NB-IoT Protocol Stack | LTE-NB Protocol Stack

This page covers NB-IoT Protocol Stack also known as LTE-NB Protocol Stack. The page describes functions of each of the layers of NB-IoT protocol stack.

Introduction:
3GPP has published NB-LTE, a narrowband cellular IoT solution which uses 200 KHz Bandwidth. It is also known as NB-IoT (Narrowband Internet of Things). It is LPWAN (Low Power Wide Area Network) radio technology which enables wide range of devices and services to be connected using cellular frequency bands. It focuses on indoor coverage, lower cost, longer battery life and to allow larger number of connected devices in a cell.

Figure-1 depicts NB-IoT network architecture with core system components. Refer NB-IoT Architecture >> for more information.

NB-IoT Protocol Stack | LTE-NB Protocol Stack

NB-IoT protocol stack has been categorized into user plane and control plane. In user-plane LTE-NB protocol stack consists of physical layer (PHY), MAC layer, RLC layer and PDCP layer. In Control-plane LTE-NB protocol stack consists of PHY, MAC, RLC, PDCP, RRC and NAS layers. Let us understand functions of NB-IoT protocol stack layers.

Physical layer: Following are the functions of physical layer.
• It enables exchange of data and control information between eNB and UE. It also enables transport of data to and from higher layers.
• The funtions performed by PHY include error detection, FEC, antenna processing, synchronization etc.
• It consists of physical signals and physical channels. Physical signals are used for system synchronization, cell identification and channel estimation. Physical channels are used for transporting control, scheduling and user payload processing from higher layers etc. Refer NB-IoT channel types >> and and NB-IoT Preamble and reference signals >> for more information.
• It uses OFDMA (with 15 KHz subcarrier spacing) in the downlink and SC-FDMA (with 3.75 KHz for single mode transmissions, 15 KHz for multi-tone transmissions) in the uplink.
• QPSK is used as highest modulation scheme.
• HD FDD and TDD are supported.

MAC layer: MAC layer takes care of cell access related messages between UE and network. This random access procedure helps in establishing RRC connection. Moreover MAC layer performs following functions.
• Mapping of logical channels on to transport channels.
• Multiplexing of MAC SDUs from one or different logical channels onto transport blocks to be delivered to physical layer on UE side.
• Error correction through HARQ retransmission.
• Priority handling between UEs by means of dynamic scheduling.
• Logical Channel prioritization.
• Transport format selection and TB size selection.

RLC layer: Following are the functions performed by RLC layer.
• Transfer of upper layer PDUs.
• Error correction through ARQ (only for AM data transfer).
• Concatenation, segmentation and reassembly of RLC SDUs (UM and AM).
• Re-segmentation of RLC data PDUs (AM)
• Reordering of RLC data PDUs (UM and AM)
• Duplicate detection (UM and AM)
• RLC SDU discard (UM and AM)
• Protocol error detection and recovery
• There are three modes supported by RLC viz. transparent mode (suitable for carrying voice), unacknowledged mode (suitable for carrying streaming traffic), and acknowledged mode (suitable for carrying TCP traffic).

PDCP layer: PDCP stands for Packet Data Convergence Protocol. PDCP layer performs following operation in the downlink and uplink. In downlink direction, it adds PDCP header to incoming data and forward it to RLC layer. In uplink direction, it removes PDCP header from incoming packet and forwards it to the IP layer. Following are the functions performed by PDCP layer.
• Transfer of data (C-plane, U-plane) between RLC and higher U-plane interface.
• Maintenance of PDCP SN, Transfer of SN status for use upon handover etc.
• ROHC (Robust Header Compression).
• In sequence delivery of upper layer PDUs at re-establishement of lower layer.
• Elimination of duplicate lower layer SDUs at re-establishement of lower layer for RLC AM.
• Ciphering and deciphering of C-plane and U-Plane data.
• Integrity protection and integrity verification of C-plabe data.
• Timer based discard.
• Duplicate discard
• For split and LWA bearers, routing and reordering
• Changes in NB-IoT PDCP compare to LTE PDCP:
-Maximum size of PDCP SDU and PDCP control PDU is 1600 bytes
-PDCP status report receive operation is not application in NB-IoT.
-In LTE, PDU carrying data from DRBs are mapped on RLC UM, but in case of NB-IoT DRBs are mapped on RLC AM.
-NB-IoT uses only 7 bit PDCP SN for DRB.

RRC layer: RRC layer specifications are slightly different compare to LTE. It is defined in TS 36.331 document.
• UE must perform transition to "RRC connected mode" before transferring any application layer data or completing any signaling procedures.
• RRC connection establishment is 3-way handshake process between UE and eNB. It is used to make transition of UE from "RRC IDLE" to "RRC Connected mode". The messages exchanged between UE and eNB required to complete RRC Connection Establishment Procedure are RRC Connection Request (UE ->eNB), RRC connection Setup (eNB->UE) and RRC setup complete (UE ->eNB).
• RRC connection establishment procedure is always initiated by UE but it can be triggered by either UE or network. RRC Connection Release is always triggered by eNB.
• The initial NAS message is transferred as part of RRC connection establishment procedure to reduce establishment delay.

Reference: 3GPP 36 series

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