Embedded Systems November 2000 Vol13_12

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I Higher Layers Logical Link Control (LLC) Media Access Control (MAC) : Home PNA PHY (1.0 PHY) Higher Layers Logical Link Control (LLC) Media Access Control (MAC) Home PNA PHY (1.0 PHY) "": Physical / SYNC Interval POTS Up DSL Downstream DSL I :.0: ::::; z z ~ 0 0 0 v; -~ Vl "' u "' Dest. l Source llength l 6 6 bytes bytes 2 Ethernet Data FICiURE 5 Shared Wireless Access Protocol (SWAP) frame·~~~~~~,l!~ CSMA/CA Isochronous Retransmission what devices are out there to share with (discovery) and th ey need to kn ow what typ e of info rmation is be ing transmitted (fo rmat) , and what is the inte nt of Lhe sending device (context). The last item can be somewhat involved . The trivia l case would be a single fun cti on devi ce, such as a printer, as a receiv- er. If some thing is sending data to a printe r, the obvious intent is to ge n- erate a print. But what about a multi- fun ctio nal devi ce such as a PC? When it receives data wha t should it do? Display? Store to di sk? Print to an attached printer? Obviously, the OSI refe rence but th ey still haven 't learned how to program th e clock on the VCR. Connecting devices to a home net- work can be no more complicated Lhan plugging in cables and operat- ing a remote control, otherwise they will not be widely accepted in the marketplace. lnteroperability Interoperability is the abili ty to con- nect two different devices from two diffe re nt manufacturers and have them perform th eir in tended fun c- tio n. Grandma can hook he r Panasonic VCR up to her ony televi- sion and they will wo rk togethe r because both are designed to meet Lhe NTSC a nalog video standard. Unfortunately, such inte roperabili ty hasn't been the case in digital net- works. Many of the current network- ing standards are based on the OSI seven-layer reference model as shown in Figure 2. Not only do devices need to be able share one and ze1·os across one or more medi a, th ey need to kn ow 68 NOVMEBER 2000 Embedded Systems Programming model has some holes Lhat need to be addressed for the home network. In order to provide true interoperabili ty, some of th e OSI layers take a more promine nt role in data exchanges than they may have in the past. In addition, some new fun ctionali ty needs to be suppli ed th at allows devi ces to ide ntify themselves and advertise the services they provide. A suggested modification to the OSI model for an interoperabili ty pro- tocol stack is shown in Figure 3. The stack has grown in complexi ty, but Lhat is the typical result of trying to make

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