Bandwidth Calculator - Network

Transfer time is the payload divided by the delivery rate, but the arithmetic is only trustworthy once the units are reconciled. Storage is quoted in bytes and links are rated in bits per second, so a megabyte becomes eight megabits before the division. A further ambiguity is decimal versus binary prefixes: network engineering follows SI, where 1 Mbps is exactly 1,000,000 bits per second, while operating systems often report file sizes in mebibytes of 1,048,576 bytes, a 4.9 percent discrepancy at the mega scale and 7.4 percent at giga. Protocol framing then removes more of the nominal rate. A standard 1500-byte Ethernet MTU carries 1460 bytes of TCP payload after the 20-byte IP and 20-byte TCP headers, and each frame also consumes 18 bytes of Ethernet header and FCS, an 8-byte preamble, and a 12-byte interframe gap, leaving roughly 94 percent goodput at the frame level before acknowledgements and retransmissions. That is why an 80 percent efficiency assumption is a common planning heuristic rather than a physical constant; real figures range from about 90 percent on a clean local gigabit link to well under 50 percent on a congested or lossy wide-area path. The other frequently overlooked limit is the bandwidth-delay product: a single TCP stream cannot exceed its receive window divided by the round-trip time, so a 64 KB window over a 100 millisecond path caps near 5.2 Mbps no matter how fast the link is. This model also assumes one continuous stream, so it understates the time needed to move many small files, where per-file open, seek, and metadata overhead can dominate.

The idea that a channel has a measurable capacity in bits per second comes from Bell Labs. Harry Nyquist established in 1924 that signalling rate is bounded by bandwidth, Ralph Hartley extended the argument in 1928, and Claude Shannon unified them in his 1948 paper A Mathematical Theory of Communication, which gave the capacity relation C equals B times the base-two logarithm of one plus the signal-to-noise ratio. Every later data rate figure, from telegraph to fibre, is measured against that ceiling. Practical rates climbed in visible steps. The Bell 103 modem of 1962 moved 300 bits per second, the V.34 standard reached 28.8 kbps in 1994, and V.90 pushed 56 kbps in 1998. Local networking followed a separate curve: Robert Metcalfe and David Boggs built Ethernet at Xerox PARC in 1973, IEEE 802.3 standardized 10 Mbps in 1983, Fast Ethernet arrived in 1995, Gigabit Ethernet in 1998, and 10 Gigabit Ethernet in 2002. The unit confusion that still plagues transfer estimates was addressed in 1998, when IEC 60027-2 introduced the binary prefixes kibi, mebi, and gibi to separate powers of two from SI decimal prefixes, though adoption remains uneven. TCP window scaling, defined by Van Jacobson, Bob Braden, and Dave Borman in RFC 1323 in 1992 and revised as RFC 7323 in 2014, removed the 64 KB window ceiling that had made long high-speed paths impossible to fill.