Cost of 802.11n
Greater Link Reliability and Predictability
Although not as widely discussed as its throughput enhancements, the 802.11n standard's ability to provide improvements in link reliability and predictability is compelling and substantially enhances application performance. With more mission-critical applications riding on the wireless network than ever before, all enterprises have an interest in ensuring the availability and reliability of their wireless connections.
This is an area where 802.11n can truly offer breakthroughs in wireless networking. 802.11n uses multiple-input multiple-output (MIMO) signal processing that relies on multiple antennas and receivers to improve the reliability of the wireless link, decreasing the likelihood that packets are dropped or lost. The reduction of dropped packets improves the link reliability, and a more consistent throughput helps ensure predictable coverage at any point in the facility. This enhanced reliability and predictability extends to existing 802.11a/b/g clients in addition to emerging 802.11n clients, delivering substantial benefits regardless of the type of devices or the speed with which the business expects to conduct a device refresh. Furthermore, the advantages of MIMO extend to both the client and the access point. Because of its ability to negate the ill effects that building materials, free-space path loss, and multipath propagation all have on the radio frequency, the introduction of MIMO represents a fundamental shift in not just WLAN design, but more importantly in user expectations for performance.
Perhaps the most talked-about improvement made possible through 802.11n is its ability to increase the throughput on a wireless network. 802.11n has the potential to offer up to five times the performance of current wireless networks. For pure 802.11n environments, testing has shown performance enhancements that deliver transmission rates of up to 300 Mbps of bandwidth per radio. Dual-band radios, operating in both 2.4-GHz and 5-GHz bands, can deliver an aggregate of 600 Mbps. In a typical deployment, businesses will see a noticeable increase in the amount of bandwidth available per client. Testing has shown a single client could experience an increase of three to five times in the average amount of dedicated bandwidth. Features including 40-MHz channels, packet aggregation, and block acknowledgement deliver the throughput enhancements of 802.11n. Additionally, the improved signal resulting from MIMO enables clients to connect at faster data rates at a given distance from the access point compared with 802.11a/b/g.
The IEEE expects to complete 802.11n in early 2008.
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/b/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