Applies only to FDDI lines.  ECHO LENGTH number of bytes of value
 ECHO DATA will be used to compose the next echo request frame which
 will be sent to the address specified by ECHO TARGET.  Hex_byte must
 be a string of exactly two hexadecimal digits.  The default ECHO DATA
 is 55.  ECHO DATA can be set in the volatile database, but it cannot
 be defined in the permanent database.