Aruba warns against 802.11n deployments

• Tags:
• Consumption,
• Lan,
• Frequency,
• Drivers

David Meyer ZDNet.co.uk

Published: 18 Oct 2007 10:47 BST

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Wireless LAN vendor Aruba Networks has warned enterprises against deploying the latest generation of Wi-Fi equipment until a number of issues have been ironed out.

Many vendors are producing wireless LAN equipment based on the draft 802.11n standard, which offers, in theory, 300Mbps throughput: a sixfold increase on current rates. Aruba currently offers no 802.11n equipment, but says it plans to in the future.

Roger Hockaday, Aruba's head of marketing for EMEA, told delegates at London's IP'07 exhibition on Wednesday that 802.11n would prove a "headache" for IT departments in its current form.

802.11n has not yet been officially ratified as a standard by the IEEE. Standardisation may not happen until as late as 2009. This delay led the Wi-Fi Alliance — a consortium of manufacturers — to start certifying equipment based on a draft version of the standard earlier this year, albeit with some misgivings about its readiness for enterprise deployment.

"There was a lot of pressure from companies who perhaps are not mainstream vendors to push people into making the decision to switch to 802.11n now," Hockaday told ZDNet.co.uk at Wednesday's event, although he conceded that the draft standard is at least ensuring interoperability between different manufacturers' products. "It is really important to set the reality of the situation. So many things are more important than the bandwidth supplied."

Nullified advantages
802.11n accomplishes higher throughput because it uses three antennae and has increased bandwidth and more advanced modulation techniques. It can also use the less crowded 5GHz band, while 802.11b/g can only use 2.4GHz.

Hockaday claimed that, because of its use of two data streams rather than one, the new design would turn some negative factors — such as a person or object getting in the way of the transmitted signal — into advantages.

"With 802.11n you have at least two data streams. Therefore if there is a pipe or a person in the way, the more reflections you get and the more likely it is that the two streams will diverge," Hockaday told delegates. "The further away you are, the more complex the radio frequency environment is, the more reflections there are — chances are the throughput will be greater."

This enhanced throughput means that, although an 802.11n access point (AP) may be more expensive than earlier versions, it will be able to service 30-40 users rather than 10-20. However, said Hockaday, the capacity advantages are nullified if some employees are using equipment which is not 802.11n compliant.

Equipment based on the 802.11b and 802.11g standards uses three non-overlapping 20MHz-wide channels in the 2.4GHz range. However, 802.11n equipment will only be able to supply one 2.4GHz channel if it is configured to use 40MHz of bandwidth. "You may find that, if you are going to spend £1,000 on an AP, you are not going to want it to run at the same speed as your legacy equipment," said Hockaday, who therefore recommended that 802.11n equipment is run only at 5GHz.

Power issues
Hockaday claimed that the increased power demands and three antennae necessary for 802.11n would prove problematic for mobile devices.

Hockaday added that power consumption would also be a major issue for local area networks. Many businesses use power over Ethernet-enabled switches to support wireless access points.

"You need two radios in the AP — one for 802.11b/g and one for 802.11n," Hockaday said. "This is going to burn 20W, and switches will only support about 13W. Second-generation APs [those currently going to market] will take about 13W with two radios, but the temperature on the cable will increase, so resistance will increase, so you will have greater power loss. There really is an issue about power consumption." He went on to predict that there would be no significant improvement to 802.11n's power problems until about 2009.

Network interference
Hockaday also claimed that initial 802.11n chips did not support dynamic frequency selection (DFS), which means that transmission could be disrupted by a range of factors, including military traffic.

Like AirMagnet's chief technology officer, Chia Chee Kuan, Hockaday suggested there could be a problem with interference with 802.11a networks, which also use the 5GHz spectrum band. "You can only run 802.11n in high-throughput mode if there are no other networks nearby," he said, suggesting that this could realistically provide a throughput of around 135Mbps. However, he added, the only environment in which 802.11n should currently be deployed in its high-throughput mode is with "a field around you".

"The problem comes when you are supporting other clients," said Hockaday. "You can go down to the interoperability modes but you won't get even 135Mbps. The high-throughput mixed format [mode] runs on 40MHz-wide channels to the end clients but... you can only send a subset of traffic at 40MHz. You are not even going to get half the throughput."

Hockaday even questioned the current demand for 802.11n's throughput, claiming that "for the majority of office applications, we are not going to use the bandwidth provided to us by 802.11n".

Hockaday also claimed that the drivers being used in current laptops that support 802.11n were not yet up to scratch. "I wouldn't like to be in the support department supporting the 802.11n clients in laptops today. 802.11n is very new and those drivers sitting inside the clients are not going to be very mature for quite some time."

The solution for those wishing to deploy 802.11n in the next year or two, Hockaday suggested, would be either to build a separate 802.11n overlay network and retain an 802.11b/g infrastructure, or to gradually phase the new network in as clients demand it.

Some vendors, such as Motorola, are shying away from supplying 802.11n-based enterprise equipment until its ratification by the IEEE, while Cisco and Colubris have started selling such equipment.

Dell has defended the nascent standard. Chief technology officer Liam Quinn said Dell was "very comfortable that the standard is stable and pretty solid".

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