This post covers Mobile and Wireless Networks by Khaldoun Al Agh, Guy Pujolle, and Tara Ali-Yahiya.

Basic Ideas

• System architecture evolution

  • Serving gateway (S-GW): the S-GW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and the packet data network gateway (PDN-GW)).
  • For idle state UEs, the S-GW terminates the downlink data path and triggers paging when downlink data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service and network internal routing information. It also performs replication of the user traffic for lawful interception.
  • MME: is the key control node for the LTE access network. It is responsible for idle mode UE tracking and paging procedures including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the S-GW for a UE at the initial attachment and at time of intra- LTE handover involving core network node relocation. It is responsible for authenticating the user. The non-access stratum signaling terminates at the MME and it is also responsible for the generation and allocation of temporary identities to UEs.
  • It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for non-access stratum signaling and handles the security key management. Lawful interception of signaling is also supported by the MME.
  • The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. Finally, the MME also terminates the S6a interface toward the home HSS for roaming UEs.
  • PDN-GW: the PDN-GW provides connectivity to the UE to external packet data networks by being the point of exit and entry of UE traffic. A UE may have simultaneous connectivity with more than one PDN-GW for accessing multiple packet data networks.
  • The PDN-GW performs policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening. Another key role of the PDN-GW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2.

• Control and user planes

  

  • The radio interface in LTE is characterized through its protocols where it can be defined by two main groupings according to the final purpose: user plane protocols and control plane protocols.
  • The first carries user data through the access stratum and the second is responsible for controlling the connections between the UE and the network and the radio access bearers. Even though separation of the control plane and the user plane was one of the most important issues of LTE design, full independence of the layers is not feasible because, without interaction between the user plane and the control plane, operators are not able to control QoS, the source/destination of media traffic or when the media starts and stops.

  

  • User plane
  • Figure shows the user plane protocol stack including the E-UTRAN and the S1 interface of a conventional, i.e. non-self backhauled, system.
  • The radio access uses the MAC, RLC and PDCP protocols. The user plane part of the S1 interface is based on the GPRS Tunneling Protocol (GTP), which uses a tunneling mechanism ensuring that IP packets destined for a given UE are delivered to the eNodeB where the UE is currently located.
  • GTP encapsulates the original IP packet into an outer IP packet which is addressed to the proper eNodeB. The S1 interface can operate over various layer 1/layer 2 technologies, e.g. fiber optic cables, leased (copper) lines or microwave links.
  • Figure also shows an example of a Transmission Control Protocol (TCP)/IP-based application, such as web browsing. The corresponding peer entities operate in the UE and at the server hosting the web application.
  • For simplicity, peer protocol entities of the server are drawn in the S-GW; however, in general, they are located somewhere in the Internet.
  • All information sent and received by the UE, such as the coded voice in a voice call or the packets in an Internet connection, are transported via the user plane.
  • User plane traffic is processed at different hierarchical levels, from eNodeB up to the core network (EPC). Also, control traffic is strictly tied to the user plane.
  • Irrespective of the reasons behind the current hierarchical architecture, for the transmission backbone, this means the higher the level of network hierarchy, the greater the amount of accumulated traffic generated. Therefore, higher level network elements will readily become the bottleneck of the network.
  • Therefore, transmission capacity should be fitted to the network hierarchy; at higher levels, high-capacity transmission means, such as fiber optics, are needed, but
  • when it comes to the edge of the network, microwave transmission becomes a more flexible and cost-effective substitution, particularly in terms of extending capacity.
  • Control Plane
  • The function of the control plane protocol is to control the radio access bearers and the connection between the UE and the network, i.e. signaling between E-UTRAN and EPC.
  • The control plane consists of protocols for control and support of the user plane functions:
  • controlling the E-UTRAN network access connections, such as attaching to and detaching from E-UTRAN;
  • controlling the attributes of an established network access connection, such as activation of an IP address;
  • controlling the routing path of an established network connection in order to support user mobility;
  • controlling the assignment of network resources to meet changing user demands.
  • In the control plane, the non-access stratum (NAS) protocol, which runs the MME and the UE, is used for control actions such as network attach, authentication, setting up of bearers and mobility management.
  • All NAS messages are ciphered and integrity is protected by the MME and UE. The RRC layer in eNodeB makes handover decisions based on neighbor cell measurements sent by the UE, pages for the UEs over the air, broadcasts system information, controls UE measurement reporting such as the periodicity of channel quality information, reports on and allocates temporary cell-level identifiers to active UEs. It also executes transfer of UE context from the source eNodeB to the target eNodeB during handover and does integrity protection of RRC messages.
  • The RRC layer is responsible for setting up and maintaining radio bearers.