Policy Control Function
The policy is simply a rule for how the 5G system can manage the different users, data sessions or different data flows. The policy themselves can be specified as different levels of granularity such as
i) some policies affect all the users in the same manner
ii) some policies are user-specific, but it affects all the services of the user in the same manner
iii) there are PDU session or IP data flow specific policies, which affect only those applications or data flow.
Now PCF is not working independently. To make the best policy level decision, it works together with many other network functions.
There are two kinds of policies
i) Session Related Schlender Policies:
The policy related to PDU sessions is known as session related policies. For example, the policies about what application traffic is supported in PDU sessions and what is forbidden are usually handled by session related policies and also what applications get higher or lower priority also comes under this policy.
ii) Non-Session Related Policy:
When the policy is related user-specific is known as a non-session related policy. For example, suppose some users are allowed to access the 5G system in a certain geography and not in some other area. In that case, that geographical description is a policy that comes under the non-session related policy.
Non-Session Related Policies
i) Access and Mobility Related Policy
- Service Area Restriction: If a device or a user is allowed to access the 5G system in certain geographical areas and not in others it is allowed to access a certain kind of service in some low graphical areas and not others. This is called a service area restriction.
- RAT/frequency selection priority When a device is accessing a 5G system, it can be seen that the 5G is available over multiple frequency bands in such a situation, the device needs guidance on which is the highest priority frequency to access. This way, the operator chooses the frequency that is expected to provide the best performance will be, which the device will choose as the preferred frequency.
ii) Policies to the UE
- UE Route Selection Policy: Suppose a device is connected to a 5G system and has an ongoing PDU session. Now, if a user opens his new application say YouTube and want to stream videos. Now the device has to decide, if the existing PDU session can be used for YouTube or if it should create a new PDU session. To help the decision like this, we have UE Route Selection Policy.
- Access Network Discovery and Selection Policy: In some scenarios, a Wi-Fi network is used to support 5G connectivity where the 5G coverage is sometimes poor. Suppose there might be a case where the devices see multiple Wi-Fi networks available. Then there should be some sense of priority in locating the Wi-Fi network and choosing the right access network. This policy will help to choose the right Wi-Fi network in a given situation.
iii) Packet Flow Description
It refers to the set of information which helps the network to detect the application traffic. We have seen that in QoS, multiple application-related traffic can be carried from the data network to gNodeB. But not all application has to be treated the same way. For example, if the operator has an agreement that if you stream music through a specific music stream service then they don’t charge you for it that means they won’t charge the data it consumes. Then we need a policy that will make the UPF, not measure the amount of data concerning that particular application. So we need a packet flow descriptor, that will help the UPF to understand what kind of character will describe a particular application.
Session Management Related Policies
- Gating Control: This type of policy is used to block or allows IP packets of certain service. For example, in some countries torrent is blocked, so all the IP packets that correspond to P2P targeting are blocked.
- Quality of Service Control: There might a case where not all the applications are allowed to be treated in the same manner. Some have higher priorities and some have lower priorities. This is defined by the QoS control, where the QoS policies define the quality of a particular authorized service should get.
Policy Flows
Whenever the PCF make decisions, it usually cold PCC rules or policy and charging control rule. But this rule cannot send directly into the network. So the PCC rule is sent by the PCF to SMF and SMF uses it to enforce different kinds of behaviour across the network. It takes the PCC rules and identifies what should be the SDF or service dataflow template that SMF should send to you UPF. What does upf need don’t know? It needs to know how it can differentiate with the different services running on the network example it needs to differentiate between the IP packet from applications like Spotify, an IP packet from YouTube or an IP packet from Internet browsing. On the other hand, gNB needs to know the quality of service profile which helps gNB to put the different quality of service flow in the right radio bearers in the wireless network. When the traffic is available, the UE need guidance on what uplink barrier to use to send the different application traffic so this one governs by the quality of service rules.
PCF Services
i) Npcf_AMPolicyControl
It provides AMF with policy information about the UE or provides access and mobility-related policy information to the UE. In addition, it also provides an access selection policy, which dictates which radio access technology should get priority in a given context. It also has different service operations such as
- Create: creates an association of AMF and PCF for a particular device.
- Update: When that association receives an update it was handled here.
- Notify: It can be used by PCF when there is no request from the AMF. At anytime PCF use this to update policies in AMF.
- Delete: To delete the association corresponding to the device for the AMF.
ii) Npcf_SMPolicyControl
It provides the SMF with policy-related information for corresponding PDU sessions. It also has service operations such as
- Create: To create an association corresponding to a particular PDU session used for PCF registration.
- Notify: If there is any policy change to related PDU sessions, then the SMF will be notified by PCF.
- Update: To update that association or update notify by the PCF when there is no request.
- Delete: To delete the association corresponding to a particular PDU session.
iii) Npcf_PolicyAuthentication
This is used to authorize an application function request and to create policies as requested by the authorized application function for the PDU session for which the application function session is bound.
iv) Npcf_BDTPolicy
To get the background data transfer policies based on the request via NEF from the application function. It updates the background data transfer policies on selection provided by the application function.