The Gr interface exists between the SGSN and the HLR. While the Gs interface exists between the SGSN and the MSC/VLR. Both of those interfaces are based on the 'SS7' protocol.

Those interfaces doesn’t pass user plane data. They only pass signaling messages. Therefore, those interfaces are having only control plane. There is no user plane exists for them.

Gr Interface

The Gr interface exists between the SGSN and the HLR.

Below is the protocol stack of the Gr interface,

MAP (Mobile Application Part)

MAP shares cellular subscriber information among different networks.

TCAP (Transactional Capabilities Application Part)

TCAP facilitates connection to an external database.

SCCP (Signalling Connection Control Part)

SCCP provides end-to-end routing. In other words, it determines the traffic is going to which application on the node, just like the TCP or UDP protocols in TCP/IP protocol stack.

Signalling Bearer

It is the SS7 protocol stack, it can be IP-based, or MTP-based, as exist in below graph,

Gs Interface

The Gs interface exists between the SGSN and the MSC/VLR. If this interface is configured, and the UE supports working in PS and CS domains, the UE will be able to make combined attach to the PS and CS domains in the same transaction.

BSSAP+ (Base Station System Application Part +)

 BSSAP+ supports signalling between the SGSN and the MSC/VLR.

SCCP (Signalling Connection Control Part)

SCCP provides end-to-end routing. In other words, it determines the traffic is going to which application on the node, just like the TCP or UDP protocols in TCP/IP protocol stack.

Signalling Bearer

It is the SS7 protocol stack, it can be IP-based, or MTP-based, as exist in below graph,