WO2006106459A1

WO2006106459A1 - A method of operating a network node of a communication network, a network node, a communication network, a computer-readable medium, and a program element

Info

Publication number: WO2006106459A1
Application number: PCT/IB2006/050980
Authority: WO
Inventors: Georgios Orfanos; Joerg Habetha
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2005-04-08
Filing date: 2006-03-31
Publication date: 2006-10-12

Abstract

A method of operating a network node (101) of a communication network (100) comprising a plurality of network nodes (101 to 104) communicating via a plurality of communication channels (105 to 107), the method comprising counting down a pre¬ determined waiting time (301) before transmitting a signal (304) via a selected one of the communication channels (105), detecting, during the counting down, whether the selected one of the communication channels (105) becomes busy, and, in case that the selected one of the communication channels (105) becomes busy during the counting down, switching to another one of the communication channels (106) and continuing the counting down on the other one of the communication channels (106).

Description

A method of operating a network node of a communication network, a network node, a communication network, a computer-readable medium, and a program element

The invention relates to the field of networks. In particular, the invention relates to a method of operating a network node of a communication network, to a network node, to a communication network, to a computer-readable medium, and to a program element. A WLAN ("Wireless Local Area Network") is a local network of a plurality of network nodes communicating in a wireless manner. A WLAN may be implemented in the frame of the standard IEEE 802.11 (see LAN MAN Standards Committee of the IEEE Computer Society, Wireless LAN Medium Access Control (MAN) and Physical Layer (PHY) specifications, IEEE Standard 802.11, 1999 Edition). IEEE 802.11 is a worldwide standard for Wireless Local Area Networks

(WLANs) constantly improving in order to cope with the raising demands of users and applications for higher throughput and Quality of Service (QoS).

In Carrier Sense Multiple Access (CSMA) methods, a so-called "backoff process" may be applied. One example of a wireless system that is based on CSMA with Collision Avoidance (CSMA/CA) is the Wireless Local Area Network (WLAN).

IEEE 802.11, respectively CSMA/CA, is based on a listen before talk scheme. Stations listen to the medium and start a packet transmission of arbitrary length (up to a certain maximum) after detecting that there is no other transmission in progress on the wireless medium. However, if two stations detect a channel as "idle" (i.e. not busy) at the same time and both send a communication message, a collision occurs.

The standard IEEE 802.11 defines a Collision Avoidance (CA) mechanism to reduce the probability of such collisions. For this purpose, before starting a transmission, a station performs a backoff procedure. The backoff procedure foresees that a station has to keep sensing the channel for an additional random time after detecting the channel as being idle for a minimum duration called Distributed Coordination Function Interframe Space

(DIFS). Only if the channel remains idle for this additional random time period, the station is allowed to initiate the transmission. The duration of this random time is determined as a multiple of a slot time (which is, for instance, 9 μs according to IEEE 802.1 Ia). Each station maintains a so-called Contention Window (CW), which is used to determine the number of slot times a station has to wait before starting a transmission.

Fig. 6 shows a scheme 600 illustrating a prior art CMSA/CA access according to IEEE 802.11. The scheme 600 shows a first station 601, a second station 602, a third station

603, a fourth station 604 and a fifth station 605 forming a communication network. For each of these stations 601 to 605, the time dependence 606 of the activities is illustrated. Fig. 6 shows Distributed Coordination Function Interframe Spaces (DIFS) 607, Backoff Downcount periods 608, Rest Backoff periods 609, and Frame periods 610. As shown in Fig. 6, the Mobile Station (MS), which has a data packet 610 to transmit, draws a random number between 0 and CW (Contention Window), which determines the duration of the Backoff timer in number of slot times. The Contention Window (CW) has a minimum starting value of 15 and it doubles after a packet collision. Its value can rise up to 255 and is decremented after a successful transfer. The doubling of the CW size reduces the probability that the same packets collide again. The receiver acknowledges a successful transmission with an Acknowledgement (ACK) frame. A collision is detected by a missing ACK frame.

While the medium is free, the station counts down the Backoff timer until it reaches zero and, after having scanned the medium as free for at least a DIFS time, starts the transmission. If during the countdown another station occupies the medium, all stations in backoff interrupt their countdown and defer until they detect that the medium is free again for a time of at least the duration of the DIFS. Then they continue the countdown at the Backoff timer starting at the previously frozen value. This ensures that stations, which deferred from channel access because their random Backoff time was larger than the Backoff time of other stations, are given a higher priority when they resume the transmission attempt. After the successful transmission of a packet, a station starts a new Backoff procedure even if no other packet is waiting in its queue for transmission. The latter is called post-Backoff.

The above medium access mechanism is applied per channel. A system can comprise multiple channels which are operating independently of one another. In IEEE 802.11 , channels are defined by different center frequencies.

Another alternative is to define channels based on the spreading code applied. In such a Code Division Multiple Access (CDMA) network, each data symbol is spread over a large bandwidth, larger than the bandwidth needed for transmission. This allows to transmit with a spectral energy that is lower than a non-spread spectrum system, a fact that allows the use of parallel transmission channels at the same time in the same frequency band. The data transmitted in the different channels can be distinguished by the use of a different spreading code for each channel. The data stream comprises a successive sequence of symbols or chips.

In conventional Direct- Sequence CDMA (DS-CDMA), each user bit is transmitted in the form of many sequential chips, each of which is of short duration, thus having a wide bandwidth. An alternative to this conventional DS-CDMA is Multi-Carrier CDMA (MC-CDMA), which is explained in Hara, S. and Prasad, R. "Overview of Multicarrier CDMA", IEEE Comm. Magazine, Vol. 35, No. 11, December 1997.

MC-CDMA is suitable for WLAN, because existing WLAN versions are so- called multi-carrier systems. A multi-carrier technique like Orthogonal Frequency Division Multiplexing (OFDM) allows for the parallel transmission of data on several subcarriers. Due to the Fast Fourier Transform (FFT) associated with Orthogonal Frequency Division Multiplexing (OFDM), MC-CDMA chips are long in time duration, but narrow in bandwidth. In this context, reference is made to Linnartz, J. "Performance Analysis of Synchronous MC- CDMA in Mobile Rayleigh Channel With Both Delay and Doppler Spread", IEEE Trans. On Vehicular Technology, Vol. 50, Issue 6, November 2001.

Each symbol of the data stream of one user is multiplied by each element of the same spreading code and is thus placed in several narrow band subcarriers. Multiple chips are not sequential, but transmitted in parallel on different sub-carriers.

US 2005/0044249 Al discloses a method and an apparatus for unifying medium access control (MAC) protocols that can unify various existed MAC protocols, e.g. slotted ALOHA protocol, carrier sense multiple access (CSMA) protocol, group randomly addressed polling (GRAP) protocol. The method comprises grouping the MAC nodes having a ready packet according to preset parameters, and transmitting packets according to the grouping result and the preset parameters. US 2005/0044249 Al discloses that MAC nodes in different groups can send their packets via different channels so as to avoid or resolve collisions. However, such conventional collision avoidance systems may suffer from limited efficiency concerning usage of channel capacity.

There may be a need for a network system allowing for an efficient signal transfer. In order to achieve the object defined above, a method of operating a network node of a communication network, a network node, a communication network, a computer- readable medium, and a program element with the features according to the independent claims are provided.

According to an exemplary embodiment of the invention, a method of operating a network node of a communication network comprising a plurality of network nodes communicating via a plurality of communication channels is provided, the method comprising counting down a pre-determined waiting time before transmitting a signal via a selected one of the communication channels, detecting, during the counting down, whether the selected one of the communication channels becomes busy, and, in case that the selected one of the communication channels becomes busy during the counting down, switching to another one of the communication channels and continuing the counting down on the other one of the communication channels. According to another exemplary embodiment of the invention, a network node for a communication network comprising a plurality of network nodes communicatively coupled via a plurality of communication channels is provided, the network node comprising a processor adapted to control or carry out the above-mentioned method.

According to still another exemplary embodiment of the invention, a communication network is provided comprising a plurality of interconnected network nodes having the above-mentioned features.

According to yet another exemplary embodiment of the invention, a computer- readable medium is provided, in which a computer program of operating a network node of a communication network comprising a plurality of network nodes communicating via a plurality of communication channels is stored, which computer program, when being executed by a processor, is adapted to control or carry out the above-mentioned method.

According to a further exemplary embodiment of the invention, a program element of operating a network node of a communication network comprising a plurality of network nodes communicating via a plurality of communication channels is provided, which computer program, when being executed by a processor, is adapted to control or carry out the above-mentioned method.

The communication schema according to embodiments of the invention can be realized by a computer program, i.e. by software, or by using one or more special electronic optimization circuits, i.e. in hardware, or in hybrid form, i.e. by means of software components and hardware components.

The characterizing features according to the invention particularly have the advantage that an efficient system of operating a network is provided in which collisions between signals transmitted at the same time or during the same time interval via the same channel by different devices can be prevented, access to the medium is accelerated and thereby packet delay reduced, and simultaneously the channel capacity is used in an efficient manner.

Namely, if a network node detects that during counting down a Backoff time, another node sends traffic onto the channel which the network node wanted to use for sending its own signals, then the network node may check whether there is another free channel on which the counting down of the Backoff time may be continued. In case that the network node finds such a presently free nun-busy channel, it may switch to this other channel without stopping the downcount. In other words, when switching, the Backoff timer may be only stopped, not reset to the initial value. Then, the countdown can be continued on this other channel, and after expiry of the remaining part of the Backoff time, the network node sends traffic over the destination channel to which it has switched.

By taking this measure, a proper compromise is made between prioritizing already waiting network nodes on the one hand, and bandwidth efficiency on the other hand. Also, security and prevention of collisions may be obtained for a network system.

According to an exemplary embodiment, a method improving the throughput and delay of prioritized stations in a network that uses Carrier Sense Multiple Access (CSMA) as a Medium Access (MAC) method is provided. Such a method may be denoted as a "Smart Backoff' method and may apply to networks with multiple data channels. These channels can be code divided, frequency divided, time divided or space divided.

In Carrier Sense Multiple Access systems, a Backoff mechanism may be applied to avoid packet collisions of several transmission stations. The Backoff mechanism according to an exemplary embodiment of the invention allows to carry out a Backoff across multiple channels. If one channel is getting busy during a Backoff countdown, the Backoff can be continued on a different channel.

Furthermore, if a transmission has been completed on one channel, a subsequent transmission can be carried out on a different channel to avoid a so-called post- Backoff. Such a scheme can significantly reduce the packet delay and increase the data throughput in a CSMA system. According to an exemplary embodiment, a system is provided in which stations carry out a decentralized Backoff mechanism to spread out their Backoff times to a minimum. According to an exemplary embodiment of the invention, this can be done very efficiently, since channel bandwidth may be used by jumping to presently unused or non- busy channels, in order to continue with counting down the Backoff time.

According to the prior art, it may happen that stations have to interrupt their Backoff countdown due to and during the transmission of other stations. For instance, one may assume a scenario where a station "B" with a relatively short Backoff period starts the transmission in a channel, in which a station "A" is presently in the countdown of its Backoff. In this case, the station "A" interrupts its Backoff timer according to the prior art, and only after the ends of the transmission of the station "B", the station "A" continues its countdown. Furthermore, after a transmission, a station has to defer again from the medium for a randomly set Backoff time, even if no other packet is in the queue waiting for transmission (so-called post-Backoff). In contrast to a scheme which increases the delay of the packet delivery in the

Backoff procedure, exemplary embodiments of the invention may provide significant advantages particularly in real time connections and in scenarios with many stations or network nodes. For this purpose, according to an exemplary embodiment, in order to enhance the performance of such networks, the so-called smart Backoff procedure may be provided. This procedure can be applied to all systems where transmission in different channels is possible. The Backoff procedure according to an exemplary embodiment of the invention thus may solve the shortcomings of the conventional Backoff, which only considers the access with the same channel and does not exploit the degree of freedom that multiple channels offer. Thus, embodiments of the invention may increase the flexibility in using the available channel capacities.

Exemplary fields of application of systems according to embodiments of the invention are all devices that apply a Backoff mechanism and have the possibility to access different communication channels. Such devices may be provided for the purpose of wireless or wired data transmission. An exemplary standard to which embodiments of the invention apply is IEEE 802.11 , particularly IEEE 802.1 In. For instance, the system according to an embodiment of the invention may be implemented in the context of a WLAN (Wireless Local Area Network).

According to an exemplary embodiment, a method for contending for access to a communication medium is provided, the method comprising a Backoff procedure in which, in case of a channel detected to become busy during counting down the Backoff time, the corresponding network node may switch to another channel. In this context, also a communication station arranged for contending for access to a communication medium in accordance with such a Backoff procedure may be provided. Furthermore, a communication system comprising a plurality of communication stations may be provided, wherein the respective communication stations are arranged for contending for access to a communication medium in accordance with a Backoff procedure as described herein.

According to an exemplary embodiment, a method of transmit Backoff for a medium access control protocol in a communication network including a plurality of stations in a plurality of code, frequency or time channels is provided. Such a method may comprise that a first station starts a Backoff on a first channel by selecting the time (slots) to wait before a transmission and starting a countdown of time (slots). In case that the Backoff of that first station is interrupted by the transmission of a second station on the first channel, the first station may search for another free channel and may continue its down-count of time (slots) on this second channel without freezing the Backoff count during the transmission of said second station. Furthermore, said second station may transmit pending data on said second channel at the completion of the down-count (eventually after an additional fixed pause time) in case of an idle medium on the second channel.

Such a method may further comprise that the first station may start a new Backoff after completion of the transmission of the same channel on which the transmission was carried out (so-called post-Backoff). Furthermore, the first station may not have to start a post-Backoff of any other channel than the channel of the previous transition, i.e. in case of data pending, immediately starting an additional transmission on one of said other channels (without performing post-Backoff on this other channel). Furthermore, in the context of such a method, the channels may be code channels defined according to Multi-Carrier Code Division Multiple Access (MC-CDMA) principles.

According to another exemplary embodiment, a communication network including a plurality of stations in a plurality of code, frequency or time channels may be provided, wherein a particular station may employ a Backoff scheme comprising starting a Backoff on a first channel by selecting the time (slots) to wait before a transmission and starting a down-count of time (slots). In case that the Backoff is interrupted by the transmission of another station on the first channel, the station may search for another free channel and may continue its down-count of time (slots) on the second channel without freezing the Backoff counter during the transmission of the other station. Then, pending data may be transmitted on said second channel at the completion of the down-count (eventually after an additional fixed pause time), i.e. in case of an idle medium on the second channel, a new Backoff may be started after completion of the transmission on the same channel on which the transmission was carried out (post-Backoff). No post-Backoff is started on any other channel than the channel of the previous transmission, i.e. in case of data pending, an additional transmission may be started immediately on one of said other channels (without performing post-Backoff on this other channel).

According to another exemplary embodiment, a communication station may be provided comprising a transmitter, a receiver, a processor and a local storage. The processor may be configured to run a Backoff scheme comprising the steps as described above referring to the communication network.

Next, further exemplary embodiments of the invention will be described.

In the following, exemplary embodiments of the method of operating a network node will be described. However, these embodiments also apply for the network node, for the communication network, for the computer-readable medium and for the program element.

The method may comprise transmitting the signal via the other one of the communication channels after having counted down the pre-determined waiting time to zero. Thus, directly after expiry of the remaining part of the waiting time, the communication message may be sent via the channel to which the network node has switched.

Furthermore, the method may comprise detecting, during the continuing of the counting down, whether the other one of the communication channels becomes busy, and in case that the other one of the communication channels has become busy during the continuing of the counting down, the network node may switch to a further one of the communication channels and may continue the counting down on the further one of the communication channels. Thus, according to the described embodiment, it is possible that the network node switches its channel a second (or even a third, a fourth, and so on) time in order to provide a sufficient bandwidth even in a scenario in which it happens multiple times that, during Backoff, the presently envisaged channel become busy. A repeated channel switching may thus further improve the performance of the entire system.

The method may further comprise transmitting the signal via the further one of the communication channels after having counted down the pre-determined waiting time to zero. Thus, even after having switched the channel multiple times, the last channel which remains free even after expiry of the Backoff time can be used for data transmission.

The method may further comprise, after having transmitted the signal via the other one of the communication channels, preventing the network node from transmitting a further signal via the one of the communication channels for a pre-determined (fixed or randomly selected) further waiting time, and enabling the network node for immediately or after a shortened further pre-determined waiting transmitting a further signal via a further one of the communication channels. In other words, a post-Backoff process may be made much more efficient according to an embodiment of the invention. After having sent the message by the communication channel which remains free during expiry of the Backoff time, this network node is prevented from sending data messages on the same channel, for priority reasons. However, such a communication node which has just sent a communication message, may use any other of (presently or potentially free) channels so as to use free capacity and bandwidth in an efficient manner. Hence, other channels than the one which has just been used for transmission can be used at once or after a shortened waiting time

(compared to the usual waiting time) for fast transmission. Thus, other free channels can be used, immediately or after briefly listening the other free channels or with a shortened backoff, before transmitting over these channels.

Alternatively, in case that it is detected that the selected one of the communication channels does not become busy during the counting down, the method may comprise transmitting the signal via the selected one of the communication channels after having counted down the pre-determined waiting time to zero on the selected one of the communication channels, and after having transmitted the signal via the selected one of the communication channels, the network node may be prevented from transmitting a further signal via the selected one of the communication channels for a pre-determined further waiting time, and the network node may be enabled for immediately or after a shortened further pre-determined waiting time transmitting a further signal via a further other one of the communication channels. Thus, other channels than the one which has been used for transmission, after the switching, can be used at once or after a shortened waiting time (compared to the usual waiting time) for fast transmission. Thus, other free channels can be used, immediately or after briefly listening the other free channels or with a shortened backoff, before transmitting over these channels. It is noted that the feature related to the post-backoff may be realized independently from the feature related to the switching, if desired, during backoff countdown, and vice versa.

The switching to another one of the communication channels may be performed based on scanning the communication channels sequentially or simultaneously. For instance, any of the network nodes may scan the available channels according to a predetermined algorithm or scheme, for example one after the other. Such a separate or sequential scanning may also be performed in a random manner. Alternatively, all of the communication channels may be monitored simultaneously, that is to say parallel in time. The latter approach is possible with code-based channels, like e.g. in the MC-CDMA embodiment, or, in case of frequency-based channels, with devices that are equipped with multiple transceivers.

The signal to be transmitted may be a data packet. Thus, it falls under the scope of the invention that in case of a collision of a data signal to be transmitted over a selected channel, the data signal is retransmitted on another, i.e. a changed, transmission channel, after expiry of the Backoff time.

According to an exemplary embodiment of the invention, the method may implement a Carrier Sense Multiple Access as a Media Access method. Carrier Sense Multiple Access (CSMA) may be denoted as a non-deterministic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium, such as an electrical bus, or a band of an electromagnetic spectrum. The term "Carrier Sense" may denote that a transmitter listens for carrier waves before trying to send. The term "Multiple Access" may denote that multiple nodes send and receive on a medium. The network node may be adapted to communicate according to the IEEE

802.11 standard (see LAN MAN Standards Committee of the IEEE Computer Society, Wireless LAN Medium Access Control (MAN) and Physical Layer (PHY) specifications, IEEE Standard 802.11, 1999 Edition). The IEEE 802.11 standard is a standard that specifies carrier sense media access control and physical layer specifications. Wireless LANs may operate in the 2.4 GHz band.

Further, the network node may be adapted to communicate according to the IEEE 802.1 In standard, being a new sub-standard of the worldwide IEEE 802.11 standard for Wireless Local Area Networks (WLANs). The IEEE 802.1 In standard is a standard that specifies a technique for establishing of wireless local networks with data rates in the range of, for instance, 540 Mbps. For instance, IEEE 802. Hn may use Multiple Input Multiple Output (MIMO) for data transmission.

The plurality of communication channels may be at least one of frequency- based communication channels, code-based communication channels, and time-based communication channels. Particularly, the plurality of communication channels of the network may be distinguished in frequency or with codes. For instance, channels are separated with codes (CDMA, "Code Division Multiple Access") or in frequency (FDMA, "Frequency Division Multiple Access").

The plurality of communication channels may be adapted for Multi-Carrier Code Division Multiple Access (MC-CDMA) or for Direct- Sequence Code Division Multiple Access (DS-CDMA).

A multi-carrier system may be particularly denoted as a system where the several sub-carriers are used for parallel transmission of data packets. According to the described embodiment, a multi-carrier mechanism may be applied to a Code Division Multiple Access (CDMA) network. In a Code Division Multiple Access (CDMA) network, each data symbol may be spread over a large bandwidth, preferably larger than the bandwidth needed for transmission. This may allow to transmit with a spectral energy that is lower than in a non-spread spectrum system. This may allow for the use of parallel transmission channels at the same time and in the same frequency band. Thus, a high capacity multi- carrier modulation technique can be implemented in a particularly advantageous manner with a standard Medium Access Control (MAC) protocol of an IEEE 802.11 WLAN.

The pre-determined waiting time may be a Backoff time. A host or node which has experienced a collision on a network may wait for an amount of time before attempting to retransmit. A random Backoff may reduce the probability that the same nodes will collide again, even if they are using the same Backoff algorithm. Increasing the Backoff period after each collision may also help to prevent repeated collisions, especially when the network is heavily loaded.

Particularly, the method may comprise pre-determining the Backoff time to be a multiple (an integer multiple) of a pre-determined time slot. Thus, according to an exemplary embodiment of the invention, the duration of the time may be a multiple of a slot time (which may be, for instance, 9 μs). Each station may maintain a so-called Contention Window (CW), which may be used to determine the number of slot times a station has to wait before starting a transmission. Before a data packet is transmitted, a random number between 0 and CW (Contention Window) may be determined, which determines the duration of the Backoff timer in a number of slot times. The Contention Window (CW) may have a minimum starting value of 15 and it may be increased (for example doubled) after a packet collision. Its value can rise, for instance up to 255, and may be decremented after a successful transfer. The increase of the CW size may reduce the probability that the same packets collide again.

The method may further comprise waiting for a pre-determined (fixed or randomly selected) pause time before transmitting the signal via the further one of the communication channels and after having counted down the pre-determined waiting time to zero. By taking this measure, collisions on the iurther channel can further be reduced. In the following, exemplary embodiments of the network node will be described. However, these embodiments also apply for the method of operating a network node, for the communication network, for the computer-readable medium and for the program element.

Particularly, the network node may be realized as a computer device, particularly as a personal computer, as a laptop computer, as a workstation, as a PDA

("Personal Digital Assistant"), or the like. However, the network node may also be realized as, for instance, a mobile phone, or the like.

The network node may further comprise a transmitter, a receiver and a local memory. The transmitter and the receiver may be coupled to the communication channels. The transmitter may transmit signals to the communication channels. The receiver may receive signals from the communication channels. The local memory may be coupled to the processor and may be adapted to store data. The local memory can be, for instance, an EEPROM. The transmitter and the receiver may also be coupled to the processor, as well as the local memory device. The processor may be a microprocessor or the like. In the following, an exemplary embodiment of the communication network will be described. However, this embodiment also applies for the method of operating a network node, for the network node, for the computer-readable medium and for the program element.

The communication network may comprise a plurality of interconnected network nodes having the above-mentioned features. The network system may be a wireless communication system for allowing a wireless communication between the plurality of network nodes, thus forming a wireless network. Nodes of such a network may communicate with each other, for instance, via a transmission of electromagnetic waves. Particularly, such a network can be a WLAN (Wireless Local Area Network). However, alternatively, the network system may be conventionally wired, i.e. the different network nodes may be connected with each other using electrical wires.

The aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment.

The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Fig. 1 shows a network system according to an exemplary embodiment of the invention.

Fig. 2 shows a communication device according to an exemplary embodiment of the invention.

Fig. 3 shows a communication scheme according to an exemplary embodiment of the invention.

Fig. 4 shows a communication scheme according to an exemplary embodiment of the invention.

Fig. 5A to Fig. 5C show the time dependence of traffic transmitted via different channels of network systems according to an exemplary embodiment of the invention.

Fig. 6 shows the time dependence of traffic transmitted via different channels of a network system according to the prior art.

The illustration in the drawing is schematically. In different drawings, similar or identical elements are provided with the same reference signs.

In the following, referring to Fig. 1, a communication network 100 according to an exemplary embodiment of the invention will be described.

Fig. 1 shows the communication network 100 comprising a first computer terminal 101 (a first station), a second computer terminal 102(a second station), a third computer terminal 103 (a third station) and a fourth computer terminal 104 (a forth station), each having a processor (not shown in Fig. 1). The computer terminals 101 to 104 are interconnected in a wireless manner via a first channel 105, a second channel 106 and a third channel 107. Via any of the communication channels 105 to 107, any of the computer terminals 101 to 104 can transmit data to any other one of the computer terminals 101 to 104 in a wireless manner. Each of the computers 101 to 104 is adapted to communicate according to the IEEE 802.11 standard, the network system 100 forming a WLAN.

In the following, referring to Fig. 2, details of the first computer terminal 101 will be described in more detail.

The computer terminal 101 comprises a microprocessor (CPU) 200, comprises at least one transmitter 201 coupled to the microprocessor 200 and wirelessly coupled to any of the communication channels 105 to 107 for transmitting data packets onto any one of these channels 105 to 107, and comprises at least one receiver 202 coupled to the microprocessor 200 and adapted to receive data packets from any one of the channels 105 to 107.

Furthermore, a rewritable memory 203 is provided and coupled with the microprocessor 200. Data may be stored in the memory 203 under control of the microprocessor 200.

The terminal computer 101 serves as a network node in the communication network 100 which comprises the plurality of network nodes 101 to 104 which are communicatively coupled via the plurality of communication channels 105 to 107.

As shown in Fig. 2, the terminal computer 101 comprises the microprocessor 200 which is adapted to carry out the method which will be described in the following.

When the terminal computer 101 plans to send a message to any one of the other terminal computers 102 to 104, the processor 200 may count down a pre-determined Backoff time before transmitting the data packet via the, for instance, first communication channel 105. While the Backoff time is counted down, the processor 200 may detect whether the first communication channel 105, via which the computer terminal 101 desires to send a communication message, becomes busy. Such an event may occur, for instance, when the second terminal computer 102 starts emitting a signal and transmitting it via the first communication channel 105 during the countdown of the Backoff time by the processor 200 of the first terminal 101.

In case that the first communication channel 105 becomes busy during the counting down, the processor 200 may switch to, for example, the second communication channel 106 and may continue the counting down of the Backoff time on this second communication channel 106.

It may further happen that, during the continuing of the counting down of the Backoff time by means of the processor 200 before sending the data packet via the second communication channel 106, also the second communication channel 106 becomes busy during this waiting time. Such an event may be detected by the processor 200. Such an event may occur, for instance, when the third computer terminal 103 starts sending a communication message via the second communication channel 106 while the processor 200 continues to count down the remaining part of the Backoff timer before sending the data packets via the second transmission channel 106. In this scenario, in which the second communication channel 106 becomes busy during the continuing of the counting down by the processor 200, the processor 200 may detect this, may switch to the third communication channel 107, and may continue the counting down on the third communication channel 107. After the expiry of the entire Backoff time, the processor 200 may finally send the data packet via the third transmission channel 107. In the following, referring to Fig. 3, another scenario will be described which may happen when operating the network system 100.

In this scenario, the first computer terminal 101 wishes to send a data packet via the first communication channel 105 and therefore starts to count down a Backoff time 301. However, before having finished with the countdown, the fourth computer terminal 104 sends a data packet 302 on the first channel 105. This can be detected by the processor 200 of the first computer terminal 101 so that the first computer terminal 101 may switch to the second, presently free, communication channel 106 to continue the counting down of the Backoff timer 301 there. Such a switch is indicated with the reference numeral 303 in Fig. 3. During the countdown of the remaining part of the Backoff timer 301, no one of the other network nodes 102 to 104 disturbs the channel 106 which thus remains free. Therefore, after expiry of the Backoff time 301, the first computer terminal 101 sends the data packet 304 via the second communication channel 106.

Independently from this, during counting down the remaining part of the Backoff timer 301, the third computer terminal 103 sends a data packet 305 via the third communication channel 107.

Coming back to the situation on the second communication channel 106, it would be conventionally necessary for the first computer terminal 101 to perform a further post-Backoff counting down before sending a further communication message. According to the described embodiment of the invention, such a post-Backoff will also be possible in case that the first computer terminal 101 would desire to send a further data packet via the already used second communication channel 106. However, in order to increase the bandwidth, it is possible for the processor 200 to detect that the first communication channel 105 is now free, thus allowing the processor 200 to send a further data packet 306 on the first communication channel 105 immediately after having sent the data packet 304 via the second communication channel 106. The jump from the second communication channel 106 to the first communication channel 105 prior to sending the further data packet 306 on the first communication channel 105 is denoted with reference numeral 307.

In the following, referring to Fig. 4, a communication scheme according to another exemplary embodiment will be described.

According to Fig. 4, four communication channels 105 to 107, 401 are shown which may be denoted as "codechannels".

A first station using a smart Backoff system according to an embodiment of the invention starts its Backoff in the usual way, see reference numeral 402 indicating counting down a Backoff time. If this Backoff is interrupted by the transmission of a data packet 403 by a second station, the first station searches for another free channel and finds the third channel 107 so that the first station performs a channel jump 404. Then, the first station continues its down-count with the Backoff timer 402.

This channel change or switch may be repeated (one or several times) until the Backoff timer 402 reaches zero and the first station is allowed to transmit its message. Hence, the smart Backoff gives prioritized stations, that are allowed to use it, a faster transmission possibility, resulting in a higher throughput and lower packet transfer delay.

A second jump is indicated in Fig. 4 with reference numeral 405 and may occur when a data packet 406 is transmitted by another station on the third channel 107. This jump 405 goes from the third channel 107 to the second channel 106 at which the countdown of the Backoff timer 402 is finished. Subsequently, the first station sends a communication data packet 407 via the second channel 106 at which the countdown of the Backoff timer 402 has been finished.

Another aspect of an embodiment of the invention concerns post-Backoff. Say again a station "A" completed a transmission on a first channel and therefore has to defer from the medium setting a post-Backoff timer. If the station "A" is allowed to use the smart Backoff, it starts a post-Backoff timer on channel 1 (on which it transmitted the last packet), but can transmit in any free channel immediately (without performing the post-Backoff), thus achieving higher Quality of Service (QoS) in terms of throughput and transmission delay. According to an exemplary embodiment, a system may be provided comprising multiple code channels. Such a system could be, for instance, a MC-CDMA or DS-CDMA system. In case of the multiple code channels, switching the channel may be a fast operation, as a switch may take place in a digital logic of the transceiver, respectively signal processor. After being blocked on one channel by a transmission of another station or having completed an own transmission, a station switches its code channel and searches for the idle channel on which to continue its Backoff, respectively start transmission. This scanning of other channels may be carried out sequentially or even in parallel.

According to another exemplary embodiment of the invention, the system may comprise multiple frequency channels, as shown in Fig. 5A, Fig. 5B, and Fig. 5C.

Fig. 5A to 5C shows smart Backoff and multiple channels with DIFS switching time.

In the scenario shown in Fig. 5A to Fig. 5C, code channel blocking through alien transmission is denoted with reference numeral 500. Furthermore, data transfer after Backoff 501 is shown, Backoff slot intervals 503, and DIFS 502.

In the described scenario, channel switching may require a little more time as analog components might be involved. However, even in this scenario switching may be improved to get a better efficiency of channel use. The switching times can be reduced or even avoided completely, particularly if certain components in the transceiver are present multiple times like, e.g. oscillators or complete transmit and receive chains. By such a redundancy, the efficiency can be further improved.

It should be noted that the term "comprising" does not exclude other elements or steps and the "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS:

1. A method of operating a network node ( 101 ) of a communication network (100) comprising a plurality of network nodes (101 to 104) communicating via a plurality of communication channels (105 to 107), the method comprising: counting down a pre-determined waiting time (301) before transmitting a signal (304) via a selected one of the communication channels (105); detecting, during the counting down, whether the selected one of the communication channels (105) becomes busy; in case that the selected one of the communication channels (105) becomes busy during the counting down, switching to another one of the communication channels (106) and continuing the counting down on the other one of the communication channels (106).

2. The method of claim 1, comprising transmitting the signal (304) via the other one of the communication channels (106) after having counted down the pre-determined waiting time (301) to zero on the other one of the communication channels (106).

3. The method of claim 1 , comprising: detecting, during the continuing of the counting down, whether the other one of the communication channels (106) becomes busy; in case that the other one of the communication channels (106) becomes busy during the continuing of the counting down, switching to a further one of the communication channels (107) and continuing the counting down on the further one of the communication channels (107).

4. The method of claim 3, comprising transmitting the signal (304) via the further one of the communication channels (107) after having counted down the pre-determined waiting time (301) to zero on the further one of the communication channels (107).

5. The method of claim 2, comprising, after having transmitted the signal (304) via the other one of the communication channels (106), preventing the network node (101) from transmitting a further signal (306) via the other one of the communication channels (106) for a pre-determined further waiting time, and enabling the network node (101) for immediately or after a shortened further pre-determined waiting time transmitting a further signal (306) via a further other one of the communication channels (105, 107).

6. The method of claim 1, comprising: in case that the selected one of the communication channels (105) does not become busy during the counting down, transmitting the signal (304) via the selected one of the communication channels (105) after having counted down the pre-determined waiting time (301) to zero on the selected one of the communication channels (105); and after having transmitted the signal (304) via the selected one of the communication channels (105), preventing the network node (101) from transmitting a further signal (306) via the selected one of the communication channels (105) for a predetermined further waiting time, and enabling the network node (101) for immediately or after a shortened further pre-determined waiting time transmitting a further signal (306) via a further other one of the communication channels (105, 107).

7. The method of claim 1 , wherein the switching to another one of the communication channels (106) is performed based on scanning the communication channels (105 to 107) sequentially or simultaneously.

8. The method of claim 1, wherein the signal (304) to be transmitted is a data packet.

9. The method of claim 1, implementing Carrier Sense Multiple Access as a Media Access method.

10. The method of claim 1, wherein the network node (101) is adapted to communicate according to the IEEE 802.11 standard.

11. The method of claim 1 , wherein the network node ( 101 ) is adapted to communicate according to the IEEE 802.1 In standard.

12. The method of claim 1 , wherein the plurality of communication channels (105 to 107) are at least one of frequency-based communication channels, code-based communication channels, and time-based communication channels.

13. The method of claim 1 , wherein the plurality of communication channels (105 to 107) are adapted for Multi-Carrier Code Division Multiple Access or are adapted for Direct- Sequence Code Division Multiple Access.

14. The method of claim 1, wherein the pre-determined waiting time (301) is a

Backoff time.

15. The method of claim 14, comprising pre-determining the Backoff time (301) to be a random multiple of a pre-determined time slot (503).

16. The method of claim 1 , comprising waiting for a pre-determined pause time before transmitting the signal (304) via the further one of the communication channels (106) and after having counted down the pre-determined waiting time (301) to zero.

17. A network node ( 101 ) for a communication network ( 100) comprising a plurality of network nodes (101 to 104) communicatively coupled via a plurality of communication channels (105 to 107), the network node (101) comprising a processor (200) adapted to control or carry out the following method: counting down a pre-determined waiting time (301) before transmitting a signal (304) via a selected one of the communication channels (105 to 107); detecting, during the counting down, whether the selected one of the communication channels (105) becomes busy; in case that the selected one of the communication channels (105) becomes busy during the counting down, switching to another one of the communication channels (106) and continuing the counting down on the other one of the communication channels (106).

18. The network node (101) of claim 17, adapted as a computer device.

19. The network node (101) of claim 17, comprising a transmitter (201), a receiver (202), and a local memory (203).

20. A communication network (100), comprising a plurality of interconnected network nodes (101 to 104) according to claim 17.

21. The communication network (100) according to claim 20, adapted for wired or wireless communication between the plurality of network nodes (101 to 104).

22. A computer-readable medium, in which a computer program of operating a network node (101) of a communication network (100) comprising a plurality of network nodes (101 to 104) communicating via a plurality of communication channels (105 to 107) is stored, which computer program, when being executed by a processor (200), is adapted to control or carry out the following method: counting down a pre-determined waiting time (301) before transmitting a signal (304) via a selected one of the communication channels (105); detecting, during the counting down, whether the selected one of the communication channels (105) becomes busy; in case that the selected one of the communication channels (105) becomes busy during the counting down, switching to another one of the communication channels (106) and continuing the counting down on the other one of the communication channels (106).

23. A program element of operating a network node (101) of a communication network (100) comprising a plurality of network nodes (101 to 104) communicating via a plurality of communication channels (105 to 107), which computer program, when being executed by a processor (200), is adapted to carry out the following method: counting down a pre-determined waiting time (301) before transmitting a signal (304) via a selected one of the communication channels (105); detecting, during the counting down, whether the selected one of the communication channels (105) becomes busy; in case that the selected one of the communication channels (105) becomes busy during the counting down, switching to another one of the communication channels (106) and continuing the counting down on the other one of the communication channels (106).