CN110856141A - Reliable and effective vehicle-mounted self-organizing network multi-channel MAC protocol - Google Patents

Abstract

The invention relates to a reliable and effective vehicle-mounted self-organizing network multichannel MAC (CRE-MAC) protocol, belonging to the technical field of vehicle-mounted self-organizing networks. It includes: the road side unit coordinates the vehicles to reserve time slots for safety message transmission and arranges the order of safety message broadcasting, thereby realizing the contention-free transmission of the safety messages on the control channel; the contention-free transmission (broadcasting) of the safety message reduces the collision probability, reduces the time required by broadcasting, realizes the bounded transmission of the safety message and also reserves more time for the reservation of the service channel; the introduction of a forwarding mechanism ensures reliable delivery of the coordination message; the CRE-MAC protocol can utilize the service channel resources to transmit non-safety messages in the whole synchronization period, effectively solves the problems of multi-channel hidden terminals and missing receiving terminals, and improves the saturated throughput and the channel utilization rate of the service channel. Compared with the traditional IEEE1609.4 protocol, the CRE-MAC protocol can ensure the reliable and timely transmission of the safety information to a great extent, improve the network throughput and reduce the transmission delay of the non-safety information.

Description

Reliable and effective vehicle-mounted self-organizing network multi-channel MAC protocol

Technical Field

The invention belongs to the technical field of vehicle-mounted self-organizing networks, and relates to a Reliable and effective vehicle-mounted self-organizing network multichannel MAC (coded Reliable and Efficient multichannel Medium Access Control, CRE-MAC) protocol.

Background

As an important component of an Intelligent Transportation System (ITS), a Vehicular Ad Hoc network (VANETs) can support wireless communication between vehicles and Road Side Units (RSUs), and plays an important role in improving vehicle-mounted communication safety and efficiency. Through vehicle-to-vehicle, vehicle-to-infrastructure communications, vehicles can exchange information to support both safety applications (e.g., emergency braking, collision warning) and non-safety applications (e.g., entertainment, in-vehicle video, navigation, etc.).

On-board ad hoc networks face various challenges due to the high speed mobility of vehicles and the constant variation in vehicle density. The main challenges facing vehicular ad hoc networks are: secure messages require timely, reliable transmission, while non-secure message transmissions typically require large and efficient throughput (e.g., map downloads, point of interest advertising) and low latency requirements (such as Quality-of-Service (QoS) sensitive video/audio streaming over voice-over-IP calls and mobile multiplayer games).

VANETs adopt ieee802.11p and IEEE1609.4 as vehicle communication protocols. However, IEEE802.11p adopts a Carrier Sense Multiple Access/collision avoidance (CSMA/CA) channel Access method, which is a contention-based channel Access method, and has no Acknowledgement (ACK) mechanism, and cannot ensure reliable and timely transmission of a safety message, especially when the vehicle density is high. The IEEE1609.4 protocol divides time into Synchronization Intervals (SI) of 100ms, each SI is composed of a Control Channel (CCH) Interval and a Service Channel (SCH) Interval of 50ms, and the vehicle periodically and constantly switches between the CCH Interval and the SCH Interval. The fixed channel switching scheme in the IEEE1609.4 protocol makes it difficult to meet the high throughput, low latency data transmission requirements for non-secure information in VANETs. For example, in a high-density car networking environment, a node does not have enough time to negotiate a service channel on a highly congested control channel. Thus, service channel utilization is affected. On the other hand, in the case of sparse vehicles but requiring a large amount of non-secure data transmission (e.g. close to the point of interest), the CCH interval may be idle for a long time, while a 50ms service channel interval is not sufficient to transmit large blocks of application data requiring high bandwidth consumption, such as voice calls, e-map downloads, etc. Thus, data faces low throughput and additional unavoidable channel switching delay issues.

In summary, the MAC layer mechanism in IEEE802.11p and IEEE1609.4 protocols in VANETs has the following disadvantages: 1) the fixed CCH interval and SCH interval are inflexible and cannot allocate time intervals according to the changing traffic flow requirements; 2) the fixed switching between the CCH interval and the SCH interval causes that the utilization rate of the bandwidth resources of the CCH and the SCH is very low and does not exceed 50 percent; 3) for a single radio transceiver, the multi-channel switching operation may generate a synchronization frame collision at the beginning of a CCH interval, and a safety message may be discarded due to no time transmission at the end of the CCH interval, so that reliable and timely transmission of the safety message cannot be guaranteed; 4) the contention-based CSMA/CA MAC mechanism on the serving channel has limited capacity to improve the SCH throughput it provides on the serving channel, while it is difficult to ensure reliable, timely transmission of the safety messages on the control channel.

In response to the deficiencies of the IEEE802.11p and IEEE1609.4 protocols, researchers have proposed many solutions. According to the document "An IEEE802.11 p-based multichannel MAC scheme with channel coordination for contextual ad hoc networks", Wanqing et al propose a VCIMAC protocol with variable CCH intervals. The VCI MAC protocol dynamically adjusts CCH intervals according to the number of nodes, but the transmission of safety messages on CCH of the protocol still adopts a CSMA/CA protocol based on competition, and when the nodes transmit on CCH, all SCH are idle, and the density of network nodes can not be guaranteed to be high, the safety messages are transmitted timely and reliably, and the service channel resources are not fully utilized. A TDMA-based MAC protocol for a reliable broadcast in VANETs proposes a VeMAC protocol specially aiming at a vehicle-mounted wireless self-organizing network scene, different transmission time slots are allocated to vehicles driving in different directions, and because the VeMAC works in a distributed mode, a node needs to send information with larger data volume to reserve a safe time slot, and the reserved time for a service channel is reduced, the improvement of the throughput of the service channel is still limited. A Coordinated multi-channel C-MAC protocol is proposed in the document "Coordinated multi-channel MAC protocol for contextual ad hoc networks", where a channel access manner based on non-competitive Time Division Multiple Access (TDMA) is used for transmission of a security message on a control channel to ensure reliable and timely transmission of the security message, but when a node transmits the security message on the control channel, all service channels are idle, and service channel resources are still not fully utilized.

In addition, the document "a Distributed Multi-Channel MAC Protocol for Ad hoc wireless Networks" shows that when the number of nodes in the system is sufficiently large, the total throughput that can be obtained by the network is not determined by the number of nodes, but by the number of channels. Therefore, the number of serving channels and the time spent on serving channels are critical factors in determining the throughput of the system.

Disclosure of Invention

The invention aims to overcome the defects of the existing vehicle-mounted self-organizing network communication method, and provides a Reliable and effective vehicle-mounted self-organizing network multichannel MAC (coded Reliable and efficient multichannel MAC, CRE-MAC) protocol, which can ensure the Reliable and timely transmission of safety information, improve the network throughput and reduce the transmission delay of non-safety information.

The purpose of the invention is realized by the following technical scheme.

A reliable and effective vehicle-mounted self-organizing network multi-channel MAC protocol comprises the following steps:

1) according to the network condition, the road side unit calculates the time T of the vehicle confirmation Interval (VII) in the next synchronization cycle_VIIAnd put it into Coordination and Length Information (CLI) data packet; at the beginning of a SI, the RSU broadcasts a CLI packet to announce T_CFI，T_VII，T_WIAnd SaSlot of each confirmed vehicle, where T_CFIDenotes the length of the Contention-FreeInterval (CFI), T_WIIndicating the length of the WSA interval, SaSlot indicating the time slot on the control channel for the acknowledged vehicle to broadcast the safety message; when there is no RSU, the node with the smallest MAC _ ID within a hop will act as a leader to perform the function of the RSU;

2) each vehicle broadcasts safety-related messages in its SaSlot: beacon or urgent message, without any collision, the forwarding node broadcasts the CLI data packet at the same time as the safety-related message;

3) during VII, the RSU performs a validation process for each newly entering vehicle to obtain a corresponding number of SaSlot, and according to the forwarding mechanism, the RSU allocates one SaSlot to each common node and three consecutive saslots to each forwarding node;

4) when a node has a non-secure message to send or request, it will select the "best" serving channel and the corresponding SerSlot on the serving channel according to its SerSlot Usage List (SUL), and then, at WSA interval, use the CSMA/CA mechanism to contend for the control channel for transmitting WSA or RFS messages;

5) when receiving an expected WSA or RFS message, if [ SCH, SerSlot ] is available, sending an Acknowledgement (ACK) message to the sending end, otherwise, sending a Non-Acknowledgement (NACK) message; if the neighbor node finds that the problem of a multi-channel hidden terminal or the problem of a receiving end loss occurs by monitoring the WSA/RFS message, the neighbor node auxiliary method is utilized to improve the channel utilization rate and the network performance;

6) the neighbor nodes hear the ACK message and update their Neighbor Information List (NIL) and SUL;

7) in the next SI, both the transmitting end and the receiving end switch to the selected SCH at the SerSlot time to transmit non-secure messages.

Further, the roadside unit calculates the time T of the Vehicle Identification Interval (VII) in the next synchronization cycle according to the network conditions in the step 1)_VII，T_VIIBy the formula T_VII＝L_total·T_rrts+m·T_cpIs calculated to obtain, wherein L_totalIndicates the total frame length, T_rrtsIndicating the time for transmitting a Reservation Request To Send (RRTS) packet, m indicating the number of rounds a node must go through before being acknowledged by the road side unit, T_cpIndicating the time to transmit a Coordination Packet (CP) Packet.

Further, the MAC _ ID in step 1) is a short MAC identifier of the node; the MAC _ ID of a node is randomly selected by the node and included in a data packet transmitted on a control channel, and if the node detects that its selected MAC _ ID is already used by another node, it reselects.

Further, the forwarding node in step 2) refers to: if there is a large truck within the RSU coverage, the large truck is selected as a forwarding node to forward the CLI packet.

Further, the working process of the forwarding mechanism in step 3) includes the following steps: s1) selecting a forwarding node according to the requirements of the forwarding node of the present invention; s2) using the formula

Calculating the number of forwarding nodes, wherein N represents the total number of nodes in the RSU coverage range; y is_fThe expansion factor of the VANETs in the forwarding mechanism is represented, the value of the expansion factor represents the scale of the VANETs, and the value is 30; s3) to the forwarding node, three consecutive SaSlot, one for transmitting the node' S own beacon or for being immediately adjacentUrgent message, two SaSlot are used to transmit one CLI packet.

Further, said step 4) will select the "best" service channel and the corresponding SerSlot on the service channel according to its SerSlot usage table (SUL); wherein the SerSlot represents the time slot for the node to send messages on the SCH; the SUL stores the SerSlots available on each SCH in the next SI; the specific method for selecting the "best" service channel SCH and the corresponding SerSlot on the service channel is as follows: m1), on the one hand, all nodes can select the SerSlot only after the RSU finishes broadcasting the CLI data packet; on the other hand, the node cannot select the SerSlot containing the SaSlot time on any SCH; m2) does not allow a node to continuously subscribe to the same SerSlot; m3) node will select the SerSlot on the SCH that has the most SerSlot available each time; if there are two or more SCHs with the same number of available serslots, the transmitting side randomly selects one of them and selects the SerSlot thereon as required in method M1) and method M2).

Further, in the step 5), the channel utilization rate and the network performance are improved by using a neighbor node assistance method, where the neighbor node assistance method is: when the neighbor node monitors the WSA/RFS message, comparing the WSA/RFS message with the NIL; if the selected SCH/SerSlot in the WSA/RFS message is found to conflict with SCH/SerSlot of other nodes, at T_waitAfter the duration, the neighbor node sends the auxiliary information to the source node.

Further, the NIL is the SCH/SerSlot used to store the MAC _ ID of the neighbor node and to transmit non-security messages in the current SI and the next SI.

Further, T is_waitBy the formula T_wait＝T_switch+XmodY_nCalculating to obtain; wherein T is_switchIndicating a radio switching delay between a transmission mode and a reception mode; x represents the MAC _ ID of the node; y is_nAnd the expansion factor of the VANETs in the neighbor node auxiliary method is represented, the value of the expansion factor represents the scale of the VANETs, and the value is 31.

Further, the auxiliary information includes [ MAC _ ID, SCH, SerSlot of node ]; wherein the MAC _ ID of the node represents the MAC _ ID of the node with collision; SCH indicates the conflicting SCH; SerSlot denotes a SerSlot in which a collision occurs.

Further, the method of updating their Neighbor Information List (NIL) and SUL in step 6) is as follows: a) in the NIL table maintained by the self, the MAC _ ID records of the transmitting node and the receiving node are respectively found, and the SCH/SerSlot value used for transmitting the non-safety message in the corresponding next SI is modified. b) Finding the SCH number negotiated by the sending node and the receiving node in the SUL table maintained by the sending node, and deleting the SerSlot negotiated by the sending node and the receiving node in the corresponding 'available SerSlot' field entry.

Compared with the communication protocol of the existing vehicle-mounted self-organizing network, the vehicle-mounted self-organizing network multichannel MAC protocol provided by the invention has the following advantages:

① under the coordination of RSU, each node can reserve a transmission time slot for the safety message without exchanging extra information, the introduction of forwarding mechanism ensures the reliable delivery of the coordination information, the proposed contention-free TDMA channel access mode greatly reduces the collision probability and the required time for transmitting the safety related message, ensures the bounded transmission delay of the safety message with high priority, solves the problem of synchronous frame collision and reduces the number of discarded frames due to expiration;

② CRE-MAC protocol uses less time to reserve the safe message transmission time slot and deliver the safe message, and reserves more time for SCH reservation of non-safe message, so that the node has more chances to reserve SCH, and the number of successful reservations is greatly increased;

③ supports data transmission on the SCH during the entire synchronization period, thereby improving the saturation throughput and channel utilization of the SCH and reducing transmission delay;

④ it can solve the hidden terminal problem of multi-channel and the lost receiving terminal problem, to improve the system throughput and reduce the channel transmission delay.

Drawings

In order to make the object, technical solution and advantages of the present invention more clear, the present invention provides the following drawings for illustration.

Fig. 1 is a frame diagram of a CRE-MAC protocol in an embodiment of the present invention.

Fig. 2(a), fig. 2(b), fig. 2(c), and fig. 2(d) are schematic diagrams of the vehicle confirmation process performed on the CCH during the vehicle confirmation interval in the embodiment of the present invention.

FIG. 3(a) multi-channel hidden terminal problem; fig. 3(b) lacks the receive side problem.

Fig. 4 is a schematic diagram of a highway scenario including an RSU and a moving vehicle with two lanes in each direction according to an embodiment of the present invention.

Fig. 5 is a comparison graph of the confirmation rates under different vehicle densities, vehicle speeds and RSU coverage in the embodiment of the present invention.

FIG. 6 is a graph comparing the validation times for various vehicle densities and vehicle speeds in accordance with an embodiment of the present invention.

Fig. 7 is a diagram illustrating transmission delay of security-related packets according to various protocols according to the embodiment of the present invention as a function of the number of nodes.

Fig. 8 is a diagram of the probability of successful transmission of a security-related message under various protocols versus a security interval in an embodiment of the present invention.

Fig. 9 is a graph of the saturation throughput on SCHs as a function of the number of nodes in the embodiment of the present invention.

Fig. 10 is a diagram illustrating the variation of the transmission delay of the non-secure packet with the number of nodes according to the embodiment of the present invention.

Fig. 11 is a graph of saturated throughput on SCHs as a function of number of nodes under various protocols in an embodiment of the present invention.

Fig. 12 is a diagram illustrating the variation of the transmission delay of the non-secure packets with the number of nodes according to various protocols in the embodiment of the present invention.

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

The CRE-MAC protocol framework of the present invention is shown in fig. 1, where we assume that each vehicle has a radio transceiver that can both send and receive messages, but not simultaneously. A Coordinated Universal Time (UTC) mechanism from the Global Positioning System (GPS) is used to synchronize time between all vehicles. Time is divided into individual Synchronization periods (SI), each of which has a duration of 100 ms.

On the control channel, one synchronization period contains two intervals: a safe interval and a WSA interval. Wherein, the WSA is a WAVE Service Advertisement (WSA).

The security interval is used for service security related messages, and in the WSA interval, the nodes perform coordination and allocation of service channels.

The safety interval is further divided into two intervals: a Contention-Free Interval (CFI) and a Vehicle Identification Interval (VII).

The contention free interval CFI contains a number of secure Slots (SaSlots) for broadcasting secure messages, which contain: beacons and emergency messages. Note: security messages are also called security-related messages.

A new synchronization cycle begins with the CFI during which the RSU broadcasts Coordination and Length Information (CLI) packets and the vehicle nodes then transmit safety-related messages. One CLI data contains: scheduling information, T, of SaSlots per vehicle_CFI，T_VIIAnd T_WIWherein T is_CFI，T_VIIAnd T_WIThe length of the CFI, the length of VII and the length of the WSA interval are indicated, respectively. By receiving the CLI packet, the node knows its transmission order in the CFI interval.

During VII, under RSU coordination, the vehicle is configured to make a SaSlot reservation based on a Dynamic Frame Slotted ALOHA (DFSA) mechanism to obtain a SaSlot for transmission at the CFI, and fig. 2(a), 2(b), 2(c), 2(d) show the reservation process, which includes the following steps.

(SS1) in fig. 2(a), we assume that each of the vehicles 1 to 4 has acquired a specific transmission time slot-SaSlot during the CFI. According to the forwarding mechanism described later in this section, the vehicle 3 is selected as a forwarding node, for which the road side unit allocates three consecutive SaSlots. The newly entering vehicles 5-9 are not assigned SaSlots in the CLI packet, so the vehicles 5-9 must obtain confirmation of the road side unit to get an opportunity to transmit the safety message. The validation process is performed by the rsu according to the DFSA mechanism, as shown in fig. 2(b) to 2 (d).

(SS2) fig. 2(b) shows Frame 1 (Frame 1) during the DFSA operation. One frame contains one Coordination Control (CC) slot and the following slots. We assume that the roadside unit knows the average speed and average density of vehicles for each lane in a two-way highway. The roadside unit estimates the number of new vehicles arriving during one synchronization period according to equation (1).

n_new＝2M·V_avg·β·T_SI(1)

Wherein, β, V_avg，T_SIAnd M represents the average density of the vehicle, the average speed of the vehicle, the length of one synchronization period SI, and the number of roads per direction, respectively. Specifying T according to the IEEE1609.4 protocol_SI＝100ms。

(SS3) the RSU allocates an initial frame size of n_newAnd broadcast it into a Coordination Packet (CP) during the CC time slot. The same number of slots is then allocated after the CC slots. We call these time Slots reserved time Slots (Reservation Slots: resslots). In order To reduce the length of each ReSlot, each newly entering vehicle randomly selects a ReSlot To broadcast a Reservation Request To Send (RRTS) packet To the road side unit for confirmation, and the road side unit does not need To reply information in the ReSlot.

The RRTS packet is evolved from the RTS packet and mainly contains the address of the source node but not the address of the destination node. The size of an RRTS packet is about 14 bytes, so the length T of one ReSlot_rrtsThe time (for transmitting one RRTS packet) is about 51 μ (assuming a data rate of 6Mbps and a Physical (PHY) header of 192 bits). Thus, the dwell time of one frame(approximately n)_new·T_rrts) Smaller, the CRE-MAC protocol may leave more time for WSA intervals. In frame 1 of fig. 2(b), two ReSlots collide and the rsu can only recognize one vehicle.

(SS4) by detecting the channel, the RSU knows the number of successful transmission slots, collision slots and free slots. And the road side unit deduces the size of the next frame, namely the number of the competitive nodes according to the time slot use condition in the 1 st frame. The number n of competing nodes is obtained according to equation (2).

Wherein, C_ratioAnd L_fRespectively representing the ratio of collision slots to total slots in a frame and the frame size declared by the roadside unit in the previous CC slot.

(SS5) after the 1 st frame ends, the roadside unit obtains the number of competing nodes n according to equation (2), n also being the size of the 2 nd frame, the value of n being broadcast in the CP.

(SS6) upon receipt of the CP, during ReSlots at frame 2, any vehicle that was not successfully acknowledged at frame 1 will randomly select a time slot and send an RRTS packet. The validation process is repeated until VII is over or all vehicles are successfully validated (e.g., as shown in fig. 2 (d)). In this way, each vehicle can obtain one SaSlot to perform contention-free transmission of safety messages.

The road side unit estimates T in the next synchronization period based on the number of newly arriving vehicles during the synchronization period_VIIAnd put it into a CLI packet. The following description is given of T_VIIAnd (4) calculating.

Optimal frame length L_fEqual to the number n of competing nodes. Thus, after one round (frame), the node is in one frame (L)_f) Probability of successful transmission of RRTS packet, P_succ,LGiven by equation (3).

Let m denote the number of rounds a node must go through before being validated by the road side unit. We design Algorithm 1 to find the total frame length L_totalI.e. the total number of ReSlots, given the number of initial competing nodes n_new。

Algorithm 1: the total frame length is obtained because each frame (round) contains one CP packet, T_VIICan be estimated by equation (4).

T_VII＝L_total·T_rrts+m·T_cp(4)

Wherein T is_cpIndicating the time to transmit a Coordination Packet (CP) Packet. Thus, T can be calculated_VIIThe value of (c).

When there is no RSU, the node with the smallest MAC _ ID within a hop will act as a leader to perform the function of the RSU, assign SaSlots to the newly arrived node and broadcast the CLI packet.

In the CRE-MAC protocol, the identity of each node is distinguished by a MAC address and a short MAC identifier (macid). The MAC _ ID is used to reduce the overhead of the node comparing the MAC addresses of each of its neighbors in the NIL and transmitting CLI packets. The MAC _ ID of a node is randomly selected by the node and included in a data packet transmitted on a control channel, and if the node detects that its selected MAC _ ID is already used by another node, it reselects.

During the WSA interval, WSA/Request for service (RFS) packets broadcast by the service provider/subscriber contain the MAC _ ID of each node, so that the MAC _ ID information of neighbor nodes is recorded in the NIL of each node. From NIL, a node knows whether its MAC _ ID is a minimum value.

During the CFI, each node broadcasts a security message: a beacon or an emergency message.

The beacon information is periodic and the confirmed vehicle broadcasts beacon information once in its SaSlot every synchronization period. While broadcasting the emergency message is event-driven, if the vehicle encounters an emergency (e.g., emergency braking, overtaking, etc.) and broadcasts the emergency message in the Saslot, no beacon is broadcast.

The RSU senses and monitors the CCH during the entire synchronization period. If the SaSlot of a particular vehicle repeats the idle for 1 second, the RSU considers that the vehicle is not within its coverage and deletes the vehicle in the next CLI packet.

The RSU collects and aggregates the information of the confirmed vehicles and puts it into the CLI packet of the next synchronization cycle. After the WSA interval is finished, the RSU broadcasts the CLI packet, and the contents include: transmission time slot SaSlot sequence, T, of each node in CFI interval_CFI，T_VIIAnd T_WIThe information of (1). T is_CFIGiven by equation (5).

T_CFI＝(2M·β·R+2n_f)·T_SaSlot(5)

Wherein R represents the coverage (diameter) of the RSU; n is_fThe number of forwarding nodes in the forwarding mechanism is represented and obtained by formula (7); t is_SaSlotIndicating the time at which a security message is broadcast, the value of which is obtained by dividing the payload of the security data packet by the channel data transmission rate, here 0.4 ms.

According to FIG. 1, T_WIGiven by equation (6).

T_WI＝T_SI-T_CFI-T_VII(6)

Although the time to broadcast the CLI packet is fixed and is only used for road side unit broadcasting, the CLI packet is not necessarily received correctly by all nodes. High density traffic, fast fading or Non Line Of Sight (NLOS) can cause transmission errors Of CLI packets. To address this problem, we introduce a forwarding mechanism to ensure reliable delivery of CLI packets. The working process of the forwarding mechanism comprises the following steps.

F1) If there is a large truck within the RSU coverage, the large truck is selected as a forwarding node to forward the CLI packet.

F2) The number of forwarding nodes is obtained using equation (7).

Wherein the content of the first and second substances,

representing an upper limit function, N representing the total number of nodes in the RSU coverage, obtained from N2M · β · R, Y_fThe expansion factor of the VANETs in the forwarding mechanism is represented, the value size of the expansion factor represents the scale of the VANETs, and the value is 30; please note that n_fAnd also the total number of forwarding nodes during CFI and VII.

F3) The RSU assigns a SaSlot to each regular node. Three consecutive SaSlot are allocated to the forwarding node: one SaSlot is used to transmit the node's own beacon or emergency message and two saslots are used to transmit one CLI packet.

Time T for transmitting a CLI packet_CLI＝0.8ms，T_CLIThe calculation method is as follows:

let us assume that the maximum number of nodes that can exist in an RSU is N_maxAnd N is_maxEqual to 200. The CLI packet includes information of SaSlot of each confirmed node, T_CFI，T_VII，T_WIAnd other information such as MAC address of RSU, RSU location, error correction code, etc. Let L_otherNumber of bytes, L, representing all other information _other30 bytes. If the maximum node number is N_maxThen at least need to

Bits to indicate the MAC _ ID of the node. For the assumed network size, a MAC _ ID of 8 bits is sufficient. Due to N_maxNode needsOne SaSlot, with 8 bits sufficient to represent a SaSlot. Since one SI is 100ms, 8 bits are sufficient to represent T separately_CFI，T_VIIAnd T_WI. Let R_dIndicating data rates on CCH and SCH, R based on considerations of security applications_dEqual to 6 Mbps. Therefore, we can obtain T by equation (8)_CLI：

Let T consider the physical layer overhead (e.g., preamble and physical layer header)_CLIIs 0.8 ms.

As shown in fig. 1, during the WSA interval, when a node has an insecure message to send or requests an insecure message, the WSA/RFS is sent through two-way handshake to negotiate and reserve SCH/service slot (SerSlot) of the next synchronization cycle.

Each service provider transmits a WSA packet containing service information and the selected SCH/SerSlot, as well as other information. Each service user may also actively broadcast an RFS packet containing its selected SCH/SerSlot for agreement with the service provider.

As shown in Table 1, each node maintains one NIL (Table 1(a)) and one SUL (Table 1 (b)).

Table 1: two data structures in the CRE-MAC protocol

(a) NIL of a node

MAC ID of a node	Current SCH/SerSlot	Next SCH/SerSlot
			ID1
	1/3,3/2	2/5,4/3,3/6
			ID2	1/6,3/4	2/3,3/3
ID3	2/4	3/1,1/5
			…	…	…

(b) SUL of a node

SCH	Available SerSlot
		1	2,4,5
2	3,5,8
		3	1,3,6
4	3,6

As shown in table 1(a), the NIL stores the MAC _ ID of the neighbor node and SCH/SerSlot used to transmit non-security messages in the current SI and the next SI.

To improve the utilization of the SCH, the CRE-MAC protocol allows each node to subscribe multiple times, which means that each node can transmit non-safety messages on the SCH multiple times during one SI.

At the start of each SI, each node copies all records of the next SCH/SerSlot in the NIL table to the current SCH/SerSlot and clears the next SCH/SerSlot record.

According to the current SCH/SerSlot, vehicles know when the neighbor nodes can perform WSA/RFS handshake on CCH, and meanwhile, the neighbor nodes can solve the problem of multi-channel hidden terminals and the problem of receiving end loss according to the current SCH/SerSlot.

As shown in table 1(b), the SUL stores SerSlots that can be used on each SCH in the next SI.

In a two-way WSA/RFS handshake, the sender selects the "best" SCH/SerSlot according to the following principle: p1) since the RSU broadcasts the CLI packet at the beginning of each SI, all nodes must switch to the CCH to receive the CLI packet. Therefore, after the RSU finishes broadcasting the CLI packet, all nodes can select SerSlots; p2) it cannot select the SerSlot including its SaSlot time on any SCH either because the node must broadcast the safety-related messages on the CCH at its own reserved SaSlot time during the CFI; p3) in order to avoid losing urgent messages on the CCH, a node is not allowed to continuously subscribe to the same SerSlot. This is because the urgent message may be repeatedly broadcast by the source node in consecutive SIs many times to alert the neighbor nodes; p4) to ensure load balancing of the SCHs, the node will select on the SCH that has the most SerSlot available each time. If there are two SCHs with the same number of available SerSlots, the transmitting side randomly selects one SCH.

When receiving an expected WSA or RFS message, if [ SCH, SerSlot ] is available, sending an Acknowledgement (ACK) message to a sending end, otherwise, sending a Non-Acknowledgement (NACK) message; if the neighbor node finds that the problem of a multi-channel hidden terminal or the problem of a lost receiving end occurs by monitoring the WSA/RFS message, the neighbor node auxiliary method is utilized to improve the channel utilization rate and the network performance.

Fig. 3(a) depicts the multi-channel hidden terminal problem: when one pair of nodes (called node pair A) executes non-safety message transmission on the SCH, and the other pair of nodes (called node pair B) simultaneously carries out SCH/SerSlot negotiation on the CCH, the node pair A does not know the negotiation information of the node pair B and selects the same SCH/SerSlot as the node pair B, so that transmission collision occurs, and the problem of multi-channel hidden terminals occurs. Node pair (V)₃,V₄) Communicating on SCH2, they do not listen to the node pair (V)₁,V₂) The channel negotiation information of (2). Therefore, when the node pair (V)₃,V₄) And node pair (V)₁,V₂) When the same SCH/SerSlot is selected for communication on SCH1, a collision occurs. The multi-channel hidden terminal problem can cause the generation of conflict on the SCH, thereby reducing the throughput of a service channel and increasing the transmission delay of non-safety messages.

Fig. 3(b) depicts the missing receiver problem: the lost receiver problem (also called the deaf-sub-receiver problem) means when a source node broadcasts a WSA/RFS packet to a specific receiver for channel negotiation, andthe receiving end is now absent either because of data transmission on the SCH with another node or because the network is unavailable. For example, as shown in FIG. 3(b), node V₃And V₄Performing non-secure message transmission on SCH while node V is₁Is in contact with node V₂The SCH/SerSlot is negotiated on the CCH. Node V₃Miss node V₁And node V₂Will send WSA/RFS to node V₁And node V at this time₁Is in contact with node V₂Performing non-secure message transmission on SCH, or node V₁Unreachable in the network (beyond node V)₃Or out of the RSU coverage), the receiver is lost. As occurs in the IEEE1609.4 protocol, in this case, the sender and other nodes, except the recipient, may have to wait a long time before acknowledging that the handshake was unsuccessful. Thus, this problem can lead to inefficient channel utilization and reduced network performance.

The method introduces the neighbor node auxiliary method to solve the problem of hiding the terminal in multiple channels and improve the network performance. When the neighbor node hears the WSA/RFS message, it is compared to the NIL. If the selected SCH/SerSlot in the WSA/RFS message is found to conflict with SCH/SerSlot of other nodes, at T_waitAfter the duration, the neighbor node sends the auxiliary information to the source node. The auxiliary information includes MAC _ ID, SCH, SerSlot of [ node ]]Wherein the MAC _ ID, SCH and SerSlot of the node respectively represent the MAC _ ID, SCH and SerSlot of the node with collision. T is_waitIs given by the following equation (9).

T_wait＝T_switch+XmodY_n(9)

Wherein T is_switchIndicating a radio switching delay between a transmission mode and a reception mode; x represents the MAC _ ID of the node; y is_nRepresenting the expansion factor of VANETs in the neighbor node assist method.

The multi-channel hidden terminal problem may be discovered by more than one neighbor node. Therefore, a collision may occur if multiple neighboring nodes transmit assistance information simultaneously. To reduce different neighbor nodes from having the same T_waitIntroducing { XmodY_nItem. { XmodY_nThe term functions like a Short InterFrame Space (SIFS). According to the provisions in the documents "IEEE Standard for information technology-local and statistical area-specific details-part 11: Wireless LAN Medium Access Control (MAC) and physical layer (PHY) specifications 6: Wireless Access technical overview environment", control packets such as ACK are transmitted after SIFS time during which normal packets cannot be transmitted. To be compatible with the regulations and save latency, we set Y in our analysis_n31. This means that, at the first T_switchThe neighboring node may then send the assistance information to the source node within the next 32 mus. Smaller or larger Y_nMay be suitable for VANETs on smaller or larger scales.

The source node updates its NIL based on the received assistance information. In the proposed CRE-MAC protocol, the destination node has to wait for T when receiving the WSA/RFS message_switchAdding Y_nCan use the self-selected [ SCH, SerSlot ]]To reply with an ACK or NACK. The method can relieve the problem of hiding the terminal in multiple channels and improve the network performance.

The problem of receiving end loss is solved by using a neighbor node auxiliary method: the neighbor node knows that the target node is communicating with another node on the SCH. At T_waitAfter the duration of (c), the neighbor node sends a message with the MAC _ ID of the destination node, SCH, SerSlot to the source node]The auxiliary information of (1). The source node that receives the assistance information updates its NIL and terminates the wait. Thus, other nodes may also terminate waiting and start their backoff processes.

The neighbor node hears the ACK message and updates its NIL and SUL.

In the next SI, both the transmitting end and the receiving end switch to the selected SCH at the SerSlot time to transmit non-secure messages.

The simulation platform employs a network simulator NS3, in which V2V and V2I communicate over an empirical rayleigh fading channel. Simulation scene bitOn a 6 km long road, there are 2 lanes in each direction, as shown in fig. 4. vehicles enter and exit the coverage area of the RSU. vehicles on each lane in each direction obey β vehicles/m (e 0.020.30)]) Poisson distribution. The vehicle speed is uniformly distributed in [80,120 ]]km/h and [60,100]km/h, variance 133. When the security-related packet size is 200 bytes and the data rate is 6Mbps, we set each SaSlot duration T in the CFI interval_SaSlotThe value is 0.4 ms. Each vehicle has a GPS and a radio WAVE communication device. All nodes are both service providers and service users. The vehicle nodes are in a saturated network environment, which means that each node has a WSA or RFS packet available after each successful execution of a reservation during a WSA interval. The simulation time was 2 minutes, and the final result was the average of each simulation result. We evaluated the proposed CRE-MAC protocol at different traffic flow densities to ensure scalability, reliability and efficiency. Table 2 lists the parameters used for the simulation.

Fig. 5 shows vehicle acknowledgement rates at different vehicle densities, speeds and RSU coverage per synchronization period. At the beginning, according to equation (1), the number of newly entering vehicles is small, so almost all entering vehicles can be confirmed by the RSU. RSU coverage is 500 meters when the vehicle density is less than or equal to 0.1 vehicle/meter, and 300 meters when the vehicle density is less than or equal to 0.18 vehicle/meter. In both cases, the vehicle confirmation rate is 100%. As the vehicle density further increases and exceeds a certain threshold, the rate of confirmation drops sharply. In this simulation, the synchronization period first ensures the transmission of the safety message and then increases the throughput of the SCH. Each SaSlot is 0.4ms in length and the synchronization period (100ms) can accommodate up to 250 confirmed vehicles in the CLI packet. When the vehicle density is 0.22(0.14) vehicle/meter and the RUS coverage is 300 meters (500 meters), 264 (280) vehicles exist in the RSU coverage. Thus, the RSU cannot allocate enough time slots in the CLI packet to all vehicles. Therefore, the rate of confirmation of the entering vehicle drops sharply. The variation in vehicle speed has little effect on the number of vehicles during the synchronization period. Thus, as shown in fig. 5, the acknowledgement rates are similar for different speeds of vehicles with the same RSU coverage.

Table 2: system simulation parameters

FIG. 6 shows the duration T of a vehicle confirmation of a new entry_VIIThe map changes as the vehicle density and the vehicle speed change. The coverage area of the RSU is 300 meters. According to equation (1), under the same vehicle density condition, a faster average vehicle speed means more vehicles enter, and finally more T is brought about_VII. However, T_VIIAre small. For various vehicle speeds, T_VIIThere is no obvious difference. For example, when the vehicle density is 0.30 vehicles/m, the average vehicle speed is 120(60) km/h, T_VIIIt is only 1.6(1.12) ms. This is because the CRE-MAC protocol employs a TDMA-based approach to transmitting security messages. During the vehicle validation interval, only newly entering vehicles compete and reserve SaSlots, and the number of newly entering vehicles during each synchronization cycle is small. On the other hand, the acknowledgement process is coordinated by the RSU and the acknowledgement duration is short. Once the specific SaSlots are assigned, the vehicles use them in the next synchronization cycle until they leave the coverage of the RSU. Short T_VIIMore time can be set aside for reserving more SerSlots and eventually the saturation throughput of the SCH can be improved.

Fig. 7 shows the process of the security-related packet delay as a function of the number of nodes under various protocols. The delay increases as the number of nodes increases. Since the CRE-MAC protocol employs contention-free transmission of TDMA for security-related messages, the delay is lower than using the IEEE1609.4 protocol based on contention transmission. For example, when the number of nodes is 80, the delays of IEEE1609.4 and CRE-MAC protocols are 208ms and 22ms, respectively. The latency of the IEEE1609.4 protocol is high. This is because, when the number of nodes is 80, a fixed 50ms is not enough to transmit all the secure data packets, and some secure data packets must wait 50ms (SCH interval) until the next CCH interval has an opportunity to be transmitted. In the CRE-MAC protocol, the time to transmit the security-related messages is less than the IEEE1609.4 protocol, since the coordination of the RSUs is relied upon without the need to exchange additional information.

Fig. 8 shows the probability of successful transmission of a safety-related message in relation to the safety interval. In the simulation, the vehicle speed was 80km/h, the vehicle density was 0.05 vehicle/m, the RSU coverage was 300m, and thus the number of vehicles was 60. It can be seen that the CRE-MAC protocol can successfully transmit security-related messages in case of a long security interval. Since the CRE-MAC protocol uses the TDMA mechanism to transmit the security related messages, the transmission time is less than using the 1609.4 protocol which transmits messages based on the contention mechanism. The 1609.4 protocol uses a fixed 50ms for transmitting security-related and WSA messages, so the transmission probability is low and remains constant after 50 ms. And because in the CRE-MAC protocol, no additional information needs to be exchanged due to the reliance on coordination of RSUs, less time is required for transmitting security-related messages. Thus, the CRE-MAC protocol may reserve more time for non-secure messages. In the CRE-MAC protocol, all security-related messages can be transmitted successfully when the security interval duration is 30 ms.

Figure 9 shows the effect of the number of nodes on the saturation throughput on SCHs in the CRE-MAC protocol. To improve utilization of the SCH, the CRE-MAC protocol allows each node to make multiple reservations and transmit non-secure data packets to the SCH during the synchronization period. Moreover, the CRE-MAC protocol further ensures high throughput performance by solving the multi-channel hidden terminal problem and the lost receiving end problem. It can be observed that throughput increases as the number of nodes increases, and then decreases as the number of nodes continues to increase. The reason is that, taking L ═ 2000Bytes as an example, when the number of nodes is less than or equal to 120, each node has a great chance to make a SerSlot reservation. However, due to the small number of nodes, at a specific time, a pair of nodes can only transmit data on a specific SCH, and a large number of SCHs are idle. As the number of nodes increases (the number of nodes is less than or equal to 120), more nodes transmit on the SCHs, the number of idle SCHs decreases, and thus throughput increases as the number of nodes increases. However, when the number of nodes becomes larger, since competition is intense, the time to reserve one SerSlot increases and the WSA interval is short, each node has little chance to make SerSlot reservation, and thus the probability of successful reservation decreases. The throughput of a heavily loaded packet is higher than that of a short loaded packet, since a packet with a large load L carries more data than a packet with a small load L.

Fig. 10 shows simulation results of the non-secure data packet delay performance varying with the number of nodes in the CRE-MAC protocol. As the number of nodes increases, the delay decreases and then increases. As the number of nodes increases, the increase in the security interval necessarily leads to a smaller WSA interval. For example, when the number of nodes is less than 120 and L is 1000 or 2000bytes, the length T of the WSA interval_WIAccounting for a major portion of packet delay. Furthermore, each node may perform at least one successful subscription and all successfully subscribed packets may be successfully transmitted on the SCH. Therefore, the transmission delay time of the data packet is 100ms or less. With further increases in the number of nodes, shorter WSA intervals result, resulting in more intense contention and fewer reservations. Nodes require more than one synchronization period to transmit data packets and thus the delay increases. For example, when the number of nodes is between 130 and 140, and L is 1000 or 2000bytes, the number of successful reservations of WSA packets is still more than the number of nodes, and thus the delay is still less than 100 ms. We can observe that different values of payload L can bring the same delay value. For example, when L3000 bytes and L4000 bytes, the delay when the number of nodes is between 10 and 60 has the same transmission delay. This is because each node can successfully subscribe at least once in one synchronization period and can successfully transmit a packet on the SCH when the next synchronization period comes. As the number of nodes increases, the transmission delay of a large-load packet is greater than that of a short load. For example, when the number of nodes is greater than 60, the transmission delay of L4000 bytes is greater than the transmission delay of L3000 bytes. This is because the number of successful reservations is the same for different loads, but with a larger number of successful reservationsThe transmission of payload packets results in less SerSlot being available on the SCH and more synchronization cycles are required to transmit the data. When the number of nodes reaches 160, the transmission of the data packets with loads of 1000 bytes, 2000bytes, 3000 bytes and 4000 bytes has the same delay. This is because the number of successful reservations is less than the number of nodes, and thus the nodes require several synchronization cycles to successfully reserve.

Fig. 11 shows simulation results of the variation of saturation throughput with the number of nodes for various protocols. As shown, in the IEEE1609.4 protocol, throughput decreases as the number of nodes increases. This is because the probability of transmission collision of data packets increases as the number of nodes increases, eventually resulting in less transmission time and opportunity for non-secure data packets. On the one hand, as can be seen from fig. 7, the proposed CRE-MAC protocol takes less time in the security interval than in the IEEE1609.4 protocol, and therefore leaves the nodes to perform SerSlot reservation and unsecured data packet transmission longer than in the IEEE1609.4 protocol. The CRE-MAC protocol shows better performance in terms of throughput than the IEEE1609.4 protocol. On the other hand, although the CRE-MAC protocol allocates some time to the forwarding nodes due to the forwarding mechanism, the CRE-MAC protocol still has higher throughput. This is because in the CRE-MAC protocol, the synchronization period contains only the guard interval and the WSA interval. Meanwhile, the CRE-MAC protocol supports data transmission in the whole synchronization period, and the channel utilization rate of the SCH is improved. For example, when L is 3000 bytes, the saturation throughput of the CRE-MAC protocol is improved by 231% compared to the IEEE1609.4 protocol.

Fig. 12 shows simulation results between the average delay of the non-secure data packets and the number of nodes under various protocols. With the IEEE1609.4 protocol, the transmission delay of a packet increases as the number of nodes increases. This is the use of a contention-based transmission mechanism for the SCH in the IEEE1609.4 protocol, where the collision probability of non-secure data packets increases as the number of nodes increases. The CRE-MAC protocol has better latency performance than the IEEE1609.4 protocol, and uses coordinated and contention-free packet transmission on the SCH than the IEEE1609.4 protocol. As the number of nodes increases, the transmission on the SCH is contention based when using the IEEE1609.4 protocol, and the contention becomes more and more intense, and therefore the delay becomes higher and higher.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and the present invention should be construed as being encompassed within the scope of the present invention by the following claims, along with the full scope of the present invention.

Claims (12)

1. A reliable and effective vehicle-mounted self-organizing network multichannel MAC protocol is provided, vehicles in the vehicle-mounted self-organizing network (vehicular Ad Hoc Networks, VANETs) all have the capability of wirelessly sending and receiving messages, and the method is characterized by comprising the following steps:

1) according to the network situation, the Road Side Unit (RSU) calculates the time T of the Vehicle Identification Interval (VII) in the next Synchronization period (SI)_VIIAnd put it into Coordination and Length Information (CLI) data packet; at the beginning of a SI, the RSU broadcasts a CLI packet to announce T_CFI，T_VII，T_WIAnd SaSlot of each confirmed vehicle, where T_CFIDenotes the length of a Contention-Free Interval (CFI), T_WIIndicating the length of the WSA interval, SaSlot indicating the time slot on the control channel for the acknowledged vehicle to broadcast the safety message; when there is no RSU, the node with the smallest MAC _ ID within a hop will act as a leader to perform the function of the RSU;

2) each vehicle broadcasts safety-related messages in its SaSlot: beacon or urgent message, without any collision, the forwarding node broadcasts the CLI data packet at the same time as the safety-related message;

3) during VII, the RSU performs a validation process for each newly entering vehicle to obtain a corresponding number of SaSlot, and according to the forwarding mechanism, the RSU allocates one SaSlot to each common node and three consecutive saslots to each forwarding node;

4) when a node has a non-secure message to send or request, it will select the "best" Service Channel (SCH) and the corresponding SerSlot on the Service Channel according to its SerSlot Usage List (SUL), and then, at WSA interval, use the CSMA/CA mechanism to contend for the control Channel for transmitting WSA or RFS messages;

5) when receiving an expected WSA or RFS message, if [ SCH, SerSlot ] is available, sending an Acknowledgement (ACK) message to a sending end, otherwise, sending a Non-Acknowledgement (NACK) message; if the neighbor node finds that the problem of a multi-channel hidden terminal or the problem of a receiving end loss occurs by monitoring the WSA/RFS message, the neighbor node auxiliary method is utilized to improve the channel utilization rate and the network performance;

6) the neighbor nodes hear the ACK message and update their Neighbor Information List (NIL) and SUL;

7) in the next SI, both the transmitting end and the receiving end switch to the selected SCH at the SerSlot time to transmit non-secure messages.

2. The MAC protocol of claim 1, wherein the Road Side Unit (RSU) in step 1) calculates the time T of Vehicle Identification Interval (VII) in the next Synchronization period (SI) according to the network condition_VII，T_VIIBy the formula T_VII＝L_total·T_rrts+m·T_cpIs calculated to obtain, wherein L_totalIndicates the total frame length, T_rrtsIndicating the time for transmitting a Reservation Request To Send (RRTS) packet, m indicating the number of rounds a node must go through before being acknowledged by the road side unit, T_cpIndicating the time to transmit a Coordination Packet (CP).

3. A reliable and effective vehicular ad hoc network multichannel MAC protocol according to claim 1, wherein said macid in step 1) is a short MAC identifier of a node; the MAC _ ID of a node is randomly selected by the node and included in a data packet transmitted on a control channel, and if the node detects that its selected MAC _ ID is already used by another node, it reselects.

4. The reliable and efficient vehicular ad hoc network multichannel MAC protocol according to claim 1, wherein the forwarding node in step 2) refers to: if there is a large truck within the RSU coverage, the large truck is selected as a forwarding node to forward the CLI packet.

5. The reliable and efficient vehicular ad hoc network multichannel MAC protocol according to claim 1, wherein the forwarding mechanism in step 3) works as follows:

51) selecting a forwarding node according to the method of claim 3;

52) using the formula

Calculating the number of forwarding nodes; wherein N represents the total number of nodes in the RSU coverage range; y is_fThe expansion factor of the VANETs in the forwarding mechanism is represented, the value of the expansion factor represents the scale of the VANETs, and the value is 30;

53) allocating three continuous SaSlots to the forwarding nodes; one SaSlot is used for transmitting the beacon of the node or the emergency message; two SaSlot are used to transmit one CLI packet.

6. A reliable and efficient vehicular ad hoc network multichannel MAC protocol as claimed in claim 1, wherein said step 4) selects the "best" Service Channel (SCH) and the corresponding SerSlot on the Service Channel according to its SerSlot Usage List (SUL); wherein the SerSlot represents the time slot for the node to send messages on the SCH; the SUL stores the SerSlots available on each SCH in the next SI; the specific method for selecting the "best" service channel SCH and the corresponding Serslot on the service channel is as follows:

61) on one hand, all nodes can select the SerSlot only after the RSU broadcasts the CLI data packet; on the other hand, the node cannot select the SerSlot containing the SaSlot time on any SCH;

62) one node is not allowed to continuously reserve the same SerSlot;

63) each time a node selects a SerSlot on the SCH that has the most SerSlot available; if there are two or more SCHs with the same number of available serslots, the transmitting side randomly selects one of the SCHs and selects the SerSlot thereon as required in method 61) and method 62).

7. The reliable and effective vehicular ad hoc network multichannel MAC protocol according to claim 1, wherein if the neighboring node in step 5) finds that a multichannel hidden terminal problem or a lost receiving terminal problem occurs by monitoring WSA/RFS messages, the multichannel hidden terminal problem is: when one pair of nodes (called node pair A) performs non-safety message transmission on the SCH, and the other pair of nodes (called node pair B) performs SCH/SerSlot negotiation on the CCH, the problem of multi-channel hidden terminals occurs; the problem of losing the receiving end is: when a source node broadcasts a WSA/RFS packet to a particular receiver for channel negotiation, the receiver is absent either because of data transmission on the SCH with another node or because the network is unavailable.

8. The reliable and effective vehicular ad hoc network multichannel MAC protocol according to claim 1, wherein in the step 5), the neighboring node auxiliary method is used to improve the channel utilization and the network performance, and the neighboring node auxiliary method is: when the neighbor node monitors the WSA/RFS message, comparing the WSA/RFS message with the NIL; if the selected SCH/SerSlot in the WSA/RFS message is found to conflict with SCH/SerSlot of other nodes, at T_waitOf (2) is continuedAfter the time, the neighbor node sends the auxiliary information to the source node.

9. The reliable and efficient vehicular ad hoc network multichannel MAC protocol as claimed in claim 8, wherein the NIL is SCH/SerSlot for storing the MAC _ ID of the neighbor node and transmitting non-safety message in the current SI and the next SI.

10. The reliable and efficient vehicular ad hoc network multichannel MAC protocol as claimed in claim 8, wherein said T is_waitBy the formula T_wait＝T_switch+X mod Y_nCalculating to obtain; wherein T is_switchIndicating a radio switching delay between a transmission mode and a reception mode; x represents the MAC _ ID of the node; y is_nAnd the expansion factor of the VANETs in the neighbor node auxiliary method is represented, the value of the expansion factor represents the scale of the VANETs, and the value is 31.

11. The reliable and efficient vehicular ad hoc network multichannel MAC protocol according to claim 8, wherein the auxiliary information includes [ node MAC _ ID, SCH, SerSlot ]; wherein the MAC _ ID of the node represents the MAC _ ID of the node with collision; SCH indicates the conflicting SCH; SerSlot denotes a SerSlot in which a collision occurs.

12. The MAC protocol of claim 1, wherein the step 6) updates their Neighbor Information List (NIL) and SUL by the following steps;

121) respectively finding MAC _ ID records of a sending node and a receiving node in an NIL table maintained by the self, and modifying an SCH/SerSlot value used for transmitting non-safety information in the corresponding next SI;

122) finding the SCH number negotiated by the sending node and the receiving node in the SUL table maintained by the sending node, and deleting the SerSlot negotiated by the sending node and the receiving node in the corresponding 'available SerSlot' field entry.

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