🚀 go-pugleaf

scogen@xenitec.on.ca (scogen@xenitec.on.ca) writes:

> We are trying to configure an SCO Unix system running some type of
> IP, (maybe TCP, maybe UPD) to operate at maximum throughput. Our goal
> is to get up to _6_ Megabits/Sec.

Well, nearly any modern Ethernet card will deliver 6 Megabits per second.
After all it's only around 600KB/s. SCO's TCP protocol stack will most
probably not be able to keep up with the card though.

> We realize that we need EISA bus machines with 32-bit Ethernet cards to do
> this.

Why ever? The 16 bit ISA bus runs at 8Mhz and transfers 16 bit across.
Taking account of all hw protocol overheads it can sustain between 5-6 MB per
second. Here you are proposing to load it with just 600KB/s, which is one
tenth of its top capacity. Think of this: my VGA card will, during continuous
graphics operation, sustain 2MB/s over the ISA bus, and my SCSI controller
will get 1.4MB/s sustained from a SCSI disk.

32 bit EISA Ethernet cards are an expensive joke, as is any "turbo" interface
between a 40MB/s 32 bit bus (EISA) and a 0.8MB/s 1 bit one (Ethernet).

    Same goes for EISA SCSI disk controllers (the most expensive SCSI disks
    top out at 4MB/s, and most cannot do more more than 1.5MB/s off the
    platter). In practice EISA is only needed for very high speed graphics
    (good luck with that) and for disk arrays, or, theoretically, for very
    high bandwidth tapes.

    The real benefits of EISA, for most people, are not in bandwidth, but
    in configurability and interrupt sharing and multiple mastering, where
    the ISA bus is badly misdesigned.

> Another important detail is that each computer will be talking over a
> Cisco router with one ethernet card per computer.

Note that you need an Ethernet card per computer anyhow :-).

Probably what you mean is that you will have a separate Ethernet cable per
computer. This of course means that each machine would have its own 0.8MB/s
Ethernet dedicated to itself. The problem is that I doubt very much that
this would buy you anything at all, as I would expect the CISCO router not
to be able to sustain more than 0.8MB/s aggregate anyhow, as routers are
normally not designed to sustain the maximum combined bandwidth of all the
Ethernets they link. Does anybody know for sure what is the maximum thruput
of a CISCO router?

I expect that in practice having a star configuration with a router in the
middle and an Ethernet to each machine will probably buy you nothing at all,
except having a single active failure point.

> This way we completely eliminate packet collisions.

This is probably completely unnecessary. Collisions are very rarely a
problem, the Ethernet will in practice share out its capacity between
machines in only slightly less than linear fashion, if packet sizes are not
too small.

If you really want for some funny reason a star configuration, with an
active hub, and no collisions and guaranteed point-to-point capacity,
consider seriously getting a Token Ring.  At least Token Ring active hubs
are designed for this task, and not as routers, and tend to be fairly fail
safe too.

> They also told us about their ES3210 card, which they claim can
> operate at _13_ Megabits/Sec. I thought that the cable can't operate
> faster than 10 Megabits/Sec. Am I missing something?

Well, they are telling you nice numbers out of thin air.

> How do I get real numbers that will tell me what to expect in the field?

Toss a coin... :-)

> I don't know how obvious my lack of network expertise is,

Very obvious. :-). But at least you know. Some other poster who also replied
to your article is not in a much better position than yourself, but does not
know. :-)

> but if you see any questions that I'm not asking, I need to know that
> also. I'm sure I'm not asking everything I need to know.

Well, now that I have answered your questions, let's tell you something more
useful.

First of all, the good news: as I have said almost any modern Ethernet card
will deliver substantially all of the Ethernet bandwidth, and this bandwidth
is at worst one fifth of the ISA bus bandwidth, so you don't need any EISA
machine.  Collisions will be in practice not a problem. You probably don't
need a CISCO router at all, you can put all the machines on the same
Ethernet, if you don't need 6Mb/s sustained bandwidth between each machine,
but only aggregate.

The bad news are that the TCP/UDP/IP sw on the host, and in particular
SCO's, will have such huge overheads and misdesigns that in practice you
will not be able to use more than a fraction of the nominal speed of the
hardware. Somebody (Van Jacobson and others) have spent some serious effort
in tuning up certain implementations of TCP/UDP/IP and can boast of being
able of getting TCP/UDP/IP end-to-end connections that exploit 90-95% of the
hardware capacity, but figures like 30-50% are far more common. I don't
remember exactly, but I seem to remember that you'd be lucky to get more than
300-400KB/s end-to-end on a TCP/UDP/IP connection from SCO's Internet
protocol stack.


Finally, what you should tell us is what is your problem, e.g. how many
machines you have got to link, how far apart they are, what is the average
packet length, and what is the maximum packet rate that you expect from each
machine, and other characteristics. Once you have told this to us, we can
tell you whether Ethernet makes any sense at all. Possible alternatives are
running TCP/UDP/IP over SCSI or over several point-to-point links, e.g.
synch, or RS-422, for example.
-- 
Piercarlo Grandi                   | ARPA: pcg%uk.ac.aber@nsfnet-relay.ac.uk
Dept of CS, UCW Aberystwyth        | UUCP: ...!mcsun!ukc!aber-cs!pcg
Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@aber.ac.uk