---
title: 'UPF Detail Function'
disqus: hackmd
---



The configuration above is for the N4 interface of the User Plane Function (UPF) in a 5G network. N4 is the interface that connects the UPF with the Session Management Function (SMF). Here is a detailed explanation of each element in the configuration:

### pfcp:
This section configures the Packet Forwarding Control Protocol (PFCP) used on the N4 interface.

1. **addr: 127.0.0.8**
- **IP addr for listening:**
The IP address used by the UPF to listen for incoming connections on the N4 interface. This is a local (loopback) IP address in this example, which means the communication will only occur within the same machine, usually for testing or special configurations.

2. **nodeID: 127.0.0.8**
- **External IP or FQDN can be reached:**
This is the IP address or FQDN (Fully Qualified Domain Name) that can be accessed externally by the SMF to communicate with the UPF. In this example, it is the same as the `addr`, using the loopback address.

3. **retransTimeout: 1s**
- **Retransmission timeout:**
The waiting time before attempting to resend an unacknowledged or unresponded message. In this example, the timeout is 1 second.

4. **maxRetrans: 3**
- **The max number of retransmissions:**
The maximum number of attempts to resend a message if no acknowledgment is received. In this example, retransmissions will be attempted up to a maximum of 3 times.

### Additional Explanation:

- **Comments:**
Comments on each line provide a brief explanation of what each parameter does. Comments begin with the `#` symbol.

- **Cannot be Set to 0.0.0.0:**
In this case, the IP address for `addr` cannot be set to `0.0.0.0`, which is usually used to listen on all network interfaces. Instead, it must be specified to a particular IP address.

### Conclusion:

This configuration sets parameters for the N4 interface of the UPF, specifying the IP address for listening to incoming connections from the SMF, setting the node ID that can be externally reached, and defining the timeout policy and the maximum number of message retransmissions. This configuration is crucial to ensure reliable communication between the UPF and SMF in a 5G network.


The configuration above is for GTP-U (GPRS Tunneling Protocol - User Plane) in a User Plane Function (UPF) within a 5G network. Here is a detailed explanation of each element in this configuration:

### gtpu:
This section configures the GTP-U protocol, which is used for transmitting user data (user plane) through the 5G core network.

1. **forwarder: gtp5g**
- **gtp5g:**
This indicates that the forwarder being used is `gtp5g`. GTP5G is an implementation of the GTP-U protocol specifically designed for 5G networks.

2. **ifList:**
This section is a list of interfaces used by the UPF to communicate with other network elements via the N3 or N9 interfaces. N3 is the interface between the UPF and the gNB (base station), while N9 is the interface between two UPFs.

- **addr: 192.168.1.130**
- **IP address for N3 interface:**
This is the IP address of the N3 interface used by the UPF to receive and send user data to and from the gNB. In this example, the IP address used is `192.168.1.130`.

- **type: N3**
- **Type of interface:**
This indicates that this interface is an N3 interface, which connects the UPF to the gNB in the 5G network architecture.

- **name: upf.5gc.nctu.me**
- **Commented name:**
This is a comment suggesting a possible hostname for the UPF. Since it is commented out (with a `#`), this setting is not active in the current configuration.

- **ifname: gtpif**
- **Commented interface name:**
This is a comment indicating a possible network interface name, but it is also commented out and not used in the active configuration.

- **mtu: 1400**
- **Commented MTU:**
This is a comment indicating a possible Maximum Transmission Unit (MTU) size for this interface. The MTU determines the maximum packet size that can be transmitted in a single network transmission, but since it is commented out, this setting is not currently used.

### Additional Explanation:

- **Comments:**
Lines starting with `#` are comments and are not executed by the system. These comments may contain additional information or alternative settings that might be needed later.

- **Multiple Connections:**
If multiple connections need to be configured, you can set `addr` to `0.0.0.0` to listen on all interfaces or list all relevant IP addresses.

### Conclusion:

This configuration sets parameters for the GTP-U interface of the UPF, specifying the IP address and type of interface used to communicate with the gNB via N3. These settings are important to ensure the UPF can efficiently send and receive user data in a 5G network.

In the configuration above, "N3" is the type of interface used by the User Plane Function (UPF) to communicate with the gNB (base station) in the 5G network architecture. Here’s an explanation of why N3 is necessary:

### Explanation of Using N3

1. **Role of UPF in 5G Network:**
The UPF (User Plane Function) is responsible for managing user plane traffic in the 5G network. The UPF receives, processes, and forwards user traffic between the 5G core network and radio elements like the gNB.

2. **Function of N3:**
The N3 interface is the communication link between the UPF and the gNB. N3 is used to send and receive user data between the radio access network (RAN) and the core network. In other words, N3 is the main path through which data from user devices is transmitted to the network and vice versa.

3. **N3 IP Configuration:**
In this configuration:

   gtpu:
     forwarder: gtp5g
     ifList:
       - addr: 192.168.1.130
         type: N3

- `addr: 192.168.1.130`: This is the IP address used by the N3 interface on the UPF. The UPF will listen for and send data on this IP address.
- `type: N3`: This indicates that the interface is of type N3, which connects the UPF to the gNB.

4. **Flexibility in Connections:**
If the UPF has multiple interfaces or IP addresses that need to be configured for N3 or N9 (another interface connecting to other network elements), we can set `addr` to `0.0.0.0` to listen on all network interfaces or list all relevant addresses. This provides flexibility in network configuration to meet various network topology requirements.

5. **Implementation of GTP-U:**
GTP-U (GPRS Tunneling Protocol - User Plane) is used to encapsulate and transmit user data through the N3 interface. Here, the `gtp5g` forwarder is used to implement GTP-U in the 5G network.

### Conclusion:

Using N3 in this configuration ensures that the UPF can communicate with the gNB to manage user traffic in the 5G network. The N3 interface is a critical path for transmitting data between user devices and the core network, so proper configuration of the IP address and interface type for N3 is essential for efficient and reliable network operations.

### About N1 (UE,OAI, and 5GC)
The N1 interface is used to connect User Equipment (UE), OpenAirInterface (OAI), and the 5G Core (5GC) because of several important functionalities and communication needs within the 5G network architecture. Here's an explanation of why N1 is used:

**Explanation of Using N1**

1. **Role of N1 in 5G Network:**
The N1 interface is the primary communication path between the User Equipment (UE) and the 5G Core Network (5GC). N1 is used for sending and receiving control signaling between the UE and network functions like AMF (Access and Mobility Management Function).

2. **Function of N1:**
- **Signaling:** N1 handles signaling between the UE and 5GC. This includes processes like UE registration, connection management, and authentication.
- **Session Management:** N1 helps in session management, such as setting up and maintaining PDU (Packet Data Unit) sessions that connect the UE to data networks.
- **Mobility Management:** N1 supports mobility management, allowing the UE to remain connected and move seamlessly between different gNB (base stations) without losing connection.

3. **UE Communication with 5GC via OAI:**
- **OpenAirInterface (OAI):** OAI is an open-source platform used to develop and test 5G network components. OAI can simulate a complete 5G network including UE, gNB, and 5GC components.
- **UE-OAI-5GC Relationship:** The UE connects to the 5GC through OAI, which acts as the access network. The N1 interface allows signaling and session management from the UE through OAI to the 5GC.

4. **Importance of Signaling on N1:**
- **Registration:** Through N1, the UE can register with the 5G network, which is necessary to obtain network services.
- **Authentication:** The authentication process ensures that only legitimate users can access the network.
- **Session Establishment:** Establishing PDU sessions to access data services after successful registration and authentication.

5. **Benefits of Using N1:**
- **Standardization:** N1 is a standardized interface in the 5G network architecture, ensuring interoperability and compatibility between various vendors and network implementations.
- **Security:** N1 includes security mechanisms for authentication and data encryption, which are essential for protecting user information.

### Conclusion:

Using the N1 interface to connect UE, OAI, and 5GC is crucial for ensuring effective and efficient signaling communication in the 5G network. N1 plays a critical role in the registration, authentication, session management, and mobility of UE within the network. This makes N1 a core component in the 5G architecture, ensuring that the UE can interact with services provided by the 5GC through an access network developed using platforms like OAI.