Overview

eNodeB architecture

Basic eNodeB Architecture source: https://docs.srsran.com/en/latest/usermanuals/source/srsenb/source/1_enb_intro.html

The srsENB application includes layers 1, 2 and 3 as shown in the figure above.

At the bottom of the protocol stack, the Physical (PHY) layer carries all information from the MAC over the air interface. It is responsible for link adaptation and power control.

The Medium Access Control (MAC) layer multiplexes data between one or more logical channels into Transport Blocks (TBs) which are passed to/from the PHY layer. The MAC is responsible for scheduling uplink and downlink transmissions for connected UEs via control signalling, retransmission and error correction (HARQ) and priority handling between logical channels.

The Radio Link Control (RLC) layer can operate in one of three modes: Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). The RLC manages multiple logical channels or bearers for each connected UE. Each bearer operates in one of these three modes. Transparent Mode bearers simply pass data through the RLC. Unacknowledged Mode bearers perform concatenation, segmentation and reassembly of data units, reordering and duplication detection. Acknowledged Mode bearers additionally perform retransmission of missing data units and resegmentation.

The Packet Data Convergence Protocol (PDCP) layer is responsible for ciphering of control and data plane traffic, integrity protection of control plane traffic, duplicate discarding and in-sequence delivery of control and data plane traffic to/from the RRC and GTP-U layers respectively. The PDCP layer also performs header compression (ROHC) of IP data if supported.

The Radio Resource Control (RRC) layer manages control plane exchanges between the eNodeB and connected UEs. It generates the System Information Blocks (SIBs) broadcast by the eNodeB and handles the establishment, maintenance and release of RRC connections with the UEs. The RRC also manages security functions for ciphering and integrity protection between the eNodeB and UEs.

Above the RRC, the S1 Application Protocol (S1-AP) layer provides the control plane connection between the eNodeB and the core network (EPC). The S1-AP connects to the Mobility Management Entity (MME) in the core network. Messages from the MME to UEs are forwarded by S1-AP to the RRC layer, where they are encapsulated in RRC messages and sent down the stack for transmission. Messages from UEs to the MME are similarly encapsulated by the UE RRC and extracted at the eNodeB RRC before being passed to the S1-AP and on to the MME.

The GPRS Tunnelling Protocol User Plane (GTP-U) layer within srsENB provides the data plane connection between the eNodeB and the core network (EPC). The GTP-U layer connects to the Serving Gateway (S-GW) in the core network. Data plane IP traffic is encapsulated in GTP packets at the GTP-U layer and these GTP packets are tunneled through the EPC. That IP traffic is extracted from the tunnel at the Packet Data Network Gateway (P-GW) and passed out into the internet.