Whoops started my own private convo with datucha... thought i would copy everyone else just to get more eyes on it and hopefully bring some clarification...
>From my understanding and using the Cisco doc the default g.711 packet size is 206bytes(including 2 - 10ms 80 byte voice samples = 160bytes or 1028bits) and using crtp is 168bytes using the calculations the pps is 64kbps/1028 bits = 50pps which using the same calculations of packet size is 1648 * 50pps = 82.4kbps total... and the same calculation using crtp same voice load so 50 pps bw calculation is total packet 1344bits * 50pps = 67.2kbps which is well below your layer 3 base of 80kbps and includes the same voice payload as the non-compressed packets at 82.4... the 64 and 8 kbps are the base layer 3 bandwidth for just the audio, not including any headers... If anyone sees it differently please let me know, i would hate to walk into the test and have this whole concept wrong... On Wed, Feb 1, 2012 at 2:05 PM, datucha123 datucha123 <datucha...@gmail.com>wrote: > > G711 64kbps and G729 8kbps are not the L3 Bandwidth for those codecs. > These are the Payload Bandwidths for those codecs. > > G711 and G729 on L3 are using 80 and 24 kbps respectavely. > > Here is the easy way to calcualte the Codec bandwidth: > > (Payload_Size + L3_Header + L2_Header) x PPS x 8 > > Where > *Payload_Size* - is the Codec Sample Size (Bit Rate). G711 at 20ms has > 160 bytes. G729 at 20ms has 20 Bytes and etc (you can find Sampling Rate > info on Cisco.com) > *L3 Header* - is the IP/UDP/RTP, which is 40 bytes, unless compressed > RTP is used. If the UDP checksum is used then the L3 Header is compresesd > up to 4 bytes, and if not used then up to 2 bytes. (UDP checksum is enabled > on a VoIP Dial-peer: "ip udp checksum" command. By default is disabled) > *L2 Header* - for Ethernet it is 18 bytes, for Frame Relay is 4 bytes > (based on QoS SRND). > *PPS* - Packets per Second. This is based on the Sampling Rate as well > in the following way: PPS = 1000 / Sample_Size in ms. > *8* - this is just bits. To convert Bytes into bits. > > > > On Thu, Feb 2, 2012 at 1:39 AM, Nathan Silvers <silver...@gmail.com>wrote: > >> Hi Everyone, >> >> Just starting out on my CCIE path and had a revelation that might not be >> huge to anyone else but if it helps one person understand I've done a good >> job... >> >> So I was struggling with Codecs how to calculate bandwidth required and >> all the header and crtp if that is involved and then Cisco throws on Layer >> 3 Bandwidth vs Overall bandwidth and from a humble voice guy who has >> focused mainly on LAN connections I am lost... >> >> http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml#topic1 >> Using the above Cisco document which can help and confuse(as they use >> median BW when it comes to the CRTP compression as it can be a 2 or 4 byte >> header depending on if you are using the UDP checksum) so my exact >> calculations were always off a few bytes.. >> >> So I was thinking about this logically and looking at the different >> pieces of the puzzle, >> Layer 3 is going to ultimately be the voice packets which is defined by >> the codec.. 711 = 64kbs, 729 = 8kbs, etc... >> Layer two includes my headers and such which add to the needed bandwidth >> depending on if they are compressed >> So if you use CRTP you only need and additional 3.6kbs for 711 which is >> the 67.6kbs bandwidth, or 11.6kbs for 729 >> Now if uncompressed the headers can add quite a bit more requiring the >> need for 18.8kbs more to put 711 at 82.8kbs and 729 at 26.8kbs >> >> So if you see a question regarding layer 3 bandwidth it is always the >> codec kbs, ie 711 is 64 and 729 is 8 regardless of any compression, the >> type of connection etc.. more of a no brainer question >> They Layer 2 bandwidth is where the savings are by chopping the 40byte >> IP/UDP/RTP header into 2 byes (or 4 bytes if UDP Checksum is enabled.) >> >> Gotta love white boards and running through a few situations.. the cisco >> doc has the calculations to go through and how changing the payload size >> can adjust the Packets Per Second which affects the required Bandwidth. If >> anyone else is struggling with this I highly recommend just writing it all >> out and trying hypothetical situations. >> >> Hope this helps someone! >> >> >> -- >> "The biggest mistake people make in life is not trying to make a living >> at doing what they most enjoy." >> >> - Malcolm Forbes >> >> Nathan Silvers >> >> >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, please >> visit www.ipexpert.com >> >> Are you a CCNP or CCIE and looking for a job? Check out >> www.PlatinumPlacement.com <http://www.platinumplacement.com/> >> > > -- "The biggest mistake people make in life is not trying to make a living at doing what they most enjoy." - Malcolm Forbes Nathan Silvers
_______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
