QCI-to-DSCP mapping cannot be fixed by 3GPP standard as it depends on transmission network design. Transmission equipments must prioritze the IP packets according to the service they are carrying, so the mapping must be configured according to TX network. QCI is either decided by HSS or PCRF. When some down link traffic comes, PGW fetches what is the corresponding QCI and accordingly sends the traffic over that particular bearer and vice versa.
DSCP is interpreted by routers outside LTE and QCI is internal to the LTE network. Hence mapping is done at the entry/exit node of the LTE network i.e., PGW.
DSCP is field in IP header, QCI is end to end parameter mapped in each part of the network in different parameters or group of parameters (in radio interface, transmission network..). DSCP value mapping to QCI is mapping for prioritizing bearers in transmission network and can be (and will be) done on P-GW, S-GW and eNB: Mapping which you configure on eNB will take place in up-link: when eNodeB has to transmit IP packet which carries GTP packet corresponding to bearer with CQI 7, eNB will add DSCP value which (according to what you have configured on eNB) corresponds to QCI7 in IP header of that IP package. Important is to emphasize that eNB itself, when receive down-link package, does not care for received DSCP in IP header (placed by S-GW), eNB does not compare recieved DSCP with configured DSCP on eNB for that QCI - moreover, those 2 DSCP values (for UL and DL) can be different, depending of transmission network. What you configure on S-GW for S1 will take place on S1 down-link in the same way as described for eNB... And analogically for other cases...
