NB-IoT Protocol Stack Explained

This page covers the NB-IoT Protocol Stack, also known as the LTE-NB Protocol Stack. It describes the functions of each layer within the NB-IoT protocol stack.

Introduction

3GPP has introduced NB-LTE, a narrowband cellular IoT solution utilizing a 200 KHz Bandwidth. This technology is commonly referred to as NB-IoT (Narrowband Internet of Things). It’s a Low Power Wide Area Network (LPWAN) radio technology that facilitates the connection of a wide array of devices and services using cellular frequency bands.

NB-IoT emphasizes indoor coverage, reduced costs, extended battery life, and the ability to support a larger number of connected devices within a cell.

Figure-1 depicts NB-IoT network architecture with core system components.

The NB-IoT protocol stack is divided into the user plane and the control plane.

• User Plane: The LTE-NB protocol stack consists of the Physical layer (PHY), MAC layer, RLC layer, and PDCP layer.
• Control Plane: The LTE-NB protocol stack includes the PHY, MAC, RLC, PDCP, RRC, and NAS layers.

Let’s explore the functions of each layer within the NB-IoT protocol stack.

Physical Layer

The physical layer performs the following functions:

• Enables the exchange of data and control information between the eNB and the UE. It also manages the transport of data to and from higher layers.
• Handles error detection, FEC (Forward Error Correction), antenna processing, synchronization, etc.
• 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.
• Employs 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 (Quadrature Phase-Shift Keying) is the highest modulation scheme used.
• Supports both HD FDD (Half-Duplex Frequency Division Duplexing) and TDD (Time Division Duplexing).

MAC Layer

The MAC (Medium Access Control) layer manages cell access-related messages between the UE and the network. The random access procedure aids in establishing the RRC connection. The MAC layer also performs these functions:

• Mapping of logical channels onto transport channels.
• Multiplexing of MAC SDUs (Service Data Units) from one or different logical channels onto transport blocks to be delivered to the physical layer on the UE side.
• Error correction through HARQ (Hybrid Automatic Repeat Request) retransmission.
• Priority handling between UEs via dynamic scheduling.
• Logical Channel prioritization.
• Transport format selection and TB (Transport Block) size selection.

RLC Layer

The RLC (Radio Link Control) layer performs the following functions:

• Transfer of upper-layer PDUs (Protocol Data Units).
• Error correction through ARQ (Automatic Repeat Request) only for AM (Acknowledged Mode) 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.
• The RLC layer supports three modes:
  • Transparent mode (suitable for carrying voice).
  • Unacknowledged mode (suitable for carrying streaming traffic).
  • Acknowledged mode (suitable for carrying TCP traffic).

PDCP Layer

PDCP stands for Packet Data Convergence Protocol. The PDCP layer performs the following operations in the downlink and uplink:

In the downlink direction, it adds a PDCP header to incoming data and forwards it to the RLC layer. In the uplink direction, it removes the PDCP header from incoming packets and forwards it to the IP layer.

The functions performed by the PDCP layer include:

• Transfer of data (C-plane, U-plane) between the RLC and higher U-plane interface.
• Maintenance of PDCP SN (Sequence Number), transfer of SN status for use upon handover, etc.
• ROHC (Robust Header Compression).
• In-sequence delivery of upper-layer PDUs at re-establishment of the lower layer.
• Elimination of duplicate lower-layer SDUs at re-establishment of the lower layer for RLC AM.
• Ciphering and deciphering of C-plane and U-Plane data.
• Integrity protection and integrity verification of C-plane data.
• Timer-based discard.
• Duplicate discard.
• For split and LWA bearers, routing and reordering.

Changes in NB-IoT PDCP compared to LTE PDCP:

• Maximum size of PDCP SDU and PDCP control PDU is 1600 bytes.
• PDCP status report receive operation is not applicable in NB-IoT.
• In LTE, PDUs carrying data from DRBs (Data Radio Bearers) are mapped on RLC UM, but in the case of NB-IoT, DRBs are mapped on RLC AM.
• NB-IoT uses only a 7-bit PDCP SN for DRB.

RRC Layer

RRC (Radio Resource Control) layer specifications are slightly different compared to LTE. These specifications are defined in the TS 36.331 document.

• The UE must transition to “RRC connected mode” before transferring any application layer data or completing any signaling procedures.
• RRC connection establishment is a 3-way handshake process between the UE and the eNB. It is used to transition the UE from “RRC IDLE” to “RRC Connected mode”.
• The messages exchanged between the UE and the eNB to complete the RRC Connection Establishment Procedure are:
  • RRC Connection Request (UE -> eNB)
  • RRC Connection Setup (eNB -> UE)
  • RRC Setup Complete (UE -> eNB)
• The RRC connection establishment procedure is always initiated by the UE but can be triggered by either the UE or the network. RRC Connection Release is always triggered by the eNB.
• The initial NAS (Non-Access Stratum) message is transferred as part of the RRC connection establishment procedure to reduce establishment delay.

Reference: 3GPP 36 series LTE