WO2019048055A1

WO2019048055A1 - Controlling access points in wireless networks

Info

Publication number: WO2019048055A1
Application number: PCT/EP2017/072624
Authority: WO
Inventors: Luis Felipe DEL CARPIO VEGA; Yu Wang
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2017-09-08
Filing date: 2017-09-08
Publication date: 2019-03-14

Abstract

A method for controlling, by a central control node (100), a plurality of time synchronized access points (200) connected to the central control node (100), wherein the plurality of access points (200) exchange data with wireless devices (50) over a wireless network, the method comprising the steps of selecting a common target time at which the plurality of access points (200) should start checking a channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission, and instructing the plurality of access points (200) to start checking the channel availability at the common target time.

Description

Controlling access points in wireless networks

Technical field

The present application relates to a method for controlling a plurality of access points by a central control node and to the corresponding control node. Furthermore, a method for operating an access point is provided and the corresponding access point. Additionally, a system comprising the central control node and the access point is provided, and a computer program comprising a program code as well as a carrier comprising the computer program.

Background Industrial networks will be based on Time-Sensitive Networking, TSN, Ethernet as the main communication technology for process or factory automation where a degree of time determinism and time synchronization are supported. In such a TSN Ethernet different access points are connected to a central control node wherein the access points exchange data with wireless devices such as devices used in a factory or production site. With TSN Ethernet a layer 2, L2 packet, can be guaranteed a level of delay and packet delivery times can be deterministic if required.

When the access points communicate with the wireless devices unlicensed bands may be used wherein in these unlicensed bands, the access to the wireless medium is allowed by following regulatory rules. Usually the wireless devices may perform listen before talk procedures also known as Clear Channel Assessment, CCA, before transmitting to the wireless medium.

Institute of Electrical and Electronics Engineers, IEEE 802.1 1 is a technology operating on unlicensed bands. The most common operating mode uses the CCA procedure implemented by sensing the medium first for a fixed amount of time and then for a variable time given by a Back-off (BO) counter. The back-off value is a random value between zero and the Contention Window, CW, which is drawn by a station which is compliant with the 802.1 1 standard and the back-off time is the back-off value multiplied by a slot time which is amended accordingly based on a physical layer numbering. When the access points transmit a frame, by way of example a beacon frame, the access points need to perform a clear channel assessment by selecting a Target Beacon Transmission Time, TBTT and back-off values which is a process which is performed for each access point. Figure 2 shows an example where three different access points transmit a beacon frame wherein the three different access points are not detecting each other's transmissions. As shown in figure 2, the first access point, AP-1 , carries out a clear channel assessment during a first time period 21 and then transmits the beacon frame 22. The second access point AP-2 has another clear channel assessment time 23 before transmitting the beacon frame, and the third access point, AP-3 has a third clear channel assessment time different from the assessment times 21 and 23 before transmitting the beacon frame 22. As shown in figure 2, different TBTT values are used by the different access points. With the introduction of OFDMA (Orthonogal Frequency-division Multiple Access) in 802.1 1 ax, a new management frame named Trigger Frame, TF, was introduced. Trigger frames are used to schedule uplink access, but since there are circumstances where uplink random access is desirable, by way of example the access point does not always know whether a station has data for uplink transmission or not, a special type of trigger frame called TF for Random Access, TF-R, was introduced.

Time determinism is hard to guarantee in unlicensed bands due to the regulatory requirements such as listen before talk discussed above. Especially the randomly selected back-off values in the CCA procedure lead to non-deterministic CCA times.

Currently the coordination of transmissions between access points is not possible with the current CCA procedures. It is by way of example not possible to send the beacon frames simultaneously by different access points which perform CCA.

Summary

Accordingly, a need exists to overcome at least some of the above-mentioned problems and to provide a possibility for a coordinated transmission at access points which carry out a listen before talk mechanism before sending data.

This need is met by the features of the independent claims. Further aspects are described in the dependent claims.

According to a first aspect, a method carried out by a central control node is provided which controls a plurality of time synchronized access points which are connected to the central control node. The plurality of access points exchange data with wireless devices over a wireless network. According to one step of the method a common target time is selected by the central control node at which the plurality of access points should start checking the channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission. Furthermore, the central control node instructs the plurality of access points to start checking the channel availability at the common target time.

Furthermore, the corresponding central control node is provided comprising a memory and at least one processing unit, wherein the memory contains instructions executable by the at least one processing unit. The control node is configured to operate as mentioned above or as discussed in more detail below. The central control node is able to coordinate the listen before talk or clear channel assessment by selecting a common target time for the different access points. Furthermore, by transmitting the common target time to the different access points the latter can apply the common target time when checking the channel availability.

Alternatively, a central control node is provided which is configured to control the plurality of time synchronized access points which are connected to the central control node. The central control node comprises a first module configured to select a common target time at which the plurality of the access points should start checking the channel availability where it is checked whether the transmission channel of the wireless network is available and free for a data transmission. The central control node comprises a second module configured to instruct the plurality of access points to start checking the channel availability at the common target time.

Furthermore, a method for operating the time synchronized access point in the network is provided in which a plurality of time synchronized access points are connected to the central control node controlling the plurality of access points which exchange data with wireless devices over a wireless network. According to one step a common target time is received at which the access point should start checking the channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission. The access point starts to check the channel availability at the received common target time.

Furthermore, the corresponding access point is provided comprising a memory and at least one processing unit wherein the memory contains instructions executable by the at least one processing unit wherein the time synchronized access point is operative to carry out the steps as mentioned above or as discussed in more detail below.

Alternatively, an access point is provided which is located in a network which is time synchronized with a plurality of other access points and wherein the access points are connected to the central control node controlling the plurality of access points and wherein the plurality of access points exchange data with wireless devices over a wireless network. The access point comprises a first module configured to receive the common target time, at which the access point should start checking the channel availability in which it is checked whether the transmission channel of the wireless network is available and free for a data transmission. The access point can comprise a second module configured to start checking the channel availability at the received common target time.

Furthermore, a system is provided comprising the central control node as discussed above or as discussed in more detail below and comprising the access point as discussed above or as discussed in more detail below.

Additionally, a computer program comprising program code is provided to be executed by at least one processing unit of a central control node or of an access point wherein execution of the program code causes the at least one processing unit to execute a method discussed above or in more detail below. Furthermore, a carrier comprising the computer program is provided wherein the carrier is one of an electronic signal, optical signal, radio signal or computer readable storage medium. It should be understood that the features mentioned above and features yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the present invention. Features of the above-mentioned aspects and embodiments may be combined with each other in other embodiments unless explicitly mentioned otherwise.

Brief Description of the Drawings

The foregoing and additional features and effects of the application will become apparent from the following detailed description when read in conjunction with the accompanying drawings in which like reference numerals refer to like elements. Figure 1 shows an example network including a central control node to which several access points are connected in a time synchronized network where a coordination of the channel sensing of the different access points is used. Figure 2 shows a schematic view of the transmission procedure used by different access points known in the art.

Figure 3 shows an example flowchart with different options to which extent the central control node shown in figure 1 coordinates the checking of a channel availability of the different access points.

Figure 4 shows an example according to an embodiment of the invention where different access points have aligned the checking of the channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission by using a common target time at which the checking of the channel availability is started.

Figure 5 shows an example flowchart of a method carried out by an access point used in the example of figure 4 for checking a channel availability.

Figure 6 shows a further embodiment of how a central control node of figure 1 coordinates the checking of the channel availability at the different access points wherein all access points check the channel availability for the same amount of time. Figure 7 shows a further embodiment of a coordination of the general availability of the different access points by the central control node wherein all access points additionally transmit the same content.

Figure 8 shows an example flowchart of a method carried out at an access point in a method when the access point operates as indicated in figure 4, 6 and 7.

Figure 9 shows an example view of how a common trigger frame is transmitted using the OFDMA transmission technology. Figure 10 shows an example message exchange occurring between the central control node and the access points when the central control node coordinates and controls a checking of the availability of the transmission channel by different access points. Figure 1 1 shows an example flowchart of a method carried out at the central control node when the central control node selects a common target time at which the different access points should start checking the channel availability and how the access points are instructed to use the common target time.

Figure 12 shows an example flowchart of a method carried out at the access point receiving the instructions transmitted by the central control node in a method of figure 1 1. Figure 13 shows an example schematic representation of a central control node configured to coordinate the checking of a channel availability of the different access points.

Figure 14 shows an example schematic representation of an access point in a network as shown in figure 1 .

Figure 15 shows another example schematic representation of a central control node coordinating the different access points when the access points check the channel availability. Figure 16 shows another example schematic representation of an access point receiving the instructions from the central control node when it should start checking the general availability for the transmission of data.

Detailed Description of Embodiments

In the following embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings which are to be illustrative only.

The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather the various elements are presented such that their function and general purpose becomes apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components of physical or functional units shown in the drawings and described hereinafter may also be implemented by an indirect connection or coupling. A coupling between components may be established over a wired or wireless connection. Functional blocks may be implemented in hardware, software, firmware, or a combination thereof.

Figure 1 shows an example schematic network in which a central control node 100 is connected in a wired network 10, such as ethernet, to a plurality of access points 200. The different access points are time synchronized access points meaning that a data packet such as a layer 2 packet can have a guaranteed level of delay and the packet delivery times can be precisely determined if necessary. The access points 200 transmit the data packets to wireless devices (50), wherein the wireless device may be associated with non-humans like a machine in a factory. However, the wireless device may be a telephone type of device, a cellular telephone, a mobile station, a cordless phone, or a personal digital assistant type of device like a laptop, notebook, notepad, tablet equipped with a wireless data connection. The wireless device may be equipped with a SIM (Subscriber Identity Module) or similar device along with a unique identification of the wireless device in a mobile communications network.

In a first step the central control node 100 can determine a list of access points 200 participating in a synchronization of a channel availability check where the different access points check whether a transmission channel of the wireless network is available and free for a data transmission. This checking of the channel availability can comprise procedures such as listen before talk or Clear Channel Assessment, CCA. Accordingly, the central control node generates a list of the access points participating in the synchronized CCA process. The access points 200 are connected to the central control node using the wired network 10 which is a time aware network and wherein the access points synchronized their clocks to the wired network's reference clock. Alternatively the access points may be synchronized via a Global Positioning System, GPS, Global Navigation Satellite System, GLOSNAS, or any alternative synchronization method/system known in the art or being developed in the future. In a next step the central control node 100 can select a common target time to begin the CCA process for each of the access points contained in the list. In a next step, the third step, the central control node 100 computes the values of the parameters needed in the Clear Channel Assessment, CCA, process for the members of the list of access points. In a fourth step the selected values of the CCA process and/or the selected target time are delivered via the network 10 to the access points 200 included in the list. The access points receiving the selected values for the CCA process and/or the selected common target time use these values for their CCA process. The values may be the same (or partially the same) for all (or a sub-set) of the access points contained in the list of access points determined by the central control node 100.

The central control node 100 may also decide the content for a common frame type to be transmitted by all or a sub-set of the access points 200 on the list of access points and send this kind of information to the access points participating in the common control procedure. The common frame can be a common beacon frame. However, the common frame type can also be an OFDMA frame where subchannels such as resource units are selected by the central control node 100, but the frame payload per subchannel is selected independently by each of the access points 200.

In a further step the central control node 100 may also decide the maximum time duration of the common frame. In the next step the access points may send an information frame to the central control node 100 reporting if the channel contention process was successful or not, wherein the information is sent over the network 10. In this context a successful process can mean that the channel was never busy during the channel contention process. A non-successful process can mean that the channel was busy at some point of the CCA assessment or during the whole time of the CCA assessment. The information about why the channel was detected busy may be also included in an information frame.

The access points detecting that the wireless medium is busy in the CCA process can either stop the transmission processes and drop the common packet.. Additionally, or as an alternative, they can continue the CCA process in the legacy CCA procedure fashion, meaning that APs fall back to perform independent CCA processes.

Furthermore, the central control node 100 may also select a modulation and coding scheme for the frame to be transmitted to assure that the transmission time for the frame will be the same at each transmitting access point 200.

Figure 3 summarizes the different options for the coordination carried out by the central control node 100 for the different access points 200. In a step S31 the control node determines the list of access points participating in the synchronized CCA process. The list may contain all access points connected via network 10 to the central control node, but the list may also contain not all the access points connected in network 10 so that some access points are not participating in the coordination process by the central control node 100. In a step S32 the central control node 100 selects the level of alignment for each of the access points. This and different options are available to the central control node. As a first option the central control node may only select a common target time in step S33 where the access points should start checking whether the transmission channel is available and free for data transmission.

In another option the central control node may not only select a common target time in step

533 but may also select a common back-off value and an inter-frame space. In this step

534 the value of these parameters needed in the clear channel assessment process are determined. As a further option the central control node may not only select the common target time and transmission parameters needed for the CCA process, but may also select in step S35 a content for a frame which is transmitted by the corresponding access point 200. In step S36 the control node 100 delivers the values determined in the different options mentioned above to the access points present in the list via the network 10

As a further option no alignment may be selected for an access point at all. For this option and for the other options the method ends in step S37. In the embodiment described above the system with the central control node and the access points may be an 802.1 1 compliant system where the computed values for the CCA parameters in step S32 are the number of back-off slots or back-off value used in channel sensing and the inter-frame space used for channel contention. The inter-frame space is an amount of time at which a channel sensing is carried out and the back-off value at which the transmission is started after it has been detected that the channel was busy and became free again. In the first option discussed in figure 4, the first level of alignment is to align the moment in time where the checking of the channel availability starts, here where the CCA process begins. In the example shown in figure 4 the central control node decides a common TBTT time and delivers this information to the involved access points. The access points receiving the common TBTT and the common values for the CCA process use the received values instead of the values they would generate themselves independently so that all access points start the channel sensing at a common point in time 41 . However, as can be seen, the different access points check whether the channel is clear for a transmission over a different period in time as the clear channel assessment time 42 of the first access point is shorter than the clear channel assessment time 43 of the third access point, wherein the latter assessment time is again shorter than the clear channel assessment time for the second access point 44. As summarized in figure 5, the access point receive the CCA alignment values such as for example the time to begin the CCA, the back-off value or the inter-frame space from the central control node 100 in step S51 and in step S52 the corresponding access point uses the received values to configure its CCA process.

Figure 6 shows a further second level of alignment as discussed above in connection with figure 3. In addition to the common target time at which the different access point should start checking the channel availability, the values used in the specific CCA process are aligned and delivered to the access points in the list. The access points receiving the common TBTT and the common values use the received values instead of the values which they would generate by themselves. Thus, as indicated in figure 6 the three access points (AP-1 , AP-2 and AP-3) all start the clear channel assessment at a common point in time 61 and the lengths of the time period for which they carry out the clear channel assessment is also the same for all access points as can be deduced by the channel assessment time 62 which is the same for all access points.

Figure 7 then shows a further level of alignment wherein the central control node further decides on the content of the frame to be transmitted. Thus, as shown in figure 7, all access points start the general assessment procedure at the same point in time 71 and for the same length of the time period 72. Additionally, the content which should be transmitted by each of the access points is also selected so that the access points (all or a sub-set from the list) transmit the same content as symbolized by the beacon frame 73.

Figure 8 summarizes the steps carried out at the access point in the embodiment shown in figure 7. In step S81 the access point receives the alignment values for the clear channel assessment such as the time to begin the clear channel assessment, the back-off values and the inter-frame space from the central control node 100. In step S82 the access point furthermore receives the details about the common frame. In figure 8 steps S81 and S82 are shown as separate steps, however, the information received may also be received in a common message.

In step S83 the access point uses the received values to configure the process in which the channel availability is checked. In step S84 after a common general assessment time 72 the common frame with the common content is transmitted in step S84. Figure 8 can also summarize the steps carried out at the access points shown in figures 4 and 6 when only the values relevant for the scenario shown in figures 4 and 6 are received in step S81 of figure 8. One example of the common content transmitted by the different access points is a trigger frame. Figure 9 shows an example where a trigger frame is transmitted simultaneously by the different access points. As shown in figure 9, every participating access point transmits in non-overlapping sub-carriers or sets of sub-carriers or resource units in downlink multipoint OFDMA fashion. This has the advantage of protecting the control signals (trigger frames) when multiple access points 200 transmit simultaneously. In the example given the common frame indicated by the central control node 100 is the trigger frame. A common frame does not necessarily mean an identical frame, but indicates the type of frame and the manner to coordinate the transmission as shown in this example. Each access point transmits on a unique sub-band to avoid interference. In the uplink scheduling period the corresponding wireless devices act according to the trigger frame's information.

Figure 10 shows a message exchange between the involved entities. In the embodiment shown in step S100 the central control node calculates the parameters which should be common to a set of access points, namely the access points for which it was determined that they should participate in the synchronized CCA process (either all or a sub-set of the access points on the list). In step S101 and S102 the central control node 100 transmits the computed values to the access points via the network 10 wherein the delivered messages can carry information about the time to begin the CCA, the back-off value, and the inter- frame space. As discussed above in connection with figure 3 to 8 the amount of information controlled by the central control node may vary from a common target time to an embodiment where also the content is determined by the central control node. In steps S103 and S104 messages may be transmitted to the different access points which may carry details on the resource allocation and the content of the common frame if the content is also controlled by the central control node. In step S105 the CCA procedure is carried out in the two access points based on the information as received from the central control node. In step S106 and S107 the two access points may simultaneously transmit the messages to the wireless devices as instructed by the central control node. In step S108 the different access points can optionally determine in addition whether the corresponding transmission channel was busy or not when the channel availability was checked. In this context success information may be generated in which the different access points indicate whether the transmission channel was busy or not. The channel availability may be considered as successful when the transmission channel was never busy when the channel availability was checked. The channel availability may be considered as not successful when the transmission channel was busy at least once when the channel availability was checked. In step S109 the different access points may transmit the corresponding success information to the central control node 100.

Figure 1 1 summarizes some of the steps carried out by the central control node for synchronizing the access procedure carried out by the different access points. In a step

51 10 the central control node at least selects a common target time in which the plurality of access points should start checking the channel availability where the different access points check whether a transmission channel is available for data transmission. In step

51 1 1 the different access points are instructed to start checking the channel availability at the common target time. The central node may also select other values/data (for example length of CCA period, data to be transmitted) and instruct APs to use also these other values as discussed above (for example in connection with figures 6 and 7).

As far as the corresponding access points are concerned reference is made to figure 12 where one access point receives the common target time as instructed by the central control node in step S120. The access point then applies the received instruction and starts checking the channel availability at the received common target time in step S121. The access point may receive also other values/data (for example length of CCA period, data to be transmitted) and use those in the CCA process and/or the transmission of data as discussed above (for example in connection with figures 6 and 7).

Figure 13 shows a schematic architectural view of a central control node 100 which can control the access points as mentioned above. The central control node comprises an interface 1 10 which is provided for transmitting user data or control messages to other entities such as the access points and which is provided for receiving user data or control messages from other entities such as the access points or any other nodes located in the network 10. The interface is especially qualified to transmit the control information to the access points with which the checking of the channel availability is synchronized. The control node 100 furthermore comprises a processing unit 120 which is responsible for the operation of the node 100. The processing unit 120 comprises one or more processors and can carry out instructions stored on a memory 130, wherein the memory can include a readonly memory, a random access memory, a mass storage, a hard disk or the like. The memory can furthermore include a suitable program code to be executed by the processing unit 120 so as to implement the above described functionalities in which the node 100 is involved. Figure 14 shows a schematic view of an access point 200 which can carry out the clear channel assessment based on the information received from the control node as discussed above. The access point 200 comprises an interface 210 configured for transmitting user data or control messages to other entities such as other access points or the central control node 100 and configured to receive user data or control messages from other entities such as the control node 100 or other access points 200. The access point furthermore comprises a processing unit 220 which is responsible for the operation of the access point 200. The processing unit 220 comprises one or more processors and could carry out instructions stored on a memory 230 wherein the memory may include a read-only memory, a random access memory, a mass storage, a hard disk or the like. The memory furthermore includes a suitable program code to be executed by the processing unit 220 so as to implement the above described functionalities in which the access point is involved. Figure 15 shows a further schematic representation of a central control node 300 wherein the central control node comprises a first module for selecting the common target time at which the different access points should start checking whether a transmission channel of the wireless network is available and free for a data transmission. Furthermore, a module 320 is provided which instructs the plurality of access points to start checking the channel availability at the common target time. The control node 300 can optionally comprise a module 330 for receiving the success information generated by the access points when checking the channel availability, wherein the success information indicates whether the transmission channel to the wireless devices was busy or not . Figure 16 shows another schematic representation of an access point 400 which comprises a first module 410 configured to receive the common target time at which the access point should start checking the channel availability. A module 420 is provided for carrying out the checking at the received common target time, i.e. to check the channel availability at the point in time indicated by the central control node. The access point 400 can optionally comprise a module 430 for generating and transmission the success information

As far as the central control node 100 is concerned the node may select a length of a time period for each of the plurality of access points during which the checking of the channel availability should be carried out. The central control node 100 can then instruct each of the plurality of access points to check the channel availability for the time period of the selected corresponding length. As indicated in connection with figure 6, the length of the time period can be the same for all access points, however, a different length may be individually determined for each access point.

Furthermore, it is possible that the control node 100 selecting the length of the time period means that at least one of a back-off value and an inter-frame space is selected for each of the access points for which the coordination should be carried out.

Furthermore, it is possible that the control node 100 selects the content for a frame to be transmitted by each of the plurality of access points 200 after the access point has successfully checked the channel availability. The different access points 200 are then instructed to transmit the frame with the selected corresponding content after the channel availability has been successfully checked. As shown in figure 7, the same content may be selected for each of the access points. However, it is also possible that a different content is selected for the different access points.

Furthermore, the control node 100 may select a maximum time duration for a frame to be transmitted by each of the access points 200 after the access points have successfully checked the channel availability. Each of the plurality of access points is then instructed to transmit the frame with the corresponding maximum time duration.

Furthermore, it is possible to select a modulation scheme and a coding scheme for a frame to be transmitted by each of the several access points 200 after the corresponding access points have successfully checked the channel availability. Furthermore, the control node 100 can determine the access points 200 among the several time synchronized access points in the network 10 for which the selecting of the common target time is carried out.

Furthermore, the control node 100 may select a frequency resource for a frame to be transmitted by each of the product access points after the corresponding access point has successfully checked the channel availability. The relative access points are then instructed to transmit the frame at the corresponding frequency resource.

As far as the access point itself is concerned the access point may receive in addition to the common target time, a length of the time period during which the checking of the channel availability should be carried out. Furthermore, the access point may check the channel availability for the received length of the time period. When the access point receives the length, it may receive at least one of a back-off value and an inter-frame space and the corresponding value is then applied when checking the channel availability.

The access point 200 may furthermore receive instructions for a content for a frame which is to be transmitted by the access point after the channel availability has been checked. The frame is then transmitted with the instructed content to the at least one wireless device 50 as shown in figure 7 where the beacon frame is used for all access points.

Furthermore, the access point 200 may receive a maximum time duration for a frame to be transmitted after having successfully checked the channel availability, the access point 200 then transmits the frame such that it has the received maximum time duration.

Furthermore, it may receive a modulation scheme and a coding scheme for a frame to be then transmitted after the checking of the channel availability. The frame is then transmitted with the received modulation scheme and coding scheme.

Furthermore, the access point 200 may determine whether a transmission channel was busy or not when the channel availability was checked. In this context it may generate a success information indicating whether the transmission channel was busy or not. This success information may then be also transmitted to the central control node 100. In this context the channel availability may be considered as being successful when a transmission channel available for the data exchange with the wireless device was never busy when the channel availability was checked. Here the channel availability may be considered as not being successful when a transmission channel available for the data transmission with the wireless device 50 was busy or at least once busy when the channel availability was checked. The central control node 100 is then informed about this fact in the transmitted success information.

The above discussed solution has the advantage that a coordination of the channel availability procedure of an access point connected to the same control node is possible. Furthermore, different levels of alignment are proposed. At the first level the time to begin the CCA procedure is synchronized wherein at the second level the time at which the CCA procedure takes place is also synchronized and at the third level the time of a frame transmission or the wireless medium is synchronized. The network can be a layer 2 network and can be of any type which is a Time-Sensitive Networking Ethernet. For a common content a common beacon framing may be sent by the different access points, this being one example use case of potential uses of the solution coordination. The advantage of this approach is to share the BSS, base station subsystem information across the covered area fast and on time and to avoid interference because a single frequency network is created at the time of the beacon transmission. This will enable more air-time for the packets in the network.

When the same content is transmitted with an access point the chance that a receiving wireless device can correctly identify the received content also increases.

Claims

1 . A method for controlling, by a central control node (100), a plurality of time synchronized access points (200) connected to the central control node (100) , wherein the plurality of access points (200) exchange data with wireless devices (50) over a wireless network, the method comprising:

selecting a common target time at which the plurality of access points (200) should start checking a channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission, - instructing the plurality of access points (200) to start checking the channel availability at the common target time.

2. The method according to claim 1 , further comprising:

selecting a length of a time period for each of the plurality of access points (200) during which the checking of the channel availability should be carried out, and instructing each of the plurality of access points (200) to check the channel availability for the time period of the selected corresponding length.

3. The method according to claim 2, wherein the same length of the time period is selected for each of the plurality of access points (200).

4. The method according to claim 2 or 3, wherein selecting a length of the time period comprises selecting at least one of a back-off value and an inter-frame space for each of the plurality of access points (200).

5. The method according to any of the preceding claims, further comprising:

selecting a content for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability, and

instructing each of the plurality of access points (200) to transmit the frame with the selected corresponding content after the channel availability has been successfully checked.

6. The method according to claim 5, wherein the same content is selected for each of the plurality of access points (200).

7. The method according to any of the preceding claims, further comprising:

selecting a maximum time duration for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability, instructing each of the plurality of access points (200) to transmit the frame with the corresponding maximum time duration.

8. The method according to any of the preceding claims, further comprising:

selecting a modulation scheme and a coding scheme for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability,

instructing each of the access points (200) to use the corresponding modulation scheme and coding scheme for the transmission of the frame.

9. The method according to any of the preceding claims, further comprising the step of determining the plurality of access points (200) among several time synchronized access points (200) for which the selecting of the common target time is carried out.

10. The method according to any of the preceding claims, further comprising

- selecting a frequency resource for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability,

- instructing each of the plurality of access points (200) to transmit the frame at the corresponding frequency resource.

1 1. A method for operating a time synchronized access point in a network in which a plurality of time synchronized access points (200) are connected to a central control node

(100) controlling the plurality of access points (200), wherein the plurality of time synchronized access points (200) exchange data with wireless devices (50) over a wireless network, the method comprising:

receiving a common target time, at which the access point should start checking a channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission,

starting to check the channel availability at the received common target time.

12. The method according to claim 1 1 , further comprising:

- receiving a length of a time period during which the checking of the channel availability should be carried out,

checking the channel availability for the received length of the period of time.

13. The method according to claim 12, wherein receiving the length comprises receiving at least one of a back-off value and an inter-frame space, wherein the received at least one of a back-off value and an inter-frame space are applied when checking the channel availability.

14. The method according to any of claims 1 1 to 13, further receiving instructions for a content for a frame to be transmitted after having successfully checked the channel availability, wherein the frame is transmitted with the instructed content to at least one wireless device after the channel availability has been successfully checked.

15. The method according to any of claims 1 1 to 14, further comprising:

receiving a maximum time duration for a frame to be transmitted after having successfully checked the channel availability,

transmitting the frame such that is has the received maximum time duration.

16. The method according to any of claims 1 1 to 15, further comprising:

- receiving a modulation scheme and a coding scheme for a frame to be transmitted after having successfully checked the channel availability,

- transmitting the frame with the received modulation scheme and coding scheme.

17. The method according to any of claims 1 1 to 16, further comprising:

determining whether a transmission channel was busy or not when the channel availability was checked,

generating success information indicating whether the transmission channel was busy or not,

transmitting the success information to the central control node (100).

18. The method according to claim 17, wherein the channel availability is considered successful when a transmission channel available for the data exchange with the wireless device was never busy when the channel availability was checked, wherein the control node is informed about this fact in the success information.

19. The method according to claims 17 or 18, wherein the channel availability is considered not successful when a transmission channel available for the data exchange with the wireless device was busy at least once when the channel availability was checked, wherein the control node is informed about this fact in the success information.

20. A central control node (100) configured to control a plurality of time synchronized access points (200) connected to the central control node (100) , wherein the plurality of access points (200) exchange data with wireless devices (50) over a wireless network, the central control node (100) comprising a memory (130) and at least one processor (120), the memory containing instructions executable by said at least one processor, wherein the central control node (100) is operative to:

select a common target time at which the plurality of access points (200) should start checking a channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission,

- instruct the plurality of access points (200) to start checking the channel availability at the common target time.

The central control node according to claim 20, further being operative to

select a length of a time period for each of the plurality of access points (200) during which the checking of the channel availability should be carried out, and instruct each of the plurality of access points (200) to check the channel availability for the time period of the selected corresponding length.

22. The central control node according to claim 21 , further being operative to select the same length of the time period for each of the plurality of access points (200).

23. The central control node according to claim 21 or 22, further being operative, for selecting a length of the time period, to select at least one of a back-off value and an inter- frame space for each of the plurality of access points (200).

24. The central control node according to any of claims 20 to 23, further being operative to select a content for a frame to be transmitted by each of the plurality of access points after successfully checking the channel availability, and to instruct each of the plurality of access points (200) to transmit the frame with the selected corresponding content after the channel availability has been successfully checked.

25. The central control node according to claim 24, further being operative to select the same content for each of the plurality of access points (200).

The central control node according to any of claims 20 to 25, further being operative to select a maximum time duration for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability, instruct each of the plurality of access points (200) to transmit the frame with the corresponding maximum time duration.

27. The central control node according to any of claims 20 to 26, further being operative to - select a modulation scheme and a coding scheme for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability,

instruct each of the access points (200) to use the corresponding modulation scheme and coding scheme for the transmission of the frame.

28. The central control node according to any of claims 20 to 27, further being operative to determine the plurality of access points (200) among several time synchronized access points (200) for which the selecting of the common target time is carried out. 29. The central control node according to any of claims 20 to 28, further being operative to

- select a frequency resource for a frame to be transmitted by each of the plurality of access points (200) after successfully checking the channel availability,

- instruct each of the plurality of access points (200) to transmit the frame at the corresponding frequency resource.

30. An access point located in a network, which is time synchronized with a plurality of other access points (200, the access points being connected to a central control node (100) controlling the plurality of access points (200), wherein the plurality of time synchronized access points (200) exchange data with wireless devices (50) over a wireless network, the access point comprising a memory (230) and at least one processor (220), the memory containing instructions executable by said at least one processor, wherein the time synchronized access point (200) is operative to:

receive a common target time, at which the access point should start checking a channel availability where it is checked whether a transmission channel of the wireless network is available and free for a data transmission,

start to check the channel availability at the received common target time.

31. The access point according to claim 30, further being operative to

receive a length of a time period during which the checking of the channel availability should be carried out,

check the channel availability for the received length of the period of time.

32. The access point according to claim 31 , further being operative to receive at least one of a back-off value and an inter-frame space for the received length and to apply the received at least one of a back-off value and an inter-frame space when checking the channel availability.

33. The access point according to any of claims 30 to 32, further being operative to receive instructions for a content for a frame to be transmitted after having successfully checked the channel availability, and to transmit the frame with the instructed content to at least one wireless device after the channel availability has been successfully checked.

34. The access point according to any of claims 30 to 33, further being operative to

receive a maximum time duration for a frame to be transmitted after having successfully checked the channel availability,

transmit the frame such that is has the received maximum time duration.

35. The access point according to any of claims 30 to 34, further being operative to

- receive a modulation scheme and a coding scheme for a frame to be transmitted after having successfully checked the channel availability,

- transmit the frame with the received modulation scheme and coding scheme.

36. The access point according to any of claims 30 to 35, further being operative to

determine whether a transmission channel was busy or not when the channel availability was checked,

generate success information indicating whether the transmission channel was busy or not,

transmit the success information to the central control node (100).

37. The access point according to a claim 36, further being operative to consider the channel availability as being successful when a transmission channel available for the data exchange with the wireless device was never busy when the channel availability was checked, and to inform the control node about this fact in the success information.

38. The access point according to claim 36 and 37, further being operative to consider the channel availability as being not successful when a transmission channel available for the data exchange with the wireless device was busy at least once when the channel availability was checked, and to inform the control node about this fact in the success information.

39. A system comprising a central control node (100) as mentioned in any of claims 20 to 29 and an access point as mentioned in any of claims 30 to 38.

40. A computer program comprising program code to be executed by at least one processor (120, 220) of a central control node (100) or of an access node (200), wherein execution of the program code causes the at least one processor to execute a method according to any of claims 1 to 19.

41 . A carrier comprising the computer program of claim 40, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.