GB/T 20851.2-2019Electronic toll collection - Dedicated short range communication - Part 2: Data link layer (English PDF)

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

China's national data link layer specification for electronic toll collection DSRC. It is Part 2 of GB/T 20851 and specifies the main parameters of the data link layer, the information frames, the requirements for establishing and releasing the dedicated communication link, and the MAC and LLC sublayers, and it applies to electronic toll collection on highways and urban roads, with reference use in automatic vehicle identification and vehicle access management. The data link layer of a tolling system has to solve a problem that ordinary networking does not face. The roadside unit is talking into a lane through which several vehicles may be present in the communication zone at once, each carrying an on-board unit that has just appeared and will disappear within a fraction of a second, and every one of them must be identified, allocated capacity, transacted with and released before it leaves. There is no time for contention resolution of the kind a normal network can afford. The medium access control is therefore centralised and deterministic: the roadside unit controls the channel completely, polls and allocates windows, and the on-board units transmit only when addressed. This part defines that mechanism - the frame structures and their fields, the addressing, the private and broadcast link establishment and release, the acknowledgement and error handling, and the parameters that bound the timing. The logical link control sublayer above it provides the service the application layer of Part 3 uses. Getting this layer wrong does not produce a slow system; it produces missed vehicles and disputed transactions, which is why the specification is national rather than left to the equipment suppliers. Issued on 10 May 2019 and in force since 1 December 2019, it replaces GB/T 20851.2-2007.

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 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.

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

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 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

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.