Understanding Long Term Evolution (LTE) can sometimes be difficult as the technology is very complex.
LTE Architecture Overview
When it comes to the EPS game, the bearer is the quarterback. The white paper notes that EPS uses these bearers to route IP traffic from a gateway in the pocket data network (PDN) to the user equipment (UE). A bearer is an IP packet flow with a defined quality of service (QoS) between the gateway and the UE. These bearers allow Internet access. They also run services such as Voice over IP (VoIP), and are often associated with a QoS.
“Multiple bearers can be established for a user in order to provide different QoS streams or connectivity to different PDNs. For example, a user might be engaged in a VoIP call while at the same time performing web browsing or FTP download.” To protect the network, the EPS network utilized its elements that have different roles.
The Core Network
Overall control of the UE within the LTE architecture is handled by the core network. The core network (also known as EPC in SAE) is also responsible for establishing the bearers.
The main components of the EPC are:
• PDN Gateway (P-GW)
• Serving Gateway (S-GW)
• Mobility Management Entity (MME)
Other functions and nodes within the EPC include the Home Subscriber Server (HSS) and the Policy Control and Charging Rules Function (PCRF). “Since the EPS only provides a bearer path of a certain QoS, control of multimedia applications such as VoIP is provided by the IP Multimedia Subsystem (IMS), which is considered to be outside the EPS itself.” To dive even deeper into the LTE architecture playing field and to gain a more technical understanding, take a look at these LTE architecture diagrams.
The Access Network
Leave it to LTE to function with not only a core network, but also an access network. The access network of LTE, known as E-UTRAN, consists of a network of eNodeBs. For normal user traffic, there is no centralized controller in E-UTRAN; therfore the E-UTRAN architecture is considered flat. According to Alcatel-Lucent, the eNodeBs are interconnected with each other by an interface known as “X2” and to the EPC by the S1 interface — more specifically, to the MME by the S1-MME interface and to the S-GW by the S1-U interface. “The protocols that run between the eNodeBs and the UE are known as the ‘AS protocols.’” The E-UTRAN is responsible for all radio-related functions, which can be summarized briefly as:
• Radio resource management (RRM) – This covers all functions related to the radio bearers, such as radio bearer control, radio admission control, radio mobility control, scheduling and dynamic allocation of resources to UEs in both uplink and downlink.
• Header Compression – This helps to ensure efficient use of the radio interface by compressing the IP packet headers that could otherwise represent a significant overhead, especially for small packets such as VoIP.
• Security – All data sent over the radio interface is encrypted.
• Connectivity to the EPC – This consists of the signaling toward MME and the bearer path toward the S-GW.
