eshop@saturn.ucsc.edu (Jim Warner) (09/18/88)
I have seen several postings asking if a transceiver cable null
modem exists. It does. It is made by American Network Connections,
Inc. of Sunnyvale, Ca.
We purchased 10 of them. Our intended use was to connect Ungermann
Bass buffered repeaters to Cisco AGS gateways. Our campus has a
cable TV type broadband network.
With somewhere around half of them deployed, it is my observation
that they aren't as good as "the real thing." That would be two
real transceivers, a short length of cable and two terminators.
While both ports on the ANC-10 are nominally identical, I've found
that most of the the direct connect devices have a preferred
direction. ("Preferred" means you want it to work.) In all cases
where the ANC-10s have trouble, the "real thing" works flawlessly.
Please no flames: I have not yet contacted ANC. My point in making
a negative recommendation is that the amount of money to be saved
with these things is so small (about $100) that "debugging" the
problem just isn't worth the effort. If you need an Ethernet
with two transceivers, your best bet it to get a real one.
For anyone that is curious about the details of the problem, when
I finally find a marginal combination, the packet loss rate is
independent of packet length. This suggests to me that the ANC-10s
provide something unusual in the way of echo back to the sending
interface. Perhaps the echo either arrives too early or too late.
The need for local echo is the reason that you can't simply wire
up a null modem cable between two DTEs without any intervening
electronics.
Jim Warner
Network Engineer
University of California, Santa Cruz