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full Contents for  TS 23.501  Word version:   16.4.0

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5.33  Support for Ultra Reliable Low Latency Communication [R16]Word-p. 313
5.33.1  General
The following features described in 5.33 may be used to enhance 5GS to support Ultra Reliable Low Latency Communication (URLLC):
  • Redundant transmission for high reliability communication.
In this Release, URLLC applies to 3GPP access only.
When a PDU Session is to serve URLLC QoS Flow, the UE and SMF should establish the PDU Session as always-on PDU Session as described in clause 5.6.13.
NOTE 1:
How the UE knows whether a PDU Session is to serve a URLLC QoS Flow when triggering PDU Session establishment is up to UE implementation.
NOTE 2:
No additional functionality is specified for URLLC in order to support Home Routed roaming scenario in this Release.
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5.33.2  Redundant transmission for high reliability communicationWord-p. 314
5.33.2.1  Dual Connectivity based end to end Redundant User Plane Paths
In order to support highly reliable URLLC services, a UE may set up two redundant PDU Sessions over the 5G network, such that the 5GS sets up the user plane paths of the two redundant PDU Sessions to be disjoint. The user's subscription indicates if user is allowed to have redundant PDU Sessions and this indication is provided to SMF from UDM.
NOTE 1:
It is out of scope of 3GPP how to make use of the duplicate paths for redundant traffic delivery end-to-end. It is possible to rely on upper layer protocols, such as the IEEE TSN (Time Sensitive Networking) FRER (Frame Replication and Elimination for Reliability) [83], to manage the replication and elimination of redundant packets/frames over the duplicate paths which can span both the 3GPP segments and possibly fixed network segments as well.
NOTE 2:
The following redundant network deployment aspects are within the responsibility of the operator and are not subject to 3GPP standardization:
  • RAN supports dual connectivity, and there is sufficient RAN coverage for dual connectivity in the target area.
  • UEs support dual connectivity.
  • The core network UPF deployment is aligned with RAN deployment and supports redundant user plane paths.
  • The underlying transport topology is aligned with the RAN and UPF deployment and supports redundant user plane paths.
  • The physical network topology and geographical distribution of functions also supports the redundant user plane paths to the extent deemed necessary by the operator.
  • The operation of the redundant user plane paths is made sufficiently independent, to the extent deemed necessary by the operator, e.g. independent power supplies.
Figure 5.33.2.1-1 illustrates an example user plane resource configuration of dual PDU sessions when redundancy is applied. One PDU Session spans from the UE via Master NG-RAN to UPF1 acting as the PDU Session Anchor, and the other PDU Session spans from the UE via Secondary NG-RAN to UPF2 acting as the PDU Session Anchor. As described in TS 37.340, NG-RAN may realize redundant user plane resources for the two PDU sessions with two NG-RAN nodes (i.e. Master NG-RAN and Secondary NG-RAN as shown in Figure 5.33.2.1-1) or a single NG-RAN node. In both cases, there is a single N1 interface towards AMF.
Based on these two PDU Sessions, two independent user plane paths are set up. UPF1 and UPF2 connect to the same Data Network (DN), even though the traffic via UPF1 and UPF2 may be routed via different user plane nodes within the DN.
In order to establish two redundant PDU sessions and associate the duplicated traffic coming from the same application to these PDU sessions, URSP or UE local configuration is used as specified in TS 23.503.
NOTE 3:
Using URSP, duplicated traffic from the application, associated to the redundant PDU Sessions, is differentiated by two distinct traffic descriptors, each in a distinct URSP rule. These traffic descriptors need to have different DNNs, IP descriptors or non-IP descriptors (e.g. MAC address, VLAN ID), so that the two redundant PDU sessions are matched to the Route Selection Descriptors of distinct URSP rules.
The redundant user plane set up applies to both IP and Ethernet PDU Sessions.
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Support of redundant PDU Sessions include:
  • UE initiates two redundant PDU Session and provides different combination of DNN and S-NSSAI for each PDU Session.
  • The SMF determines whether the PDU Session is to be handled redundantly. The determination is based on the policies provided by PCF for the PDU Session, combination of the S-NSSAI, DNN, user subscription and local policy configuration. The SMF uses these inputs to determine the RSN which differentiates the PDU Sessions that are handled redundantly and indicates redundant user plane requirements for the PDU Sessions in NG-RAN.
  • Operator configuration of UPF selection ensures the appropriate UPF selection for disjoint paths.
  • At establishment of the PDU Sessions or at transitions to CM-CONNECTED state, the RSN parameter indicates to NG-RAN that redundant user plane resources shall be provided for the given PDU sessions by means of dual connectivity. The value of the RSN parameter indicates redundant user plane requirements for the PDU Sessions. This request for redundant handling is made by indicating the RSN to the NG-RAN node on a per PDU Session granularity. PDU Sessions associated with different RSN values shall be realized by different, redundant UP resources. Based on the RSN and RAN configuration, the NG-RAN sets up dual connectivity as defined in TS 37.340 so that the sessions have end to end redundant paths. When there are multiple PDU Sessions with the RSN parameter set and with different values of RSN, this indicates to NG-RAN that CN is requesting dual connectivity to be set up and the user plane shall be handled as indicated by the RSN parameter and the associated RAN configuration. If the RSN value is provided to the NG-RAN, NG-RAN shall consider the RSN value when it associates the PDU Sessions with NG-RAN UP.
  • NOTE 4:
    The decision to set up dual connectivity remains in NG-RAN as defined today. NG-RAN takes into account the additional request for the dual connectivity setup provided by the CN.
  • Using NG-RAN local configuration, NG-RAN determines whether the request to establish RAN resources for a PDU Session is fulfilled or not considering user plane requirements indicated by the RSN parameter by means of dual connectivity. If the request to establish RAN resources for PDU Session can be fulfilled by the RAN, the PDU Session is established even if the user plane requirements indicated by RSN cannot be satisfied. If the NG-RAN determines the request to establish RAN resources cannot be fulfilled then it shall reject the request which eventually triggers the SMF to reject the PDU Session establishment towards the UE. The decision for each PDU Session is taken independently (i.e. rejection of a PDU Session request shall not release the previously established PDU Session). The RAN shall determine whether to notify the SMF if the RAN resources indicated by the RSN parameter can no longer be maintained and SMF can use that to determine if the PDU Session should be released.
  • In the case of Ethernet PDU Sessions, the SMF has the possibility to change the UPF (acting as the PSA) and select a new UPF based on the identity of the Secondary NG-RAN for the second PDU Session if the Secondary NG-RAN is modified (or added/released), using the Ethernet PDU Session Anchor Relocation procedure described in clause 4.3.5.8 of TS 23.502.
  • The SMF's charging record may reflect the RSN information.
  • The RSN indication is transferred from Source NG-RAN to Target NG-RAN in the case of handover.
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5.33.2.2  Support of redundant transmission on N3/N9 interfacesWord-p. 316
If the reliability of NG-RAN node, UPF and CP NFs are high enough to fulfil the reliability requirement of URLLC services served by these NFs, but the reliability of single N3 tunnel is considered not high enough, e.g. due to the deployment environment of backhaul network, the redundant transmission may be deployed between PSA UPF and NG-RAN via two independent N3 tunnels, which are associated with a single PDU Session, over different transport layer path to enhance the reliability.
To ensure the two N3 tunnels are transferred via disjointed transport layer paths, the SMF or PSA UPF should provide different routing information in the tunnel information (e.g. different IP addresses or different Network Instances), and these routing information should be mapped to disjoint transport layer paths according to network deployment configuration. The SMF indicates NG-RAN and PSA UPF that one of the two CN/AN Tunnel Info is used as the redundancy tunnel of the PDU Session accordingly. The redundant transmission using the two N3/N9 tunnels are performed at QoS flow granularity and are sharing the same QoS Flow ID.
During or after a URLLC QoS flow establishment, if the SMF decided that redundant transmission shall be performed based on authorized 5QI, NG-RAN node capability and/or operator configuration, the SMF informs the PSA UPF and NG-RAN to perform redundant transmission via N4 interface and N2 information accordingly. In this case, NG-RAN should also provide different routing information in the tunnel information (e.g. different IP addresses), and these routing information should be mapped to disjoint transport layer paths according to network deployment configuration.
NOTE 1:
The NG-RAN node capability to support the redundant transmission on N3/N9 can be configured in the SMF per network slice or per SMF service area.
If duplication transmission is performed on N3/N9 interface, for each downlink packet of the QoS Flow the PSA UPF received from DN, the PSA UPF replicates the packet and assigns the same GTP-U sequence number to them for the redundant transmission. The NG-RAN eliminates the duplicated packets based on the GTP-U sequence number and then forwards the PDU to the UE.
For each uplink packet of the QoS Flow the NG-RAN received from UE, the NG-RAN replicates the packet and assigns the same GTP-U sequence number to them for redundant transmission. These packets are transmitted to the PSA UPF via two N3 Tunnels separately. The PSA UPF eliminates the duplicated packet based on the GTP-U sequence number accordingly.
NOTE 2:
How to realize the sequence number for support of GTP-U duplication over N3/N9 is up to stage 3.
NOTE 3:
For redundant transmission on N3/N9 interfaces, reordering is not required on the receiver side.
The PSA UPF and NG-RAN may transmit packets via one or both of the tunnels per QoS Flow based on SMF instruction.
NOTE 4:
The AMF selects an SMF supporting redundant transmission based on the requested S-NSSAI and/or DNN.
During UE mobility, when the UE moves from NG-RAN supporting redundant transmission to NG-RAN not supporting redundant transmission, the SMF may release the QoS flow which are subject to redundant transmission.
Figure 5.33.2.2-1 illustrates the case that the redundant transmission is performed only on N3 interface. These packets are transmitted to the NG-RAN via two N3 Tunnels separately. The RAN node and PSA UPF shall support the packet replication and elimination function as described above.
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Two Intermediate UPFs (I-UPFs) between the PSA UPF and the NG-RAN may be used to support the redundant transmission based on two N3 and N9 tunnels between a single NG-RAN node and the PSA UPF. The NG-RAN node and PSA UPF shall support the packet replication and elimination function as described above.
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In Figure 5.33.2.2-2, there are two N3 and N9 tunnels between NG-RAN and PSA UPF for the URLLC QoS Flow(s) of the same PDU Session for redundant transmission established during or after a URLLC QoS flow establishment. In the case of downlink traffic, the PSA UPF duplicates the downlink packet of the QoS Flow from the DN and assigns the same GTP-U sequence number to them. These duplicated packets are transmitted to I-UPF1 and I-UPF2 via N9 Tunnel 1 and N9 Tunnel 2 separately. Each I-UPF forwards the packet with the same GTP-U sequence number which receives from the PSA UPF to NG-RAN via N3 Tunnel 1 and N3 Tunnel 2 respectively. The NG-RAN eliminates the duplicated packet based on the GTP-U sequence number. In the case of uplink traffic, the NG-RAN duplicates the packet of the QoS Flow from the UE and assigns the same GTP-U sequence number to them. These duplicated packets are transmitted to I-UPF1 and I-UPF2 via N3 Tunnel 1 and N3 Tunnel 2 separately. Each I-UPF forwards the packet with the same GTP-U sequence number which receives from the NG-RAN to PSA UPF via N9 Tunnel 1 and N9 Tunnel 2 respectively. The PSA UPF eliminates the duplicated packets based on the GTP-U sequence number.
The I-UPFs inserted on one leg of the redundant paths shall not behave in an UL CL or Branching Point role.
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5.33.2.3  Support for redundant transmission at transport layerWord-p. 317
Redundant transmission can be supported within the 5G System without making any assumption on support for protocols such as IEEE FRER in the application layer (DN only) at the same time it can be supported without requiring redundant GTP-U tunnel over N3. The backhaul provides two disjoint transport paths between UPF and NG-RAN. The redundancy functionality within NG-RAN and UPF make use of the independent paths at transport layer. Support of redundant transmission at transport layer requires no 3GPP protocol impact.
Following are the required steps:
  • UE establishes the PDU session for URLLC services. Based on DNN, S-NSSAI, knowledge of supporting redundant transmission at transport layer and other factors as described in clause 6.3.3, SMF selects a UPF that supports redundant transmission at transport layer for the PDU session. One N3 GTP-U tunnel is established between UPF and NG-RAN.
  • The knowledge of supporting redundant transmission at transport layer can be configured in the SMF, or be configured in UPF and then obtained by the SMF via N4 capability negotiation during N4 Association setup procedure.
  • For DL data transmission, UPF sends the DL packets on N3 GTP-U tunnel. Redundant functionality in the UPF duplicates the DL data on the transport layer. Redundant functionality in the NG-RAN eliminates the received duplicated DL data and sends to NG-RAN.
  • For UL data transmission, NG-RAN sends the received UL packets on N3 GTP-U tunnel, the Redundant functionality in the NG-RAN performs the redundant handling on the backhaul transport layer. The Redundant functionality in the UPF eliminates the received duplicated UL data and sends to UPF.
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5.33.3  QoS Monitoring to Assist URLLC ServiceWord-p. 318
5.33.3.1  General
In this release, the QoS Monitoring is applied for packet delay measurement. The packet delay between UE and PSA UPF is a combination of the UL/DL packet delay on Uu interface and UL/DL packet delay between NG-RAN and PSA UPF. The NG-RAN is required to provide the QoS Monitoring on UL/DL packet delay on Uu interface. The QoS Monitoring on UL/DL packet delay between NG-RAN and PSA UPF can be performed on different levels of granularities, i.e. per QoS Flow per UE level, or per GTP-U path level, subject to the operators' configuration, and/or 3rd party application request, and/or PCF policy control for the URLLC services.
The PCF generates the authorized QoS Monitoring policy for a service data flow based on the QoS Monitoring request if received from the AF. The PCF includes the authorized QoS Monitoring policy in the PCC rule and provides it to the SMF.
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5.33.3.2  Per QoS Flow per UE QoS Monitoring
SMF may activate the end to end UL/DL packet delay measurement between UE and PSA UPF for a QoS Flow during the PDU Session Establishment or Modification procedure.
The SMF sends a QoS Monitoring request to the PSA UPF via N4 and NG-RAN via N2 signalling to request the QoS monitoring between PSA UPF and NG-RAN. The QoS Monitoring request may contain monitoring parameters determined by SMF based on the authorized QoS Monitoring policy received from the PCF and/or local configuration.
The NG-RAN initiates the measurement of UL/DL packet delay on Uu interface based on the QoS Monitoring request from SMF. NG-RAN reports the UL/DL packet delay result on Uu interface to the PSA UPF in the UL data packet or dummy UL packet.
If the NG-RAN and PSA UPF are time synchronised, the one way packet delay monitoring between NG-RAN and PSA UPF is supported based on transferring time stamp in the GTP-U header. The PSA UPF and NG-RAN encapsulate the QoS Monitoring Packet (QMP) indicator which indicates the packet used for UL/DL packet delay measurement and the local time in the GTP-U header. Based on the received time stamp in the GTP-U header and the local time, the PSA UPF calculates the UL packet delay and DL packet delay between NG-RAN and PSA UPF. NG-RAN encapsulates the DL packet delay result and the UL/DL packet delay result on Uu interface in the GTP-U header of UL data packet sent to PSA UPF. The NG-RAN sends a dummy UL packet as the monitoring response packet to the PSA UPF if there is no UL data packet available. The PSA UPF reports the result to the SMF based on conditions specified in SRR.
NOTE:
When does the NG-RAN sends the dummy UL packet as monitoring response to PSA UPF depends on NG-RAN's implementation.
If the NG-RAN and PSA UPF are not time synchronised, it is assumed that the UL packet delay and the DL packet delay between NG-RAN and PSA UPF is the same. The PSA UPF creates and sends the monitoring packets to the RAN:
  • The PSA UPF encapsulates in the GTP-U header with QMP indicator and the local time T1 when the PSA UPF sends out the DL monitoring packets.
  • The NG-RAN records the local time T1 received in the GTP-U header and the local time T2 at the reception of the DL monitoring packets. The NG-RAN initiates UL/DL packet delay measurement on the Uu interface.
  • When receiving the UL packet from UE or when the NG-RAN sends the dummy UL packet as monitoring response, the NG-RAN encapsulates QMP indicator, the UL/DL packet delay result of Uu interface, the time T1 received in the GTP-U header, the local time T2 at the reception of the DL monitoring packet and the local time T3 when NG-RAN sends out this monitoring response packet to the UPF via N3 interface, in the GTP-U header of the monitoring response packet.
  • The PSA UPF records the local time T4 when receiving the monitoring response packets and calculates the round trip and UL/DL packet delay between NG-RAN and anchor PSA UPF based on the time information contained in the GTP-U header of the received monitoring responsepacket. The PSA UPF calculates the UL/DL packet delay between the NG-RAN and the PSA UPF based on the (T2-T1+T4-T3)/2. The PSA UPF calculates the UL/DL packet delay between UE and PSA UPF based on the received UL/DL packet delay results of Uu interface and the calculated UL/DL packet delay between RAN and PSA UPF. The PSA UPF reports the result to the SMF based on some specific condition, e.g. when threshold for reporting to SMF is reached.
If the redundant transmission on N3/N9 interfaces is activated, the UPF and NG-RAN performs QoS monitoring for both UP paths. The UPF reports the packet delay of the two UP paths respectively to the SMF.
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5.33.3.3  GTP-U Path MonitoringWord-p. 319
The SMF can request to activate QoS monitoring for the GTP-U path(s) between all UPF(s) and the (R)AN based on locally configured policies. Alternatively, when a QoS monitoring policy is received in a PCC rule and the QoS monitoring is not yet active for the DSCP corresponding to the 5QI in the PCC rule, the SMF activates QoS Monitoring for all UPFs currently in use for this PDU Session and the (R)AN. The SMF sends the QoS monitoring policy to each involved UPF and the (R)AN via N4 interface and via N2 interface respectively.
A GTP-U sender performs an estimation of RTT to a GTP-U receiver on a GTP-U path by sending Echo messages and measuring time that elapses between the transmission of Request message and the reception of Response message. A GTP-U sender computes an accumulated packet delay by adding RTT/2, the processing time and, if available, an accumulated packet delay from an upstream GTP-U sender (i.e. an immediately preceding GTP-U sender in user plane path) thus the measured accumulated delay represents an estimated elapsed time since a user plane packet entered 3GPP domain.
It is expected that a GTP-U sender determines RTT periodically in order to detect changes in transport delays. QoS monitoring is performed by a GTP-U end-point (UP function) that receive and store QoS including a packet delay budget parameter for QoS flow by comparing a received accumulated packet delay with the stored QoS parameter possibly also taking into the account the measured delay of GTP-U path to next GTP-U end-point processing time. If the GTP-U end-point determines that the packet delay exceeds the requested packet delay budget then the node triggers QoS monitoring alert signalling to a control plane network function, e.g. SMF or to an OA&M function.
NOTE:
Echo Request message and Echo Response message are sent outside GTP-U tunnels (the messages are using TEID set to 0). If underlying transport is using QoS differentiation (e.g. IP DiffServ) then it is up to the implementation to ensure that the Echo messages are classified correctly and receive similar treatment by the underlaying transport as GTP-U GTP-PDUs carrying QoS flows (user data).
QoS Monitoring can be used to measure the packet delay for transport paths and map the QoS Flows to appropriate network instance, DSCP values as follows:
  • Packet delay measurement is performed by using GTP-U Echo Request/Response as defined in the TS 28.552, in the corresponding user plane transport path(s), independent of the corresponding PDU Session and the 5QI for a given QoS flow, for a specific URLLC service.
  • RAN measures UL/DL packet delay within RAN (e.g. including delays of F1-U, Xn-U, Uu interfaces for NR) and calculates UL packet delay of N3 interface. RAN provides the UL packet delay comprising the packet delays of RAN (including delays of F1-U, Xn-U, Uu interfaces for NR) and N3 interface towards SMF (via N2).
  • The PSA UPF calculates the UL/DL packet delay of N3/N9 interface (N9 is applicable when I-UPF exists).
  • UPF and RAN reports QoS Monitoring result to the SMF based on some specific conditions, e.g. first time, periodic, event triggered, when thresholds for reporting towards SMF (via N4) are reached.
  • UPF does measurement of network hop delay per transport resources that it will use towards a peer network node identified by an IP destination address (the hop between these two nodes) and port. The network hop measured delay is computed by sending an Echo Request over such transport resource (Ti) and measuring RTT/2 when Echo Response is received.
  • UPF maps {network instance, DSCP} into Transport Resource and measures delay per IP destination address and port. Thus, for each IP destination address, the measured delay per (network instance, DSCP) entry is determined.
  • The UPF performing the QoS monitoring can provide the corresponding {Network instance, DSCP} along with the measured packet delay for the corresponding transport path to the SMF.
  • Based on this, SMF can determine QoS Flow mapping to the appropriate {Network instance, DSCP} considering {5QI, QoS characteristics, ARP} for the given QoS flow.
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