in one layer, e.g. DCC_access
DCC_access mechanism: functionality of DCC_access usually using several DCC_access components
NDL database: database that contains DCC_access configuration parameters, input parameters and output parameters
reference parameter: parameter controlled by a control loop
3.2 Symbols
For the purposes of the present document, the symbols given in [1], [2], [3], [5] and the following apply:
acPrio access priority
NOTE: Provided by network layer or derived as specified in [1].
dB(x) decibel function: 10·log (x)
cl variable for channel load
cs variable for carrier sense threshold
ds variable for DCC sensitivity
MIN(x ; …;x ) minimum function, returns its lowest argument
1 N
minCL(Δt) minimum channel load for time time period Δt
MAX(x ; …;x ) maximum function, returns its largest argument
1 N
maxCL(Δt) maximum channel load for time time period Δt
M measured number of packets
pkt
N number of probes
p
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8 ETSI TS 102 687 V1.1.1 (2011-07)
N number of OFDM symbols in the preamble
PR
N data bits per OFDM symbol
DPMS
p
pow(p, x) exponentiation: x
S signal power level
S signal level threshold
th
T packet air time
AIR
T average time for channel access
CA
T measuring interval
m
T probing interval
p
txQ number of DCC_access transmit queues

3.3 Abbreviations
For the purposes of the present document, the abbreviations given in [1], [2], [3], [5] and the following apply:
DCC Decentralized Congestion Control
DCC_access DCC component of the access layer
DCC_app DCC component of the facility layer
DCC_mgmt DCC component of the management layer
DCC_net DCC component of the network layer
D-CCA CCA sensitivity for DCC
DPSK Digital Phase Shift Keying
DSC DCC sensitivity control
NDL Network Design Limits
SM State Machine
SNR Signal to Noise Ratio
TAC Transmit Access Control
TDC Transmit Datarate Control
TPC Transmit Power Control
TRC Transmit Rate Control
VANET Vehicular Ad-hoc NETwork
4 Decentralized congestion control overview
4.1 DCC operational requirements
"Decentralized congestion control" (DCC) is a mandatory component of ITS-G5 stations operating in ITS-G5A and
ITS-G5B frequency bands to maintain network stability, throughput efficiency and fair resource allocation to ITS-G5
stations. DCC requires components on several layers of the protocol stack and these components jointly work together
to fulfil the following operational requirements:
• Provide fair allocation of resources and fair channel access among all ITS stations in the same communication
zone.
• Keep channel load caused by periodic messages below pre-defined thresholds.
• Reserve communication resources for the dissemination of event driven high priority messages.
• Provide fast adoption to a changing environment (busy / free radio channel).
• Keep oscillations in the control loops within well-defined limits.
• Comply to specific system requirements, e.g. reliability.
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9 ETSI TS 102 687 V1.1.1 (2011-07)
4.2 DCC architecture
The DCC architecture is displayed in Figure 1. It consists of the DCC components:
• DCC_access located in the access layer;
• DCC_net located in the networking and transport layer;
• DCC_app located in the facility layer;
• DCC_mgmt located in the management layer.
The components are connected through the DCC interfaces 1 to 5 as shown in Figure 1. These interfaces are mapped to
the corresponding cross layer interfaces as described in TS 102 723 [i.2], [i.3], [i.4] and the present document.

Figure 1: DCC Architecture
The present document specifies DCC_access, which comprises the DCC mechanisms transmit power control (TPC),
DCC sensitivity control (DSC), transmit rate control (TRC) transmit datarate control (TDC) and DCC access control
(TAC)
Additionally DCC_access services are specified that are offered to other DCC components using interface 1 [i.3] and
interface 2 [i.2]. These services are a transmit model (Clause 5.6), a receive model (Clause 5.7) and channel probing
(Clause 6.2) and transmit packet statistics (Clause 6.3).
NOTE: The term packet is used on the access layer and the network layer to indicate that usually the same
payload is transported.
4.3 Network design limits (NDL)
An operational requirement of DCC is to keep the actual channel load below predefined limits that are part of the
Network Design Limits (NDL, Clause A.4). The NDL are used to configure DCC_access. The NDL are stored in the
NDL database that contains all relevant information used by DCC_access, i.e. configuration parameters, controlled
parameters and DCC status information.
The NDL database is part of DCC_mgmt, i.e. the management layer is responsible for maintaining the information
(configuration parameters). Data exchange between DCC_mgmt (including NDL database) and the other layers is
described in the corresponding interface documents (TS 102 723 [i.2], [i.3], [i.4]).
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10 ETSI TS 102 687 V1.1.1 (2011-07)
The NDL database includes:
• ranges of the controlled parameters (minimum and maximum values);
• design limits, i.e. default and target values of the controlled parameters;
• regulatory limits and device dependent parameters (e.g. max. transmit power);
• model parameters, e.g. parameters of the transmit model, channel model and receive model;
• internal control loop parameters, e.g. signal level thresholds and time constants.
The controlled parameters and the measured parameters are written to the NDL database, especially:
• reference values, i.e. the average target value used by DCC_access transmit queuing for per packet control;
• channel load measures.
4.4 DCC_access functional view
Figure 2 shows the functional view of DCC_access with the building blocks:
• transmit queuing (Clause 6.1), which enhance the standard 802.11 queues by DCC mechanisms;
• channel probing (Clause 6.2) to collect statistics on the communication channel;
• transmit statistics (Clause 6.3) to observe the behavior of the own ITS station;
• control loop (Clause 6.4) that adapt the behavior of the own ITS station to the actual channel load.

Figure 2: DCC_access functional view
DCC_access relies on measured values for the channel load (Channel probing) and on statistics about transmitted
packets (Transmit statistics).
The transmit statistics of DCC_access shall take into account all packets that are transmitted, including packet
repetitions, RTS, CTS and ACK packets.
The Control loop manages reference parameters according to the DCC_access mechanisms TPC, TRC, TDC and DSC.
The reference parameters are:
• TPC → reference transmit power: NDL_refTxPower
• TRC → reference packet interval: NDL_refPacketInterval
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11 ETSI TS 102 687 V1.1.1 (2011-07)
• TDC → reference datarate: NDL_refDatarate
• DSC → reference D-CCA sensitivity: NDL_refCarrierSense
• TAC → reference queue status NDL_refQueue
Their usage is specified in Clause 5.
Packets are classified at network layer, which provides the access priority (acPrio) per packet via interface 1 (see
Figure 1). Additionally each packet that arrives from the network layer has a preset value of transmit power and
datarate.
The block Transmit queuing in Figure 2 assigns the packets to the corresponding MAC transmit queue (see [1]).
Enqueuing a packet to the MAC transmit queue shall not occur more frequently than specified by TRC.
On enqueuing a packet to its MAC transmit queue the preset values are compared with the current reference values of
TPC, TRC and TDC and modified if necessary (see Clause [4]).
The DSC reference parameter NDL_refCarrierSense is used to control the CCA (see [1]).
DCC_app and DCC_net are out of scope of the present document. They may affect DCC_access indirectly by
dynamically modifying the NDL database (Clause 4.3), i.e. the control loop configuration parameters. DCC_access
provides a set of status parameters to these upper layers via the NDL database.
5 DCC access mechanisms
5.1 Transmit power control
5.1.1 TPC parameters
"Transmit power control" (TPC) is based on transmit power thresholds listed in Table 1.
The signal power thresholds depend on the selected channel from ITS-G5A or ITS-G5B and the selected transmit
queue.
These thresholds are part of the NDL and shall be maintained by DCC_mgmt (Clause 4.2).
All transmit signal power thresholds shall be of type ndlType_txPower as specified in Table A.1.
Table 1: Transmit power thresholds
Transmit power thresholds Definition
NDL_minTxPower minimum transmit power (e.i.r.p.)
NDL_maxTxPower maximum transmit power (e.i.r.p.)
NDL_defTxPower(acPrio) default transmit power (e.i.r.p.)
NDL_refTxPower(acPrio) reference transmit power (e.i.r.p.)

NDL_minTxPower is the minimum transmit signal power that can be selected by DCC_access.
NDL_maxTxPower is the maximum transmit signal power that can be selected by DCC_access, considering the
maximum possible power and the maximum power allowed by regulation. Thus this value depends on the selected
channel.
NDL_defTxPower is the default value for NDL_refTxPower, i.e. the transmit power that is used if no preset value is
available.
NDL_refTxPower is the reference parameter set by the DCC_access mechanism TPC.
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12 ETSI TS 102 687 V1.1.1 (2011-07)
The NDL transmit signal power thresholds shall fulfil the following relations:
EQ 1: NDL_minTxPower ≤ NDL_refTxPower ≤ NDL_maxTxPower
EQ 2: NDL_minTxPower ≤ NDL_defTxPower ≤ NDL_maxTxPower
5.1.2 TPC Operation
The transmit power (effTxPower) of a packet can be set per-MSDU as described in [2]. This presetting of the network
layer is modified by TPC according to the following rules:
• On reception via DCC interface 1 the packet is assigned to the corresponding transmit queue defined by the
per-MSDU priority (acPrio).
• The preset per-MSDU value of effTxPower is corrected according to the following relation:
EQ 3: effTxPower = MIN(NDL_refTxPower(acPrio), effTxPower)
5.2 Transmit rate control
5.2.1 TRC parameters
"Transmit Rate Control" (TRC) is based on packet timing thresholds listed in Table 2.
The packet timing thresholds depend on the selected channel from ITS-G5A or ITS-G5B and the selected transmit
queue. Timing thresholds are divided into packet duration thresholds and packet interval thresholds.
Packet duration thresholds shall be of type ndlType_packetDuration as specified in Table A.1. Packet interval
thresholds shall be of type ndlType_packetInterval as specified in Table A.1. The thresholds are part of the NDL and
shall be maintained by DCC_mgmt (Clause 4.2).
Table 2: Packet timing thresholds
Packet timing thresholds Definition
Packet duration thresholds
NDL_maxPacketDuration(acPrio) maximum duration (air time) of a packet
Packet interval thresholds
NDL_minPacketInterval minimum interval between packets
NDL_maxPacketInterval maximum interval between packets
NDL_defPacketInterval(acPrio) default interval between packets
NDL_refPacketInterval(acPrio) reference interval between packets

NDL_maxPacketDuration is the maximum allowed air time of a packet (T ).
AIR
T is derived from the preamble duration and the number of data bits per OFDM symbol N (see note 1):
AIR DBPS
EQ 4: N = (T +T ) / T
PR PREAMBLE SIGNAL SYMBOL
EQ 5: T = (N + N )*T
AIR PR SYMBOL SYMBOL
with T , T , T and N as specified in [1].
PREAMBLE SIGNAL SYMBOL SYMBOL
NOTE 1: N ≈8*packetLength/N with packetLength as the number of bytes of the packet as encoded in the
SYMBOL DBPS
SIGNAL field of the PPDU (see [1]). N is the number of data bits in an OFDM symbol dependent on
DPSK
MCS.
NOTE 2: N = 5
PR
NOTE 3: T = 8 μs for 10 MHz channels.
SYMBOL
NDL_maxPacketDuration shall be of type ndlType_packetDuration as specified in Table A.1.
NDL_minPacketInterval is the minimum packet interval that can be selected by DCC_access.
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13 ETSI TS 102 687 V1.1.1 (2011-07)
NDL_maxPacketInterval is the maximum packet interval that can be selected by DCC_access.
NDL_defPacketInterval is the default value for NDL_refPacketInterval, i.e. the packet interval that is used if TRC is
inactive.
NDL_refPacketInterval is the reference parameter set by the DCC_access mechanism TRC.
The packet timing thresholds shall fulfil the following relations:
EQ 6: NDL_minPacketInterval ≤ NDL_refPacketInterval ≤ NDL_maxPacketInterval
EQ 7: NDL_minPacketInterval ≤ NDL_defPacketInterval ≤ NDL_maxPacketInterval
The packet intervals shall be measured between subsequent starts of packets.
NDL_maxPacketDuration shall be small compared to NDL_minPacketInterval (see [i.1]).
The packet intervals are of type ndlType_packetInterval as specified in Table A.1.
5.2.2 TRC Operation
The packet air time of a packet (T ) is derived from the packet length according Clause 5.2.1.
AIR
• On reception via DCC Interface 1 the packet is assigned to the corresponding transmit queue defined by the
per-MSDU priority (acPrio).
• In case that T exceeds NDL_maxPacketDuration the packet shall be dropped.
AIR
NOTE 1: TDC could be used to decrease T and avoid packet drops (Clause 5.3.2).
AIR
• If NDL_refPacketInterval(acPrio) > 0 the configured packet interval shall be ensured.
Ensuring the packet interval means that there shall be a time interval of at least NDL_refPacketInterval(acPrio)
between the transmission start of the current packet (from queue acPrio) and the transmission start of the previous
packet.
NOTE 2: Transmissions of the previous packet also includes packet repetitions, RTS, CTS or ACK packets.
NOTE 3: The time for arbitration can be neglected.

5.3 Transmit datarate control
5.3.1 TDC parameters
"Transmit Datarate Control" (TDC) is based on datarate thresholds listed in Table 3.
The datarate thresholds depend on the selected channel from ITS-G5A or ITS-G5B and the selected priority.
All datarate thresholds shall be of type ndlType_dataRate as specified in Table A.1. They shall be maintained by
DCC_mgmt (Clause 4.2).
Table 3: Packet datarate thresholds
Packet datarate thresholds Definition
NDL_minDatarate minimum datarate
NDL_maxDatarate maximum datarate
NDL_defDatarate(acPrio) default datarate
NDL_refDatarate(acPrio) reference datarate

NDL_minDatarate is the minimum datarate that can be selected by DCC_access.
NDL_maxDatarate is the maximum datarate that can be selected by DCC_access.
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14 ETSI TS 102 687 V1.1.1 (2011-07)
NDL_defDatarate is the default value of NDL_refDatarate, i.e. the value that is used if TDC is inactive.
NDL_refDatarate is the reference parameter set by the DCC_access mechanism TDC.
The datarate parameters are specified via the MCS value as specified in [2].
The datarate parameters shall fulfil the following relations:
EQ 8: NDL_minDatarate ≤ NDL_refDatarate ≤ NDL_maxDatarate
EQ 9: NDL_minDatarate ≤ NDL_defDatarate ≤ NDL_maxDatarate
5.3.2 TDC Operation
The transmit datarate (effTxDatarate) of a packet can be set on a per-MSDU basis as described in [2]. This reference
setting is modified by DCC_access according the following rules:
• On reception via Interface 1 the packet is assigned to the corresponding transmit queue defined by the
per-MSDU priority (acPrio).
• The preset per-MSDU value effTxDatarate is corrected according to the following relation:
EQ 10: effTxDatarate = MAX(NDL_refDatarate(acPrio), effTxDatarate)
Furthermore the following TDC functionality shall be provided
• While the air time T of a packet exceeds NDL_maxPacketDuration the datarate effTxDatarate shall be
AIR
increased, but not above NDL_maxDatarate.
5.4 DCC Sensitivity control
5.4.1 DSC parameters
"DCC Sensitivity Control" DSC is based on sensitivity thresholds listed in Table 4. All sensitivity thresholds shall be of
type ndlType_rxPower as specified in Table A.1. They shall be maintained by DCC_mgmt (Clause 4.2).
The sensitivity thresholds are used to determine whether the transmitter is clear to send or not (Table 4). They depend
on the selected channel from ITS-G5A or ITS-G5B (see [i.1]).
Table 4: DCC sensitivity thresholds
Receive signal thresholds Definition
NDL_minCarrierSense minimum D-CCA sensitivity
NDL_maxCarrierSense maximum D-CCA sensitivity for DCC
NDL_defCarrierSense default D-CCA sensitivity
NDL_refCarrierSense reference D-CCA sensitivity

The "Clear Channel Assessment for DCC" (D-CCA) shall indicate a busy channel during a reception of a packet with
receive level greater than NDL_refCarrierSense. In case that the preamble portion was missed, the D-CCA shall hold
the carrier sense signal busy for any signal above NDL_refCarrierSense.
NDL_minCarrierSense is the minimum D-CCA sensitivity that can be selected by DCC_access.
NDL_maxCarrierSense is the maximum D-CCA sensitivity that can be selected by DCC_access.
NDL_defCarrierSense is the default value of NDL_refCarrierSense, i.e. the value that is used if DSC is inactive.
NDL_refCarrierSense is the reference parameter set by DCC_access mechanism DSC.
The thresholds for D-CCA sensitivity shall fulfil the following relations:
EQ 11: NDL_ minCarrierSense ≤ NDL_refCarrierSense ≤ NDL_maxCarrierSense
ETSI
15 ETSI TS 102 687 V1.1.1 (2011-07)
5.4.2 DSC operation
DSC has an impact on CCA by applying the reference parameters NDL_refCarrierSense instead of the installed
receiver sensitivity and the -65 dBm carrier sense threshold defined in [1]. This modified version of CCA is called
D-CCA.
NOTE 1: The receiver sensitivity is not modified, only the thresholds for CCA. This provides the same chance for
channel access to ITS stations with high sensitive receivers. The transmitter is allowed to transmit
although a far distant transmitter might be active at the same time.
Whenever the DCC_acces control loop changes its state (Clause 6.4.2) the D-CCA reference parameter
NDL_refCarrierSense is modified accordingly (Clause 6.4.4).
NOTE 2: DSC gives priority in channel access to higher priority messages by setting a higher D-CCA sensitivity
threshold NDL_refCarrierSense for the "priority" transmit queues and DCC prevents channel congestion
resulting from external interference blocking the transmitter due to a too sensitive carrier sensing.
5.5 Transmit access control
5.5.1 TAC parameters
"Transmit access control" (TAC) is the DCC_access mechanism that support the operational requirement of fair channel
access. In case of high channel load the TAC is more restrictive to ITS-stations that transmit many packets. This is done
using the DCC_access transmit queueing (Clause 6.1).
The DCC_mgmt shall maintain the queueing parameters as shown in Table 6 (Clause 4.2). The parameters shall be
maintained per channel.
Table 5: Queueing parameters
Receive signal thresholds Definition
NDL_numQueue Number of transmit queues in DCC_access
NDL_refQueueStatus(acPrio) Status of transmit queue

NDL_numQueue is the number of available transmit queues (Clause 6.1).
NDL_refQueueStatus(acPrio) is the reference parameter set by the DCC_access mechanism TAC. It is an array of
length NDL_numQueue. An array element NDL_refQueueStatus(acPrio) indicates the status of the transmit queue with
priority acPrio.
5.5.2 TAC operation
The transmit queues are ordered according the priority such that the highest priority queue has priority index q = 0. The
actual transmit statistics (A.2) are compared with the statistics of the DCC transmit model (Clause 5.6).
If too many packets are sent with priority index less or equal q the corresponding queue is marked as closed, i.e.:
EQ 12: NDL_refQueueStatus(q) = CLOSED if txChannelUse(q) ≥ NDL_tmChannelUse(q)
otherwise the queue is OPEN, i.e.:
EQ 13: NDL_refQueueStatus(q) = OPEN if txChannelUse(q) < NDL_tmChannelUse(q)
Packets with priority acPrio that arrives at a closed transmit queue (NDL_refQueueStatus(acPrio) = CLOSED) shall be
dropped.
NOTE: Packet drops can be avoided by
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