Layer 2 header issues ; voice call bandwidth calculation

 

Hai Mark ,

 

I have been going through your ccie voice advanced technologies .In Qos you have mentioned that frame relay header size in 4 bytes which is true according to srnd .This was a fact that i was not at all  able to digest because for the past 4 years according to me it was 6 byte header followed by 3 byte trailer .Why I was asking you this question is because I was following IP Telephony Self Study Qos which is written by Wendell odom .Within this book it is explicitly mentioned in page 477 that frame relay header is 6bytes + 3 byte Trailer. Also within cisco official curriculum books that talked about how to calculate the bandwidth for a voice call ,there also the information for frame relay header is 6 bytes .For ethernet trunk its 22+4=22 bytes ,ppp its 6-9 bytes etc .Things changed when I have seen Qos SRND .

Which document shold i follow ,Qos SRND or cisco official curriculum ?

 

whats the reason for ambiguity regarding this information ?

Hope to receive an answer from you .

 Thanks for your valuable time .

 Regards,

 Adam Luckey

 

Comments

  • Hi Adam,

    Good observations. I will try to explain with brevity both why I refer to that document as well as possibly why it might state 4 bytes.

    The overwhelmingly primary reason I refer (or one could even say "lean") so heavily on that document, is not that it is the newest QoS guide (CUCM SRND as well as the "MediaNet" SRND and other publications have much newer and more accurate information), nor that it is without fallibility (it does contain some errors) -- it is because it is the document that is on the CCIE Candidates desktop in PDF form in the actual CCIE Voice lab exam. Not in the Cisco Documentation web site that you have a link to from the candidate desktop -- on the desktop as a standalone PDF (along with the CUCM SRND). Which logically means that the person/people in charge of populating that desktop with icons (such as router & switch terminal shortcuts, notepad, calculator, and a shortcut to the online DocCD) must think that it's fairly important, and probably use that information in their grading (they have admitted plainly at Cisco Live - Networkers in past years that they do). 

    As for the 4 bytes, the QoS Scheduler mechanism in Cisco IOS only accounts for 4 of the 6/7 bytes of FR header in it's calculations, and that is the real reason we and that QoS SRND refer to that.

    For a more in-depth look at exactly which header parts (and therefore which bytes) the IOS scheduler accounts for, please see this excellent post by on of our esteemed authors (Petr Lapukhov) who actually reverse engineered Cisco's IOS QoS Scheduler: Computing VoIP Traffic Bandwidth Consumption.

    Some other blog articles by Petr dealing with QoS relevant to the CCIE Voice exam can be found here:


    Kind Regards,

    Mark Snow, CCIE #14073
    (Voice, Security)
    Instructor
    Internetwork Expert, Inc.
    Toll Free: 877-224-8987
    Outside US: +1 775 826 4344

    - docendo discitur



    On Jun 27, 2011, at 6:26 AM, adam.luckey222 wrote:
     

    Hai Mark ,

     

    I have been going through your ccie voice advanced technologies .In Qos you have mentioned that frame relay header size in 4 bytes which is true according to srnd .This was a fact that i was not at all  able to digest because for the past 4 years according to me it was 6 byte header followed by 3 byte trailer .Why I was asking you this question is because I was following IP Telephony Self Study Qos which is written by Wendell odom .Within this book it is explicitly mentioned in page 477 that frame relay header is 6bytes + 3 byte Trailer. Also within cisco official curriculum books that talked about how to calculate the bandwidth for a voice call ,there also the information for frame relay header is 6 bytes .For ethernet trunk its 22+4=22 bytes ,ppp its 6-9 bytes etc .Things changed when I have seen Qos SRND .

    Which document shold i follow ,Qos SRND or cisco official curriculum ?

     

    whats the reason for ambiguity regarding this information ?

    Hope to receive an answer from you .

     Thanks for your valuable time .

     Regards,

     Adam Luckey

     



    INE - The Industry Leader in CCIE Preparation
    http://www.INE.com

    Subscription information may be found at:
    http://www.ieoc.com/forums/ForumSubscriptions.aspx

Sign In or Register to comment.