This blog post includes the details related to Network Functions as shown in System Service Based Architecture. For your convenience, the picture of 5G System Service Based Architecture is described below once again.
The session management function (SMF) is part of control plane function within 5G Core Network. The main responsibilities of SMF are as follows
- PDU Session Management
- IP Address Allocation
- GTP-U Tunnel Management
- Downlink Notification Management
PDU Session Management includes setup, modification and release of PDU sessions. For ease of understanding a PDU session is equivalent to the concept of EPS Bearer in 4G. It is logical connection between UE and the exit of User Plane Function (UPF) towards a specific Data Network (External network, or Internet)
An example of PDU Session concept is shown below
- PDU Session management requires SMF to complete signalling towards the UE, Base Station and User Plane Function (UPF).
- SMF can signal directly with the UPF but requires the AMF to support signalling towards the UE and Base Station
- SMF uses Non Access Stratum (NAS) signalling messages to communicate with the UE. Both the base station and AMF act as transparent routers for these messages which are relayed between the SMF and UE.
- UE can initiate the setup of a PDU Session send a NAS: PDU Session Establishment Request. Assuming a successful setup, SMF responds with a NAS: PDU Session Establishment Accept message.
- Signalling towards the base station requires the AMF to transfer messages using Next Generation Application Protocol (NGAP)
- AMF and Base Station are responsible for managing NGAP signalling procedures but content is relayed to and from the SMF when those procedures relate to Session Management
IP Address Allocation depends upon the type of PDU Session.
- A PDU session can be setup to transfer either IPv4, IPv6, Ethernet or Unstructured data types.
- UE can request a specific data type within the NAS: PDU Session Establishment Request message. The allocated data type is confirmed within the NAS: PDU Session Establishment Accept message.
- IP address allocation is applicable to PDU Sessions which transfer IPv4 or IPv6 packets.
GTP-U Tunnel Management refers to the management of user plane GTP-U tunnel between the Base Station and UPF.
- GTP-U tunnels are used to transfer user plane data between the Base Station and UPF.
- A GTP-U tunnel operates by adding a set of IP/UDP/GTP-U headers to the user plane data packets. The IP layer is used to route the packets between the Base Station and UPF. The UDP layer is used to provide connectionless data transfer and to specify the port number for the GTP-U layer
- The GTP-U layer specifies the Tunnel Endpoint Identifier (TEID) which links the user plane packet to a specific PDU Session.
General concept of GTP-U tunnel is illustrated below.
User plane packet include its own IP header if the PDU session data type is IP. The additional IP header is only used to route the packet through the GTP-U tunnel and is removed by the UPF or base station
Downlink Notification Management refers to the initiation of paging procedure.
- A UE needs to be paged if downlink data arrives at UPF after UE has been released to RRC Idle mode.UPF recognises that the UE does not a GTP-U tunnel towards the Base Station. It triggers the UPF to inform the SMF that downlink data has arrived for the UE.
- SMF eventually informs the AMF from where the Network Triggered Service Request procedure is initiated or UE is paged.
- SMF is responsible for selecting an appropriate UPF during the setup of a PDU session. Selection for appropriate UPF depends upon appropriate load, geographic location, PDU session type etc.
- SMF is responsible for selecting an appropriate Policy Control Function (PCF) during setup of a PDU session. It selects the PCF to provide the policy for a specific PDU session. The selection criteria depends upon Data network name used by PDU session, configured information etc.
- SMF requests subscription information from Unified Data Management (UDM) function during the setup of a PDU Session.
- SMF provides support for interfacing with network charging system both for Online Charging System (OCS) and Offline Charging System (OFCS).
1. 3GPP TS 29.502
2. 3GPP TS 29.508