Microwave Engineering Europe - March 2008 - (Page 33)

WIRELESS INFRASTRUCTURE 33 to be 1 Mbit per second (Mbps) before the added power losses begin to outweigh the gains from shortening the time on air. ANT uses a radio with a bandwidth of 1 Mbps while ZigBee employs one based on the IEEE802.15.4 standard with a bandwidth of 250 kbit per second (kbps). That means a ZigBee radio has to spend four times longer on air to send a set amount of data compared to ANT. However, the advantages of bandwidth can easily be negated by an inefﬁcient protocol. Power consumed by the radio when ‘on’ will have the biggest effect on overall power consumption because this will usually be an order of magnitude higher than the power consumed by the radio when ‘off’ (although this ﬁgure is important too due to the amount of time a radio will spend in this state). The problem is that the radio will diligently transmit what it’s told to transmit by the protocol and unless the data is packaged in a way that optimizes ‘off time per bit of data sent’, the proportion of the time the radio spends ‘on’ will rise signiﬁcantly. The challenge, therefore, is to design an efﬁcient protocol. This means using shorter packets per given payload. The efﬁciency of a protocol is measured by the ratio of overhead to payload. Typically ZigBee’s protocol efﬁciency is 20 percent (although it is possible to vary the payload length to improve efﬁciency). In comparison, ANT’s efﬁciency is 47 percent. Because of the interoperability requirement, ZigBee’s stack is comparatively large. While being less than a quarter the size of Bluetooth’s protocol it is an order of magnitude larger than ANT’s. This increases the external microcontroller burden, demanding microcontrollers that are more powerful and increasing system cost. ANT requires 2 kbyte of host microcontroller system resources compared to 100 kByte for ZigBee. (ANT’s requirement for external host resources is eliminated when using an ANT technology called SensRcore.) Many enterprising engineers have attempted to improve on ZigBee by taking the IEEE802.15.4 radio and creating their own protocol. This has led to a slew of proprietary protocols running on so-called “ZigBee” radios. (This is a misnomer as the protocol’s called ZigBee, not the radio.) Many of these proprietary solutions feature lower power consumption than a true ZigBee alternative. Yet in some ways they are the worst of all worlds. By using a proprietary protocol, a supplier loses one of ZigBee’s key assets, that of interoperability, and by using what is designed to be a “standard” radio platform they face engineering compromises at the physical layer, such as comparatively high transmission current and modest bandwidth. If a proprietary approach is to be successful, it must be demonstrably superior to the standards-based version to compensate for the loss of the advantage of interoperability and the conﬁdence that stems from a standardised, multi-vendor approach. This is the challenge that has long faced ANT with its highly optimized protocol and a proprietary 1 Mbps radio optimized for ultra-low power operation. ANT has proved Microwave Engineering ● March 2008 ● www.mwee.com 031_032-033_034_MWEE.indd 33 22/02/08 12:07:35 http://www.microsemi.com http://www.mwee.com

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Microwave Engineering Europe - March 2008 News Contents Comment Wireless Infrastructure: A Direct Conversion I/Q Demodulatordrives Favorable Basestation Cost-performance Metrics Wireless Infrastructure: Mobile World Set to Reshape the Internet RF Amplifiers: Latest Advances in RF Amplifiers Include a CMOS PA Operating at 77 GHz and Significant Advances in PAs for WiMAX and Broadband Applications Many Applications Still Require Unique Performance Benefits of BeO ACE Automated Circuit Extraction Returns to Real Design by Exploring Design Alternatives and Changes in Seconds Exceeding the Standard for Wireless Sensor Networks Products Calendar

Microwave Engineering Europe - March 2008

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