In a way that is better understood , Divided into 5 layer
application layer ： Directly provide services for user application process , To the transport layer to the application of information processing （ decrypt , How to show it to you , For example, text messages ： Regular script ? running script ?）, And then I can show you .
Transport layer ： Responsible for transport network layer information to application layer , The main functions include division and reuse .
Divide and use ： Is to receive from the network layer separately delivered to the application layer corresponding process （ There is a port number ）
multiplexing ： All the data coming from the application process （ Header information , Yes IP??? And port , Add port only ???） Package , It is then sent to the network layer
network layer ： It provides communication services between different hosts on the switching network , Yes IP agreement , Choose the appropriate route （ route ）, Data transmitted from the source host , Can find the destination host through the router in the network .（ A protocol , More conceptual , It's just teaching you how to find your way ）.
data link layer ： Data transmission between two hosts , It's always on one link after another , Between two adjacent points , It's point-to-point direct transmission . Special link layer protocol is needed , The data link layer hand over the network layer IP Datagrams are assembled into frames （framing）, On a link between two adjacent nodes “ transparent ” Transmit frame to ground （frame） Data in （ There is synchronization information ： Has it been delivered completely , Address information ： Where to send it ? error control ： There is a mistake , Stop right now , retransmission ）.
physical layer （physical
layer）： The unit of data transmitted on the physical layer is bits , The task of the physical layer is to transparently transport the bit stream .（ It can be understood as a physical medium , Such as twisted pair , Coaxial cable , optical cable , Wireless channel, etc , Very small, very small units ,1
bit 1bit Down the road ）.
major function ：
（1）gNB：Radio Resource Management, Wireless resource management ;Radio Bearer Control, Wireless bearer control ;Radio
Admission Control Wireless access control ; Connection Mobility Control Connection Mobility Control ;Measurement
configuration & Provision; Measurement configuration and regulation ;Dynamic Resource Allocation Dynamic resource allocation （ arrange ）
（2）AMF：NAS (Non-access stratum) Security Non Access Layer Security ;idle State Mobility
Handling Idle state move processing
（3）UPF：Mobility Anchoring Mobility anchoring ;PDU handling Protocol data unit processing
（4）SMF（Session Management function）：UE IP address allocation UE IP Address assignment ; PDU
Session Control Protocol data unit session control
4.3 Network Interfaces
NG User Plane：The NG user plane interface (NG-U) is defined between the NG-RAN
node and the UPF. The user plane protocol stack of the NG interface is shown on
Figure 188.8.131.52-1. The transport network layer is built on IP transport and
GTP-U is used on top of UDP/IP to carry the user plane PDUs between the NG-RAN
node and the UPF.
NG-U, connect NG-RAN and UPF,NG Interface user interface protocol stack
The transport network layer is located in the IP Above the transport layer , use GPRS Tunneling
Protocol-User（GPRS Tunnel protocol ： user , stay UDP/IP above ） Pass in NG-RAN Node and UPF User interface data protocol unit .
NG-U provides non-guaranteed delivery of user plane PDUs between the NG-RAN
node and the UPF.
NG-U Used UDP agreement , There is no guarantee of complete accuracy of transmission .
NG Control Plane：
The NG control plane interface (NG-C) is defined between the NG-RAN node and
the AMF. The control plane protocol stack of the NG interface is shown on
Figure 184.108.40.206-1. The transport network layer is built on IP transport. For the
reliable transport of signalling messages, SCTP is added on top of IP. The
application layer signalling protocol is referred to as NGAP (NG Application
Protocol). The SCTP layer provides guaranteed delivery of application layer
messages. In the transport, IP layer point-to-point transmission is used to
deliver the signalling PDUs.
NG-RAN Node and AMF between , The transport network layer is located in the IP Above the transport layer , For reliable signaling transmission ,SCTP Add in IP Above the layer , The application layer signaling protocol is called NGAP,SCTP（Stream
Control Transmission Protocol） The reliable transmission of application layer message is ensured , In transit IP Layer to point transmission is used to transmit signaling protocol data units .
NG-C provides the following functions:
* NG interface management;
NG Interface management
* UE context management;
UE Context management
* UE mobility management;
UE Mobility management
* Transport of NAS messages;
NAS information transmission
* PDU Session Management;
Protocol data unit session management
* Configuration Transfer;
* Warning Message Transmission.
Warning message transmission
Control related NGAP, Yes AP existence , use TCP/IP agreement ???
Xn User Plane：The Xn User plane (Xn-U) interface is defined between two NG-RAN
nodes. The user plane protocol stack on the Xn interface is shown in Figure
220.127.116.11-1. The transport network layer is built on IP transport and GTP-U is
used on top of UDP/IP to carry the user plane
PDUs.Xn-U( User interface ) In two NG-RAN Between nodes , The transport network layer is located in the IP Above the transport layer , use GTP-U agreement （UDP/IP） To transmit the user interface PDUs
Xn Control Plane：The Xn control plane interface (Xn-C) is defined between two
NG-RAN nodes. The control plane protocol stack of the Xn interface is shown on
Figure 18.104.22.168-1. The transport network layer is built on SCTP on top of IP.
The application layer signalling protocol is referred to as XnAP (Xn
Application Protocol). The SCTP layer provides the guaranteed delivery of
application layer messages. In the transport IP layer point-to-point
transmission is used to deliver the signalling PDUs.
Xn-C In two NG-RAN Between nodes , The application layer signaling protocol is called XnAP,SCTP（Stream Control Transmission
Protocol） Layer protocol ensures the reliable transmission of application layer information . stay IP Layer to point transmission is used to transfer signaling data
Xn interface management; Xn Interface management ;
UE mobility management, including context transfer and RAN
paging:UE Mobility management , Including context transfer and RAN paging ：
Dual connectivity. Dual connection （4G and 5G???）
4.4 Radio Protocol Architecture
Wireless protocol architecture
* User Plane
The figure below shows the protocol stack for the user plane, where SDAP,
PDCP, RLC and MAC sublayers (terminated in gNB on the network side) perform the
functions listed in subclause 6.
SDAP（Service Data Adaptation Protocol）,PDCP（Packet Data Convergence Protocol）,
RLC（Radio Link Control）and MAC Sublayer ends at gNB Network side
* Control Plane
PDCP, RLC and MAC（subclause 6）,
RRC（subclause 7）,radio resource control
NAS（Non-access stratum） control protocol（ Non access layer control protocol ）
NAS control protocol (terminated in AMF on the network side) performs the
functions listed in 3GPP TS 23.501 ), for instance: authentication, mobility
management, security control… authentication , Mobile management and security control
4.5 Multi-RAT Dual Connectivity
many RAT Dual connection （Radio Access Technology）
NG-RAN supports Multi-RAT Dual Connectivity (MR-DC) operation whereby a UE in
RRC_CONNECTED is configured to utilise radio resources provided by two distinct
schedulers, located in two different NG-RAN nodes connected via a non-ideal
backhaul and providing either E-UTRA (i.e. if the node is an ng-eNB) or NR
access (i.e. if the node is a gNB). Further details of MR-DC operation can be
found in 3GPP TS 37.340 .
NG-RAN Support multiple access and dual connection operation , So a UE stay RRC_CONNECTED It is configured to utilize the wireless resources provided by two different schedulers , The scheduler is located in two different NG-RAN Node （ Through non ideal return connection ） And provide E-UTRA perhaps NR Access .