Sign in Subscribe

MM sub Mobility Management sublayer

Last updated on 

The Mobility Management (MM) sublayer is one of the sublayers of the Radio Resource Control (RRC) layer in the LTE (Long-Term Evolution) and NR (New Radio) communication systems. The MM sublayer is responsible for managing the mobility of mobile devices between different radio access technologies and different base stations or cells within the same radio access technology. In this article, we will explain the MM sublayer in detail, including its functions, protocols, and procedures.

Introduction

The MM sublayer is responsible for the management of mobility-related functions, such as handover, tracking area update, and cell reselection. The MM sublayer is located in the RRC layer, which is responsible for controlling the radio resources used by the user equipment (UE). The RRC layer is located above the MAC (Medium Access Control) layer and below the PDCP (Packet Data Convergence Protocol) layer in the LTE and NR protocol stacks.

Functions of the MM Sublayer

The MM sublayer performs several functions related to the mobility of the UE. These functions include:

2.1 Handover

The MM sublayer is responsible for initiating and managing handovers between different cells or base stations. Handover is the process of transferring an ongoing communication session from one cell or base station to another without interrupting the communication. The MM sublayer is responsible for initiating the handover procedure and selecting the target cell or base station based on different criteria, such as signal quality, load balancing, and mobility history.

2.2 Tracking Area Update

The MM sublayer is responsible for updating the tracking area (TA) of the UE. The TA is a geographical area consisting of one or more cells, and the UE is required to inform the network about its current location within the TA. The MM sublayer is responsible for initiating the tracking area update procedure and selecting the new TA based on different criteria, such as the UE's mobility history and the network's load balancing.

2.3 Cell Reselection

The MM sublayer is responsible for selecting the best cell or base station for the UE to camp on, based on different criteria, such as signal quality, interference level, and load balancing. The MM sublayer is also responsible for initiating the cell reselection procedure when the UE moves to a new location, and selecting the new cell or base station based on the same criteria.

2.4 Mobility Management Signaling

The MM sublayer is responsible for managing the signaling messages related to mobility management, such as handover signaling, tracking area update signaling, and cell reselection signaling. The MM sublayer is responsible for initiating, processing, and forwarding these messages to the appropriate entities within the network.

Protocols used in the MM Sublayer

The MM sublayer uses several protocols to perform its functions related to mobility management. These protocols include:

3.1 RRC (Radio Resource Control) Protocol

The RRC protocol is used to control the radio resources used by the UE. The RRC protocol is responsible for managing the connection between the UE and the network, including the establishment, maintenance, and release of the connection. The RRC protocol is also responsible for managing the UE's mobility-related functions, such as handover, tracking area update, and cell reselection.

3.2 NAS (Non-Access Stratum) Protocol

The NAS protocol is used to exchange control signaling messages between the UE and the core network. The NAS protocol is responsible for managing the UE's identity, security, and mobility-related functions, such as location update and authentication.

3.3 S1AP (S1 Application Protocol)

The S1AP protocol is used to exchange control signaling messages between the eNodeB (evolved NodeB) and the MME (Mobility Management Entity). The S1AP protocol is responsible for managing the UE's mobility-related functions, such as handover, tracking area update, and paging.

Procedures used in the MM Sublayer

The MM sublayer uses several procedures to perform its functions related to mobility management. These procedures include:

4.1 Handover Procedure

The handover procedure is initiated by the MM sublayer when the UE needs to switch to a new cell or base station due to deteriorating signal quality or changing network conditions. The handover procedure involves several steps, including:

  • Measurement: The UE performs measurements on neighboring cells to determine the best target cell or base station for handover.
  • Triggering: The UE sends a handover request message to the source eNodeB to request handover to the target cell or base station.
  • Preparation: The source eNodeB prepares the handover by allocating resources for the UE in the target cell or base station.
  • Execution: The UE and the network perform the handover by transferring the ongoing communication session from the source cell or base station to the target cell or base station.
  • Completion: The handover is completed when the UE receives a handover complete message from the target eNodeB.

4.2 Tracking Area Update Procedure

The tracking area update procedure is initiated by the MM sublayer when the UE moves to a new TA. The tracking area update procedure involves several steps, including:

  • Registration: The UE sends a location update message to the network to register its new location within the new TA.
  • Authentication: The network authenticates the UE's identity and verifies its location within the new TA.
  • Update: The network updates the UE's location information in its database and assigns new radio resources to the UE within the new TA.

4.3 Cell Reselection Procedure

The cell reselection procedure is initiated by the MM sublayer when the UE moves to a new location within the same TA. The cell reselection procedure involves several steps, including:

  • Measurement: The UE performs measurements on neighboring cells to determine the best cell or base station to camp on.
  • Evaluation: The UE evaluates the quality of the signal and other criteria, such as load balancing and mobility history, to select the best cell or base station.
  • Selection: The UE selects the best cell or base station and sends a cell reselection request message to the network.
  • Confirmation: The network confirms the cell reselection and assigns new radio resources to the UE in the selected cell or base station.

Conclusion

The MM sublayer is an important sublayer in the LTE and NR communication systems, responsible for managing the mobility of mobile devices between different radio access technologies and different base stations or cells within the same radio access technology. The MM sublayer performs several functions related to mobility management, such as handover, tracking area update, and cell reselection. The MM sublayer uses several protocols and procedures to perform these functions, including the RRC protocol, NAS protocol, and S1AP protocol. The MM sublayer plays a crucial role in ensuring the smooth and efficient operation of the LTE and NR networks.