In 5G, a PDU Session Type determines the kind of data connectivity established between a user’s device and the network. It specifies whether the session supports IPv4, IPv6, dual-stack IPv4/IPv6, Ethernet, or unstructured data formats, ensuring tailored connectivity for various applications like internet access, industrial IoT, or special-purpose networks. Each session type is uniquely configured to meet the data and service requirements of the user.
The concept of a PDU Session in 5G can be compared to the “EPS Bearer” in LTE. It is a critical component that provides end-to-end user plane connectivity between the User Equipment (UE) and the Data Network Name (DNN).
A PDU Session in 5G is a logical connection established between the User Equipment (UE) and a Data Network (DN). This session provides end-to-end user-plane connectivity, enabling the transmission of user data packets across the 5G core network. Each session is uniquely identified using a PDU Session ID and supports only one specific type of Protocol Data Unit (PDU) at a time, such as IPv4, IPv6, Ethernet, or Unstructured.
PDU Sessions are managed dynamically through NAS SM signaling, which allows their establishment, modification, or release based on user or network requirements. By facilitating differentiated Quality of Service (QoS) for each flow within the session, PDU Sessions ensure optimal performance and reliability for diffrent 5G applications needs.
Key Features of PDU Sessions
- End-to-End Connectivity: A PDU Session ensures that the UE can communicate seamlessly with the DNN.
- QoS Flows:
- Each PDU Session can include one or multiple Quality of Service (QoS) flows.
- These flows have a one-to-one mapping with QoS profiles, ensuring consistent quality for packets within a specific flow.
- PDU Session Types:
- IPv4
- IPv6
- IPv4v6
- Ethernet
- Unstructured
- Single PDU Type Per Session: A PDU Session supports only one type of PDU, as requested by the UE during its establishment.
Key Components of a PDU Session
- PDU Session ID: A unique identifier for each session, assigned by the UE.
- S-NSSAI (Slice Identifier): Specifies the network slice for the session.
- DNN (Data Network Name): Indicates the target data network.
- SSC Mode: Defines session longevity and user-plane anchor management.
- User Plane Security Information: Dictates whether user-plane ciphering and integrity protection are active.
Overview of PDU Session Types
1. IP-Based PDU Session type:
This type of PDU session is same as IP based session that was supported by EPC in LTE networks which are IPv4, IPv6 and Dual-Stack IPv4v6 These types support traditional internet services and provide advanced features like IPv6 multi-homing.
Session Types:
- IPv4: Supports legacy internet services with IPv4 addressing.
- IPv6: Enables advanced internet features like multi-homing.
- Dual-Stack (IPv4v6): Supports both IPv4 and IPv6 simultaneously.
IP Address Management:
- The Application Function (AF) provides IP addresses and prefixes for IPv4, IPv6, or IPv4v6 sessions.
- The PDU Session Anchor serves as the IP anchor point for the allocated IP address/prefix.
UE IP Address Management: The UE requests a PDU Session type based on its IP stack capabilities:
- Dual-stack UE: Requests IPv4, IPv6, or IPv4v6 based on configuration or policy.
- IPv4-only UE: Requests “IPv4.”
- IPv6-only UE: Requests “IPv6.”
- Unknown IP capability: Requests “IPv4v6” by default.
- The SMF (Session Management Function) finalizes the PDU Session type based on the DNN (Data Network Name) configuration and operator policies.
Multi-Homing Support:
- For IPv6 multi-homing, multiple IP anchors are supported to ensure redundancy and improved performance.
- For non-multi-homed sessions, a single IP anchor is used.
Session and Service Continuity (SSC) Modes:
- SSC Mode 1: The network preserves the UE’s connectivity, retaining the IP address during the session.
- SSC Mode 2: The network may release the session, including the allocated IP address, as part of service termination.
The IP-Based PDU Session Type ensures robust internet connectivity for 5G devices, supporting legacy and advanced IP features. With flexible address management and support for dual-stack operations, it caters to a wide range of use cases, from basic browsing to high-performance, multi-homed networking.
2. Ethernet PDU Session Type
The Ethernet PDU (Protocol Data Unit) session type enables the 5G System (5GS) to handle Ethernet frames, which are essential for delivering certain types of services. It’s a type of session that allows 5G networks to handle Ethernet traffic, much like a virtual Ethernet cable connecting the user device to the network.
Ethernet frames are forwarded between the device and the network, and depending on the operator’s setup, the system might map multiple devices to the same network interface or dedicate one interface per session.
Unlike traditional sessions, Ethernet PDU Sessions don’t assign IP or MAC addresses to devices. Instead, the network relies on MAC addresses from the devices themselves and can enforce rules for allowed addresses or VLANs.
This setup is ideal for services like industrial IoT or private networks, where Ethernet-based communication is critical.
Applications:
- Remote office connectivity to corporate networks.
- Industrial IoT devices linked to factory LANs.
- Fixed Wireless Access (FWA) for residential broadband.
3. Unstructured PDU Session Type
The Unstructured PDU (Protocol Data Unit) Session type provides a flexible way to deliver raw data between a user device (UE) and the Data Network (DN) through 5G networks. It is a session type designed to carry raw, unstructured data between the device and the network. Think of it as a basic data pipe without predefined formats or protocols.
The session uses tunneling (e.g., UDP/IPv6) to transport data between the UE and its destination in the DN. For uplink traffic, the data is encapsulated and sent to the DN. For downlink traffic, the network encapsulates the data, and the UPF decapsulates it before delivering it to the UE.
Unlike other PDU Session types, the UE doesn’t directly manage IPv6 prefixes. Instead, the SMF handles address management and mapping, simplifying the UE’s role. This session type is ideal for scenarios where applications need raw data delivery without the overhead of complex protocol structures, such as specialized industrial use cases or proprietary systems
- Supports IoT protocols like LoWPAN, MQTT, and CoAP
- Treats user data as unstructured bits, with no specific format.
- SSC Modes: Supports SSC modes 1 and 2, not mode 3.
- The SMF provides the MTU value during PDU Session establishment, which the UE uses to configure payload size.
- Each Unstructured PDU Session supports only one 5G QoS Flow.
- Applications: Ideal for IoT use cases where protocol-specific processing isn’t required.
The Unstructured PDU Session type provides a streamlined mechanism for delivering raw data in 5G networks. By leveraging PtP tunneling and centralized IPv6 management, it supports efficient data transport while offering flexibility for specific application needs.
Attribuites of a PDU Session
| PDU Session attribute | May be modified later during the lifetime of the PDU Session | Notes |
|---|---|---|
| S-NSSAI of the HPLMN | No | (Note 1) (Note 2) |
| S-NSSAI of the Serving PLMN | Yes | (Note 1) (Note 2) (Note 4) |
| DNN (Data Network Name) | No | (Note 1) (Note 2) |
| PDU Session Type | No | (Note 1) |
| SSC mode | No | (Note 2) The semantics of Service and Session Continuity mode is defined in clause 5.6.9.2 |
| PDU Session Id | No | |
| User Plane Security Enforcement information | No | (Note 3) |
| Multi-access PDU Connectivity Service | No | Indicates if the PDU Session provides multi-access PDU Connectivity Service or not. |
|
NOTE 1:
If it is not provided by the UE, the network determines the parameter based on
default information received in user subscription. Subscription to different
DNN(s) and S-NSSAI(s) may correspond to different default SSC modes and different
default PDU Session Types.
NOTE 2:
S-NSSAI(s) and DNN are used by AMF to select the SMF(s) to handle a new session.
Refer to clause 6.3.2. The DNN may include both the Network
Identifier and the Operator Identifier, see TS 29.502. See
more details of the DNN usage and applicability, e.g. when full DNN or only Network
Identifier is applied, in relevant stage 3 specifications.
NOTE 3:
User Plane Security Enforcement information is defined in
clause 5.10.3.
NOTE 4:
The S-NSSAI value of the Serving PLMN associated to a PDU Session can change
whenever the UE moves to a different PLMN, while keeping that PDU Session.
|
||
Reference :