Monday, March 24, 2008

802.11n for Enterprise

Enterprise equipment developers have done their research and are incorporating additional technologies into 802.11n equipment and by doing so should garner some impressive wireless network performance characteristics:

  1. The physical data rate selection algorithm (whew) has the tough job of determining what data transfer rate should be used based on the measured signal strength. 802.11a/g uses 12 steps from 1Mbps to 54Mbps. Whereas 802.11n has a total of 88 incremental data rate steps, which provides a more granular drop off when the signal strength weakens.
  2. 802.11a/g uses transmit diversity which is useful and logical as the device transmits from the antenna that displayed the best reception characteristics during the last receive cycle. 802.11n uses spatial multiplexing, a technique that divides the information to be transmitted into independent and separately encoded data signals called streams. Each data stream is then transmitted from an independent antenna. The 802.11n standard allows up to four transmit streams. This is an interesting concept that increases data transmission capacity by multiplexing or reusing the space dimension multiple times.
  3. In order to be able to transmit multiple data streams, designers had to rework OFDM, which is the digital multi-carrier modulation scheme used by 802.11a/g. MIMO-OFDM used by 802.11n devices is the results. MIMO-OFDM is viewed by many as the most singularly significant development of 802.11n.
  4. 802.11a/g networks do not work well in multipath environments as I discussed in an earlier post. Basically having multiple—slightly different in phase or timing due to the environment—copies of the same transmitted RF signal arrive at the receiving antenna, drives the receiver nuts to put it politely. Since the real world is mostly a multipath environment, 802.11n was developed to make use of the slight differences exhibited by the arriving RF signals to distinguish the different data streams being transmitted.
  5. Channel size is one determinant of how much data can be passed over a wireless link. The 802.11a/g standard uses 20 MHz channels and history has proven that amount of bandwidth to be a limiting factor. In the past few years equipment developers have tried to improve the physical transfer rate of data by using proprietary technology which combined adjacent channels to support greater data rates. The 802.11n standard describes how to use the much wider 40 MHz channels—that are easy to implement, cost effective, and only require moderate increases in digital signal processing. If properly implemented, 40-MHz channels can provide greater than two times the usable channel bandwidth.
  6. By design, TCP/IP traffic requires error-free transmission of data and one of the controls used to regulate the processing of traffic is the ACK bit. The ACK bit is sent by the receiver to acknowledge receipt of each frame, which turns out to be significant management overhead just for receipt verification. One way to improve throughput would be to devise a way to acknowledge the receipt but more efficiently. That is exactly what 802.11n does with the Block-ACK. By removing the need for one acknowledgment frame for every data frame, the amount of overhead required for the ACK frames, as well as preamble and framing, is reduced.
  7. No stone was unturned when the developers were looking at ways to improve throughput and efficiency. Even the lowly guard interval was tweaked. The guard interval is used to prevent data loss from propagation anomalies as well as interference created if the following transmission starts too soon. Can that interval be reduced? It would help throughput, even if just slightly. 802.11n specifies two guard intervals 400ns and 800ns. If optimal conditions exist the 802.11n device will drop down to the 400ns guard interval to reduce what is considered unnecessary idle time.
The 802.11n equipment developers are not satisfied with just these improvements. Smart antennas, multiple radios, and mesh technology are some heavy duty technologies that are being added to enterprise 802.11n appliances, which will allow 802.11n gear to evermore approach wired network parameters.

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