This attribute defines the type of clocking source used for synchronizing the E3 transmit clock: local means direct use of the local clock oscillator at its centre frequency; line means use of a signal phase-locked to the incoming E3 line rate; module means the use of a signal phase-locked to the CP reference clock. Note that the multiport cell-based cards have one common clock source for each of the three ports. For this reason, the following rules apply for the value of clockingSource across the board's three ports: All three clockingSource values must be 'local' OR All three clockingSource values must be 'module' OR One clockingSource value must be 'line' and the other two must be 'otherPort'. The default value for this attribute will be module if a CP clock is available, and local otherwise.