The term bandwidth–distance product (or bandwidth–length product) is often used in the context of optical fiber communications. It is usually defined as the product of the length of a fiber-optic link and its maximum signal bandwidth. That bandwidth is strongly related to the data rate (in Gbit/s), with a conversion factor which depends on the used modulation format. The bandwidth–distance product is typically limited by the phenomenon that the bit error rate rises sharply for too high data rates.
The concept of the bandwidth–distance product is helpful e.g. for comparing the performance of different types of fiber-optic links. However, the definition by no means implies that the achievable bandwidth–distance product is independent of the chosen fiber length. Whether this is the case, depends on the circumstances. It can be true for a link based on multimode fiber, the capacity of which is limited by intermodal dispersion; indeed, the bandwidth–distance product (based on the modal bandwidth) is mostly used in that domain. (A typical value may then be of the order of a few GHz·km for a graded-index fiber, or an order of magnitude less for step-index fibers.) On the other hand, in a link based on single-mode fiber, which is limited by chromatic dispersion and which does not contain means for dispersion compensation, a doubling of the data rate may enforce a reduction in the fiber length by a factor of four. The bandwidth–distance product is then effectively reduced by a factor of two. However, the transmission distance may of course also be doubled by concatenating two fiber-optic links with electronic regeneration between them.
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