7 Replies Latest reply: Dec 28, 2008 1:52 PM by B Haines RSS

    Clock Rate on DCE



      Hi All,



      Exactly what is Clock rate ?



      All I know about it is that it is used in CSU/DSU type of device that provides clocking for the line to the router. Clocking is typically provided by DCE network to routers. Thats it !



      I was trying to configure 3 routers (A, B, C) to a router (M) above them. I set the clock rate of 64k on the interface of M going to A. And set clock rate on B and C going to M. As a result, only link between M & A was working but not between M&B, M&C.



      But when i reset the clock rate on all M interface and remove clock rate from B and C, link was working fine.



      Can you please let me know what could be the possible reason.


















        • 1. Re: Clock Rate on DCE

          On reading your posting, I used Packet Tracer 5.0 to design your scenario and believe you me, it worked perfectly. All the serial interfaces were up as well as the line protocols.


          First of all, I think you should check your configurations well, making sure that the serial interfaces between the routers are having the correct subnetting and the links do not have the shutdown command on them.


          Also, make sure you have the correct and matching encapsulation on the serial interfaces and that, you are configuring the right interfaces with the DCE cabling but not the DTE. You can use the show controllers command to see which interface is connected as the DCE.


          I believe this could be of help.



          • 2. Re: Clock Rate on DCE
            Scott Morris - CCDE/4xCCIE/2xJNCIE


            show controllers | include DCE|DTE



            That'll help track down which side should be responsible. Another method is to put clock rate on BOTH sides. The command won't generate an error, but will not appear in the configuration of the side which has the DTE cable!












            • 3. Re: Clock Rate on DCE

              Hi Richa


              What is clockrate? To send you a message I need you to differentiate between two or more states.


              Think of morse code. It has a gap, a dot and a dash. So long as the receiver can tell the diffrence between a dot and a dash you can send at any speed. Just think how fast people can text.


              Modern communication uses two states off and on but what is the difference between offoffoffononon and off off offf on on on. So did I send 01 or 0001111. The answer is the clockrate.


              So If I send 000 in one second and I tell you the clockrate is one second then you know I actually sent 0. If the clockrate is six per second then I sent 000000.


              Early communication would sent a start bit (or two) eight bits and a stop bit. So As soon as you saw the circuit voltage go high you would start you clock. You then measure the voltage for the next eight clocks and final check for the last clock.


              At Higher speed you would have a start character followed by lots of data and terminated by a stop character. So you would receive a block or frame of data.


              So normally one end provides the clock and the other end listens. The old cable contains a clock pin or clock pins because clocks can wander. Normally the telco provides the clock but in your set up M would provide the clock and you need to code it on the controller or serial statement depending on the hardware. Telco normally uses an atomic clock so if you need a clock for the router use the telco clock.


              Modems will try different clockrate with each other until they find one that the interconnecting copper (or aluminium) can support. This is called stepping down.


              Regards Conwyn

              • 4. Re: Clock Rate on DCE
                Scott Morris - CCDE/4xCCIE/2xJNCIE


                To take that a little step further, one has to wonder about if we don't know what the clock rate is, how do we understand what the clock rate is that's being sent to us? In the modem world, it was a statically set thing. (You can try this with your console port settings to understand).



                Serial links can send anywhere from 300 baud (symbols per second - even lower in the REALLY OLD days!) on up to 115200 baud. Console ports default to 9600 baud. So what happens if you set your terminal program to 2400 baud? Or 14400? You'll still receive information just fine. You will simply interpret it incorrectly. That's what all the wierd stuff on your screen is!



                On T1/E1 lines, we actually reverse engineer the clocking by understanding a common sequence (called framing code) in order to figure out what makes sense. Again, SOME static configuration, such as B8ZS/ESF in the T1 world will tell us our linecode and framing types. Then we simply have to FIND it! (Some more complex issues can be introduced here, but I'm resisting the urge to cause explosions this fine Christmas Day!!!)



                But it's all about understanding the message being sent to us.









                PS. Merry Christmas, or Happy Holidays to you all!



                • 5. Re: Clock Rate on DCE


                  Thanks all of you for elaborating Clock rate for me.



                  Conwyn you really help me to understand the concepts of clock rate. Nice explaination.






                  Merry Christmas to you all ! Have nice holidays.









                  • 6. Re: Clock Rate on DCE

                    Hi Richa


                    Scott is correct. When I started modems were 300 bits per second. I worked on a teletext system which allowed you to send blocks (characters) to a television at 1200 and had a keyboard to enter data at 75 bits per second. It was a public system with page numbers but there were also private page numbers. The system was used for stock management and payroll systems.


                    Regards Conwyn

                    • 7. Re: Clock Rate on DCE
                      B Haines


                      Thanks for the 'set clock rate on all interfaces' info. I was unaware that you could do that! LoL