Two frequency bands are available on WLANs: 2.4 GHz and 5 GHz.

The 2.4 GHz frequency band is the Industrial, Scientific, and Medical (ISM) open frequency band. Interference sources in the 2.4 GHz frequency band include cordless phones, baby monitors, microwave ovens, wireless cameras, bluetooth devices, infrared sensors, and fluorescent light ballasts.

Compared with 2.4 GHz frequency band, 5 GHz frequency band has fewer interference sources and more devices begin to use the 5 GHz frequency band, such as cordless phones, radars, wireless sensors, and digital satellites.

In most cases, microwave ovens work at the frequency band ranging from 2.4 to 2.5 GHz, which overlaps the 2.4 GHz frequency band used by WLAN devices. In addition, the power of microwave ovens ranges between 800 W and 2000 W, which is much higher than the transmit power of APs and STAs. Even though interference shielding is performed, microwave ovens still have severe interference on WLAN devices. Microwave ovens greatly reduce the throughput of WLAN devices if they are within a distance shorter than 8 meters around WLAN devices.

The power of cordless phones is about 3 W, which is higher than the AP's transmit power. According to the test analysis on the interference caused by cordless phones on WLAN devices, when the distance between cordless phones and APs (or STAs) is within 1 meter, interference increases significantly. When the distance is shorter than 0.5 meter, WLAN devices are even offline and the cordless phone voice is not clear. Therefore, you are advised to deploy cordless phones more than 2 meters away from APs or STAs.

The transmit power of wireless cameras ranges from 500 mW to 1000 mW. In indoor scenarios, wireless cameras may affect the WLAN network but have lighter interference than microwave ovens and cordless phones. Therefore, you are advised to deploy wireless cameras far away from WLAN devices during WLAN planning.

Bluetooth devices use the frequency hopping spread spectrum (FHSS) technology and 1 MHz channel bandwidth. If a bluetooth device is sending data at the frequency band overlapping with a WLAN channel that is being monitored by a WLAN device, the WLAN device selects a random backoff period. During this period, the bluetooth device changes to work at a non-overlapping channel, allowing the WLAN device to send data. Therefore, bluetooth devices have small interference on WLAN devices. This interference can be ignored during WLAN planning.

Wi-Fi is a trademark of the Wireless Local Area Networks Alliance (WLANA). It is actually not a standard and only ensures that products using this trademark can interoperate with each other. As most Wi-Fi products use the IEEE 802.11b standard, Wi-Fi usually refers to 802.11b. Wi-Fi is a new technology that uses the WLAN protocol.

Wi-Fi can provide wireless coverage in an area with a radius of up to 90 m (300 inches), while the WLAN can provide wireless coverage in an area with a radius 5 km (with antennas used). The biggest advantage of Wi-Fi is its high transmission speed (up to 11 Mbit/s). Wi-Fi is a short-distance wireless transmission technology applicable to offices and households.

• Multiple input multiple output (MIMO) is an antenna system that consists of M transit antennas and N receive antennas. The MIMO technology allows spaces to become the resources used to improve performance and increases the coverage range of the wireless system.

  The MIMO system generates multiple spatial flows with each antenna generating a maximum of one spatial flow. The single in single out (SISO) system sends or receives one spatial flow (one copy of signals) at a time. The MIMO technology allows multiple antennas to send and receive multiple spatial flows (multiple copies of signals) simultaneously and to differentiate the signals sent to or received from different spaces. An 802.11n device supports up to 4x4 MIMO, a maximum of four spatial flows, with a rate of up to 600 Mbit/s.

• The maximal ratio combining (MRC) technology improves the signal quality of the receive end.

  In MRC, the same signal from the transmit end is received by the receive end through multiple paths (multiple antennas) because the receive end receives this signal using multiple antennas. Generally, among multiple paths, there is one path providing better signal quality than the other paths. The receive end uses a certain algorithm to allocate different weights to receiving paths. For example, the receive end allocates the highest weight to the receiving path providing the best signal quality, which improves the signal quality of the receive end. When none of multiple receiving paths can provide better signal quality, the MRC technology can ensure better receive signals.

• The beamforming or Transmit Beam Forming (TxBF) technology produces the strong directional radiation pattern based on the strong correlation of the spatial channel and wave interference principle, making the main lobe of the radiation pattern adaptive to point to the wave direction. This technology improves the SNR, system capacity, and coverage range. Beamforming or TxBF is an optional feature in the 802.11n standard.

  Beamforming includes explicit beamforming and implicit beamforming. Explicit beamforming requires the receive end to send information about the received signal to an AP. The AP then adjusts the transmit power to the optimal value according to the signal information. This function increase the SNR of the receive end and improves the receiving capability. Implicit beamforming allows an AP to automatically adjust the transmit power to increase the SNR of the receive end based on channel parameters without requiring the receive end to work with the AP. Currently, mainstream terminals do not support beamforming.

• Space time block coding (STBC) transmits multiple copies of one data flow in wireless communication. STBC uses many antennas to produce multiple receive versions of data, improving data transmission reliability. Among these data copies, optimal copies are combined to provide most reliable data. This redundancy increases the chance of using one or more copies of received data to correctly decode the received data. STBC combines all the copies of received signals to produce the useful data.

• The MIMO technology provides the system with the spatial multiplexing gain and spatial diversity gain.

  In spatial multiplexing, multiple antennas are used on the received end and transmit end and multipath components in spatial communication is used, allowing signals to be transmitted over multiple data channels (MIMO sub-channels) in the same frequency band. This technology makes the channel capacity linearly increase with the growing number of antennas. This increase in channel capacity does not require additional bandwidth and does not consume additional transmit power. Therefore, spatial multiplexing is an efficient means to improve channel capacity and system capacity.

  In spatial multiplexing, serial-to-parallel conversion is performed on the transmitted signal to produce several parallel signal flows, which are than transmitted using their respective antennas in the same frequency band simultaneously. Due to the use of multipath propagation, each transmit antenna produces a unique spatial signal for the receive end. After the receive end receives the mixed signals of data, it differentiates these parallel data flows based on the fading between different spatial channels. Spatial multiplexing requires the spacing between transmit and receive antennas to be greater than the distance, ensuring that each sub-channel of the receive end is an independently fading channel.

The channel bandwidth in HT20 mode is 20 MHz, and the channel bandwidth in HT40 mode is 40 MHz. Two neighboring 20 MHz channels are bundled to form a 40 MHz channel. One channel functions as the main channel, and the other as the auxiliary channel. The main channel sends Beacon packets and data packets, and the auxiliary channel sends other packets. When the HT40 mode is used in the 2.4 GHz frequency band, there is only one non-overlapping channel. Therefore, you are not advised to use the HT40 mode in the 2.4 GHz frequency band.

Two neighboring 20 MHz channels can be bundled into a 40 MHz channel. If the center frequency of the main 20 MHz channel is higher than that of the auxiliary channel, 40MHz-minus is displayed; otherwise, 40MHz-plus is displayed. For example, if the center frequency 149 and the center frequency 153 reside on two 20 MHz channels, 149plus indicates that the two 20 MHz channels are bundled to form a 40 MHz channel.

Generally, the WLAN coverage range various according to the environment. When no external antenna is used, the WLAN coverage range is about 250 meters. In the semi-open space or the space with a compartment, the WLAN coverage range is about 35 to 50 meters. When external antennas are used, the WLAN coverage range increases with the antenna gain and is determined according to customer requirements. If an outdoor antenna and amplifier are used, the WLAN coverage range can reach up to several tens of kilometers.

SSID: service set identifier.

BSS: basic service set, an area covered by an AP. Each BSS is identified by a BSSID. The simplest WLAN contains only one BSS, and all STAs are in the BSS. To enable the STAs to communicate, disable STA isolation.

When the radio chip sends data in OFDM modulation mode, it divides a frame into different data blocks to send. To ensure data transmission reliability, the guard interval (GI) is used between the data blocks to ensure that the receive end correctly parses each data block. During spatial propagation, the delay will occur on wireless signals at the receive end because of multipath. If subsequent data blocks are transmitted fast, these data blocks will interfere with the original data block. The GI is used to avoid such interference. The common GI is 800 ns, whereas the short GI defined in the 802.11n standard is 400 ns, which increases the physical connection rate by 11%.

The direct sequence spread spectrum (DSSS) technology has advantages in high-reliability applications, whereas the frequency hopping spread spectrum (FHSS) technology has advantages in low-cost applications. Generally, DSSS fast transmits data in full-band mode, and allows for a higher transmission frequency in the future. The DSSS technology applies to a fixed environment or applications requiring high transmission quality. Therefore, DSSS wireless products are usually used in wireless plants, wireless hospitals, network communities, and campus networking. The FHSS technology applies to the endpoints requiring fast mobility. Because the FHSS transmission range is small, more FHSS devices than DSSS devices are required in the same transmission environment, which requires a high cost.

WLAN rate refers to the wireless rate of data transmissions between APs and STAs or between bridges and downstream nodes. Devices on both ends work in half-duplex mode, that is, they can only receive or send data at a time. The WLAN rate is the sum of upstream and downstream rates. Common users mainly use Internet access services to browse web pages, most of which is downstream traffic. In this case, the WLAN rate refers to the downstream rate.

802.11e defines Quality of Service (QoS) for the wireless LAN, which provides the required service quality for voice and multimedia applications and enhances network performance. Wi-Fi Multimedia (WMM) defines four access categories, including voice, video, best effort, and background to optimize network communication quality and ensure stable access of corresponding applications to network resources. The WMM standard is a subset of IEEE 802.11e.

MSDU is short for MAC service data unit.

MPDU is short for MAC protocol data unit.

In wireless network security, an MSDU is an Ethernet packet. After integrity check MIC, framing, packet buffering in PS mode, encryption, serial number assignment, CRC checksum, and MAC header are added to an MSDU, the MSDU becomes an MPDU. An MPDU is a data frame encapsulated using 802.11.

The A-MSDU technology aggregates multiple MSDUs into a large payload. Typically, when an AP or a STA receives an MSDU from the protocol stack, it tags the MSDU with an Ethernet header, called the A-MSDU subframe. The A-MSDU technology encapsulates multiple A-MSDU subframes into an MPDU, which is called an A_MPDU subframe, in accordance with the 802.11 protocol, as shown in the following figure:

The A-MPDU technology aggregates several A_MPDU subframes encapsulated in accordance with the 802.11 protocol. Sending several MPDUs at a time reduces the PLCP preamble and header required to send each 802.11 packet, increasing the system throughput, as shown in the following figure.