WO2006109213A1

WO2006109213A1 - Communication system operating according to the carrier sense multiple access with collision detection (csma/cd) protocol

Info

Publication number: WO2006109213A1
Application number: PCT/IB2006/051032
Authority: WO
Inventors: Georgios Orfanos; Joerg Habetha
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2005-04-11
Filing date: 2006-04-04
Publication date: 2006-10-19

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 (302) simultaneously in a plurality of the communication channels (105 to 107) before transmitting a signal (303) via at least one of the communication channels (105).

Description

COMMUNICATION SYSTEM OPERATING ACCORDING TO THE CARRIER SENSE MULTIPLE ACCESS WITH COLLISION DETECTION (CSMA/CD) PROTOCOL

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 2003/0145095 Al discloses a method for implementing a plurality of backoff counters on a hardware backoff counter for use in implementing a prioritized message transmission network. A message with a smallest backoff time is selected and placed into the hardware backoff counter. When the hardware backoff counter expires, the message is transmitted. Whenever the communication medium becomes busy, the backoff time for every message is updated.

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 simultaneously in a plurality of the communication channels before transmitting a signal via at least 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 a multi-channel parallel backoff process is provided in which a backoff time or a waiting time is counted down from an initial value to zero parallel in time in at least two of a plurality of independent communication channels. Thus, a plurality of communication channels may be monitored simultaneously, and in the statistical average it may be possible to reduce the time which a station or network node has to wait before it is allowed to send a signal or communication message over one of the parallel monitored channels. The particular channel or channels over which the communication message is in fact sent may be selected based on the criteria on which communication channel or channels the waiting time has expired at first. In other words, a plurality of possible communication channels are defined or selected as those channels on which the waiting time is counted down simultaneously. Thus, a network node has multiple chances for sending a communication message over one of the selected communication channels. The signals may then be sent over this or those communication channel(s) at which the waiting time has expired at first.

In a scenario in which a corresponding communication channel has become busy in the meantime, for instance because another network node has started to send a signal over one of these communication channels, the down-count of the backoff time may be stopped as long as this communication channel remains busy. After this transmission is finished, the count down of the backoff time for the corresponding communication channel may be resumed. Hence, the occurrence of such an event and/or the length of the backoff time for a particular channel may be criteria based on which it depends whether such a channel is in fact used for transmitting the communication message, or not.

By taking this measure, the throughput and the bandwidth of the system may be increased and the average waiting time until a network node is able to send a signal over the network may be reduced. Simultaneously, a reasonable rule of prioritization may be achieved since no signal transmission is allowed before expiry of the waiting time for at least one of the plurality of communication channels.

Thus, according to an exemplary embodiment of the invention, a parallel backoff mechanism for medium access carrier sensing may be provided. In this context, a method may be provided to improve the throughput and delay of prioritized stations in the network that uses Carrier Sense Multiple Access (CSMA) as a Medium Access (MAC) method. Such a method which may also be denoted as a "parallel backoff' particularly applies to networks with multiple data channels. These channels can be code-based, frequency-based or time-based.

In carrier sense multiple access systems, a backoff mechanism may be applied to avoid packet collisions of several transmitting stations. The parallel backoff mechanism according to an exemplary embodiment allows to start multiple backoffs on different channels in parallel. If one channel is getting idle during a backoff countdown, the station can choose the corresponding channel and can abandon all backoff processes on other channels. Alternatively, it can continue to countdown of the backoff on the other channels to obtain multiple (sequential or parallel) transmission opportunities on the channels.

The scheme according to an exemplary embodiment of the invention can significantly reduce the packet delay and increase the data throughput in a CSMA system. Thus, embodiments of the invention relate to a system in which stations carry out a decentralized backoff mechanism to spread out the access times to a medium. In contrast to conventional CSMA systems, in which access is restricted to a single channel in parallel, embodiments of the invention scan or monitor more than one of a plurality of communication channels parallel in time so as to overcome limits of an achievable throughput to the capacity of a single channel. The packet delay which is depending on the traffic and number of stations contending on the respective channel according to the prior art may be significantly reduced according to embodiments of the invention.

Multiple channels can be bundled to achieve a higher data throughput per station. This channel bundle can be treated again as a single but bigger channel. A single backoff procedure would be carried out on this channel bundle. However, it may be more efficient, according to an exemplary embodiment of the invention, not to treat the channel bundle as a single channel but as a set of separate channels. Separate backoff counters can then be started on these channels. This parallel backoff can increase the throughput if multiple channels can be used in parallel, and it can also be used by stations that are only able to access one channel at a time to reduce their packet delay.

According to an exemplary embodiment of the invention, multiple backoff processes are started in parallel on different channels. The backoff processes do not necessarily have to have the same backoff parameters (like backoff time, length of a slot, etc.). What may happen (even if the backoff parameters are the same on all channels) is that, depending on the traffic on the channels, some backoff countdowns will end earlier than others.

According to an exemplary embodiment, a station may then start multiple transmissions in parallel on the different channels, on which the backoff has been completed. Particularly, there are three different scenarios foreseen: Firstly, a station may start transmission on a single channel, which may be the first channel on which a backoff is completed. The backoff on the other channels may then be abandoned.

Secondly, a station may start transmission on each channel independently, once a backoff has been completed. Thus, transmission may be performed on more than one channel at the same time, without a synchronization. For instance, as soon as the backoff time has expired for a first communication channel, transmission may be started on the first communication channel. Later, as soon as the backoff time has expired for a second communication channel, transmission may be started also on the second communication channel, and so on.

Thirdly, a station may start transmission on multiple channels in parallel, but waits until the backoff on a certain number of (two, three, to up to all) channels has been completed to transmit the data in parallel (for instance at a higher "bundle data rate"). For instance, when the backoff time has expired for a first communication channel, no transmission may be started on the first communication channel. Later, as soon as the backoff time has expired for a second communication channel, transmission may be started on the first communication channel and on the second communication channel simultaneously.

Such a procedure can be expanded for stations using n out of m (n≤m) channels, where the countdown is not interrupted in p (p≤n) cases, leading the station to start parallel transmissions on d (d≤p) channels.

Embodiments of the invention are intended for any system with multiple channels. However, one exemplary embodiment is a system, where channels are defined either in code or time. The reason is that it may be difficult (even though not impossible) for a station to scan multiple frequency channels in parallel. Exemplary fields of application of the invention are all devices that apply a backoff mechanism and have the possibility to access different communication channels. Such devices can be adapted for the purpose of wireless or wired data transmission.

An exemplary standard to which the invention may be applied is IEEE 802.11, particularly IEEE 802.1 In. 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 a parallel countdown is performed in a plurality of channels at the same time. Correspondingly, a communication station arranged for contending for access to a communication medium in accordance with the above backoff procedure is provided. Furthermore, a communication system comprising a plurality of communication stations is provided, wherein respective communication stations are arranged for contending for access to a communication medium in accordance with the above described backoff procedure.

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 and a plurality of code, frequency or time channels is provided, wherein a station that has the intention to transmit data starts multiple backoff processes on different channels in parallel by selecting, on each of these channels, the time (slots) to wait before a transmission and by starting a countdown of time (slots). Referring to the described embodiment, the backoff process may run independently and the station may start its transmission on the channel, on which the respective backoff process ends at first, and it abandons all other of the backoff processes for the same transmission.

Additionally or alternatively, the backoff process may run independently and the station may start a transmission on several of the channels, on which a backoff process has been started, after the backoff process has been completed on the respective channel.

Additionally or alternatively, the station may start a transmission on a group of the channels, on which a backoff process has been started, only after the backoff process on each of the channels of the group has been completed. In the context of the previously described embodiment, the group of channels may comprise all channels on which the station has started a backoff process for the transmission.

According to an exemplary embodiment of the method, the channels are code channels defined according to Multi-Carrier Code Division Multiple Access (MC-CDMA) or Direct Sequence Code Division Multiple Access (DS-CDMA) principles.

A communication network according to another exemplary embodiment of the invention includes a plurality of devices and a plurality of code, frequency or time channels, wherein devices employ a backoff scheme, and wherein a device that has the intention to transmit data starts multiple backoff processes on different channels in parallel by selecting, on each of these channels, the time (slots) to wait before a transmission and by starting a downcount of time (slots).

According to another exemplary embodiment, a communication device is provided which comprises a transmitter, a receiver, a processor and a local storage. The processor may be configured to run a backoff scheme wherein, when the device intends to transmit data, multiple backoff processes may be started on different channels in parallel by selecting, on each of these channels, the time (slots) to wait before a transmission and by starting a countdown of time (slots).

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 program element and for the computer- readable medium.

The pre-determined waiting time may be counted down in the plurality of the communication channels independently from one another. When the countdown is interrupted in one channel, for instance since another communication node of a communication network sends a communication message over a particular one of the communication channels, this does not (necessarily) have to interrupt the countdown on the other channels as well. By taking this measure, a "competition" of the countdown on the different communication channels may be achieved, so that, after a very short time, the desired signal may be sent over one or more of the communication channels at which transmission is currently appropriate due to the traffic on the entire system.

At least one parameter indicative of the pre-determined waiting time may differ for different of the plurality of the communication channels. In other words, the waiting times (length, division into a slots, etc.) may be different for the different communication channels. As already mentioned, the waiting times and their properties do not (necessarily) have to be the same for all the communication channels. This may increase the flexibility of the entire system. However, it is also possible that the waiting times and/or their properties are identical for a part of or for all of the communication channels. The pre-determined waiting time may be counted down simultaneously in all of the communication channels before transmitting the signal via the at least one of the communication channels. Thus, not only a part of the communication channels, but all available communication channels may be taken into account as a potential communication channel for transmitting a communication message. This may decrease the delay before sending the communication message on the network.

The method may comprise, when the waiting time has been counted down to zero for a first one of the plurality of the communication channels, transmitting the signal via the first one of the communication channels in which the waiting time has firstly been counted down to zero. Thus, when the waiting time has expired for one of the communication channels, this communication channel is free for transmitting the communication message of the network node and may thus be used for transmission.

The method may comprise, when the waiting time has been counted down to zero for a first one of the plurality of the communication channels, transmitting the signal via the first one of the communication channels in which the waiting time has firstly been counted down to zero, and abandoning the counting down of the waiting time in all other of the plurality of the communication channels. By taking this measure, all other communication channels may be left free for sending a communication message by any one of the other network nodes of the communication system, which may increase the efficiency of the entire system. According to this embodiment, as soon as a free channel has been found and the waiting time has passed for this channel, transmission is carried out via this channel, and all other channels are disregarded for this message.

The method may further comprise, when the waiting time has been counted down to zero for a respective one of the plurality of the communication channels, transmitting the signal via the respective one of the communication channels in which the waiting time has been counted down to zero, so that the transmission is performable simultaneously by a group of more than one of the plurality of communication channels for which the respective waiting time has already expired. In other words, after having counted down the waiting time for one of the communication channels, the processes on the other communication channels do not necessarily have to be abandoned, but it is possible to continue counting down the waiting time on these remaining channels as well. If on a second one of the communication channels the waiting time has expired to zero, (a part of) the communication message or another communication message of the network node may also be sent via this second channel. This procedure may go on until a (pre-determined) number of communication channels are used for transmitting one or more messages, wherein remaining communication channels may then remain used. In the latter channels, the processes may then be abandoned.

The method may further comprise, when the waiting time has been counted down to zero for a respective one of the plurality of the communication channels, transmitting the signal via the respective one of the communication channels in which the waiting time has been counted down to zero, so that the transmission is performable simultaneously by a group of a pre-determined number larger than one of the plurality of the communication channels. According to this embodiment, even if the first one of the communication channels has counted down its corresponding waiting time to zero, the message is not yet sent. In contrast to this, this communication channel which is ready and which is reserved for the transmission of the communication message waits until at least one further of the communication channels becomes free. Only when a predetermined number of the communication channels has become free by expiry of the corresponding waiting times, the message is sent in parallel over all of these communication channels.

The method may further comprise, when the waiting time has been counted down to zero for each of a plurality of the communication channels of a group of communication channels, transmitting the signal simultaneously by the group of the plurality of the communication channels. Thus, a synchronized transmission of the communication message may be realized which might increase the stability of the system.

Still referring to the described embodiment, the group of the plurality of the communication channels may include all of the plurality of the communication channels in which the waiting time has been counted down simultaneously. Thus, all communication channels may send the communication message starting with the same instant in time, which requires expiry of the waiting time in each of the channels.

The signal to be transmitted may be a data packet. Thus, it falls under the scope of the invention that a data signal is transmitted via one or via a plurality of transmission channels after expiry of the Backoff time for this or these channels.

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.

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 of the invention 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 store data. The local memory can be, for instance, an EEPROM. The transmitter and the receiver may also be coupled with the processor, as well as the local memory device. The processor may be a microprocessor or the like.

In the following, an 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 of the invention 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 multi-channel parallel backoff scheme according to an exemplary embodiment of the invention.

Fig. 4 shows a multi-channel parallel backoff scheme according to an exemplary embodiment of the invention.

Fig. 5 shows a multi-channel parallel backoff scheme 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 a 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 a 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. The first terminal computer 101 may be operated under the control of the processor 200 and within the communication network 100 formed by the interconnected network nodes 101 to 104 which communicate via the plurality of the communication channels 105 to 107.

The processor 200 may count down a pre-determined backoff time simultaneously in all of the communication channels 105 to 107 before transmitting a signal via at least one of the communication channels 105 to 107. For instance, the same backoff time may be counted down by the first terminal 101 for each of the communication channels 105 to 107.

It may happen that, during the countdown, the second terminal computer 102 starts to send a communication message via the second communication channel 106. This may have the consequence that the countdown of the backoff time in the second communication channel 106 is interrupted or frozen. It may further happen that the fourth computer terminal 104 starts sending a communication message via the third communication channel 107 during the countdown. Also the countdown of the backoff time by the first computer terminal 101 on the third channel 107 will then be stopped. Thus, it will be the first channel 105 which remains free during the entire countdown of the backoff time by the first computer terminal 101, so that the backoff time countdown will finish first for the first communication channel 105. After expiry of the backoff time in the first communication channel 105, the processor 200 of the first computer terminal 101 will send the communication message over the first communication channel 105. The backoff time is counted down in the plurality of communication channels 105 to 107 independently from one another. According to the described embodiment, parameters indicating the backoff time for the communication channels 105 to 107 are identical.

As has been described, when the waiting time has been counted down to zero for the first one of the plurality of communication channels 105 to 107, in the present case for the first communication channel 105, the communication messages is transmitted via the first one of the communication channels 105 in which the waiting time has firstly been counted down to zero.

In the following, referring to Fig. 3, a multi-channel parallel backoff scheme 300 according to an exemplary embodiment of the invention will be described.

Fig. 3 illustrates a communication network in which four communication channels 105 to 107, 301 are provided. For each of these communication channels 105 to 107, 301 it is shown at which times a network node counts down an assigned backoff time 302 in a corresponding one of the communication channels 105 to 107, 301, and at which times a data message or frame is sent as a data transmission 303 via one or more of the communication channels 105 to 107, 301. Fig. 3 illustrates the case where multiple independent transmissions are started once the respective backoffs are completed. Thus, the configuration of Fig. 1 relates to a parallel backoff with asynchronous parallel data transmission. According to the scenario of Fig. 3, when the backoff time 302 has been counted down to zero for a respective one of the plurality of communication channels 105 to 107, 301, the data packet 303 is transmitted via the respective one of the communication channels 105 to 107, 301 in which the backoff time 302 has been counted down to zero, so that the transmission is performed simultaneously by a group of four communication channels 105 to 107, 301. In other words, whenever a channel 105 to 107, 301 is free and has not become busy during counting down the backoff time 302, data are sent through this channel. This is a very efficient way of using the entire channel capacity.

In the following, referring to Fig. 4, a parallel backoff scheme 400 with synchronous parallel data transmission will be explained.

According to Fig. 4, a transmission is only started after a certain number of backoffs has been completed. The reason for the latter could be, for instance, that the station is not capable of transmitting on different channels independently. In this case, the station has to switch to a "broader" channel once all channels have become idle.

In the configuration of Fig. 4, the backoff times 302 expire for each of the communication channels 105 to 107, 301 independently. Only on expiry of the backoff time 302 for the last one of the four channels (in the present case communication channel 301) data transmission 303 occurs parallel in time in all four communication channels 105 to 107, 301. Therefore, the communication channels 105 to 107 wait until the last communication channel, here the communication channel 301, has successfully finished the backoff count down. Thus, only when the waiting time or backoff time 302 has been counted down to zero for each of the plurality of the communication channels 105 to 107, 301, the signal 303 is transmitted simultaneously by the communication channels 105 to 107, 301.

In the following, referring to Fig. 5, a parallel backoff scheme 500 with a single channel data transmission will be explained.

Fig. 5 illustrates an embodiment in which the parallel backoffs are used to gain faster access to the medium. The channel on which the backoff is completed first will be used. This method may be used for devices which are only capable of transmitting on one channel at a time.

Thus, according to Fig. 5, when the backoff time 302 has been counted down to zero for a first one of the plurality of the communication channels 105 to 107, 301, in the present case the third communication channel 107, the signal 303 is transmitted via the first one of the communication channels 107 in which the waiting time or backoff time 302 has firstly been counted down to zero. All other countdowns may be abandoned.

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 predetermined waiting time (302) simultaneously in a plurality of the communication channels (105 to 107) before transmitting a signal (303) via at least one of the communication channels (105).

2. The method of claim 1, wherein the pre-determined waiting time (302) is counted down in the plurality of the communication channels (105 to 107) independently from one another.

3. The method of claim 1, wherein at least one parameter indicative of the predetermined waiting time (302) differs for different of the plurality of the communication channels (105 to 107).

4. The method of claim 1, wherein the pre-determined waiting time (302) is counted down simultaneously in all of the communication channels (105 to 107) before transmitting the signal (303) via the at least one of the communication channels (105).

5. The method of claim 1, comprising, when the waiting time (302) has been counted down to zero for a first one of the plurality of the communication channels (105 to 107), transmitting the signal (303) via the first one of the communication channels (107) in which the waiting time (302) has firstly been counted down to zero.

6. The method of claim 1, comprising, when the waiting time (302) has been counted down to zero for a first one of the plurality of the communication channels (105 to 107), transmitting the signal via the first one of the communication channels (105) in which the waiting time (302) has firstly been counted down to zero, and abandoning the counting down of the waiting time (302) in all other of the plurality of the communication channels (105 to 107).

7. The method of claim 1, comprising, when the waiting time (302) has been counted down to zero on one or more of the plurality of the communication channels (105 to 107), transmitting the signal (303) via the respective one or ones of the communication channels (106) in which the waiting time (302) has been counted down to zero, so that the transmission is performable simultaneously by a group of more than one of the plurality of the communication channels (105 to 107).

8. The method of claim 1, comprising, when the waiting time (302) has been counted down to zero for a respective one of the plurality of the communication channels (105 to 107), transmitting the signal (303) via the respective one of the communication channels (106) in which the waiting time (302) has been counted down to zero, so that the transmission is performable simultaneously by a group of a predetermined number larger than one of the plurality of the communication channels (105 to 107).

9. The method of claim 1 , comprising, when the waiting time (302) has been counted down to zero for each of a plurality of the communication channels (105 to 107) of a group of communication channels (105 to 107), transmitting the signal (303) simultaneously by the group of the plurality of the communication channels (105 to 107).

10. The method of claim 9, wherein the group of the plurality of the communication channels (105 to 107) includes all of the plurality of the communication channels (105 to 107) in which the waiting time (302) has been counted down simultaneously.

11. The method of claim 1 , wherein the signal (302) to be transmitted is a data packet.

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

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

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

15. 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.

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

17. The method of claim 1 , wherein the pre-determined waiting time (302) is a

Backoff time.

18. The method of claim 17, comprising pre-determining the Backoff time (302) to be a multiple of a predetermined time slot.

19. 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 (302) simultaneously in a plurality of the communication channels (105 to 107) before transmitting a signal (303) via at least one of the communication channels (105).

20. The network node (101) of claim 19, adapted as a computer device.

21. The network node ( 101 ) of claim 20, comprising a transmitter (201 ), a receiver (202), and a local memory (203).

22. A communication network (100), comprising a plurality of interconnected network nodes (101 to 104) according to claim 19.

23. The communication network (100) according to claim 22, adapted for wired or wireless communication between the plurality of network nodes (101 to 104).

24. A computer-readable medium, in which a computer program for 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 (302) simultaneously in a plurality of the communication channels (105 to 107) before transmitting a signal (303) via at least one of the communication channels (105).

25. A program element for 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 control or carry out the following method: counting down a pre-determined waiting time (302) simultaneously in a plurality of the communication channels (105 to 107) before transmitting a signal (303) via at least one of the communication channels (105).