TR1034™ Fax Boards
Do I need a CSU
for my T1?
Engineer – Core Products
November 18, 1997
Revised May 5, 2000
Revised April 29, 2005 (TechArts)
What Is a CSU?
A CSU, or Channel
Service Unit, is a compact piece of customer premises equipment that can be
installed inline on a T1 trunk immediately in front of a network interface
device such as the Brooktrout TR1034. The range of functions that a CSU may perform
varies widely, but at a minimum it provides line conditioning, loopback and
diagnostic capabilities. The signals present on a T1 are susceptible to
distortion as loop length increases and various types of interferences are
encountered. The CSU reinforces these signals, keeps track of errors, and
provides test modes to help resolve line problems. Another important function
that many CSUs provide is called "keep alive" which, as the name implies, will
keep the T1 link up even if your network interface device goes down. This helps
to avoid calls to your carrier to ask them to re-enable a T1 that they shut
down when they detected a problem with your equipment.
available from a variety of manufacturers in both rack-mount and free-standing
configurations. Prices typically range from $500 - $1000 depending on features.
Oftentimes the company where the T1 is being installed will have a preferred
CSU; at other times the carrier providing the T1 will offer a recommendation.
What Is Not a CSU?
There is a great
deal of feature overlap between CSUs and other devices. Here are a few such
devices that you may encounter:
A DSU, (Data
Service Unit), is a device that converts a digital signal from
another format such as serial to the bipolar format used by T1. This function
is not normally needed outside of the data world since telephony interface
boards produce a signal that is already in T1 format. Many CSUs, however, have
DSU functionality built-in and are thus referred to as CSU/DSUs.
A "smart jack" is
a device increasingly used by carriers as their demarcation point. This device,
like a CSU, provides diagnostic/loopback functions that the carrier can control
remotely. Unlike a CSU, however, a smart jack does not provide line
A repeater is a
device that digitally regenerates a T1 signal. It is used at one-mile intervals
along a copper T1 land line. It is a dumb device that neither monitors the T1
signal for errors nor has loopback capability. Every CSU contains a built-in
When Do I Need a CSU?
The short answer
to this question is, "Unless you’re connected to a PBX, you need a CSU."
However, in many cases a CSU may not be strictly necessary. Consider the
following points when making a decision about a particular installation:
factor driving the need for a CSU is the loop length, or the distance from the
network interface to the source of the T1 signal, which can be a PBX, a central
office switch, or a repeater. If that distance is more than about 500 feet, a
CSU may be required in order to maintain a clean signal. This rule is not as
straightforward as it sounds, however, because it is often difficult to
determine loop length. You can’t simply measure "as the crow flies," because the
wiring may take a very roundabout path from point A to point B.
Another factor to
consider is that you may have a hard time getting help from your carrier unless
a CSU is in place. If you call to report a problem and you’re not using a CSU,
oftentimes the first thing they’ll tell you is to install one and see if the
problem goes away. If the problem remains after the CSU is installed, you will
at least have a device that can generate some data to help determine the nature
of the problem.
Finally, the CSU
provides a degree of isolation between the network and your interface device.
Burnt-out line drivers on cards are not unheard of, and the causes of this can
range from lightning strikes to poor quality lines. A CSU is usually easier and
cheaper to replace than your fax board.
So, there are a
number of situations where you can get away with not having a CSU. Like a
filtered and surge protected power strip however, a CSU provides relatively
inexpensive insurance against a variety of problems when the quality of your T1
trunk is unknown, which is most often the case unless you’re directly connected
to a PBX in the same room.
©2004 Brooktrout, Inc, All right reserved.