This counts Errored Blocks on this SONET/SDH port: the number of error-detected blocks (at whatever block granularity applies to this signal's rate) recorded during the bin, per the MIB's description. EB is the block-based error tally that feeds the G.826-style performance parameters for this port -- because it counts blocks rather than seconds, it can distinguish a bin with a few, heavily-corrupted seconds from one with light errors smeared across many seconds, which plain ES on the same bin cannot; because this bin is the still-open, currently-accumulating interval, its value is only a partial count until the bin's Duration elapses and it rolls into history -- polling it twice a few minutes apart is the fastest way to catch a fault building up in near real time. An EB count of 494 for this bin, spread across relatively few ES for the same interval, would suggest a small number of seconds carried a disproportionate share of the errors on this port.