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毕业论文网 > 外文翻译 > 电子信息类 > 电子科学与技术 > 正文

蓝牙MESH标准:概述和实验评价外文翻译资料

 2022-08-08 19:28:32  

The Bluetooth Mesh Standard: An Overview and

Experimental Evaluation

Mathias Baert *, Jen Rossey, Adnan Shahid and Jeroen Hoebeke

Department of Information Technology, Ghent University/imec, Technologiepark-Zwijnaarde 15, 9052 Ghent, Belgium; jen.rossey@ugent.be (J.R.); adnan.shahid@ugent.be (A.S.); jeroen.hoebeke@ugent.be (J.H.) * Correspondence: mathias.baert@ugent.be

Received: 28 June 2018; Accepted: 20 July 2018; Published: 25 July 2018

Abstract: Mesh networks enable a many-to-many relation between nodes, which means that each node in the network can communicate with every other node using multi-hop communication and path diversity. As it enables the fast roll-out of sensor and actuator networks, it is an important aspect within the Internet of Things (IoT). Utilizing Bluetooth Low Energy (BLE) as an underlying technology to implement such mesh networks has gained a lot of interest in recent years. The result was a variety of BLE meshing solutions that were not interoperable because of the lack of a common standard. This has changed recently with the advent of the Bluetooth Mesh standard. However, a detailed overview of how this standard operates, performs and how it tackles other issues concerning BLE mesh networking is missing. Therefore, this paper investigates this new technology thoroughly and evaluates its performance by means of three approaches, namely an experimental evaluation, a statistical approach and a graph-based simulation model, which can be used as the basis for future research. Apart from showing that consistent results are achieved by means of all three approaches,

we also identify possible drawbacks and open issues that need to be dealt with.

Keywords: BLE; Bluetooth Mesh; round-trip-time; IoT; scalability; performance; interoperability

1. Introduction

Bluetooth Low Energy (BLE) first appeared in the Bluetooth 4.0 standard [1], which made a clear difference between Classic Bluetooth and the Low Energy variant (also labeled Bluetooth Smart). Low-power wireless communication scenarios could use this new technology to satisfy the necessities of communication while saving more power. In recent years, BLE has profiled itself as one of the leading technologies for the Internet of Things [2] and has been implemented as a household feature in current smartphones, tablets, laptops, etc. This way, users could immediately interact with these BLE products without having to purchase an additional gateway. With each new version of the technology, the achievable throughput, range and energy efficiency have improved significantly [3].

However, the whole Bluetooth Low Energy design was focused on these phone-device interactions, resulting in typical point-to-point, star-based network topologies, with the phone being the central of the network. This approach did not have the mesh capabilities offered by its low-power competitors such as ZigBee and Thread, both running on top of IEEE 802.15.4 in the 2.4 GHz band. Meshing enables a topology where each node in the network can talk to every other node, directly or via multi-hop communication. Such a network is self-organizing, self-healing and enables path diversity.

Consequently, several attempts were made to add meshing capabilities to BLE, resulting in different techniques, which are discussed in detail in [4]. This survey states that there are still some open issues regarding BLE mesh networking. One of them is the interoperability between the different implementations, an issue that can only be resolved when BLE meshing is standardized. This is exactly

what happened recently, when the Bluetooth SIG group released a mesh network specification based on BLE by making use of its advertising capabilities [5].

Sensors2018, 18, 2409; doi:10.3390/s18082409 www.mdpi.com/journal/sensors

As such, it is relevant to find out how this new standard operates, tackles the other open issues identified in [4] and handles the disadvantages of using the advertising capability of BLE as a basis for communication. This is exactly the key contribution of this paper. Apart from giving the reader an overview on how the BLE meshing specification operates, insights are given in its performance by means of three approaches: an experimental evaluation, a statistical approach and using a theoretical model.

The rest of the paper is organized as follows. In Sections 2 and 3, we introduce the reader to Bluetooth Low Energy itself as well as the new Bluetooth Mesh standard. Section 4 briefly describes related work. In Section 5, we first take a statistical approach to calculate the round-trip-time (RTT),

which is further used to validate our experiments presented in Section 6. This section also contains an overview of the used hardware and measuring environment. The experiments focus on the RTT as a metric. Section 7 introduces a theoretical model for the Bluetooth Mesh standard. The model is validated through the experiments in the previous section. Finally, we give some concluding remarks and discuss open issues in Section 8.

2. Background on Bluetooth Low Energy

As stated in Section 1, BLE is the low-power variant of Classic Bluetooth. It operates in the same 2.4 GHz ISM band and uses frequencies between 2402 and 2480 MHz. The used spectrum is divided into several channels, but, unlike Classic Bluetooth, BLE uses channels of 2 MHz spacing, which leads to 40 usable channels. This is shown in Figure 1. These 40 channels are divided into three primary advertisement channels (37, 38 and 39), avoiding the main channels used by Wi-Fi, and 37 connection oriented channels. BLE offers t

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蓝牙MESH标准:概述和实验评价

Mathias Baert *, Jen Rossey, Adnan Shahid and Jeroen Hoebeke

Department of Information Technology, Ghent University/imec, Technologiepark-Zwijnaarde 15, 9052 Ghent, Belgium; jen.rossey@ugent.be (J.R.); adnan.shahid@ugent.be (A.S.); Jeroen.hoebeke@ugent.be (J.H.) * Correspondence: mathias.baert@ugent.be

收到日期: 2018年6月28日;审核日期: 2018年7月20日;发布时间: 2018 年 7 月 25 日

摘要:Mesh网络支持节点之间的多对多通信,这意味着网络中的每个节点可以使用多跳通信和多个路径与所有其他节点进行通信。由于它支持传感网络和执行网络的快速传输,因此它是物联网 (IoT) 中的一个重要组成部分。近年来,利用蓝牙低功耗(BLE)作为实施此类Mesh网络的基础技术,引发了人们的兴趣,但是由于缺乏通用标准,各种 BLE Mesh解决方案无法相互兼容。近年来随着蓝牙Mesh统一标准的出现,这种情况发生了变化。但是目前并没有此标准如何运行、执行以及如何解决有关 BLE Mesh网络的其他问题的详细描述。因此,本文对这项新技术进行了深入的研究,并通过实验评价、统计方法和基于图形的仿真模型三种方法对其性能进行了评价,作为今后研究的基础。除了表明通过所有三种方法可以取得一致的结果外,我们还找出了一些需要解决的潜在缺点和一些未解决的问题。

关键词: 低功耗蓝牙;蓝牙Mesh;往返时延;物联网;可扩展性;性能;兼容性

简介

蓝牙低功耗 (BLE) 首次出现在蓝牙 4.0 标准 [1]中,它与经典蓝牙和低能耗变体(也称为蓝牙智能)之间存在明显的差异。低功耗无线通信方案可以利用这项新技术满足通信需求,同时节省更多电力。近年来,BLE 是物联网的领先技术之一 [2],并已作为家用功能在当前的智能手机、平板电脑、笔记本电脑等设备中应用。这样,用户就可以利用这些 BLE 产品进行即时交互,而无需购买额外的网关。得益于蓝牙新版本的提供了更优异的技术,蓝牙设备可实现的吞吐量,通信范围和能源效率已显著提高 [3]。

然而,整个蓝牙低功耗设计侧重于手机终端交互,从而产生典型的点对点、星型网络拓扑,手机变成了网络的核心。这种协议没有其低功耗竞争对手(如 ZigBee 和 Thread)提供的Mesh组网功能,这两种协议都在 2.4 GHz 频段的 IEEE 802.15.4 之上运行。Mesh支持拓扑,网络中的每个节点可以直接或通过多跳通信与所有其他节点通信。这种网络是可以自我组织、自我修复的,并且支持路径多样性。

结果,开发者多次尝试向 BLE 添加Mesh功能产生了不同的技术,在 [4] 中详细讨论了这些技术。此调查指出,在 BLE Mesh网络方面仍然存在一些悬而未决的问题。其中之一是不同系统之间的兼容性,这个问题只有在标准化 BLE Mesh时才能解决。最近,蓝牙SIG组利用其广播功能[5]发布了基于BLE的Mesh规范。

因此,了解新标准如何运作、解决 [4]中确定的其他未解决问题以及处理使用 BLE 的广播功能作为沟通基础的缺点是相关的,这正是本文的主要阐述内容。除了向读者概述 BLE Mesh规范的工作原理外,还通过三种方法(实验评估、统计方法和使用理论模型)对其性能进行了洞察。

论文的其余部分按如下方式组织。在第2 节和第 3节中,我们将读者介绍蓝牙低功耗本身以及新的蓝牙Mesh标准。第4节简要介绍了相关工作。在第 5节中,我们首先采用统计方法计算往返时延 (RTT),进一步用于验证我们第6节提出的实验6。本节还概述了使用的硬件和测量环境。实验的重点是RTT作为一个指标。第 7部分介绍了蓝牙网格标准的理论模型。通过上一节中的实验验证了模型。最后,我们在第8条作一些总结意见,并讨论悬而未决的问题8。

蓝牙低功耗背景

如第 1节所述,BLE 是经典蓝牙的低功耗变体。它们都在 2.4 GHz ISM 频段中工作,使用 2402 到 2480 MHz 之间的频率。使用的频谱分为多个信道,但与经典蓝牙不同,BLE 使用 2 MHz 间距的信道,从而产生 40 个可用信道。如图 1所示。这40个信道分为3个主要的广播信道(37、38、39),避开了Wi-Fi使用的信道和37个传输信道。BLE提供了两种设备之间的通信模式:广播或建立连接。

图 1. BLE信道分发图示

这种广播模式意味着一个或多个设备在这三个信道上以特定时间间隔(当作为broadcaster或advertisement角色时)广播信息,而广播范围内的其他设备在广播频道上扫描,一次扫描一个信道(固定时间间隔,固定频段),以可能获取该信息(即sanner角色)。如图 2 所示2。

图 2. 说明 BLE 广播和扫描模式 [6]。

设备简历连接后以同步的方式使用其他 37 个信道。一个设备作为主设备(或中央设备)角色,并管理与多个从设备(或外围设备)的连接,如图3所示。主服务器通过 TDMA 方案管理和同步这些连接,每个连接都使用 FHSS 技术来确保稳定性。信息在发送时以特定时间间隔在单个频段之间跳转,每次发送都使用与上一次不同的频段。

图 3. BLE 连接导向模式的插图 [6]

两种通信技术都有其用途,两者都用于实现BLE Mesh网络。一般来说,广播角色有广播和扫描两种状态,每个节点都是可拓展的以确保将接收的数据包重新广播到网络中。实现连接的建立主要依赖于节点可以同时作为中央和外围设备,从而将多个星形拓扑链接为网状拓扑。对于这两种方法,已经存在几种方法实现Mesh拓扑[4],每种方法都有其自身的优点和缺点。根据用例,其中一个可以提供更令人满意的解决方案。于是,蓝牙特殊兴趣组 (SIG) 选择使用广播模式作为蓝牙网格标准的核心基础技术,下一节将详细讨论。

3. 蓝牙Mesh标准基础知识

3. 1. 蓝牙Mesh概念

从概念上讲,蓝牙Mesh标准被定义为发布/订阅模型,发布者可以发布到某个群组地址,订阅者可以订阅一个或多个感兴趣的主题。如图 4 所示4,其中交换机可以发布到特定主题,灯可以订阅一个或多个地址。这个概念是实施标准的灵感来源。蓝牙Mesh网络中的节点可以订阅一个或多个地址(存储在订阅者列表中),并发布到一个特定地址(存储在发布地址中)。该标准定义了两种主要地址类型:单播地址和群组地址。当每个节点成为网络的一部分并唯一标识此节点时,将为每个节点提供单播地址。群组地址包括一组节点。

每个节点在订阅列表中都有其单播地址。如果节点要加入特定组,它还要将相应的群组地址添加到订阅列表中。另一个节点可以使用其特定的单播地址或使用节点已订阅的组地址向此节点发送消息。此信息存储在发送节点的发布地址中。

图 4. 通过发布/订阅模型对蓝牙网格标准的概念定义 [5]

3.2 蓝牙Mesh拓扑

为了能够连接这些不同的发布者和订阅者,标准中创建了一个Mesh拓扑网络。本节的其余部分将分步概述此类拓扑中存在的所有不同类型的节点。

该标准使用 BLE 广播和扫描作为实现通信的基础技术。要在蓝牙Mesh网络中进行通信,将使用泛洪机制。默认情况下,泛洪机制可确保网络中的每个节点重复传入消息,以便进一步中继它们,直到到达目标节点。与正常的 BLE 广播相比,蓝牙Mesh节点不会根据广播间隔发送数据包。他们在随机生成的回退时间后直接发送数据包。

要扫描传入数据包的播发通道,网格节点使用 100% 占空比。这意味着Mesh网络中的节点始终进行扫描,除非它们正在发送数据包。对于扫描,仍使用扫描间隔和窗口。扫描窗口等于扫描间隔,以确保节点永远不会停止扫描。扫描间隔可确保节点在播发通道之间切换以进行扫描。

该标准使用新型 BLE 播发数据包在网状网络中进行通信,该网络仅支持支持 BLE 和蓝牙网格的设备。幸运的是,该标准还定义了向后兼容性功能,以确保不支持蓝牙网格的 BLE 设备也可以成为蓝牙Mesh网络的一部分。此功能基于 BLE 连接。在此旁边,蓝牙网格节点可以实现一些可选功能,用于管理和增强通信,如图5所示。这些功能在以下小节中作了说明。

图 5. 蓝牙Mesh标准中每个可选功能的示例。

3.2.1 继电器功能

如果没有适当的管理,标准中使用的泛洪机制将严重降低可扩展性、稳定性等。为了防止这种情况,引入了中继功能。简而言之,只有启用中继功能的节点才会将接收到的消息转发到网络中。

该标准还引入了一个消息缓存,可确保中继仅中继一次特定消息,以及消息的'生存时间'(TTL) 字段。仅当消息不在缓存中且其 TTL 大于 1 时,中继节点才会中继消息。在进一步中继到网络中之前,它总是将 TTL 减少 1。

在图5中,此功能在左侧指示。上面的开关只能通过中间的开关到达下面的灯。中间开关配置为充当中继节点。

3.2.2 代理功能

该标准定义了不支持蓝牙Mesh网络的 BLE 设备的向后兼容性功能。这样,本机 BLE 设备(如智能手机)也可以与蓝牙网状网络连接。这是通过代理功能实现的。启用代理功能的节点可通过两种方式进行通信:使用默认 BLE 广播功能和使用使用 BLE 连接的向后兼容性功能。

如图5中间部分所示。智能手机不支持蓝牙Mesh组网。它仍可以通过充当代理的代理节点灯与下面的灯连接。智能手机和代理节点灯之间的通信是使用向后兼容性功能完成的。代理节点灯使用默认 BLE 广播功能将信息进一步转发到网络中(从而转发到下面的灯中)。这也解释了为什么代理节点也应该是中继节点。

3.2.3 友谊功能

讨论的最后一个功能是友谊功能,它由两个子功能组成:朋友节点和低功耗节点。由于此标准中使用泛洪机制,节点使用 100% 占空比来扫描不同的信道。这降低了 BLE 广播的低功耗能力,从而限制了对功率敏感应用的实用性。

为了应对这一限制,引入了'友谊'功能。想要成为Mesh网络一部分的功率受限设备无法以 100% 的占空比扫描(例如,由于能量有限)。但是,网络中的其他节点(例如,电源供电的灯泡)可以协助此节点,使其仍可以是网络的一部分。两个节点可以建立友谊,电源受限设备充当低功耗节点,灯泡节点充当朋友节点。

朋友节点负责两件事:存储其低功耗节点的传入消息,以及将从低功耗节点接收到的消息中继到网络中。因此,每个朋友节点也应该是中继节点。低功耗节点在一定时间间隔内向好友节点询问新消息。使用此功能,低功耗节点不需要进行 100% 占空比扫描,可以节省电量。

在图5的右侧,显示了两个节点:一个灯具节点和一个传感器节点。灯充当朋友节点,传感器充当低功耗节点,他们建立友谊关系。

3.3 蓝牙Mesh堆栈

蓝牙Mesh网络中的每个设备都实现图6所示的蓝牙Mesh堆栈。此堆栈由分层体系结构组成,具体由以下层组成:

  • 蓝牙低功耗核心规范:该标准基于 BLE 规范构建,同时使用面向广播和连接机制。

承载层:Mesh消息需要基础的通信系统来进行传输和接收。承载层定义了网络PDU如何由给定的通信系统进行处理。这时定义了两个承载层,即广播承载层和GATT承载层。广播承载层利用蓝牙低功耗的GAP广播和扫描功能来传送和接收mesh PDU。GATT承载层使用BLE连接的机制。

  • 网络层:此层负责中继、网络级的安全等。
  • 传输层:在此层上对较大的消息进行分段和重新组合。这层也进行应用程序层的安全处理。
  • 访问层:定义应用程序层(应用程序、模型和基础模型)如何利用上层传输层。它可确保应用程序的正确接收来自上层传输层的传入消息,并确保从应用程序传出的消息正确转发到下面的传输层。
  • (基础)模型层:两个最高层构成所谓的模型的定义。模型是特定方案的标准化规范。这可能与Mesh网络(基础模型)的配置和管理功能以及常用的特定用户方案(例如,照明开关的标准化)相关。在这些较高层上,蓝牙Mesh网络设备可以定义为基础模型和其他几个模型的组合。每个模型都表示应用程序的一部分,它们共同构成了整个设备的运作形式。

在此堆栈之上,实现了应用程序。该标准还定义了几种强制安全措施(网络和应用程序层安全性、密钥刷新等)以及用于预配和配置设备作为Mesh网络一部分的机制。这些标准的这些方面超出了本文的范围。在我们的实验中,我们总是假定一个预配和配置完全的Mesh网络。

图 6. 蓝牙Mesh标准的分层体系结构 [5]

3.4. 蓝牙Mesh

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