CSU/DSU

Aside from setting a clock rate for synchronization what do CSU/DSU's actually do?

Try this .

Forgive me I asked that question poorly. I actually seen that page already I was not satisfied with the definition. Not because it's wrong or incorrect but because it leaves unanswered questions in my mind(which I suppose I should have better articulated).


So far what I understand about a CSU/DSU component is:

CSU/DSU (Channel Service Unit/Data Service Unit)

Generally speaking a CSU/DSU sits between a router and telco connection.

CSU/DSU takes a lan frame and makes it a wan frame.
CSU/DSU matches the clock rate of a router to the telco line.

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(copypasta)
Channel Service Unit (CSU) receives and transmits signals from and to the WAN line and provides a barrier for electrical interference from either side of the unit

The DSU manages timing errors and signal regeneration. The DSU provides a modem-like interface between the computer as Data Terminal Equipment (DTE) and the CSU.

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So I guess given the information I already knew the CSU/DSU does have a valid job. I just don't understand why the router can't do the job without an external piece of hardware. Does the router not understand it can't use a lan frame over a wan link?

So I guess given the information I already knew the CSU/DSU does have a valid job. I just don't understand why the router can't do the job without an external piece of hardware. Does the router not understand it can't use a lan frame over a wan link?

I'm definitely not the expert in this area, but I'll give it a shot:

As you know, the requirements and physical limitations of a LAN are very different from that of a WAN, hence completely different protocols/solutions had to be made for various cases. CSU/DSU is only one of the devices that can be used (for example, when you purchase a T1 or T3 leased line). A router would need either an internal module fitted (Such as the Cisco WIC-1DSU-T1 interface card) or an external CSU/DSU device.

Other devices such as Modems or an ISDN Terminal Adapter are also WAN devices that can be used to ultimately interconnect your LAN to a WAN

So basically a CSU/DSU can more or less be viewed as a wan "adapter"?


It adapts the clock rate to whatever the ISP/Telco allows.
It adapts your normal frame output and discards it for a frame suited to the medium (whatever that may be) on the ISP/Telco side.

And the reason it's external vs. built in (or at least a card) I'm guessing and correct me if I'm wrong is there are many different types of Wan terminations that the ISP/Telco could offer and new link technologies come along every once and a while.

Thats good seeing that you have read widely like that. most people dont go into detail with CSU/DSU like you have.

Ok, enough of that. The CSU/DSU does not have to be an external device, nowadays they come integrated into the routers as a cisco measure of simplifying the network by reducing the hardware needed for configuration. therefore these routers can now, as you say, "know they are routing through the WAN. But the basic function is really for setting the clock rate.
In previous cases the CSU was located at the ISP router and the DSU (a router with a WIC-1DSU-T1 interface card) located at the customer premise equipment. Nowadays you can get one thats either a CSU or a DSU. hope this helps enlightens things abit.