Clock rate on routers

[vishalpatil86]

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!

[lgijssel]

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]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

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In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

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]

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.

[jen.medyas]

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

[Daniel Mercado]

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.

[jen.medyas]

Hello Daniel,

Thanks for the reply, it was really helpful.

I would like to know a last thing. Having a clock rate, is it something mandatory ? I mean, if it defines the speed with which each device must emit, does it mean we always have one in a link ?

Thanks

[Daniel Mercado]

I cannot say that there will always be a clock signal on a serial line, there may not be. Additionally, to help with phase shift, there usually is a serial clock transmit external (SCTE) signal as well (don't worry about this one). If there are no cabling issues and the telco configured the CSU correctly, there will be a clock signal on the line. If a clock signal is not detected, the devices will default to a set value, 64000 bits/s on Cisco devices. Even if both devices run at the same value, if they don't have a way of synchronizing their clocks, you may still end up with a phase shift in the signal. Both clocking mismatches and phase shifts cause errors that can be seen in the "show interface xx" command. Of special interest to clocking are CRC, framing errors, and aborts. CRC is pretty easy to understand if you re-read my original post and read a bit about what CRC was design for. If your clocking is totally off, you will misread the 8 bits in a stream completely (like my previous example of interpreting "10" as "1100"). If your shift is of, you may read 7 bits or less correctly and then "fall off" or "bump into" one of the bits. "Fall off" would be when your phase shift is enough that you end up reading the next 0 instead of the 1 you just missed and "bump into" would be the inverse. Both of these would make your cyclic redundancy check fail. Depending on how off your shift is, you may end up with a lot of discarded messages.

[jen.medyas]

I understand now the concept thanks. But still, what will happen if I have 2 routers R1 and R2 and configure a clock rate in both of them (same WAN link) ? For example, x for R1 and y for R2 ? ( I already try it on gns, I thought it would return an error msg but it did not )
What will happen then ? will they negotiate a new clock ? or will they choose the small one ? I understand that the clock is essential to send/receive "correct" data so that mismatch will surely have a bad effect ( I guess...).
Thanks again

[Daniel Mercado]

Hello Jen,

I am unaware of any feature that would detect and autocorrect the mismatch in any modern IOS versions. I also think that the devices would have to cycle through all the possibilities until they can understand what is coming across the line. I have come across this in labs and in the real world. The mismatch may also be so severe that the interface will just continue to reset because it only sees garbage coming in. A severe mismatch would be like two devices speaking different languages while none is bilingual.

I have also used GNS3 for CCNA R&S and Security and quite a bit of CCIE lab studies, but moved away from it coincidently because you will not see many layer 1 and layer 2 issues or errors. I know that many CCIE R&S continue to use GNS3 to maintain many skills, but there are many things you will be missing out on your way there if you don't get your hands dirty with real devices while studying. I say that as a warning, because I have also seen many people stay stuck in basic functionalities because they did not and do not train on real equipment; even some CCIEs.

With that warning said, get yourself a great workbook, such as those offered by https://micronicstraining.com/ if they offer what you are pursuing and can afford one and then start to purchase equipment on sites like ebay.com. I know someone may discourage you from doing so, but you will learn a lot more on your terms and schedule. If you do it right, you will actually save some money this way. I am not trying to sell a particular brand, but I have found Narbik's way of writing the labs to be enlightening. I have even had situations where our engineering team was stuck and spent endless amounts of time going through Cisco guidance with no resolve and then came back home and understood the solution after a quick look at his books. Their pretty good, I also tried INS and I think they are OK and more affordable when I checked.

Don't hesitate to ask more. I'm happy to help.

[jen.medyas]

Thank you very much. I'll see if I can buy an used router from eBay. Thanks again !

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I haven't tried it, but on real equipment back-to-back, I would expect the link to not function.  Setting the clock rate, or not, also determines whether the interface is the DCE side or the DTE side.  I wouldn't expect DCE<>DCE to work.

[angelawright]

By assigning a clock rate to the router, the timing is set. This allows a router to adjust the speed of its communication operations, thereby synchronizing with the devices connected to it.