In order to reach the full capacity of a TP1 network, areas can be connected together via a TP1 backbone line. The TP1 backbone line is limited to one TP1 segment, which shall NOT be extended by means of TP1/TP1 couplers. In order to connect a TP1 area to the TP1 backbone line, a TP1/TP1 coupler is to be used. The TP1 area main line is to be connected to the secondary side of this TP1/TP1 coupler.

Network capacity: [capacity backbone line] + [capacity area main line] x 15 + [capacity line] x 15 x 15

- the capacity taken by the 15 TP1/TP1 couplers from the backbone line (primary side) is 15 x 1 = 15 devices
- per TP1 area:
- the capacity taken by the TP1/TP1 coupler for the connection with the backbone line (secondary side) is 1 device
- the capacity taken by the 15 TP1/TP1 couplers from the main line (primary side) is 15 x 1 = 15 devices

- per TP area and per TP1 line:
- the capacity taken by the TP1/TP1 coupler for the connection with the main line (secondary side) is 1 device
- the capacity taken by the 3 TP1/TP1 coupler from the line (primary side + secondary side) is 3 x 2 = 6 devices

This brings the total capacity of a TP1 network to: [ 64 - 15 ] + [ 64 - 1 - 15 ] x 15 + [ (4 x 64) - 1 - 6 ] x 15 x 15 = 56794 devices.**Area 0**

In order to be complete, it needs to be stated that the backbone line is also the main line of area 0. This means that in total maximum 30 TP1/TP1 couplers can be connected to the backbone line.

This brings the total capacity of a TP network to: { [ 64 - 30 ] + [ (4 x 64) - 1 - 6 ] x 15 } + { [ 64 - 1 - 15 ] x 15 + [ (4 x 64) - 1 - 6 ] x 15 x 15 } = 60514 devices.**Check**

This calculation can also be reversed:

Total capacity: capacity of all lines + capacity of all main lines - capacity taken by all backbone couplers, line couplers and repeaters

[16 x 15 x 256] + [16 x 64] - [ (15 x 2) + (16 x 15 x 2) + (16 x 15 x 3 x 2) ] = 60514 devices.

The backbone line is also a main line (of area 0)