TECHNICAL FIELD The present invention relates to the field of LAN-systems, and is particularly suitable for wireless such systems. The invention makes it possible to use devices from different kinds of LAN-systems in a shared system.

BACKGROUND OF THE INVENTION A LAN (Local Area Network) usually consists of at least one central unit, a so called Access Point, AP, and a number of user devices, mobile or stationary.

The interaction between the AP and the devices can be controlled in a number of ways, two of which are centralized control (CC) and distributed control (DC). In a CC-system the AP schdules who may transmit, when, and for how long, while in a DC-system, the various devices listen to detect if somebody else is transmitting. If a device in a DC-system doesn't detect transmissions from others, it may start its own transmissions after a certain period of time.

Since different kinds of AP: s are used in the two systems, a user from one of the systems who enters an area where the other kind of system is in use will not be able to access the LAN in that particular area. One obvious solution to this problem is to design a device which has dual functionality, i. e. functionality for either of the two systems. However, such a device is likely to be expensive, and possibly also bulky.

DISCLOSURE OF THE INVENTION As described above, there exists a need for a LAN-system, in particular a wireless LAN-system, in which devices from both CC and DC-systems can interact with one and the same AP, at the same time as the devices are not made unreasonably expensive for the user. In particular, it would be of

interest to find a solution in which existing devices could function with one and the same AP without any modifications.

This problem is solved by the present invention in that it provides a central unit, an AP, for use in a LAN system, which system can comprise users from a first LAN system using the CC principle and users from a second system which uses the DC principle, the AP comprising means which allow it to function as a central unit in both the first and the second said systems. The AP according to the invention further comprises means for transmitting information to the users of at least one of the said systems, said information permitting users of one of the systems to transmit during a certain interval of time.

Thus, the AP according to the invention may either transmit information to the CC-devices to remain silent during a certain period of time, which will automatically lead to the DC-devices detecting the silence, thereby enabling them to transmit, or the AP can transmit information to the DC-devices, informing them of the next point in time when they may transmit or alternatively to stop transmitting. Accordingly, in the central unit according to the invention it would be advantageous to let the means for transmitting information to the users of one of the systems comprise means for transmitting this information to users of the CC-system using an existing message in the CC-system, thereby informing users of the CC-system of a period of time during which they may not transmit, thus giving the users of the DC-system the opportunity to transmit. The use of an existing message is not a necessity, but enables the invention to function with a minimum of redesign.

It is also possible to let the AP according to the invention comprises means for transmitting said information to the users of the DC-system, thereby informing the DC-users of periods of time during which they may transmit.

Suitably, this is done by transmitting the information to the users of the DC- system via information in an existing message in the DC system.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described below in more detail, with reference to the appended drawings, in which Fig 1 shows a schematic drawing of a LAN system with an AP according to the invention, and Fig 2-4 show various implementations of messages from an AP according to the invention, and Fig 5-6 show various implementations of a version of the invention, and Fig 7-8 show implementations of another version of the invention.

EMBODIMENTS Fig 1 very schematically shows a Wireless Local Area Network system (WLAN) with an AP according to the invention. As shown in the drawing, the system can comprise users both from a CC-system and from a DC-system.

An example of a CC-system is the HiperLAN 2 system, H/2, and an example of a DC-system is the IEEE 802. 11 system. These systems will from now on be used in this description to facilitate the understanding of the invention. In order to enable the system to function, the AP according to the invention has been given functionality to enable it to function as an AP in both the H/2- system and in the 802.11-system.

In the H/2-system, the interface between the devices and the AP is defined by a fixed frame structure for TDD (Time Division Duplex), using a 2 ms long frame which is transmitted repeatedly in a 16 frame multiframe structure. The frame comprises a number of logical channels, controlled by the AP in order to schedule the traffic in the system, both uplink and downlink. Two of these channels are the Broadcast Control Channel (BCCH) and the Frame Control Channel (FCCH), which are read by the H/2 devices in order to obtain information on when in the frame to transmit and receive.

As opposed to the H/2-system, the air interface in the 802.11-system uses two mechanisms, known as Point Coordination Function (PCF) and Distributed Coordination System (DCF). The PCF mechanism is an optional feature of the 802.11-system and has similarities to the H/2 system in the sense that it is a CC system. However, the structure of the PCF is not similar to the H/2 system. Since the PCF is an optional feature of the 802.11-system, it is not widely used and hereinafter only the DCF operation will be described. For the DCF function, when a device in the 802.11 system, being either an AP or station, wants to transmit data, it will do so if it senses that no other devices in the system have transmitted for a certain period of time, i. e. that the system has been"quiet"for that period of time. To signal the identities for the AP, e. g. the basic service set identification (BSSID), and necessary information to all users, the AP transmits a so called Beacon. The 802.11 system also utilizes a so called Network Allocation Vector (NAV) in order to enhance the air interface function: when a device starts a transmission, it also transmits a NAV, which indicates the length of the transmission covering both the own transmission and a possible response transmission from the peer station or AP.

According to the invention, the AP (preferably an H/2 AP) has been configured so that it can also support WLAN devices from the 802.11 system.

However, the AP must also be provided with means for preventing the H/2 devices from transmitting, so that the 802.11 devices will detect the absence of transmissions, thus allowing them to transmit in turn. Preferably this should be done using existing messages in the H/2-system, so that the invention can be used without modifying existing devices.

According to the invention, the AP can schedule a silent period for the H/2 devices in two ways, using existing messages:

The first way is to schedule a silent period over multiple frames, which can be done by means of the existing H/2 message called"AP-Absence". The silent period may be dynamically allocated each time, or fixed allocated such that certain frames are always silent. The H/2-devices will decode the message, and avoid the use of the channel during the period, leaving it free for 802.11.

This is illustrated in Fig 2, which shows a number of H/2 MAC (Medium Access Control) frames, and an AP absence message sent from the AP, indicating start and duration of"AP-absence"to the H/2-devices.

The absence period is also shown in fig 2. During this absence period or silent period for the H/2 part of the system, the 802.11 devices will detect silence, and will thus begin to transmit in their normal fashion. The absence message can be sent once, or repeatedly in order to raise the probability of reception.

In an alternative version of the invention, the AP can schedule silent periods for the H/2-devices within a H/2 MAC frame. This can also be done in combination with the method described earlier. Silence within an H/2 MAC frame is schduled by the AP and broadcast by it to the devices by means of the FCCH information Element (IE)"empty part in frame", which contains a pointer to where in the frame the empty period starts, and also a duration field, indicating the duration of the silent period.

In order to even further improve the function of the invention, another problem might need to be addressed: during a"non-silent"H/2 MAC frame, there may be short periods of silence, or the 802.11 devices could have such a weak signal reception that they perceive the system as being silent, and thus begin to transmit. In order to prevent this from happening, it would be possible to let the AP transmit a BEACON and a NAV value during H/2- periods. The NAV which is transmitted in this way indicates the end of the silent period for the H/2-devices. As an option to signal a BEACON, a dummy

signal may also be transmitted, since all 802.11 signals contain a NAV field.

A signal which may be utilized as dummy signal is the so called null frame.

Since this Beacon, along with the NAV, will be transmitted during an H/2- period, the AP will preferably schedule a downlink so called LCCH (Link Control Cannel) in the frame to a non-existing device or connection, in which the Beacon can be transmitted.

The transmission of such Beacons and NAV by an AP is shown in Fig 3, where a number of H/2 MAC frames are shown, and an AP which transmits a Beacon to the 802.11 units, where the Beacon contains a NAV which expires at the next start of an absence period. Alternatively, subsequent such Beacon messages can be sent, as illustrated in fig 4.

In order to further reduce the probability that the 802.11 devices start to transmit during the H/2 MAC frames, it is possible to let the AP transmit"filler information", i. e. dummy PDU (Protocol Data Unit) in the MAC frame where no transmission is otherwise scheduled, thus making it more difficult for the 802.11-devices to detect absence of information.

In the system described above, there is no way of scheduling the end of a transmission for an 802.11 device, which means that there is a risk that an 802.11 device, once it starts to transmit, may continue its transmissions into periods of time intended for H/2-devices. Also, the 802.11 devices may detect absence of information during periods of time intended for the H/2 devices, and start to transmit.

According to the invention, these problems may be addressed by letting the AP periodically transmit the Beacon signal at the start of the"802.11 phase", i. e. the period of time intended for use by the 802.11 devices. The 802.11 phases may also be made to exist periodically, as shown in figs 5a, 5b and fig 6.

Fig 5a shows an example of scheduling, in which there is an 802.11 frame (thin lines) in each H/2 MAC frame (thick lines), with the AP transmitting a Beacon as described above at the beginning of each of the 802.11 phases.

Alternatively, the 802.11 phases can occur more irregularly, as for example shown in fig 5b, in which the 802. 11 phases are schduled in every other H/2 MAC frame. Yet another alternative, as illustrated in fig 6, is to let the 802.11 phase occur during so called"absent frames"in the H/2 MAC frames, with the absent frames being created by means of a signal from the AP to the H/2 MAC-devices, for example the signal AP absent, indicating the start and duration of the absent frame.

According to the invention, the existing Beacon signal to the 802.11 devices is preferably modified to contain information regarding the 802.11 phase, such as its duty cycle, duration or periodicity. In addition, the 802.11 devices are also preferably modified in this version of the invention, in order to prevent them from detecting unused parts of H/2 MAC frames, and start transmission in those parts. A suitable modification to the 802.11 devices is that they at certain points in time, for example at startup, handover between different AP: s or at power up must detect whether or not there is a Beacon being transmitted. If no Beacon is detected, a modified device will not be allowed to transmit. A detected Beacon signal will also be used by the modified 802.11 devices in order to compute the duration of their transmissions, so that these transmissions do not"spill over"into time periods schduled for other activities, such as for example H/2 MAC transmissions.

As a furher alternative, it is suggested that the 802.11 phase is not announced in the FCCH when parts pf the MAC frame is used for the 802.11 phase. This can be achieved since unused parts of the MAC frame is not addressed in the FCCH header. An exception is the"empty parts in frame", which addresses unused parts, but is used to allow mobile terminals to make

measurements when no traffic is required. By assigning part of a MAC frame for an 802.11 phase and not addressing it by any means in the Frame Control CHannel, H2 devices will not utilize the 802.11 phase.

In another version of the invention, the 802.11 phase is combined with a number of H/2 MAC frames into a so called"super frame structure", with a preferable duration of modulus 2 milliseconds. In this version of the invention, the 802.11 devices operate in the infrastructure Basic Service Set (BSS) mode and under normal rules of the standard, thus enabling the 802.11 phase to use both the DCF function as described earlier, and the PCF (Point Co-ordination Function).

Also in this version of the invention, the beacon signal mentioned earlier is made use of, as a standard beacon message, with the addition of a new parameter in the beacon signal. This new parameter contains the duration from the start of the super frame to the start of the first H/2 frame in the super frame.

A super frame structure according to this version of the invention is shown in Fig 7. As explained earlier, the total duration of the proposed super frame is 2k multiplied by the duration of an H/2 MAC frame, which is 2 milliseconds.

The exponent k is an integer. The total duration of the proposed super frame is thus 2k * 2 ms, with the duration of the 802.11 phase being N*2 ms, and the duration of the H/2 MAC part of the super frame being M * 2 ms, where N and M are integers which can be changed between one super frame to the next, as needed.

The Beacon signal in the super frame may be transmitted according to the standard rules of the 802.11 system, or at certain targeted transmit times, which would preferably be at the start of each super frame, suitably with the 802.11 phase following the beacon signal, and the H/2 MAC frames thus being transmitted as the last part of the super frame, as shown in Fig 7.

Preferably, the 802.11 devices are prevented from starting transmissions which would cross the boundary of the first H/2 frame. This is achieved by requiring a 802.11 station to calculate the duration of a tentative frame exchange transmission, including any acknowledgement from a peer AP or station, and prevent transmission if the duration would cross the start boundary of the H/2 phase.

To prevent 802.11 transmissions during the H/2 phase, 802.11 should defer transmission until the end of the H/2 phase. One way of achieving this is shown in Fig 8: At the start of an 802.11 phase of the super frame, the 802.11 devices set their Network Allocation vector (NAV) to equal the duration of the H/2 phase of the super frame.

A H/2 MAC devices which have successfully conducted a sleep request exchange with the AP, and entered a low power mode, will (wake up at a so called"wake up"frame, to detect whether the AP has pending data for the H/2 MAC device. In order to make sure that the H/2 devices wake up during H/2 phases of the super frames and at the same time making it possible to vary the integers N and M described above, i. e. to vary the length of the respective phases of the super frame, a minimum value can be set for the integer M which defines the length of the H/2 MAC part of the super frame.

The H/2 MAC devices can then be set to wake up during the part of the super frame defined by the minimum value of M, thus ensuring that they do not wake up during a part of the super frame used for 802.11 purposes. The minimum value of the integer M should be chosen bearing in mind that the H/2 MAC device, when awakening, should have enough time left in the super frame to decode data from the AP. A suitable minimum value for M is two.

The invention is not limited to the embodiments described above, but may be freely varied within the scope of the appended claims.