Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!xylogics!bu.edu!bu-it!kwe
From: kwe@bu-it.bu.edu (Kent England)
Newsgroups: comp.dcom.lans
Subject: Re: Need source for broadband modem and data link bridge II
Keywords: Ungermann-Bass, broadband modem, bridge
Message-ID: <61503@bu.edu.bu.edu>
Date: 27 Jul 90 20:42:39 GMT
References: <15721@s.ms.uky.edu>
Sender: news@bu.edu.bu.edu
Reply-To: kwe@bu-it.bu.edu (Kent England)
Organization: Boston University
Lines: 65

In article <15721@s.ms.uky.edu>, ericd@ms.uky.edu (Eric B. Durbin) writes:
> I seem to be getting some conflicting information here... let me
> clarify what I need:
> 
> Diagram
>                           XXXXXXXXX               XXXXXXXXX
> ============XXX-----------XXXXXXXXX===============XXXXXXXXX===== local
ethernet
> Ungermann-        coax    Broadband  transceiver   Datalink
> Bass                        Modem      cable        Bridge
> Cable
> 

	Ah, now I see what you mean by "Ungermann-Bass".  When you say
"Ungermann-Bass cable" I see a CATV system.  There are a variety of flavors.

	If you have an Ungermann-Bass set-up, it is probably mid-split,
with channel translators that take a single 6 MHz TV channel.  U-B uses a
5 Mbps/6 MHz system, that fits conveniently within a single TV channel
pair.

	You can have hi-split and lo-split CATV systems as well.  In case
you don't understand the split;  when you are running data, you have to
have one channel for signals sent
"up" the cable toward the head-end (root of the CATV system) and another
channel for signals sent "down" the cable.  The "split" involves picking
the frequency location of the guard band between "up" and "down" and the
shift between the two channels.  A translator takes the signal from "up"
and rebroadcasts it "down" the cable, so all stations hear everything.

	If your CATV system is set-up U-B style, then you only have a
6MHz channel pair (probably mid-split, probably 4A/R channel pair) and
a translator already in place.  Use a U-B Buffered Repeater in place of
the "broadband modem" in the diagram above.

> 
> I also know that a Broadband Modem is made by Chipcom Corp. in MA, and
> Datalink Bridges are made by DEC (ie LAN Bridge 150).  Chipcom also
> makes a combined broadband modem and datalink bridge called Ethermodem
> III Bridge.
> 

	Chipcom uses the IEEE spec approach.  (They may have something
proprietary as well.)  If you get the Chipcom or other IEEE compliant
gear, you get a full 10 Mbps in *three* contiguous channels (18 MHz),
one "up" and the other "down".  Three channel pairs altogether.

	A U-B single channel translator won't work with this.  You
would have to change to a translator from Chipcom or get a translator
that shifts bigger chunks of bandwidth.  (What do they call those kind
of translators?)

	Also note that the IEEE spec approach has distance limitations,
1800 meters from the head-end max, as I recall.  [Check that, I could
be wrong.]

	So, how Ungermann-Bass is your CATV system?  Would you be 
willing to set up new channel assignments, or are you running HBO on
those frequencies?  Can you replace the U-B translator(s) with
whatever the Chipcom gear requires?

	There's a lot to working with broadband.  :-)  I hope this
explanation helps more than confuses.

	--Kent