IEEE 802.11: n

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802.11n

IEEE 802.11n was an amendment to IEEE 802.11-2007 as amended by IEEE 802.11k-2008, IEEE 802.11r-2008, IEEE 802.11y-2008, and IEEE 802.11w-2009. It builds on three IEEE.11 standards. They are the multiple-input multiple-output (MIMO), a 40 MHz channel in the physical layer (PHY), and frame aggregation to the MAC layer.

Frame aggregation is a feature of IEEE 802.11e and 802.11n wireless LAN standards that increases throughput by sending two or more data frame in a single transmission.

MIMO is a method of using multiple antennas to coherently resolve more information than is possible with a single antenna which increases data rate. One way it does this is through use of Spatial Division Multiplexing (SDM), which spatially multiplexes multiple independent data streams. This SDM transfers multiple independent data streams simultaneously within one spectral channel of bandwidth.

MIMO technology requires a separate radio-frequency chain and analog-to-digital converter for MIMO antenna making it more expensive than non-MIMO systems.

Its purpose is to improve network throughput over the previous two standards (802.11a and 802.11g) with a significant increase in the maximum net data rate from 54 Mbit/s to 600 Mbit/s with use of four spatial stream at a channel width of 40 MHz. 802.11n standardized support for multiple-input multiple-output and frame aggregation and security improvements among other features. It can be used in the 2.4 and 5 GHz frequency bands.

Number of Antennas

The number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link. The 802.11n draft allows up to 4 x 4 : 4. This means the radio can transmit on four antennas and receive on four, and it can send and receive 4 data streams.

Deployment Strategies

To achieve maximum output, a pure 802.11n 5 GHz network is recommended. The 5 GHz band has substantial capacity due to many non-overlapping radio channels and less radio interference as compared with the 2.4 GHz band. An 802.11n-only network may be impractical since many users need support for legacy equipment such as 802.11b/g. When using a mixed-mode system, an optimal solution would be to use a dual-radio access point and place the 802.11b/g traffic on the 2.4 GHz radio and the 802.11n traffic on the 5 GHz radio. This setup assumes that all the 802.11n clients are 5 GHz capable, which isn't a requirement of the standard. Quite a few wifi-capable devices only support the 2.4 GHz, such as iPhone 4s, and there is no practical way to upgrade them to support 5 GHz. A technique called "band steering" is used by some enterprise-grade APs to send 802.11n clients to the 5 GHz band, leaving the 2.4 GHz band for legacy clients. Band steering works by responding only to 5 GHz association requests and not the 2.4 GHz requests from dual-band clients.

40 MHz in 2.4 GHz

The 2.4 GHz ISM band is fairly congested. 802.11n provides an option to double the bandwidth per channel to 40 MHz which results in slightly more than double the data rate. When in 2.4 GHz, enabling this option takes up to 82% of the unlicensed band, which in many areas may prove to be infeasible.

The specification calls for requiring one primary 20 MHz channel as well as a secondary adjacent channel spaced + or - 20 MHz away. The primary channel is used for communications with clients incapable of 40 MHz mode. When in 40 MHz mode, the center frequency is actually the mean of the primary and secondary channels.

Local regulations may restrict certain channels from operation. For example, channels 12 and 13 are normally unavailable for use as either a primary or secondary channel in North America.

802.11n Goals

(1) Up to 600 Mbps data rate

(2) MIMO (Multiple input, multiple output antennas)

(3) Channel bonding

(4) High Throughput (HT) options

(5) Maintain backward compatibility with existing 802.11a/b/g

802.11n Improvements

(1) Shorter Guard Interval (GI) - reduced from 800ns to 400ns

Short Guard Interval

(2) Channel bonding

(3) Spatial Multiplexing - at least 2 spatial streams and up to 4 (MIMO), each stream arrives at the receiver with different amplitude (signal strength) and phase

(4) Modified OFDM - 52 sub-carriers (instead of 48 used in previous versions)

(5) Improved forward error correction - 5/6 coding rate instead of the old 3/4 rate

(6) New Modulation Coding Scheme (MCS) - There are 77 possible modulation schemes instead of the 12 available in 802.11b/g. Some of these are compulsory and some are optional.

MCS selects the following based on RF channel conditions

(a) 8 data rates

(b) bonded channels

(c) multiple spatial streams

(d) different guard intervals

(e) modulation types

(7) Frame Aggregation Mechanisms

(a) Aggregate - MAC Service Data Unit (A-MSDC) - wraps multiple Ethernet frames in an 802.11 frame up to 8KB, performed in software

A-MSDU

(b) Aggregate - MAC Protocol Data Unit (A-MPDC) - allows bursting 802.11 frames up to 64KB, performed in hardware

A-MPDU

(8) Block-Acknowledge Mechanism - confirm a burst of up to 64 frames with a single Block ACK (BA) frame or combined acknowledgements can be requested by sending a Block ACK Request (BAR)

(9) Backwards Compatibility - There are three modes

(a) Legacy - b/g compatibility with "Clear-to_send" to self

Legacy Mode

(b) Mixed - a/g compatibility with legacy header

Mixed Mode

(c) Greenfield - No backwards compatibility

Greenfield Mode

Note: 5-15% of the throughput is reduced by the WPA2 encryption