- This topic has 5 replies, 1 voice, and was last updated 13 years, 6 months ago by Webmaster.
5th August 2007 at 22:52 #48571rizGuest
As i have seen the VoIP calculator offered by this web site, lines to VoIP bandwidth calculator and Earlan to VoIP calculator, and read the tutorials on site. the thing i am not able to get is that why layer 2 header is not being added while calculating the bandwidth of headers being added(RTP,UDP,IP)as this will not give the actuall bandwidth required (and any packet has to pass through layer 1 via datalink layer). and in eg. for G.711 the 16kbps added(if we take 50 samples per sec) for headers for every 64kbps will not sufficient bandwidth if we omit layer 2 header. although the layer 2 is changeable and used according to requirement.
Riz khan6th August 2007 at 10:02 #48572WebmasterGuest
It’s true that the free online calculators do not take the datalink protocol into account. The datalink protocol could be PPP or ATM etc. and would add to the overall badwidth requirement.
But, the protocols you mention are not layer 2. IP is a layer 3 protocol (network). UDP is a layer 4 protocol (transport) and RTP is a layer 5 protocol (session). These three protocols are taken into account by the calculators.
For more information about the layered approach, you can study a document we published online.
Downloadable versions which also take layer 2 into account are available for purchase.6th August 2007 at 12:45 #48573rizGuest
yes, thats what i want to ask that while calculating overall bandwidth you add layer5,4,3 but why you are ignoring(not adding) layer 2 header in it. which will definitely not give correct bandwidth.6th August 2007 at 15:20 #48574WebmasterGuest
We omit layer 2 to keep the calculator simple. It is, after all, a free resource.
The effect that layer 2 has depends on what datalink protocol is used. Sometimes, it is neglibigle; sometimes it is significant.6th August 2007 at 18:15 #48575rizGuest
plz tell when datalink layer makes a significant contribution and when it is negligible and how much contribution does it makes.
and 2nd question is that in calculator you have use Earlang B formula out side the IP netork(i.e. you find only call blocking prbablity in PSTN network and after this you have add the header bandwidth and is telling total bandwidth while ignoring the blocking probablity with in the IP network (PLP:paket loss probablity due to overflow in queues and others)).16th August 2007 at 16:30 #48576WebmasterGuest
Your first question: It depends on the minimum payload a lay 2 protocol can support. If the minimum payload is large, then changing the size of other headers and the RTP payload makes no difference. You’ll need to research your specific layer 2 header.
Your second point: Yes, that’s true. But, in my opinion, not significant in the real world.