To understand Roaming architecture in GPRS, there are a few terms we have to understand,

Outbound roaming

The outbound roaming user simply means that you are travelling outside your country, but you still need to connect to the network of your country while you are outside. This means you are an outbound roamer.

Inbound roaming,

This is the visitor who is coming to your country, while he still needs to connect to the network of his own country. This means that this visitor is an inbound roamer.

Home PLMN 'HMPLN'

The home PLMN, is the PLMN the user originally belongs to.

Visited PLMN 'VPLMN'

The Visited PLMN, is the PLMN the user is currently visiting.

Roaming scenario

To understand the architecture of the scenario lets review the nodes tasks.

The SGSN is responsible for managing the location of the MS, and keep the session up while the MS is moving from one place to another.

While the GGSN is responsible for connecting to the PDN.

Now, let’s take the inbound roaming scenario as example to understand the roaming architecture,

the incoming visitor from a foreign country exist in your country, and he needs to connect to his network that exist in his country.

Therefore, the MS connects through the SGSN of the network the MS currently exist in (the VPLMN). Wile, the SGSN connects to the GGSN that exist in the visitor’s home country (HPLMN). Then, the GGSN of the HPLMN connects to the PDN of the HPLMN.

Please note that the interface between the SGSN and the GGSN in this case will be the 'Gp interface'.