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Community Member

Maximum cable length for an E1 X21 cable

Cisco only do 3m cables for serial interface to DCE/DTE.

I’ve seen some X21 extension cables that would allow us a longer distance between our router and X21 interface. Has anyone used these and are we likely to see any problems if we use them. ( if anyone knows of any single cable solution I’d really like to hear about that as well but I haven’t been able to find one. )

We need an overall distance of about 10m and the line will be an E1 (2Mb)

Community Member

Re: Maximum cable length for an E1 X21 cable

cisco does have longer cables , but the core is not individually screened and will give lots of errors between your sites when used at 2mg.

You could always use individually screened cables to extend the 3m cables.

Community Member

Re: Maximum cable length for an E1 X21 cable

We have done that extensively and run into a number of problems.

These problems were analyzed by our local carrier.

Since the clocking is only generated by the DCE, it tracels with a delay to the DTE. There the delayed clock is used for the TX data which travels back to the DCE. So the TX data arrives at the DCE with twice the delay time.

But the DCE compares this delayed TX against its own undelayed clock.

If the timing is wrong you run into problems.

At 2mb the crucial length is between 11m-17m. It also depends on the cable,

the chips ... you could get problems earlier and at lower speeds.

We usually try to reduce the length, once i succeded in increasing the length.

I hope it will answer your question

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