lanes, and Transport and User Layer Separation One important part of the 3GPP architecture is the differences between the control and user planes, and transport and user layers. This terminology is sometimes quite confusing, but understanding this is an important part of understanding the 3GPP architecture, and how it works. We will first look at the user and control planes, and then look at the transport and user layers. Control and User Planes The 3GPP architecture is divided into the control and user planes. The control plane takes care of the signaling in the network. It transports the messages that are needed to attach a subscriber to the network, authenticate and authorize the user, open and close connections, and perform handover signaling. As the name implies, the control plane transports signals that control the network and the mobile station. The user plane, on the other hand, is responsible for transporting the user packets through the network. Depending on the interface, the control and user plane can be realized with completely different protocols, or different parts of the same protocol. This is due to the very different nature of the control and user planes. The control plane messages are sent when there is something to signal, and to refresh the state in the network. It is under heavy load, especially when the network connection is set up and also during handovers. The user- plane load is dependent on the download and upload speed of the subscriber. Transport and User Layers The transport layer refers to the IP network and the related transport protocols used to transport the control and user planes in the 3GPP network. This is the network that connects the network elements to each other. End users do not have access to this network. The transport layer network of the operator is connected to other operators' networks to enable control- and user-plane connections during roaming. Basics of the 3GPP Technologies The user layer consists of the users' packets. This includes the users' IP packets and t