JakubFloty´nski,RafałKrenz AWirelessMeshNetworkenablingtheimplementationofaDeterministicMediumAccess

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POZNA ´NSKIE WARSZTATY TELEKOMUNIKACYJNE 2011 1

A Wireless Mesh Network enabling the

implementation of a Deterministic Medium Access

Jakub Floty´nski, Rafał Krenz

Abstract—Wireless Mesh Networks (WMNs) are an emerging approach to reliable and ﬂexible communication. Currently, they are usually based on existing technologies designed for local area networks, which do not provide satisfactory solutions for trans-mitting QoS-sensitive data. The implementation of a deterministic medium access enabling QoS for WMNs has speciﬁc demands. The paper introduces a WMN satisfying the requirements.

Index Terms—wireless mesh network, software components, deterministic medium access.

I. INTRODUCTION

A wireless mesh network (WMN) is a data transmission network which has a structure with no hierarchy (the structure of mesh [1]). Particular nodes of the WMN are not dependent on each other, as they are, e.g., in networks with access points or with a tree structure. Each WMN node is capable of relaying data even if it is neither transmitter, nor receiver. Two types of the mesh nodes differing in functionality are distinguished. A router is a mesh node, the main goal of which is to relay data between different networks. Particular router interfaces may be compatible with various network technologies, which enable combining of heterogeneous networks. The hardware requires the use of long-life batteries or connection to external power sources. The client is a second type of a WMN node. It uses one wireless interface connected to a single network. The client can be battery powered because of its lower hardware complexity in comparison with the router.

A WMN is flexible in configuration. Mesh nodes are able to self-organise and self-configure. The WMN’s fault tolerance is high because of the number of nodes and links between them. The breakdown of a couple of devices does not necessarily prevent data transmission and does not result in a breakdown of the entire network. Also, the transmission rates may be higher in comparison to networks with single paths between nodes. The network’s long range is achieved through a large number of neighbouring WMN nodes, instead of high-power transmitters. The WMN is an appropriate solution for public hotspots, measurement or control networks, etc.. Nowadays, WMNs comprise a research area that includes, e.g., security architectures [2][3], channel reservation [4], MAC protocols [5], routing protocols [6], scheduling [7] and power control. The WMN described in this paper is based on well established

J. Floty´nski is with the Department of Information Technology, the Poznan University of Economics, e-mail: ﬂotynski@kti.ue.poznan.pl.

R. Krenz is with the Chair of Wireless Communications, Poznan University of Technology, e-mail: rkrenz@et.put.poznan.pl.

Krzysztof Bakowski, Robert Kotrys and Marcin Rodziewicz are with the Chair of Wireless Communications (Poznan University of Technology) and they have participated in the WMN testbed development.

low power, short range technologies for wireless local area networks - 802.11a/b/g.

A number of Medium Access Control (MAC) protocols used in 802.11 networks may be adopted to WMNs, but they do not usually provide Quality of Service (QoS) or are not adjusted to the ﬂat and distributed structure of the WMN. Some deterministic medium access protocols for WMNs, e.g., TDMA [8] or reservation [9] based which have speciﬁc software requirements have been proposed.

The main contribution of this paper is the proposal of hardware and software components enabling implementation of the deterministic medium access for WMNs.

The paper is structured as follows. First, a short overview of WMN standard drafts, protocols and implementations is presented. Second, requirements for WMN components which enable the implementation of a deterministic medium access are discussed. Third, tests of the WMN which verify its behaviour are described. Finally, conclusions and proposals for future research are presented at the end of the article.

II. STATE OF THE ART OFWIRELESSMESHNETWORKS

This section presents the main standards, protocols and some examples of wireless mesh network implementations.

A. WMN Standards

As with wireless networks, different standards of WMNs have been proposed for personal (802.15.5), local (802.11s [9]) and metropolitan area networks (802.16a). The WMN described in this paper is based on 802.11a/b/g LAN technol-ogy, hence the 802.11s standard draft will be described more exhaustively.

While 802.11a/b/g networks are single-hop, 802.11s intro-duces multi-hop communication between nodes. The draft touches on such issues as: medium access, topology learning and discovering, routing, conﬁguration, management and secu-rity. The description of MAC layer distinguishes two protocols: compulsory Enhanced Distributed Channel Access (EDCA) and optional Mesh Coordinated Channel Access (MCCA).

B. WMN protocols

A number of protocols have been designed for 802.11a/b/g networks. Some of them may be adapted to WMNs. This ar-ticle considers only single-channel communication protocols. The most common MAC protocol for WLANs is the Dis-tributed Coordination Function (DCF) [10]. It relies on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).

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Before transmitting, a node checks whether the channel is free. If it is, the transmission occurs, or else the node waits a random period of time and tries once again. DCF is a fully distributed protocol - no common coordinator is used. In addition, no trafﬁc classes are distinguished - sending emails may be done in the same way as streaming voice. It is not possible to reserve the channel in advance, which implies that DCF does not support QoS.

Point Coordination Function (PCF) [11] is a protocol using an Access Point as a network coordinator granting channel access to only one node at the moment. It guarantees QoS and makes PCF suitable for applications which crucial concern is maintaining the constant stream of data transmitted. The lack of distribution is a drawback in comparison to DCF. Unlike DCF, PCF is rarely implemented.

The 802.11s draft describes the Enhanced Distributed Chan-nel Access (EDCA) protocol as a compulsory medium access method for WMNs. It is similar to DCF, but introduces prior-ities into the data transmitted over the network. Prioritization is performed using different expectation periods. Statistically, nodes transmitting critical data (with a higher priority), wait for medium access shorter than other nodes. There is no way to reserve medium access in advance. EDCA does not guarantee QoS, but makes it only more probable.

Mesh Coordinated Channel Access [9] (MCCA, known also as Mesh Deterministic Access [22]) is an optional medium access method speciﬁed by 802.11s. The protocol is fully distributed and deterministic. It allows nodes to reserve chan-nel access in advance. There are four phases of MCCA. First, during advertisement all nodes broadcast periods already reserved for transmission they know about. Second, during reservation nodes attempting to transmit ask data recipients for permission. Third, transmission occurs. Finally, when all data delivered, the reservation may be cancelled.

Another example of a deterministic MAC protocol is a TDMA-based protocol described in [8]. It disables CSMA/CA in a WMN and transmits packets in time slots assigned for a particular node in advance. Like MCCA, the method requires precise clock synchronization among mesh nodes.

C. WMN implementations

A number of WMNs have been developed for research and industrial purposes, e.g., WMN for studies of network coding, bandwidth estimation, scheduling, routing or adaptive coding and modulation [12]. Other examples are proposed to improve trafﬁc control [13] or to enable immediate reactions for emergency forces [14].

WMN testbeds described in the literature have a couple of common features. First, they are often developed to provide a wide area of network access. Second, they are usually based on proven 802.11 technology implemented by cheap devices. The networks are built using low-performance computers equipped with low-capacity memories. To provide an individual network protocol, existing network card drivers are often modiﬁed, hence Linux is the most popular solution as an open-source system. To enable an easy and efﬁcient management of mesh nodes, the operating system is commonly loaded to nodes from the server.

The testbeds often utilize the MadWifi [15] driver which pro-vide WLAN for Linux. MadWifi does not support mesh mode, so it is updated in existing WMN implementations. Contrary to MadWifi, WifiMesh [16] for FreeBSD and open80211s [17] for Linux are both projects implementing WMN. The drivers are still enriched with new capabilities like authentication, encryption or congestion signalling. The MAC protocol of the drivers uses CSMA/CA. Additionally, open80211s intro-duces EDCA traffic priorities supporting QoS. Although the implementations are based on the same draft, they are not compatible and cannot cooperate.

III. REQUIREMENTS FORWIRELESSMESHNETWORK

COMPONENTS

Although different implementations of WMNs are proposed, they do not provide strong support for QoS, which is highly recommended for such applications as real time systems, voice or video streaming. On the one hand, EDCA implemented by open80211 increases QoS, but does not guarantee it, so it is not sufﬁcient for some appliances. On the other hand, a deterministic medium access (e.g., MCCA or TDMA-based) is still not available in existing WMN drivers. This paper addresses the design of a wireless mesh network which enable the implementation of a time-based deterministic medium access. The implementation of a deterministic MAC protocol has some speciﬁc demands for WMN components.

1) Use of cheap, reliable, widely used, technologies and standards: Technologies and standards used should be well

known and have extensive documentation to avoid problems that are difﬁcult to solve.

2) Open-source device drivers: It is a difﬁcult and complex

task to implement device drivers from scratch. Existing open-source device drivers may be modiﬁed to develop a desirable deterministic MAC protocol.

3) Simplicity of adding and removing WMN nodes:

Mod-iﬁcation of the WMN structure should be possible without interruption of the network’s activity. New nodes should receive the required software, set proper parameters and start cooperation with the network ﬂuently.

4) Simplicity of software management: Any changes of the

operating system, drivers, conﬁguration tools or other software crucial for the WMN may be centrally managed and inﬂuence many nodes simultaneously. The set up of a single node should also be possible, but done only when necessary.

5) Simplicity of nodes management: Node parameters

should be modiﬁed individually or in a group while the network is run. Changes are visible immediately after intro-duction; no restart of the whole WMN is required.

6) Precise clock synchronization: Deterministic MAC

pro-tocols mentioned above requires precise synchronization of mesh nodes to avoid conﬂicts between simultaneous transmis-sions.

IV. COMPONENTS OF AWIRELESSMESHNETWORK

This section describes the hardware and software compo-nents of the WMN testbed developed at Chair of Wireless

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Communications (Poznan University of Technology) consist-ing of 29 nodes. The selection of the components has been done with regard to the requirements listed above to enable the implementation of the deterministic medium access. The WMN construction covers both hardware and software con-ﬁguration.

A. Hardware of the WMN

The network is based on cheap, reliable and well recognized 802.11a/b/g standards. Each node is based on an Alix main-board, which is an integrated device including CPU, RAM, Ethernet interface and PoE interface. Such devices are less efficient, but sufficient and much cheaper for WMNs than typical mother boards installed in PCs. Nodes are equipped with a few wireless network interfaces. The wired interface is used for flexible node management while the wireless connection is disabled. The testbed contains a Network At-tached Storage server dedicated to centralized data storage and software management, e.g., storing and loading an operating system.

B. Software of the WMN

The WMN testbed includes the following software compo-nents.

1) Device drivers: perform two main tasks - control the

hardware and implement MAC protocols. Open80211s is the most advanced of available drivers. It incorporates a driver for the MAC layer mac80211, a conﬁguration driver

-cfg80211, an interface for cfg80211 - nl80211 and drivers for

wireless network cards with Atheros chips - Ath5k and Ath9k. It implements mesh mode, so only a particular MAC protocol needs to be added. The driver determines the operating system installed on the nodes - it must be Linux. Open80211s has two important disadvantages. It is still under development, so subsequent versions may be not compatible. Moreover, the available documentation is very poor, which can cause problems during the implementation.

2) The operating system of WMN nodes: has a crucial

impact on other WMN software components. Only Linux dis-tributions have been considered with regard to the open80211s driver. Additionally, a distribution should be convenient for devices with restricted capabilities (slow CPU and limited amount of memory). A few systems have been taken into account: Slax, Gentoo, Xubuntu and Debian. The first two have very limited requirements (about 200 MB free memory), but they are rarely used and have no extensive documentation. The last two (Xubuntu and Debian) are frequently used, so their documentation is rich. The systems may be installed without graphical user interface and they require about 300 MB of memory. Aside from the operating system, an appropriate core version must also be chosen. The choice was based on tests described later. Different core versions were tested for Xubuntu and Debian to check the compatibility with open80211s. The configuration including Debian and the core 2.6.36.1 works properly, contrary to configurations using Xubuntu and other core versions.

3) The bootloader of the operating system: is important

for the ﬂexibility of software management in the WMN. The primary issue is whether a software upgrade has to be done individually for each mesh node. Three solutions have been considered as described below. First, the operating system may be installed individually on each WMN node. Second, the operating system may be mounted on nodes from the server using the Network File System protocol without copying data. Third, the operating system may be copied to nodes from the server.

The first solution is very inefficient - any changes of software must be introduced individually to each node, but it does not require any additional software loading the system. This is not a drawback of the two other approaches. Mounting the system has the most reduced requirements for the nodes, but has high demands for the server and network connections, because of the high network traffic generated. In addition it is not fault tolerant - the crash of the server disables the network. A few boootloaders enabling that approach are available, e.g.,

Grub [23]. Copying the entire system to the nodes causes high

trafﬁc only while starting the network and is fault tolerant, so the solution was chosen for the WMN. It is implemented using Clonezilla [18] for Linux. Clonezilla is installed on the server. The mainboard of the nodes supports Preboot Execution Environment (PXE), which downloads a Clonezilla agent directly after reboot. The agent downloads the operating system and launches it.

4) Conﬁguration tools: are responsible for setting the

pa-rameters of the WMN nodes. Local and remote configuration tools are utilized for the WMN. The first group is used to set parameters locally, directly on the node. Ifconfig is a standard Linux tool responsible for the management of network inter-faces: the assignment of IP addresses, masks, gateways, listing of available interfaces, turning on and down, setting MTU, creating virtual network interfaces, and managing interruptions called by the device. Iw is a configuration tool for WMNs compatible with open80211s. It enables network scanning, creation and removal of virtual interface, setting bit-rates, transmitting power, network id, a power saving mode and a few modes of work for WMN nodes. OpenSSH is used for remote spreading and installing new code among different WMN nodes. It is used while programming the drivers. The package is comprised of tools for remote access to the shell (SSH) and file copying (SCP, SFTP).

5) Synchronization protocol: is a crucial software

compo-nent for correct operation of time-domain MAC protocols. Two standardized solutions have been considered. Network Time Protocol (NTP) is the most popular time synchronization protocol with client-server architecture that does not need efficient servers or network connections. The accuracy of NTP is about 0.1 ms for fast LAN networks and up to a few tens of milliseconds for the Internet [19]. NTP is not sufficient for the WMN project which requires a precision level up to a few microseconds. Precision Time Protocol (PTP) is a synchronization protocol developed especially for LAN networks with high accuracy requirements that cannot be satisfied using NTP. Unlike NTP, PTP is a fully distributed protocol. The network is divided into several segments, each

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Fig. 1. Test cases for the WMN

with a master node from which other nodes get the current time. PTP is precise enough for the WMN - to about a few microseconds [20]. An open-source PTP implementation for Linux [21] is used in the project.

V. TESTCASES FOR THENETWORK

Tests of the WMN aim at the verification of the coopera-tion among the WMN components: the hardware, operating system, drivers and configuration tools. The tests use the WMN consisting of four nodes. Three tests were performed as described below. In each test, the transmission of a file using SFTP protocol is performed.

6) Transmission in the chain: (Fig. 1a) was successful

-data was transmitted from node 1 to node 4 and vice versa.

7) Dynamic network path selection: (Fig. 1b) assumes

nodes are connected in the triangle. At the beginning, paths in the network are established dynamically. The path between nodes 1 and 3 is established through node 2. After falling down node 2, the new path is established through node 4. The table of the paths is selected by using iw. The test succeeded - ﬁrst, the path 1-2-3 was established and the transmission occurred, second, the path 1-4-3 was established and the transmission occurred.

8) Static network path selection: (Fig. 1c) assumes nodes

connected as previously. The dynamic path selection is dis-abled. Iw is used to establish the path between nodes 1 and 3 through node 4. The test succeeded - the path 1-4-3 was set and the transmission between nodes 1 and 3 occurred.

VI. CONCLUSIONS ANDFUTURERESEARCH

In this paper the WMN testbed implementation was pro-posed. First, the hardware and software components were described. Second, the behaviuor of the network was veriﬁed by the tests. The developed WMN satisﬁes requirements for the implementation of the time-based deterministic medium access, which is desirable especially for delay critical appli-cations.

Future works may cover the implementation and testing of different deterministic MAC protocols on the network developed. A problem may occur with regard to maintaining compatibility with future versions of the Linux core and

the used version of open80211s. Second, starting the nodes simultaneously may expose the problem of downloading the operating system, which may be solved by introducing delays for starting particular nodes.

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