Linda Crosby (lcrosby@ALMSA-1.ARPA)
Mon, 8 Dec 86 11:00:16 CST

Here is the consolidation of the meaningfull replys to our query to
the TCP/IP mailing list.

(And before anyone asks..Mr. Savacool originated the query,
I was the 'pipeline' since I am on the TCP-IP mailing list and
Mr. Savacool is not.)

Thank you all,

Linda J. Crosby


The original question was :

> I am in the process of establishing a TCP/IP network that will
>run on Broadband ethernet. The prototype installation will consist of
>4 mainframe computers (VAX 780) and 8 or 16 users using IBM-PC clones. The
>highwater mark could be as many as 8 mainframe computers and 400 IBM-PC
>clone users.
> I would like to know if 400 users, as described above, running the
>standard suite of DOD protocols (TCP/IP, TELNET, FTP, SMTP) can be
>supported on a single ethernet ? How many connections would be
>reasonable before the network begins to degrade ? Is anyone familiar
>with a broadband ethernet, similar to what we are proposing, that is
>configured with 400 connections ? Is this reasonable ?
> I would like to here from anyone who has covered this ground
>before. A successfull installation elsewhere would be a big
>confidence builder.

     I would like to thank all those who were kind enough to answer.
The replys ranged from several people who felt compelled to point out
that I didn't know what I was talking about when I talked about
ethernet (broadband or baseband) to many people who shared some genuine
insight about their own experiences.

     To those who still think that ethernet is only baseband I suggest
you contact ChipCom Corp, SYTEK, DEC or any of the other vendors now
selling the ethernet on broadband products that were approved by the
IEEE 802.3 committee on September 19, 1985.

     Why do I want to do ethernet on broadband and not just regular
baseband ethernet ? Simple, We have a large investment in a
broadband cabling plant and it is not reasonable for us to lay new
cable, it is much more reasonable for us to hang transceviers off of
our existing cable to provide the same function.

     The following are the messages that I found addressed my original
question. Once again thanks for the many replys. We are going to do
some of this soon (government procurements take at least 6
months) so I will have some real world experience within the year.

Bob Savacool

[ The opinions expressed herein are my own. I have no affiliation
with any company or any products mentioned. ]


Resent From: Cpt Brian Boyter ut-ngp!boyter <>

We have several large departmental Ethernets which we would like to
interconnect. At the same time, we would like to provide Ethernet
capability throughout our campus. One option which we are exploring
is to run Ethernet on our existing broadband cable system, which currently
supports video and Sytek LocalNet 20.
Because of the size of the resulting Ethernet, and because multiple protocols
will be used (TCP/IP, Lat-11, DDCMP), we believe we will also need to have
some bridges placed at critical locations. It appears that there are several
broadband Ethernet, and Ethernet bridge products on the market. Among the
vendors are Chipcom, Ungermann Bass, Applitek, DEC, and CMC.

I'd be interested in hearing other folks' experiences with these products.
Particularly helpful would be any recommendations or "gotchas" in configuring
such a network.


From: John Lekashman <lekash@AMES-NASB.ARPA>

Its somewhat reasonable. It all depends on what these users are going
to be doing. If you intend to bring up remote file servers on vaxes
for pc's, and some sort of network file system, then you will probably
lose when the count of PC's gets above about 20. If you intend to
use the pc's as terminals and login to the vaxes for random computational
needs, then the count can probably go into several hundred.
This is also true if the primary traffic is going to mail between

One approach that you can use that is extensible upward for a long time
is to properly subnet into groups. This does mean some additional
hardware, and choosing the correct software. For example, you could
group 25 to 50 pc's that are likely to have traffic between them
on one cable, along with an associated larger machine (incidentally,
a vax is only a mini, or at best a 'super-mini') and feed this
into a packet switch on the same cable to another cable that N other
such things exist on. (In your case N becomes 8) In this way,
high traffic flows along each of the separate ethernets, and lower
density of flow from group to group. Vaxes with appropriate
software, (eg 4.3 unix from berkeley is one such OS that we use here
for such a purpose) can function as a subnet gateway. If you need
it, I can point you at several vendors who will sell packet switches,
about 10K each. These can switch at about 3 megabits currently,
with 1000 byte packets, so its not a terrible performance loss.


From: Bob Napier <rwn@ORNL-MSR.ARPA>

We are assisting Lowry AFB (Denver) in evaluating/installing a broadband
network of up to 500 nodes for Office Automation.

I will be happy to provide you information on our experiences, although we
are just awaiting bids to a RFP.


From: Charles Hedrick <>

We have 3 DEC-20's, 3 Pyramids, and a 785 on one Ethernet. I don't
think you will have any problem. It's real hard to saturate
an Ethernet, unless you are using diskless machines, which do their
paging over the network.


From: Jeffrey C Honig <$>

I'm planning on doing something similar but, at least initially, on a
smaller scale. I'm planning on making extensive use of LANBRIDGE 100's
to separate traffic into groups with a backbone connecting them. That
could increase the traffic you could handle.

I'm planning on using Chipcom modems on the broadband, how about you?
I've researched the matter fairly carefully and have a paper that
describes my research and plans. I can mail you a copy if you are


From: Dan Lynch <>

 The answer to your question is not easy. I could easily work
up a scenario that would bog down -- like using the Ether
for remote file sharing (ala diskless workstations). But if every host
is just using the Ether for mail, some random FTPs and some Telnetting
then it could work well. The real question is what are those PC
users doing? Using them as terminal emulators to get to the VAXen?
Or as real hosts and only sending mail and spreadsheets around?

Anyway, You should contact Charles Hedrick at Rutgers (Anyway, You should contact Charles Hedrick at Rutgers (Hedrick@Rutgers)
as he has a huge installation and knows a lot.
Can you give me your US Mail address so I can send you a brochure on
the upcoming TCP/IP Interoperability conference? (March 87).


From: Dick Karpinski <>

No problem. Each pair of nodes can use about 1 megabit/sec but unless
they're gonna really pass a bunch of data around, I'd expect to see
loadings like 2% and 4% for such a small net. On baseband you get to
use like 70-80% before it degrades much. It's 2-4 times worse on the
5 megabit ethernet on broadband, but you still have a safety factor
of 4-8 before serious degradation.

But why use broadband at all?? The connections are more expensive and
less speedy and bigger too. Buffered repeaters and filtered repeaters
like the DEC LANBridge handle length problems (and traffic levels too
with the filtering) at costs like $4k and $7K respectively.

We have about 30 VAXen and Suns etc and just about can't see the traffic.


From: Bill Nowicki <>

        I am in the process of establishing a TCP/IP network that will
        run on Broadband ethernet.

In a way, "Broadband ethernet" is an oxymoron. Ethernet is
baseband. There are several companies that build systems that are
compatible with Ethernet transciever specs, but use broadband
signalling instead of baseband signalling. Although this is an analog
issue that is mysterious to software types like me, the fact
that you modulate and demodulate should not help collision problems, so
you have the usual Ethernet length restrictions. Of course you can
exceed the length restrictions, with possible collision problems.

        The prototype installation will consist of 4 mainframe
        computers (VAX 780) and 8 or 16 users using IBM-PC clones.

Your definition of "mainframe" is interesting, since the workstation on
my desk is twice as fast, and is the middle of our line. At any
rate, we have many Ethernets with up to about 100 Sun-3 machines. It is
interesting that bandwidth is not the first limitation. The main
reason you don't want more than about 100 machines is that one faulty
machine can bring the whole network down. The probability that someone
shorts the cable, or starts to continuously broadcast, or at least has
bad collision detection circuitry, becomes pretty close to one with
over 100 nodes.

Of course this might be because Ethernet networks just evolve, while
broadband networks are usually "planned". We make each floor of each
building its own Ethernet, with additional Ethernets for labs. You can
make a Sun into a gateway just by sliding in another Ethernet
controller. It also helps to use transceiver multiplexor boxes such as
the ones made by TCL, to reduce the number of actual taps, (less likely
to short the cable). So you probably can put 400 PCs onto a single net,
but do you want to?


This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:37:00 GMT