Embedded Systems November 2000 Vol13_12

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i i = TABLE 2 Technology Jini JetSend HAVi Universal Plug and Play Open Services Gateway Initiative Upper-layer protocols Data Transport Re uirements IP over any media Any reliable transport IEEE 1394 Any media Any media lnteroperability Services Provided Discovery/ service lookup Data representation and negotiation Discovery/service identification/ service API Discovery/service identification/ service API Service API Related Standards Or -!Information Comment Requires Java Virtual Machine, and is in the works Developed by Hetwlett-Packard Driven by Sony, Grundig, Phillips, and other A/V companies Driven by Microsoft Java-dependent Somewhat to the consternation of FIGURE Jini communications model its two main competitors, Home1:tf has defined the Shared Wireless Access Protocol (SWAP). SWAP includes sup- port for Carrier Sense Multiple Access/ Collision Avoidance (CSMA/ CA) similar to the 802.11 standard for high speed data and a Time Division Multiple Access sti ll relatively new and there are inter- operabili ty problems with many of the implementations because the standard origin ally let developers choose between two different physi- cal layers. The interoperabi li ty issues have been recently addressed and 802.llb is starting to come down in cost, making it more attractive to home users. Blue tooth is the name of an indus- try consortium backed by Intel, IBM, and others. The goal of Bluetooth is to provide low cost (les than $10) "personal area networks" (or piconets). At its current 780Kbps, the data rate of Bluetooth is likely too low for most imaging applications, but adequate for file sharing and print- ing. Most troubling from a home net- working standpoint is the limi ted range of 10 meters. There is the abili- ty to extend the range to over 100 meters by upping the power levels of the transmitters, but this will have an impact on power con umption that significantly impacts th e mobil e device market-the prime target for Bluetooth. The HomeRF Working Group has the support of Hewlett-Packard, Intel, and others. It is based on a subset of 802. llb and provides a 2Mbps throughput. It also specifies the Shared Wireless Access Protocol (SWAP), which provides in teroperabil- ity with other devices and supports sev- eral voice channels as well as data. Each of the e technologies have certain features in common. They all operate in the unli ce nsed 2.4GHz ISM band, perform frequen- cy hopping (so they don't step all over each other si nce they operate in the same band) , and trace some of th eir heritage to the 802.llb specification. In addition, each of them specifies only the media access and lin k layer portion of the proto- col stack, so existin g upper layer protocols such as TCP / IP can run on top of them. 72 NOVEMBER 2ooo Embedded Systems Programming (TDMA) portion similar to the DECT standard to support voice and other streaming data. At a 50 hop per sec- ond rate, SWAP has a 20ms frame as shown in Figure 5. Bluetooth uses a much faster hop- ping rate (1,600 hops/ second) at its MAC layer and also provides the link layer functionality. In addition, Bluetooth provides an RFCOMM layer, similar to the IrDA's IrCOMM layer which provides a legacy serial interface. See Figure 6. Bluetooth allows ad hoc networks called piconets. The Logical Link Control portion of the stack provides fo r discovery of other nodes within range. This allows nodes to. form a point-to-point or point-to-multipoint connection with each other. The details of the services provided and the format and context of the trans- mitted data is left to the application using the Bluetooth stack. Although the portability and wire- less nature of RF is very attractive, none of the RF solutions curren tly provides the bandwidth required for high performance imaging or multi- . media systems. These solutions will

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