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Understanding IEEE 802.11n

The AX411 Access Point software provides support for IEEE standard 802.11n – Draft 2.0. This standard enables higher throughput, improved reliability, and improved range.

By default, the access point is configured to operate in a b/g/n mode which enables pre-n clients to associate with the access point. The n clients can also associate in this mode, where they can take advantage of the 802.11n enhancements. You can override the default mode of operation and tune various aspects of the 802.11n standard.

The following sections describe in more detail the various 802.11n-specific configurable options.

Radio Mode

The radio mode determines the type of wireless clients that can connect to the access point. The 802.11n access point supports 802.11b, 802.11g, 802.11a, and 802.11n clients. The radio can be configured to support only one type of client or to use a mixed mode in which different types of clients can connect to the radio.

The AX411 Access Point has two radios: radio 1 is set to operate at 5 GHz and radio 2 is set to operate at 2.4 GHz.

Radio 1 supports the following modes:

Radio 2 supports the following modes:

Channel Bandwidth

The 802.11n specification allows the use of a 40-MHz wide channel. This enables higher data rates to be achieved versus rates obtainable using the “legacy” channel bandwidth of 20 MHz. However, when using a wider channel bandwidth there are fewer channels available for use by other 2.4-GHz or 5-GHz devices. To restrict the use of the channel bandwidth to a 20-MHz channel, you can configure the channel bandwidth. This setting applies to either the 2.4-GHz or 5-GHz bands.

Some regulatory domains do not support the use of 40-MHz channel bandwidth. If the access point is operating in a regulatory domain that does not support a 40-MHz channel bandwidth, then this setting will not apply.

Primary Channel (40-MHz Channel Bandwidth Only)

A 40-MHz channel can be considered to consist of two 20-MHz channels that are contiguous in the frequency domain. These two 20-MHz channels are often referred to as the “primary” and “secondary” channels. The primary channel is used for n clients who only support 20-MHz channel bandwidth and legacy clients.

When the access point is configured to use 40-MHz channel bandwidth, you can specify the location of the primary channel—either the upper half or lower half of the 40-MHz channel. When the user selects a 40-MHz channel, the channel choice will always refer to the primary channel.

For example, if the 40-MHz channel is in the 5-GHz band and you have selected channel 36 and specified the primary channel as “upper,” then the primary channel would exist at channel 40, the secondary channel would exist at channel 36, and the center frequency of the 40-MHz channel would exist at channel 38.

Transmission Rates

The 802.11n specification defines a new set of transmission rates to enhance throughput for 802.11n wireless clients. These rates are defined as modulation and coding scheme (MCS) indexes. MCS indexes have different meanings depending on the size of the channel bandwidth in use. The access point software and hardware supports MCS indexes 0-15, and 32 which allows for a maximum transmission rate as high as 270 Mbps. Transmission rates are not configurable.

The access point software and hardware is capable of transmission rates as high as 54 Mbps for legacy devices. When the access point is configured to operate in a “mixed” radio mode (for example, 802.11b/g/n mode), the access point sends and receives frames based on the type of client. By default, the access point always selects the optimum rate for communicating with the client based on wireless network conditions. Note that in a mixed radio mode, you are still allowed to select supported and basic rates. The 802.11n clients are backward compatible with legacy transmission rates (a rate defined in an 802.11 standard prior to 802.11n) and can communicate using these legacy rates.

Default selected values for legacy basic rates are as follows:

Default selected values for legacy supported rates are as follows:

Protection

The 802.11n specification provides protection rules to guarantee that 802.11n transmissions do not cause interference with legacy stations or access points. By default, these protection mechanisms are enabled. However, you can turn off these protection mechanisms.

With protection enabled, protection mechanisms are invoked if legacy devices are within range of the access point. This causes more overhead on every transmission, which has an impact on performance. There is no impact on performance if there are no legacy devices within range of the access point.

Note: Care should be taken when turning protection off because legacy clients or access points within range can be affected by 802.11n transmissions. This setting does not affect a client’s ability to associate with the access point.

There are also protection mechanisms for 802.11g transmissions to provide similar, interference-free operation of legacy 802.11b clients and access points. When you configure the protection setting, both the 802.11n and 802.11g protection mechanisms are affected.

Guard Interval

An additional technique used to improve throughput in 802.11n transmissions is to shorten the guard interval. The guard interval is a time interval inserted between orthogonal frequency division multiplexing (OFDM) symbols in which no valid data is transmitted.

The purpose of this guard interval is to reduce intersymbol and intercarrier interference (ISI and ICI). The 802.11n specification allows for a reduction in this guard interval from 800 nanoseconds (defined in the 802.11a and 802.11g specifications) to 400 nanoseconds. This can yield a 10 percent improvement in data throughput. When you shorten the guard interval, the access point will transmit using a 400 nanoscond guard interval when communicating with clients that also support the short guard interval. This setting is only configurable when one of the 802.11n modes is selected.

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