Understanding the 3G UMTS Protocol Stack

The UMTS protocol stack is a layered structure that dictates how data is processed and transmitted across the network. Each layer in the stack, from the physical layer to the application layer, performs specific functions to ensure reliable communication. Understanding the protocol stack is vital for developers and network engineers focusing on 3G network design and optimization.

The 3G protocol stack consists of Access Stratum (AS) and Non-Access Stratum (NAS).

Access Stratum supports OSI layer 1 to 3. It consists of PHY (layer 1), MAC (layer 2), RLC (layer 2) and RRC (layer 3).

NAS, or the core network part, is divided based on circuit-switched (CS) or packet-switched (PS) functionalities. For CS functions, it consists of CM (Connection Management) and MM (Mobility Management) layers. The CM layer takes care of CC (Call Control), SM (Supplementary Services), and SMS (Short Message Service). This applies to the UMTS CS control plane stack.

For the UMTS CS user plane stack, the NAS part does not include CM and MM layers but it does include the application data layer protocol end-to-end (between UE-NodeB-RNC-MSC-Remote user).

For PS functions, it consists of SM (Session Management) and GMM (GPRS Mobility Management) layers. This is for the UMTS PS control plane stack.

For the PS user plane stack, the AS part incorporates PDCP (Packet Data Convergence), and the NAS part incorporates packet protocol data (IP/PPP/…) and packet data applications (FTP/HTTP/..).

PDCP does compression of IP headers and may or may not exist in the UMTS protocol stack. Let’s understand the generic functions of each of these 3G protocol layers.

1. Physical Layer (Layer 1)

• Functions:
  • Provides the interface between the radio network and the user equipment (UE).
  • Converts data packets from higher layers into a format suitable for transmission over the air interface.
  • Handles modulation, coding, spreading, and multiplexing of data.
  • Manages physical channels such as uplink and downlink transmission.
  • Responsible for power control, error detection, and error correction.
• Key Components:
  • Transmission and reception of signals.
  • Handling of channel coding, rate matching, and transport format selection.

2. Data Link Layer (Layer 2)

The Data Link Layer in UMTS is divided into three sublayers:

a. MAC (Medium Access Control) Sublayer

• Function:
  • Manages access to the shared radio resources.
  • Handles logical channel multiplexing and mapping of logical channels to transport channels.
  • Provides services such as priority handling, scheduling, and HARQ (Hybrid Automatic Repeat Request).
• Key Components:
  • Ensures reliable data transfer between the physical layer and higher layers.
  • Supports segmentation and reassembly of data packets.

b. RLC (Radio Link Control) Sublayer

• Function:
  • Ensures reliable delivery of data between the UE and the network by providing error correction through retransmission.
  • Segments and reassembles packets, ensuring data is transferred in the correct sequence.
  • Provides three modes of operation:
    • Transparent Mode (TM): No additional protocol control information.
    • Unacknowledged Mode (UM): No retransmission, used for real-time services.
    • Acknowledged Mode (AM): Error-free transmission with retransmissions.
• Key Components:
  • Flow control and error handling.
  • Concatenation, padding, and segmentation of data.

c. PDCP (Packet Data Convergence Protocol) Sublayer

• Function:
  • Performs header compression to reduce the overhead on IP packets, increasing the efficiency of data transfer.
  • Supports ciphering and integrity protection, providing security for user data.
  • Manages the transmission of IP packets in both user and control planes.
• Key Components:
  • Compression and decompression of IP headers.
  • Security functions like encryption and integrity checks.

3. Network Layer (Layer 3)

The Network Layer in UMTS is divided into two primary sublayers.

a. RRC (Radio Resource Control) Sublayer

• Function:
  • Manages the establishment, maintenance, and release of radio connections.
  • Controls the configuration of the lower layers (Physical, MAC, and RLC).
  • Handles mobility management, handovers, and QoS (Quality of Service) settings.
  • Manages signaling for power control, measurement reporting, and cell reselection.
• Key Components:
  • Establishes and releases radio bearers.
  • Manages the connection between the UE and the network.

b. NAS (Non-Access Stratum)

• Function:
  • Resides in the core network and handles functions like mobility management and session management.
  • Manages communication between the UE and the core network components like the MSC (Mobile Switching Center) and SGSN (Serving GPRS Support Node).
• Key Components:
  • Mobility management, including registration, location updates, and paging.
  • Session management, including context establishment and data transfer.

Conclusion

A comprehensive understanding of the 3G UMTS protocol stack equips professionals with the knowledge needed to enhance network performance and resolve protocol-related issues. It is a key element for ensuring robust and reliable communication in 3G networks.