Question about Serial max speeds and length

Discussion in 'Cisco' started by DigitalBlade, Mar 16, 2007.

  1. DigitalBlade

    DigitalBlade Guest

    Hi there,

    This is my first post on this group, but I've been following it for a
    while and find it very helpful. I am a CCNA student and was just
    having a bit of a problem trying to understand some concepts of serial
    interfaces.

    First of all, I was wondering about speed. In most situations, the
    Cisco curriculum uses serial interfaces to interconnect routers. But
    let's say you have a network of 100 computers behind a router and they
    are trying to get to the internet through that serial interface. I
    can't help but notice that the 2 mbps or so is painfully slow for a
    network of that size. I've come across an interface known as High
    Speed Serial Interface, which solves that problem by introducing a 52
    Mbps bandwidth. But on the downside, maximum length for that cable is
    50 feet.

    Which brings me to my second point of confusion. If the diagrams
    interconnect regional office with Headquarters, one would expect
    exceptional lengths (miles and miles), but any kind of serial
    interface has a very limited length (usually 10 feet). So how can this
    diagram possibly be true? As far as I am aware, only Fiber optic
    allows you these lengths, but for a smaller company this is extremely
    expensive.

    Would anyone be able to clear that up for me? Thanks in advance
     
    DigitalBlade, Mar 16, 2007
    #1
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  2. DigitalBlade

    chris Guest


    > Which brings me to my second point of confusion. If the diagrams
    > interconnect regional office with Headquarters, one would expect
    > exceptional lengths (miles and miles), but any kind of serial
    > interface has a very limited length (usually 10 feet). So how can this
    > diagram possibly be true? As far as I am aware, only Fiber optic
    > allows you these lengths, but for a smaller company this is extremely
    > expensive.
    >
    > Would anyone be able to clear that up for me? Thanks in advance
    >


    Companies that connect offices via point to point links do so over a telco
    network. Your serial cable only needs to reach to the NTE from the telco
    which is usually less than two metres. From there traffic is carried over
    the telco network to the other end where another NTE will link into the far
    end router via another short serial cable.

    Chris.
     
    chris, Mar 16, 2007
    #2
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  3. "DigitalBlade" <> writes:
    >First of all, I was wondering about speed. In most situations, the
    >Cisco curriculum uses serial interfaces to interconnect routers. But
    >let's say you have a network of 100 computers behind a router and they
    >are trying to get to the internet through that serial interface. I
    >can't help but notice that the 2 mbps or so is painfully slow for a
    >network of that size. I've come across an interface known as High
    >Speed Serial Interface, which solves that problem by introducing a 52
    >Mbps bandwidth. But on the downside, maximum length for that cable is
    >50 feet.


    Right, HSSI is pretty standard for DS3 (45Mbps) interfaces.

    You're dealing with serial connections back to back in the labs,
    because it bypasses/eliminates what you normally buy from a telco
    provider. In real life, you'd hook those serial interfaces up to
    something else in the middle, having the same exact things on either end.

    Its not typical to go running serial lines everywhere, thats what they
    did in the 70's & 80's :) They only go from your router to the telco
    interface typically.

    Also, 2Mbps may be the best you can afford for an office to office
    connection depending on your requirements. Its not exactly cheap to go
    long distances, and you'll end up paying by the Mbit...

    >Which brings me to my second point of confusion. If the diagrams
    >interconnect regional office with Headquarters, one would expect
    >exceptional lengths (miles and miles), but any kind of serial
    >interface has a very limited length (usually 10 feet). So how can this
    >diagram possibly be true? As far as I am aware, only Fiber optic
    >allows you these lengths, but for a smaller company this is extremely
    >expensive.


    Because the diagram eliminates the telco, and thats who is going to
    provide your fiber. Yes, telcos run on fiber, and thats exactly how
    they go the distance.

    But, depending on what you get from the telco, and how, you'll be
    running serial (or equivilent) back from the telco termination box
    back into the router.

    But in the lab, there's no point in trying to become a telco to
    simulate real life, when you can go back-to-back to simulate what you
    see in real life at one layer up than what telco provides.
     
    Doug McIntyre, Mar 16, 2007
    #3
  4. DigitalBlade

    DigitalBlade Guest

    Thank you both very much for your detailed explanations. This really
    cleared it up for me.
     
    DigitalBlade, Mar 17, 2007
    #4
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