SMF — Session Management Function — the function that owns a PDU session

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The core network

Where it sits

Sits atLevel 2 of the hierarchy · The core network · explained
Learn firstNetwork function · Reference point — 3 concepts in the full chain, see the paths
UnlocksPDU session
Primary clauseTS 23.501 §6.2.2
Used inref-core 10 · ref-session 10 · ref-ue-state 3 · ref-ue-mobility 2 · index 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. The Session Management Function — the owner of a PDU session from establishment to release. TS 23.501 §6.2.2. It is the function that decides everything about a connection and touches none of its packets.

The functionality list, abridged:

Session Management — establishment, modification and release, "including tunnel maintain between UPF and AN node" · UE IP address allocation & management · DHCPv4/DHCPv6 server and client · ARP and IPv6 Neighbour Solicitation proxying for Ethernet sessions · Selection and control of UP function · Configures traffic steering at UPF · Termination of interfaces towards policy functions · Termination of SM parts of NAS messages · Downlink Data Notification · "Initiator of AN specific SM information, sent via AMF over N2 to AN" · Determine SSC mode of a session · charging control at the UPF.

"Initiator of AN specific SM information, sent via AMF over N2 to AN" is the line that reaches the RAN. That is the QoS profile — the 5QI and ARP for each QoS flow — travelling from the SMF, through the AMF as an opaque payload, over N2, into the base station's scheduler. It is the single instruction the core gives the RAN about how to treat a device's traffic, and everything in this folder's scheduling and link adaptation sits underneath it.

"Downlink Data Notification" is the other seam. When a packet arrives for a device that is CM-IDLE, the UPF buffers it and tells the SMF, which tells the AMF, which pages the device. One line in a functionality list, and it is the whole reason a phone rings.

"Termination of SM parts of NAS messages" is the counterpart to the AMF's transparent proxying: 5GSM messages are written by the device for the SMF, and the SMF is where they are actually read.

Read on

This concept is read as part of one argument in ref-core, alongside the rest of its group.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand SMF (level 2) you first need 3 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 5G Core
1 Network function · Reference point
Immediately before SMF: Network function, Reference point.
Keep going — where this sits on the route
◀AMFstep 13 · level 2
Step 14 of 118level 2 · The core network
UDMstep 15 · level 2▶
Level 2 → 3 · this unlocks
PDU session
↳ all 9 concepts at level 3 · level 2 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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