actual bandwidth on serial links between two routers

Unanswered Question
Mar 8th, 2009

Hi everybody!

if I connect two routers back to back by serial interfaces, then what would be the actual bandwidth? The serial interface defaults to 1.544 Mbps which is not the actual bandwidth.

Thanks a lot!

I have this problem too.
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Richard Burts Sun, 03/08/2009 - 12:37

There is another factor that is much more important than the distance in determining the actual bandwidth when 2 routers are connected back to back using serial interfaces. One of the routers must function as DCE and must supply clocking. The actual bandwidth will depend on the clock rate configured on the router functioning as DCE (subject, of course, to the capability of the serial interface and to the distance).



sarahr202 Sun, 03/08/2009 - 14:57

Thanks everyone who responded to my post.

So two variables, clock rate on dce and distance determine the speed of the serial link.

if we have two routers are connected back to back by very very short serial cable, and clock rate is set to 56000, then the bandwidth would be around 56 kbps?

thanks a lot!

Richard Burts Sun, 03/08/2009 - 15:31

Yes, if you set the clock rate for 56000 then your bandwidth would be around 56kbps.

And to clarify a bit the link posted by Adam is discussing the maximum distance over which you can drive serial signaling without something to boost or regenerate the signal. It is not a case that longer distance slows down the bandwidth. If you configure clock rate (bandwidth) for 56000 and go further then the maximum signaling distance it is not that it goes slower, it is that it does not go at all.

So I would say that there is really one variable about bandwidth rather than 2.



Giuseppe Larosa Sun, 03/08/2009 - 14:28

Hello Sarah,

the bandwidth is a logical parameter that is used by routing protocols and modular QoS, but it is the clock rate command given on the interface with the DCE cable side that decides the real speed between the two routers.

Using back to back connections in lab setups is quite common.

In the case of Frame Relay encapsulation additional tricks are needed to make the link to work.

Hope to help



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