WO2007054874A2

WO2007054874A2 - Multi-channel wireless mesh networks

Info

Publication number: WO2007054874A2
Application number: PCT/IB2006/054110
Authority: WO
Inventors: Hans-Jurgen Reumerman; Guido Hiertz; Thomas Junge
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2005-11-09
Filing date: 2006-11-03
Publication date: 2007-05-18
Also published as: WO2007054874A3

Abstract

A protocol is defined for coordinating the allocation of time-slots (TS) in a network that comprises sub-networks (BSS 1-4) that operate on a plurality of channels (A, B, C). A channel sampling sequence (A-C-B) is defined for the network, and each sub-network includes a station that switches channels in accordance with this channel sampling sequence. The channel sampling sequence (A-C-B) defines periods of time within which coordination information (311, 312, 313) is communicated on each channel (A, B, C). The coordination information (311, 312, 313) includes an allocation of time slots for traffic between sub-networks (BSS1-4), and traffic within sub-networks (BSS1-4). Preferably, the coordination information (311, 312, 313) also includes the channel sampling sequence (A- C-B), so that new sub-networks (BSS 1-4) can learn the sequence and join the network dynamically by communicating allocation requests consistent with this channel sampling sequence (A-C-B). Procedures are defined for sub-networks (BSS 1-4) to enter or leave the network that provides for efficient channel utilization with minimal overhead.

Description

MULTI-CHANNEL WIRELESS MESH NETWORKS

This invention relates to the field of communications, and in particular to a method and device that facilitate communications among and within neighboring wireless networks that are configured to operate on multiple channels.

Wireless communication devices have limited range, thereby restricting the extent of wireless networks to a limited area. If the limited area includes a base station that is connected to a wide-area network, this base station can serve to relay messages between the wide-area network and each of the stations within the wireless network, thereby extending the effective communication range of each station to the range of the wide-area network. In many situations, however, the limited area may not include a station that is connected to a wide area network, but may include a station that is within range of another wireless network. In this case, it is desirable to use that station to relay messages between these neighboring networks, in the hope of eventually reaching a network that includes a base station that is connected to the wide-area network.

FIG. 1 illustrates an example network that can be created by providing relaying services between neighboring wireless networks, hereinafter termed sub-networks. In the IEEE 802.11 standard, these sub-networks are termed "Basic Service Sets" (BSS).

FIG. 1 illustrates four sub-networks, BSSl, BSS2, BSS3, BSS4. Each of these sub- networks include a plurality of stations, S. In these example sub-networks, at least one station is defined as an access point, AP, and is configured to manage communications within the sub-network.

The access point AP effectively defines the sub-network, because stations 'join' a sub-network by locating a nearby access point and requesting permission from the access point AP to join the sub-network. Generally such access points AP are able to relay messages within the sub-network, so that any station that can communicate with the access point is also able to communicate with any other station within the sub-network.

Each sub-network also includes a station that serves as a mesh point MP, and is configured to communicate in each of two or more neighboring/overlapping sub-networks. The mesh point MP serves as a relay for messages from the sub-network to the neighboring sub-network. If a station is configured to be both a mesh point and an access point, it is termed a mesh access point, MAP. Any of a variety of protocols could be used to manage communications within each sub-network and between neighboring networks. For communications efficiency, a protocol that uses a time-slot allocation scheme to minimize interference/collisions among the transmissions from each station is generally preferred. This allocation must encompass stations in a number of different sub-networks, because the transmission range of an individual station will generally extend beyond the station's particular sub-network. Additionally, the range of interference from a given station will extend beyond that station's reception range, and unless otherwise notified, it will be unaware of the effects its transmissions are having on its distant neighbors. For example, a station S 101 in sub- network BSSl in FIG. 1 may not be able to clearly detect transmissions from a station S 104 in sub-network BSS4, yet its transmissions may interfere with the ability of another station S 114 in sub-network BSS4 to receive communications from that station S 104. Thus, even if station S 101 is configured to "back off" whenever it detects transmissions from other stations, so as not to purposely interfere, it may still interfere with communications in sub- network BSS4, because it is unaware of some or all of the transmissions within that subnetwork.

To reduce the potential for interference, different channels can be used in different sub-networks. These different channels can be provided by using different frequencies, different spread-spectrum codes, or other channel differentiating techniques common in the art. The use of multiple channels, however, significantly complicates the coordination of communications among the sub-networks. In a multi-channel environment wherein each sub-network operates at a potentially different channel, each mesh point MP must be able to communicate on at least two channels, and this communication must by coordinated within each of the sub-networks. That is, in a typical non- interacting environment wherein independent networks operate on different channels, each network operates independently. In the subject environment of interacting networks, however, because each mesh point MP acts as a relay between sub-networks, some coordination between the sub-networks is required, even though the sub-networks are operating on different channels. Additionally, even though different channels may be used, the number of available channels is not limitless. Therefore, it is likely that some sub-networks within a range of mutual interference may be operating on the same channel, and coordination of these communications is desirable to maximize communication efficiency. It is an object of this invention to provide a method and system for coordinating Communications among sub-networks in a multi-channel environment. It is a further object of this invention to provide a method and system for coordinating communications among sub-networks that conform to the IEEE 802.11 or WiMedia/MBOA MAC protocol, or variations thereof.

These objects, and others, are achieved by a protocol that coordinates the allocation of time-slots in a network that comprises sub-networks that operate on a plurality of channels. A channel sampling sequence is defined for the network, and each sub-network includes a station that switches channels in accordance with this channel sampling sequence. The channel sampling sequence defines periods of time within which coordination information is communicated on each channel. The coordination information includes an allocation of time slots for traffic between sub-networks, and traffic within subnetworks. Preferably, the coordination information also includes the channel sampling sequence, so that new sub-networks can learn the sequence and join the network dynamically by communicating allocation requests consistent with this channel sampling sequence. Procedures are defined for sub-networks to enter or leave the network that provides for efficient channel utilization with minimal overhead.

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: FIG. 1 illustrates an example network comprising a plurality of connected subnetworks.

FIG. 2 illustrates a timing diagram of an example multiple sub-network protocol in a single-channel network.

FIG. 3 illustrates a timing diagram of an example multiple sub-network protocol in a multi-channel network, using a channel sampling sequence.

FIG. 4 illustrates an example flow diagram for selecting a channel from among multiple channels to provide a balanced allocation of channel utilization within a network. FIG. 5 illustrates an example flow diagram for eliminating gaps in a beacon period. Throughout the drawings, the same reference numeral refers to the same element, or an element that performs substantially the same function. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.

In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

As noted above, to minimize interference among sub-networks within interference range of each other, an allocation of time-slots among requesting transmitters in a network comprising multiple wireless sub-networks is generally preferred. Copending U.S. patent application 60/753,852, "DISTRIBUTED MEDIUM ACCESS PROTOCOL FOR MESH WIRELESS NETWORKS", filed 12 May 2005 for Guido R. Hiertz, Sebastian Max,

Francesc Dalmases, and Hans J. Reumerman, Attorney Docket PH000953US1, describes one such scheme, and is incorporated by reference herein. In this protocol, time is divided into superframes, and each superframe includes two frames, one for traffic between subnetworks, herein termed "mesh traffic", and one for traffic within each sub-network, herein termed "sub-network traffic. Each superframe includes an initial "Beacon Period" BP wherein the allocation of timeslots within the subsequent frames is announced to each recipient of the superframe. Transmitters are not permitted to transmit during the traffic periods except during their allocated time slot. A transmitter requests an allocation of one or more time slots by detecting vacant/available time slots within the desired mesh or sub- network traffic period, and communicating its reservation of one or more of the available time slots in the traffic period during a vacant/available beacon period. The reservation of a time slot also preferably includes an identification of the intended receiver, and the reservation is 'confirmed' by the receiver acknowledging that it will be available to receive the transmission(s) during the indicated time slot(s). FIG. 2 illustrates an example timing diagram corresponding to above referenced patent application. In this example, three time periods are illustrated: a beacon period 210, a mesh traffic period 220, and a sub-network traffic period 230.

The beacon period 210 includes a number of time periods 1, 2, 3, etc. during which a beacon signal can be sent. The shaded/speckled blocks in this beacon period 210 indicate "occupied" or "busy" beacon signal time periods, and the unshaded blocks indicate

"vacant" or "available" time periods. Each occupied block period includes a beacon signal 211, 212, 216, etc. that identifies a reservation/allocation of one or more time slots TS 201 in the traffic periods 220, 230. For example, beacon signal 211 reserves a time slot for the transmission of message 211 '; beacon signal 212 reserves two contiguous time slots for the transmission of message 212'; and beacon signal 216 reserves two distinct time slots for the transmission of messages 216', 216". As noted above, each beacon signal 211, 212, etc. preferably includes an identification of the transmitter of each message 211 ', 212', the time period(s) required, an identification of the intended recipient(s) of the message, and so on. One of ordinary art will recognize that alternative protocols may also be used to coordinate traffic between and within sub-networks, the allocation of time-slots being well known in the art.

If multiple channels are available for use in a network of wireless sub-networks, communications can occur in parallel, rather than serially, as in a single channel network, but the coordination of communications within a multi-channel network requires coordination among the multiple channels. Of particular note, the stations that facilitate communications between sub-networks, herein termed mesh points MP, need to be able to communicate with each other, and each of these sub-networks may be operating on a different communication channel.

FIG. 3 illustrates an example timing diagram for coordinating communications among sub-networks that employ multiple-channels, in accordance with an aspect of this invention. Three channels A, B, C are illustrated in this example. As illustrated in FIG. 3, time periods 1, 2, 3, etc. within the beacon period 310 are allocated to each of the network channels A, B, C. These periods are allocated in a known, or knowable, sequence that is repeated in each beacon period 310, so that each mesh point MP can monitor or communicate beacon signals at each relevant channel at these defined time periods.

In the example of FIG. 3, the channel sampling sequence is a continuous repetition of sequence A-C-B, with one time period being provided for beacon signals on each channel. One of ordinary skill in the art will recognize that multiple beacon signal time periods can optionally be provided during each sampling period, so as to allow more information flow as each channel is sampled, particularly if there is substantial overhead associated with synchronizing receivers and transmitters with each change of channel. One of ordinary skill in the art will also recognize that the beacon time periods on each channel that are not included in the channel sampling sequence, such as the two time periods in gap 301 between periods 1 and 4 on channel A, can be used for beacon signals that solely address sub-network traffic on that channel. In like manner, the channel sampling sequence need not include contiguous time periods, and need not continually repeat, particularly if the amount of mesh traffic is predicted to be substantially less than the amount of sub-network traffic. That is, for example, the channel sampling sequence can be defined as a given number of repetitions of A-C-B-(null)-(null), indicating that time periods 1-3 are allocated for mesh traffic beacon signals, 4-5 are not allocated for mesh beacons, then 6-8 are again allocated (to channels A-C-B) for mesh beacons, and so on.

In a preferred embodiment, the channel sampling sequence is negotiated and defined as each channel is added or deleted from the network. That is, for example, when two subnetworks determine that they are in proximity of each other and can each operate on different channels, at least one mesh point is selected and the beacon signal time periods for mesh traffic on each channel is identified. Preferably, the agreed upon mesh traffic beacon signal time periods, herein termed the channel sampling sequence, is communicated regularly on each channel, so that when another sub-network discovers this network in its proximity, it can signal its intent to join the network during an available beacon signal time period. If this new sub-network operates on a different channel from the first and second sub-networks, the mesh points in the sub-networks negotiate a new channel sampling sequence that includes this new channel, and this new channel sampling sequence is again communicated regularly on each of the channels to allow other sub-networks to signal their intent to join the network. Additionally, if the current channel sampling sequence is deemed inefficient or ineffective by a particular mesh point, that mesh point can initiate a request for renegotiation and redefinition of the channel sampling sequence.

The communication of the channel sampling sequence can be performed in a variety of forms. In a straightforward embodiment, each beacon signal includes an identifier of its type, one type being defined as a beacon signal that includes the channel sampling sequence. Within that signal, an explicit identification of each channel in the intended sequence order is provided in vector form; eg: (channelA, channelC, channelB).

Alternatively, a set of predefined channel sequences can be defined, and the beacon signal can be configured to contain an identifier of the particular predefined sequence. For example, a table of predefined channel sequences can be published, and the beacon signal need only provide an index to the table, identifying the selected sequence. Optionally, one or more default sequences can be defined, and absent receipt of a sequence-identifying beacon, mesh points can be configured to use the appropriate default. These and other techniques for communicating the current channel sampling sequence in a network will be evident to one of ordinary skill in the art. When a new sub-network joins the network using one of the defined channels, a renegotiation of the channel sampling sequence is not required. The mesh point of the new sub-network merely selects the channel and communicates its traffic by reserving traffic time slots via the beacon signals protocol, as discussed above. To receive mesh traffic from other channels, the mesh point monitors the channels during the beacon period in accordance with the communicated channel sampling sequence, and responds appropriately to beacon signals that identify the mesh point as an intended receiver on the corresponding channel.

Preferably, when a new sub-network joins the network using one of the defined channels, it selects the channel so as to minimize overall network congestion. FIG. 4 illustrates an example flow diagram for selecting a channel from among multiple channels to provide a balanced allocation of channel utilization, while also minimizing interference.

At 410, the received power at the mesh point is determined by monitoring each channel. Generally, monitoring the beacon signals at each channel is sufficient for determining the received signal strength, although the traffic periods may also be monitored. The channel that exhibits the lowest received power is preferred, except if that channel is also the busiest. Thus, at 420, the number of occupied/reserved time slots is determined for each channel. If, at 430, all of the channels have the same number of occupied time slots, the channel having the lowest power is selected, at 450; otherwise, the busiest channel(s) is(are) eliminated from consideration, at 440, before the channel with the lowest power is selected, at 450.

The determination of power and occupied time slots can be specific to the intended use of the channel by the mesh point. If, for example, the mesh point is merely choosing a channel to use for communicating mesh traffic, the power and occupied time slots associated with the mesh-traffic period may be considered. If, on the other hand, the mesh point also acts as an access point, and is choosing a channel to use for communicating subnetwork traffic, the power and occupied time slots associated with the sub-network period may be considered. In like manner, invocation of the channel selection process can also be specific to the intended use of the channel by the mesh point. If the channel is being selected for communicating mesh traffic, the mesh point may invoke the selection process each time it has mesh traffic to communicate; if, on the other hand, the selection is used to set the channel for the use of stations in the sub-network, the mesh access point may only invoke this process when it first joins the network, or when it determines that the selected channel has become inefficient or ineffective.

Preferably, mesh points will use the first occurring empty time period in the beacon period to communicate its beacon signal. In most protocols, once a time slot is reserved for communicating traffic, the reservation of that time slot in each subsequent frame or superframe remains in effect until the time slot is explicitly released, thereby avoiding the need to repeatedly negotiate ownership of the time slot.

In a preferred embodiment, the size of the beacon period is dynamically adjusted, to reduce the amount of time consumed by communicating the reservation information. Preferably, the beacon period need only extend long enough to provide one empty beacon time in each channel. To minimize the size of the beacon period, all of the occupied beacon signal time periods for each channel should occur at the beginning of the beacon period, and empty time periods between occupied beacon time periods (i.e. beacon signal gaps) should be eliminated. To eliminate these gaps, the mesh points should be configured to change their beacon time periods as required. To avoid a "ripple" effect, wherein a mesh point moves down one time period to fill the gap, then another moves down one time period to fill the gap that the first move created, and so on, the process of FIG. 5 is preferably used to fill gaps in the beacon period.

Each mesh point monitors the beacon time periods according to the channel sampling sequence, and notes the occurrence of gaps. If, at 510, the mesh point is the owner/user of the last occupied beacon time period on a channel, and one or more earlier vacant beacon time periods exists on this channel, at 520, the mesh point acquires the earliest vacant beacon time period on the channel, at 530. Once acquired, the mesh point uses this acquired time period to transmit a beacon signal that reserves the time slots that had been reserved by this mesh point at the later beacon time slot, at 540, and cancels the beacon signal at the later beacon time slot, at 550. Preferably, the determination of whether a gap exists, at 520, includes a latency criteria, such that a gap is not determined to be present until the beacon time period is vacant for at least a given number of consecutive beacon periods. In like manner, the beacon period is not reduced until the trailing empty beacon time periods are determined to be empty for a given number of consecutive beacon periods. Alternatively or optionally, the gap-elimination process and beacon period reduction process may be invoked only when a sub-network leaves the network, or when a substantial number of gaps are detected. The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within the spirit and scope of the following claims. In interpreting these claims, it should be understood that: a) the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; b) the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; c) any reference signs in the claims do not limit their scope; d) several "means" may be represented by the same item or hardware or software implemented structure or function; e) each of the disclosed elements may be comprised of hardware portions (e.g., including discrete and integrated electronic circuitry), software portions (e.g., computer programming), and any combination thereof; f) hardware portions may be comprised of one or both of analog and digital portions; g) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; h) no specific sequence of acts is intended to be required unless specifically indicated; and i) the term "plurality of an element includes two or more of the claimed element, and does not imply any particular range of number of elements; that is, a plurality of elements can be as few as two elements.

Claims

CLAIMSWhat is claimed is:

1. A method of coordinating communications among a plurality of subnetworks (BSS1-4) (BSS1-4) operating on a plurality of channels (A, B, C) comprising: defining a channel sampling sequence (A-C-B), and communicating coordination information (311, 312, 313) associated with each channel of the plurality of channels (A, B, C) on each channel in accordance with the channel sampling sequence (A-C-B).

2. The method of claim 1 , wherein the coordination information (311, 312, 313) includes an identification of the channel sampling sequence (A-C-B).

3. The method of claim 2, wherein the channel sampling sequence (A-C-B) is explicitly contained in the coordination information (311, 312, 313).

4. The method of claim 2, wherein the channel sampling sequence (A-C-B) is a selected sequence of a plurality of predefined channel sampling sequences, and the coordination information includes an index to the selected sequence.

5. The method of claim 1, wherein at least one transceiver in each sub-network in the plurality of sub-networks (BSS 1-4) is configured to switch channels in accordance with the channel sampling sequence (A-C-B).

6. The method of claim 5, wherein the at least one transceiver is configured to communicate traffic between sub-networks (BSS 1-4) of the plurality of sub-networks (BSS 1-4).

7. The method of claim 6, wherein the coordination information (310) includes a first allocation (320) of time-slots (TS) for communicating the traffic between the subnetworks (BSS 1-4).

8. The method of claim 7, wherein the coordination information (310) includes a second allocation (320) of time-slots (TS) for communicating traffic within each subnetwork.

9. The method of claim 6, wherein the at least one transceiver is configured to select a channel (A, B, C) for communicating the traffic, based on a received signal strength from other transceivers on each of the channels (A, B, C).

10. The method of claim 9, wherein the at least one transceiver is configured to select the channel based on a number of time-slots (TS) reserved for transmissions on each channel (A, B, C).

11. The method of claim 1 , wherein the sub-networks (BSS 1 -4) include Basic Service Sets that conform to an IEEE 802.11 standard.

12. The method of claim 1, including: defining a beacon period (BP) within a frame for communicating the coordination information (311, 312, 313), and defining a traffic period (220, 230) within the frame for communicating message traffic.

13. The method of claim 12, wherein the traffic period (220, 230) is further divided into: a mesh traffic period (220) for communicating message traffic between the sub-networks (BSS 1-4), and a sub-network traffic period (230) for communicating message traffic within the sub-networks (BSS 1-4).

14. The method of claim 12, wherein the coordination information (311, 312, 313) is communicated via beacon signals at time periods within the beacon period (BP) in accordance with the channel sampling sequence (A-C-B).

15. The method of claim 14, including reducing (510-550) a duration of the beacon period (BP) by relocating (540) beacon signals at a later time period within the beacon period (BP) to an earlier time period within the beacon period (BP).

16. A device comprising: a receiver that is configured to switch channels among a plurality of channels (A, B, C) in accordance with a channel sampling sequence (A-C-B), and a transmitter that is configured to coordinate communication of message traffic (320, 330) via beacon signals (311, 312, 313) that are transmitted in accordance with the channel sampling sequence (A-C-B).

17. The device of claim 16, wherein the receiver is configured to receive the channel sampling sequence (A-C-B).

18. The device of claim 16, wherein the transmitter is configured to transmit the message traffic at time slots (TS) that are defined in the beacon signals (311, 312, 313).

19. The device of claim 16, wherein the receiver is configured to receive the message traffic on a first channel of the plurality of channels (A, B, C), and the transmitter is configured to transmit the message traffic on a second channel of the plurality of channels (A, B, C).

20 The device of claim 16, wherein the transmitter is configured to select (410- 450) a channel of the plurality of channels (A, B, C) for communicating the message traffic based on strengths of signals received by the receiver at each of the plurality of channels (A, B, C).