Here, we illustrate the GPRS interfaces between the MS, the SGSN, and the GGSN in 3G network in both 'control plane', and 'data plane'.

• The 'control plane' is where the signaling messages are passing.
• While the 'data plane' is where user data is passing.

User Plane interfacesin 3G

Below diagram shows the interfaces exist between the MS and the GGSN in user plan in 3G network,

The RNS means 'Radio Network Subsystem'. And it contains the RNC and the NodB.

Gn interface

• SGSNs and GGSNs use the 'Gn interface' in the communication between each other.
• This interface is named 'Gp interface' in case the SGSNs exist in different PLMN than the GGSNs.
• The protocol stack for Gn/Gp interface shows that it uses TCP/IP in the network packet routing  between different nodes.
• While, it uses the GTP-U protocol 'GPRS Tunneling Protocol' in the application layer.
• GTP-U refers to 'GPRS Tunneling Protocol for the user plane', it tunnels user data between the SGSNs and the GGSNs in the user plan.

IuPS interface

The IuPS interface is the interface between the RNC and the SGSN in the user plan.

• The protocol stack of the IuPS interface in user plan, is the same as the Gn interface.
• This is because when using Direct Tunnel, the RNC will communicate directly with the GGSN in the user plan, instead of the SGSN. Therefore, the protocol stack between them should be the same.

IP/UDP

• Responsible for routing packets between the RNC and the SGSN.

GTP-U

• GTP-U refers to 'GPRS Tunneling Protocol for the user plane', it tunnels user data between the SGSNs and the GGSNs in the user plan.

Uu interface

It is the radio interface between the MS and the NodeB.

RLC

• 'Radio Link Control', it controls the logical links between the MS and the RNS over the radio interface.

PDCP

• 'Packet Data Convergence Protocol' , it transfers user data between the MS and the RNS.

Using of Direct tunnel

In case of using the 'Direct Tunnel', the RNC will communicate directly with the GGSN in the user plan. in this case the IuPS interface on the RNC will communicate with the Gn interface on the GGSN, as exist in below graph,

Control Plane interfaces in 3G

Below graph shows the interfaces exist in the control plan between the MS and the SGSN.

The IuPS interface is the interface between the RNC and the SGSN, and the Uu interface is the interface between the MS and the NodeB.

IuPS interface

• The IuPS interface in the control plan is different than the user plan. In control plan, IuPS is based on the SS7 protocol. And it contains the following protocols,

SCCP

• 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,

RANAP

• 'Radio Access Network Application Part', it handles signalling messages like 'RAB establishment' between the RNC and the SGSN.

GMM/SM

• Responsible for Mobility Management (MM) and Session Management (SM) functionality between the MS and the SGSN.

Uu interface

The Uu interface is the interface between the MS and the RNS.

RLC

• 'Radio Link Control', it controls the logical links between the MS and the RNS over the radio interface.

RRC 

• 'Radio Resource Control', establishes, maintains and releases 'Radio Resource connections' between the RNC and a mobile station.