Light Reading says the upcoming edition of Unstrung Insider notes that the two WiMax specifications (802.16-2004 and 802.16e), were originally supposed to be backwards compatible. But they're not.

802.16 Task Group E (802.16e) is charged with developing an amendment to the current 80.16-2004 standard that covers 'Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands.'

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"This implies 802.16e would be used for both fixed and mobile access – and according to several sources, dual fixed-mobile operation is still the goal," writes the report's author, Gabriel Brown, chief analyst for the Unstrung Insider.

But it's an open secret in the industry that the mobile specification will not initially interoperate with the fixed WiMax products.

The mobile standard will implement a new physical (PHY) layer on the chipset and enhancements to the media access control (MAC) to make it more suitable for mobile applications. "In effect then, there will be two types of incompatible WiMax – one that’s been widely implemented today for fixed use, and another that is still in draft form for mobile/portable," writes Brown.

Chipmaker Intel, one of the leading proponents of mobile WiMax, confirms that the two specifications will not work together.

"Yes, that's exactly right, they do use different PHYs," an Intel spokeswoman tells Unstrung. She describes the two specs as "separate but complimentary," saying that Intel envisages fixed WiMax being used as a system to backhaul data for mobile WiMax, when 802.16e products become available.