This thesis is one of the answers to two questions as follows :
1) Why isn't there a overall roaming including all the mobile phone operators (cellulars and PCSs) in Korean mobile communications?
2) Why and in what conditions does a partial roaming happen?
To analyze, network coverage is regarded as a vertical quality difference.
In answer to these questions, the conclusions are as follows :
1) A overall roaming doesn't happen because it makes the service homogenous and price competition stronger and finally reduces the profit of the operators
2) In 3-firm competition a partial roaming happens because the roaming between two firms makes the quality of their service most different from that of the other's and raises their profit in case that the width of the quality difference that they can choose is sufficiently small.
