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An Infrared Network for Mobile Computers
Norman Adams, Rich Gold, Bill N. Schilit, Michael M. Tso, and Roy Want
- System overview: ParcTab PDAs (128x64 pixel, very
impoverished CPU) communicate with picocell
infrared tranceivers, using pulse position modulation and
CSMA at a speed of either 9.6 Kb/s or 19.2 Kb/s. Tranceivers are
connected to gateway nodes over a serial line at 38.4 Kb/s;
gateways are connected to the rest of the world.
- Agents: Each ParcTab has a tab agent associated with it.
The tab agent is responsible for maintaining location information
for its ParcTab, and routing packets to and from the ParcTab.
Sun RPC is used to communicate between ParcTabs, tab agents, and
applications. The tab agent ensures at-most-once RPC semantics
to the ParcTab, but communications from the ParcTab is
unreliable. (Typically pen-events - onus on user to repeat.)
- T-RPC: T-RPC is the RPC mechanism used to deliver commands
("T-Cmds") to the ParcTab. Each T-RPC round-trip includes
an Ethernet, a serial line, and an IR transmission, but end-to-end
latency is less than 250ms. Reliable delivery to the ParcTab is
implemented at this layer, in order to simplify lower layers'
design. T-Cmds are typically bitmap transmissions and audio
events - the ParcTab is barely more than a terminal.
- Location information: The tab agents having knowledge of
their ParcTab's location implies that location specific contextual
information is available to applications.
The ParcTab system was one of the first systems built that implements some
of the Xerox PARC's ubiquitous computing philosophy. It also demonstrated
that PDA class devices can be useful and integrated with the rest of the
world, with the help of a proxy agent and intelligent applications.
- The proxy agent only provides routing and location information.
This implies applications must be fully ParcTab aware.
- Authentication and privacy issues were conspicuously absent.
- There was a lack of discussion on interesting applications for
this network and distributed system; such examples would reveal
the utility of their design.
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