 EDFAs therefore allow a single-channel optical link to be upgraded in bit rate by replacing only equipment at the ends of the link, while retaining the existing EDFA or series of EDFAs through a long haul route.Furthermore, single-wavelength links using EDFAs can similarly be upgraded to WDM links at reasonable cost.The EDFA's cost is thus leveraged across as many channels as can be multiplexed into the 1550 nm band.