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Basic data | Standard ID | GB/T 20851.2-2019 (GB/T20851.2-2019) | | Description (Translated English) | Electronic toll collection -- Dedicated short range communication -- Part 2: Data link layer | | Sector / Industry | National Standard (Recommended) | | Classification of Chinese Standard | L79 | | Classification of International Standard | 35.100.20 | | Word Count Estimation | 30,395 | | Date of Issue | 2019-05-10 | | Date of Implementation | 2019-12-01 | | Older Standard (superseded by this standard) | GB/T 20851.2-2007 | | Quoted Standard | GB/T 7421-2008; GB/T 15629.2-2008; GB/T 20839-2007; GB/T 20851.1-2019 | | Issuing agency(ies) | State Administration for Market Regulation, China National Standardization Administration | | Summary | This standard specifies the main parameters, information frame, dedicated communication link establishment and cancellation, MAC sublayer and LLC sublayer requirements of the data link layer dedicated to electronic charging. This standard is applicable to electronic toll collection systems for highways and urban roads, and can be used for reference in the fields of automatic vehicle identification and vehicle exit management. | GB/T 20851.2-2019: Electronic toll collection -- Dedicated short range communication -- Part 2: Data link layer
---This is a DRAFT version for illustration, not a final translation. Full copy of true-PDF in English version (including equations, symbols, images, flow-chart, tables, and figures etc.) will be manually/carefully translated upon your order.
Electronic toll collection--Dedicated short range communication--Part 2. Data link layer
ICS 35.100.20
L79
National Standards of People's Republic of China
Replaces GB/T 20851.2-2007
Dedicated short-range communication for electronic toll collection
Part 2. Data link layer
Part 2. Datalinklayer
Published on May 10,.2019
2019-12-01 Implementation
State Administration of Market Supervision
Published by China National Standardization Administration
Contents
Foreword III
1 Scope 1
2 Normative references 1
3 Terms and definitions 1
4 Symbols and abbreviations 1
5 Link layer main parameters 2
6 Information frame 2
7 Establishment and cancellation of dedicated communication links 5
8 MAC sublayer 5
9 LLC Sublayer 9
Appendix A (Normative Appendix) Point-to-point dedicated communication link establishment process 23
References 25
Foreword
GB/T 20851 `` Special short-range communication for electronic toll collection '' is divided into 5 parts.
--- Part 1. physical layer;
--- Part 2. Data link layer;
--- Part 3. Application layer;
--- Part 4. Equipment application;
--- Part 5. Test methods for main parameters of the physical layer.
This part is the second part of GB/T 20851.
This section is drafted in accordance with the rules given in GB/T 1.1-2009.
This section replaces GB/T 20851.2-2007 "Special short-distance communication for electronic charging part 2. Link data layer"
Compared with GB/T 20851.2-2007, in addition to editorial changes, the main technical changes are as follows.
--- Modified the content of normative references (see Chapter 2, Chapter 2 of the.2007 edition);
--- Modify the parameter definition and parameter value of the main parameter Tu of the link layer, and modify the parameter value of N1 (see Chapter 5,.2007
Chapter 5 of the annual edition);
--- Removed the main parameters of the link layer T3 and N2 (see Chapter 5 of the.2007 edition);
--- Increased the definition and value of the main parameters of the link layer T4a, T4b, Tr1, Tr2 (see Chapter 5);
--- Modified the value of bits 3-0 in the downlink MAC control domain (see 6.3.1,.2007 version 6.3.1);
--- The value of the uplink MAC control domain bit 4-0 has been modified (see 6.3.2,.2007 version 6.3.2);
--- Added provisions for time window management in the MAC sublayer (see 8.1).
This section is proposed and managed by the National Intelligent Transportation System Standardization Technical Committee (SAC/TC268).
This section was drafted by. Highway Research Institute of the Ministry of Transport, Zhongguancun CCCC Guotong Intelligent Transportation Industry Alliance, Beijing CCCC Guotong
Intelligent Transportation System Technology Co., Ltd., Beijing Sutong Technology Co., Ltd., Shenzhen Jinyi Technology Co., Ltd., Beijing Juli Technology Co., Ltd.
Co., Ltd., Shanghai Changjiang Intelligent Data Technology Co., Ltd., Beijing Wanji Technology Co., Ltd., Shenzhen Chenggu Technology Co., Ltd., Guangzhou
Aite Communication Equipment Co., Ltd.
The main drafters of this section. Chen Bingxun, Liu Hongwei, Zhang Beihai, Liu Yongping, Gui Jie, Li Wei, Zhao Yuyang, Zhou Jian, Wu Zhaojiong, Zhang Yujun,
Lu Liyang.
The previous versions of the standards replaced by this section are.
--- GB/T 20851.2-2007.
Dedicated short-range communication for electronic toll collection
Part 2. Data link layer
1 Scope
This section of GB/T 20851 specifies the main parameters, information frames, and dedicated communication chains of the data link layer for short-distance communication dedicated to electronic toll collection.
Requirement for establishment and revocation of routes, MAC sublayer, LLC sublayer.
This section applies to electronic toll systems for highways and urban roads. References can be made to areas such as automatic vehicle identification and vehicle access management.
2 Normative references
The following documents are essential for the application of this document. For dated references, only the dated version applies to this article
Pieces. For undated references, the latest version (including all amendments) applies to this document.
GB/T 7421-2008 High-level data link control (HDLC) procedures for remote communication and information exchange between information technology systems
GB/T 15629.2-2008 Information technology systems for remote communication and information exchange specific requirements for local area networks and metropolitan area networks
Part 2. Logical link control
GB/T 20839-2007 General terms for intelligent transportation systems
GB/T 20851.1-2019 Short-distance communication dedicated to electronic charges Part 1. Physical layer
3 terms and definitions
The terms and definitions defined in GB/T 20839-2007 and GB/T 20851.1-2019 apply to this document.
4 Symbols and acronyms
4.1 Symbols
The following symbols apply to this document.
V (RB). Receive success or failure state variable.
V (RI). Receive sequence state variable.
V (SI). Send sequence state variable.
4.2 Abbreviations
The following abbreviations apply to this document.
ACK. Acknowledge
ACn. Acknowledged Command/Response
BST. BeaconServiceTable
C/R. Command/Response
F. Final
FCS. Frame Check Sequence (FrameCheckSequence)
LID. Link Identifier
LLC. Logical Link Control
LPDU. Logical Link Control Protocol Data Unit (LLCProtocolDataUnit)
LSDU. Logical Link Control Service Data Unit (LLCServiceDataUnit)
LSB. Least Significant Bit
M. Modifier function bit
MAC. Medium Access Control
MSB. Most Significant Bit (MostSignificantBit)
OBU. OnBoardUnit
PDU. Protocol Data Unit
P/F. Polling/Final
RSU. Roadside Unit
UI. UnnumberedInformation
VST. Vehicle Service Table
5 Link layer main parameters
The main parameters of the link layer are shown in Table 1.
Table 1 Main parameters of the link layer
Parameter parameter definition parameter value
160 μs minimum interval between T1 downlink frame and the next adjacent uplink frame
Minimum interval between T2 uplink frame and subsequent adjacent downlink frame 32μs
Tu public uplink window duration 3ms
N1 dedicated link establishment request delay counter 0 ~ 2
T4a Maximum time before a dedicated uplink window transmission starts 6.84ms
T4b Maximum time before a common uplink window transmission starts 480 μs
Tr1 common downlink frame retransmission interval 10ms
Dedicated downlink frame retransmission interval with the same Tr2 link identifier 10ms
N3 internal transfer counter-
N4 acknowledge timer-
Note. This section does not specify the parameter values of N3 and N4.
6 Information frame
6.1 Information frame format
Data link layer information exchange should be performed in the form of a frame.The frame contains the MAC address, MAC control domain, LPDU (optional), and frame checksum.
section. According to the type of LPDU, it is further divided into frames containing command LPDUs and frames containing response LPDUs, and information frames containing command LPDUs.
The structure is shown in Figure 1, and the structure of the information frame containing the response LPDU is shown in Figure 2.
Figure 1 Information frame structure with command LPDU
Figure 2 Information frame structure with response LPDU
The frame structure without the LPDU field is shown in Figure 3.
Figure 3 Frame structure without LPDU field
6.2 Frame Encapsulation
6.2.1 Frame Encapsulation Format
The information frame adopts synchronous transmission mode, and the frame encapsulation format is shown in Figure 4.
Figure 4 Encapsulation format of the second layer information frame
6.2.2 Frame start flag and frame end flag
The format of the frame start flag and frame end flag are binary sequences 01111110.
The frame end flag of the previous frame cannot be used as the frame start flag of the next frame.
If the receiving end receives multiple consecutive frame start flags, the last one is used as the start of the frame.
6.2.3 Preamble and Postamble
For the preamble and postamble, see Chapter 6 and Chapter 7 of GB/T 20851.1-2019.
6.2.4 Transparent transmission
The information between the start of frame flag and the end of frame flag, excluding the start of frame flag and the end of frame flag, should be interpolated to zero.
a) The sending end inserts a "0" after sending five "1s" in succession;
b) The receiver checks the sixth bit when it receives five "1" s in a row.
--- If the sixth bit is "0", delete the "0"; if it is "1", check the seventh bit;
--- If the seventh bit is "0", it indicates the start or end flag; if it is "1", the receiving end regards the information frame as invalid
Frame and discard.
6.2.5 MAC Address
Media access addresses are divided into broadcast MAC addresses and dedicated MAC addresses. Broadcast MAC address is used by RSU for all OBUs
Access; The dedicated MAC address is used to access a specific OBU.
The broadcast MAC address value should be 32-bit all "1" bits. 0xFFFFFFFF; the private MAC address value should be 32-bit non-all "1" bits.
6.3 MAC Control Domain
6.3.1 Downlink MAC Control Domain
The downlink MAC control domain is used by frames sent by the RSU. The format of the downlink MAC control field is shown in Table 2.
Table 2 Downlink MAC control domain
Bit identifier meaning value
7 D/U direction identifier 0. Downlink
6 Whether L LPDU exists 1. exists; 0. does not exist
5 C/R command/response 0. command
4 Q broadcast information, only valid if MAC address is all 1.
0. Do not seek to establish a dedicated link;
1. seeking to establish a dedicated link
3 ~ 0-Reserved as 0
6.3.2 Uplink MAC Control Domain
The uplink MAC control field is used by frames sent by the OBU. The format of the uplink MAC control field is shown in Table 3.
Table 3 Uplink MAC control domain
Bit identifier meaning value
7 D/U direction identifier 1. Uplink
6 Whether L LPDU exists 1. exists; 0. does not exist
5 C/R command/response 1. response
4 ~ 0-Reserved as 0
6.4 LPDU format
LPDU format see 9.3.
6.5 Frame check sequence
There should be a 16-bit FCS before the end of frame flag. The calculation range of FCS includes MAC address, MAC control domain and LPDU.
The FCS shall comply with the 16-bit frame check sequence defined in GB/T 7421-2008. The generator polynomial is X16 X12 X5 1,
The initial value used is 0xFFFF.
6.6 bit order
The frame start flag, frame end flag, MAC address, MAC control domain, and LPDU shall be transmitted in the order of the first LSB. FCS
Transmission should start from the MSB.
7 Dedicated communication link establishment and cancellation
7.1 Dedicated communication link establishment
The communication between RSU and OBU supports both broadcast and point-to-point.
In broadcast mode, there is no need to establish a dedicated communication link between the RSU and the OBU.The broadcast MAC address is used as the link identifier.
The OBU can receive messages from the RSU.
In the point-to-point mode, a dedicated communication link needs to be established between the RSU and the OBU, and the link is uniquely identified by a dedicated MAC address.
The process of establishing a dedicated link is as follows.
a) RSU periodically broadcasts specific information with Q of 1;
b) After receiving this information, the OBU in the communication area randomly delays N1 time units Tu;
c) the OBU sends information including its MAC address to the RSU;
d) After the RSU confirms the receipt of a valid frame, it registers the corresponding OBU MAC address and uses the MAC address and the corresponding OBU
Communication;
e) After the OBU receives the downlink frame with its own OBUMAC address, the dedicated link is successfully established.
The establishment of the point-to-point dedicated communication link is in accordance with Appendix A.
7.2 Deactivation of dedicated communication link
The cancellation of the dedicated communication link and the cancellation of the OBUMAC address by the RSU are determined by the RSU logic.
8 MAC sublayer
8.1 Time window management
8.1.1 Overview
Time window management is divided into dedicated uplink window allocation and multiple public uplink window allocations, which distinguishes
MAC address of the downlink frame. The broadcast MAC address is assigned a public uplink window, and the private MAC address is assigned a private
The uplink window and window management overview are shown in Figure 5.
Figure 5 Overview of window management
8.1.2 Downlink window
The RSU allocates a downlink window when sending an information frame.
The downlink window starts with the first bit of the preamble and ends with the end flag of the Layer 2 information frame transmitted on the downlink.
The last bit (if there is a postamble, it is the last bit).
If the previous window is an uplink window, the downlink window should start after the previous window ends and T2 is added.The uplink window
See Figure 6 for the timing of the downlink window afterwards.
Figure 6 Timing of the downlink window after the uplink window
8.1.3 Uplink window
8.1.3.1 Overview
The uplink window is divided into a public uplink window and a dedicated uplink window.
The public uplink window can be used by any OBU in accordance with 8.1.3.2, and a dedicated uplink window can only be used by one
OBU is used.
8.1.3.2 Common uplink window
Each downlink frame with a broadcast address is assigned three consecutive common uplink windows.
The starting point of a common uplink window. If the common uplink window is the first window after the downlink window, then
The allocation window now ends after T1 of the downlink frame; if the immediately preceding window is a common uplink window, it appears in the immediately preceding window.
The end time of the adjacent window.
The duration of the common uplink window is Tu, and the corresponding windows are Tu0, Tu1, Tu2. Preamble of common uplink information frame
The first bit of the code should be sent within T4b within a randomly selected corresponding public uplink window, and the entire information frame should be completely in one
In the common uplink window, the timing of the common uplink window is shown in Figure 7.
Figure 7 Timing of the common uplink window
If the time required to send an information frame within the currently selected public uplink window cannot be met, you should wait for the next public downlink
Three common uplink windows are allocated for the information frame of the link, and one of them is randomly selected according to the average distribution.
8.1.3.3 Dedicated uplink window
Only OBUs with the same OBUMAC address as the MAC of the downlink that is assigning the window can use the dedicated uplink
window.
A dedicated uplink window starts after the T1 time of the end of the downlink window allocated to the dedicated uplink window.
The link window timing is shown in Figure 8.
The first bit of the preamble of the dedicated uplink information frame should be sent within T4a of this window.
Within a dedicated uplink window. If the time required to send an information frame within the currently allocated dedicated uplink window cannot be met
Request, the end of the dedicated uplink window for this allocation, and should wait for the next dedicated uplink window.
Figure 8 Dedicated uplink window timing
8.2 MAC service primitives
The MAC sublayer provides the following service primitives to the LLC sublayer.
a) MAC.request. The LLC sublayer sends to the MAC sublayer, requesting to send an LPDU;
b) MAC.indication. The MAC sublayer sends to the LLC sublayer to indicate that an LPDU was successfully received.
The MAC sublayer service primitives in RSU and OBU are B-MAC primitive and M-MAC primitive, respectively. See the MAC service primitives
Table 4.
Table 4 MAC service primitives
Service primitive entry description
B-MAC.request
Prototype B-MAC.request (MAC address, LPDU)
The functional LLC sublayer sends to the MAC sublayer and requests to send an LPDU to the OBU
parameter
MAC address. The MAC address to be sent to the OBU. It can be a dedicated MAC address or a broadcast MAC address.
LPDU. Link Protocol Data Unit
B-MAC.indication
Prototype B-MAC.indication (MAC address, LPDU)
The functional MAC sublayer sends to the LLC sublayer, indicating that a valid frame was successfully received
parameter
MAC address. the content of the MAC address field in the received frame;
LPDU. Link Protocol Data Unit
M-MAC.request
Prototype M-MAC.request (MAC address, LPDU)
The functional LLC sublayer sends to the MAC sublayer to request an LPDU to be sent to the RSU
parameter
MAC address. the dedicated MAC address of the OBU;
LPDU. Link Protocol Data Unit
M-MAC.indication
Prototype M-MAC.indication (MAC address, LPDU)
The functional MAC sublayer sends to the LLC sublayer, indicating that a valid frame was successfully received
parameter
MAC address. the content of the MAC address field in the received frame;
LPDU. Link Protocol Data Unit
8.3 MAC Service Description
8.3.1 RSUMAC Service
8.3.1.1 Dedicated link establishment
The RSU periodically broadcasts specific BST information with Q equal to 1. The OBU responds to the BST information with a dedicated MAC address.
After the RSU confirms the receipt of a valid frame, it registers the MAC address of the OBU and uses that MAC address to communicate with the OBU;
See 8.3.2.1 for the process.
8.3.1.2 Frame reception
8.3.1.2.1 Checking the validity of the frame
The MAC sublayer shall check the validity of all received frames and shall meet the following requirements.
a) The start of frame flag and the end of frame flag are in accordance with 6.2.2;
b) After removing the 0 bits inserted to maintain transparency, the number of bits in the frame is an integer multiple of 8, and not more than N2 eight bits
Bit group
c) Contains a valid private MAC address.If a private link has been established, this MAC address should be the same as that of the private link.
MAC addresses match;
d) contains a MAC control domain that meets the requirements;
e) Include a valid FCS.
8.3.1.2.2 Information reception
If the L bit of the received valid frame is 1, it indicates that the frame contains an LPDU. Extract LPDU and MAC address from the frame
The content of the domain is transmitted to the LLC sublayer in the form of B-MAC.indication.
If the L bit of the received valid frame is 0, it indicates that the frame does not contain LPDU.
8.3.1.3 Frame transmission
RSUMAC receives the LPDU provided by LLC and constructs a frame according to the frame format.The L bit of the MAC control domain should be set to 1, D
The bit should be set to 0. If it is a broadcast message, set Q according to whether to seek to establish a dedicated link. The frame is then passed to the lower layers.
8.3.2 OBUMAC Service
8.3.2.1 Dedicated link establishment
After receiving the broadcast from the broadcast MAC address and Q equal to 1, the OBU sends a message to the RSU with a dedicated MAC address.
After the OBU receives the first downlink frame that matches its MAC address, it indicates that the dedicated link is established.
8.3.2.2 Frame reception
8.3.2.2.1 Checking the validity of the frame
The MAC sublayer shall check the validity of all received frames and shall meet the following requirements.
a) the start of frame mark and the end of frame mark shall comply with the provisions of 6.2.2;
b) After removing the 0 bits inserted to maintain transparency, the number of bits in the frame is an integer multiple of 8, and not more than N2 eight bits
Bit group
c) contains a valid address field indicating a valid MAC address;
d) contains a MAC control domain that meets the requirements;
e) Include a valid FCS.
8.3.2.2.2 Information reception
If the L bit of the received valid frame is 1, it indicates that the frame contains an LPDU. Extract LPDU and MAC address from the frame
The content of the domain is transmitted to the LLC sublayer in the form of M-MAC.indication.
8.3.2.3 Frame transmission
OBUMAC receives the LPDU provided by LLC and constructs a frame according to the frame format.The L and D bits of the MAC control domain
Should be set to 1. The frame is then passed to the lower layers.
9 LLC Sublayer
9.1 Overview
The LLC generates a command PDU and a response PDU for transmission, and interprets the received command PDU and response PDU. LLC stipulated
Features include the following.
a) initialization of control information;
b) organize data flow;
c) interpret the received command PDUs and generate appropriate response PDUs;
d) LLC sublayer error control and error recovery.
The LLC sublayer specifies the protocol process of information and control transmission between peer entities, and its logical link control operations include GB/T 15629.2-
Type 1 and Type 3 specified in.2008.
Type 1 operation specifies an unconfirmed connectionless service with minimal protocol complexity. Provides basic data recovery at the upper layer
Use this type of operation for the complex sequence function.
Type 3 operation specifies a data unit exchange service to confirm connectionless mode, which allows a station to transmit data at the same time.
Request data back.
9.2 LLC Sublayer Service Specification
9.2.1 General
The LLC sub-layer specifies the services that LLC sub-layer users require of the LLC sub-layer, and these services make LLC sub-layer users available to LLC
The sublayer exchanges packets.
The LLC sublayer provides two service methods.
a) Unconfirmed connectionless method. This data transmission service provides a set of methods to enable data link user entities to adopt unconfirmed
Switch LSDUs without establishing a connection at the data link layer. The data transmission can be point-to-point, multicast or broadcast;
b) Confirm connectionless mode. This data unit exchange service provides a set of methods to enable the data link user entity to establish data without
In the case of link connection, LSDUs are exchanged and confirmed at the LLC sublayer. The data is exchanged point-to-point.
9.2.2 Overview of Interaction Process
9.2.2.1 Unconfirmed connectionless service
The primitives related to unconfirmed connectionless data transfer are.
DL-UNITDATA.request
DL-UNITDATA.indication
DL-UNITDATA.request is passed from the LLC sublayer user to the LLC sublayer, and the request is sent using an unconfirmed connectionless method.
LSDU.
DL-UNITDATA.indication is passed from the LLC sublayer to the LLC sublayer user, indicating the arrival of an LSDU.
9.2.2.2 Confirm the connectionless service
9.2.2.2.1 Confirming Connectionless Data Transmission
The primitives related to confirming the connectionless data unit delivery service are.
DL-DATA-ACK.request
DL-DATA-ACK.indication
DL-DATA-ACK_STATUS.indication
DL-DATA-ACK.request is passed from the LLC sub-layer user to the LLC sub-layer, requesting the use of a data unit to confirm the connectionless mode
The transmission process sends an LSDU.
DL-DATA-ACK.indication is passed from the LLC sublayer to the LLC sublayer user, indicating the arrival of a command PDU, the
Except when PDUs are used only for resynchronization.
DL-DATA-ACK-STATUS.indication is passed from the LLC sublayer to the users of the LLC sublayer, conveying the corresponding
Execution result of DL-DATA-ACK.request.
9.2.2.2.2 Confirm connectionless data exchange
The primitives related to confirming the connectionless data unit exchange service are.
DL-REPLY.request
DL-REPLY.indication
DL-REPLY-STATUS.indication
DL-REPLY.request is passed from the LLC sub-layer user to the LLC sub-layer, requesting that the data units be exchanged using a confirmation connectionless method
The process returns an LSDU from a remote site or exchanges LSDU between sites.
DL-REPLY.indication is passed from the LLC sublayer to the LLC sublayer user, indicating the arrival of a command PDU.
DL-REPLY-STATUS.indication is passed from the LLC sublayer to the users of the LLC sublayer, conveying the corresponding DL-
The execution result of REPLY.request.
9.2.2.2.3 Confirm the connectionless data to be updated
The primitives related to confirming the connectionless update data update service are.
DL-REPLY-UPDATE.request
DL-REPLY-UPDATE-STATUS.indication
DL-REPLY-UPDATE.request is passed from the LLC sublayer user to the LLC sublayer, asking the LLC sublayer to save one
LSDU, and sent when other sites request LSDU later.
DL-REPLY-UPDATE-STATUS.indication is passed from the LLC sublayer to the users of the LLC sublayer.
The execution result of the corresponding DL-REPLY-UPDATE.request.
9.2.3 Detailed service specifications
9.2.3.1 General
The detailed service specification specifies the LLC's service primitives and their parameters, showing the logic of the information transfer relationship ...
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