As the ITU Recommendations do not define in what order VC12 timeslots are arranged in a VC4, currently there are two timeslot numbering policies: Sequential Mode (ITU-T) and interleaved.
By Order
The timeslot numbering by order is shown in Figure 1.
NOTE: 
The calculation formula for VC12 numbering by order is as follows: VC12 number = TUG3 number + (TUG2 number - 1) x 3 + (TU12 number - 1) x 21
Interleaved
The timeslot numbering in the interleaved mode is shown in Figure 2.
The calculation formula for VC12 numbering in the interleaved mode is as follows: VC12 number = (TUG3 number - 1) x 21 + (TUG2 number - 1) x 3 + TU12 number
The Timeslot Numbering Policy for Interconnection with Third-Party Equipment
When equipment from different vendors requires 2 Mbit/s service interconnections, service configuration is not easy as the same VC12 in a VC4 can be numbered differently depending on the numbering policy used by a vendor. As a result, you need to translate the VC12 number.
The Web LCT processes timeslot numbering as follows:
- The Web LCT does not allow you to query and set the timeslot mode for a single NE. Hence, the by order mode is used by default on the Web LCT regardless of which mode is set on the NE.
- In the Web LCT, a button is added to switch the timeslot mode. The switching operation only switches the display of the user interface. This change is not applied to the NE and the timeslot configuration on the NE is not affected.