GB2427981A - Optimising a contention period of a superframe for a wireless network - Google Patents

Abstract

A communications controller is described which provides flexible control of, for instance, an IEEE802.11(n) superframe. The communications controller provides control of the length of the superframe and the proportion of the superframe reserved as the Contention free Period, on the basis of an optimisation. A preferred approach to optimisation is also described, based on non-linear optimisation and more particularly a barrier method.

Description

1 2427981 Wireless Communications System This invention relates to

apparatus, methods, processor control code and signals for establishment of wireless communications in a network. More particularly, but not exclusively, it relates to aspects of the technology supported by the IEEE8O2.11 standards, especially concerning WLAN.

The IEEE8O2. 11 standard (in particular, the digest entitled "IEEE Wireless LAN Edition - A compilation based on IEEE Std 802.1 1TM-1999 (R2003) and its amendments") describes and defines technology for the implementation of wireless network based communication between stations (STA). In the context of the standard, a network is referred to as a Basic Service Set (BSS). Two topologies of BSS are supported. Firstly, an "infrastructure" topology defines one of the stations of the BAA as an Access Point (AP). An access point is provided to act as a bridge into a wider LAN or even the internet. Secondly, in an "independent" topology, a group of stations form a BSS on an ad hoc basis.

The 802.11 standard defines technology in two layers of the OSI model, namely the MAC layer and the PHY layer. The MAC layer presents a MAC Service Access Point (SAP) to higher layers of the OS! model, accepting MAC Service Data Units (MSDUs).

The MAC is capable of fragmenting a received MSDU if this allows for more robust transmission of data. After the MAC has processed a (possibly fragmented) MSDU, it presents MAC Protocol Data Units (MPDUs) to the PHY SAP.

In order to manage access to the communications medium, the standard provides a Distributed Coordination Function (DCF). Using the DCF, each station in the BSS competes for access to the resource on an equitable basis with all others, using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). As described in section 9.1.1 of the above referenced standard, a station using the DCF monitors the medium to determine if another station is transmitting and, if not, backs off for a minimum specified time period (which in practice is randomly generated) to determine that the medium remains free for use, before attempting to commence transmission itself.

Transmissions are conducted with either a two-phase (DATA/ACK) or fourphase (RTS/CTS/DATAIACK) handshake. In each case, error recovery is achieved through Automatic Repeat reQuest, ARQ. The four phase handshake is particularly appropriate to networks containing hidden or exposed nodes - i.e. stations that cannot be directly reached by a transmitting station.

Prioritisation, for example of the right to send an ACK in reply over another STAs right to initiate a new exchange, is achieved through Inter Frame Spaces (IFS) - the short IFS (SIFS), DCF IFS (DIFS), PCF IFS (PIFS) and Extended/Emergency IFS (EIFS).

Further description of the structure and use of the different types of IFS can be found in the above referenced version of the standard.

Use of the 802.11 standard compliant technology in an infrastructure type topology only requires provision of the DCF mechanism. However, a centralised Point Coordination function (PCF) can be optionally provided, to offer enhanced operation. This is of particular use for traffic with particular Quality of Service requirements, such as loss, delay or jitter.

The PCF offers contention-free access to a particular, nominated station. This station is implemented with a Point Coordinator (PC) functionality. Typically the Point Coordinator (PC) resides on the AP.

In order to allow PCF and DCF to coexist, the standard specifies use of a superframe, illustrated in figure 1. The superframe comprises a beacon (B) announcing the start of a contention free period (CFP), which is then followed by a contention period (CP).

During the CFP, the Access Point has contention free access to the medium. Then, in the CP, the station in question reverts to operation in accordance with the DCF.

The implementation of PCF provides an access point with the prospect of suitable access to a medium, but it further introduces certain deficiencies. For instance, if a station successfully contends for access to the medium during the CP there is no way of bounding the transmission time of that station, apart from the upper limit on MPDU- size. This can lead to unnecessary and undesirable delay of the beacon and consequently the ensuing contention free period. This clearly impacts the ability of PCF to support delay- and jitter-sensitive traffic.

It is thus desirable to find an arrangement which resolves issues arising through use of the PCF and DCF in combination. It is further desirable to find an arrangement which does this without requiring substantial change to the existing standard technology, as this would have an undesirable impact on compatibility.

A MAC protocol for use with the IEEE8O2. 11 defined technology is emerging in the draft standard IEEE8O2.lle. This draft standard is intended to enhance the PCF, through provision of a Hybrid Coordination Function (HCF). The HCF offers a Transmit Opportunity (TXOP), which grants access to the medium for a defined period of time (to address the delayed-beacon issue of PCF mentioned above). A Hybrid Controller (HC), which replaces the PC in 802.1 le, also has the ability to poll in the CP as well as the CFP. Stations are also prohibited from transmitting over a defined Target Beacon Transition Time (TBTT), again to address the delayed beacon issue.

Inmost published discussion of IEEE8O2.ll, and most particularly the concept of a superframe, the superframe is typically characterised by two parameters: CFPpip, which determines the rate at which the super-frame repeats (i.e. when new beacons are issued), and CFPMAX, which determines the length of the CFP. CFPREP is expressed in absolute length, in milliseconds. Typically, as described in "Self-adaptive transmission scheme of integrated services over an IEEE 802.11 WLAN," (C. Li, J. Li, and X. Cai, Electronics Letters, vol. 40, pp. 1596, 2004) CFPREP is in the range 20-lOOms.

Further, CFPMc is normally expressed as a ratio of the maximum CFP period to the CFPREP period.

The polling process within the CFP is initiated by the PC/HC sending either a simple CF-Poll packet to the first STA to be polled or, if the AP has data to send to the STA, a combined Data+CF-Poll packet. Having received a CF-Poll, the STA then has the right to send a single buffered frame (constrained by the explicit TXOP value in the CF-Poll packet, as of lie). The STA can then acknowledge the Data (if any) with a CF-Ack or, if it has data to send (that will fit within the TXOP), a Data+CF-Ack frame. As a further example of a multi-purpose transmission, the PC can then send a combined Data+CF-Ack+CF-Poll frame, acknowledging the transmission from the previous STA, polling the next one and sending it some data, in one operation. This process continues until the PC concludes the CFP with a CFP-End transmission (which can also be combined with CFP-Ack to the last polled STA if needed); the DCF-based CP then begins. For HCF, to distinguish from PCF operation, the letters "QOS" preface all CFPoll/CF-Ack messages; some additional parameters are employed to link the poll to a specific traffic stream identifier.

STAs associate with the AP by means of an initial handshake. A STA can indicate during association that it doesn't required to be polled now, or ever. That is, it is possible for a STA not to be included on the polling list. Whereas, in PCF, the list of STAs to be polled is the entire list of STAs that have associated with the AP, in HCF, a more dynamic registration process is used. In the latter case, a STA sends a TXOP- request to the AP in a QoS-Info field during a preceding TXOP. This preceding TXOP is either a polled transmission or a contended transmission; it will be evident to the reader that the very first TXOP must be obtained through contention. A TXOP can be anywhere between 32 and 8160 ts in length, in 32s increments, but constrained as follows: * There is a BSS-wide upper TXOPlimit (broadcast in the beacon) * There is a TXOPlimit[AC] for each AC (subject to the BSS-wide limit) set by the HC (via information fields in the beacon), for use in the EDCAICP phase of the superframe (note that the HC itself is not bound by this limit) * An explicit poll-specific TXOP can be granted to a specific station for a specific

exchange (via a field in the CF-Poll frame).

Traffic Streams, TS, are also set up in advance via the management plane, to give the HC advance notice of the requirements of the applications in the BSS. TSs are characterised by a data structure called a TSpec, or Traffic Specification, which includes minimum, nominative and peak (where appropriate) values for MSDU size, service interval, data rate, delay and required raw PHY rate.

Further information is available in the standards, specifically section 9. 3 of IEEE8O2. 11 2003 referenced above, and in corresponding sections of the draft IEEE8O2. lle Standard.

A series of publications by Changle Li and colleagues describe the discrete selection of pre-defined HCF parameters through pre-defined lookup tables indexed by the number of active A/V stations and the maximum allowable delay of the applications and the improvements that result. The look-up values have been found empirically. The publications are: "Self-adaptive transmission scheme of integrated services over an IEEE 802.11 WLAN," (C. Li, J. Li, and X. Cai, Electronics Letters, vol. 40, pp. 1596, 2004); "Performance Analysis of IEEE 802.11 WLAN to Support Voice Services," (C. Li, J. Li, and X. Cai, Proceedings of the 18th International Conference on Advanced Information Networking and Application (AINA 04), vol. 2, pp. 343 - 346, 2004); "A study of self-adaptive transmission for integrated voice and data services over an IEEE 802.11 WLAN," (C. Li, M. Li, and X. Cai, 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2004. PIMRC 2004., vol. 3, pp. 1922 - 1926, 2004); and "Performance evaluation of IEEE 802.11 WLAN - high speed packet wireless data network for supporting voice service," (C. Li, J. Li, and X. Cai, presented at 2004 IEEE Wireless Communications and Networking Conference, 2004. WCNC., 2004).

It is desirable to provide a way of controlling communication in a system involving a hybrid between contention based access and access controller based access with improved balance between the two modes of access.

An aspect of the invention provides communication control means for establishing a communications network with one or more communications stations across a communications medium, the communications network effecting communication by means of successive instances of a data frame structure, the data frame structure defining a first period during which the communication control means is operable to control access to the communications medium and a second period during which the access to the communications medium is allowed on a contention basis, wherein the communication control means further comprises means for determining a proposed optimum lengths of the first period and the second period and means for defining the first period and the second period on the basis of the proposed optimum lengths.

In a preferred embodiment, the means for determining optimum lengths comprises means for performing a constrained optimisation.

In a preferred embodiment, the optimisation is constrained by performance parameters of the system.

In a preferred embodiment, the optimisation is a non-linear optimisation.

In a preferred embodiment, the optimisation employs a barrier method.

Another aspect of the invention further provides a method of controlling access to a communications medium, comprising means for establishing a data frame structure for successive definition in the communications medium, the data frame structure defining a first period during which access to the communications medium is controlled centrally and a second period during which access to the communications medium is allowed on a contention basis, further comprising determining a proposed optimum length of the first period and the second period and defining the first period and the second period on the basis of the proposed optimum lengths.

In another aspect of the invention, wherein a communications network is established on the basis of a communications superframe comprising a contention period and a contention free period, there is provided a superframe controller including means to determine a performance characteristic, means to determine on the basis of non-linear optimisation superframe characterisation parameters and means for deploying the superframe in accordance with the determined superframe characterisation parameters across the network.

In another aspect of the invention, a general purpose computer, including wireless communications means for transmitting and receiving wireless communications signals, is configured by computer executable instructions to operate as either an access controller or a superframe controller as described above. The computer executable instructions can be introduced as a computer program product, storing information defining such computer executable instructions. The product can comprise a storage medium, such as an optical or magnetic disk, or a signal, such as an internet based download.

Further preferred features of these aspects of the invention will now be set forth by the following description of specific embodiments of the invention, provided by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic timing diagram showing the definition of a superframe in the context of the 802.11(n) standards; Figure 2 is a schematic diagram of a wireless communications system in accordance with a specific embodiment of the invention; Figure 3 is a schematic diagram of an access point of the system illustrated in figure 2; Figure 4 is a flow diagram of a process of characterising a superframe in an access controller of the access point illustrated in figure 3; Figure 5 is a flow diagram of a constraint definition subprocess of the process illustrated in figure 4; Figure 6 is a graph of performance of the process of the specific embodiment for different starting criteria; Figure 7 is a graph of exemplary results for a first optimised variable, for the specific embodiment of the invention; Figure 8 is a graph of exemplary results for a second optimised variable, for the specific embodiment of the invention; and Figure 9 is a flow diagram setting out an exemplary optimisation process called in a step of the process illustrated in figure 4.

With reference to Figure 2, an IEEE8O2. 11 compliant wireless network 10 comprises an access point 100. The access point 100 is connected to an external network (such as the Internet), in this example by means of a broadband modem. It will be appreciated that other alternative arrangements can be made for the access point to establish connection to an external source of streaming packet-based data.

The access point 100 establishes wireless communication, in accordance with the 802.lle Standard, with nodes A 102, B 104 & C 106. The access point 100 is thus configured to route data between the external network and the respective nodes 102, 104, 106.

In this embodiment, by way of example only, Node A 102 is a portable laptop computer, Node B 104 is a desktop computer and Node C 106 is a multimedia device (e.g. set-top box) operable in conjunction with a television or hi-fl system. Each of these nodes 102, 104, 106 is equipped with a 802.11 g wireless LAN network adaptor and configured to communicate with the access point 100.

Referring to Figure 3, the access point 100 is equipped with a broadband modem 202 as previously described, to establish connection to the Internet. The broadband modem 202 is connected to a general purpose bus 204, which in turn connects to the components of the access point including working memory (combining RAM and ROM function, as required) 206, a processor 208, a wireless network access controller 230 and a mass storage device 216. The access controller 230 is, in turn, connected to an antenna 212.

User operable input devices 220 are further provided, in communication with the processor 208. The user operable input devices 220 comprise any means by which an input action can be interpreted and converted into data signals.

Audio/video output devices 222 are further connected to the generalpurpose bus 204, for the output of information to a user. Audio/video output devices 222 include any device capable of presenting information to a user, for example, a speaker and a video display unit.

The operation of the access point 100 will now be described. On the basis of execution of an application residing at a network node, data is retrieved from the external network via a connection established through the broadband modem 202. The manner in which the modem establishes connection is not relevant to the present invention, and can be of a conventional nature.

The retrieved data is then stored in the RAM 206. The access controller 230, determines, on an ongoing basis, access to the network by the access point 100 and by the other nodes 102, 104, 106. The access controller 230 is operable to define the superframe, as previously described in figure 1, but also to determine the length of the superframe and the proportion thereof which is reserved as the CFP.

To do this, the access controller 230 operates in accordance with the process illustrated in figure 4.

In order to develop the objective function by which the access controller 230 determines an optimal specification for the superframe, certain assumptions must be made. The figures now used in this specific example are from an IEEE8O2. 11 b compliant system as used in the Li et al. publications referred to above.

It will be clear to the reader of these documents that these are viable parameters for typical systems presently available, and allow a fair comparison of performance of the specific embodiment of the invention with the arrangements offered in those documents. Clearly, it will be evident to the skilled person that an actual implementation will use the parameters appropriate to the implementation in question.

For example, if an aspect of the present invention is being implemented in an 802.11 n system then parameters suitable for such technology would be used instead.

For clarity, the general example, with algebraic representations of the parameters, will be set out in due course. The assumptions from the cited prior art set out above, and hence the example parameters used for the remainder of this section, include: An IEEE8O2.l lb system with a datarate of 2Mbps; Standardised polled-traffic payload sizes of 200bytes; Ten data stations each generating frames with a mean size of 1000 bytes at a rate of 7.5 frames per second; No bit-errors, interference, captureeffect or hidden terminals; and RTS/CTS handshaking and power-saving disabled throughout.

Some specific parameters used in the present example are given in Table 1: Table 1: Model Parameters Parameter Value Parameter Value Slot 0.O2ms PIFS 0.O3ms SIFS 0.Olms DIFS 0.O5ms PLCP Header! MAC Header 28 bytes 0. 192ms Preamble Mean data MSDU 1000 bytes A/V MSDU 200 bytes Data rate 2Mbps Control rate 1Mbps Beacon 160 bytes ACK 14 bytes From these parameters, the characteristics of a standard data exchange in each phase of the superframe can be identified, as illustrated in step S 1-2 of Figure 4.

Using these standard exchanges with their implicit overheads, utility functions expressing the utilisation or wastage of each phase are derived by calling a procedure in step Si -4. This captures both the inherent efficiency of the exchange and the efficiency of the fit of the exchanges within the superframe structure. This procedure is illustrated as a two part process in figure 5.

Firstly, in step S2-2, the overhead incurred during the polling phase, V, the number of stations to be polled, is expressed in terms of CFPM, CFP and N, as shown in equation 1: _[o.o2N +{(cFP *CFP)_2228N}]/ (1) /(CFP * CFP) Then, in step S2-4, the utilisation of the contention period, L(Nc1), is similarly expressed in terms of CFPM, CFPpp and NCTX, the number of stations transmitting during the contention period, as equation 2 shows: L N - I [(o.674N) + {((i - CFPIX)CFP)- 4.978N}/ 1 (2) ( CTx) - [ /((1 CFPIX)CFPP)] These functions give the upper bounds on performance in different configurations, for example in the graph illustrated in figure 6, V is considered for different values of CFPMAX.

Further constraints could, in alternative versions of the specific embodiment, be expressed, such as capturing the minimal sizes of the CFP and CP as specified to the standard (as specified in IEEE8O2.1 1 2003, specifically at paragraph 9.3.3.3.

Specifically, one constraint on the characterisation of the superframe is that the CFP has to be at least large enough to contain polled exchange (CFPoI1+Data, CFP011+Ack) comprising the largest payload possible in each direction, plus a Beacon and a CF-End.

Similarly, the CP has to be large enough to contain an exchange (plus ACK) of the largest payload possible. The largest payload is 2312 bytes in 802.1 lb (employing WEP, as stated in the standard).

Although the standard does not specify clearly the data rates for which transmission is to be enabled, in order to meet the objective of ensuring sufficient space for an exchange by any station, the data rate must be the lowest data rate possible - in this case, 1Mbps. Similarly, the standard does not clearly state whether time must be allowed for inter- frame spacing (IFS); however it would be evident to the skilled person that that the IFS must be included to complete the exchanges successfully. For the purposes of defining the largest possible size of the CFP duration, the standard includes SIFS (and two slot durations) in the calculation of the CP size, which suggests that this is an appropriate arrangement.

Hence, the CFP must be at least: CFPMIN PIFS + Beacon + Data + SIFS + Data + SIFS + CF-End (3) = 0.03 + 1.696 + 18.912 + 0.01 + 18.912 + 0.01 + 0.352 = 3 9.922 ms And the CP must be at least: CPMIN DIFS + Data + SIFS + ACK (4) = 0.05 + 0 + 18.912 + 0.01 + 0.304 = 19.276 ms It should be noted that, in the standard, it is stated that this is true "when operating with a CW of aCWmin". The back-off mechanism draws a random value from the range [0, CW], where CW is the exponentially increasing value (increasing on each failed transmission) that is initialised at aCWmin, which is 31 for the 802.11 b PI-IY (paragraphs 9.2.4 and 18.3.3 of the standard).

These additional constraints suggest that a number of other operational restrictions would, in some specific embodiments of the invention be appropriate, including that CFPREP ? 6Oms, and that combinations such as (CFPREp1 OOms, CFPM0.95) are invalid (as such a combination would leave a contention period duration ofjust 5ms).

Once all desired constraints have been derived in accordance with step S 1-4, the process continues in step S 1-6, by constructing an objective function in accordance with optimisation practice. The derived utilisation functions L and V can be used directly in the following objective function: Obj(x) = (1- L(Nc))2 + (V(Np))2 (5) Then, taking into account the objective function of equation 5, the function of equation 6 below is optimised in step S1-8 to derive the apparently most desirable combination of CFPMAJ( and CFPREP. The manner in which this is achieved will now be described.

In the following, CFPMAX = x, with the assumption that CFPREP (y) is equal to the delay D. Using the data exchange characteristics L and V derived in step S1-2 gives: ( ( Np(CbCa)*'l2 i +11- I (6) 1-x) L xy) On establishment of the optimisation, the optimisation problem is a minimisation of f0(x,y) , subject to: CFPmm - xy = 0 CP - (1- x)y = 0 0 =x =1 0 =y =D Considering the 802.llb constraints set out in the four publications by Li et a!. above, the parameters have the following values M =0.674 standardised data exchange overhead, ms H =4.978 standardised data exchange, ms N 10 number of data stations Cb=2.228 polled exchange duration, ms Ca = 0.02 polled exchange overhead, ms CFPmIn = 39.922 CFPM at 1Mbps, ms (from equation 3) CPmin = 19.276 CPMIN at 1Mbps, ms, (from equation 4) D E {75, 87.5, 100, 112.5, ... 200} delay values under consideration N E {2, 4, ... 20} numbers of polling stations under consideration P. = 0.0075 packet rate of the contending traffic, used to estimate the effective number of contending stations The "Barrier Method" is then used to solve the optimisation problem stated above. In simple terms, the barrier method uses an "indicator function", formed by modifying the inequality constraints, to indicate whether the solution is feasible or not. Slight variations are applied to a parameter of the indicator function(s) is (are) successively and iteratively, with each iteration performing Newton's Method for that set of values.

In further detail, the barrier method is applied to this multivariate optimisation problem which seeks global minima of the objective function t0 (x, )* Due to the constraints on this optimisation, the choice of solution is restricted to a subset of feasible solutions defined by g(x) = 0 and h(x) =0, where both g and h can be multi-valued functions. In the present non-linear case, provided thatf is twice continuously differentiable, solutions can be found using a mixture of analytic and numerical methods.

The optimisation function is re-expressed in vector form (as it is multivariate), and the constraints form the barriers after which the method is named. Gradients and Hessians (second derivatives) of the optimisation function and the barrier functions are taken.

Starting points (xO and yO) are found to initialise the Barrier Method and optimal methods for x and y can then be found.

Referring to figure 9, the barrier method is outlined qualitatively in steps si to s7, and detailed using pseudo-code in Table 1 below.

In step si, the loss function is expressed as in equation 5. In step s2, the constraints of the problem are chosen, as discussed above. In step s3, parameters 1, p and inner and outer tolerances of the algorithm,e, are initialised. Typical values might for example be t = 0.5, p = 2, e = 0. 000 1, and e, = 0.0001.

In step s4, a starting vector x that satisfies the constraints is chosen. In step s5, Newton's method is run until inner tolerance e, is met. Once inner tolerance s, is met, outer tolerance c, is evaluated; if the outer tolerance is also met, then in step s6 the new near-optimal vector x is taken as the output of the process. However, if outer tolerance e is not met, then in step s7 the logarithmic barrier accuracy parameter t is increased by a factor p, and the Newtonian process re-started using the last vector x.

Accuracy parameter t provides a trade-off between convergence performance and the number of iterations required for convergence. As t increases it provides a better approximation to an indicator function (see below), but at the cost of slower convergence.

It will be clear to a person skilled in the art however that, particularly in the case of pre- computed sequences, the barrier method may be initialised with a relatively high value of t and eliminate step s7.

Although the barrier method is appropriate to convex problems and the skilled person will appreciate that the present problem is not convex, the barrier method is still appropriately used by suitable selection of constraints.

given strictly feasible x, t > 0, z > 1, > 0, > 0 repeat 1. Newton'c method (x. > 0) a. x= -V2f(x)'Vf(x) A2 = Vf (x)H x h. quit if A2/2 < return x x c. line search (determine 5) d. x x+/3ix 2. x x 3. quit if p/t < eo 4. t:== pt Table 1: Psuedo code for barrier method optimisation for the cost function of equation 3.

For a given set of constraints (specifically the aforementioned 802.1 lb parameters and the assumptions in the papers by Li et a!.) ranges ofvalues as shown in figures 7 and 8 are then produced.

The access controller 230 is operable to apply the above described processes to operating parameters during operation of the network. In that way, changes to the operating parameters can be responded to by adjustment, if optimal to do so, of either or both of the superframe repetition period and the proportion of the superframe reserved for the CFP.

The access controller 230 of the described specific embodiment is shown as implemented as a discrete unit, but it will be appreciated that it could be integrated with one or more other functional units of the access point. Further, it would be possible for the function of the access controller to be distributed. The non-linear optimising determination functionality could be provided in one node, while the superframe definition functionality could be provided by entirely separate means.

The access controller is shown as implemented by hardware, and particularly by integrated circuit (IC) devices. However, it will be appreciated by the skilled person that some or all of the function of the access controller could optionally be implemented in a general purpose hardware unit configured by a suitable software product. Such a software product could be introduced by means of optical (or other) storage media, by means of a signal such as a download, or by any other way of enabling software to become stored on a computer apparatus. In such circumstances, the suitable computer apparatus could be IEEE8O2. 11 compliant, or even IEEE8O2.1 le compliant, with further functionality in accordance with a specific embodiment of the invention to be provided by means of the software configuration.

Whereas the described embodiment illustrates operation of the optimisation process by the access controller in a dynamic environment, an alternative embodiment could provide a memory resource, such as a look up table, storing values of CFPMAJ and CFPREP for all possible configurations of the parameters. The currently most appropriate values of CFPMAJ( and CFPREP could then be looked up by the access controller 230 and applied to the system.

For the avoidance of doubt, the present invention is not limited to applications involving 802.11 e technology - it can equally be applied to 802.11 technology involving PCF.

Further, it can be applied to any other communications technology in which a balance is struck between a period wherein access is centrally controlled and a period in which access is contended for.

Further variations, modifications and additional features will be apparent to the skilled man considering the above disclosure and no statement above is intended to limit the scope of protection sought for the invention, which is to be determined by reference to the appended claims, interpreted in the light of, but not specifically limited to, the above

description of specific embodiments.

Claims (12)

1. CLAIMS: 1. A communication controller for establishing a communications

   network with one or more communications stations across a communications medium, the communications network effecting communication by means of successive instances of a data frame structure, the data frame structure defining a first period during which the communication control means is operable to control access to the communications medium and a second period during which the access to the communications medium is allowed on a contention basis, wherein the communication controller further comprises means for determining a proposed optimum length of the first period and of the second period and means for defining the first period and the second period on the basis of the proposed optimum lengths.
2. 2. A controller in accordance with claim 1 wherein the means for determining optimum lengths comprises means for performing a constrained optimisation.
3. 3. A controller in accordance with claim 2 wherein the means for determining optimum lengths comprises means for performing an optimisation constrained by performance parameters of the system.
4. 4. A controller in accordance with claim 3 wherein the means for determining optimum lengths comprises means for performing a non-linear optimisation.
5. 5. A controller in accordance with claim 4 wherein the means for determining optimum lengths comprises means for performing an optimisation employing a barrier method.
6. 6. A controller in accordance with claim 1 wherein the means for determining said proposed optimum lengths comprises storage means storing predetermined data calculated on the basis of a prior optimisation, for reference with regard to one or more look-up parameters.
7. 7. A controller in accordance with any preceding claim wherein the controller is IEEE8O2. 11 compliant.
8. 8. A method of controlling access to a communications medium, comprising establishing a data frame structure for successive definition in the communications medium, the data frame structure defining a first period during which access to the communications medium is controlled centrally and a second period during which access to the communications medium is allowed on a contention basis, further comprising determining a proposed optimum length of the first period and the second period and defining the first period and the second period on the basis of the proposed optimum lengths.
9. 9. A superframe controller operable to establish a superframe in a communications network established on the basis of a communications superframe comprising a contention period and a contention free period, the controller including means for determining a performance characteristic, means for determining, on the basis of non- linear optimisation, superframe characterisation parameters and means for deploying the superframe in accordance with the determined superframe characterisation parameters across the network.
10. 10. A computer program product comprising computer executable instructions which, when executed by a general purpose computer provided with means for establishing wireless communications, cause said computer to be configured either to operate as an access controller in accordance with any of claims 1 to 7, or to perform the method of claim 8, or to operate as a superframe controller in accordance with claim 9.
11. 11. A computer program product in accordance with claim 10, comprising a storage medium storing information defining the computer executable instructions.
12. 12. A computer program product in accordance with claim 10, comprising a computer receivable signal bearing data defining the computer executable instructions.