Probably the most exciting development in wireless LANs is the arrival of the new high capacity 802.11n radio link. While it holds the promise of a five-fold increase in raw transmission capacity, migrating to the new “standard” will impact a number of important areas in the network design. I put standard in quotes because we still don’t have a real 802.11n standard; the IEEE 802.11n committee probably won’t ratify the current draft until sometime next year. In the meantime, the Wi-Fi Alliance has developed a certification plan for products built to the Draft 2.0 standard.
Normally we would caution enterprise users to steer clear of pre-standards, but the Wi-Fi Alliance has changed the rules of the game. Given the Alliance’s track record in getting interoperable products on the market, and their assurance that products built to the Draft 2.0 certification will interoperate with those based on the final standard when it is ratified, this looks to be a case where it’s safe to deploy products based on the Draft.
Me-Oh-My-Oh MIMO!
The major development in 802.11n that accounts for its boost in transmission capacity is the use of an optional Multiple Input-Multiple Output (MIMO) antenna system. The basic trick of a MIMO transmitter is that it can send multiple simultaneous radio transmissions, all of which occupy the same frequency channel; the 802.11n implementation can send up to four simultaneous signals, in which case one-quarter of the bits would be carried on each.
Normally if you have four transmitters all sending on the same channel, they will interfere with each other and nobody gets through. The trick in a MIMO system is that the transmitting antennas are placed some distance apart, a technique called spatial multiplexing. As the signals are originating from different points in space, the receiver will be able to distinguish each of them by its unique arrangement of multipath images (i.e. the original signal and the delayed echoes of that signal produced by its bouncing off obstacles in the environment). The amazing thing is that the transmitters do not have to be spaced that far apart to produce recognizably different images; the minimum spacing is one-half wavelength or about 3 inches for 2.4 GHz or 1.5 inches for 5 GHz transmitters.
As well as having multiple transmitters or outputs, a MIMO system can also have multiple receivers or inputs, and the energy of each transmit signal detected at each receiver is combined to improve reception; this capability also can be used to improve the performance of legacy a/b/g transmissions. The result is that a MIMO system can increase the transmission capacity through spatial multiplexing and increase the range and the reliability by combining the receive signals from each input antenna.
Besides the inherent advantage of MIMO transmission, 802.11n incorporates some other important features. First, 802.11n can operate in either the 2.4 GHz ISM or the 5 GHz U-NII bands. It can also use the standard 20-MHz channel employed in 802.11a or g, or a 40-MHz double channel. The maximum transmission rate is 289 Mbps in a 20-MHz channel, and 600 Mbps in a 40-MHz channel. None of the existing chipsets can generate four channels, so it is important to determine the maximum rate your devices can actually achieve. The standard also reduces some of the timer intervals and adds other efficiencies to the MAC protocol. Finally, the 802.11n devices can share channels with legacy a/b/g devices, though that will result in a significant reduction in throughput for the n-devices.
So Where’s The Rub?
Unfortunately, moving to 802.11n will involve more than just a straight equipment swap. While we’re only now getting our feet wet on 802.11n, here’s the first wave of issues we have uncovered.
That’s the easy stuff. The real decisions will center around how you plan and manage your wireless LAN network going forward. We noted that 802.11n can operate in either the 2.4 GHz or 5 GHz bands, so which will you use? Fundamental to that decision will be whether you intend to support legacy a/b/g devices on the same network with your n-devices.
The knee-jerk response from many consultants has been to leave your existing b/g devices where they are in the increasingly congested 2.4 GHz band, and use the 5 GHz band with its 23 available 20 MHz channels for n-devices. Unfortunately, many customers were looking at deploying WLAN voice services in the 5 GHz band using 802.11a handsets; we don’t expect 802.11n-capable voice handsets for a few years at least. Should we change our voice deployment plan to use 2.4 GHz for voice and migrate our legacy data devices to 802.11n in the 5 GHz band, or should we divide our 5 GHz channels between 802.11a voice devices and high-capacity 802.11n devices?
Conclusion
Wireless LANs have been one of the great technology success stories for the past decade, and that success has been driven by progressive advances in interoperable products. With the introduction of 802.11n, we are starting to feel the outside edges of the envelope, and can clearly see the limitations created by the requirement to maintain backwards compatibility with transmission rates that can go as low as 1 Mbps.
While public Hot Spot operators must continue to support every Wi-Fi device ever made, enterprise customers have greater flexibility in defining company-wide standards. I always tell my clients to set all of their access points to g-only and then hang around the Help Desk to see who calls; when someone calls, give them a 802.11g-card! We might wind up doing something similar with 802.11n, but if we plan the migration correctly, we shouldn’t have to cut our legacy users off at the knees.
This is a public forum. CMP Media and its affiliates are not responsible for and do not control what is posted herein. CMP Media makes no warranties or guarantees concerning any advice dispensed by its staff members or readers.
Community standards in this comment area do not permit hate language, excessive profanity, or other patently offensive language. Please be aware that all information posted to this comment area becomes the property of CMP Media LLC and may be edited and republished in print or electronic format as outlined in CMP Media's Terms of Service.
Important Note: This comment area is NOT intended for commercial messages or solicitations of business.
