Cisco Support Community
Community Member

csu/dsu and modem


Is there any technical difference between a modem and csu/dsu. What exactly does a csu/dsu does? Is it functionally similar to a modem? In practical networks, how are they deployed? Thanks for your assistance in advance.




Re: csu/dsu and modem

At a very high level, modems and CSU/DSUs are "sort of" the same in that they convert a signal of one type into a signal of another type.

They are generally NOT considered to do the same thing at any functional level.

Modems (a contraction of MODulator and DEModulator) for computer / data communication started out as acoustic devices, dial teh phone of the other side, put the handset in the cradle of the modem ...and it would convert the serial data into acoustic sounds that could be sent across a voice circuit.

The problem was that you can only encode so much stuff into an audible signal. I think the highest speed acoustic I ever saw was 1200 baud (Racal-Vadic). MOst were 300 baud or 110 baud.

Modems eventually evolved into direct line-connnect devices (when law/regulation permitted). The direct-connect devices could use more complex signals and so could handle higher data rates; but their baisc function was to convert serial data into something the telephone network would interpret as audible signaling.

CSU/DSU is a combination device (just like it looks; a Channel Service Unit and a Digital Service Unit).

You can buy the individual units (or could, technology has sorta moved forward a bit).

A CSU does things like "Line Protection" it save your equipment from line surges from the outside, like near-hit lightning), it also provides some equalization to the signal (in and out).

The Telcos send all their signals out at full strength to reach the longest distances... if your location is too close, the signal is "too loud" ... so you could add some "Line Build Out" (LBO) to attenuate the signal, for example. The CSU also provides a looping point for testing that can be remotely commanded (by the Telco/carrier).

The DSU is also a testing / looping point, and basically provides the serial<=>T1 signal conversion.

There are also special setups where you could have multiple DSUs and onl one CSU, or something called "Add/Drop" CSU (aka "Drop & Insert" CSU) used to feed multiple devices from a single T1 (like a PBX and a router).

There's much more to both systems (modems VS. CSU/DSU), but this is just to give you an idea of how much different they are.

As far as practical use, modems are typically relagated to the "backup circuit" role, along with ISDN (ISDN is probably more popular).

CSU/DSUs tend to be primary WAN links of T1 or Fractional T1. External CSU/DSUs, most often connect to the device (PBX/router) via serial connection, most often a V.35 signaling scheme with a Winchester connector (large rectangular thing with locking screws). Internal T1 connections (like a Cisco WIC-T1 DSU) presents itself as an RJ48 jack; the T1 connects directly from the NIU/SJ and looks like an RJ45 Ethernet jack, but is physically slightly different) ... by regulation and convention it still has CSU/DSU components to permit testing and line compatibility, the CSU/DSU is just "built-in."

Good Luck


Community Member

Re: csu/dsu and modem


Thx for such a detailed explanation. Well, i have a few queries still .. hope you will find time to answer these too ;-)

(a) suppose, i have a Leased Line between Sydney and Singapore. So, does this mean, there will be 4 CSU/ DSU's in between. 2 @ telco end and 2 @ my end. Does this kind of setup does not require a modem ? Because the signals have to be modulated/ demodulated, right?

(b) any link which mentions the physical connections of router to csu/ dsu and to modem, please let me know.

Thx again...

Hall of Fame Super Gold

Re: csu/dsu and modem


when it comes to how a circuit is provisioned, you practically cannot assume anything. Telco may be using CSU/DSU but call them with another name. They may be dropping off a small fiber mux. They may be using HDSL or G.SHDSL and the devices used for that have again different names. There is so much variety in the field that it is almost impossible to keep track of how things are locally done in each contry, each telco, each area.

Of course if you were to work for a service provider these things would matter but you would access to the information as you need.

So if you are trying to learn I would focus on how things are from the user point of view, that is much simpler.

Either you are given an E1 interface (with the only variables of balanced/unbalanced, framing, clock source), or you are given a classical "serial" interface like V.35.

The first option is better because it eliminates an external box, and let the router to see the real performances of the circuit.

Hope this helps, please rate post if it does!

Community Member

Re: csu/dsu and modem

Hello Paolo

You said "Either you are given an E1 interface (with the only variables of balanced/unbalanced, framing, clock source), or you are given a classical "serial" interface like V.35."


Could you explain in which instance telco will provide serial interface? For example, if i am connecting via Frame Relay, you are saying that telco can either give a T1/E1 interface or a Serial Interface for connection. So my question is in which scenario they will provide a serial interface like V.35. From what i understand there is a limitation for the total length of V.35 cable. So i am confused about the use case in which Telco provide a V.35 cable for wan connectivity. I know this is a very basic question and I hope to get some reply.

CreatePlease to create content