[ih] More Topology, Packet Radio

Lawrence Stewart stewart at serissa.com
Tue Aug 31 05:56:34 PDT 2021

I can contribute a few bits of information about the Packet Radio Network.

In 1978 I designed the 1822 interface for the Xerox Alto.  It was used to connect to the Bay Area Packet Radio Network and for connecting PARC-MAXC2 to the Arpanet.

The Radios used an entirely different low level protocol than the IMPs.  It was called CAP, for Channel Access Protocol.  CAP was notable for a very small MTU - it had an 11 (16-bit) word header and up to 116 words of data.

PARC used the PRNet for a while to encapsulate PUP traffic between the PARC building and the Xerox Advanced Systems Devision (Ben Wegbreit and Charles Simonyi) building.

I wrote the CAP driver in Mesa, for connection to Hal Murray’s Mesa Gateway code.  It may still be around, in the files Paul McJones put up on the CHM servers at http://xeroxalto.computerhistory.org/Indigo/Alto-1822/.index.html <http://xeroxalto.computerhistory.org/Indigo/Alto-1822/.index.html>
The BCPL test software for the 1822 is definitely there. 

I don’t know what language the radio code used. It was written by Collins Radio and they had (from SRI accounts) a truly stone age attitude about it.  The master version was kept in a box of cards in the manager’s office.

I found the writeup of the Xerox work in IEN-78 at http://www.watersprings.org/pub/rfc/ien/ien78.pdf <http://www.watersprings.org/pub/rfc/ien/ien78.pdf>


I guess I am surprised by the comments here about the subleties of the 1822 distant host signaling.  I don’t think the Alto board had optoisolaters and it did work in both local and distant host modes, but was never tried with very long cables or ground problems.

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