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Clock rate on routers

vishalpatil86
Beginner
Beginner

Hi,

am reading some books for the CCNA..however...i can't seem to fully understand what exactly is the clock rate used for...when I configure a serial interface I always set up a clock rate of 56000 but I don't know what exactly it does? Any ideas?? THANK YOU!!!


And also i have another question, are serial connections only to connect other routers?

Serial interface is the only interface to which we can give a clock rate??


thank you!

12 REPLIES 12

lgijssel
Engager
Engager

The clock rate serves to synchronize the receiver on the other side of the link.

Each clock pulse signals the transmission of a bit.

Note that it is mainly used in lab setups. In the real world, network providers almost always deliver the clock signal.

And also i have another question, are serial connections only to connect other routers?

No, they are also used on other equipment like multiplexers for example.

Serial interface is the only interface to which we can give a clock rate??

As far as my practical experience goes, Yes.

Note that per standard only DCE interfaces are generating/sending  a clock.

You could perhaps configure it on a DTE but it would not do anything because there is only a clock input pin.

http://en.wikipedia.org/wiki/Data_circuit-terminating_equipment

http://en.wikipedia.org/wiki/Data_terminal_equipment

regards,

Leo

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

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Posting

Clock rate is used by the sender/receiver so it "knows" how often signally may change to indicate ones and zeros.  Higher clock rates mean higher (faster) bits per second transmission rates.  (It's also used to "know" signally boundaries.)

If your working in lab with back-to-back routers, ping across the link at 56K and then increase the clock rate to the maximum.  If you ping again, you should see the ping times decrease as it takes less time to send a frame/packet across the serial interface.

Daniel Mercado
Beginner
Beginner

This command tells the router how many samples it should take per seconds from the recieving interface. The the Telco provides the clocking to your router to ensure that there is no significant phase shift between the transmitted signal and the received signal, which would result in data loss. For example, if the intended message is a "binary 10" and the Telco transmits a 1 for 2 seconds and a 0 for 2 seconds, but your router is set to sample once per second, your router will interpret a binary 1100 which in no way represents the intended message. Of course, I am oversimplifying the process but I hope this helps.

Hi,

This command tells the router how many samples it should take per seconds from the receiving interface.

Does it mean that the router is, in this case, a DTE ?

Also, the clock command is issued to "apply" my own clock rate ( the router's one, here ), if the Telco is providing his own clock rate, which one will be kept ?

Thanks

Hello Jen,

That first statement can help add to the original confusion. I should have stated, "This command, on the DCE side, tells the routers on the link how many samples they should take per seconds and how often they should send on that same link." Basically, it is used to control how often both DCE and DTE devices should read and "write" or receive and transmit on that link.

The only thing you have to really worry about for CCNA and as a network administrator are the following facts.

* The cable will determine which side will be the DCE.

* All serial interfaces will be the DTE by default and will have a default clock rate of 64000.

* You would only have to determine which side is the DCE by looking at the cable or with show controllers commands and only if you are labing or if you are working for the Telco.

*The DCE will send a clocking signal to the line and the DTE will receive it by default and adjust the clock rate automatically by default.

As a network admin at the customer premise (CCNA R&S), you will not have to configure the clock rate. As a network admin for the Telco (CCNA SP), you will. There could be more to it, but it is enough for the CCNA R&S and CCNP R&S exams and most real world setups. I have not taken any of the SP training or exams.

Please let me know if you need more help and I will try my best to explain.