EXFO FTB-8500 Series for FTB-500 - Manual (Page 208)

Traffic Analyzer Tabs
194 FTB-8500 Series and FTB-8120NGE/8130NGE
FC RX
Error Analysis

Block (FC 10x): The number of frames received with an errored block
condition.

Symbol (FC 1x, 2x, and 4x): A Symbol error is declared when an
invalid symbol excluding the SOF and EOF is detected.

FCS: The number of received Framed Layer 2 or Framed Layer 1
(with xPAT) having an invalid FCS.

Oversize: An Oversize error is declared when a Framed Layer 2 with a
valid FCS exceeds 2148 bytes in length.

Undersize: An Undersize error is declared when a Framed Layer 2
with a valid FCS is smaller than 36 bytes in length.
Total Error Count: Indicates the total number of errors including all the
above errors.