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                   Inter-Office Memorandum

        To         Fairbairn, Kay, Ingalls, Goldberg, Shoch     Date            November 8, 1979


        From       Larry Tesler                                 Location        Palo Alto


        Subject    Ethernet Plans                               Organization    PARC/SSL


XEROX

        Hardware

        Bruce and I have verified empirically that the 8086 can keep up with the burst of data at the
        beginning of an Ethernet packet. This includes WAITing for a packet to start, checking the
        destination host number for "broadcast" or "me", and storing the whole packet into main memory.
        \Ve had to use a software FIFO; that is, we interspersed ~AOVW instructions into the pre-REP
        instruction stream to keep ahead of the hardware. We may have been too conservative, so the
        FIFO may be able to go away in the real thing. (Note: The tests Doug and Dan did were invalid
        because they ANDed "broadcast" to "me" instead of ~Ring; they stored into local memory instead
        of into main memory; and they had other minor bugs. Does anyone want to 'critique our tests?)

        I have thought of one improvement to the board. If, after turning on the receiver, the software
        could have the option, instead of going into a WAIT, to just get interrupted at the next start-of-
        packet; and if the first word of the packet were kept in the buffer until it was read by the 8086
        (even in the face of other stuff from the wire pounding on it to move over); then, although we
        would surely miss the rest of the packet; we could discover that someone was trying to tell us
        something and we could then go into a real listen loop hoping for a retransmissioll. \Vith t..~is
        capability, we could use the 8086 for other purposes when the host machine had no intention to use
        the Ethernet, but still respond to unexpected requests from other hosts.

        \Ve have agreed that the Notetaker I Ethernet board should be readied for stitchweld now, but not
        actually stitchwelded until PUP Level 0 software is essentially completed. 'The latter will probably
        happen around the end of the year. By doing a N otetaker I Ethernet board (or several), we hope to
        get early experience that should greatly benefit both the Notetaker II Ethernet design and all future
        Notctaker Ethernet software.

        Software

        Jim Althoff has agreed to start writing Notetaker Ethernet software December 1. He will work
        almost half-time for several months implementing board diagnostics, PUP Level 0, PUP Levell,
        and finally some higher level protocols. We expect to discover hardware problems in the proccss.
        Jim will report them to Doug and Ron promptly. Jim has other network and process scheduler
        projects to keep him busy while he is waiting for any hardware fixes to be done. (W c have takcn
        Jim off the BSP project and have agreed that the BSP, though desired ASAP, is not critically
        needcd until June, and that Bruce can handle the Dorado implementation without Jim's help.)

        We hope that enough will be known about the use of the board by March 1 that we can confidently
        recommend an architecturc for the Notetaker II Ethernet Among the issues to explore are: How
        rcliable is the analog phase decoder? ,\Vere the synchronizatiion and interrupt schemes successful?
        How bad is main memory contention? What processor will do what job (e.g., who checksums)?
        Can the Ether 8086 do work of other kinds when the Ether is not in use? Can we run "full-
        duplex" in software?

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