What is the Function of CSU/DSU device and what is its relation with clockrate?

jash.exejash.exe Registered Users Posts: 5 ■□□□□□□□□□
Hi, I am having a bit confusion in a topic, What exactly is the function of CSU / DSU Device?

I am answered with different answers from my friends, teachers and google, all answers are different,

For now what I have studied from answers.com and wikipedia is that CSU/DSU is a modem like device used to connect in-house network to dedicated internet (T1, E1 etc) while teachers are focusing on bandwidth allocation, that csu/dsu are only used for bandwidth allocation like the ptcl (ISP) provides us 10mb dsl, 4mb and other different bandwidth options is only because of CSU/DSU.

If you can provide me exact process of what CSU / DSU device do, and how it cannot have any other alternative solution, This would help me a lot. Thanks


  • sides14sides14 Member Posts: 113
    The use of a CSU/DSU is mostly used as a point for a customer defined connection from the provider. There are a couple of different clock settings associated with a CSU/DSU (internal, recovered, loop). When you are receiving a signal (clocking) from a service provider, you should always use recovered clocking (prevents slips because of different synchronization between you and the provider). Internal clocking works okay (not very reliable) if you are providing the reference, but for the other devices will need to be set for recovered. As for speeds, a CSU/DSU is used primarily for TDM based circuits. Other speeds (Ethernet) use an entirely different synchronization medium (IEEE158icon_cool.gif. Once you get to the point of needing Ethernet based timing to provide reliable service, you are getting into major issues regarding jitter and wander performance for equipment.
  • Forsaken_GAForsaken_GA Member Posts: 4,024
    One of the major issues of confusion I run into with folks when they first run into WAN protocols where clocking is an issue is understanding that clocking has nothing to do with time, it has to do with timing. Subtle difference, but there. I always equate it to a metronome in music, it provides the pacing for the circuit. The higher the clock rate, the more bandwidth flows across per tick.

    A CSU/DSU is, effectively, a media converter. It takes your WAN circuit, receives the info, and then converts it into something you can use at the LAN level. Conceptually, it's a bridge between your WAN and LAN connections.
  • ThunderPipeThunderPipe Member Posts: 120
    Great explanation Forsaken. I'd recommend posting this bit of info in the CCNA forums for our newer networking studiers. They say you don't truly understand something until you can explain it in simplistic terms....or something like that. I'd say you have one hell of an understanding.
  • drkatdrkat Banned Posts: 703
    Great explanation! Lets take it a bit further on the timing... On a wan link you have a master/slave clock source - one is going to give timing.. and they other is going to receive timing. In order for the two sides to talk flawlessly they need to be in sycn - this is where the concept of timing comes from, however a T1 is a different signal than ethernet and vice versa so in order to do this we need to have a CSU/DSU - - it takes the incoming signal and converts it (as Forsaken said)

    If you have a router with a T1 and you do a show controllers T1 you'll see the clocking information. If one side say is giving time (master) and the other side is receiving (but set to master) - You will see Controlled Slip Seconds (CSS) which are "slips" in synchronization. This may prove useful in your studies.

    On the different options.. You can effectively bring in a T1 that does both voice/data and break off the DS0's using a Channel Bank (CSU) - adtran comes to mind; You can effectively do the same with an ONT (fiber) any type of "media converter" you can perform this.

    In your case of DSL - the phone company would break off a port on their DSLAM and send it to your house, the DSLAM syncs (clocks) for the DSL modem and they talk to each other - your can bring in both voice/data across the line to the DSLAM and then break the channels for voice and send them one way and the data another; it's all in the card configuration on the DSLAM.

    I hope this helps, feel free to ask any more questions
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