IEEE 802.11n

Demand for wireless LAN hardware experienced phenomenal growth in the years following the ratification of the IEEE 802.11a standard in the summer of 1999, evolving quickly from novelty into necessity. The initial expansion was driven primarily by users connecting notebook computers to networks at work and to the Internet at home, as well as at coffee shops, airports, hotels, and other mobile gathering places. However, by the time wireless chipset shipments had passed the 100 million unit mark in 2005 – a more than tenfold increase from 2001 shipments of less than 10 million units – the growing pervasiveness of Wi-Fi was pushing the technology beyond the PC and into consumer electronics applications like Internet telephony, music streaming, gaming and even photo viewing and in-home video transmission, and the existing Wi-Fi network infrastructure was beginning to feel the strain.

Fortunately this situation had been anticipated and the IEEE had approved the creation of the IEEE 802.11 Task Group N (802.11 TGn) during the second half of 2003, charged with the development of an amendment to the 802.11 standard whose minimum throughput requirement represented an approximate 4x leap in WLAN throughput performance compared to current 802.11a/g networks. The Task Group voted to unanimously approve the resultant draft 802.11n standard in January 2006. Though the standard is unlikely to be ratified before 2007, the specification is stable enough for draft-n Wi-Fi cards and routers to reach the marketplace during the course of 2006.

The draft 802.11n specification differs from its predecessors in that it provides for a variety of optional modes and configurations that dictate different maximum raw data rates. This enables the standard to provide baseline performance parameters for all 802.11n devices, while allowing manufacturers to enhance or tune capabilities to accommodate different applications and price points. With every possible option enabled, 802.11n could offer raw data rates up to 600 Mbit/s. However, WLAN hardware does not need to support every option to be compliant with the standard. For example, the expectation is that most draft-n WLAN hardware available during 2006 will be capable of supporting raw data rates up to 300 Mbit/s.

In the 802.11n draft, the first requirement is to support an OFDM implementation that improves upon the one employed in the 802.11a/g standards, using a higher maximum code rate and slightly wider bandwidth. This change improves the highest attainable raw data rate to 65 Mbit/s from 54 Mbit/s in the existing standards.

One of the most widely known components of the draft specification is known as Multiple Input Multiple Output, or MIMO. MIMO exploits a radio-wave phenomenon called multipath: transmitted information bounces off walls, doors, and other objects, reaching the receiving antenna multiple times via different routes and at slightly different times. Uncontrolled, multipath distorts the original signal, making it more difficult to decipher and degrading Wi-Fi performance. MIMO harnesses multipath with a technique known as space-division multiplexing. The transmitting WLAN device actually splits a data stream into multiple parts, called spatial streams, and transmits each spatial stream through separate antennas to corresponding antennas on the receiving end. The current 802.11n draft provides for up to four spatial streams, even though compliant hardware is not required to support that many.

Doubling the number of spatial streams from one to two effectively doubles the raw data rate. There are trade-offs, however, such as increased power consumption and, to a lesser extent, cost. The draft-n specification includes a MIMO power-save mode, which mitigates power consumption by using multiple paths only when communication would benefit from the additional performance. The MIMO powersave mode is a required feature in the draft-n specification.

Another optional mode in the 802.11n draft effectively doubles data rates by doubling the width of a WLAN communications channel from 20 MHz to 40 MHz. The primary trade-off here is fewer channels available for other devices. In the case of the 2.4GHz band, there is enough room for three non-overlapping 20MHz channels. Needless to say, a 40MHz channel does not leave much room for other devices to join the network or transmit in the same airspace. This means intelligent, dynamic management is critical to ensuring that the 40MHz channel option improves overall WLAN performance by balancing the high-bandwidth demands of some clients with the needs of other clients to remain connected to the network.

With all the optional modes and back-off alternatives, the array of possible combinations of features and corresponding data rates can be overwhelming. To be precise, the current 802.11n draft provides for 576 possible data rate configurations. In comparison, 802.11g provides for 12 possible data rates, while 802.11a and 802.11b specify eight and four, respectively.

The table below compares the primary IEEE 802.11 specifications:

802.11a 802.11b 802.11g 802.11n
Standard Approved July 1999 July 1999 June 2003 Not yet ratified
Maximum Data Rate 54 Mbit/s 11 Mbit/s 54 Mbit/s 600 Mbit/s
RF Band 5 GHz 2.4 GHz 2.4 GHz 2.4 GHz or 5 GHz
Number of Spatial Streams 1 1 1 1, 2, 3, or 4
Channel Width 20 MHz 20 MHz 20 MHz 20 MHz or 40 MHz

The draft 802.11n specification was crafted with the previous standards in mind to ensure compatibility with more than 200 million Wi-Fi devices currently in use. A draft-n access point will communicate with 802.11a devices on the 5GHz band as well as 802.11b and 802.11g hardware on the 2.4GHz frequencies. In addition to basic interoperability between devices, 802.11n provides for greater network efficiency in mixed mode over that offered by 802.11g.