PDU Session Establishment in 5G

The original document was written here.

A PDU session is a logical connection from between UE and the 5GC (more precisely with UPF in the 5GC) that connects users to any Data Network (DN). The DN may be the internet, IMS, or any enterprise network.

Important Points about PDU Session

• There are two components of a PDU session:

1. The UE to NG-RAN connection is called Data Radio Bearer (DRB)
2. NG-RAN gNB to UPF connection in 5GC is an N3 tunnel.

• The SMF manages the PDU session.
• Each PDU session has its own N3 tunnel between NGRAN and 5GC. For example, if there are two PDU sessions, there would be two N3 tunnels.
• One or more PDU sessions from a UE could be connected to the same UPF.
• The data that flows through PDU session is mapped to a QoS Flow. QoS Flow determines how user packets are treated as per QoS requirements.
• One PDU session can have multiple QoS flows. (QoS Flow is the lowest granularity of a traffic flow where QoS and charging can be applied. QoS Flow is identified by QFI within a PDU session.)
• In the figure below, there are two QoS flows, hence two DRBs
  (e.g. one QoS Flow for IMS, the other for a voice call). But there is only one N3 tunnel that transports both QoS flows. (N3 tunnel is established when a UE connects to a DN and is maintained till it goes idle.)