Ultra Cloud Core 5G User Plane Function, Release 2020.02 - Configuration and Administration Guide

Bias-Free Language

The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.

Book Contents
Find Matches in This Book
Available Languages
Download Options

Book Title

Ultra Cloud Core 5G User Plane Function, Release 2020.02 - Configuration and Administration Guide

Chapter Title

GTP-U Support

Results

Updated:
April 30, 2020

Chapter: GTP-U Support

GTP-U Support

This chapter covers the following topics:

Feature Summary and Revision History

Summary Data

Table 1. Summary Data

Applicable Product(s) or Functional Area

5G-UPF

Applicable Platform(s)

VPC-SI

Feature Default Setting

Enabled – Always-on

Related Changes in this Release

Not Applicable

Related Documentation

Not Applicable

Revision History

Table 2. Revision History
Revision Details Release

First introduced

2020.02.0

Feature Description

3GPP specifies provisions for UEs capable of supporting both 5G and 4G NAS to connect to E-UTRAN and 5G core network.

To forward data (G-PDUs and End Marker packets) during an EPS to 5GS handover, the SMF:

  • Provisions one PDR per E-RAB (that supports data forwarding for at least one QoS flow).

  • Creates and associate one QER with each PDR, including the QFI IE set to the QFI value of one of the QoS flows mapped to the E-RAB, to request the UPF to insert a GTP-U PDU Session Container extension header including the QFI.

Data forwarding during handovers between 5GS and EPS is supported as follows (see, 3GPP TS 38.300):

  • For 5G to 4G handover, the source NG-RAN node sends one or several end-markers including one QFI of those QoS flows mapped to the same E-RAB and sends the end-marker packets to the UPF over the PDU session tunnel. UPF removes the QFI and maps to an appropriate E-RAB tunnel towards SGW.

  • For 4G to 5G handover, the source eNB forwards the received end markers in the EPS bearer tunnel to the SGW, which forwards them to the UPF. The UPF adds one QFI among the QoS flows mapped to that E-RAB to the end-markers and sends those end-markers to the target NG-RAN node in the per PDU session tunnel.

How it Works

Call Flows

Initial Attach on E-UTRAN via MME and S-GW

Initial attach on E-UTRAN/EPS follows the procedure defined in 3GPP TS 23.401, Section 5.3.2.1.

The following diagram shows the call flow derived from 3GPP reference for initial attach on E-UTRAN/EPS.

Table 3. Initial Attach on E-UTRAN via 5G Core Call Flow
Step Description

1

At Step 9, SMF+PGW-C perform a UPF selection and perform N4 Session Establishment procedure. Since this session is a 4G session connecting to SMF+PGW-C, separate CN tunnel is created for each bearer and QFI is not sent in the QER and PDR, correlation ID might be present.

2

At Step 18, SMF+PGW-C performs N4 Session Modification to update the eNodeB TEID on the data path to the UPF.

The 3GPP specifications provide mechanisms to achieve mobility of a UE from LTE to 5G NR and vice versa. This mobility is achieved in two different architectures – with and without N26 interface between AMF and MME.

5G to EPS Handover with N26 Interface

5G to EPS handover with N26 interface is defined in 3GPP TS 23.502, Section 4.11.1.2.1. The following diagram shows the detailed call flow for N26 interface.

Table 4. 5G to EPS Handover with N26 Interface Call Flow
Step Description

1

In Step 2b, the SMF+PGW-C sends the N4 Session modification to the UPF to establish the CN tunnel for each EPS bearer. The bearer mapping to the 5G QoS and PCC rules received from PCC is already present with SMF. The SMF also contains the bearer IDs obtained from the Bearer ID Allocation procedure. SMF+PGW-C creates new PDRs for the N4 session and gets TEID allocated for each bearer as required by the 4G system.

2

In Step 10b, SMF+PGW-C sends N4 Modification Request to UPF to create additional PDRs and FARs to receive the redirected DL data over the indirect tunnel from NG RAN and forward them to eNodeB. The uplink PDRs in this case has the QFI to match forwarded DL data from NG RAN and the associated QER does not have the QFI as data needs to be forwarded to eNodeB. Also, the FAR redirects the received data to eNodeB over appropriate tunnel based on the QFI.

3

At Step 11, for the QoS flows indicated in QoS Flows for Data Forwarding, NG-RAN initiates data forwarding through the UPF based on the CN Tunnel Info for Data Forwarding per PDU Session. Then the UPF maps data received from the data forwarding tunnel(s) in the 5GS to the data forwarding tunnel(s) in EPS and sends the data to the target eNodeB through the Serving GW.

4

In Step 15, the SMF sends N4 Modification Request to UPF to activate the DL data path to E-UTRAN. At this time, both the indirect tunnel and the direct DL path are activated towards eNodeB.

5

At Step 21, the SMF sends N4 Modification Request to the UPF to delete the indirect forwarding tunnel.

Other call flows related to EPS to 5G and 5G to EPS handover with N26 interface, or without N26 interface are defined in 3GPP 23.502, Section 4.11.1.2.1 and Section 4.11.2.

Was this Document Helpful?

FeedbackFeedback

Contact Cisco