** | R | NZ | 5-bit exponent | 9-bit mantissa |
** +----+----+------------------+-------------------------------+
**
-** R = reserverd (written as 0)
+** R = reserved (written as 0)
** NZ = 0 if 0 cells/sec; 1 otherwise
**
** if NZ = 1, rate = 1.mmmmmmmmm x 2^(eeeee) cells/sec