Transmission Spectrum for unit cell vs device

[Nayab Shiraz]

https://docs.quantumatk.com/tutorials/transmission_gr_mos2/transmission_gr_mos2.html

The tutorial in this link says that transmission spectrum for a perfect periodic system  can be calculated from unit cell and detailed device setup is not required. Can I use this method to study the transmision spectrum of carbon nanotubes adsorbed with metal atoms?
Is the transmission given by this method is for an infinite tube and if I make a device that is for finite tube?

[Petr Khomyakov]

This method is valid for truly periodic system only. If you have a single scatterer or scattering region, you should build a device configuration with perfect leads connected to this scattering region. 

[Nayab Shiraz]

So there is no need to construct a device which is made by repeating the same unit cell?