September 6th, 2010
802.11n For Dummies
IEEE 802.11n-2009 is an amendment to the IEEE 802.11-2007 wireless networking standard to improve network throughput over the two previous standards — 802.11a and 802.11g — with a significant increase in the maximum raw data rate from 54 Mbit/s to 600 Mbit/s with the use of four spatial streams at a channel width of 40 MHz. 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 to the 2.4 GHz band.
A quick summary of 802.11n is below.
802.11n supports both 2.4 and 5 GHz modes and is backward compatible with all legacy 802.11a, b, or g devices. Just make sure to look for the Wi-Fi logo. Slightly modified logos exist that have tag lines like “with some n features”, “dual-stream n”, and “multi-stream n” providing additional tested capabilities of devices.
The main goals of 802.11n is to improve range and throughput. In most cases a single access point will provide coverage for an entire home at wired Ethernet speeds (100 Mbps). Data rates up to 600 Mbit/s are achieved only with the maximum of four spatial streams using a 40 MHz-wide channel. Various modulation schemes and coding rates are defined by the standard and are represented by a Modulation and Coding Scheme (MCS) index value.
802.11n hardware electronics are better than previous generation equipment and results in improved performance for 802.11n and legacy devices. New hardware includes digital signal processing (DSP) that provides the highest throughput possible in all RF conditions. The transmitter and receiver use precoding and postcoding techniques, respectively, to achieve the capacity of a MIMO link.
802.11n can combine two 20 MHz channels in 2.4 and 5.0 GHz bands. This is usually called channel-bonding and results in higher throughput. The specification calls for requiring one primary 20 MHz channel as well as a secondary adjacent channel spaced ±20 MHz away. The primary channel is used for communications with clients incapable of 40 MHz mode.
802.11n uses MAC aggregation mode. By increasing the amount of frames that carry user data vs. management data 802.11n increases throughput and efficiency. Because of 802.11 protocol overheads, like the contention process, interframe spacing, PHY level headers (Preamble + PLCP) and acknowledgment frames throughput can suffer for applications like VoIP and other real-time apps.
Support for multiple data (or “spatial”) streams to achieve higher throughput. Each spatial stream requires a discrete antenna at both the transmitter and the receiver – multiple-input multiple-output (MIMO). MIMO technology requires a separate radio frequency chain and analog-to-digital converter for each MIMO antenna which translates to higher implementation costs compared to non-MIMO systems.
Support for multiple transmit and receive antennas to increase range. The number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link. The “a x b:c” notation helps identify what a given radio is capable of. The first number (a) is the maximum number of transmit antennas or RF chains that can be used by the radio. The second number (b) is the maximum number of receive antennas or RF chains that can be used by the radio. The third number (c) is the maximum number of data spatial streams the radio can use. A radio that can transmit on two antennas and receive on three, but can only send or receive two data streams would be 2×3:2.
[source]
For more technical details see http://www.cs.washington.edu/homes/dhalperi/pubs/mimo_for_dummies.pdf
Related posts:
- WiFi For Dummies
- Wireless Network Performance – Channel Planning
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Filed under: WLAN Design, WLAN Standards
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