ESCON (Enterprise Systems Connection) is a marketing name for a set of IBM and vendor products that interconnect S/390 computers with each other and with attached storage, locally attached workstations, and other devices using optical fiber technology and dynamically modifiable switches called ESCON Directors. In IBM mainframes, the local interconnection of hardware units is known as channel connection (and sometimes as local connection to distinguish it from remote or telecommunication connection). ESCON's fiber optic cabling can extend this local-to-the-mainframe network up to 60 kilometers (37.3 miles) with chained Directors. The data rate on the link itself is up to 200 Mbps (million bits per second) and somewhat less when adapted to the channel interface. Vendor enhancements may provide additional distance and higher amounts of throughput.
In the early 1990s, ESCON replaced a much slower and more cumbersome system of interconnecting mainframes and attached devices known as the bus-and-tag cable. Bus-and-tag cable was copper, used a parallel bit attachment, and is usually referred to as a parallel environment in comparing it to the ESCON environment, which uses serial bit-by-bit technology.