GB2414899A

GB2414899A - WLAN load balancing and handoff

Info

Publication number: GB2414899A
Application number: GB0506916A
Authority: GB
Inventors: Russell John Haines; Darren Phillip Mcnamara
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2004-06-01
Filing date: 2004-06-01
Publication date: 2005-12-07
Also published as: GB0506916D0

Abstract

The present invention relates to mobile station load balancing and hand-offs in multiple wireless local area networks (WLAN). The present invention provides a base station comprising: a first transceiver for communicating with a number of mobile stations over a first wireless network (BSS1); a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same air-interface protocol but on different channels; means for allocating a said mobile station to a said transceiver in order to balance a mobile station load parameter between said transceivers. There is also provided means for handing-off a said mobile station from one said transceiver to the other said transceiver, and additionally means for selecting said channels from a plurality of potential channels according to a quietness parameter.

Description

24 1 4899 M&C Folio: GBP90061A Document: 1002609

WLAN LOAD BALANCING

Field of the Invention

The present invention relates to mobile station load balancing and handoffs in multiple wireless local area networks (WLAN).

Background of the Invention

Wireless connectivity for voice calls and data access and transfer is becoming increasingly important. Various mechanisms exist for servicing this need including cellular phone networks and wireless local area networks (WLAN). A WLAN may be provided by a single base station coupled to a telephone line such as in a home for example, or in a distributed wireless communication network a number of (often overlapping) WLAN provide wide coverage for example in an airport lounge or office.

These WLAN are connected to a wired backbone network which typically provides access to external networks such as the Internet or internal client servers such as databases. The backbone network, such as an Ethernet for example, also provides the communication link to support the transfer of mobile terminals between WLAN when these terminals move between respective coverage areas in the distributed wireless network. Examples of such a network include extended service sets (ESS) using an IEEE802.11 protocol and comprising multiple basic service sets (BSS) or WLAN coupled together via a backbone link.

Various mechanisms exist for determining when a mobile terminal should transfer from one WLAN to another, including the location of the terminal. In addition, the WLAN of a wider distributed wireless network will typically overlap, so that there is some latitude for transferring terminals between two WLAN, both of which could service it. For example one WLAN may be servicing a high number of terminals and be "over-loaded" whereas the other WLAN may have some extra capacity. In this case, it is possible to transfer one or more terminals from the overloaded WLAN to the other WLAN in order to balance the load between them. Various load balancing algorithms exist to determine when to transfer a terminal between overlapping WLAN.

Patent document US2004003107 discloses a load measurement method for determining the loading and capacity on a variable capacity channel by measuring the times at which packets are queued for transmission, and have their transmission completed, or by measuring these times in addition to the arrival times of the packets. The times may be measured using a device driver or other operating system component. The measurement may be performed in a centralized or distributed fashion for multi-access or point to point channels.

Patent document W02004004227 discloses a load balancing method for load balancing in a wireless communication network. A load control device is used which is external to the subscriber terminals, and is adapted to process information related to the load in the wireless communication network and to instruct roaming of a subscriber terminal from an associated access point to another access point. Access point status information determined in the various access points is received and communication status information related to the access points is determined. The subscriber terminal processes these information into roaming support information, which are in turn processed in the load control device into an access point related load based roaming analysis. On this basis, it is decided by the load control device, whether a subscriber terminal is to be associated with another one of the access points.

Patent document W02004004226 discloses a system of access point (AP) initiated hand-off which provides dynamic load balancing of network bandwidth between access points in an 802.11 wireless LAN. The access point generates and monitors average bandwidth utilization of client devices connected to the access point. The average bandwidth utilization for each client device is aggregated and selected clients are forced to roam to other access points if the aggregate bandwidth is equal or exceeds a threshold.

Patent document US2003139197 discloses a load-balanced multi-AP WLAN. A network having distribution of access point loading includes access points to which mobile stations can associate themselves based upon access point beacon signal levels and loading levels for the various access points. A mobile station receives beacon signals from various access points and determines a signal strength for the received beacon signals. The mobile station also receives access point loading information from the access points. The mobile station associates with an access point based upon the access point beacon signal strengths and the access point loading information Patent document US2003134642 discloses a load balanced WLAN with association rejections. A network includes access points that admit/terminate mobile station associations based upon the loading level of the access point and/or whether a mobile station can associate with a further access point. Mobile stations transmit information indicative of the access points to which they can associate. The access points determine whether to admit/terminate a mobile station association based upon access point loading.

Patent document EPl 156623 also discloses a load balanced WLAN. A communication system with a number of access points and a number of network stations arranged to communicate with one of the access points through a wireless communication protocol, each access point is able to: monitor its access point traffic load and transmit an access point traffic load parameter to the network station and the network station is able to: monitor its network station traffic load; store a network station traffic load parameter: receive access point traffic load parameters from the access points; select a communication connection with one of the access points using a predetermined cost function taking the access point traffic load parameters and the network station traffic load parameters into account.

In an alternative approach, overlapping WLAN using different airinterface protocols are employed. For example patent document US2004013128 a multi-mode access point (AP) and a method of controlling access between that AP and one or more clients is disclosed. In a first time period, the AP commands the client(s) not to send data on a first channel, using, for example, the 802.1 la Wireless Local Area Network (WLAN) Standard. The AP is enabled, in that first time period, to receive data on a second channel using say, the 802.1 lb WEAN standard. At the end of the first time period, the AP switches so that the first channel is commanded to be silent whilst data can instead be sent via the second channel. Data queuing for a given channel can take place at the or each client when that channel is commanded to be silent, for subsequent transmission when that channel is enabled again.

Patent document also discloses a multi-mode Access Point (AP, 110 fig. 2) that supports both 802.1 la and 802.1 lb Wireless Local Area Network (WLAN) standards simultaneously by use of Contention Free Periods (CFP), wherein the Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) mechanism is suspended for a given time period while each station waits for a request ("poll") from the Access Point before transmitting a frame. Frames are not transmitted, unless a poll is received, if a network allocation vector (NAY) has a value above zero. Those frames not transmitted by the transceiver commanded to stay silent are stored in a queue for later retrieval. Access to both 2.4 GHz and 5 GHz band stations at the same time via two respective antennae (502, 504, fig.3) and two physical layers (405, 415) is thus provided without the need for separate Media Access Control (MAC) interfaces in the Access Point.

In a further alternative approach a multi-channel AP is used in which different types of traffic are allocated to dedicated channels. Patent document US6393261 discloses a multi-channel access point for use in a wireless network having a system backbone and a plurality of mobile terminals. The access point includes a communication circuit coupling the access point to the system backbone, and a first transceiver for wirelessly communicating with at least one of the plurality of mobile terminals on a first communication channel. In addition, the access point includes a second transceiver for wirelessly communicating with at least another of the plurality of mobile terminals on a second communication channel different from the first communication channel. This additional channel can be used when the first channel is full, or the two channels can be used to separate data and voice calls.

Despite these various different approaches to improving wireless communications using distributed wireless networks, there is still a growing need to further optimise this giving its increasing popularity. s

Summary of the Invention

In general terms the present invention provides a multi-channel wireless access point especially for IEEE802.11 air-interface protocols, but also suitable for others such as IEEE802.15 WPAN, HiperLAN or other WLAN's. The access point or base station provides two channels, basic service sets (BSS) or WLAN's using the same air-interface protocol or technology. For example a base station or single physical access point provides two logical access points or BSS in the case of 802.11 technology. The coverage areas of these two logical access points or WLAN's are substantially co- located. The base station is also preferably coupled to a backbone such as a wired Ethernet connection to form part of a larger distributed network of WLAN's covering a larger geographical area such as an office or airport terminal. The base station is additionally arranged to allocate mobile terminals or stations such as mobile phones, PDA's or laptop computers between the two channels in order to balance the load on the two WLAN's. This may simply be an attempt to keep substantially equal numbers of mobile terminals associated with each channel, or the allocation may be more sophisticated by for example equalising estimated bandwidth usage, minimising error rate or interference, or maximising signal strength.

In particular in this first aspect the present invention provides a base station according to claim 1.

This provides a mechanism for active load-balancing of a multi-channel or multiple WLAN providing base station. It also allows handing off of mobile terminals from one WLAN to the other WLAN for other reasons, for example because the mobile terminal has completed one type of call (eg voice) and has started another type of call (eg email) which is handled by the other WLAN.

Advantageously the hand-off mechanism of the first aspect and the load balancing mechanism of the first aspect can be combined, although this isn't essential.

Preferably the air-interface technology is one of the IEEE802.11 protocols, and the association, re-association and de-association mechanisms are used for hand-off.

There is also provided a method of operating a base station according to claim 19.

There is also provided a network comprising a base station having a first transceiver for communicating with a number of mobile stations over a first wireless network (BSS1), a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same air-interface protocol but on different channels, and means for handing off a said terminal from one said network to the other said network; the network further comprising a number of mobile terminals.

There is also provided a mobile terminal for communicating with a base station having a first transceiver for communicating with a number of mobile stations over a first wireless network (BSS1), a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same air-interface protocol but on different channels, and means for handing off a said terminal from one said network to the other said network.

There is also provided a mobile terminal according to claim 16.

There is also provided a mobile terminal according to claim 17.

In general terms in a second aspect, the present invention provides a multi-channel wireless access point especially for IEEE802. 11 airinterface protocols, but also suitable for others such as IEEE802.15 WPAN, HiperLAN or other WLAN's. The access point or base station provides two channels, basic service sets (BSS) or WLAN's using the same air-interface protocol or technology. For example a base station or single physical access point provides two logical access points or BSS. The coverage areas of these two logical access points or WLAN's are substantially colocated. The base station is preferably also coupled to a backbone such as a wired Ethernet connection and may be part of a larger distributed network of WLAN's covering a larger geographical area such as an office or airport terminal. The base station is additionally arranged to select a said channels by stepping through a series of predetermined potential channels and determining activity on said channel by another WEAN or mobile terminal, then determine the order of quietness of said channels, and select the best channel or channels for the WEAN or WLAN's provided by said access point.

Preferably the activity determining is performed over at least one beacon interval.

Preferably the selected channel is further arranged to be furthest apart (for example in terms of frequency) from the most active channels.

Brief Description of the Drawings

Embodiments are now described with respect to the following drawings, by way of example only and without intending to be limiting, in which: Figure 1 shows a known distributed wireless network having overlapping BSS; Figure 2 shows conceptually a wireless network having overlapping BSS according to an embodiment; Figure 3 shows the architecture of a base station according to an embodiment; Figure 4 shows a flow chart for PCF/HCF Load Balancing; Figure 5 shows a flow chart for DCF Load Balancing; Figure 6 shows a sequence diagram for a STA-Originated HandOff; Figure 7 shows a sequence diagram for a STA-Assisted Hand-off; Figure 8 shows a sequence diagram for an AP-Cornmanded Hand-off; Figure 9 shows the architecture of a mobile terminal according to an embodiment; Figure 10 shows a flow chart for dynamic frequency selection; and Figure 11 shows a flow chart for DFS maximal spread.

Detailed Description

Figure 1 shows a known distributed wireless network arrangement suitable for an office or public space such as an airport lounge. The network comprises two or more wireless local area networks (WLAN) or basic service sets BSS1 and BSS2 in the case of IEEE802.11. Each BSS is provided by a separate base station or access point API and AP2 respectively. Each access point is coupled to other access points in the larger wireless network by a backbone communications link 1 such as a wired Ethernet for

example.

The basic service sets BSS1 and BSS2 of adjacent access points API and AP2 will overlap in coverage area as shown. However as long as the two BSS's use different channels, there is no significant interference between the two. The upper WLAN BSS 1 has two connected or associated mobile stations TO and Tot, whilst the lower WLAN BSS2 has two different mobile stations T32 and T42 associated with it's access point AP2. It can be seen that mobile stations TO and T32 are located in the area of overlap between the two BSS, and so could be associated with either access point. If one of the access points (AP1) becomes overloaded, for example because mobile terminal TL requires a large bandwidth, then the other terminal TO may lose its connection or its bandwidth may reduce below a minimum threshold causing it to seek another connection with a different access point (in this case AP2). Alternatively one of the forced handoff schemes mentioned previously may be employed to move this terminal to another base station (for example AP2). . As described previously, various load balancing algorithms exists to determine when this should be done. The handoff will involve the "new" access point (AL2) determining whether it has sufficient bandwidth to accommodate the new terminal Tot, and whether its wireless link with TO is adequate, for example has a minimum signal strength. If this is sufficient and there is capacity then the "new" access point AP2 signals to the "old" access point AL1 to handoff with instructions for the mobile terminal T2', for example the new channel frequency.

Referring now to figure 2, a wireless network according to an embodiment operating according to an IEEE802.11 protocol is show and comprises a base station or access point AP3 having two air-interface modems or wireless adapters 11 in order to provide two co-located WLAN's or basic service sets BSS3 and BSS4. Thus the access point AP3 is effectively a double or dual access point, or comprises two logical access points (APE and AP32 not shown). The base station AP3 also comprises a connection to a backbone 1, similar to that described above with respect to figure 1.

The logical access points providing the two logically separate WLAN BSS3 and BSS4 may be addressed individually by the rest of the wider wireless network coupled by the backbone communications link 1. Alternatively, the access point AP3 may be accessed in a unitary fashion, the access point AP3 itself then dividing mobile terminals between the two WLAN BSS3 and BSS4.

The co-located BSS accommodate a number of mobile terminals T53, T63, T74, and T84; in this case having two terminals associated with one of the overlapping WLAN BSS3 and two associated with the other co-located WLAN BSS4. It can be seen that because of the co-located nature of the two BSS, the mobile terminals could be associated with either.

The co-located BSS are termed here a "super-BSS" (or more generally a super-WLAN), and can comprise any number of BSS. The super-BSS may also form part of an ESS (or more generally a distributed wireless network) having additional access points providing further BSS from different locations. Further, whilst the BSS of the super- BSS are co-located in the sense that they originate from the same location or physical access point, they may have differing coverage areas. For example one BSS may have a smaller radius and correspond to a higher QoS or rate, or one of the BSS may be directional whilst the other is omni-directional. This might correspond to a smaller coverage, higher rate zone within an area, for example a meeting room area within the larger office.

The access point or base station AP3 is arranged to load share the terminals T5-T8 between the two WLAN BSS3 and BSS4, for example such that a roughly equal amount of bandwidth is utilised by each. Thus for example the first WLAN BSS3 may have one, two or three terminals associated, while the other BSS (BSS4) has a complementary number (three, two or one in the drawing). The access point AP3 may allocate mobile terminals newly entering the joint coverage area to one or the other BSS depending on a load sharing algorithm. Suitable known load sharing algorithms have already been described above, and any of these could be used. The communication between separate AP takes place in the case of embodiments within the (dual) base station rather than across a backbone network as previously. Preferably the access point AP3 may also force handoff of a terminal from one of its WLAN (BSS3) to another (BSS4) in order to more evenly distribute the load of already associated terminals.

These functions and example implementations are described in more detail below.

It can be appreciated then that this arrangement gives a high degree of flexibility within the wider network as well as that provided by the individual access point, and additionally boosts the amount of bandwidth that is available within the service area of an access point. Whilst two co-located BSS have been shown, additional co-located BSS could be added to the access point (AL3) to further improve these advantages.

Figure 3 shows a preferred architecture for the (double or dual) access point of figure 2.

The access point AP3 comprises a backbone modem or adapter 14 such as an Ethernet modem, router functionality 13, and two wireless adapters or BSS modems 1 la and 1 lb coupled respectively to two antenna elements 12a and 12b. The access point also comprises a load balancing algorithm or function 15 and a mobile terminal allocation

table 16.

In this embodiment the two WLAN BSS1 and BSS2 provided by the BSS modems 11 and antennas 12 are logically independent and separately addressable by the wider network connected via the backbone communication link 1. Messages or packets addressed to a mobile terminal (not shown) are forwarded via the backbone 1 to the Ethernet modem 14 which forwards the data and destination address to the router 13.

The router 13 determines which terminal T5-T8 the packet is intended for, then determines from the terminal allocation table 16 which BSS (1 or 2) the terminal is currently associated with, and forwards the packet to the appropriate BSS modem 1 la or 1 lb. The preferred air-interface technology is based on one of the IEEE802.11 WLAN based protocols, for example a, b, g and n. Both the WLAN BSS1 and BSS2 use the same protocol, for example 802.11n, but operate on different channels. In the case of 802.11 technology this means the BSS operate on different frequencies, however other air- interface protocols may utilise different frequency hoping sequences, different modulation or different time slots. In 802.11, each terminal associated with a particular BSS (BSS1) has to contend for "air-time" with the other terminals (T5 and T6) and modem (1 la) in known manner. Similarly the terminals associated with the other BSS (BSS2) contend with different terminals (T7 and T8) and the other modem (1 lb). There is no contention across the basic service sets because they operate on different channels and hence are independent of each other.

A standard IEEE802.11 Basic Service Set (BSS) operates on a single channel, whether that be a low-rate lMbps physical channel in the 2.4GHz ISM band (original 802.11 FHSS or DSSS), a high-rate 54Mbps physical channel in the 5GHz UNII band (802.11 a high-rate PHY) or the postulated high-throughput lOOMbps logical channel in the 2.4 or 5GHz bands (802.11 Task Group n).

Each of these channels must be shared between all of the users or stations (STA) in that BSS, with all users competing and contending for that single resource. As the bandwidth requirements or amount of data to be transferred to each terminal (STA) increases, the medium (i.e. the channel) gets busier and busier until the Access Point (AP; assuming an infrastructure BSS) is unable to service all STA and the system reaches its maximum throughput level.

There are many channels available to IEEE802.11 WLANs; three parallel WLANs in adjacent non-overlapping channels can be supported in the 2.4GHz band (although there are about a dozen centre frequencies) and many more in the 5GHz band. Two or more BSSs operating on different non-overlapping channels do not mutually interfere.

In the case of a standard 802.11 implementation, the above described embodiment is specification-compliant as an access point with two (or more) transceivers each operating on a different frequency, is effectively working as a two (or more) access points in a single box. In the case of the 802.11n working proposal, there may be operational modes where the system can use two (or more) 20MHz channels in parallel to achieve the high throughput required of the tin standard. Thus a single mobile terminal may be associated with both BSS in order to provide the higher bandwidth.

The embodiment intelligently uses a multi-transceiver architecture such that it is present as multiple mutually exclusive BSS operating on different frequencies. This may also allow the use of a greater bandwidth by a mobile subscriber terminal by allowing this to access both channels. This arrangement is in contrast to a proposal to simply extend from an existing DSSS / OFDM system occupying a 20MHz channel to one occupying a 40MHz channel. Although this also gives a welcome increase in the amount of bandwidth available, it would be in violation of some regional regulations regarding the occupation of channels by a single system (e.g. the regulations in Japan, set by the Association of Radio Industries and Businesses (ARIB)).

As discussed above, there is a requirement in the embodiment for a routing means 13 to route incoming data to the correct terminal T. In a preferred arrangement, the functionality implicit in the operation of the backbone Distribution System of an Extended Service Set (ESS) as specified in the core 802.11 specification is utilised - see IEEE Computer Society, "Information technology - Telecornmunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE Std 802.11, July 1999 [the basic 802.11

specification].

The first embodiment of a load-balancing algorithm is based around an 802. 11 PCF / 802.11e HCF implementation, where mobile terminals STA associated with a BSS submit resource requests to be incorporated into a round-robin polling by the AP (PC/HC) as described in Patent Application Number US2004003107, the contents of which are hereby incorporated or the person skilled in the art is directed to. PCF is the Point Coordination Function, an overlay to the core 802.11 DCF (Distributed Coordination Function) where the Point Coordinator (a role taken on by the AP) can seize the medium and poll data out of STAB. This is useful if there's a transmission time that the STAs are aiming for. HCF, the Hybrid Coordination Function, is the outcome of the (as yet draft) 802.11e specification. It has two components, EDCA which is a QoS-enhanced version of DCF, and HCCA which is a QoS version of PCF. In this case, the AP simply sorts the bandwidth requirements in order of size, and allocates them evenly across the number of constituent BSSs within the "super-BSS" as shown in figure 4. Specifically in the case of 802.11e HCF control, it is also desirable to further sort the requirements according to Access Category (AC) because, for example, it would be undesirable to have multiple AC_VOs (voice traffic) on the same physical channel, as their elevated priority and periodic nature may cause contention problems and affect other traffic flows.

Figure 5 shows an alternative embodiment which supports the more anarchic DCF mode of operation in which all stations contend for access on an equal basis, regardless of whether they're the AP, a QoS-STA with some short-lifetime traffic, or a regular STA being used by someone reading their email. This requires that the AP monitor the amount of traffic being generated on the medium by each mobile station (STA) or terminal (T) within the BSS by logging the MAC addresses and duration of each exchange on the medium i.e. keeping a record of the transmit-time over a sliding window for each STA. The transmission time for an exchange is available in the Duration field in the MAC header of either the RTS or the DATA packet according to whether RTS/CTS is in use or not, i.e. the AP doesn't have to sit there timing every transmission. The method then sorts this log and evenly distributes the STAs as with the first arrangement.

Other embodiments could include simply equalising the number of associations on each BSS.

In a farther alternative, load balancing of certain types of traffic or calls can be implemented. For example where video calls are required for two mobile stations, one can be assigned within the first WLAN (BSSl) and the other within the second WLAN (BSS2). Video calls require a "regulated" approach of assigned transmission times so as not to delay the arrival ofreal time packets. By contrast "bursty" calls such as web- surfing or emailing can be dealt with stochastically, the associated packets being fitted in when there is available bandwidth. Load balancing the video calls in this way, or more generally traffic types, can improve the quality of service (QoS). In a worst case the timing of the two video calls might require that they both transmit at the same time so that they clashed with every transmission. Even if they're not aligned, the combination of their bandwidth usage and the regular periodicity of the transmissions would leave very little space for the bursty traffic, resulting in large latency and delay jitter for the latter. By putting the two video calls on two BSSs, there's a much larger window for the contention-based traffic.

Thus the embodiments can be used to balance a load parameter, based for example simply on the number of mobile stations; bandwidth levels; or available QoS service levels, that is trying to equalise the number of high QoS calls, or using a QoS parameter for each service level and trying to equalise the sum of these parameters for each BSS.

In a further alternative, interference levels might be balanced for example by maintaining a similar signal to noise and interference level for each call, or the cumulative calls in each BSS.

The preferred embodiments also utilise a hand-off mechanism whereby a mobile terminal STA is transferred from one WLAN (BSSI) to another (BSS2). In one embodiment the multiple BSSs of the super-BSS should be configured as a single ESS, as this then means that hand-offs between the BSSs are BSS-transitions (defined somewhat logically, in 5.4.2.1 of the 802.11 basic specification, as a transition where a STA moves between BSSs in the same ESS, and which results in, theoretically, no disruption in service).

An STA-assisted mechanism can then be used where the STA actually reassociates with the second BSS - a first approach. Alternatively, an APcontrolled procedure commanding a hand-off can be used - a second approach. As a further alternative, an entirely AP-controlled procedure which is backwards-compatible with legacy devices could be used - a third approach. The first two approaches have the advantage that the reassociation/commanded-hand-off routes are faster and result in less disruption in service. It is also a smoother process, especially if the implementation of an association rejection in a particular STA necessitated referral back to the user.

With an 802.11e system with QAPs/QSTAs ("QoS-enabled AP" and "QoS-enabled STA" respectively), the QAP should verify that no DLP arrangements exist with a QSTA that it is about to move to a new channels Similarly, in the first approach, a QSTA would be responsible for ensuring that any outstanding DLP agreements have been torn down. DLP or Direct Link Protocol is an 1 le extension in which, rather than (wastefully) sending all packets via the AP as you have to in an infrastructure 802.11 network, DLP enables you to link up two QSTAs directly, which halves the bandwidth consumed, and improves the delay.

The first approach uses the reassociation mechanism described in 5.4.2.3 of the basic 802.11 specification (which the person skilled in the art will be familiar with) to support the STA requesting a move from one AP to another. The STA could identify unilaterally that the channel is becoming loaded above a particular threshold (by examining the proportion of time that the medium is active from physical and virtual carrier sense schemes (the Clear Channel Assessment mechanism and the Network Allocation Vector respectively) as illustrated in figure 6. Alternatively the AP could command the STA to perform a reassociation through a new message as shown in figure 7. In this case the AP determines the channel loading and selects one or more stations or terminals STA to command to handoff.

The second approach takes the view that as the first approach necessitated the addition of a new message to invoke the existing reassociation mechanisms, there may as well be an entirely new command that explicitly commands a hand-off to a particular frequency, cutting out part of the reassociation process (specifically, having to find the other BSS), as shown in figure 8. The frequency will be known to the AP, for example this may be stored in the terminal allocation table.

A schematic of an architecture for a mobile base station adapted to carry out these three hand-off mechanisms are shown in figures 9a, 9b and 9c. Figure 9a shows the process in the terminal where the terminal itself initiates the handoff, and corresponds to figure 6.

The terminal monitors the channel loading of the channel it is using until this exceeds a predetermined threshold. The terminal then locates another BSS, for example by listening for its beacon and monitoring the load on the associated channel. This will most likely be the co-located BSS as this will have the strongest carrier signal. Having established a suitable new BSS, the terminal sends a re-association request including its current BSS identifier. The access point associated with the BSS then determines whether it will accept the terminal, and if so forwards a confirmation. The terminal then switches channel and begins monitoring the new channel.

Figure 9b illustrates the AP initiated handoff using re-association. Here the terminal receives a dedicated command from the AP to re-associate. This may be generated by the AP for example when it determines the channel loading is above a given threshold.

The terminal then searches for a new BSS as described with the previous process.

Having found a suitable BSS, the terminal then forwards a re-association message, which if accepted by the AP, results in a confirmation message. The terminal then switches channels, and listens for any further commands to re-associate from the new AP.

Figure 9c shows an access point initiated handoff using a dedicated handoff command.

The command includes a BSS identifier for the new BSS, as well as its frequency. The terminal then requests association with the new BSS access point, and awaits a confirmation. The terminal then switches channel and awaits any further hand-off commands from the new (logical) access point. This method has the advantage that the "old" access point or BSS provider can nominate the new access point of BSS provider, and so can nominate one of its other BSS within the super- BSS. This guarantees that the terminal will be handed off to another BSS within the super-BSS.

The final (third), backwards-compatible approach relies on the fact that a disassociation notification (5.4.2.4 of the 802.11 basic spec) can be invoked by either party, so the AP can tell a STA that its association has been revoked. Therefore, once the need for a STA to be moved from one BSS to another has been identified, the AP can revoke the STA's association. The STA will then try to find another BSS to associate to. The exact behaviour in this situation is not specified in the standard, which leaves a possibility that the STA will attempt to re-associate with the first BSS - which the AP can deal with because an association response (to an association request) (7.2.3.4/5) can be negative (e.g. Result Code "REFUSED" 10.3.6.2.2, and reason code 12, "Association denied due to reason outside the scope of this standard", in Table 19 on page 54, which can also be cited as the disassociation reason, see 10.3.8.1.2).

Alternatively, the STA should locate one of the other constituent BSSs of the super-BSS on the grounds that their transceivers are co-located with the original BSS, so the signal strength of their Beacon frames should be the highest of those received (if the STA selects the BSS on that basis). As previously mentioned, the BSS-selection procedure is not specified in the STA, so the exact behaviour of the STA cannot be predicted. Not only is there no means to guarantee which of the constituent BSSs would be selected (assuming there were more than one), there is no way to guarantee that the STA will not venture off to another BSS entirely; the hope though is that any other BSS will reject is association requests, and the STA will eventually find its way back to one of the constituent BSSs. If there is an absolute need for the STA to move to a particular constituent BSS, then association requests to other constituent BSSs can be successively rejected to shepherd the STA to the right one. Having (finally) associated with the correct constituent BSS, the AP informs the nebulous Distribution System (DS) via the IAPP (Inter Access Point Protocol) which is responsible for routing downlink traffic to the right AP en route to the STA (≈11.3.2).

Whilst the preferred embodiments have been described as utilising both the load balancing algorithm and a hand-off mechanism in combination, it is possible that each of these processes can be used independently of the other. For example the load balancing could be used just for allocation of new terminals to a BSS (1 or 2), without utilising hand-off to transfer a terminal from one BSS to another. Alternatively existing backbone based hand-off mechanisms could be used, with each BSS being a separate logical entity in the larger network. Similarly, the hand-off mechanism could be used without the prompting of a load balancing algorithm. For example, the terminal may be handed over from one BSS to the other co-located BSS when it requires a voice connection instead of a data connection for example, or simply when the BSS it is currently associated with becomes full or over-loaded.

Preferred embodiments also utilise a Dynamic Frequency Selection (DFS) mechanism.

When selecting additional channels (or, for that matter, the original channel), the AP is arranged to re-tune the transceiver in question to each potential channel in turn, and listen for at least a beacon interval to identify any other BSSs and any other occupiers in that channel. Having scanned all potential channels, the AP then orders them in order of quietness and selects the best ones, as shown in figure 10. This ordering may be achieved by use of a quietness parameter which can be based on for example one or more of the following: a traffic level measure, for example packet transfer rate; an interference measure, for example a Received Signal Strength Indication (RSSI) or Received Signal Level (RxLev) can be the best way of determining the quietness of a channel; a quality of service measure for example a measure if whether a channel can still support 2 high QoS calls, or a gross "amount of bandwidth remaining", or a channel with high bandwidth consumption but low repeat rate.

An additional criteria to the ordering process, is that the ordering is biased to favour as wide a spread of channel numbers as possible, to minimise any mutual adjacent channel interference between constituentBSSs. This process consists of taking the set of clear channels (above a certain threshold), ordering them by channel number, and selecting the points farthest apart within that set as illustrated in figure 11.

Whilst this DFS mechanism is advantageously used with the load-balancing and hand- off embodiments, it can also be used independently of these.

Further, whilst the embodiments have been described with respect to a centralised group of WEAN, it is also applicable to ad hoc networks of WLAN that communicate without a wired backbone network. Thus for example access points with co-located channels in an ad hoc network can also make use of the load balancing, hand-off and DFS embodiments.

The skilled person will recognise that the above-described apparatus and methods may be embodied as processor control code, for example on a carrier medium such as a disk, CD- or DVD-ROM, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications embodiments of the invention will be implemented on a DSP (Digital Signal Processor), ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). Thus the code may comprise conventional programme code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re- programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog _ or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a field-(re)programmable analogue array or similar device in order to configure analogue hardware.

The skilled person will also appreciate that the various embodiments and specific features described with respect to them could be freely combined with the other embodiments or their specifically described features in general accordance with the above teaching. The skilled person will also recognise that various alterations and modifications can be made to specific examples described without departing from the scope of the appended claims.

Claims

CLAIMS: 1. A base station comprising: a first transceiver for

communicating with a number of mobile stations over a first wireless network (BSS1); a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same air-interface protocol but on different channels; means for handing-off a said mobile station from one said transceiver to the other said transceiver.
2. A base station according to claim 1 wherein the first and second wireless networks are substantially co-located.
3. A base station according to claim 2 wherein each wireless network has an associated coverage area, and wherein the two coverage areas substantially overlap.
4. A base station according to claim 1, 2 or 3 further comprising means for communicating with one or more other base stations in an extended wireless network or distributed WLAN over a backbone network.
5. A base station according to any one of claims 1 to 4 wherein said protocol is IEEE 802.11a, b, g, or n; such that the transceivers operate on different frequencies to provide two independent basic service sets (BSS1, BSS2).
6. A base station according to any one of claims 1 to 5 wherein said handing-off means comprises means for instructing a said mobile terminal to terminate communication with a said transceiver.
7. A base station according to claim 6 wherein said instruction is a IEEE802.11 based dis-association message.
8. A base station according to claim 6 or 7 wherein said instructing means is further arranged to instruct said terminal to start communicating with the other said transceiver.
9. A base station according to claim 8 wherein said further instruction is a IEEE802. 11 based re-association message.
10. A base station according to any one of claims 1 to 9 further comprising means for allocating a said mobile station to a said transceiver in order to balance a mobile station load parameter between said transceivers.
11. A base station according to claim 10 wherein said mobile station load parameter is based on one of the following: number of mobile stations; bandwidth levels; interference levels; available QoS service levels; a combination of these.
12. A base station according to any one of claims 1 to 11 further comprising means for selecting said channels from a plurality of potential channels according to a quietness parameter.
13. A base station according to claim 12 wherein the quietness parameter comprises one of the following: a traffic level measure; an interference measure; a quality of service measure; a combination.
14. A base station according to claim 12 or 13 wherein said selection means comprises means for stepping through a series of predetermined potential channels and determining activity on said channel by another wireless network or mobile station, means for determining an order of said quietness parameter of said channels, and means for selecting a channel or channels having the best parameter.
15. A base station according to claim 14 further comprising means for selecting a channel furthest from the channel with the worst quietness parameter.
16. A mobile station comprising: means for communicating with a base station over a first wireless network (BSS1); means for receiving a command from said base station to terminate said communication on said wireless network means for determining another wireless network associated with said base station and suitable for said communication; means for switching said communication from said first network to the other network.
17. A mobile station comprising: means for communicating with a base station over a first wireless network (BSS 1); means for determining a loading parameter for said network; means for determining another wireless network associated with said base station and suitable for said communication when said parameter exceeds a threshold; means for switching said communication from said first network to the other network.
18. A mobile station according to claim 16 or 17 wherein said switching means comprises means for sending an IEEE802.11 re-association request for the other network.
19. A method of operating a base station comprising a first transceiver for communicating with a number of mobile stations over a first wireless network (BSS1), and a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same airinterface protocol but on different channels; the method comprising: handing-off a said mobile station from one said transceiver to the other said transceiver.
20. A method according to claim 19 wherein the first and second wireless networks are substantially co-located.
21. A method according to claim 20 wherein each wireless network has an associated coverage area, and wherein the two coverage areas substantially overlap.
22. A method according to claim 19, 20 or 21 further comprising communicating with one or more other base stations in an extended wireless network or distributed
23. A method according to any one of claims 19 to 22 wherein said protocol is IEEE 802.11 a, b, g, or n; such that the transceivers operate on different frequencies to provide two independent basic service sets (BSS1, BSS2).
24. A method according to any one of claims 19 to 23 wherein said handingoff comprises instructing a said mobile terminal to terminate communication with a said transceiver.
25. A method according to claim 24 wherein said instruction is a IEEE802. 11 based dis-association message.
26. A method according to claim 24 or 25 wherein said instructing further comprises instructing said terminal to start communicating with the other said transceiver.
27. A method according to claim 26 wherein said further instruction is a IEEE802.11 based re-association message.
28. A method according to any one of claims 19 to 27 further comprising allocating a said mobile station to a said transceiver in order to balance a mobile station load parameter between said transceivers.
29. A method according to claim 28 wherein said mobile station load parameter is based on one of the following: number of mobile stations; bandwidth levels; interference levels; available QoS service levels; a combination of these.
30. A method according to any one of claims 19 to 29 further comprising selecting said channels from a plurality of potential channels according to a quietness parameter.
31. A method according to claim 30 wherein the quietness parameter comprises one of the following: a traffic level measure; an interference measure; a quality of service measure a combination.
32. A method according to claim 30 or 31 wherein said selection comprises stepping through a series of predetermined potential channels and determining activity on said channel by another wireless network or mobile station, determining an order of said quietness parameter of said channels, and selecting a channel or channels having the best parameter.
33. A method according to claim 32 further comprising selecting a channel furthest from the channel with the worst quietness parameter.
34. A method of operating a mobile station comprising means for communicating with a base station over a first wireless network (BSS1), the method comprising: receiving a command from said base station to terminate said communication on said wireless network determining another wireless network associated with said base station and suitable for said communication; switching said communication from said first network to the other network.
35. A method of operating a mobile station comprising means for communicating with a base station over a first wireless network (BSS1), the method comprising: determining a loading parameter for said network; determining another wireless network associated with said base station and suitable for said communication when said parameter exceeds a threshold; determining another wireless network associated with said base station and suitable for said communication; switching said communication from said first network to the other network.
36. A method according to claim 35 further comprising determining a loading parameter for said network; and wherein said determining is initiated when said parameter exceeds a threshold.
37. A method according to claim 35 or 36 wherein said switching comprises sending an IEEE802. 11 re-association request for the other network.
38. Program code for controlling a programmable apparatus to carry out the method of any one of claims 19 to 37.
39. A program product comprising the code of claim 38.

39. A program product comprising the code of claim 38. :

: l ( Amendments to the claims have been filed as follows 1. A wireless local area network base station comprising: a first transceiver for communicating with a number of mobile stations over a first wireless network (BSS1); a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same air-interface protocol but on different channels; means for handing- off a said mobile station from one said transceiver to the other said transceiver.

2. A base station according to claim 1 wherein the first and second wireless networks are substantially co-located.

3. A base station according to claim 2 wherein each wireless network has an associated coverage area, and wherein the two coverage areas substantially overlap.

4. A base station according to claim 1, 2 or 3 further comprising means for communicating with one or more other base stations in an extended wireless network or distributed WEAN over a backbone network.

5. A base station according to any one of claims 1 to 4 wherein said protocol is IEEE 802.11a, b, g, or n; such that the transceivers operate on different frequencies to provide two independent basic service sets (BSS1, BSS2).

6. A base station according to any one of claims 1 to 5 wherein said handing-off means comprises means for instructing a said mobile terminal to terminate communication with a said transceiver.

7. A base station according to claim 6 wherein said instruction is a IEEE802.11 based dis-association message. l

8. A base station according to claim 6 or 7 wherein said instructing means is Farther arranged to instruct said terminal to start communicating with the other said transceiver.

9. A base station according to claim 8 wherein said further instruction is a IEEE802.11basedre-association message.

10. A base station according to any one of claims 1 to 9 further comprising means for allocating a said mobile station to a said transceiver in order to balance a mobile station load parameter between said transceivers.

11. A base station according to claim 10 wherein said mobile station load parameter is based on one of the following: number of mobile stations; bandwidth levels; interference levels; available QoS service levels; a combination of these.

12. A base station according to any one of claims 1 to 11 further comprising means for selecting said channels from a plurality of potential channels according to a quietness parameter.

13. A base station according to claim 12 wherein the quietness parameter comprises one of the following: a traffic level measure; an interference measure; a quality of service measure; a combination.

14. A base station according to claim 12 or 13 wherein said selection means comprises means for stepping through a series of predetermined potential channels and determining activity on said channel by another wireless network or mobile station, means for determining an order of said quietness parameter of said channels, and means for selecting a channel or channels having the best parameter.

15. A base station according to claim 14 further comprising means for selecting a channel furthest from the channel with the worst quietness parameter. : r At,

16. A mobile station comprising: means for communicating with a wireless local area network base station over a first wireless network (BSS 1); means for receiving a command from said base station to terminate said communication on said wireless network means for determining another wireless network associated with said base station and suitable for said communication; means for switching said communication from said first network to the other network.

] 7. A mobile station comprising: means for communicating with a wireless local area network base station over a first wireless network (BSS] ); means for determining a loading parameter for said network; means for determining another wireless network associated with said base station and suitable for said communication when said parameter exceeds a threshold; means for switching said communication from said first network to the other network.

18. A mobile station according to claim 16 or 17 wherein said switching means comprises means for sending an IEEE802.11 re-association request for the other network.

]9. A method of operating a wireless local area network base station comprising a first transceiver for communicating with a number of mobile stations over a first wireless network (BSS1), and a second transceiver for communicating with a number of mobile stations over a second wireless network (BSS2), wherein the first and second transceivers are arranged to operate the same air-interface protocol but on different channels; the method comprising: handing-off a said mobile station from one said transceiver to the other said transceiver.

20. A method according to claim 19 wherein the first and second wireless networks are substantially co-located.

21. A method according to claim 20 wherein each wireless network has an associated coverage area, and wherein the two coverage areas substantially overlap.

22. A method according to claim 19, 20 or 21 further comprising communicating with one or more other base stations in an extended wireless network or distributed 23. A method according to any one of claims 19 to 22 wherein said protocol is IEEE 802.11 a, b, g, or n; such that the transceivers operate on different frequencies to provide two independent basic service sets (BSS1, BSS2).

24. A method according to any one of claims 19 to 23 wherein said handingoff comprises instructing a said mobile terminal to terminate communication with a said transceiver.

25. A method according to claim 24 wherein said instruction is a IEEE802. 11 based dis-association message.

26. A method according to claim 24 or 25 wherein said instructing further comprises instructing said terminal to start communicating with the other said transceiver.

27. A method according to claim 26 wherein said further instruction is a IEEE802.11basedre-association message.

28. A method according to any one of claims 19 to 27 further comprising allocating a said mobile station to a said transceiver in order to balance a mobile station load parameter between said transceivers. l l

33.;. 1: ( 29. A method according to claim 28 wherein said mobile station load parameter is based on one of the following: number of mobile stations; bandwidth levels; interference levels; available QoS service levels; a combination of these.

30. A method according to any one of claims 19 to 29 further comprising selecting said channels from a plurality of potential channels according to a quietness parameter.

31. A method according to claim 30 wherein the quietness parameter comprises one of the following: a traffic level measure; an interference measure; a quality of service measure a combination.

32. A method according to claim 30 or 31 wherein said selection comprises stepping through a series of predetermined potential channels and determining activity on said channel by another wireless network or mobile station, determining an order of said quietness parameter of said channels, and selecting a channel or channels having the best parameter.

33. A method according to claim 32 further comprising selecting a channel furthest from the channel with the worst quietness parameter.

34. A method of operating a mobile station comprising means for communicating with a wireless local area network base station over a first wireless network (BSS1), the method comprising: receiving a command from said base station to terminate said communication on said wireless network determining another wireless network associated with said base station and suitable for said communication; switching said communication from said first network to the other network.

35. A method of operating a mobile station comprising means for communicating with a wireless local area network base station over a first wireless network (BSS1), the method comprising: determining a loading parameter for said network; A\ determining another wireless network associated with said base station and suitable for said communication when said parameter exceeds a threshold; determining another wireless network associated with said base station and suitable for said communication; switching said communication from said first network to the other network.

36. A method according to claim 35 further comprising determining a loading parameter for said network; and wherein said determining is initiated when said parameter exceeds a threshold.

37. A method according to claim 35 or 36 wherein said switching comprises sending an IEEE802. 11 re-association request for the other network.

38. Program code for controlling a programmable apparatus to carry out the method of any one of claims 19 to 37.