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CSU/DSU's why?

DerekAustin26DerekAustin26 Member Posts: 275
in CCNA & CCENT
Why are CSU/DSU's only used for T1's or better ?
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    tierstentiersten Member Posts: 4,505
    Because they're what you need to connect to a digital circuit.
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    DerekAustin26DerekAustin26 Member Posts: 275
    But ISDN and DSL are connected to a digital circuit so why dont they just use CSU/DSU's?
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    tierstentiersten Member Posts: 4,505
    DSL isn't digital. It goes over your regular analog phone line.
    ISDN is a digital line but it isn't a leased line like a T1.

    A CSU/DSU is just what you use for a digital leased line. There isn't much else to it. If you have <blah> type of line then you need <blah> device to connect to it.
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    astorrsastorrs Member Posts: 3,139 ■■■■■■□□□□
    Many routers (or router modules) these days have the CSU/DSU built in.

    This really isn't any different from the NIC in your computer incorporating the MAU directly into the card freeing up the need for external transceivers and AUI ports.
    twitter: @astorrs
    profile: linkedin.com/in/astorrs
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    kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
    tiersten wrote: »
    DSL isn't digital. It goes over your regular analog phone line.

    Hate to disagree, but based on the Digital part of Digital Subscriber Line, DSL is a digital connection.
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    DerekAustin26DerekAustin26 Member Posts: 275
    Im not arguing here, im just trying to get an understanding based on the material i've looked over... DSL isnt digital? But why is it a "Digital Subscriber Line" ?

    And if you look at Wikipedia DSL vs POTS diagram.. DSL carries digital data on the POTS analog line to your modem which sits between another modem where the signal is converted to an ALL Digital Line straight to the ISP.
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    msteinhilbermsteinhilber Member Posts: 1,480 ■■■■■■■■□□
    DerekAustin26 wrote: »
    But ISDN and DSL are connected to a digital circuit so why dont they just use CSU/DSU's?

    Instead ISDN uses an ISDN modem and DSL uses a DSL modem. Think of the CSU/DSU as a modem so to speak I guess.
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    tierstentiersten Member Posts: 4,505
    kalebksp wrote: »
    Hate to disagree, but based on the Digital part of Digital Subscriber Line, DSL is a digital connection.
    The line itself is an analog line. The higher frequency portion of it is used to transmit/receive your DSL signal.
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    kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
    tiersten wrote: »
    The line itself is an analog line. The higher frequency portion of it is used to transmit/receive your DSL signal.

    I understand that, but what defines whether a signal is analog or digital? Can't the upper frequencies use a digital signal along with the analog signal for voice?
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    DerekAustin26DerekAustin26 Member Posts: 275
    So i can clear this up.. The Illustration of whats happening should look like so..

    PC
    (digital)----|DSL MODEM|~~~~(analog pots line)~~~~|DSL MODEM|
    (All Digital Line)
    ISP


    Can you provide an illustration of whats going on here?



    Cuz i know Cable looks like:

    PC
    (MODEM)~~~~~~~~~~~~~~~~(Modem)
    End User or ISP

    T1 looks like:

    PC----(Router)---(CSU/DSU)
    (CSU/DSU)----ISP
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    DerekAustin26DerekAustin26 Member Posts: 275
    The Illustration of whats happening on a DSL line should look like so..

    PC
    (digital)----|DSL MODEM|~~~~(analog pots line)~~~~|DSL MODEM|
    (All Digital Line)
    ISP


    Anyone know if this is correct? If so, please provide me an illustration plz.
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    kalebkspkalebksp Member Posts: 1,033 ■■■■■□□□□□
    DerekAustin26, there's a diagram of DSL topology on this page: Introduction to DSL

    Based on Fast Guide to DSL, DSL is not converted to analog. I don't know whether that page is correct, if anyone has anymore information on the subject I'd love to know. For some reason I really want to know the answer to whether DSL is analog or digital.
    Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting it to you.
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    DerekAustin26DerekAustin26 Member Posts: 275
    Thanks bud... I feel the same way! i cant stand not knowing all the pieces of the puzzle! Let me know if you find out anything at all when your 100% sure. Thanks
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    Morty3Morty3 Member Posts: 139
    Afaik, DSL is a way of transmitting digital signals on the telephone (pots) line. A normal modem (oldschool ones that made that annoying noise that we all got nightmares about) would have
    done what is illustrated above, converted digital signals to analog for transmission and the analog to digital for the computer to understand.

    Correct me if I'm wrong so that I can learn!
    CCNA, CCNA:Sec, Net+, Sonicwall Admin (fwiw). Constantly getting into new stuff.
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    networker050184networker050184 Mod Posts: 11,962 Mod
    Morty3 wrote: »
    Afaik, DSL is a way of transmitting digital signals on the telephone (pots) line. A normal modem (oldschool ones that made that annoying noise that we all got nightmares about) would have
    done what is illustrated above, converted digital signals to analog for transmission and the analog to digital for the computer to understand.

    Correct me if I'm wrong so that I can learn!

    This is my understanding as well, but I am by no means an expert in the DSL arena.
    An expert is a man who has made all the mistakes which can be made.
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    rossonieri#1rossonieri#1 Member Posts: 799 ■■■□□□□□□□
    wow ... nice discussion.

    IMHO - from what i've read, understood, and what i've do :
    from a general link subscriber point of view - a DSL is only a link that what they've subscribed and paid for - dont care much about p2p frame-relay, or ISDN-BRI, PRI/T1/E1, 10mbps metro-ether etc.

    but, from telecommunication/technical point of view - as above links explained - DSL works by transmitting digital signal over copper wire - dont care its a coaxial like those used by cable-TV or basic "analog" phone cable like those used by ADSL/VDSL etc.

    a native digital link from the provider to the customer (let say PC) dont need "modem" - but they only need signal-adapter which is CSU/DSU. but nowadays - a modem (which is signal converter from analog to digital) - can be pronounced as CSU/DSU (which is the real native digital-to-digital interface adapter/network termination).

    every digital link devices should have this CSU/DSU (either internally built-in or externally attached) - if they dont - they wont be able to use the line - even if they looked like a very small "combo modem/router".

    common example to this is your cable "modem" - its actually an external CSU/DSU to feed the line (adapt, or to change the link interface) into your linksys ethernet router. i've used to have a coaxial NIC so i dont have to use the "cable-modem" to use the link.

    ADSL over POTS "modem" - its a CSU/DSU although it runs over 2 wires copper telephone cable. the digital signal itself has been generated by both the CSU/DSU and the DSLAM.

    there are some terminology - i've used to learn the digital networking like NT1, NT2, Terminal adapter and so forth - so feel free to search around to know what they are & what they are for.

    and 1 more, there are few misconception for some about ISDN and T1 in north america.
    a T1 in north america mainly refers to 1.5mbps HDLC link, while the T1 or known as an E1 for the rest of the world refers to 1.5mbps and 2mbps ISDN Primary link (not HDLC).

    please do correct me if i'm wrong,

    HTH.
    the More I know, that is more and More I dont know.
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    DerekAustin26DerekAustin26 Member Posts: 275
    Well I spoke with a CCIE and he said that a CSU/DSU is basically just a modem EXCEPT it does not "modulate/demodulate" - It is used for digital networks because it synchronizes the clock speed and it multiplexes.. Hence the 24 channels for a T1; the 672 channels for a T3. This a Modem itself cannot multiplex.
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    DerekAustin26DerekAustin26 Member Posts: 275
    Any other questions, comments, concerns, corrections, illustrations? Anything?
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    dtlokeedtlokee Member Posts: 2,378 ■■■■□□□□□□
    DerekAustin26 wrote: »
    Well I spoke with a CCIE and he said that a CSU/DSU is basically just a modem EXCEPT it does not "modulate/demodulate" - It is used for digital networks because it synchronizes the clock speed and it multiplexes.. Hence the 24 channels for a T1; the 672 channels for a T3. This a Modem itself cannot multiplex.


    So his description basically is "it's like a modem but it doesn't do anything a modem does"?

    A CSU/DSU converts the electrical signal from the router or other DTE into a signal that follows the signaling form of the T1 and applies the framing for the channels. Depending on the use all 24 channels can be used for a single connection, or it can be broken into multiple connections and you can use a MUX to mix the signals from different sources (TDM or Stat-TDM.) In most cases of data you don't use a MUX because the data is transmitted as IP traffic (or some other protocol) which handles the multiplexing of multiple data streams.
    The only easy day was yesterday!
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    DerekAustin26DerekAustin26 Member Posts: 275
    Well it replaces the modem if your using a T1 Circuit or greater. It connects to DTE devices like a modem and it interfaces with the WAN like a modem. Only thing it didnt do was "modulate/demodulate" which is the primary function of a modem i know. But he was saying that CSU/DSU's are akin to a digital modem and used with T1's because they require multiplexing, also CSU does the exact FUNCTION of a modem, but it doesn’t do it LIKE a modem does. A DSU is simply using all 24 DS0’s in the DS1, but it still has to keep track of timing and synchronization of the individual channels. (comments are encouraged)

    Now my only question is.. Why aren't they used with ISDN??? ISDN is all digital and it uses 3 channels I believe. I've heard of NT1's for ISDN, but why those and what do they do?
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