The Access Network of LTE, known as E-UTRAN, is composed of a network of eNodeBs, which manage normal user traffic without a centralized controller, leading to a flat network architecture. eNodeBs are interconnected through the X2 interface and connected to the EPC via the S1 interface, specifically using the S1-MME interface for the MME and the S1-U interface for the S-GW. The protocols operating between the eNodeBs and the UE are called Access Stratum (AS) protocols.
E-UTRAN handles all radio-related functions, which include:
- Radio Resource Management: This involves managing radio bearers, including radio bearer control, radio admission control, radio mobility control, and the scheduling and dynamic allocation of resources to UEs in both uplink and downlink.
- Header Compression: This ensures efficient use of the radio interface by compressing IP packet headers, reducing overhead, which is especially beneficial for small packets like VoIP.
- Security: All data transmitted over the radio interface is encrypted to ensure security.
- Connectivity to the EPC: This includes signaling towards the MME and maintaining the bearer path towards the S-GW.
These functions enable E-UTRAN to manage the radio interface effectively, ensuring secure and efficient communication between UEs and the EPC.
How E-UTRAN is different from previous Generations?
In LTE, the network functions are concentrated within the eNodeBs, each managing multiple cells. Unlike previous generations where the radio controller was separate, LTE integrates this function directly into the eNodeB. This integration allows for tighter interaction between different protocol layers of the radio access network, which reduces latency and enhances efficiency.
By distributing control, LTE eliminates the need for a high-availability, processing-intensive central controller, reducing costs and avoiding single points of failure. Additionally, since LTE does not support soft handover, there is no need for a centralized data-combining function.
As the UE moves, the network must transfer all UE-related information, or UE context, along with any buffered data from one eNodeB to another. This process requires mechanisms to prevent data loss during handover, which are facilitated by the X2 interface.
The S1 interface, which connects the Access Network to the Core Network (CN), features an important concept known as S1-flex. This concept allows multiple CN nodes (MME/S-GWs) to serve a common geographical area, forming a mesh network with the eNodeBs in that area.
For example, an eNodeB can be served by multiple MME/S-GWs, as shown with eNodeB#2 in Figure above. This setup creates an MME/S-GW pool, and the area covered by such a pool is called a pool area. This arrangement enables load sharing among multiple CN nodes and eliminates single points of failure. The UE context typically remains with the same MME as long as the UE is within the pool area.