The frames reached at the frame relay network and forwarded on the basis of DLCI. All the VCs are defined prior to the traffic on the network on the basis of DLCI (This is just like the MPLS network as packets are forwarded on the basis of Labels).
Now my Question is How can MPLS is used in frame relay network or what are its affects in the presence of DLCI (As DLCIs are also performing the same task and also works on layer 2) or these two are different equivalent technologies ?
The answer is : MPLS is working on FR like it's working on ethernet. Because the FR and Ethernet are running on Layer2. It's told that MPLS is working at layer 2.5 , meaning that is between Layer 3 , and Layer 2. So in order to work it needs a layer2 forwarding in your case FR's DLCI. On top of the Layer 2 it is the MPLS header.