PDU Session: (Protocol Data Unit)
A PDU Session provides connectivity between applications on a UE and a DN such as the 'Internet' or private corporate networks.
PDU Session Establishment:
PDU session occurs when:
- a new PDU session is created
- a PDU session is handover from non-3GPP access.
- Ex: Wifi etc..
- Initiated by UE:
- Can be triggered by network
PDU Session Call Flow:
Lets discuss each step in brief:
1.) UE sends session establishment request to RAN --> AMF, which contains the Info about which DN(data network) the device is requesting a PDU session towards and what kind of session type it is looking for:
- Session type:
- IP based
- Ethernet
- unstructure Also recognize that if it is a new PDU session or an old one(hand over). In case of new PDU session, AMF will see what kind of session the device had requested, to what data network it is trying to connect and AMF will select a appropriate SMF based on the type of request. If the device request a session handover, it mean it is not a new session so the AMF will look for UE context based on the corresponding previous SMF session
2.) Now the AMF contact to SMF to create SM context and contact UDM for getting necessary subscription data and subscribing to further updates of subscriber data. And then UDM response to AMF for subscriber request.
3.) Now in this step, all the authentication, authorization, identification and validation part takes place for PDU session.
4.) In this step, SMF selects an appropriate PCF and create a policy session for fetching necessary PCC rules from PCF and also select an appropriate UPF to connect with correct DN and creates a connectivity with corresponding UPF. At this point SMF also selects the IP address for the devie.
5.) In this step, SMF sends the accepted Message towards the AMF together with the information related to quality of service and also the tunnel between the gnodeb and UPF and this information is sent via AMF.
6.) Now at this point, the quality of service and tunneling information has reached the RAN. The RAN creates connectivity towards the device and replace back to the AMF with the information on the end point of the RAN. This meaning all the necessary setup has been complete for sending the uplink data from the device towards 5G network. So the first uplink data can now be sent.
7.) In this step, the AMF has previously received the wrong end of the tunnel information from RAN, now we forward it to the SMF and SMF to the UPF, so that UPF will know what is the tunnel end point in the RAN. So it esh=tablish connectivity from UPF to the RAN. The GTP tunnel is now complete, which means now the UPF can forward the the first downlink data from DN to UE. So, at this point the downlik data connection is complete.
8.) Now as both uplonk and downlink data connection is successful, the AMF can go and register itself with the UDM that this is the SMF that is corresponding to the PDU session that was correctly created.
With this registration, the PDU session establishment procedure will be complete.