Packet Core Procedures

In this chapter, we will consolidate the concepts and fundamental knowledge from the previous chapters and map them to real 5G system procedures. We will see how the fundamentals of cloud-native infrastructure give shape to the communications between the various network functions in a producer-consumer relationship to bring about the reality of the 5G system and various procedures that enable a 5G device to get access to mobile broadband. All the call flows explained in detail here are expected to be implemented in code that has been built based on cloud-native development methods; these are mainly container-based and have a producer-consumer method of providing and consuming services.

In Chapter 2, we learned about the role of the network functions in the 5G packet core, and in Chapter 3, Building Blocks – Cloud Native Infrastructure, we learned about the services offered by each of those network functions. In this chapter, we will see how all the...

PDU session establishment in 5G

In Chapter 2, we discussed a Protocol Data Unit (PDU) session, wherein we learned that a PDU provides end-to-end connectivity to user equipment (UE) to avail of data services. A PDU session can be imagined to be a pipe that is associated with every UE and is identified by a PDU session ID; this pipe is used for the user data to flow between the 5G packet core to the UE. The PDU session ID is generated by the UE. Figure 7.1 shows a PDU session along with various Quality of Service (QoS) flows associated with it:

Figure 7.1 – PDU session in 5G

The establishment of a PDU session involves the UE along with a number of network functions. These network functions exchange messages with each other, and when all these message transactions are successful, a PDU session is finally established.

Figure 7.2 describes the procedure through which this end-to-end connectivity is established. It can be noticed here that the participating...

PDU session establishment with 4G/N26/IWF

3GPP has specified provisions for the UEs capable of supporting both 5G and 4G NAS to connect to the E-UTRAN and 5G core network. An SMF capable of the EPS interworking support would act as a PGW-C+SMF and would have the S5/S8 interface with SGW to receive the 4G session creation request.

Figure 7.7 – 4G interworking function

All the other interfaces involved in a 4G session creation (for example, Gx, Gy, Gz, etc.) are replaced by the corresponding 5G core SBI interfaces (Npcf, Nudm, and Nchf). The SMF+PGW-C provides information over N4 to the UPF+PGW-U related to the handling of traffic over S5-U.

This option will be used typically by network operators when most of their network has a 5G core deployed. In this case, it makes sense to slowly retire the legacy of the 4G-based core and leverage the interworking function of the 5G core to cater to 4G devices.

In Figure 7.7, we see how interworking is...

IMS 5G PDU session establishment

Voice and video calls are use cases that need to be supported by every service provider network. The IP Multimedia Subsystem (IMS) enables these services in a telecom network. In this section, we will see how voice, video, and rich communication services are provided with the help of the IMS in 5G, which is very similar to the support of voice in 4G (VoLTE). Figure 7.12 shows the establishment of an IMS PDU session in 5G:

Figure 7.12 – IMS 5G PDU attach in 5G – part 1

Let’s look at these steps in detail:

Step 1: The UE initiates the N1 message with the PDU session establishment request by including a container identifier, P-CSCF IPv4/v6 Request contained within the ePCO (extended Protocol Configuration Options) IE, and DNN set to IMS.

Step 2: The AMF sends the Nsmf_PDUSession_CreateSM context request to the SMF by including N1 and N2 messages as multipart, along with the ePCO IE if it is received...

VoNR call flow

In the previous section, we saw how an IMS PDU session is established. In this section, we will go over the VoNR call flow, which is used when a user places a voice call.

Figure 7.25 – VoNR call flow – part 1

Step 1: The UE initiates SIP registration toward the called party via UPF, and P-CSCF through the IMS core network in the backend.

Step 2: P-CSCF sends Npcf_PolicyAuthorization_Update to the PCF to enforce policies, modify service information, apply gate control for traffic shaping and policing, modify subscription to SDF notification/deactivation, and so on.

Step 3: The PCF sends an HTTP POST request (/callbacks/v1/smPoliciesUpdate
Notification/imsi-123xxxxxx:1:1591309591702656285/update) to the SMF to update PCC rules (SIP VoNR audio), policy control request triggers, SDF, and charging-related information.

Step 4: The SMF processes the received PCC rules and sends a 204 No Content (200 OK) message for...

Wi-Fi attach

One of the prime features of the 5G core network is its capability to inter-work with different types of access networks, both 3GPP-based and non-3GPP-based. One such network is Wi-Fi. The 5G radio can coexist with Wi-Fi access to cover coverage gaps. Hence, denser access can be planned with complementing radio access technologies. This is how Wi-Fi and New Radio (5G radio) can complement each other and do not need to be treated as competing technologies.

Figure 7.29 – Wi-Fi attach – part 1

We will now go over the Wi-Fi attach procedure:

Step 1: The UE sends the IKE_SA_AUTH exchange message to ePDG (IWF-interworking function) to create the IPSec tunnel.

Step 2: The UE sends the IKE_SA_AUTH exchange message to the SMF+PGW-C as a GTP Create Session request by including the P-CSCF IPv4 or IPv6 request and DNS IPv4 or IPv6 container identifier in APCO IE options. The ePDG checks the UE capability information to determine whether...

Summary

In this chapter, we have seen the various procedures in 5G and how they come together to service a subscriber. We have gone over how a PDU session is established in 5G, and we have gone over how to connect a 4G radio to work with a 5G interworking function. We have seen how to establish a voice call over New Radio and finally, we have seen how Wi-Fi access integrates with a 5G system. By the end of this chapter, we now understand the 5G call flows in detail and have developed a near-practical understanding of the messages exchanged between the various network functions to bring about a data or a voice call in 5G.

In the upcoming chapters, we will delve further into 5G-NSA and 5G-SA system architectures that leverage these procedures. The next chapter introduces exclusive VoNR concepts.