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Data between UE and Network (or another UE) go through various components on the data path. In most case, the resource allocation and the data path is configured statically or semi-statically. We cannot say all those components are optimized for each individual user or each individual use case (e.g, rush time traffic, regular hour traffic, eMBB, IoT etc). However, in ideal case where you can configure resource allocation and parameters of the components along the data path dynamically (by automation), we may define a set of parameters of all the components on the data path in most optimal way for specific UEs or specific use cases. The specific set of parameters assigned for the UEs or use cases is called a 'Slice' of the network. Network Slice is a logical concept of splitting all the resources along the data path into multiple sets, each of which is optimized for specific UEs or use cases.
I don't think there is any specification which explicitely defines all the details on how to implement these slices. But by the nature of dynamic configuration changes, we would easily guess that all the network elements along the data path should be constructed in such a way to change the configuration by remote control and automation. That's why the concept of network slicing can easily be achievable in 5G where most of the network components are designed for virtualization and the components along the transport (e.g, routers) is designed as SDN (Software Defined Network).
Slicing vs 5QI(QoS) vs DNN(APN)
Since Network Slicing is a concept of configuraing all the component along the end to end communication path, it can do everything that QoS(5QI in case of NR) and DNN(APN). That is, a certain aspect of network slicing and QoS and API overlaps. Due to this, we are often confused with these three concepts.
In short, I would say Network Slicing would be a superset of QoS and APN. That is, Network Slicing can do everything that QoS and APN do, but not in vice versa, meaning that there are certain aspect of Network Slicing that cannot be done by QoS or APN.
Let's first think of Network Slicing and APN. Basically APN is controlling network parameters (or configurations) by switching the traffic to various different gateway on the core network. We can assign different APN(gateway) depending on MNO (Mobile Network Operator) or MVNO (Mobile Virtual Network Operator). In other way, we can assign different APN(gateway) depending on the service types (e.g, internet, mms, sms, voice etc). We can do all of these with Network Slicing as well. As you may notice here, APN has control only over the components after the gateway, but it cannot control over the component before the gateway. For example, APN does not have any control over radio access network, whereas Network Slicing can controll the configurations not only for APN path but also for Radio Access path as well.
Now let's compare Network Slicing with QoS. Difference between Network Slicing and QoS is more subtle since QoS is also controlling the data flow end-to-end to meet a certain SLA(Service Level Agreement). But there are still some of the network slicing feature that cannot be done by QoS. For example, both Network Slicing and QoS can differentiate SLA between different application type (e.g, SLA between VoNR and eMBB), but QoS cannot differentiate the SLA within the same application. For example, QoS cannot differentiate the SLA between one UE (subscriber, also called a tenant) for the same application (e.g, VoNR) whereas Network Slicing can do.
Network Slicing vs Private Network
As you may notice from previous sections, there are many different ways of slicing the network. Let's suppose the Opeartor Y sliced network as illustrated below. In this example, we see a slice (i.e, dedicated network) for BMW group and FIAT group. From the point of BMW group and FIAT, it would be as if they have their own private network. That is, Network Slicing can be used as a way of implementing Private Network in wider sense of meaning. You may not say this cannot be a private network since the Operator Y would use licensed spectrum. I also agree that a keyword for Private Network was 'Unlicensed Spectrum', but in many case the meaning of technical terms varies (expands in most case) and I think the meaning of Private Network also has evolved to encompass various use case like this. I think this type of Private Network would attract attention not only from private company but also from Network Operators. To be hones, I don't think the private network based on Unlicensed Spectrum would be a strong motivation to existing operators, but this type of private network would be attactive to the established network operators as well.
Signaling for Network Slicing
Source : Recreated from MAS5G: Move Around Smartly in 5G (IEEE)
Followings are the IE (Information Elements) involved in the signaling shown above.
Identification of Network Slice
The identification of a Network Slice is indicated by a NAS Information Element called S-NSSAI as structured as below. S-NSSAI is made up of two field SST (Slice/Service Type) and SD (Service Differentiator). SD is an optional field. SST has 8 bit field length implying that it can indicates a total of 255 different slice types.
The SST field may have standardized and non-standardized values. Values 0 to 127 belong to the standardized SST
range. Values 128 to 255 belong to the Operator-specific range.
The SD field has a reserved value "no SD value associated with the SST" defined as hexadecimal FFFFFF. In certain
protocols, the SD field is not included to indicate that no SD value is associated with the SST.
< 23.003 - Figure 28.4.2-1: Structure of S-NSSAI >
The SST standardized in 3GPP as of now are as shown. Only 4 (as of 23.501 version 16.7.0) out of 127 possible types that can be configured by SST field.
< 23.501 - Table 18.104.22.168-1 - Standardised SST values >