Mobile Packet Core

There are two types of roaming, 'Inbound' and 'Outbound'.

Suppose that you are working as 'PS core engineer' in a mobile operator X. 

Inbound Roaming

Inbound Roaming means a user coming from an outside network to your network.

In this case, he will connect to the access network, and the SGSN of your network.

Then, the SGSN of your network will connect to the GGSN of the user's mobile operator.

In this case, your network is the VPLMN, and the user's network is the HPLMN.

Outbound Roaming

Outbound Roaming mens a user from your network is travelling away out of your network.

In this case, he will connect to the access network, and the SGSN of the mobile operator he currently exists in.

Then, the SGSN of this mobile operator will connect to the GGSN of your network.

In this case, your network is the HPLMN, and the network the user is traveling to is the VPLMN.