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Sunday, February 8, 2015

Data Flow Across LTE Protocol Layers


The LTE (long term evolution) air interface provides connectivity between the user equipment (UE) and the eNB (eNodeB). It is split into a control plane and a user plane. Among the two control plane signalings, the first is provided by the access stratum (AS) and carries signaling between the UE and the eNB. The second carries non-access stratum (NAS) signaling messages between the UE and the MME (mobility management entity), which is piggybacked into an RRC (radio resource control) message. The user plane delivers the IP (Internet protocol) packets to and from the EPC (evolved packet core), the S-GW (serving gateway), and the PDN-GW (packet data network gateway).

The structure of the lower layer protocols for the control and user planes in AS are the same. Both planes utilize the protocols of PDCP (packet data convergence protocol), RLC (radio link control), and MAC (medium access control), as well as the PHY (physical layer) for the transmission of the signaling and data packets.


The figure below, illustrates the radio interface protocol stack. The protocol stacks reside in both the UE and the E-UTRAN (evolved universal terrestrial radio access network).



And exactly how data flow across these layers?

HSDPA Data Flow



The figure besides  illustrates an example of the data flowing on the downlink HSDPA from the application layer to the PHY layer. In this simplified figure, the data are coming from application layer maps into RLC packets. After headers are added and segmentation of the IP packets is performed, the RNC sends the data into the MAC layer. The MAC maps one or more RLC PDUs into a HARQ process.

Each MAC PDU in a HARQ process is sent over the HSDPA channel HS-PDSCH. HS-SCCH informs the UE of the control information of this transmission with user-specific H-RNTI.

Once the UE decodes the HS-PDSCH transmission correctly, based on the CRC attached in the TB, it sends HS-DPCCH ACK alongside the CQI to indicate the channel conditions. NodeB uses this feedback to decide on the size of the packets to send. NodeB retransmits any TB on an HARQ process that failed HS-PDSCH CRC, when it receives NACK from the UE. Any data not recovered in the HARQ process, can then be recovered at the RLC layer by detecting any holes in the PDUs’ SN.

LTE Data Flow


The figure illustrates an example of the data flowing on the downlink LTE from the application layer to the PHY layer.

In this simplified figure, the data come from application layer maps into the PDCP SDUs for ciphering and then into RLC packets. The RLC adds headers and segments the PDCP SDUs into RLC PDUs. The MAC maps one or more RLC PDUs into a HARQ process, depending on the CQI received from the UE. Each MAC SDU in a HARQ process is sent over the PDSCH channel. The PDCCH informs the UE of the control information (DCI) of this scheduling instance with a user-specific C-RNTI.

Once the UE decodes the PDSCH transmission correctly, it sends the PUCCH or PUSCH ACK alongside the CQI and other MIMO-related channel information. The eNB uses this feedback to decide on the size of the packets. When the eNB receives NACK from the UE on a particular HARQ, a retransmission is performed on the same HARQ process to recover the data first at lower layers. If the lower layer is unable to decode the data on the HARQ level, the RLC can then request a retransmission of the RLC PDUs as a new transmission on the lower layer. The UE sends the successfully decoded packets to the upper layer for processing.



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