In frame mode, MPLS uses a 32-bit label that is inserted between the Layer 2 and Layer 3 headers i.e layer 2 encapsulations like HDLC, PPP, Frame Relay & ATM (atm can work in both cframe as well as cell mode)
In MPLS over ATM networks, routers are connected to ATM-based provider networks consisting of ATM switches that forward data based on virtual circuits (VCs) provisioned on the ATM switches. Cell-mode MPLS uses the virtual path identifier/virtual channel identifier (VPI/VCI) fields in the ATM header as the label value.
I suggest you get hold of these 2 books for MPLS
MPLS & VPN Architectures -- Jim Guichard ISBN: 1-58705-112-5
MPLS Fundamentals -- Luc De Ghein
HTH
Narayan
