Frame Relay>>>>>Do we need a CSU/DSU?

Discussion in 'Cisco' started by Paul.Parish, Mar 27, 2007.

  1. Paul.Parish

    Paul.Parish Guest

    I'm reading through the Cisco Press book CCNA INTRO and it mentions
    Frame Relay but doesn't mention a Frame Relay network using a CSU/
    DSU. It mentions a DTE and Frame Relay switch(which provides
    For some odd reason I remember seeing a CSU/DSU at my old work where
    we had Frame Relay. It also might be possible we had T1 instead.


    Do most Frame Relay networks include a CSU/DSU? My guess is NO
    because the frame relay switch provides clocking.

    Thank you!
    Paul.Parish, Mar 27, 2007
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  2. Paul.Parish

    Scooby Guest

    Technically No, Reality Yes.

    Many examples will show you how you can set up your own frame relay
    environment by connecting routers back to back. However, the Frame Relay
    switch will be handled by the telco. Since they can't readily connect a
    direct cable between your devices, they need some other transport, such as a
    T1 or DS3 to get from them to you. It is not that Frame Relay needs the
    CSU/DSU, but the T1 or DS3 would.

    Since you mentioned T1 above.... Don't confuse the T1 with the L2 protcol.
    Frame-Relay can use a T1, same as PPP and HDLC.

    Hope that helps,

    Scooby, Mar 27, 2007
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  3. So what you are saying is the frame relay runs over T1?

    My understanding is that Frame Relay is basically a partial T1 with
    virtual circuits and ability to have mulitpoint connections.

    So with Frame Relay you need only 1 CSU/DSU at each site as opposed to
    5(1 for every T1 line going out).
    ciscopimpenator, Mar 27, 2007
  4. Paul.Parish

    Scooby Guest

    Yes, Frame Relay runs over T1. DS0 and DS3 are other options and of course
    E1. Not sure if other methods are availabe. T1, in your example, is not
    the Frame Relay speed, but the circuit speed - BIG difference. The T1 can
    be full or partial - a T1 is made up of 24 (64k) channels and you can order
    any number of them for your circuit. 4 Channels would give you a 256k
    circuit. From that, you can order a certain amount of CIR for your Frame
    Relay. The maximum for either burst or CIR would be the physical circuit

    And, yes, you only need one CSU/DSU per site, not one for each pvc. The
    Frame switch does all the mapping for you. So, even if you are full mesh,
    you are still really Hub and spoke. That said, you may be connected to
    different frame switches at different locations and the pvcs are mapped

    The bottom line is that you need some physical connectivity to get from your
    equipment to the frame switch. A T1 is a very common option. The T1 will
    go from your site to the frame switch (not to your other locations).

    Does that help any better?
    Scooby, Mar 28, 2007
  5. So T1 is the physical cable that goes from your site to the telco.
    Frame Relay is the Layer 2 protocol that runs over the T1.

    Next question...

    What Layer 2 protocol do most people use when leasing a full T1?
    HDLC? PPP?

    So in essense if you use Frame Relay you will always need at least one
    CSU/DSU at the customer site?

    ciscopimpenator, Mar 28, 2007
  6. Guys, don't confuse him more than he's already confused...

    T1, Fractional T1, OC3, Leased Lines, Ethernet, Fiber Optics are examples of
    Transmitting Media. They define speed, distance, physical media used, etc.

    Frame Relay, PPP, 802.3, etc. are Datalink Layer protocols. They define how
    data are transmitted over the physical media.

    Some protocols are Media-dependent, and some are not. You may have Frame
    Relay, PPP or HDLC running over T1, Fractional T1 or T3, etc.

    And speaking about your particular question... What is CSU/DSU? Basically
    it's modem, which converts router's digital signals into the specific
    physical media. You cannot transmit digital signals over analog T1 media,
    so, you need CSU/DSU. Another question is that some routers may have
    internal CSU/DSU card, and some may require external CSU/DSU box. So, it's
    matter of what model is your router.

    Good luck,

    CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, Sun SCSA,
    Checkpoint CCSA, etc.
, Mar 28, 2007
  7. Paul.Parish

    Paul.Parish Guest


    Next question:

    Which Layer 2 protocol is used when a customer leases a T1 line?
    HDLC? PPP?
    I imagine it varies between telco provider, but what is the generally
    accepted L2 protocol in practice?

    Thank you,
    Paul.Parish, Mar 28, 2007
  8. Paul,

    If you have really "Dedicated Leased Line" (dead copper coming from Location
    1 to Location 2), then you can use whatever you want. However in real life
    you have to use whatever your Telco provider supports. For example, MCI
    supports Frame Relay, and don't like PPP, and AT&T recommend PPP and offer
    Frame Relay as an exception for additional charge.

    Good luck,

    CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, Sun SCSA,
    Checkpoint CCSA, etc.
, Mar 28, 2007
  9. Yes, Frame-Relay is one of several layer-2 protocols that can run over
    a T1 among other media types.

    Some prefer HDLC, some prefer PPP, some prefer Frame-Relay.
    Yes, you'd generally need a CSU/DSU for any type of T1 that comes
    in. Frame-Relay, ATM, clear channel, etc. etc.

    NB: almost all gear comes with cards with built-in CSU/DSUs. You
    rarely would ever see a standalone unit anymore. Although in existing
    implementations, you'll likely see many kinds. Probably won't get back
    far enough to start seeing standalone CSU's though :)
    Doug McIntyre, Mar 28, 2007
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