IEEE 802.11s Wireless Mesh Networks Anastasios Daniilidis, Khalil Khalil Dept. of Communication Systems, Lund University, Box 118 SE-221 00, Lund, Sweden {d04ad, e02kk}@student.lth.se Abstract Nowdays, WLAN technology cove rs a wide variety ofdevices and applications used by many users around the globe. The most common standard in WLANs is the well known IEEE 802.11 which comprises a family of standards that each offer different kinds of characteristics. Neither the infrastructure nor the ad hoc mode made it possible to cov- er the demands of usage of WLANs. Demands, like mobility, pushed the standards over the edge and the term mesh net- working was born. Devices no longer have to be in the same area where an access point is. They can gain access to ser- vices by other nearby access points that denote the same network. Key features like QoS, power management, securi- ty and routing implement existing, new or hybrid techniques to deploy an integrated mesh network. This article makes a presentation of the mesh concept and introduces the new member of the 802.11 family, i.e. 802.11s, and especially the concept of Mesh Deterministic Access (MDA). 1. Introduction Wireless LANs is the key technology to most services, in combination with propriate devices offered by networkproviders and manufacturers respectively. However prob- lems like coverage area still remain and mesh networking is here to solve this problem among others. WMNs are com- prised by mesh routers and mesh clients called the nodes[2]. Each node has the ability to operate both as a host and as a router that can provide packet forwarding in behalf of an- other node that is out of range of transmision with its desti- nation. The scale of the network determines its type which can be a WLAN, a WMAN or even a WPAN. The IEEE 802.11 working group, WG, works on wire- less mesh networks in the task group, TGs, that produce the standards for mesh networks. Hence, the wireless mesh standard 802.16a is developed for WMAN, the 802.11s for WLAN and the 802.15.5 for WPAN[3]. The focus is on 802.11s where the TGs define an extended service set (ESS) of a number of devices (nodes) interconnected to each oth- er, enabling automatic topology learning and dynamic path configuration. Mesh connectivity is established by apply- ing multihop mesh techniqu es to spec ify a wirel ess dis- tribution system (WDS) building a wireless infrastructure among nodes. Each node that supports mesh functionality is called a mesh point(MP) and it supports functionalities like neighbor discovery , channel selection and association f orm- ing with its neighboring MPs. The implementation is based on top of the current PHY layer of IEEE 802.11a/b/g/n and operates at the 2.4 - 5.0 GHz frequency bands. However, the current routing and MAC techniques limit the WMN and its capabilities. Thus, enchancements or new deployed techniques have to be implemented by a WMN perspective. Such a MAC enchancement is the Mesh Determistic Ac- cess that is an access method which allows MPs to access the channel at specific times (called MDAOPs) with lower contention that would otherwise be possible[4]. 2. Wireless Mesh Networks As mentioned earlier, WMNs consist of two types ofnodes: mesh routers and mesh clients. The difference be- tween a conventional router and a mesh router, apart from the mes h functi ona lit y , is tha t the lat ter can ach ie ve the same coverage with lower transmission power through multi-hop communications. As regards to mesh clients, they also have neccesary mesh functions and can thus behave as a router. On the other hand, gateway or bridge functions do not ex- ist in these nodes. Additionally, mesh clients have only one wireless interface. 2.1. WMNs Architecture Three different categories distinguish in the WMNs ar- chitecture, based on the functionality of the nodes. • Infrastructure/backbone WMNs
Anastasios Daniilidis, Khalil KhalilDept. of Communication Systems,
Lund University,Box 118 SE-221 00, Lund, Sweden
{d04ad, e02kk}@student.lth.se
Abstract
Nowdays, WLAN technology covers a wide variety of devices and applications used by many users around the
globe. The most common standard in WLANs is the wellknown IEEE 802.11 which comprises a family of standardsthat each offer different kinds of characteristics. Neither theinfrastructure nor the ad hoc mode made it possible to cov-er the demands of usage of WLANs. Demands, like mobility, pushed the standards over the edge and the term mesh net-working was born. Devices no longer have to be in the samearea where an access point is. They can gain access to ser-vices by other nearby access points that denote the samenetwork. Key features like QoS, power management, securi-ty and routing implement existing, new or hybrid techniquesto deploy an integrated mesh network. This article makes a
presentation of the mesh concept and introduces the newmember of the 802.11 family, i.e. 802.11s, and especiallythe concept of Mesh Deterministic Access (MDA).
1. Introduction
Wireless LANs is the key technology to most services,in combination with propriate devices offered by network providers and manufacturers respectively. However prob-lems like coverage area still remain and mesh networking ishere to solve this problem among others. WMNs are com-prised by mesh routers and mesh clients called the nodes[2].Each node has the ability to operate both as a host and as arouter that can provide packet forwarding in behalf of an-other node that is out of range of transmision with its desti-nation. The scale of the network determines its type whichcan be a WLAN, a WMAN or even a WPAN.
The IEEE 802.11 working group, WG, works on wire-less mesh networks in the task group, TGs, that producethe standards for mesh networks. Hence, the wireless meshstandard 802.16a is developed for WMAN, the 802.11s forWLAN and the 802.15.5 for WPAN[3]. The focus is on
802.11s where the TGs dene an extended service set (ESS)of a number of devices (nodes) interconnected to each oth-er, enabling automatic topology learning and dynamic pathconguration. Mesh connectivity is established by apply-
ing multihop mesh techniques to specify a wireless dis-tribution system (WDS) building a wireless infrastructureamong nodes. Each node that supports mesh functionality iscalled a mesh point(MP) and it supports functionalities likeneighbor discovery, channel selection and association form-ing with its neighboring MPs. The implementation is basedon top of the current PHY layer of IEEE 802.11a/b/g/n andoperates at the 2.4 - 5.0 GHz frequency bands. However,the current routing and MAC techniques limit the WMNand its capabilities. Thus, enchancements or new deployedtechniques have to be implemented by a WMN perspective.Such a MAC enchancement is the Mesh Determistic Ac-
cess that is an access method which allows MPs to accessthe channel at specic times (called MDAOPs) with lowercontention that would otherwise be possible[4].
2. Wireless Mesh Networks
As mentioned earlier, WMNs consist of two types of nodes: mesh routers and mesh clients. The difference be-tween a conventional router and a mesh router, apart fromthe mesh functionality, is that the latter can achieve the samecoverage with lower transmission power through multi-hopcommunications. As regards to mesh clients, they also have
neccesary mesh functions and can thus behave as a router.On the other hand, gateway or bridge functions do not ex-ist in these nodes. Additionally, mesh clients have only onewireless interface.
2.1. WMNs Architecture
Three different categories distinguish in the WMNs ar-chitecture, based on the functionality of the nodes.
The gure 1 above shows an infrastructure/backboneWMN. The dashed and solid lines denote wireless and
wired connections respectively. As it can be seen in thegure, different kinds of clients connect to the meshrouters that form the infrastructure. The radio technol-ogy used by the mesh routers varies signicantly. Fur-thermore, routers apply self-conguring, self-healinglinks among themselves to form the backbone net-work. They can also connect to the internet by gate-way functions. Conventional clients can connect to themesh routers either by using the Ethernet interface orby using the same radio technology as the routers.
• Client WMNs
Figur 2. Client WMNs.
In this type of architecture, no mesh router exists. In-stead, conventional devices establish peer-to-peer net-works among them to constitute the actuall network performing routing and conguration functions as wellas providing end-user applications to customers. Thereexists one single radio interface among the devices anda packet is forwarded to its destination by hoppingthrough devices.
• Hybrid WMNs
Figur 3. Hybrid WMNs.
As the name of the architecture denotes, this is the casewhere the network comprises by both infrastructureand client mesh networks. The clients can access thenetwork by other clients or by routers providing im-proved connectivity and coverage within the WMN.
2.2. Characteristics of WMNs
This section describes the characteristics of mesh net-works as the following:
• Multi-hop WMN: To provide greater coverage andnon-line-of-sight (NLOS) among nodes, the multi-hopfunction becomes indispensable[5].
• Support for ad hoc networking, and capability of self-forming, self-healing, and self-organization: Theseproperties result in enchanced network performanceand gradual growth.
• Mobility dependence on type of mesh nodes: Minimalmobility of mesh routers but mesh clients can be sta-tionary or mobile nodes.
• Multiple types of network access: WMNs can sup-port backhaul access to the Internet and peer-to-peercommunications[6].
• Dependence of power-consumption constraints on thetype of mesh nodes: Mesh clients require power ef-cient protocols in contrast to mesh routers.
• Compatibility and interoperability with existing wire-less networks: WMNs built based on IEEE 802.11technologies have to be compatible with the IEEE802.11 standards. Additionaly, such networks must beinter-operable with other types of wireless networks,e.g. WiMAX, ZigBee[7] and cellular networks.
The section below describes the main functions of WMNs[1].
2.3.1. Mesh Topology Creation
On activation, an MP searches for potential mesh net-works present within its coverage area. When such a net-work is found, the MP associates with the network. If nonetwork is found, the MP must be able to create a new one.There are two types of network discovery: the active and the passive approach. After network discovery, basic connectiv-ity of the nodes is maintained within the network. Further-more, beacon messages are send for topology maintenance.
2.3.2. Routing
Routing is a very important feature of WMNs since itallows communication between MPs. The IETF MANETWG is concetrated on standarizing IP (layer 3) routing pro-tocol functionality for wireless ad hoc networks. In con-sideration are taken two types of protocols. These are the proactive and the reactive routing protocols. The formertype of protocols is useful in small networks and the lat-ter is preferred when the network size is large. Since thesize of a mesh network can not be determined, neither typeof protocol can be used. A solution to this problem is touse a hybrid protocol . Mesh networks can alternatively useadaptive routing protocols .
2.3.3. Security
As in other wireless networks, security is also a greatissue in mesh mode. To guarantee a safe communicationchannel, security measures must be taken. These measurescontrol
• Condentiality and integrity: Data sent by the air in-terface have to be protected against eavesdroping andmodications.
• Unauthorized access: MPs have to authenticate them-selves rst to successfully join a network.
• Denial of service (DoS) attacks: Caused when an MPmisbehaves in routing packets, e.g. as in the case of ablack hole [8].
2.3.4. Quality of Service
Applications set bounds in parameters like packet loss,throughput, delay, and jitter in order to work properly. Thus,QoS mechanisms have to be employed to meet those de-mands.
In WMNs, it is distinguished two kinds of QoS issues.Firstly, QoS has to be present both in case of access net-work trafc and backbone trafc. This means that call ad-mission control (CAC) has to be present for the rst caseand a mechanism to differentiating the two types of trafc.Secondly, a mechanism is required obove L2 to guaranteeQoS over multi-hops. The MAC layer can only guarantee aminimum service level for inter-MP trafc.
End-to-end Qos is supported by ow control which istightly linked with CAC where the former uses ow identi-ers and the latter checks for service-level requirements inpacket headers.
2.3.5. Power Efficiency
Many devices are battery-driven and that has an effecton the up-time of a device. In turn, up-time depends on bat-tery capacity and device power consumption. Power-saving
mechanisms aim to extend the up-time as much as possible.Since a device can do many different things, there should
also be propriate solutions for each case. Inactive devicescould enter a sleep mode, trafc-forwarding devices couldhave scheduled wake-up mechanisms and so on. Anotherway of dealing with this issue is to adapt transmission pow-er depending on tranmission range.
Nevertheless, there is a more efcient way used in meshnetworks called power-aware routing, where network rout-ing paths are oprtimized for power consumption[9].
3. IEEE 802.11s
Figur 4. IEEE 802.11s network architecture
The proposed network architecture is depicted in g. 4.The 802.11 TGs denes an extended service set (ESS), usu-ally refered to as a mesh network. Every IEEE 802.11-basedentity (AP or STA) that fully or partially supports meshfunctionality is refered to as a mesh point (MP). Minimal
MP operations include neighbor discovery, channel selec-tion, and forming associations with neighbors. A WDS isformed by MPs and mesh links that connect the MPs. Thisway, the ESS is distinguished from the BSS, dened in thelegacy IEEE 802.11[10]. MAPs are specic MPs but canact as APs as well. MPPs is another type of MPs that hasthe ability of interconnecting other WMS with the network it belongs to. Furthermore, it can act as a bridge/gatewayof the mesh network and other networks in the DS. Such aWMN is uniquely identied by a mesh ID assigned to everyMP to represent an ESS.
3.1. Medium Access Coordination Function
Figur 5. 802.11s MCF sublayer
The Medium Coordination Function (MCF) componentsare shown in g. 5. The sublayer is built on top of the PHYlayer where no modications have been made. The 802.11sMAC sublayer is an amendment to IEEE 802.11 to createa WDS. This section will cover the most important parts of MFC.
Focused on peer-to-peer discovery of MPs, this serviceset (SS) enables automatic topology learning, establisheslinks and forms a dynamic data delivery path across MPs.
• Topology discovery and formation: A new candidatenode initially gathers information from neighboringnodes either by active scaning (i.e. sending probe mes-sages) or by passive listening (i.e. by receiving period-ic beacons). Finally, two peers form a partial or a fullmesh topology by associating with each other.
• Path selection protocol: Formally, a L2 path selec-tion protocol is used to handle unicast and broad-cast/multicast data delivery. On the other hand, MPs
might be mobile or nonmobile and thus a hybrid rout-ing protocol supporting both proactive and on-demandschemes is more suitable for such a network[11]. Thus,the hybrid scheme uses the ad hoc on-demand vector(AODV) and the optimized link state routing (OLSR)to reach the goal. To make the routing prototcols morerobust against link failures, radio aware metrics areproposed.
• Forwarding scheme: The trafc in a WMN consistsof 4-address data frames. When a MP receives suchframes, it checks for the frame authenticity and thedestination MAC address before forwarding. In theMAP arrive the 3-address frame which is convertedto a 4-address format and then it is forwarded to itsdestination. The support of forwarding multicast andbroadcast trafc is also supported.
3.1.2. Medium Access Coordination
The proposal is to use the enchanced distributed channelaccess (EDCA) mechanism as medium access coordinationwhich is the re-use of previous MAC encahncements, i.e.802.11e. The MAC mechanisms support congestion con-trol, power saving, synchronization and beacon collisionavoidance. The proposed mechanisms shall make it possi-ble to enable multiple channel operations in multiradio orsingle radio as well as mixed environments. Furthermore,there must be compatibility with legacy devices.
Optional MAC enchancements include Mesh Determin-istic Access (MDA) that is a reservation-based determinis-
tic mechanism, Common Channel Framework (CCF) that isa multi-channel operation mechanism, Intra-mesh Conges-tion Control and power management.
3.1.3. Mesh Conguration and Management
Since the deployment of self-conguring paths and linkscan be unmanaged, it is required the use of autonomic man-agement modules.
The purpose of management is to ensure a free of prob-lems network operation. A mesh point that may fail doesnot effect the overall network performance but it has to bemanaged anyway.
3.2. Use Cases
3.2.1. Residential Case
In this model, the primary purposes for the mesh network are to create low-cost, easily deployable, high performancewireless coverage throughout the digital home. The meshnetwork is intended to eliminate RF dead-spots and areasof low-quality coverage. High bandwidth applications tend
to be used but also simple ones, e.g. video streaming andwireless printers.
3.2.2. Office Case
The objective in the ofce case is to create low-cost, eas-ily deployable wireless networks that provide reliable cov-erage and performance. A wireless mesh LAN becomesuseful in areas where Ethernet cabling does not exist oris cost prohibitive. Companies reduce their costs in asso-ciation with cable and time of installation. Furthermore,they can benet also from increase in employee productivi-ty through expanded connectivity to key network resources.
3.2.3. Campus / Comamunity / Public AccessCase
Mesh networks can in this case provide connectivity over
large geographic areas in low cost, higher bandwidth inter-net access in contrast to the traditional methods and locationbased services for information and safety purposes.
3.2.4. Public Safety Case
Access to emergency and municipal safety personnelsuch as re, police, and hospital is important if a corre-sponding incident occurs. The network can be used forvideo surveillance, tracking emergency workers with bio-sensors, voice and data communication between emergecypersonnel and so on.
3.3. Fundamentals of MAC
EDCA is a mandatory mechanism in 802.11e that isreused in 802.11s to provide prioritized QoS services. Onthe other hand, MDA is an optional mechanism that can beused in 802.11s. Both are described in the following twosections below.
3.3.1. EDCA
Enhanced Distributed Channel Access is an extensionof Distributed Coordination Function (DCF). Thus, DCF isthe basis for EDCA. QoS stations (QSTAs) access the medi-um using 8 different user priorities (UPs). This means thatpackets sent by the QSTAs are assigned a priority value, be-fore entering the MAC. These packets are then mapped tothe four rst-in rst-out (FIFO) queues,called access cate-gories (ACs) implemented in EDCA, according to gure 6.
For each AC, an enhanced variant of the DCF, namel-ly the enhanced distributed channel access function (ED-CAF), contends for TXOPs using a set of EDCA param-eters. Transmission opportunity (TXOP) is dened as the
Figur 6. Priority mapping.
interval of time when a particular QSTA has the right to ini-tiate transmission. Eack AC behaves like a virtual station: itcontends for medium access starting its backoff timer aftersensing the medium idle after AIFS, (AIFS being the cor-responding DIFS in DCF). The rule that applies is that theAC with the lowest AIFS has the higher priority. The differ-ent parameters are used to give a low-priority class a longerwaiting time[12]. An illustration of the queues is shown ingure 7 and the timing relationship in gure 8.
Figur 7. Implementation model.
Figur 8. The timing relationship for EDCA.
3.3.2. MDA
In Mesh Deterministic Access (MDA) scheme, involvingMPs have to support sychronization. MDA sets up time pe-riods, called MDAOPs, to prevent MPs of initiating trans-
mission sequences in case they interfere with each oth-ers transmissions or receptions. MPs that set up MDAOPsaccess the medium by using the MDA access parametersCWMin, CWMax, and AIFSN within these periods.
A Mesh DTIM interval comprises of MDAOPs, see g-ure 9. Such an interval is set up between the MDAOP ownerand the addressing MP. After the MDAOP is set up:
• The MDAOP owner uses CSMA/CA and backoff toobtain a TXOP using the MDACWmin, MDACWmax,and MDAIFSN parameters. The ranges of values of theparameters are identical to those used in EDCA.
• Both the MDAOP owner and the addressed MP adver-tise the MDAOP. Except the MDAOP owner, all otherMPs should not initiate transmissions during the TX-OP initiated in the MDAOP.
Figur 9. MDAOPs during DTIM.
A sender MP has the ability to establish a set of MDAOPs each identied by a unique ID called the MDAOPSet ID. Such a set id has to be unique for the sending MP,
so that the MDAOP set ID and the senders MAC addressuniquely identify an MDAOP set in the mesh. A MDAOPSet ID can also handle set up and teardown of the the entireset of MDAOPs in an MDAOP set.
TXOPs also exist in MDA but since it is obtained by aMP in a MDAOP, it is called MDA TXOP. Such an TX-OP is required to end within the MDAOP it origianlly wasobtained.
Neighborhood MDAOP times for a MP are those TX-RX times that are advertised by neighboring MPs, forminga set of MDAOPs currently used in the neighborhood. Thus,a sender cannot set up new MDAOPs within these times.
Neighbor MDAOP interfering times for a MP in relationto another MP are the times when the former cannot set upMDAOPs with the latter. Thus, creating MDAOPs withinthese times can and will result in interference.
The MDA access fraction, at a MP, is dened as the ratioof the total duration of its Neighborhood MDAOP Times"ina Mesh DTIM interval to the duration of the Mesh DTIMinterval. It exists to make sure that a new MDAOP set doesnot cause the MAF of another MP to exceed a MAF lim-it. If the limit is exceeded, the MDAOP request should berefused.
Upon MDAOP setup[13] the following packet formatsare used.
Figur 10. Request/Reply setup frame formats.
At this point it can be stated that every new entity hasa view over the entire mesh network. However, an MDAManager exists to allow end-to-end ows using MDA fea-tures and is responsible for
• Path computation.
• Invoking MDAOP Setup Procedure on node along thepath.
The MDA Manager can make path computation in thefollowing two different ways:
•
Using Dijkstra algorithm.• Using Ford algorithm.
In conclusion, the MDA does not perform well for thefollowing reasons.
1. Partial overlapping of MDAOPs.
2. Interference with other MDAOPs which are not con-sidered in Neighborhood Times
3. DTIM fragmentation.
Thus, Dynamic Relocation is en enchancement to MDA
confronting the rst two problems mentioned above.
4. Summary
The article presented the concept of Wireless Mesh Net-works and referred to the different implementations of eachtype. The new amandement of 802.11, i.e. 802.11s, lookspromising in bringing a new wave in wireless LANs. Thearea of implementations is wide and usages in the futurecan be meet almost everywhere. We have seen that the basis
for 802.11s in the MAC layer, EDCA, comes from its ances-tor, i.e. 802.11e, and that the PHY layer remains untouched.The optional mechanism MDA showed some weaknessesbut the Dynamic Relocation was developed to cover someof them.
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