![]() The GTP is a group of IP-based transport protocols that are used to carry IP packets within GSM, UMTS, and LTE networks. One GTP tunnel is established for each radio bearer in order to carry user traffic between the eNB and the selected S-GW. The S1-U consists of GTP-U protocol running on top of user datagram protocol (UDP), which provides best-effort data delivery. ![]() The S1-MME is responsible for EPC bearer setup and release procedures, handover signaling, paging, and NAS signaling transport. Each SCTP association between an eNB and an MME can support multiple UEs. The S1-MME carries S1 application protocol (S1-AP) messages, using SCTP 2 over IP to provide guaranteed data delivery. The S1 interface supports a multi-point connection among MMEs/S-GWs and eNBs. The RLC and MAC sublayers both restart in a new cell after the handover.įigure 3.6. Data protection during handover is the responsibility of the PDCP sublayer. In the absence of any centralized control node, data buffering during handover due to user mobility in the E-UTRAN must be performed in the eNB itself. The E-UTRAN user-plane protocol stack is shown in Figure 3.6, consisting of the packet data convergence protocol (PDCP), radio link control (RLC), and medium access control (MAC) sublayers that are terminated in the eNB on the network side. It is the main controlling function in the AS responsible for establishing the radio bearers and configuring all lower layers using RRC signaling between the eNB and the UE. The radio resource control (RRC) protocol is known as layer 3 in the AS protocol stack and in 3GPP RAN terminology. The lower layers perform the same functions as for the user-plane with the exception that there is no header compression function for the control-plane. This includes both access stratum (AS) and NAS protocols. The protocol stack for the control plane between the UE and MME is shown in Figure 3.5. ![]() A 3GPP-specific tunneling protocol called GPRS tunneling protocol (GTP) is used over S1 and S5/S8 interfaces. Different tunneling protocols are used across different interfaces. An IP packet for a UE is encapsulated in an EPC-specific protocol and tunneled between the P-GW and the eNB for transmission to the UE. The main functions in each entity and their termination points in the network have been shown in the figure. The functional split between the EPC and E-UTRAN is shown in Figure 3.4.
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