An Infrared Network for Mobile Computers

Norman Adams, Rich Gold, Bill N. Schilit, Michael M. Tso, and Roy Want

One-line summary:

Overview/Main Points

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.

Relevance

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.

Flaws

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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