802.11n speeds wireless LANs
- 30 October, 2006 08:13
As a proposed wireless standard for high-throughput enhancements, 802.11n has been viewed primarily as a consumer technology. However, 802.11n has key applications applicable to the enterprise and is widely expected to drive the next generation of deployments.
Enterprise-class, bandwidth-intensive applications like ERP and CRM systems, workgroup computing applications, and some wireless backhaul applications require throughputs larger than current 802.11 technologies can provide. In addition to throughput, 802.11n will significantly enhance the reliability and range of existing 802.11 networks. The standard defines procedures by which throughputs greater than 100Mbps and significant range improvements also are possible.
Work began on the standard in late 2003, and earlier this year Draft 1.0 was published. The next version, Draft 2.0, is expected to be available in January 2007. The Wi-Fi Alliance, in response to market pressure, has changed its plan of record to certify prestandard 802.11n devices no later than first half of 2007. The IEEE expects to complete 802.11n in early 2008.
The market now is inundated with early implementations based on Draft 1.0, but enterprise customers would be wise to wait until Wi-Fi-certified products become widely available to avoid forklift upgrades. Existing 802.11n devices have severe interoperability concerns and have no guarantee of eventual interoperability with the ratified 802.11n.
802.11n calls for completely new hardware on clients (wireless LAN cards and adapters) and infrastructure (access points). In some cases, the high throughputs of 802.11n pose a significant scalability challenge for products that perform encryption and decryption on the wireless switch, requiring forklift upgrades.
Some of the important features that are included in current 802.11n draft are multiple-input multiple-output (MIMO), channel bonding and frame aggregation.
MIMO is the ability to transmit two or more unique radio streams simultaneously, delivering two or more times the data rate per channel. MIMO enhances spectral efficiency by using the same amount of channel width to derive significantly higher throughputs. In addition to spectral efficiency, MIMO mitigates multipath, a longstanding cause of 802.11 interference.
Multipath is a propagation phenomenon by which multiple radio signals reach receiving antennas by bouncing off of objects along the way. Traditional 802.11 networks degrade in the presence of multipath. 802.11n MIMO technology will use multipath constructively, dramatically improving indoor wireless performance and reliability.
Channel bonding is a controversial feature in the current 802.11n draft. Traditional 802.11 technologies use a 20MHz-wide channel to transmit and receive. However, 802.11n proposes a way to double to 40MHz the channel width used.
Channel bonding as defined in 802.11n Draft 1.0 doesn't work around traditional 802.11a / g traffic gracefully. This causes severe problems in the 2.4GHz spectrum, where there are only three effective channels. There is an industry effort underway to limit channel bonding technologies to only the 5GHz spectrum, where there is a wider array of channels.
802.11 has significant inefficiencies in channel acquisition and back-off delays. Sometimes more than 50% of the time is spent on the back-offs before transmission. Frame aggregation is being proposed as a way to alleviate these deficiencies. With frame aggregation, once a station acquires the medium for transmission, potentially long packets can be transmitted without significant delays between transmissions. Frame aggregation has been proposed at the levels of media access control and physical layer.
When deployed properly in the enterprise, 802.11n has the potential to increase significantly the overall throughput, range and reliability of the network.