WO2016082283A1 - 一种基于双无线网卡的工业移动网络ap切换方法 - Google Patents

一种基于双无线网卡的工业移动网络ap切换方法 Download PDF

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WO2016082283A1
WO2016082283A1 PCT/CN2014/095571 CN2014095571W WO2016082283A1 WO 2016082283 A1 WO2016082283 A1 WO 2016082283A1 CN 2014095571 W CN2014095571 W CN 2014095571W WO 2016082283 A1 WO2016082283 A1 WO 2016082283A1
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handover
signal strength
switching
mobile terminal
wireless network
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PCT/CN2014/095571
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English (en)
French (fr)
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曾鹏
刘金娣
李栋
李志博
俞雪婷
于海斌
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中国科学院沈阳自动化研究所
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Priority to EP14906697.9A priority Critical patent/EP3226615B1/en
Priority to US15/527,633 priority patent/US10154445B2/en
Publication of WO2016082283A1 publication Critical patent/WO2016082283A1/zh

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/16—Performing reselection for specific purposes
    • H04W36/18—Performing reselection for specific purposes for allowing seamless reselection, e.g. soft reselection
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/0005—Control or signalling for completing the hand-off
    • H04W36/0083—Determination of parameters used for hand-off, e.g. generation or modification of neighbour cell lists
    • H04W36/00837—Determination of triggering parameters for hand-off
    • H04W36/008375—Determination of triggering parameters for hand-off based on historical data
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/08—Reselecting an access point
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/24—Reselection being triggered by specific parameters
    • H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/24—Reselection being triggered by specific parameters
    • H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
    • H04W36/302—Reselection being triggered by specific parameters by measured or perceived connection quality data due to low signal strength
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W84/00—Network topologies
    • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
    • H04W84/10—Small scale networks; Flat hierarchical networks
    • H04W84/12—WLAN [Wireless Local Area Networks]

Definitions

  • the invention relates to the field of AP handover in an industrial mobile network, in particular to an industrial mobile network AP handover method based on dual wireless network cards.
  • WiFi technology has been widely used due to its low cost and quick deployment.
  • Research on industrial control network based on WiFi has also been carried out.
  • the 802.11 standard is not clearly defined for the handover problem caused by the movement of the terminal node, but is implemented by each WiFi device manufacturer.
  • the latest 802.11r standard refines the technical specification of inter-AP handover, it only describes the handover procedure of the mobile terminal (MN) from disconnecting from the original AP to accessing the new AP. Due to the limited coverage of WiFi, in the mobile application field, end users will frequently switch from one AP to another AP. Therefore, how to implement fast autonomous inter-AP handover becomes the key to further development and application of WiFi.
  • the switches that make up the wired network infrastructure take time to learn the location of each MAC address and then determine the data exchange path.
  • the terminal needs to continuously switch between multiple WiFi nodes in the process of transmitting and receiving data by the mobile terminal.
  • the WiFi network connection of the mobile terminal has jumped, the data flow to the mobile terminal will still point to the wrong target address, and this will continue until the switch learns the new location of the mobile terminal, which will result in the end user.
  • the data flow continues to be interrupted.
  • the system needs to perform continuous scanning. When the speed increases, the switching time becomes a major problem.
  • the research results show that the switching delay between APs is at least several hundred ms, and more than 90% of the delay comes from the scanning phase, and the authentication and association consumes less than 10%. To minimize the scanning delay, the implementation can be achieved.
  • the AP switches quickly.
  • the present invention proposes an industrial mobile network AP switching method based on dual wireless network cards, adopting dual wireless network card hardware devices and a pre-learning mechanism to reduce the switching delay to meet the industrial network. The need for non-disruptive transmission.
  • Step 1 During the learning process, the mobile terminal stores the AP basic information and the switching sequence scanned on the moving track, and saves it to the storage unit of the mobile terminal;
  • Step 2 During the process of continuously moving the mobile terminal, the probe packet is continuously sent to the surroundings, and if the feedback information is received, it proves to enter the coverage of other APs;
  • Step 3 When scanning a new AP access point, the mobile terminal invokes the history storage information, determines which AP the next hop should switch to, and then performs signal strength scanning on the next hop AP to monitor the AP signal strength in real time, and Compared with the current AP signal strength;
  • Step 4 When the signal strength of the next hop AP is greater than the strength of the signal of the currently accessed AP, the handover algorithm is triggered, that is, the function of the dual wireless network card is exchanged by the bonding technology;
  • Step 5 Modify the routing table of the mobile terminal, and send a successful handover message to the controller.
  • Step 6 After receiving the successful handover message, the controller modifies the corresponding flow table and stores the information in the topology information of the controller.
  • Step 7 When the mobile terminal moves outside the coverage of the original AP, the dual network card of the terminal is in the state of redundant backup function, that is, only one network card is active, and when one is down, the other is immediately switched from the backup to the active state until scanning After the new AP, proceed to the next handover process.
  • the learning process records the basic information of the AP, including: the relative location of the AP, the SSID of the AP, the address of the AP, and the channel of the AP.
  • the timing of the switching in the step 4 is:
  • the fast-moving terminal node is located in the coverage of each AP for a short period of time.
  • the mobile terminal node scans a new AP, it enters the signal strength comparison phase when the NAP signal strength is greater than or equal to OAP. , then switch to connect to the next hop AP access point.
  • the handover includes three phases: a handover preparation phase, a handover phase, and a handover completion;
  • a new AP signal is found in the process of transmitting data between the MN and the OAP, and then the channel used by the NAP is queried, and the signal strength of the NAP is scanned for a specific channel and compared with the OAP signal strength in real time.
  • the MN sends an authentication and association message to the NAP.
  • the MN sends a handover request to the controller, and after the controller confirms the handover, the command to modify the flow table is delivered.
  • the AP node modifies the flow table, the switch is completed and the data can be transmitted normally.
  • the AP and the mobile terminal have multiple antenna combinations according to characteristics of different antennas, including an AP omnidirectional antenna/MN directional antenna, an AP directional antenna/MN omnidirectional antenna, and an AP directional antenna/MN directional antenna.
  • the mobile terminal uses a dual wireless network card.
  • the channels communicated between the mobile terminal and the neighboring APs do not overlap.
  • the invention has the following advantages and beneficial effects: the problem of AP switching based on the SDN industrial backhaul network is solved, and the fast and seamless switching between different access points is realized.
  • the invention adopts dual wireless network card mechanism and bonding technology for different network cards Assign different tasks and continuously switch the working status between the dual NICs for seamless soft switching.
  • the pre-learning mechanism Through the pre-learning mechanism, the switching target of the next hop is clarified, and the ping-pong effect in the switching process caused by reflection and refraction is avoided.
  • the present invention proposes a handover timing decision algorithm and an overlapless soft handover channel allocation mechanism for an industrial application scenario. These mechanisms can greatly reduce the switching delay and packet loss rate, and meet the application requirements of industrial scenarios.
  • FIG. 1 is a schematic diagram of an industrial backhaul network of the present invention
  • FIG. 2 is a schematic diagram of a handover decision algorithm according to the present invention.
  • FIG. 3 is a timing diagram of a handover process of the present invention.
  • Figure 5 is a flow chart of the handover of the present invention.
  • the invention realizes a non-disruptive switching method in an industrial mobile network architecture
  • the industrial mobile network architecture is an industrial backhaul network architecture based on SDN, and the architecture is as shown in FIG. 1 .
  • the industrial backhaul network based on SDN supports the openflow protocol, and realizes data storage and forwarding through a flow table.
  • the mobile terminal node is connected to the AP through the WiFi. Due to the limited coverage of the AP, the mobile terminal needs to continuously switch between the APs.
  • the present invention includes a pre-learning mechanism, a handover timing decision method, a non-repetitive channel allocation mechanism, and an overall handover procedure, which will be described in detail one by one.
  • the pre-learning mechanism proposed by the present invention stores the basic information of the AP, the switching sequence of the terminal, and the like in the terminal mobile terminal for the basic fixed feature of the industrial application scenario. Scanning for a specific handover target AP, obtaining the signal strength of the next hop access AP, and preparing for the handover decision algorithm.
  • the pre-learning process refers to the testing process performed on the trajectory before the mobile terminal is not officially working. Due to the characteristics of the industrial scenario targeted by the application, the AP location is relatively fixed and the channel used by the AP is also fixed.
  • the learning process mainly focuses on recording the basic information of the AP, including: the relative location of the AP, the SSID of the AP, the address of the AP, and the channel of the AP.
  • the AP information is saved and maintained by the terminal MN.
  • the information format is as follows:
  • the invention proposes a handover timing decision algorithm and studies the trigger timing of the handover.
  • the proposed handover timing decision algorithm is based on the active scanning method, taking into account the change of the received signal strength of the mobile terminal and the AP, and the history information and current mobility of the handover of the mobile terminal.
  • the terminal receives the signal strength of the AP, which ensures that the handover is completed in time and reduces the ping-pong effect in the handover process.
  • the invention is based on a soft handover method of a dual wireless network card, and therefore needs to select a suitable soft handover channel allocation mechanism.
  • a non-overlapping channel allocation mechanism is adopted. Since both the AP and MN RF modules can select omnidirectional antennas and directional antennas, various combinations are possible depending on the characteristics of different antennas. Including AP omnidirectional antenna/MN directional antenna, AP directional antenna/MN omnidirectional antenna, AP directional antenna/MN directional antenna.
  • the first two channel allocation mechanisms have requirements for the relative positions of the two wireless network cards on the MN. When the moving speed of the mobile terminal increases, the spacing of the dual network cards is also required to increase. Therefore, the first two channel allocation mechanisms are not suitable for the case where the MN's cell volume is small.
  • the AP directional antenna/MN directional antenna mechanism can be used in applications where the MN is small in size and the spacing between the two antennas is small.
  • the fast-moving terminal node Due to the limited coverage of the AP, usually only a few hundred meters, the fast-moving terminal node is located in the coverage of each AP for a short time.
  • OAP current access point
  • the switch timing decision is to study when the mobile terminal triggers the handover, which is most beneficial for fast handover.
  • the handover starts too late, that is, the MN starts to switch when the distance from the OAP is too large, the OAP may not be able to provide a reliable wireless connection to the MN before the handover is completed. At this time, the system will be disconnected, the handover fails, and data loss occurs. serious.
  • the handover is not started as early as possible. If the NAP signal is not strong enough when attempting to switch, the NAP cannot be determined, and the neighbor AP discovery needs to be repeated, which takes a lot of time and increases the handover delay. Therefore, choosing the appropriate switching opportunity is of great significance for successfully completing the handover and reducing the handover delay.
  • Figure 2 shows the decision of the handover timing decision.
  • FIG. 3 shows the process by which the MN switches between OAP and NAP.
  • the switch consists of three phases: the switch preparation phase, the switch phase, and the switch completion.
  • the NAP is clear, and the MN and the external network are always in a connected state during the terminal process, so that ping-pong switching and connection interruption do not occur.
  • the invention adopts the signal strength comparison method to perform the handover decision, and when the signal strength of the NAP is greater than the OAP, the handover is decisively performed.
  • this method is simple, the application scenario algorithm considered for the present invention is highly efficient and does not affect the switching effect.
  • the non-overlapping channel allocation mechanism is adopted in the solution to reduce the number of scanning channels and thus reduce the handover delay.
  • This solution adopts dual-link soft handover. Therefore, two wireless network cards need to be provided on the MN to provide two wireless links.
  • the AP can be a standard device supporting the IEEE802.11 protocol.
  • both the AP and MN RF modules can select omnidirectional antennas and directional antennas, various combinations are possible depending on the characteristics of different antennas. Including AP omnidirectional antenna/MN directional antenna, AP directional antenna/MN omnidirectional antenna, AP directional antenna /MN directional antenna.
  • the first two channel allocation mechanisms have requirements for the relative positions of the two wireless network cards on the MN, that is, there must be enough spacing to complete one handover.
  • the fast switching algorithm When the MN moves at a speed of 40 km/h, the fast switching algorithm is adopted, the switching delay is about 200 milliseconds, and the MN travels about 2 meters during the switching, that is, the distance between the two antennas is required to be at least 2 meters; when the motion of the MN When the speed is increased, the distance between the two network cards is required to be larger. Therefore, the above two channel allocation mechanisms are not suitable for the case where the MN's cell volume is small.
  • the AP directional antenna/MN directional antenna mechanism can be used in applications where the MN is small in size and the spacing between the two antennas is small.
  • the directional antenna of the AP in Figure 4 adopts different directions: AP1, AP3, AP5, ... AP2n+1 directional antenna orientation is consistent with the direction of motion of the MN, and its signal coverage area is indicated by a solid line, using channel 1; AP2
  • the directional antennas of AP4, AP6, ..., AP2n are opposite to the direction of motion of the MN, and the signal coverage area is indicated by a broken line, using channel 6.
  • the distance between adjacent APs is substantially equal, and the coverage areas between APs with the same antenna direction do not overlap, and the coverage areas between adjacent APs with different antennas partially overlap.
  • the antenna direction of the MN wireless network card N1 is the same as the direction of its movement, and the antenna direction of the N2 is opposite to the moving direction. It is required that N1 can only be connected with AP2n (n is a positive integer), and N2 can only be connected with AP2n-1 (n is a positive integer). At this time, the switching between N1 and N2 is not completed automatically, but needs to be controlled.
  • N1 When the MN is in the picture position, N1 is connected to AP4, and N2 is connected to AP1.
  • the switching process is as follows:
  • N2 moves out of the signal coverage of AP1, enters the signal coverage of AP3, and N2 switches from AP1 to AP3, and the handover process is completed during the shadowed portion of the MN through the vertical line in the figure.
  • the signal of AP4 is in the enhancement, and N1 remains connected to AP4.
  • N1 moves out of the signal coverage of AP4, enters the signal coverage of AP6, and N1 switches from AP4 to AP6, and the handover process is completed during the shadowed portion of the MN through the horizontal line in the figure. At this time, the signal strength of AP3 is sufficient to maintain the connection with N2.
  • Step 1 During the learning process, the mobile terminal stores the basic AP information and the switching sequence scanned on the moving track, and saves it in a specific storage unit, so as to prepare for the call in the formal working process;
  • Step 2 The mobile terminal keeps moving, adopts the active scanning method, and continuously sends the probe packet to the surroundings. If the feedback information is received, it proves to enter the coverage of other APs;
  • Step 3 When scanning to a new AP access point, the mobile terminal invokes the history storage information to determine which AP the next hop should switch to, and then performs signal strength scanning for the specific AP, and monitors the AP signal strength in real time, and the current AP signal strength comparison;
  • Step 4 When the signal strength of the next hop AP is greater than the strength of the signal of the currently accessed AP, the handover algorithm is triggered, that is, the function of the dual wireless network card is exchanged by the bonding technology;
  • Step 5 Modify the routing table of the terminal node, and send a successful handover message to the controller.
  • Step 6 After receiving the successful handover message, the controller modifies the corresponding flow table and stores the information in the topology information of the controller.
  • Step 7 When the mobile terminal moves outside the coverage of the original AP, the dual network card of the terminal is in the state of redundant backup function, that is, only one network card is active, and when one is down, the other is immediately switched from the backup to the active state until scanning After the new AP, proceed to the next handover process.

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  • Computer Networks & Wireless Communication (AREA)
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Abstract

本发明提供了一种基于双无线网卡的AP切换方法，由移动终端主动扫描AP信号强度，然后将扫描到的信号强度与当前接入AP的信号强度进行比较，如果信号强度大于当前AP信号强度，则开始切换，否则继续扫描直至成功切换为止。为了降低切换时延，针对工业应用场景中移动路线基本固定的特点，本发明提出了预学习机制，即在正式应用前，对应用场景的AP位置、切换顺序等信息进行学习，并存储到终端节点，以便正式应用中进行有针对性的扫描和切换。本发明解决了工业移动网络场景下的AP快速切换问题；采用双无线网卡实现无中断切换；针对特定的工业应用场景，采用预学习机制，进一步降低扫描时延。

Description

一种基于双无线网卡的工业移动网络AP切换方法 技术领域
本发明涉及工业移动网络的AP切换领域，具体的说是一种基于双无线网卡的工业移动网络AP切换方法。
背景技术
随着控制、通信、计算机、网络等技术的飞速发展，企业的信息集成技术得到快速发展，涵盖从工厂的现场控制到产品的存储运输过程中。工业网络在提高生产速度、管理生产过程、合理高效加工等领域起到越来越关键的作用。近年来，以太网进入工业控制领域，出现了大量基于以太网的工业控制网络。同时，随着无线技术的发展，基于无线的工业控制网络逐渐兴起。
WiFi技术因其成本低廉、部署快捷等优点而得到广泛应用，基于WiFi的工控网络研究也已经开展。针对终端节点移动而造成的切换问题，802.11标准并没有做出明确规定，而由各WiFi设备厂家自行实现。虽然最新802.11r标准细化AP间切换的技术规范，但它仅描述移动终端(MN)从断开与原AP连接到接入新AP的切换过程。由于WiFi覆盖范围有限，在移动应用领域中，终端用户将频繁地由一个AP切换到另一个AP，因此如何实现快速自主的AP间切换成为WiFi进一步发展和应用的关键。
当数据包在网络中传输的时候，组成有线网络基础构架的交换机需要时间来学习每个MAC地址的位置，然后决定数据的交换路径。在快速移动的应用场景下，移动终端发送和接收数据的过程中，终端需要在多个WiFi节点之间不断的快速切换。当移动终端的WiFi网络连接已经跳转的时候，去往移动终端的数据流仍将指向错误的目标地址，这种情况会一直持续直到交换机学习到该移动终端新的位置，这将导致最终用户的数据流持续中断。为了保证移动终端在WiFi节点之间的及时切换，系统需要进行持续的扫描。当速度增加的时候，切换时间就会成为一个主要的问题。研究结果表明，AP间切换延迟至少在数百ms以上，并且90％以上的延迟来源于扫描阶段，而认证和关联所消耗的时间还不到10％，为此尽量降低扫描时延才能给实现AP快速切换。
随着WiFi移动应用研究的不断深入，自主AP间切换吸引越来越多的关注，但是研究发现WiFi还不能实现快速自主的AP间切换。
发明内容
针对基于WiFi的工业网络存在的切换问题，本发明提出了一种基于双无线网卡的工业移动网络AP切换方法，采用双无线网卡硬件设备和预学习机制来降低切换时延，以满足工业网络的无中断传输的需求。
本发明为实现上述目的所采用的技术方案是：一种基于双无线网卡的工业移动网络AP切换方法，包括以下步骤：
步骤1：在学习过程中，移动终端将移动轨迹上扫描到的AP基本信息及切换顺序进行存储，并保存到移动终端的存储单元中；
步骤2：在移动终端不断移动的过程中，不断向周围发送探测包，如果收到反馈信息，则证明进入其他AP覆盖范围；
步骤3：当扫描到新的AP接入点时，移动终端调用历史存储信息，判断下一跳应该切换到哪个AP，然后针对下一跳AP进行信号强度扫描，实时监测该AP信号强度，并与当前AP信号强度比较；
步骤4：当下一跳AP信号强度大于当前接入的AP的信号的强度时，则触发切换算法，即通过bonding技术将双无线网卡的功能进行调换；
步骤5：修改移动终端的路由表，并给控制器发送成功切换消息；
步骤6：控制器接收到成功切换消息后，修改相应的流表，并将信息存储到控制器的拓扑信息中；
步骤7：当移动终端移动到原AP覆盖范围外，终端的双网卡处于冗余备份功能状态，即只有一个网卡处于活动，当一个down机后另一个马上由备份转为活动状态，直到扫描到新的AP后，进入下一次切换过程。
所述学习过程记录AP的基本信息包括：AP相对位置，AP的SSID，AP的地址，AP的信道。
所述步骤4中切换的时机为：
由于AP覆盖范围有限，因此快速移动的终端节点位于每个AP覆盖范围内的时间很短，当移动终端节点扫描到新的AP后，则进入信号强度比较阶段，当NAP信号强度大于等于OAP时，则进行切换，连接到下一跳AP接入点。
所述切换包含了三个阶段：切换准备阶段、切换阶段、切换完成；
切换准备阶段是MN与OAP传输数据的过程中发现新的AP信号，然后查询NAP使用的信道，并针对特定信道对NAP的信号强度进行扫描，并与OAP信号强度进行实时比较。切换阶段是MN扫描到NAP的信号强度大于等于OAP时，MN向NAP发送认证、关联消息，成功接受响应消息后，MN向控制器发送切换请求，控制器确认切换后，下发修改流表命令；响应AP节点修改完流表后则切换完成，可以正常传输数据。
所述AP和移动终端根据不同天线的特性有多种天线组合，包括AP全向天线/MN定向天线、AP定向天线/MN全向天线、AP定向天线/MN定向天线。
所述移动终端采用双无线网卡。
所述移动终端与相邻AP之间通信的信道不重叠。
本发明具有以下优点及有益效果：解决了基于SDN工业回程网的AP切换问题，实现了不同接入点间的快速无缝切换。本发明采用双无线网卡机制和bonding技术，为不同的网卡 分配不同的任务，并不断切换双网卡间的工作状态，以实现无缝软切换。通过预学习机制，明确了下一跳的切换目标，避免了由反射折射等造成切换过程中的乒乓效应。另外，本发明提出了针对工业应用场景的切换时机决策算法和无重叠软切换信道分配机制。这些机制能够极大的降低切换时延和丢包率，满足工业场景的应用需求。
附图说明
图1为本发明的工业回程网示意图；
图2为本发明的切换决策算法示意图；
图3为本发明的切换过程时序图；
图4为本发明的无重叠信号分配示意图；
图5为本发明的切换流程图。
具体实施方式
下面结合附图及实施例对本发明做进一步的详细说明。
本发明在工业移动网络体系架构中实现无中断切换方法，该工业移动网络架构是基于SDN的工业回程网架构，体系结构如图1所示。基于SDN的工业回程网支持openflow协议，通过流表实现数据的存储转发。移动终端节点通过WiFi与AP相连，由于AP覆盖范围有限，移动终端需要不断的在各个AP间进行切换。
本发明包括预学习机制、切换时机决策方法、无重复信道分配机制和整体切换流程等，接下来将逐一详细记述。
1.预学习机制
本发明提出的预学习机制，针对工业应用场景的移动路线基本固定的特点，将AP的基本信息，终端的切换顺序等存储到终端移动终端中。针对特定的切换目标AP进行扫描，获取下一跳接入AP的信号强度，为切换决策算法做准备。
预学习过程指的是移动终端未正式工作前，在轨迹上进行的测试过程。由于该应用针对的工业场景的特点，AP位置相对固定，AP使用的信道也固定。学习过程主要是侧重于记录AP的基本信息，主要包括：AP相对位置，AP的SSID，AP的地址，AP的信道。AP信息由终端MN保存维护。信息格式如下：
编号 SSID IPaddress MACaddress channel NextAP
1 AP1 192.168.2.4 8C-89-A5-C1-9D-5A 1 AP2
2 AP2 192.168.2.6 8C-89-A5-D6-3C-7B 6 AP3
3 AP3 192.168.2.8 8C-89-A5-B2-2A-4D 1 AP4
实际切换过程中通过对MN(移动终端)中保存的AP信息进行利用，可以很快的发现 切换目标AP，如此可以进一步降低切换时延。实际工作中切换和数据传输是通过两个并行的天线实现的，具体实现方法会在下文中详细记述。
2.切换时机决策机制
本发明提出了切换时机决策算法，研究切换的触发时机，提出的切换时机决策算法是基于主动扫描方法的，考虑到移动终端与AP接收信号强度的变化，以及移动终端切换的历史信息和当前移动终端接收到AP的信号强度，既可以确保切换及时完成，同时减少切换过程中的乒乓效应。
本发明是基于双无线网卡的软切换方法，因此需要选择合适的软切换信道分配机制，本发明中采用无重叠信道分配机制。由于AP和MN的射频模块都可以选择全向天线和定向天线，根据不同天线的特性，可以有多种组合。包括AP全向天线/MN定向天线、AP定向天线/MN全向天线、AP定向天线/MN定向天线。前两种信道分配机制方式对于MN上两块无线网卡的相对位置都有要求，当移动终端运动速度增大时，要求双网卡的间距也增大。因此，前两种信道分配机制不适合MN的单体体积较小的情况。AP定向天线/MN定向天线机制可以用于MN体积较小、两天线间距很小的场合。
由于AP的覆盖范围有限，通常只有几百米，快速移动的终端节点位于每个AP覆盖范围内的时间很短，当MN离开当前接入点(Old AP,OAP)一定距离时，就需要进入切换状态，并且在彻底离开OAP的覆盖范围前完成切换，连接到下一跳接入点(Next AP,NAP)。切换时机决策就是研究移动终端何时触发切换最有利于实现快速切换。
若切换开始过晚，即MN离开OAP的距离过大时才开始切换，则在切换完成之前，OAP可能已经不能为MN提供可靠的无线连接，此时系统会断连，切换失败，造成数据丢失严重。切换也并非越早开始越好，如果在试图切换时，NAP信号还不足够强，不能确定NAP，需要重复进行邻居AP发现，花费大量的时间，也会增加切换延时。因此，选择合适的切换时机对于成功完成切换并降低切换时延有着重要意义，附图2给出切换时机决策示意图。
附图3给出了MN在OAP与NAP间进行切换的过程。切换包含了三个阶段：切换准备阶段、切换阶段、切换完成。鉴于本发明采用了预学习机制和双网卡硬件设备，NAP是明确的，且终端过程中MN与外网始终处于连接状态，因此不会出现乒乓切换和连接中断的情况。本发明采用信号强度比较法进行切换决策，当NAP的信号强度大于OAP时则果断进行切换。这种方法虽然简单，但是针对本发明所考虑的应用场景算法效率很高，并且不会影响切换效果。
3.无重叠信道分配机制
鉴于切换时延主要由扫描时延构成，本方案中采用无重叠信道分配机制，减少扫描信道数，进而减少切换时延。本方案采用双链路软切换，因此MN上需要配备两块无线网卡以提供两条无线链路，AP可以是支持IEEE802.11协议的标准设备。
由于AP和MN的射频模块都可以选择全向天线和定向天线，根据不同天线的特性，可以有多种组合。包括AP全向天线/MN定向天线、AP定向天线/MN全向天线、AP定向天线 /MN定向天线。前两种信道分配机制方式对于MN上两块无线网卡的相对位置都有要求，即必须有足够完成一次切换的间距。当MN以40km/h的时速运动时，采用快速切换算法，一次切换时延约为200毫秒，在切换期间MN行进约2米，即要求两天线间的距离至少为2米；当MN的运动时速增大时，要求双网卡的间距更大。因此，上述两种信道分配机制不适合MN的单体体积较小的情况。AP定向天线/MN定向天线机制可以用于MN体积较小、两天线间距很小的场合。
附图4中AP的定向天线采用了不同的方向：AP1、AP3、AP5……AP2n+1的定向天线朝向与MN的运动方向一致，其信号覆盖区域用实线表示，采用信道1；AP2、AP4、AP6……AP2n的定向天线朝向与MN的运动方向相反，其信号覆盖区域用虚线表示，采用信道6。相邻AP间的距离基本相等，天线方向相同的AP之间覆盖区域不重叠，而相邻且天线不同向的AP之间覆盖区域部分重叠。
MN的无线网卡N1的天线方向与其移动的方向一致，N2的天线方向与其移动方向相反。要求N1只能与AP2n(n是正整数)相连接，N2只能与AP2n-1(n是正整数)相连接，此时N1与N2的切换不是自动检测完成，而需要进行控制。
当MN在如图位置时，N1与AP4连接，N2与AP1连接。当MN继续向图示方向移动时，切换过程如下：
1)当MN到达AP3所在的位置时，N2移出AP1的信号覆盖范围，进入AP3的信号覆盖范围，N2从AP1切换到AP3，切换过程在MN经过图中竖线的阴影部分期间完成。此时，AP4的信号在增强中，N1保持与AP4的连接。
2)当MN到达AP4所在的位置时，N1移出AP4的信号覆盖范围，进入AP6的信号覆盖范围，N1从AP4切换到AP6，切换过程在MN经过图中横线的阴影部分期间完成。此时，AP3的信号强度足够保持与N2的连接。
4.切换步骤：如图5所示。
步骤1：学习过程中，移动终端将移动轨迹上扫描到的AP基本信息及切换顺序进行存储，并保存到特定的存储单元中，以备正式工作过程中的调用；
步骤2：移动终端不断移动，采用主动扫描方法，不断向周围发送探测包，如果收到反馈信息，则证明进入其他AP覆盖范围；
步骤3：当扫描到新的AP接入点时，移动终端调用历史存储信息，判断下一跳应该切换到哪个AP，然后针对特定AP进行信号强度扫描，实时监测该AP信号强度，并与当前AP信号强度比较；
步骤4：当下一跳AP信号强度大于当前接入的AP的信号的强度时，则触发切换算法，即通过bonding技术将双无线网卡的功能进行调换；
步骤5：修改终端节点的路由表，并给控制器发送成功切换消息；
步骤6：控制器接收到成功切换消息后，修改相应的流表，并将信息存储到控制器的拓扑信息中；
步骤7：当移动终端移动到原AP覆盖范围外，终端的双网卡处于冗余备份功能状态，即只有一个网卡处于活动，当一个down机后另一个马上由备份转为活动状态，直到扫描到新的AP后，进入下一次切换过程。

Claims (7)

  1. 一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，包括以下步骤：
    步骤1：在学习过程中，移动终端将移动轨迹上扫描到的AP基本信息及切换顺序进行存储，并保存到移动终端的存储单元中；
    步骤2：在移动终端不断移动的过程中，不断向周围发送探测包，如果收到反馈信息，则证明进入其他AP覆盖范围；
    步骤3：当扫描到新的AP接入点时，移动终端调用历史存储信息，判断下一跳应该切换到哪个AP，然后针对下一跳AP进行信号强度扫描，实时监测该AP信号强度，并与当前AP信号强度比较；
    步骤4：当下一跳AP信号强度大于当前接入的AP的信号的强度时，则触发切换算法，即通过bonding技术将双无线网卡的功能进行调换；
    步骤5：修改移动终端的路由表，并给控制器发送成功切换消息；
    步骤6：控制器接收到成功切换消息后，修改相应的流表，并将信息存储到控制器的拓扑信息中；
    步骤7：当移动终端移动到原AP覆盖范围外，终端的双网卡处于冗余备份功能状态，即只有一个网卡处于活动，当一个down机后另一个马上由备份转为活动状态，直到扫描到新的AP后，进入下一次切换过程。
  2. 根据权利要求1所述的一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，所述学习过程记录AP的基本信息包括：AP相对位置，AP的SSID，AP的地址，AP的信道。
  3. 根据权利要求1所述的一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，所述步骤4中切换的时机为：
    由于AP覆盖范围有限，因此快速移动的终端节点位于每个AP覆盖范围内的时间很短，当移动终端节点扫描到新的AP后，则进入信号强度比较阶段，当NAP信号强度大于等于OAP时，则进行切换，连接到下一跳AP接入点。
  4. 根据权利要求1所述的一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，所述切换包含了三个阶段：切换准备阶段、切换阶段、切换完成；
    切换准备阶段是MN与OAP传输数据的过程中发现新的AP信号，然后查询NAP使用的信道，并针对特定信道对NAP的信号强度进行扫描，并与OAP信号强度进行实时比较。切换阶段是MN扫描到NAP的信号强度大于等于OAP时，MN向NAP发送认证、关联消息，成功接受响应消息后，MN向控制器发送切换请求，控制器确认切换后，下发修改流表命令；响应AP节点修改完流表后则切换完成，可以正常传输数据。
  5. 根据权利要求1所述的一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，所述AP和移动终端根据不同天线的特性有多种天线组合，包括AP全向天线/MN定向天线、AP定向天线/MN全向天线、AP定向天线/MN定向天线。
  6. 根据权利要求1所述的一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，所述移动终端采用双无线网卡。
  7. 根据权利要求1所述的一种基于双无线网卡的工业移动网络AP切换方法，其特征在于，所述移动终端与相邻AP之间通信的信道不重叠。
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EP3226615B1 (en) 2020-01-22
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US20170339615A1 (en) 2017-11-23
US10154445B2 (en) 2018-12-11
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