Can 802.11n devices work only in 802.11n mode? Can 802.11n devices work in 802.11bg mode

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802.11n devices can work in multiple modes including 802.11bg mode.

Other related questions:
Can a laptop equipped with an 802.11b/g NIC associate with an 802.11n AP
If an 802.11n AP is not configured to work in pure mode (for example, the only mode), the laptop can associate with the AP. However, 802.11b STAs transmit packets at a low rate. The time taken by an 802.11n STA to transmit a 1000-byte packet at a rate of 300 Mbit/s is 1/27 of the time taken by an 802.11b STA at a rate of 11 Mbit/s. If many 802.11b/g STAs connect to an 802.11n AP, AP performance deteriorates. To ensure performance of the entire WLAN, 802.11n APs deny access of 802.11b STAs.

What is the working process of 802.11n short GI
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%.

802.11n and its configuration
802.11n and its configuration. As the core of the 802.11n physical layer, multiple-input multiple-output (MIMO) technology enables 802.11n to send multiple radio signals during wireless transmission and form multiple spatial streams using multipath effect. Therefore, the data transmission speed is greatly increased. In addition, MIMO helps 802.11n obtain diversity gain and multiplexing gain, effectively extending the coverage distance and improving the transmission rate. The 802.11n standard defines one to four spatial streams for MIMO technology. For example, two spatial streams improve the transmission rate by two times, and four spatial streams by four times, reaching 600 Mbit/s. The number of spatial streams varies on different products. 802.11n can operate on both 2.4 GHz and 5 GHz frequency bands. Delay will occur at the receiving end when wireless signals are transmitted in space because of multipath. If subsequent data blocks are transmitted fast, these data blocks will interfere with original ones. The general inverter (GI) is used to avoid such interference. The common GI is 800 us, while the short GI defined in the 802.11n standard is 400 us, which increases the physical connection rate by 11%. Currently, the mainstream AP products are all 802.11n-compliant. Configuration: [HUAWEI]wlan [HUAWEI-WLAN-view]radio-profile name test [HUAWEI-WLAN-radio-prof-test]channel-mode fixed //Manually configure the channel. [HUAWEI-WLAN-radio-prof-test]80211n guard-interval-mode short //Change the mode to short preamble to increase rate [HUAWEI-WLAN-radio-prof-test]undo legacy-station enable //Reject access of traditional 802.11a/b/g STAs (supported in V200R005). [HUAWEI-WLAN-radio-prof-test]quit [HUAWEI-WLAN-view]ap xx radio 1 //Enter the AP view. Or, [HUAWEI]interface wlan-radio 0/0/1 //Enter the radio interface for Fat APs. [HUAWEI-WLAN-radio-0/0/1]channel 40mhz xx xx can be set to 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, or 161. Note: 1. You are not advised to configure the 40 MHz mode on the 2.4 GHz frequency band. 2. The security profile must be configured as WPA2 + CCMP, because WEP is incompatible with 802.11n, and TKIP may affect the user access rate.

Why did STAs fail to search 802.11n signals after the AP is enabled with 802.11n
This is because the IEEE 802.11n draft protocol does not support a high throughput rate of the WEP or TKIP unicast ciphers. If a STA uses the WEP or WPA-TKIP encryption mode, the data transmission rate will decrease to 54 Mbit/s. In WEP or TKIP mode, if the STA uses the 802.11b/g/n NIC to associate with APs, it can only be associated with 802.11g APs. As a result, the displayed signal transmission rate is 54 Mbit/s but not the 802.11n rate. This is restricted by the IEEE 802.11 standard.

Why does a STA still receive 802.11b/g signals after 802.11n is enabled on an AP
According to IEEE 802.11n drafts, data cannot be transmitted at a full rate when passwords are unicast in WEP or TKIP encryption mode. If these encryption modes (such as WEP and WPA-TKIP) are used, the maximum data transmission rate is only 54 Mbit/s. Similarly, in TKIP or WEP encryption mode, an 802.11 b/g/n NIC can only associate with an AP in 802.11g mode. Therefore, the signal rate is 54 Mbit/s instead of the rate in the 802.11n protocol.

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