Cellular IoT support and evolution for the 5G System
Substantial E-UTRAN/EPC evolution has been achieved in 3GPP to enable the "Cellular Internet of Things" (CIoT). In particular, eMTC (WB-E-UTRAN) and NB-IoT have been designed in RAN WGs in Rel-13 and enhanced in Rel-14. The corresponding system architecture aspects have been designed for EPC in Rel-13 and Rel-14. These system architecture aspects apply to both NB-IoT and eMTC (WB-E-UTRAN).
As documented in TS
23.501 [1] clause 5.31, the following CIoT features have been introduced in
R16:
- Control Plane CIoT 5GS Optimisation (CP CIoT 5GS Optimisation) is
used to exchange user data between the UE and the SMF as payload of a NAS
message in both uplink and downlink directions, avoiding the establishment of a
user plane connection for the PDU Session (i.e. avoiding the need for Data
Radio Bearer and N3 tunnel). Early Data Transmission (EDT), i.e. sending user
data in RRC Message 3 is supported for mobile originated Control Plane CIoT 5GS
Optimisation.
- User Plane CIoT 5GS Optimisation (UP CIoT 5GS Optimisation)
supports transfer of user plane data from CM-IDLE without the need for using
the Service Request procedure to establish an Access Stratum (AS) context in
NG-RAN and UE. UP CIoT 5GS Optimisation is enabled by the Connection Suspend
and Connection Resume in CM-IDLE with Suspend procedures. EDT is also supported
for UP CIoT 5GS Optimisation.
- UE and network negotiate whether to use CP
CIoT 5GS Optimisation and/or UP CIoT 5GS Optimisation as well as N3 data
transport and header compression for CP CIoT 5GS Optimisation during the
Registration procedure. The UE indicates its Preferred and Supported Network
Behaviour, i.e. the UE indicates which of the aforementioned features the
UE supports and whether it prefers to use CP CIoT 5GS Optimisation or UP CIoT
5GS Optimisation. In response, the network indicates which of those features it
supports for this UE.
- Non-IP Data Delivery (NIDD) refers to mobile
originated (MO) and mobile terminated (MT) communication between UE and an
Application Function (AF) where the user data is considered unstructured (also
referred to as "non-IP"). NIDD is enabled using an unstructured PDU
session between UE and NEF and the NEF's NIDD API on the N33/Nnef reference
point for data delivery from/to AF. Alternatively, non-IP data is delivered
using an unstructured PDU session between UE and UPF and a Point-to-Point N6
tunnel between UPF and AF. The NEF also supports distribution of mobile
terminated messages to a group of UEs based on the NIDD API.
- Reliable Data Service (RDS) may be used between UE and
NEF or UPF, respectively, for unstructured PDU Sessions. RDS provides a mechanism
for the NEF or UPF to determine if the user data was successfully delivered to
the UE and for the UE to determine if the data was successfully delivered to
the NEF or UPF. When a requested acknowledgement is not received, RDS
retransmits the packet.
- Extended Discontinuous Reception (DRX) for
CM-IDLE and CM-CONNECTED with RRC-INACTIVE enables the UE to reduce its power
consumption while still being available for MT data and/or network originated
procedures within a certain delay dependent on the negotiated DRX cycle value.
In CM-IDLE state the following DRX cycles are supported: up to almost 44
minutes (for eMTC) and up to almost 3 hours (for NB-IoT). In CM-CONNECTED with
RRC-INACTIVE, DRX cycles of up to 10.24 seconds are supported.
- Enhancements for the Mobile Initiated
Connection Only (MICO) mode of rel.15:
- MICO mode with Extended Connected Time
enables an AMF that is aware of pending or expected MT traffic to keep the UE
in CM-CONNECTED state and to request the RAN to keep the UE in RRC-CONNECTED
state for an Extended Connected Time period to ensure that the downlink data
and/or signalling is delivered to the UE before the UE is released.
- MICO mode with Active Time is similar to
the UE Power Saving Mode (PSM) defined for EPS [2], i.e. UE and AMF negotiate
an Active Time value, which dictates for how long the UE is reachable for
paging upon entering CM-IDLE. Once the Active Time has elapsed, the UE can
enter MICO mode, i.e. become unreachable for paging.
- MICO mode and Periodic Registration Timer
Control enables the network to align a UE's Periodic Registration Updates with
an expected DL communication schedule for the UE. This is achieved by the
Strictly Periodic Registration Timer Indication which the network can provide
to the UE to avoid that the Periodic Registration Timer is restarted by the UE
when the UE enters CM-CONNECTED.
- High latency communication refers to
mechanisms that may be used to handle mobile terminated (MT) communication with
UEs being unreachable while using power saving functions (e.g. extended DRX or
MICO mode with Active or Extended Connected Time). High latency communication
is supported by extended buffering of downlink data in the UPF, SMF or NEF when
a UE is using power saving functions in CM-IDLE and is not reachable.
Alternatively, high latency communication is supported through different AF
notifications. An AF may for example subscribe to receive UE reachability
notifications so that the AF then waits with sending the data until it gets a
notification that the UE has become reachable.
- Support for Monitoring Events enables
AFs to acquire information such as whether a UE is roaming or to determine when
a USIM is changed to a different ME. The complete list of monitoring events is
documented in TS 23.502 [3] clause 4.15.3.1.
- External parameter provisioning
refers to functionality originally defined in Rel-15 already, which enables an
AF to provision information such as Expected UE Behaviour parameters or other
service parameters to the network (see TS 23.502 [3] clause 4.15.6.1). In
Rel-16, additional Expected UE Behaviour parameters were introduced (e.g. an
indication whether the UE is stationary or mobile), which can be used by the
AMF and/or SMF for various purposes, e.g. by AMF for paging optimisations. In addition,
an AF can provide Network Configuration parameters (e.g. Maximum Response Time)
to the network, which in turn can be used to derive parameters for system
procedures (e.g. to derive the Active Time or Extended Connected time for MICO
mode). The complete list of Expected UE Behaviour parameters and Network
Configuration parameters are documented in TS 23.502 [3] clause 4.15.6.3 and
4.15.6.3a, respectively.
- PDU session handling during inter-RAT
idle mode mobility to and from NB-IoT allows the SMF to maintain a PDU
session, disconnect a PDU session with a reactivation request or to disconnect
a PDU session without reactivation request when the UE moves between a
"broadband" RAT (e.g. NR or WB-E-UTRAN) and a "narrowband"
RAT (NB-IoT).
- System aspects of the Enhanced Coverage
RAN feature as specified in TS 36.300 [4], in particular storage of Paging
Assistance Data for UEs supporting Enhanced Coverage and providing of Paging
Assistance Data to RAN during paging has been introduced. Specific subscribers can
also be restricted to use the Enhanced Coverage feature through Enhanced
Coverage Restricted information that is stored in the UDM. Based on the latter,
AMF informs UE, RAN and SMF about the use (or not) of the Enhanced Coverage
feature.
- The rate of user data sent to and
from a UE can be controlled in two ways:
- Serving PLMN rate control allows the
serving network to protect its AMF and the Signalling Radio Bearers in the
NG-RAN from the load generated by NAS Data PDUs by indicating a limit for the number
of NAS Data PDUs per time unit to the UE and UPF/NEF.
- Small Data Rate Control allows HPLMN
operators to offer customer services such as "maximum of Y messages per
day" and is also based on indicating a number of packets per time unit
limitation to UE and UPF/NEF for enforcement.
- Congestion control for control plane data
transfer enables the AMF to restrict, e.g. during high-load situations, the
use of the control plane for data transmission (i.e. for Control Plane CIoT 5GS
Optimisation). In particular, AMF may provide a Control Plane data back-off
timer to the UE. While the Control Plane data back-off timer is running, the UE
is not allowed to initiate any data transfer via Control Plane CIoT 5GS
Optimisation.
- Service Gap Control can be used to control
the frequency at which UEs can access the network, e.g. to alleviate peak load
situations. Service Gap control is realized by AMF indicating a Service Gap
Time to a UE; the UE then stays in CM-IDLE mode for at least the whole duration
of the Service Gap timer before triggering Mobile Originated user data
transmission.
- Inter-UE QoS for NB-IoT targets UEs
that are using Control Plane CIoT 5GS Optimisation and are accessing the
network via NB-IoT. The feature allows NG-RAN to prioritise resource allocation
between those UEs based on the subscribed NB-IoT UE Priority that NG-RAN may
retrieve from the AMF.
- Differentiation of Category M UEs
enables the network to identify traffic to/from Category M UEs, e.g. for
charging differentiation and subscription-based access control. This
functionality is based on a Category M indication from UE to NG-RAN during RRC
Connection Establishment and subsequently, an LTE-M Indication to the AMF in
the Initial UE Message. Based on this, AMF considers the RAT type to be LTE-M
and informs SMF, SMSF and PCF accordingly for subsequent use of the LTE-M RAT
type, e.g. in CDRs. A subscription parameter for subscription-based access
restriction for LTE-M is also introduced.
- Selection, steering and redirection
between EPS and 5GS allows a network that supports CIoT features in both
EPC and 5GC to steer UEs from the CN type that the UE is attempting to register
with to the other CN type (e.g. from 5GC to EPC) due to operator policy, e.g.,
due to roaming agreements, Preferred and Supported Network Behaviour, load
redistribution, etc. It is assumed that operators configure the steering
policies in EPC and 5GC such that steering UEs back and forth between EPC and
5GC is avoided.
References
[1] TS 23.501, “System architecture
for the 5G System (5GS)”
[2] TS 23.401, “General
Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial
Radio Access Network (E-UTRAN) access”
[3] TS 23.502, “Procedures
for the 5G System (5GS)”
[4] TS 36.300, "
Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal
Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2"