MS – BTS – BSC – TRC – MSC
MS – BTS : using Um interface
BTS – BSC : using Abis interface
BSC – TRC : using Ater interface
TRC – MSC : using A interface
Um is GSM radio link
for A, Ater, Abis generally using E1 link, but it is also mixed with microwave, ATM even STM link depending to planning and what hardware type and vendor which u are using now.