Question about Serial max speeds and length

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

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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.


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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...

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.

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Doug McIntyre

Thank you both very much for your detailed explanations. This really cleared it up for me.

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