Survey of Routing Protocols and Channel Assignment protocols in Wireless Mesh Networks by ijcsis


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									                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                              Vol. 8, No. 6, September 2010

         Survey of Routing Protocols and Channel
     Assignment protocols in Wireless Mesh Networks
                                    Vivek M Rathod, Suhas J Manangi, Satish E, Saumya Hegde
                                  {vivekmrathod, suhasjm, hsitash444, hegdesaumya}
                                      National Institute of Technology Karnataka – Surathkal

Abstract: This paper is a survey on wireless mesh networks.              A. Client WMN:
Here we mention the basics of wireless mesh network, their               Client mesh networks or simply ad-hoc networks are formed
purpose, channel assignment techniques and routing                       by client devices with no supporting fixed infrastructure.
protocols. This survey is prepared towards helping those                 Each node plays same role and participates in packet
working on the relationship between channel assignment                   forwarding.
and routing protocols.
                                                                         B. Infrastructure WMN:
Keywords: Wireless Mesh Networks, Routing protocols,                     In contrast to client WMN, infrastructure WMN consists of
Channel Assignment, Multi Hop, Multi Radio.                              routers and client devices. The routers are interconnected
                                                                         via wireless links to form a multi-hop backhaul
                    I.    INTRODUCTION                                   infrastructure. One or more routers are connected to the
A wireless mesh network (WMN) [1] is a communication                     wired network and are called gateways of the WMN.
network made up of radio nodes organized in a mesh                       Generally mesh router has two or more radio interfaces. One
topology. The nodes which constitute the WMN are in                      of which is an access interface for the clients to access the
adhoc mode so as to realize mesh topology.                               network. The second radio interface is a relay interface for
Wireless mesh architecture is an effort towards providing                forwarding and routing data packets. This is basically used
high-bandwidth network over a specific coverage area.                    for inter-router communication. Client devices associate
Wireless mesh architecture’s infrastructure is, in effect, a             themselves with the nearest mesh router to access the
router network minus the cabling between nodes. It's built               network. They don’t participate in routing or relaying of
of peer radio devices that don't have to be cabled to a                  packets. Therefore even if two clients are within the wireless
wired port like traditional WLAN access points (AP) do.                  range of each other, they cannot directly communicate. It
The traditional WLANs consist only of single hop end-to                  has to happen through their respective routers.
end connection (i.e., between the client and access point).
In contrast, Mesh architecture sustains signal strength by               C. Hybrid WMN[8]:
breaking long distances into a series of shorter hops.                   This architecture is the combination of Infrastructure and
Intermediate nodes not only boost the signal, but                        client meshing; clients can access the network through
cooperatively make forwarding decisions based on their                   mesh routers as well as by directly meshing with other
knowledge of the network, i.e. perform routing. Such                     mesh clients. A hybrid WMN is an extension to the
architecture may with careful design provide high                        Infrastructure WMN. In a hybrid WMN the clients not
bandwidth, spectral efficiency, and economic advantage                   only connect to the wireless backhaul, but also serve as a
over the coverage area.                                                  gateway to for the clients which are located too far from
This paper is organized in the following sections.                       the wireless mesh router. Therefore a hybrid WMN is
     1. Types of wireless mesh networks (network                         more robust and more scalable than the previous two. A
         architectures).                                                 well-built hybrid WMN would enable fast, cheap and easy
     2. Essential characteristics of WMN.                                deployment of networks, leading to interesting
     3. Components of WMN and their alternatives.                        applications such as emergency networks.
     4. Routing purposes, problems and protocols.
     5. Areas for research.                                                III.   ESSENTIAL CHARACTERISTICS OF WMN
                                                                         WMN has mainly following essential characteristics:
             II.    NETWORK ARCHITECTURE                                    1. Multihop and Multi-pathing: multiple paths
The types of network structures being used for WMNs can                         between two point in a WMN leads to the increase
be classified into three types in a very broad sense.                           in bandwidth. This increase in the bandwidth is
     1. Client wireless mesh networks                                           because the RTT (round trip time) for shorter paths
     2. Infrastructure wireless mesh networks                                   (hops) is less than that of a single end to end path.
     3. Hybrid wireless mesh networks [8].                                      Multiple packets can travel simultaneously

                                                                                                    ISSN 1947-5500
                                                              (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                             Vol. 8, No. 6, September 2010
           between two ends. Multi-pathing strips the data to           radio routers’ backhaul support provides the best hybrid
           be sent to a destination and sends these chunks via          WMN; ubiquitous connectivity but with multiple levels of
           multiple paths, which increases the throughput               redundancy built in.
           significantly. It also provides robustness to paths,
           because there is always an alternative unless the                      V.     MULTI RADIO MESH ROUTER
           destination itself is not connected to the network.          There are mainly 4 types of Multi Radio Mesh Routers:
    2.     Self-healing, self-forming and self-organising:                  1. Single unit mesh router
           Since most of the nodes of the WMN are mobile,                   2. split wireless router
           the WMN is always aware of its surroundings. It                  3. Multi-Channel Multi-Transceiver single radio
           dynamically changes the routing paths based on the               4. low cost mesh router construction
           current state of the network. If a participating node
           quits, the network is reconfigured so as to keep the         A. Single Unit Mesh Router:
           remaining nodes connected. Similarly the dynamic             Single unit mesh router is a single package with multiple
           changes in the network must also take place based            radios in it. All these radios’ operate in non overlapping
           on the network traffic at different routes.                  channels. Some of these could be used to relay packets
    3.     Compatibility and interoperability: The WMNs                 between routers, while the others to provide connectivity to
           built on the IEEE 802.11 standards must be capable           the clients or client adhoc network. Even though the radios
           of supporting conventional Wi-Fi clients.                    operate in non-overlapping channels, the practical results
    4.     Cost Factor: WMNs can be very cost effective                 have shown that there is a significant amount of interference
           because we can build and configure a WMN with                between them due to the near-field effect, resulting in
           minimal existing resources. A WMN could provide              reduced throughput
           an effective and good internet bandwidth to the
           group of users who share a single internet link.             B. Split Wireless Router:
                                                                        Split mesh router is a network (wired) of two or more
         IV.   COMPONENTS OF WIRELESS MESH                              single radio routers. This design has gained motivation
                             NETWORKS                                   from the limitations of the single unit multi-radio routers.
A WMN consists of two types of wireless nodes. Mesh                     We refer to the single radio routers which are part of split
Routers and Mesh Clients. The Mesh Routers have                         router as nodes hereafter.
improved computational, communication and power                         The commercially available single-radio routers often
resources as compared to Mesh Clients. Mesh Routers are                 provide multiple interface technologies like the Ethernet,
generally static and form the multi-hop backhaul network                fiber or ATM. Two or more such units are connected via a
with optional access to other auxiliary networks. In addition,          backhaul using one of the available interface options like
Mesh Routers are also typically equipped with multiple                  the Ethernet. Since the separation between these nodes is
wireless network interfaces (IEEE 802.11[3]) and are                    determined by the cable length forming the backhaul, the
therefore able to establish high capacity connections. Mesh             interference can be significantly reduced by increasing the
Clients are mobile devices, which take advantage of the                 distance between them. This is an effective solution for
existing communication infrastructure provided by the Mesh              the interference due to near-field effect in the single unit
Routers.                                                                mesh router.
                                                                        Since our mesh router unit is a combination of 3-single
A. 1-Radio VS Multi-Radio Approaches:                                   radio routers, we need a software abstraction by which the
In 1-radio approach the participating nodes have only one               assembly appears like a single unit to the network. Each
radio each. Consider a network where both the clients and               single radio router must here be aware of the neighbors of
the mesh routers have only one radio, and then mesh                     the other two.
routers would not be able to listen to the backhaul and the
client simultaneously. Collisions would be very frequent.               C. Multi-Channel Multi Transceiver Single Radio[4]:
This will result in very low throughput. Thus one radio                 In this kind of routers, a radio includes multiple parallel RF
WMN is inferior to multi-radio infrastructure mesh                      front-end chips and baseband processing modules to support
networks in Multihop situations. In the case of 1 radio ad              several simultaneous channels. On top of the physical layer,
hoc mesh networks, available bandwidth is reduced by                    only one MAC layer module is needed to coordinate the
50% with each hop: bandwidth available at the 3rd hop is                functions of multiple channels. So far no multi-channel
1/8 of the available capacity. However, while one-radio ad              multi-transceiver MAC protocol has been proposed for
hoc mesh networks are unsuitable for Multihop situations,               WMNs.
they are useful in one-hop situations for quickly
establishing p2p communications. Conversely, 2-radio                    D. Low Cost Mesh Router Construction
infrastructure meshes are ideal for Multihop situations                 A low cost router can be set up using two USB or PCI radio
with no restriction on the number of hops. Thus One radio               cards on a low-end computer. But this would also require a
mobile client mesh network combined with two or more                    MAC layer which supports multiple NICs simultaneously.

                                                                                                   ISSN 1947-5500
                                                             (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                            Vol. 8, No. 6, September 2010
                                                                       D. Maxflow Based Channel Assignment Routing:
  VI.       CHANNEL ASSIGNMENT IN MULTI RADIO                          MCAR is an improvement over centralized channel
                            ROUTERS [2] [4]                            assignment algorithm. The interdependence among channel
In case of a two radio router network, there is no much                assignments across the whole network is taken into account
flexibility, because one of the radios is used to                      by first identifying the groups of links that need to be
communicate with other mesh routers on same channel                    assigned the same channel in order for the number of
and the other radio is used to communicate with clients.               different channels on every router not to exceed the number
But if we have more than two radios on each mesh router,               of radios. Then, the actual channel assignment stage exploits
we could use one for communicating with clients and the                the result of the first stage to assign channel in such a way
other radios could be intelligently assigned different                 that no replacement of previously assignments are
channels so that they form channel diversified routes                  necessary.
among mesh routers. Whether or not two routers are
neighbors is decided by the channels assigned to them.                 E. Topology and Interference aware Channel Assignment:
The channel assignment can be made based on link quality               This algorithm undergoes two phases. One is Topology
and the topology.                                                      discovery and the other is channel selection.
There are various algorithms proposed for the channel                  Topology discovery: Prior to the channel assignment the
assignment problem. They can be classified into two                    topology is discovered. Topology discovery for every
categories;                                                            router is the identification of band-specific set of
     1. Interference-aware channel assignment(IACA)                    neighboring routers and the measurement of quality of
     2. Traffic-aware channel assignment (TACA)                        link to each of these neighbors. Each router tunes itself to
Some of the algorithms are:                                            various channels on which band topology is to be
     1. Identical channel assignment                                   discovered. This activity is co-ordinated by the channel
     2. Hybrid channel assignment                                      management server. The link quality is measured by ETT
     3. Centralized channel assignment                                 (estimated transmit time).
     4. Maxflow based channel assignment routing                       Channel selection: Dijkstra's shortest path algorithm is
          (MCAR)                                                       used in TIC to discover frequency-diversified routes
     5. Topology and          interference-aware channel               between the gateway and routers. The interference
          assignment (TIC)                                             between mesh links is generated using conflict-graph
                                                                       model. For generating the above model interfering mesh
A. Identical Channel Assignment:                                       links have to be identified in the first place. Thus the data
In this method first radio is assigned channel 1, second is            generated in the first phase (topology discover) can be
assigned next non overlapping channel and so on. Though                used to construct conflict graph. Thus the interfering links
this preserves connectivity, this method in no way makes               are assigned non-overlapping channels.
any effort in reducing interference.                                   Cross-layer work: In most of the situation the throughput
                                                                       of configured WMN depends on both the channel
B. Hybrid Channel Assignment:                                          assignment and routing algorithm chosen. So there is a lot
In this strategy some radios are statically assigned channels          of research in developing the cross-layer protocols which
while other radios are assigned channels dynamically.                  deals with the channel assignment and routing jointly.

C. Centralized Channel Assignment:                                                      VII.    Routing Protocols [9]
In this method the links are visited in some order and a               Routing protocols lie at the heart of designing a WMN
common channel is assigned to interfaces on both the ends.             network. They, in simple terms, specify the relay routes for
If all the interfaces of the end node are already assigned a           packets in the network. Most of the protocols neglect the
channel and they don’t share any common channel, then it is            traffic between the mesh nodes and only         consider the
necessary to replace one on the channel assignments. This              traffic between the nodes and the internet.
ends up in a recursive channel assignment procedure. The               Network Asymmetry: This is the situation in which
visit can be in the decreasing order of the number of links            forward direction of a network is significantly different
falling in the interference range and the least used channel in        from the reverse direction in terms of bandwidth, loss rate,
that range is selected (interference aware). It could also be          and latency. Forward path routing protocols are effective
based on the estimated link flow rates(traffic aware).The              in routing of the packets from the mesh nodes to the
algorithm might then visit all the links in decreasing order of        gateway of the WMN, backward routing protocols are
expected link flow rate and select the channel which                   effective in routing the packets from the internet to the
minimizes the sum of expected flow rates of all the links in           mesh nodes.
the interference region that are assigned the same channel.            Some of the most popular protocols being used are AODV
                                                                       and OLSR.

                                                                                                  ISSN 1947-5500
                                                                  (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                 Vol. 8, No. 6, September 2010
A. Ad-hoc On Demand Distance Vector Routing(AODV):                          this protocol is that intermediate nodes can lead to
The Ad hoc On Demand Distance Vector (AODV) routing                         inconsistent routes if the source sequence number is very
algorithm is a routing protocol designed for ad hoc mobile                  old and the intermediate nodes have a higher but not the
networks. AODV is capable of both unicast and multicast                     latest destination sequence number, thereby having stale
routing. It is an on demand algorithm, meaning that it                      entries. Also multiple RouteReply packets in response to a
builds routes between nodes only as desired by source                       single RouteRequest packet can lead to heavy control
nodes. It maintains these routes as long as they are needed                 overhead. Another disadvantage of AODV is that the
by the sources. Additionally, AODV forms trees which                        periodic beaconing leads to unnecessary bandwidth
connect multicast group members. The trees are composed                     consumption.
of the group members and the nodes needed to connect the                    B. Optimized Link State Routing Protocol(OLSR):
members. AODV uses sequence numbers to ensure the                           It is a proactive protocol. The Optimized Link State
freshness of routes. It is loop-free, self-starting, and scales             Routing Protocol (OLSR) [1] [5] is a proactive routing
to large numbers of mobile nodes.                                           protocol. Every node sends periodically broadcast
AODV builds routes using a route request / route reply                      "Hello"-messages with information to specific nodes in
query cycle. When a source node desires a route to a                        the network to exchange neighborhood information. The
destination for which it does not already have a route, it                  information includes the nodes IP, sequence number and a
broadcasts a route request (RREQ) packet across the                         list of the distance information of the nodes neighbors.
network. Nodes receiving this packet update their                           After receiving this information a node builds itself a
information for the source node and set up backwards                        routing table. Now the node can calculate with the shortest
pointers to the source node in the route tables. In addition                path algorithm the route to every node he wants to
to the source node's IP address, current sequence number,                   communicate. When a node receives an information
and broadcast ID, the RREQ also contains the most recent                    packet with the same sequence number twice he is going
sequence number for the destination of which the source                     to discard it. In these routing tables he stores the
node is aware. A node receiving the RREQ may send a                         information of the route to each node in the network. The
route reply (RREP) if it is either the destination or if it has             information is only updated:
a route to the destination with corresponding sequence                           1. A change in the neighborhood is detected.
number greater than or equal to that contained in the                            2. A route to any destination is expired.
RREQ. If this is the case, then it unicast a RREP back to                        3. A better (shorter) route is detected for a
the source. Otherwise, it rebroadcasts the RREQ. Nodes                                destination.
keep track of the RREQ's source IP address and broadcast
ID. If they receive a RREQ which they have already                          The difference from OLSR to LSR (Links State Protocol)
processed, they discard the RREQ and do not forward it.                     is that OLSR relies on multi-point relays (MPR). MPR is a
If the RREP propagates back to the source, then nodes set                   node which is selected by its direct neighbor (one
up forward pointers to the destination. Once the source                     hop).The first idea of multipoint relays is to minimize the
node receives the RREP, it may begin to forward data                        flooding of broadcast messages in the network. An
packets to the destination. If the source later receives a                  information packet should not be sent twice in the same
RREP containing a greater sequence number or contains                       region of the network. MPR helps to optimize and reduce
the same sequence number with a smaller hop count, it                       that problem. Each node informs its direct neighbors (one
may update its routing information for that destination and                 hop) about its MPR set in the "Hello"-messages. After
begin using the better route.                                               receiving such a "Hello"-message, each node records the
As long as the route remains active, it will continue to be                 nodes MPR Selector that selects it as one of their MPRs.
maintained. A route is considered active as long as there                   The second idea is that the size of the hello messages is
are data packets periodically travelling from the source to                 reduced. It includes only the neighbors that select node N2
the destination along that path. Once the source stops                      as one of their MPR nodes. In this way partial topology
sending data packets, the links will time out and                           information is propagated. Node N2 can be reached only
eventually be deleted from the intermediate node routing                    from its MPR selectors.
tables. If a link break occurs while the route is active, the               Advantages:
node upstream of the break propagates a route error                              1. Minimal latency.
(RERR) message to the source node to inform it of the                            2. Ideal in high density and large networks.
now unreachable destination(s). After receiving the                              3. OLSR achieves more efficiency than classic LS
RERR, if the source node still desires the route, it can                             algorithms when networks are dense.
reinitiate route discovery.                                                      4. OLSR avoids the extra work of "finding" the
Advantage and disadvantages:                                                         destination by retaining a routing entry for each
The main advantage of this protocol is that routes are                               destination all the time, thus providing low
established on demand and destination sequence numbers                               single-packet transmission latency.
are used to find the latest route to the destination. The                        5. OLSR can easily be extended to QoS monitoring
connection setup delay is less. One of the disadvantages of                          by including bandwidth and channel quality

                                                                                                       ISSN 1947-5500
                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                              Vol. 8, No. 6, September 2010
        information in link state entries. Thus, the quality             F. Gateway Source Routing (GSR):
        of the path (e.g., bandwidth, delay) is known                    In this protocol forward path information from the packets
        prior to call setup.                                             that arrive at the gateway is reused. In the routing header
Disadvantages:                                                           of every packet, the intermediate hops from the mesh node
    1. When the network is sparse, every neighbor of a                   to the gateway are recorded. These paths are then stored in
        node becomes a multipoint relay. The OLSR then                   the gateways. To route packets to a mesh node, the mesh
        reduces to a pure LS protocol.                                   gateway inverts the recorded forward path and copies it to
    2. High control overhead (reduced by MPR usage) .                    the packet header. The gateway then sends the packet to
    3. Higher computation.                                               the first node of the backward path. Each node updates the
    4. Storage.                                                          path in the header by removing its entry and forward the
    5. Implementation complexity.                                        packet to the given next hop until the packets reaches the
C. Backward Routing Protocol:                                            GSR requires that a packet towards a host in the internet is
The above mentioned protocols are designed to route in                   first sent by a mesh node in order to establish the
the direction from the mesh nodes to the internet.                       backward path. This should not be a problem when we
However most services generate asymmetric traffic and                    assume that majority of communication is initiated by the
the amount of the downstream from the servers in the                     mesh nodes.
internet to the mesh nodes far exceeds the upstream.                     If a mesh node has to act as a server, a dedicated
Therefore some routing protocols are proposed which take                 addressing mechanism will have to be used.
care of this backward traffic. Backward path routing in
more involved than routing on forward path because the                   G. Hierarchical Routing Protocols:
data addressed at any host in the internet only needs to be              Some of the protocols which differ from the above are
forwarded to the gateway, while the backward routing                     hierarchical routing and geography based routing.
protocol needs to address each node individually.                        In hierarchical routing, a certain self-organization scheme
There are three main families of backward routing                        is employed to group network nodes into clusters. Each
protocols; reactive hop-by-hop routing, proactive hop-by-                cluster has one or more cluster heads. Nodes in a cluster
hop routing, and proactive source routing.                               can be one or more hops away from the cluster head.
                                                                         Since connectivity between clusters is needed, some nodes
D. AODV-CGA:                                                             can communicate with more than one cluster and work as
This extended AODV protocol allows the use of multiple                   a gateway. When the node density is high, hierarchical
gateways to the internet. It shares most of the mechanisms               routing protocols tend to achieve much better performance
with the well-known AODV protocol. The addition to the                   because of less overhead, shorter average routing path,
existing AODV which is made is that, all gateways are                    and quicker set-up procedure of routing path. However,
connected to a dedicated router that acts as a proxy to the              the complexity of maintaining the hierarchy may
internet. This router has two tasks:                                     compromise the performance of the routing protocol.
     1. On the forward path, it sends route on behalf of                 In WMNs, a mesh client must avoid being a cluster head
          hosts in the internet;                                         because it can become a bottleneck due to its limited
     2. On the backward path, it initiates route requests for            capacity.
          nodes in the wireless mesh network.
                                                                         H. Geographical Base Routing:
E. Proactive Field-based Routing (PFR):                                  Compared to topology-based routing schemes, geographic
Wireless mesh nodes periodically exchange beacons.                       routing schemes forward packets by only using the position
These beacons contain a list of all known destinations with              information of nodes in the vicinity and the destination
their respective field value. When a new destination                     node. Thus, topology change has less impact on the
appears, it announces its presence with beacons to its                   geographic routing than the other routing protocols. Early
neighbors in order to establish a field. With this                       geographic routing algorithms are a type of single-path
mechanism, a field on the network is constructed for every               greedy routing schemes in which the packet forwarding
destination. This field assigns a value to every node in the             decision is made based on the location information of the
network; the destination bears the maximum value.                        current forwarding node, its neighbors, and the destination
Packets are then routed along the steepest gradient                      node. However, all greedy routing algorithms have a
towards the destination.                                                 common problem, i.e., delivery is not guaranteed even if a
Advantages and disadvantages:                                            path exists between source and destination. In order to
This protocol ensures loop freedom. This protocol enables                guarantee delivery, planar-graph-based geographic routing
nodes to consider multiple routes to the destination. This               algorithms have been proposed recently. However, these
protocol also has a drawback. Since it proactively                       algorithms usually have much higher communication
maintains all routes, it incurs communication overhead                   overhead than the single-path greedy routing algorithms.
even if the traffic is too low.

                                                                                                    ISSN 1947-5500
                                                                           (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                          Vol. 8, No. 6, September 2010
                 VIII.    CONCLUSIONS
Here we mentioned the basics of wireless mesh network,
their purpose, various techniques involved and the area of
the research in wireless mesh networks. Further research can
be done studying the relationship between channel
assignment techniques and routing protocol. These two
areas will influence one another and an efficient
combination can be possibly found.

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[5]. Routing Packets into Wireless Mesh Networks. Baumann R,
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[8]. Hybrid routing with periodic updates in wireless mesh networks.
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