N3 and N9 User Plane Separation

Users prefer to keep the N3 and N9 network separately in a 5G network on SMF or UPF that handles both non-roaming subscribers and home routed subscribers.

If the UPF does not detect whether an UE is an outbound roamer or a homer, the SMF passes information to the UPF about the interface to use for the subscriber. To overcome this scenario, SMF allows N3 interface on one network and S5-U/S8-U and N9 interface on another network, to allow users to maintain a separate RAN network from core network especially on the data path.

The SMF populates source interface type IE (3GPP interface type) in PDI IE in N4 messages while creating PDR with an appropriate interface type. The format of IE is as below:

Bits

Octets

8

7

6

5

4

3

2

1

1 to 2

Type = 160 (decimal)

3 to 4

Length = n

5

Spare

Interface Type Value

6 to n+4

These octets are present only if explicitly specified

Following table describes the 3GPP Interface type and its values.

Interface Value

Values (Decimal)

S1-U

0

S5 /S8-U

Note
If there is separation of roaming and non-roaming traffic, this value should only used for the S5-U interface and "S8-U" (decimal 19) should be used for the S8-U interface.

1

S4-U

2

S11-U

3

S12

4

Gn/Gp-U

Note
If there is separation of roaming and non-roaming traffic, this value should only be used for the Gn-U interface and "Gp-U" (decimal 20) should be used for the Gp-U interface.

5

S2a-U

6

S2b-U

7

eNodeB GTP-U interface for DL data forwarding

8

eNodeB GTP-U interface for UL data forwarding

9

SGW/UPF GTP-U interface for DL data forwarding

10

N3 3GPP Access

11

N3 Trusted Non-3GPP Access

12

N3 Untrusted Non-3GPP Access

13

N3 for data forwarding

14

N9 (or N9 for non-roaming

Note
If there is separation of roaming and non-roaming traffic, this value should only be used for N9 non-roaming interfaces and (decimal value "21") should be used for N9 roaming interfaces.

15

SGi

16

N6

17

N19

18

S8-U

19

Gp-U

20

N9 for roaming

21

Iu-U

22

N9 for data forwarding

23

Sxa-U

24

Sxb-U

25

Sxc-U

26

N4-U

27

SGW/UPF GTP-U interface for UL data forwarding

28

N6mb/Nmb9

29

N3mb

30

N19mb

31

Spare

32–63

How it Works

Following functions can occur during N3 and N9 User Plane Separation:

  • SMF provides:

    • S5-U as source interface type in the Packet Detection Information (PDI) of uplinks PDRs for homers attaching via 4G RAT.

    • N3 3GPP Access as source interface type in the PDI of uplinks PDRs for homers attaching through 5G RAT.

    • S5-U as source interface type in the PDI of uplinks PDRs for homers attaching through Wi-Fi RAT. This is done as an optimization to avoid tunnel recreation during 4G < - > Wi-Fi handovers.

  • During Hand Over (HO) scenarios:

    • From 5G to 4G/Wi-Fi: A Tunnel is recreated and N4 modification goes with uplink PDRs PDI having source interface type set to S5-U.

    • Fron 4G/Wi-Fi to 5G: A Tunnel is recreated and N4 modification goes with uplink PDRs PDI having source interface type set to N3.

    • Similarly, the uplink PDRs for dedicated flow/bearers get created with either N3 or S5-U source interface type depending on the RAT.

  • UPF handles N4 messages without source interface type and allocates tunnel from interface, which is not associated to any interface for backward compatibility reasons.

Configuring Interface Types

To configure the UPF ingress interface type, use the following configuration:


configure 
   context context_name 
      user-plane-service service_name 
         [ no ] associate gtpu-service gtpu_service_name upf-ingress interface-type [ S5-U | S8-U | S2b-U | n3 3Gpp Acces | n3 | n9 | n6 | SGi ]  
         end 

NOTES:

  • associate gtpu-service gtpu_service_name : Associates the GTP-U service with the user plane service.

  • upf-ingress : Configures the interface type as UPF ingress.

  • interface-type [ S5-U | S8-U | S2b-U | n3 3Gpp Acces | n3 | n9 | n6 | SGi ] : Configure the relevant GTP-U ingress interface type.

The following is an example:

associate gtpu-service n3-upf-in upf-ingress
associate gtpu-service n3-upf-in upf-ingress interface-type S5-U | S8-U | 3 3Gpp Access | n9-s5-s8
| N3 for data Forwarding | N9 | N6 | SGi