z Direct–sequence spread-spectrum (DS or DSSS) z Orthogonal Frequency Division Multiplexing (OFDM)
The OFDM system provides 802.11a wireless local area network (WLAN) with data rate communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. The support of transmitting and receiving at data rates of 6, 12, and 24 Mbit/s is mandatory. The system uses 52 subcarriers that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Forward error correction coding (convolutional coding) is used with a coding rate of 1/2, 2/3, or 3/4.
D. 802.11 Management Architecture
802.11 management architecture is composed of three component: the MAC layer management entity (MLME), a physical-layer management entity (PLME), and a system management entity (SME). In the wireless world, station must identify a compatible network before joining it. The process of identifying existing networks in the area is called scanning. Several parameters are used in the scanning procedure, such as BSSType (independent or infrastructure), BSSID (individual or broadcast), SSID (“network name”), ScanType (active or passive), ChannelList, ProbeDelay, and Min ChannelTime and MaxChannelTime. A scanning report is generated at the conclusion of a scan. The report lists all the BSSs that the scan discovered and their parameters. After compiling the scan result, a station can elect to join one of the BSSs.
Before this can happen, both authentication and association are required.
2 Dynamic Frequency Selection
channel interference. DCA techniques are becoming important with the tremendous growth of demand for wireless multimedia applications because this scheme can adapt better variation of traffic density than the Fixed Channel Selection (FCS) scheme. In this paper, we propose a scheme of dynamic channel selection (DCS) for IEEE 802.11 WLAN. In our paper, we consider the infrastructure BSSs with an AP as a unique decision maker of the DCS within an 802.11 WLAN. That is, we will not consider the ad hoc mode of WLANs in our paper. If two co-located Basic Service Sets (BSSs) operate at the same channel, which are referred to as overlapping BSSs, it is not easy to support Quality-of-Service (QoS) due to the possible contentions among overlapping BSSs. By having the DCS function implemented, an Access Point (AP) can determine the best channel to work at, and initiate the switch of all the stations (STAs) associated with its BSS to the newly selected channel.
B. Dynamic Channel Selection (DCS) Mechanism
The proposed DCS mechanism for 802.11h is composed of the following seven components[4]:
• Initiation of a channel selection
• Request of channel measurements by AP
• Channel measurement process
• Measurement reports from STAs
• Decision making by AP
• Channel switch announcement by AP
• Switching into the new channel
In the following, we discuss each component in detail.
I. Initiation of a channel selection: There are two cases when a channel selection can be initiated: (1) when a BSS is formed by an AP; and (2) when the AP and/or a STA of a BSS experiences the bad channel condition persistently. For the second case, when to initiate a channel selection is not to be defined in the standard, but implementation-dependent. However, we will need to define some feedback mechanism from the STAs to the AP so that the STAs can report the channel status, which they experience, to the AP. That is, it is possible that a subset of the STAs of a BSS, excluding the AP, may be in an overlapping region with a neighboring BSS, thus experiencing a lot of contentions from the STAs in the neighboring BSS. Moreover, the
feedback mechanism may incorporate a channel selection request by a STA, which experiences a bad channel condition. The AP will utilize the channel status information or channel selection request from STAs in order to determine when to begin a channel selection.
II. Request of channel measurements by AP: In order to select the best channel to run a BSS, the AP needs to know the status of other channels as well as the current channel. While the status of the current channel should be available to the AP in order to initiate a channel selection, the AP needs to collect the information about other channels. This will be done via the channel measurements of other STAs. While the channel measurement by the AP itself may be possible by disrupting the service of a BSS for a short time, we believe that it is not desirable since the AP should serve other STAs all the time. The first step in collection of the channel status information is to request the channel measurements to STAs. A new management frame is needed to be defined in order for the AP to request measurements at other channels to a set of STAs associated with its BSS when it decided to initiate a channel selection. This request can be either of unicast, multicast, and broadcast. The management frame will specify (1) when to begin measurements, (2) which channels to measure, (3) how long to measure, and (4) how to measure.
III. Channel measurement process: The measurement of a channel will be in two forms: (1) detection of other BSSs, (2) measurement of Received Signal Strength Indicator (RSSI). There are a number of primitive parameters for scan request, which should be specified by the AP when a channel measurement is requested. These include: ScanType: either active (the STA sends a probe frame and expects a response from a BSS) or passive (the STA simply listens to the channel, trying to detect some frames) scanning. ChannelList: a list of channels to examine.
MinChannelTime: the minimum time to spend on each channel.
MaxChannelTime: the maximum time to spend on each channel.
Knowing whether there are existing BSSs in specific channels is not good enough in order to determine the best channel to run a BSS. In the case of existing BSSs detected, knowing how close the STAs belonging to those BSSs is desirable while in case of no BSS detected, knowing the noise or interference level is desirable. When an 802.11 non-compliant system is up and running in a channel, the existence of such a system should be detectable not as a BSS, but as a co-channel interference. The 802.11 PHYs define a parameter named received signal strength indicator (RSSI),
which ranges from 0 through RSSI maximum. This parameter is a measured by the PHY sublayer and it is indicative of the energy observed at the antenna used to receive the current PLCP Protocol Data Unit (PPDU). RSSI is measured during the reception of the PLCP preamble.
The value of RSSI is reported to the local MAC entity as a parameter of the primitive PHY-RXSTART.indicate (RXVECTOR), which is an indication by the PHY sublayer to the local MAC entity that the PLCP has received a valid start frame delimiter (SFD) and PLCP Header. Based on this information, we expect to be able to use the RSSI in order to indicate how close the STAs of existing BSSs are from the channel measuring STA at least relatively.
IV. After the completion of a channel measurement, the STA, which was requested to measure the channel(s), should report the result to the AP.
The result will include all three parts of the measurements described in the previous subsection.
V. After hearing back from STAs, the decision whether to move out of the current channel or not should be made by the AP while the rule is implementation-dependent. The decision will involve the three things: (1) whether to move or not, (2) to which channel if decided to move, and (3) when to move. In order to determine whether to move, the AP will have to compare the status of other channels with that of the current channel in terms of STAs’ measuremnt reports. The channel switch instant may be affected by the status of sleeping STAs since it is desired to inform all the STAs within the BSS of the channel switch decision.
VI. Once the AP determines to switch the channel, it must announce it to every STA in the BSS. It may transmit a broadcast frame several times indicating when and to which channel the STAs should jump. We may want to define a new management frame for this announcement, or another possibility is to add this information into a beacon (by defining a new field in the beacon frame format). If a STA misses all these frames, which may happen due to not-so-common reasons, it will be suddenly
‘disconnected’ from the AP after the AP move into the new channel. Then, the STA will have to re-associate with the AP by scanning all the channels. This channel switch announcement may require the acknowledgement from some STAs, especially, STAs with real-time isochronous connections. Since a service disruption of such connections is highly undesirable, ensuring the channel switch decision reception by such STAs is recommended. In this case, we need to define a new control