This presentation is an introduction to the 5G network slicing based on 3GPP Release 15 standards

1. 5G Network Slicing
Sridhar Bhaskaran
3GPP Standards Expert (SA2, CT4, CT3 and CT1)

2. Network Slicing in 3GPP - History
Release 13 Release 14 Release 15
● Dedicated Core Networks
(DECOR) introduced
● Selection and Redirection of
MME based on subscribed UE
usage type
● MME further selects SGW / PGW
based on UE usage type
● One UE = one UE usage type
● No indicator in RRC to let eNB
select right MME
● All changes are core network
centric
● Enhanced DECOR (eDECOR) ⇒
Introduces UE assisted Dedicated Core
Network selection
● UE provisioned with a default Dedicated
Core Network ID (DCN ID) by HPLMN
● Serving Network provides a DCN ID for
that PLMN
● UE stores per PLMN DCN ID
● DCN ID carried in RRC
● eNB selects right MME based on DCN
ID
● MME selects SGW / PGW based on UE
usage type
● One UE = one DCN ID
● All PDN connections of the UE in same
DCN
● Brand new 5G core network and system
architecture
● One UE can connect to multiple slices
● AMF (equivalent of MME) is common to all
slices
● Slice identified by Specific Network Slice
Selection Assistance Information (S-NSSAI)
● S-NSSAI contains Slice Type and Slide
DIfferentiator
● NSSAI = Set of S-NSSAI
● UE provisioned with a configured NSSAI per
PLMN
● UE provided with allowed NSSAI by serving
PLMN
● UE can be connected upto 8 S-NSSAIs
(slices) simultaneously
● Requested NSSAI carried in RRC during
initial access (when 5G-GUTI not available)
● Selection of NFs based on S-NSSAI

3. Slicing in LTE
DECOR and eDECOR

4. How DECOR Works?
UE eNB Initial MME
Target
MME
HSS
Attach Request
Authentication Information Request
Authentication Information Answer
(Auth vectors, UE usage type)
Check if this MME serves the UE usage type.
If not initiate NAS redirectionReroute NAS Message Request
(MMEGI of target MME, NAS
message)
Initial UE Message(NAS Message - Attach Request)
Attach Procedure Continues with Target MME

5. Some key aspects of DECOR
● UE usage type is mapped to a Dedicated Core Network (DCN)
● MMEs that belong to a DCN are grouped together under a MME Group Id (MMEGI)
● Within a set of Tracking Areas (TA List), there can be multiple DCNs (and hence multiple MMEGI)
● During SGW selection MME uses TAI + UE usage type in DNS query to discover an SGW that
belongs to the DCN.
● During PDN Connection establishment, the MME uses APN + UE usage type in the DNS query to
discover PGW that serves the APN and belongs to the DCN.

6. Enhancements to DECOR (eDECOR) - R14
UE eNB
Target
MME
HSS
RRC Message (DCN-ID, NAS(Attach Request))
Select MMEGI based on DCN-ID
received in RRC
Attach Procedure Continues with Target MME
S1AP Initial UE Message (NAS
message(Attach Req), DCN ID)
Attach Accept (DCN-ID)
● UE stores the DCN-ID provided by MME in per PLMN DCN-ID
● UE uses that DCN-ID in subsequent initial NAS messages (Attach, TAU) in that PLMN

7. Limitations of DECOR / eDECOR
● One UE = one DCN ID at a given time
● So at any given time one UE can be connected to only one DCN
● All PDN connections (even if it belongs to different APN) part of same DCN
● Simultaneous use of same application network (APN) via different 3gpp core
networks not possible
○ E.g One UE simultaneously connected to multiple VPN (i.e APN name
could be same) ⇒ In this case network isolation for each VPN not
possible.
● UE selection of a slice based on application not possible (i.e UE can not tell
over RRC which PDN connection needs to be routed to which DCN as all
PDN connections are routed to same DCN)

8. Slicing in 5G

9. 5G Core Network Architecture
Courtesy: http://www.3gpp.org/news-events/3gpp-news/1930-sys_architecture

10. EPC vs 5G Network Slicing
UE RAN MME SGW
PGW
(APN1)
PGW
(APN2)
PGW
(APN3)
● 1 UE - connect to one Dedicated Core Network (DCN)
● 1 DCN can support multiple applications (APN)
● Same application support in multiple DCNs require repeated
configurations for same APN but different DCN in DNS
UE RAN AMF
SMF1
SMF2
SMF3
UPF1 DN-1
UPF2 DN-2
UPF3 DN-3
● 1 UE - can connect to multiple core network slices
● Each slice identified by an S-NSSAI
● AMF is common to all slices UE uses
● SMFs specific to each slice
● SMFs selected via NRF specific to the slice (S-NSSAI)
● NRFs + SMFs can be in different administrative domain from AMF
● SMFs select UPF
● Traffic routing of each slice is independent and isolated
● RAN supports slicing at the radio
● Network Slice Selection Policies provided to UE to
select a slice for a given application
LTE - Evolved Packet Core (EPC) 5G Core Network (5GC)
10

11. 5G Control Plane Protocol Stack

12. Why MM - SM Split?
● Unlike EPC, the NAS stack in 5G is split between AMF and SMF. The SM
part of NAS stack is handled at SMF.
● Why is this split needed?
○ Reason: Network slicing
○ Some slices don't need session management at all (e.g some IoT UEs
that only support SMS).
○ For such cases, there is no need to have SM stack in the slice.

13. Key Concepts
● S-NSSAI: Specific Network Slice Selection Assistance Information. An S-NSSAI
identifies a network slice. It contains
○ Slice/Service Type (SST) - refers to expected network slice behavior in terms of
features and services.
○ Slice Differentiator (SD) - optional information - complementing SST to
differentiate among multiple slices of same Slice type.
● NSSAI: A collection of S-NSSAI.
● Configured NSSAI: NSSAI provisioned in the UE applicable to one or more PLMNs.
● Allowed NSSAI: NSSAI provided to the UE by serving PLMN via NAS signaling.
● Requested NSSAI: NSSAI provided by UE to serving PLMN during Registration
● NSSP: Network Slice Selection Policy - contains policy rules to let UE select an
HPLMN specific S-NSSAI for an application type.
● URSP: UE Route Selection Policy - NSSP is contained within URSP in UE
● NSI: Network Slice Instance - a collection of NF instances that serve a given S-NSSAI.
NSI is not visible to UE and the RAN. It is a completely core network concept

14. What is Stored in UE?
● Configured NSSAI per PLMN (including HPLMN) and/or Configured NSSAI
applicable to many PLMN(s) [i.e standard NSSAI to use for any PLMN]
○ Configured NSSAI provided by HPLMN initially = Set of Subscribed
S-NSSAI
● For Configured NSSAI provided by serving PLMN, some rules to map
HPLMN provided S-NSSAI to serving PLMN configured S-NSSAI is also
provided.
● List of rejected S-NSSAI by serving PLMN at PLMN level and/or current
Registration Area (set of TAs) level.
● NOTE: Number of configured NSSAI the UE can store other than that of
HPLMN provided NSSAI is upto UE implementation.
● Ref: Clause 5.15.4 of 3GPP TS 23.501

15. Requested NSSAI Selection in UE - During
Registration Procedure
Registration
Procedure Starts
Is Allowed NSSAI for
the serving PLMN
available?
Is Configured NSSAI for
the serving PLMN
available?
Requested NSSAI = Allowed
NSSAI or its subset
Requested NSSAI =
Configured NSSAI or its
subset
Yes
Yes
No
No
Requested NSSAI = NULL
Reference: Clause 5.15.5.2 of
3GPP TS 23.501

16. Requested S-NSSAI Selection in UE - During
PDU Session Establishment
PDU Session Establishment
Procedure Starts
Is URSP with
NSSP available?
Is mapping from Configured
NSSAI of HPLMN to Allowed
NSSAI available?
Derive a S-NSSAI based on NSSP
(NOTE: URSP with NSSP is provided by
HPLMN). The derived S-NSSAI is of
HPLMN scope
No S-NSSAI included in PDU Session
Establishment Request (NAS SM
Message)
Include an S-NSSAI mapped to an
S-NSSAI in Allowed NSSAI
Include an appropriate S-NSSAI
from Allowed NSSAI
Yes
No
Yes
No
Reference: Clause 5.15.5.3 of
3GPP TS 23.501

17. Role of AMF
● During Registration Procedure:
○ Query UDM to get subscribed NSSAI
○ Verify if S-NSSAI(s) in requested NSSAI are permitted based on Subscribed NSSAI (AMF to
map requested S-NSSAI to Configured NSSAI for HPLMN for roamers)
○ If allowed NSSAI not available in AMF UE context
■ Either query NSSF to determine Allowed NSSAI (or)
■ Based on local configuration determine if the AMF can serve the UE
● During PDU Session Establishment Procedure:
○ If NRF for the S-NSSAI included by UE already available at AMF - query the NRF to select an
SMF from that slice instance
○ Else query the NSSF to select a slice instance for the S-NSSAI and get an NRF for that slice
instance. Then query the NRF to select the SMF.
● At any time:
○ Update UE with new Configured NSSAI for the Serving PLMN along with mapping of this
configured NSSAI to Configured NSSAI for the HPLMN.
■ Uses UE Configuration Update NAS procedure.
○ Update the URSP (and NSSP) in the UE when requested by PCF.

18. Role of NSSF
● During Registration Procedure:
○ Based on requested NSSAI and subscribed NSSAI provided by AMF select the network slice
instance(s) to serve the UE for the set of S-NSSAI.
■ Correspondingly select the set of NRFs to be used in each of these selected network
slice instance(s)
○ Alternatively NSSF may defer the selection of network slice instance(s) for S-NSSAI to PDU
session establishment procedure.
○ Select a target AMF set or list of candidate AMFs
○ Determine allowed NSSAI
○ Provide mapping of each S-NSSAI of allowed NSSAI to subscribed S-NSSAI(s), if required
○ Return the above to AMF
● During PDU Session Establishment Procedure:
○ Based on requested S-NSSAI, select a network slice instance.
○ Determine the NRF to be used in that slice instance (within same PLMN)
○ Return the above to AMF

19. Role of NRF
● NRF is a Network Repository Function - a registry that maintains the NF profile and the NF
services supported by each NF that has registered with it - within that network slice instance.
● During Registration Procedure:
○ AMF may query the NRF provided by NSSF to select target AMF or an NRF that is locally
configured at AMF, to get the list of candidate AMFs.
● During PDU Session Establishment Procedure:
○ AMF queries the NRF from the network slice instance corresponding to the UE requested
S-NSSAI to select an SMF from that network slice instance.

20. AN - AMF Interaction for Supported S-NSSAI
Access Network
(AN)
AMF
AN to AMF: S-NSSAIs supported
in each TA.
AMF to AN: Set of S-NSSAIs that
the AMF can connect to.
N2 Association Setup / Update
Procedure

21. Registration Procedure in Non
Roaming/Roaming Case
UE NG-RAN AMF NSSF NRF UDM
Registration Req over RRC
(included Req NSSAI from
allowed/configured NSSAI if
GUTI not available) Route to AMF as per
R-NSSAI or to a default AMF
Get slice specific subscription data
Slice selection request (Req NSSAI, Sub NSSAI, TAI, Mapping of Req
NSSAI to HPLMN NSSAI for roamers)
Slice selection response (Allowed NSSAI, Mapping information of
allowed NSSAI to HPLMN NSSAI for roamers, NRF, Target AMF set,
rejected S-NSSAI with cause)
Query NRF and find target AMF / redirect registration to target AMF and
continue rest of registration procedure

22. PDU Session Establishment Procedure in Non
Roaming Case
UE AMF NSSF NRF SMF
PDU Session Establishment
Request(DNN, S-NSSAI) Query NSSF if NRF for
S-NSSAI is not
available
Return NRF address
for the selected
Network Slice Instance
Query NRF to select SMF (DNN, S-NSSAI)
Return SMF Address
PDU Session Establishment Procedure Continues with Selected SMF

23. PDU Session Establishment Procedure - SMF Selection in
Home Routed Roaming Case (Option 1)
AMF vNSSF vNRF
hNRF (slice
Level) hNSSF
Query vNSSF if vNRF
for S-NSSAI is not
available
Return vNRF address
for the selected
Network Slice Instance
Query vNRF to select vSMF (DNN, requested
S-NSSAI) and hSMF (DNN, requested S-NSSAI
mapped to HPLMN S-NSSAI), hNRF infor NF Discovery Request (DNN)
Slice selection request
Slice selection response (appropriate hNRF to use)
NF Discovery Response (v-SMF, h-SMF address)

24. PDU Session Establishment Procedure - SMF Selection in
Home Routed Roaming Case (Option 2)
AMF vNSSF vNRF
hNRF
(PLMN
Level)
Slice specific
local hNRF
Query vNSSF if vNRF
for S-NSSAI is not
available
Return vNRF address for
the selected Network Slice
Instance
Query vNRF to select vSMF (DNN, requested
S-NSSAI) and hSMF (DNSS, requested
S-NSSAI mapped to HPLMN S-NSSAI) NF Discovery Request (DNN, S-NSSAI)
Directly query slice instance specific NR
if local configuration of slice instance
specifc NRF based on S-NSSAI / NSI-ID
available.
NF Discovery Response (v-SMF, h-SMF address)

25. Use Cases Enabled by 5G Slicing
1 UE - common AMF - but multiple slices with slice specific SMF, UPF and PCF
Courtesy: http://www.3gpp.org/news-events/3gpp-news/1930-sys_architecture

26. Other Use Cases Enabled by 5G Slicing
● For vertical applications - operators can spawn SMF, UPF, PCF in separate slice
instance(s) for that vertical market and route UE traffic for those vertical applications.
● Testing of new features in the network by deploying a specific slice and configuring a
specific set of UEs to use that slice (through UE Configuration Update NAS
procedures).

27. Thank You