Patent Application
20050250507
1. Field of the Invention
The present invention relates to channel assignments in 802.11 wireless networks.
2. Description of the Related Art
IEEE 802.11 has emerged as the standard protocol for wireless Ethernet communication. While 802.11 is becoming widely used, it was not originally designed for such large scale use. Problems due to interference that may afflict large scale wireless communication networks were not given their due attention in the formation of the IEEE 802.11 standard.
802.11 is currently limited to static channel assignment (SCA). Under SCA, a channel used for data communication is fixed and data transfer between a first transceiver and a second transceiver occurs on only that channel regardless of channel utilization and interference. This can lead to a condition where a channel is overloaded while other channels are underutilized. An overloaded channel often experiences more interference than an underutilized channel.
There are numerous detrimental consequences interference can have on a wireless communications network, such as increased packet error rate, reduced network throughput and longer transfer delays. Communication systems using the IEEE 802.11 standard with SCA can therefore have a channel suffering from reduced network throughput and longer transfer delays due to interference while other channels are underutilized.
In one exemplary embodiment of the present invention, a first transceiver transmits a request-to-send message over a plurality of communication channels. A second transceiver receives the request-to-send messages, and chooses or selects one of the communication channels based on the received request-to-send messages. For example, in one embodiment, the channel over which the request-to-send message having the highest signal-to-noise ratio was received is selected. The second transceiver then sends a clear-to-send message over the selected channel. Based on receipt of this clear-to-send message, the first transceiver assigns the selected channel to transmit data packets.
As will be appreciated, this methodology of channel assignment adapts to conditions present in the communication system and may increase system throughput while reducing interference.
The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, wherein like reference numerals designate corresponding parts in the various drawings, and wherein:
An antenna 140 of a second transceiver 20 receives signals, such as signals transmitted by the antenna 130 of the first transceiver 10. Signals received by the antenna 140 are demultiplexed into their respective channels by a second combiner/demultiplexer 145 and sent to respective second MAC/PHY units 160. As with the first MAC/PHYS unit 110 in the first transceiver 10, the second MAC/PHYS units 160 in the second transceiver 20 corresponds to each channel (e.g., three MAC/PHYS unit 160 each corresponding to one of the three non-overlapping channels in IEEE 802.11b). The implementation of the second MAC/PHY units 160 are well known in the art and will not be further described. At each MAC/PHY unit 160, messages are extracted from the channel. Extracted RTS messages are sent to a second DCA unit 170.
Returning to
The antenna 130 of the first transceiver 10 receives signals, such as signals transmitted by the antenna 140 of the second transceiver 20. Signals received by the antenna 130 are demultiplexed into their respective channels by the first combiner/demultiplexer 125 and sent to respective MAC/PHY units 110. Each MAC/PHY unit 110 extracts messages from their respective channel. The extracted CTS message is sent to the first DCA unit 100.
Receipt of a CTS message on the selected channel indicates to the first DCA unit 100 to use the selected channel for communication. The first transceiver 10 will then being sending traffic (e.g., packets) over the selected channel. The first DCA unit 100 also resets the unselected channels; namely, the first DCA unit 100 resets the first MAC/PHY units 110 associated with the unselected channels. This brings the reset MAC/PHY units 110 to a state as if no RTS messages were sent by those units.
It should be understood that within the exemplary embodiment of the present invention described above, the first transceiver 10 represents the source terminal and the second transceiver 20 represents the destination terminal. However, it should be understood that the second transceiver 20 may represent the source terminal and the first transceiver 10 may represent the destination terminal in another exemplary embodiment of the present invention.
In an exemplary embodiment of the present invention, the first transceiver 10 and the second transceiver 20 may encounter a situation in which no RTS message is properly received by the second DCA unit 170. A situation may also arise in which the channels for the received RTS messages are considered inappropriate for selection. One reason a received RTS message may be considered inappropriate is a low signal-to-noise ratio (SINR). When the second DCA unit 170 does not receive an RTS message and/or does not receive an appropriate RTS message, a CTS message is not sent to the first transceiver 10 by the second transceiver 20. After a time-out period in which the first DCA unit 100 does not receive a CTS message from the second transceiver 20, the first DCA unit 100 instructs the first MAC/PHY units 110 to retransmit the RTS messages over their respective communication channels to the second transceiver 20 in order to select a channel on which to transmit.
In an exemplary embodiment of the present invention, the timer period Tr may be determined based on a characteristic of the data transmission. For example, a small Tr value may indicate a small delay before data transmission is initiated over a selected channel. However, the channel selected may not be the best channel when using a small Tr value. A large Tr value may have a longer initial delay before data transmission begins as compared to a small Tr value, but a second DCA unit 170 using a large Tr value may select a better channel with superior performance characteristics as compared to a second DCA unit 170 using a small Tr value.
DCA gain refers to the performance increase between a transceiver using DCA over a transceiver using SCA. In general, the DCA gain may benefit from a large Tr value when a large amount of data is to be transmitted. The initial transmission delay is relatively insignificant when a large data transfer is required. Similarly, the DCA gain may benefit from a small Tr value when a small amount of data is required for transmission. The initial transmission delay is relatively significant when a small data transfer is required.
According to another exemplary embodiment of the present invention, a packet transmission over the selected channel may be unsuccessful. The packet is then re-transmitted on the selected channel. If normal operation is resumed following the re-transmission of the data packet, the data transmission continues normally on the selected channel. However, if there is a second unsuccessful packet transmission attempt, the selected channel is reset and the first DCA unit 100 instructs the MAC/PHY units 10 to re-transmit the RTS messages on the multiple communication channels to the transceiver 20 in order to select a new channel for transmission.
In another embodiment of the present invention, the first DCA unit 100 instructs the MAC/PHY units 10 to re-transmit the RTS messages on the multiple communication channels in order to select a new channel after a number of data packets have been sent by the first transceiver 10.
According to another exemplary embodiment of the present invention, a registration and authentication process is executed on a set one of the multiple communication channels prior to using the DCA according to the present invention.
According to another exemplary embodiment of the present invention, the first DCA unit 170 initiates DCA only if the length exceeds a fixed threshold amount (L). The threshold amount L may be user-specified or a design parameter set by the system designer. If the length of the packet to be transmitted exceeds the threshold amount L, DCA may be initiated, such as illustrated in
Hidden terminal interference is caused by the simultaneous transmission of two transceivers where each transceiver is unaware of the transmission of the other transceiver, and the transmission of both transceivers are received by the same destination transceiver. This interference lowers the system throughput and increases the average packet delay.
According to an exemplary embodiment of the present invention, a hidden terminal problem may be avoided using DCA by selecting a channel which does not currently carry traffic, if such a channel is available. Thus, the number of transceivers simultaneously transmitting to a destination transceiver may be reduced.
In an exemplary embodiment of the present invention, DCA is not activated when the length of the packet for transmission does not exceed the threshold amount L. In this embodiment, the packet is transmitted according to conventional SCA techniques.
Further, a system designer may specify a small threshold amount L value to realize DCA gain. However, a threshold amount L value that is too small may cause unnecessary delay before data transmission for small data packets, and may waste bandwidth due to the DCA operations.
According to another exemplary embodiment of the present invention, a transmitting DCA transceiver may communicate with a receiving transceiver that does not have DCA capability. The DCA transceiver may operate normally as if the transceiver had DCA capability. A CTS message would be received by the DCA transceiver only on one of the multiple communication channels because the non-DCA transceiver will only respond on the one fixed channel.
A determination that the non-DCA transceiver does not have DCA capability may be made by the DCA transceiver after receiving a CTS message only on the one, same channel for a period of time. The DCA transceiver may then turn off the MAC/PHY's for other channels.
According to another exemplary embodiment of the present invention, a receiving DCA transceiver may be implemented with a transmitting transceiver that does not have DCA capability. The DCA transceiver may operate normally as if the transceiver had DCA capability. A RTS message would be received by the receiving DCA transceiver only on one of the multiple communication channels. A determination that the non-DCA transceiver does not have DCA capability may be made by the DCA transceiver after receiving an RTS message only on the one, same channel for a period of time. The DCA transceiver may then turn off the MAC/PHY's for other channels.
Next, a method of selecting a channel as set forth in step S316 of
The inventive techniques of the present invention will allow transceivers using DCA to avoid interference inherent in transceivers using SCA, and therefore reduce the negative effects interference may inflict on a 802.11 wireless communication system.
The exemplary embodiments of the present invention being thus described, it will be obvious that the same may be varied in many ways. For example, the exemplary embodiments may apply to 802.11 wireless networks in which each of the multiple communication channels do not overlap with one another. It should be further understood that the exemplary embodiments may apply to 802.11 wireless networks in which at least one of the multiple communication channels overlap with another of the multiple communication channels. Such variations are not to be regarded as a departure from the spirit and scope of the exemplary embodiments of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
