ILLUSTRATION BY ANDY GILMORE

short timescales. Also, a single human operation sometimes translates to many computer operations (that is, round-trips). For example, opening a single Web page usually requires many round-trips, even if you are getting only a single large object (for example, a large picture).

 

Propagation, Bandwidth, Latencies, and hops The traversal of the fiber loop between New York and San Francisco presumes a data-transfer unit of a single

encoded binary digit of information. The lower bound for that traversal would be 2 × 19, or 38ms (or 368 million instructions). The time for this bit to travel from its source to its destination and back again is called its propagation delay.

Propagation delay is important, but compared with the much more common metric of bandwidth—measured in bits per second—it is rarely quoted as a figure of merit. At least partially, this is because the observed propagation delay depends on context, whereas

bandwidth (say of a fiber-optic transmission system) can be measured in isolation. Bandwidth can also be increased through engineering (for example, through encoding schemes for transmission systems that encode multiple bits per symbol) and thus is more attractive as a figure of merit to those who build transmission systems. Finally, bandwidth is a measure of work, which is attractive to purchasers.

Bandwidth can also affect latency, which is distinct, in my view, from propagation delay; the propagation