Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!elbereth.rutgers.edu!ron.rutgers.edu!ron
From: ron@ron.rutgers.edu (Ron Natalie)
Newsgroups: comp.dcom.lans
Subject: Re: cisco-to-cisco over 1.5M serial?
Keywords: T1, cisco, CSU/DSU, is this trip really necessary?
Message-ID: 
Date: 8 Dec 88 16:50:54 GMT
References: <1751@csun.edu>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 38

I'd go with one of the slick CSU/DSU pairs on the market today.  The Verilink
one seems to be pretty good for the price.

The CSU (customer service unit) is a device that sits between the customer
equipment and the phone companies circuit and provides some diagnostic
services like remote loopback.  For T1 speeds, they also provide enforcement
to complience with the various 1's density requirements and such.  The input
and output to a CSU look the same.  Usually a DB-15 connector which have two
pairs of pins actually in use, one for the transmit side and one for receieve.
Presumably you can get away without this box on your own wire, but it's nice
to have anyhow.

The DSU (data service unit) is what you think of a MODEM in the digital
service.  It talks V.35, RS-232, or RS-449 to the CISCO box and converts that
into the transmit and recieve signals to the CSU.  The DSU is infact a modem
that will get your data as far as the next step in the digital facility.  The
DSU has the appropriate connector for the data equipment side (DB-25 for RS-232
DB-37 for 449, whatever the V.35 connector is called) and a DB-15 with the two
pairs going to the CSU.

You can get both in the same box usually called a CSU/DSU or sometimes an ISU
(Integrated Service Unit).  It used to be that the phone company provided both,
then just the CSU, and now you pretty much can do what you please.  To work with
the CISCO box, you are going to at least need the DSU.  You may be able to do
without the CSU, but having one is convenient for debug purposes.

The confusuion stems from the fact that while DSU's are common for slower speeds
like 56K and slower, it is only recently that boxes like the cisco T1 units want
to talk T1 data rates directly into a single channel.  In the past T1 was always
made up of multiplexed slower circuits, and the multiplexor had a part to it that
played the roll of DSU, requiring only a CSU for the T1 line.

My recommendation is for the new VeriLink CSU/DSU pair that they call the Connect 1.
We can get them (University Price) for about $2500 each end.  We know the comparable
product from Digital Link works because Cisco used one of our Digital Link modems
to test out their boards with originally and we ran with them for quite some time.

-Ron