Institute of electrical and electronics engineers (IEEE) 802.11 is one of widely spread standards related to a wireless local area network (WLAN), which is set by IEEE and broadly defines a physical (PHY) layer and a media access control (MAC) layer. IEEE 802.11 supports various frequency bands as well as a frequency band of 2.4 GHz. With regard to several standards of IEEE 802.11, set times, use frequency bands, frequency bandwidths, and transmission speeds thereof are as shown in the following table 1.
TABLE 1UsefrequencyFrequencyTransmissionStandardsSet timesbandsbandwidthsspeedsIEEE 802.1119972 GHz20 MHz2MbpsIEEE 802.11bOctober,2 GHz20 MHz11/22Mbps1997IEEE 802.11aOctober,2 GHz20 MHz54Mbps1999IEEE 802.11gJune, 20002 GHz20 MHz54MbpsIEEE 802.11nSeptember,2/5 GHz  20/40 MHz  600Mbps2009
In Ethernet for wired communications, a transmitting terminal transmits a packet through a certain port and at the same time receives a packet through another port. If the transmitting terminal receives only its own packet, it is regarded that packet collision does not occur. If the transmitting terminal receives another packet besides its own packet, it is informed that the packet collision occurs. That is, in Ethernet, the transmitting terminal can sense the packet collision in a receiving terminal
However, a wireless device of the IEEE 802.11 system operates in a half-duplex manner, so that a transmitting terminal cannot sense packet collision in a receiving terminal. Thus, the transmitting terminal of the IEEE 802.11 first ascertains whether a channel is idle, and then performs communication.
In IEEE 802.11, there are broadly two methods of ascertaining whether the transmitting terminal is transmitting a packet through a channel. A first method is to set up an inter frame space (IFS). A station senses that the channel is idle, and then performs transmission after the lapse of the IFS. This is because a remote station may start transmission at a point of time when sensing that the channel has already been idle.
A second method for ascertaining whether the channel is idle is to set up a contention window. The contention window is configured as a unit of slot on a time base, and a station ready for transmitting a packet is on standby during the contention window, in other words, an arbitrary time (backoff interval). After the lapse of this arbitrary time, the station senses the channel, i.e., ascertains whether the channel is idle. At this time, the packet is transmitted if the channel is idle, but the contention window is set up again if the channel is being used by another wireless device.
FIG. 1 is a view for explaining a method of ascertaining whether a channel is idle in the IEEE 802.11 system.
Referring to FIG. 1, it is shown that an MAC station of a station 3 (STA3) is the first to receive a packet. However, the station 3 transmits the packet after waiting a data inter frame space (DIFS). A station 1 (STA1) reserves the transmission of the packet since it senses that the channel is being occupied by the station 3 at a point of time when waiting a DIFS after the packet arrives at its own MAC station.
After the station 3 completes the transmission of the packet, each of the stations (STA1, STA2 and STA5) waits the DIFS, and is then on standby during the backoff interval arbitrarily selected by each station. If the station 2 (STA2) is the first station of which the backoff interval is elapsed, the station 2 performs the transmission of the packet but other stations wait with their own stored backoff intervals until the station 2 completes the transmission of the packet.
After the lapse of the DIFS after the station 2 completes the transmission, other stations STA1, STA4 and STA 5 are on standby during the stored backoff intervals, respectively. If the backoff interval of the station 4 STA4 is equal to that of the station 5 STA5, the station 4 and the station 5 transmit packets at the same time and thus a packet collision happens. In this case, the station 1, the station 4 and the station 5 wait the DIFS again, and then the station 1 having the shortest backoff interval starts the transmission of the packet.
In the IEEE 802.11 system, two fundamental problems arise because of wireless environmental characteristics. A first problem is a hidden node. Referring to FIG. 2, if there are three stations, a station A transmits a packet to a station B, but a station C cannot receive the packet. The station C senses a channel in order to transmit a packet to the station B and transmits the packet to the station B as sensing that the channel is idle, but at this time a packet collision occurs in the station B. In this case, the station C regards the station A as the hidden node.
A second problem is an exposed node. Assume an environment where there are four stations as shown in FIG. 3. In such an environment, a station B tries to transmit a packet to a station A, and a station C tries to transmit a packet to a station D. When the station C senses a channel, it is determined that the channel has already been occupied. However, the station C does not have to wait until the channel becomes idle, since the station A is out of a transmission region of the station C. In this case, the station B regards the station C as the exposed node.