Bandwidth-delay product

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In data communications, bandwidth-delay product refers to the product of a data link's capacity (in bits per second) and its end-to-end delay (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the amount of data "on the air" at any given time, i.e. data that have been transmitted but not yet received. Sometimes it is calculated as the data link's capacity times its round trip time^[1].

Obviously, the bandwidth-delay product is higher for faster circuits and for long-delay links such as GEO satellite connections. The product is particularly important for protocols such as TCP that guarantee reliable delivery, as it describes the amount of yet-unacknowledged data that the sender has to duplicate in a buffer memory in case the client requires it to re-transmit a garbled or lost packet.^[2]

A network with a large bandwidth-delay product is commonly known as a long fat network (shortened to LFN and often pronounced "elephant"). As defined in RFC 1072, a network is considered an LFN if its bandwidth-delay product is significantly larger than 10⁵ bits (~12 kB).

[edit] Examples

Customer on a DSL link, 1 Mbit/s, 200 ms one-way delay: 200 kbit = 25 kB
High-speed terrestrial network: 100 Mbit/s, 100 ms: 10 Mbit = 1.25 MB
Server on a long-distance 1 Gbit/s link, average one-way delay 300 ms = 300 Mbit = 37.5 MB total required for buffering