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Software change supercharges 802.11 chip

Posted: 12 Jan 2004 ?? ?Print Version ?Bookmark and Share

Keywords:software package? prism? nitro? 802.11g? wireless lan?

A new software package for use with the Prism Nitro 802.11g wireless LAN (WLAN) chip promises to substantially increase throughput, in one mode of particular interest to home media network applications, to as much as 140Mbps.

The software from parent Globespan/Virata exploits existing features in the Nitro chip, according to DAve Feller, Prism's director of WLAN products. This means that the fundamental media data rate, the modulation scheme and the 802.11 protection mechanisms remain untouched. The new software also leaves the device fully compatible with existing 802.11b or .11g equipment.

But the new code boosts throughput in several ways. First, it enables a packet-bursting mode in the Nitro chip that fits packets into what would otherwise be dead air between slow packets. This feature can improve throughput even with legacy 802.11 devices.

More ambitious techniques are used as well. The new code uses a packet concatenation scheme to create "super" packets for 802.11g clients, eliminating much of the inefficiency in sending what is basically stream data - such as audio or video - through the packet-based network.

For further throughput, the software enables a proprietary data compression scheme that, when communicating with another Prism Nitro chip, substantially increases effective throughput.

Feller estimated that these techniques together could give the Prism Nitro XM (for Xtreme Media) release effective throughput ranging from 70Mbps in the immediate vicinity of the access point to about 25Mbps at 200ft. That speed is approached by only the Atheros SuperG 3.3 device, according to Feller, which he said uses its own style of compression and concatenation algorithms, but with less throughput.

To reach the full 140Mbps claimed at the top line, Prism has turned to an altogether proprietary technique: the new software establishes a peer-to-peer link directly between clients when they are passing data from one client to another. This is in contrast to the normal 802.11 architecture in which the data would move from the sending client to the access point, and then to the receiving point. "In effect, this doubles the available data rate, because you aren't transmitting the same data twice," Feller said.

In this mode the software sets up the peer-to-peer link automatically and transparently to the user while maintaining a side-channel connection to the access point. This preserves the security and power-management features of the network while eliminating the need to repeat the data transmission.

Feller explained that this mode would be used, for example, in home network applications where a user might send data to a printer from a digital camera, or might transmit a video stream from a DVD player in one room to a display in another.

Typically, these links would be over shorter distances than the link from the devices back to the access point, so the data rate would improve from sheer proximity as well as from the existence of the direct link.

Prism also claims that the direct-link connections capability gives the Nitro-XM throughput in client-to-client transfers that would require 140Mbps from conventional 802.11 devices-hence the original claim. "This is a bout all that can be wrung out of 802.11g," Fellers said. "The next step in performance will have to wait for 802.11n."

- Ron Wilson

EE Times

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