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X.21 - a collection :

X.21 is a CCITT recommendation for operation of digital circuits.

The X.21 interface operates over eight interchange circuits (i.e. signal ground, DTE common return, transmit, receive, control, indication, signal element timing and byte timing) their functions is defined in recommendation X.24 and their electrical characteristics in recommendation X.27.

Frage : I have two cisco 2500 and the cable to hook them up back to back. I am inquiring about the bandwidth parameter on the serial interfaces. Can I actually set the bandwidth between the two routers and will that actually be the bandwidth of the link between the routers? I am trying to test the time it takes to do a backup over a similutated T-1 link. Thank you for your help.


Antwort : Aug 21, 2000


Yes, it should work as the same bandwidth you set on the interfaces. Go to the interface you want to use and set up the clock rate to your desire speed, and the interface type as dce. Apply this to only one of the interface not both !

Frage : I have 2 x 2610's and wish to connect them back to back, any idea how to do it? I have a DCE to DTE cable but I cannot remember how to set up the serial ports.


Antwort : Jun 29, 2000

for the DCE device interface Serial0 ip address bandwidth 500000 clockrate 500000 --- for the DTE device ! interface Serial0 ip address bandwidth 500000

Works fine, last long time. Don't forget to seup some routing or you will not pass traffic no matter what.

Anmerkung zu einem G.703 zu X.21 Wandler :

The (xxxxxx) is a basic PCM converter unit which provides a conversion from either a BNC or RJ45 (das ist G.703) interfaced network to a standard X.21 interface presented on a 15 way ‘D’ type socket.

It is simple to fit and there are no internal options to set as the unit is designed to only function with a PTT supplied network, which will provide the timing.

The front panel Line indicator allow the user to identify that valid HDB3 data is being received. If Green, valid HDB3 signal are detected otherwise the LED is red. If errors are detected in the HDB3 encoding of the data, creating a bipolar violation, the Line indicator will flash Yellow for each violation.


As the X21 interface only accommodates a single clock signal, used for both transmit and receive data, it is important that all data is referenced to a single source.


Normally this source is derived from the PCM network so equipment at each end of the link must be set to slave timing.

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