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1. WO2010083660 - METHOD, DEVICE AND SYSTEM FOR ASSIGNING CHANNELS IN WIRELESS MESH NETWORKS

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

METHOD, DEVICE AND SYSTEM FOR ASSIGNING CHANNELS EV WERELESS

MESH NETWORKS

Field of the Invention

[0001] The present invention relates to the field of Wireless Local Area Networks, and in particular to a method and a device and a system for Assigning Channels in Wireless Mesh Networks.

Background of the Invention

[0002] While IEEE 802.1 Ig only supports a maximum of 3 non-overlapping channels in the 2.4 GHz band, IEEE 802.11a has much more resources: Depending on the regulatory domain, up to 12 orthogonal channels are available in the region around 5.5 GHz. As the higher frequency increases the pathloss, transmission powers of up to 30 dBm (Decibels referenced to one milliwatt) are allowed for outdoor usage. Therefore, IEEE 802.11a provides an excellent foundation for city-wide Wireless Mesh Networks (WMNs).

[0003] As described in the market overview, three different WMN deployment concepts are possible, each building upon the predecessor: Single-radio WMNs, Dual-radio WMNs, and Multi-radio WMNs.

[0004] Single-radio WMNs: Each Mesh Point (MP) and Access Point (AP) is equipped with one radio only. As the IEEE 802.11 Medium Access Control (MAC) does not foresee a multi-channel operation, and no such operation can be expected in the near future, it is the only option for the single-radio WMN to use one frequency only, both for the communication to the associated Stations (STAs) in the Basic Service Sets (BSSs) as well as for the mesh links. Therefore, there is interference from BSS to BSS, from BSS to the mesh links and between the mesh links. This interference is the major limiting factor of the WMN performance and is responsible for the poor efficiency of IEEE 802.11 -based WMNs. [0005] Dual-radio WMNs: Using dual radio MPs/ APs, two optimization approaches become possible. First, MPs and APs can use one radio for the mesh backbone and the other one for the BSS, which allows to separate the mesh backbone and the BSSs by using different frequency channels. Hence, the user-controlled STAs can no longer interfere with the provider-controlled mesh, which allows for more reliable operation. Second, the MPs/ APs are no longer forced to use the same frequency channel for their BSSs, which reduce the inter-BSS interference. Commonly, dual-radio MPs/ APs are equipped with one IEEE 802. Hg radio for the BSSs and one IEEE 802.11a radio for the mesh backbone. Frequency planning for the BSSs is a known problem from the standard operation of Wireless Local Area Networks (WLANs) and therefore not in the scope of this patent; frequency planning for the mesh is not necessary, as one frequency channel must be shared to allow for connectivity of the mesh network. Therefore, mesh links still interfere with each other, reducing the capacity of the WMN.

[0006] Multi-radio WMNs: Only in multi-radio WMNs the full capability of a multi-channel operation can be used to enhance the network capacity: While one radio is dedicated to the

BSS, r radios are available for the mesh backbone, r more than or equal to 2. Therefore, it becomes possible to use more than one frequency for the mesh without loosing the connectivity, hence the term Multi-Radio Multi-Channel (MRMC) WMNs.

[0007] Due to size limitations and antenna separation, the number of radios per MP/ AP is limited; usually, 2 or 3 radios are available for the mesh network and 1 for the BSS. With up to 12 frequency channels in IEEE 802.11a, a planning of the frequency/radio assignment is required in Multi-radio WMNs. Summary of the Invention

[0008] The present invention is directed to a method, a device and a system for Assigning Channels in Wireless Mesh Networks for planning of the frequency/radio assignment which is required in Multi-radio WMNs.

[0009] Accordingly, the embodiment provided in the present invention is realized through the following technical solutions.

[0010] In an embodiment, the present invention provides a method for Assigning Channels in Wireless Mesh Networks, which includes the following steps: channel assignment is created from initial topology of the network by iteratively determining a bottleneck node and expanding a channel; the channels of the channel assignment are merged according to the occupancy of each channel until channel number of the channel assignment is equal to the maximum number of channels; the merged channel assignment is distributed to the network.

[0011] In an embodiment, the present invention further provides a device for Assigning Channels in Wireless Mesh Networks, which includes: an expand module, configured to create channel assignment from initial topology of the network by iteratively determining a bottleneck node and expanding a channel; a reduction module, configured to merge the channels of the channel assignment according to the occupancy of each channel until channel number of the channel assignment is equal to the maximum number of channels; a distribution module, configured to distribute the merged channel assignment to the network.

[0012] In an embodiment, the present invention further provides a system for Assigning Channels in Wireless Mesh Networks, which includes: more than one AP/MP, configured to communicate with each other and execute channel assignment distributed by a channel assignment device; the channel assignment device, configured to create channel assignment from initial topology of the network by iteratively determining a bottleneck node and expanding a channel; merge the channels of the channel assignment according to the occupancy of each channel until channel number of the channel assignment is equal to the maximum number of channels; distribute the merged channel assignment to the network.

[0013] It can be seen from the above technical solutions that, a radio with a new channel is added to the bottleneck node or the neighbor of the bottleneck node without the maximum number of available channels considered, and then the channels of the WMN are merged until channel number of the WMN lowers to the maximum number of channels, so the channel assignment finally distributed to the WMN has a channel number equal to the maximum number of channels. In this way, the method, the device and the system for Assigning Channels in Wireless Mesh Networks can output channel assignment which not only fulfills the constraints of the maximum channel number but also makes the WMN more efficient.

Brief Description of the Drawings

[0014] The drawings required in the descriptions of the embodiments of the invention or the prior art will be introduced briefly in order to explain the technical solutions in the embodiments or the prior art more clearly. Evidently, the drawings described below are merely illustrative of some embodiments of the invention, and those ordinarily skilled in the art can further derive from these drawings other drawings without any inventive effort.

[0015] FIG 1 illustrates a flow chart of a method for assigning channels in a WMN according to a first embodiment of the present invention;

[0016] FIG 2 illustrates a flow chart of a method for assigning channels in a WMN according to a second embodiment of the present invention;

[0017] FIG 3a ~ 3h illustrate schematic views of a network split example according to the second embodiment of the present invention; [0018] FIG 4a ~ 4f illustrate schematic views of an expansion example according to the second embodiment of the present invention;

[0019] FIG 5a ~ 5e illustrate schematic views of an merge example according to the second embodiment of the present invention;

[0020] FIG 6 illustrates a schematic structural view of a device for assigning channels in a WMN according to a third embodiment of the present invention;

[0021] FIG 7 illustrates a schematic structural view of a system for assigning channels in a WMN according to a fourth embodiment of the present invention;

[0022] FIG 8a illustrates a result view of Saturation throughput in the 20 evaluation scenarios; [0023] FIG 8b illustrates a result view of Saturation throughput in the 20 evaluation scenarios for one dedicated mesh radio & channel.

Detailed Description of the Invention

[0024] The technical solutions in the embodiments of the invention will be described below clearly and fully with reference to the drawings in the embodiments of the invention. Evidently, the described embodiments are only a part but not exhaustive of embodiments of the invention. Any other embodiments which will occur to those ordinarily skilled in the art in light of the embodiments in the invention here without any inventive effort shall fall within the scope of the invention.

[0025] A technical solution will be described in a first embodiment of the invention taking a method for assigning channels in a WMN as an example. According to the first embodiment of the invention as illustrated in Figure 1, a method for assigning channels in WMN includes the following steps:

[0026] In step 101, a bottleneck node of the WMN is determined. [0027] With the status information of the WMN, such as the initial topology of the WMN, the traffic of each link, and the rate of each link, there are many methods to determine the bottleneck node. For example the node which is one hop connected with the node that has wired link with the external network is determined as the bottleneck node. Another method to determine the bottleneck node is using occupancy of each node to evaluate the bottleneck node. The occupancy of one node means the fraction of time blocked by and transmitting to its neighbor nodes in the same channel. When a channel has a full load, its occupancy is 1.0; a channel has overload with occupancy greater than 1.0; otherwise if the channel still has unused load remaining, its occupancy is less than 1.0. There will be c occupancies for a multi-radio node, where the c indicates the channel number of the multi-radio node. The node which has an occupancy greater than 1.0 is the bottleneck node of the WMN. If there are more than one node or no node having an occupancy greater than 1.0, the node which has the highest occupancy on any channel is determined as the bottleneck node.

[0028] The occupancy of a node can be computed with the following equations: occupancy = fracTx + fracBusy; fracTx is sum of loads of all outgoing links of the node, and fracBusy is sum of loads of all neighboring nodes; and loads = offered trffic per link / current rate per link used.

[0029] The bottleneck node of the WMN is determined with the network topology as input. The network topology includes all information that is required to compute the occupancy. Initially, each node is equipped with one mesh radio and one BSS radio; and all mesh radios are tuned to the same frequency so that the nodes can communicate with each other. This represents the typical dual-radio WMN.

[0030] In step 102, whether radio number of the bottleneck node reaches the maximum number of radios is determined. If radio number of the bottleneck node does not reach the maximum number of radios, turn to step 103; otherwise, if radio number of the bottleneck node reaches the maximum number of radios, turn to step 105.

[0031] After the bottleneck node (node bn) is determined in step 101, suppose channel Cb is the channel on which the bottleneck node bn has the highest occupancy. Two different options are now possible: Either, the new radio is added directly to node bn, enhancing node bn's capacity; or the new radio is added to a node in the near neighborhood (a neighbor node of the node bn), lowering the interference on node bn by moving some links of this node to a new channel. [0032] If node bn has fewer radios than the maximum number of radios, the first option is selected, as this option has the maximum impact on the occupancy of node bn; and the new radio is added directly to node bn in step 103. If radio number of node bn reaches the maximum number of radios, it means node bn has maximum radios and it is not possible to extend node bn; then the second option is selected, and a candidate node is searched in the near neighborhood of the node bn. After the candidate node is determined by searching, the new radio is added to the candidate node. With a new channel assigned to the new radio of the candidate node, several links of the candidate node are switched to the new channel and then turn to step 101.

[0033] This search for the candidate node requires a candidate node to have traffic on the channel Cb and to be within the reception range of node bn; therefore, the addition of a radio to the candidate node has an effect on the occupancy of node bn. Furthermore, the candidate node has fewer radios than the allowed limit, which means the radio number of the candidate node is less than its maximum number of radios.

[0034] If more than one candidate node exists, the one with the highest cumulated offered traffic may be selected to add a new radio; if no candidate node exists, there is no possibility to decrease the occupancy of node bn and thus the search stops. As radio number of the bottleneck node reaches the maximum number of radios and no candidate node exists, turn to step 105. Usually, when more than one candidate node exists, anyone of the candidate nodes can be selected to add a new radio. [0035] In step 103, if radio number of the bottleneck node does not reach the maximum number of radios, a new radio is added to the bottleneck node.

[0036] Before the step 101, anyone of the nodes in the WMN, including the bottleneck node and the candidate node, has been at least using one radio for the mesh backbone; and some nodes may at least use two radios, one radio for the BSS and the other for the mesh backbone. In the Multi-radio WMN, every node has 2 radios at least for the mesh backbone. The maximum number of radios limits the available radios a node can use. When a new radio is added, the bottleneck node or the candidate node uses an unused radio for the mesh backbone. A new radio is added to the bottleneck node or the candidate node by using a new radio, such as in the step 103. [0037] In step 104, with a new channel assigned to the new radio of the bottleneck node, several links of the bottleneck node are switched to the new channel and then turn to step 101.

[0038] After the bottleneck node starts using a new radio, a new channel is assigned to the new radio of the bottleneck node. Some links of the bottleneck node are selected and switched to the new channel. So the links of the bottleneck node are partly switched to the new channel.

[0039] In step 105, the channels of the WMN are merged according to the occupancy of each channel until channel number of the WMN lower to the maximum number of channels. [0040] As the limited number of available channels is not considered during the channel assignment, the configuration resulting form the first stage (the step 101-104) will have for most networks more channels than the maximum number of channels in the service area. Therefore, the channel number of the WMN is reduced by merging the channels. The channels are merged while most of the capacity improvements and the connectivity are maintained.

[0041] After computing the occupancy of each node on each channel, the two least occupied channels (cl and c2) are determined; a channel occupancy is measured as the maximum occupancy of any node on this channel. After finding those two least occupied channels cl and c2, cl and c2 are merged, for example all links from cl are reassigned to c2, reducing the channel number by one. Then if the channel number of the WMN greater than the maximum number of channels, another two least occupied channels are determined and merged; the determine and merge is repeated to lower the channel number of the WMN until the channel number of the WMN is equal to the maximum number of channels.

[0042] In step 106, the channel assignment is distributed to the WMN. [0043] The addition of new radios to nodes, the bottleneck node or the candidate node, can increase the capacity of the bottleneck node; this is done by adding a new radio either to the bottleneck node or to a node in the direct neighborhood of the bottleneck node, which lowers the interference on the bottleneck node. With each added radio, a new channel is assigned and selected links are switched to this new channel, lowering the occupancy of the bottleneck node.

[0044] The first stage (the step 101-104) of the method is only concerned with this addition of new radios; it stops if the bottleneck node and all its neighbor nodes have reached the maximum number of radios. At the first stage, the restriction of the channel number is not considered. Hence, a valid output of the first stage is a channel assignment that uses more channels than available. Therefore, the second stage of the method (the step 105) reduces the channel number by merging iteratively the two least used channels. At the second stage, the channel number of the WMN is reduced to equal to or less than the maximum number of channels; and then the channel assignment, which has a channel number equal to or less than the maximum number of channels, is distributed to the WMN to configure the network. [0045] The first embodiment describes a method for assigning channels in a WMN. This method gives a technical solution for planning of the frequency/radio assignment which is required in Multi-radio WMNs. And a radio with a new channel is added to the bottleneck node or the neighbor of the bottleneck node without the maximum number of available channels considered, and then the channels of the WMN are merged until channel number of the WMN lowers to the maximum number of channels, so the channel assignment finally distributed to the WMN has a channel number equal to the maximum number of channels. This method described in the first embodiment can output channel assignment which not only fulfills the constraints of the maximum channel number but also makes the WMN more efficient.

[0046] A technical solution will be described in a second embodiment of the invention taking a method for assigning channels in a WMN as an example. According to the second embodiment of the invention as illustrated in Figure 2, the channel assignment method includes the following steps:

[0047] In step 201, a bottleneck node of the WMN is determined, and a channel is expanded.

[0048] The channel expansion is the first major part of the channel assignment method. By neglecting the restriction of the number of available orthogonal channels and only considering the maximum number of radios per node (which is given by cost and/or implementation limits), it extends a dual-radio WMN until no more radios can be added. In the step 201, the channel expansion may include several steps to add several new radios to different nodes.

[0049] Network split is the fundamental operation during the channel expansion to the MRMC WMN. The network split allows to add one more new radio to a given node (node i) and switch links of node p to the new radio on a new channel without requiring any other new radio in the network. As an input to the network split , it requires node i, a set of links L = {(/, j) I j e N} connected to node i and a new channel,. All links between node i and node j are contained in L, where node j can be any node in the network N. During the network split, first a link (Ll) between node i and node q is taken out of L, and switched to the new channel. Secondly, node q's links on the same channel as Ll are switched to the new channel. Thirdly, for node q's neighbor nodes except node i, all the links on the same channel as Ll of the neighbor nodes except node i are switched to the new channel; and then the neighbor's neighbors except node q and so on. In this way, node i splits the network into two separated layers, one with the old channels and the other one with the new channel. Depending on the connectivity of node i, large parts of the network may be moved to the layer with the new channel. Besides during the network split, at first more than one link, such as Ll between node i and node j and L2 between node i and node m, may be taken out of L, and then L2 is dealt with similarly to Ll so as to split the network into two separated layers, one of which has Ll and L2.

[0050] The network split conserves connectivity in the network: If the network was connected before the split, it still is connected after the split with a new channel added. Both layers of the network, the one on the old channels and the one on the channel are connected, and the two layers are connected via the new radio of node i.

[0051] Figure 3a~3h show a network split example. In Figure 3b~3h, the real lines are old channels and the dashdotted lines are new channels. Figure 3a shows a network with node i (the black node) and other 6 nodes, where the network split is done. There are 7 possible splits for the example network in 3a: 4 splits with one link of node i at a new channel, 3 splits with

2 links of node i at a new channel. Figure 3b~3e show 4 splits with one link of node i at a new channel and Figure 3f~3h show 3 splits with two links of node i at a new channel. Besides, in the splits showed by Figure 3b~3h, the real lines can be used as new channels and the dashdotted lines as old channels.

[0052] Figure 3b~3h shows all 7 possible splits that can be applied to the black node (node i) of the network in Figure 2a: As node i has 4 links, each of the links can be switched separately to a new channel; additionally, a combination of 2 links can be switched. For a node with k links, there are in total 2k~l - 1 different possible splits. In WMNs which are deployed for coverage, k seldom exceeds 5; thus, an enumeration of all splits has size 15.

[0053] Based on the network split operation, Figure 4a~4f show an expansion example with several successive channel expansion steps. For the expansion example it is assumed that the maximum number of radios per node is three, two radios for mesh and one for BSS. In Figure 4a~4f, different kinds of lines correspond to different channels, and the number near a node means the radio number of the node for mesh. The maximum number of radios is assumed as 2 radios for mesh. As the node may not have a radio for BSS, the radio for BSS is not considered in Figure 4a~4f.

[0054] Figure 4a shows the initial topology of the network. Starting with the network in Figure 4a, the black node is identified as the bottleneck node, and all the nodes in the network are using the same channel (channel 1) for mesh backbone, here node a (the black node in Figure 4a) being the bottleneck node in Figure 4a; hence, the split is performed at node a. With four links, seven different splits are possible for node a: four splits where only one link and the attached sub-tree switch; or three splits where two links switch simultaneously.

[0055] As node a has seven different splits, the occupancy of node a with seven different splits is computed. By comparing the occupancy of the bottleneck node with seven different splits, the split with which node a has a lowest occupancy is selected, for example the split indicated in Figure 4b is selected. Then as showed in Figure 4b, a new radio is added to node a with a new channel (channel 2) expanded, and now the node b (the black node in Figure 4b) becomes the bottleneck node. [0056] Similarly, the next two splits to the expansion in Figures 4c and 4d are computed. In Figures 4c, a new radio is added to node b with a new channel (channel 3) expanded; In Figures 4d, a new radio is added to node d with a new channel (channel 4) expanded. Then, the bottleneck node of the network in Figure 4d is again node a, which has already reached the maximum number of radios, two radios for mesh. Therefore, the split is applied at one of the four neighbors of node a; the node (node d) right of node a is selected and a new channel (channel 5) is expanded to node d in Figures 4e. Similarly, the split to the network in Figure 4e is applied to the node (node e) left of node a, and a new channel (channel 6) is expanded to node e in Figures 4f.

[0057] Finally, the network in Figure 4f has reached the maximum stage of expansion: The bottleneck is again node a, and no neighbor of the bottleneck node is a valid candidate node, because they have either reached the maximum number of radios or a split would not reduce the occupancy. Thus, the expansion finishes with a (2, 6)-MRMC configuration, where the expansion result is a topology configuration with 6 channels and every node's maximum number of radios for mesh is two.

[0058] The step 201 can be executed iteratively for times until there is no more expansion able to be done, and for each time one bottleneck node of the WMN is determined, and one channel is expanded. In step 201, when a bottleneck node of the WMN is determined, a channel is expanded to the bottleneck node or one candidate node among the bottleneck node's neighbors. If radio number of the bottleneck node reaches the maximum number of radios and no candidate node exists in the neighbors, the expansion is finished with a topology configuration as an output. The topology configuration contains information of nodes and channels in the WMN.

[0059] In step 202, the channels of the WMN are merged according to the occupancy of each channel until channel number of the WMN lowers to the maximum number of channels.

[0060] With the topology configuration determined in the step 201, the channels of the WMN are merged until channel number of the WMN lower to the maximum number of channels.

[0061] Figure 5a~5e show a merge example with the step-by-step channel reduction down to two channels. For the merge example it is assumed that the maximum number of channels is two in the WMN. In Figure 5a~5e different kinds of lines correspond to different channels, and the number near a node means the radio number of the node for mesh. [0062] Figure 5a shows a network resulting from the step 201. The network in Figure 5a has the same topology configuration as the network in Figure 4f determined in the step 201.

[0063] Starting with the network in Figure 5a, the occupancy of each node on each channel is computed, and the two least occupied channels (channel 1 and channel 3) are determined; a channel occupancy is measured as the maximum occupancy of any node on this channel. The occupancy of channel 1 or channel 3 is lower than other channels', such as channel 2, channel 4, channel 5 or channel 6. After finding those two least occupied channels (channel 1 and channel 3), channel 1 and channel 3 are merged, for example all links from channel 3 are switched to channel 1 as showed in Figure 5b, reducing the channel number by one. Figure 5b shows a network has 5 channels, one less than the initial network in Figure 5a.

[0064] Then in the network showed in Figure 5b, the channel occupancy of each node on each channel is computed, and the two least occupied channels are determined as channel 2 and channel 6. Channel 6 is the channel between node c and node e. The channel 2 and channel 6 are merged in Figure 5c where all links from channel 6 are switched to channel 2.

[0065] Similarly, the next two merges are showed in Figures 5d and 5e. The channel occupancy in Figures 5c is computed, and channel 1 and channel 4 are merged in Figures 5d; the channel occupancy in Figures 5d is computed, and channel 2 and channel 5 are merged in Figures 5e.

[0066] Finally, the network in Figure 5e has two channels only. The channel number of the network in Figure 5e to the maximum number of channels is lowered to the maximum number of channels. The merge is finished with a topology configuration in Figure 5e as an output. The topology configuration contains information of nodes and channels in the WMN. [0067] In step 203, the channel assignment is distributed to the WMN.

[0068] After the step 202, the topology configuration with channel number equal to the maximum number of channels is determined. Then the channel assignment of the topology configuration is distributed to the WMN, and the WMN nodes work as the channel assignment. [0069] The second embodiment describes a method for assigning channels in a WMN. This method gives a technical solution for planning of the frequency/radio assignment which is required in Multi-radio WMNs. And a radio with a new channel is added to the bottleneck node or the neighbor of the bottleneck node without the maximum number of available channels considered, and then the channels of the WMN are merged until channel number of the WMN lowers to the maximum number of channels, so the channel assignment finally distributed to the WMN has a channel number equal to the maximum number of channels. This method described in the first embodiment can output channel assignment which not only fulfills the constraints of the maximum channel number but also makes the WMN more efficient.

[0070] A technical solution will be described in a third embodiment of the invention taking a device for assigning channels in a WMN as an example. According to the third embodiment of the invention as illustrated in Figure 6, the channel assignment device includes the following modules: [0071] an expand module 61, configured to create channel assignment from initial topology of the network by iteratively determining a bottleneck node and expanding a channel;

[0072] a reduction module 62, configured to merge the channels of the channel assignment, which is created by the expand module 61, according to the occupancy of each channel until channel number of the channel assignment is equal to the maximum number of channels;

[0073] a distribution module 63, configured to distribute the merged channel assignment, which is merged by the reduction module 62, to the network.

[0074] The expand module 61 may include the following units:

[0075] a storage unit 610, configured to store the status of the network, such as the topology of the network, the traffic of each link, and the rate of each link;

[0076] a determine unit 611, configured to determine a bottleneck node of the network according to the status of the network which is stored by the storage unit 610;

[0077] an addition unit 612, configured to add a new radio to the bottleneck node, which is determined by the determine unit 611, with a new channel assigned to recompose the topology of the network which is stored by the storage unit 610, if radio number of the bottleneck node is less than the maximum number of radios. [0078] The determine unit 611 and the addition unit 612 cooperate with each other to iterate the steps they are configured to do. The steps, which the determine unit 611 and the addition unit 612 are configured to do, are iterated to recompose the topology of the network until radio number of the bottleneck node is equal to the maximum number of radios so as to create a channel assignment which may be the topology of the network stored by the storage unit 610.

[0079] The expand module 61 may include the following units:

[0080] a storage unit 610, configured to store the status of the network, such as the topology of the network, the traffic of each link, and the rate of each link;

[0081] a determine unit 611, configured to determine a bottleneck node of the network according to the status of the network which is stored by the storage unit 610;

[0082] an addition unit 612, configured to add a new radio to the bottleneck node, which is determined by the determine unit 611, with a new channel assigned to recompose the topology of the network which is stored by the storage unit 610, if radio number of the bottleneck node is less than the maximum number of radios; [0083] an second addition unit 613, configured to add a new radio to a neighbor node of the bottleneck node, which is determined by the determine unit 611, with a new channel assigned to recompose the topology of the network which is stored by the storage unit 610, if radio number of the bottleneck node is equal to the maximum number of radios and radio number of the neighbor node is less than the maximum number of radios. [0084] The determine unit 611, the addition unit 612 and the second addition unit 613 cooperate with each other to iterate the steps they are configured to do. The steps, which the determine unit 611, the addition unit 612 and the second addition unit 613 are configured to do, are iterated to recompose the topology of the network until radio number of the bottleneck node and the neighbor nodes is equal to the maximum number of radios so as to create a channel assignment which may be the topology of the network stored by the storage unit 610. Here, the neighbor nodes above may be some neighbors of the bottleneck node.

[0085] The addition unit 612 or the second addition unit 613 may include the following units:

[0086] a radio addition unit, configured to add a new radio to the node which may be the bottleneck node or a neighbor node;

[0087] a channel assign unit, configured to assign a new channel to the new radio of the node which is added by the radio addition unit;

[0088] a link switch unit, configured to switch several links of the node to the new channel, which is assigned by the channel assign unit, to recompose the status of the network. [0089] The reduction module 62 may include the following units:

[0090] a compute unit 621, configured to compute the occupancy of each node on each channel of the channel assignment;

[0091] a second determine unit 622, configured to determine the two least occupied channels, that have less occupancy than other channels of the channel assignment, according to the occupancies which are computed by the compute unit 621 ;

[0092] a merge unit 623, configured to merge the two least occupied channels which are determined by the second determine unit 622.

[0093] The third embodiment describes a device for assigning channels in a WMN. This device gives a technical solution for planning of the frequency/radio assignment which is required in Multi-radio WMNs. And a radio with a new channel is added to the bottleneck node or the neighbor of the bottleneck node without the maximum number of available channels considered, and then the channels of the WMN are merged until channel number of the WMN lowers to the maximum number of channels, so the channel assignment finally distributed to the WMN has a channel number equal to the maximum number of channels. This device described in the first embodiment can output channel assignment which not only fulfills the constraints of the maximum channel number but also makes the WMN more efficient.

[0094] A technical solution will be described in a fourth embodiment of the invention taking a system for assigning channels in a WMN as an example. According to the fourth embodiment of the invention as illustrated in Figure 7, the channel assignment system includes:

[0095] more than one AP/MP 71, configured to communicate with each other and execute channel assignment distributed by a channel assignment device 72;

[0096] the channel assignment device 72, configured to create channel assignment from initial topology of the network by iteratively determining a bottleneck node and expanding a channel; merge the channels of the channel assignment according to the occupancy of each channel until channel number of the channel assignment is equal to the maximum number of channels; distribute the merged channel assignment to the network.

[0097] The channel assignment device 72 in the fourth embodiment can the device for assigning channels in a WMN described in a third embodiment.

[0098] Evaluation Metric

[0099] In the evaluation of the proposed channel assignment above, we will use the saturation throughput of multiple scenarios as evaluation metric. To define this metric, consider a service area with a WMN deployment. We position in this service area s STAs; each STA has the same offered traffic, t Mb/s, this requirement is partitioned into 90% downlink traffic from a single, imaginary "Internet" node which has direct connection to all APs, and 10% uplink traffic from the STA to the "Internet" node. This traffic splitting shall model the typical requirement of Internet browsing with high download and small upload-requirements.

[00100] With increasing offered traffic t, some links of network become saturated. Depending on the topology, it is be still possible to increase the offered traffic for some STAs, which do not use the links around the network's bottleneck; furthermore, the IEEE 802.11 MAC does not incorporate mechanisms for fairness among links or paths. [00101] For example, with an offered traffic below 0.3Mb/s, the throughput follows the offered traffic graph exactly: the network is able to transport 100%. Above 0.3Mb/s, the mean throughput slowly deviates from the optimal curve, but still increases. Therefore, the point where the throughput leaves the offered traffic is not a meaningful metric, as it does not capture the following further increase. Hence, the saturation throughput is defined as the maximum offered traffic per STA such that the mean throughput per STA is at least 90% of the offered traffic. To find this point using event-driven simulation, which can only evaluate the mean throughput for a given offered traffic, a binary search procedure is used: Starting with a small initial t, 0.05Mb/s in the example, t is increased successively by a factor of 2 until less than 90% is carried successfully (here at 0.8Mb/s with 65%. Then, a lower and an upper bound is known and the binary search can be continued until the two bounds are within 0.05% of each other - which is the case at the points 0.4125Mb/s and 0.425Mb/s in the example. The final saturation throughput is the mean of the two bounds, 0.418Mb/s. On average, around 10 successive simulations are required to find this saturation throughput for one WMN; these simulations cannot run in parallel as they depend on each other.

[00102] Upper Bounds and Lower Bounds

[00103] To judge the gain resulting from the addition of radios and/or channels to a WMN, the lower and the upper bounds of the saturation throughput are determined first.

[00104] For the upper bound evaluation, a ratio of 0 MPs per AP is applied, that is the network consists of APs only and is no longer a WMN with multi-hop routes. Each AP creates a BSS, selecting one of the available 3 non-overlapping channels in the 2.4 GHz range by random. STAs scan the frequency range and associate to the AP with the highest reception power.

[00105] As the network does not need multi-hop routes, the achieved saturation throughput is the upper bound for the topology. Figure 8a shows the saturation throughput for the 20 scenarios; the scenario ID is sorted by the saturation throughput in increasing order. It can be seen that the saturation is above IMbs per STA for all scenarios, with a mean of 1.5Mbs. In all following evaluations, the saturation throughput will be given relative to this upper bound, i. e. between 0 and 1.0. Of course, the evaluation will use the same BSS channels, so that the results are independent from the BSS channel selection algorithm.

[00106] Figure 8b shows the lower bound saturation throughput for four different ratios of MPs per AP. The lower bound for the channel assignment algorithm results from a WMN which is based on dual-radio nodes: Each MP and AP is equipped with two radios, one for the BSS and the other for the mesh backbone. To ensure connectivity, all mesh backbone radios have to use the same frequency. In the following, the type of MRMC WMN will be denoted by giving the maximum number of radios and available channels as tuple (r, c); Hence, this configuration is a (1, I)-MRMC WMN. [00107] The change of the number of MPs per AP has a huge effect on the network topology: It directly influences the average path length from an MP to an AP, but also the network deployment costs, as the installing of cables to a AP is the major cost factor that differentiates APs from MP. In the extreme, only one AP is used; in this case, the size of the service area determines the average path length. In our evaluation, this size is one square kilometer (consider deliverable 1 for more details).

[00108] In Figure 8b, each of the samples represents the lower bound for the channel assignment algorithm for a selected scenario and MPs to AP ratio; as the algorithm works with r > 1 and c more than or equal to r, it should be easily possible to surpass this limit.

[00109] Figure 8b shows Saturation throughput in the 20 evaluation scenarios for one dedicated mesh radio & channel, i. e. a dual-radio WMN.

[00110] Saturation Throughput [00111] To evaluate the performance of the MRMC configuration algorithm, firstly the saturation throughput of a (r, c)-MRMC WMN is compared with (i) the upper bound, given by a network consisting of APs only, and (ii) the lower bound, given by a (1, I)-MRMC WMN.

[00112] All in all, The comparison with the upper bound limit allows for judging the effect of the introduction of multi-hop paths in the WMN in contrast to a traditional, cellular network without MPs. The closer the individual results reach 1.0, the less difference to a traditional network is perceptible - with reduced costs, of course. Hence, the comparison with the upper bound is usable for the final judgment of the feasibility of WMNs.

[00113] The corresponding comparison with the lower bound is especially interesting as it shows how efficient the additional radios and channels can be used in comparison to a WMN that demands the least resources, i. e. one channel and one radio only.

[00114] Regarding the results, the following conclusions can be drawn:

[00115] All configurations in any scenario increase the saturation throughput in comparison to the lower bound. [00116] A deployment with 5 MPs per AP is able to reach nearly 100% of the saturation throughput as the upper bound using a maximum of 3 radios together with 10 channels. 50% of the upper bound can be reached with 15 MPs per AP and 2 or 3 radios using 10 channels, but not with only one AP in the service area.

[00117] Solutions above may fulfill the following constraints in WMNs:

[00118] 1. Radio-to-Channel: The number of orthogonal channels that can be assigned to any node is bounded by the maximum number of radios the node has;

[00119] 2. Channel-to-Link: Two nodes that communicate with each other directly should share one common channel; [00120] 3. Network Connectivity: Each MP in the WMN has at least one reliable path to at least one AP;

[00121] 4. Load Distribution: The expected load of the links of a node should be distributed equally so that each link, but also each channel is utilized similarly;

[00122] 5. Interference Minimization: Due to the limited number of available channels and radios per node, interference between links cannot be completely eliminated; hence, the effects of interference on the IEEE 802.11 MAC protocol should be minimized.

[00123] Though the method and the device and the system for assigning channels in a WMN have been disclosed by some exemplary embodiments of the present invention hereinabove, anybody skilled in the art can make some modifications and variations without departing from the spirit and scope of embodiment the present invention. Therefore, the specification should not be understood as the limitation to the present invention.

[00124] The skilled person in the art will readily appreciate that the present invention may be implemented using either hardware, or software, or both. Embodiments within the scope of the present invention also include computer-readable media for carrying or having computer-executable instructions, computer-readable instructions, or data structures stored thereon. Such computer-readable media can include physical storage media such as RAM, ROM, other optical disk storage, or magnetic disk storage. The program of instructions stored in the computer-readable media is executed by a machine to perform a method. The method may include the steps of any one of the method embodiments of the present invention. [00125] The above embodiments are provided for illustration only and the order of the embodiments can not be considered as a criterion for evaluating the embodiments. In addition, the expression "step" in the embodiments does not intend to limit the sequence of the steps for implementing the present invention to the sequence as described herein.

[00126] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications and variations may be made without departing from the scope of the invention as defined by the appended claims and their equivalents.