During typical capacity planning processes i see a perhaps simplistic consideration for estimating only the page or data sizes that are expected to pass through the network pipes. That means like in regular application development the average HTTP page response size is 30KB, or the XML web service data size averages 100KB, such figures are taken raw and multiplied by the forecasted volume to get the expected bandwidth requirements.
It seems application developers and architects tend to forget, or are totally unaware of, the fact that application data are encapsulated into lower-layer transport layers for delivery. So they seldom consider the bandwidth eaten up by header overheads in TCP/IP packets and Ethernet frames.
I feel these overhead should be considered during bandwidth planning, but do not know how to accurately calculate these. Is there an basic formula or process that factors these overhead percentages? So far I have not seen any article that discusses this matter.