Performance Monitoring traffic direction selector.
coToCpe(2) measures from CO's active trunk port to
CPE's active trunk port.
cpeToCo(3) measures from CPE's active trunk port to
CO's active trunk port.
To measure from one CPE to another, the user should
designate the source CPE as cpeTx(4) and the
destination CPE as cpeRx(5).
The reflector CPE must be activated before the
source CPE.