Network Slice is a concept within 3GPP CT and its history can be traced back to R13/R14 with the introduction of static Slicing in LTE networks. It is being said that the real business case of Network Slicing will come with the arrival of 5G, although in Rel15 Stage3 release in Dec, 2017 still the complete definition with use case mapping is missing but it is being said R16 Stage3 coming in Q2 2018 it will be available. 5G network must address the Network Slicing from dynamic slicing point of view which can be provisioned, managed and optimized through Orchestrator in real time for different use cases like Massive IoT, Ultra Reliable and enhanced MBB.
But what is the consistent definition of Network Slice? Because still I find the term means different to different teams and hence in this paper I will quest for a deep dive to come to a common definition and how it can be implemented in a consistent manner. In addition I want to answer one common question should we delay slicing now and wait for 5G Rel16 Stage 3 or we can start with enablement of simple slicing scnerio that can be upgradable to 5G as we move it.
Frankly speaking Slicing is not something only needed in 5G the prior networks do need and somehow support them due to one very reason which is business case like 4G EPC deployed in Oil industry, Corporate banks connectivity etc. However in the 4G era there are some limitations like only support FUP RAN channels or not-shared RAN channels can be used. In a nutshell it means only a static slice can be provisioned mainly define well ahead of time mostly based on APN /IMSI. Since 5G is mostly about verticals and 3GPP CT3 is doing a fantastic job to define details of how a dynamic network slicing will be about. Please refer to 3GPP TR 28.801 just widening up the Options for runtime slice instance selection. IN 5G in addition to DNN (the 5G equivalent of an APN) we can use IMSI + NSSAI comprising up to 8 S-NSSAI for mapping the access session to a slice instance, also MANO will be evolved to support NSSF-Manager for handling all SLA/KPI and management part.
Hence we will try to analyze evolution of network slicing as we evolve future and software defined networks. The detailed understanding of concept requires understanding the whole concept involving the following dimensions.
1. Understanding Slice requirements from TR22.891
- The operator shall be able to create and manage network slices that fulfil required criteria for different market scenarios.
- The operator shall be able to operate different network slices in parallel with isolation that e.g. prevents data communication in one slice to negatively impact services in other slices.
- The 3GPP System shall have the capability to conform to service-specific security assurance requirements in a single network slice, rather than the whole network.
- The 3GPP System shall have the capability to provide a level of isolation between network slices which confines a potential cyber-attack to a single network slice.
- The operator shall be able to authorize third parties to create, manage a network slice configuration (e.g. scale slices) via suitable APIs, within the limits set by the network operator.
- The 3GPP system shall support elasticity of network slice in term of capacity with no impact on the services of this slice or other slices.
- The 3GPP system shall be able to change the slices with minimal impact on the ongoing subscriber’s services served by other slices, i.e. new network slice addition, removal of existing network slice, or update of network slice functions or configuration.
- The 3GPP System shall be able to support E2E (e.g. RAN, CN) resource management for a network slice.
Figure 1 Understanding Slice Requirements in NFV/SDN enabled Future Networks
It seems clear even if the Network slicing standard is not locked the existing NFV/SDN architecture can be used to enable it at least as over lay to provide resource isolation as Tenant level.
2. Understand Network Slicing from NFV/SDN Point of view
NFV/SDN is to enable agile delivery of service using multi-tenant provisioning in Common NFVI, one basic Slicing concept can be understood by provision multi VNF for different use cases like for Massive IoT we can have a light weight C-SGN combining both Control and User Plane while for uRLLC it can mean a distributed VNF’s using CUPS architecture to deliver real experience to the Edge. Even for eMBB it can mean end service can be delivered by avoiding vFW or other NE’s in order to build a big pipe to deliver video and live broadcast use cases. Some new additional function in Rel15/Rel16 can be used to deliver slice from Telco point of view like décor is considered to be an early enabler for the Slicing but obviously it do not come with end to end provision and monitor/management functions which can only be promised in 5G
Figure 2 Understanding Slicing context in NFV/SDN
The Above figure explains a good Reference to understand Network Slicing, for those familiar with NFV/SDN the key change is the addition of Network Slice layer. This layer may be part of NSD /VNFFGH or exist as an independent layer. Since one Service can comprise of multiple slices as needed so this layer is required even if 1:1 relation exist, it is abstraction layer to offer flexibility.
Services ⇒ It is a business service something which will be offered to end customer
Network Slice instance-NSI ⇒ a collection of resources from below layer s that defines a slice
Sub Network instance NSSI ⇒ consider it as a group of related VNF/PNF
Both the NSI and NSSI is delivered as part of NSD. Hence a Network Slice Instance (NSI) may be composed by none, one or more Network Slice Subnet Instance (NSSI), which may be shared by another NSI. Similarly, the NSSI is formed of a set of Network Functions, which can be either VNFs or PNFs.
Figure 3 Network Slice components
3. Network Slice enablement from 5G
The real enablement of Dynamic Network slicing will come with 5G to ensure dedicated services to each customer for their customized use case or segment. The complete advantage of network slicing can only be achieved when whole Telco Network is virtualized including RAN and possibly RF modules also because the UE will request a slice based on specific ID that needs to be identified by RF, RAN to ensure it can be delivered end to end otherwise the whole concept of network slice is like a static overlay based on VPN/VRF as is implemented today. This is the main reason why all Tier1 CSP’s want to accelerate NFV/SDN use cases before 5G to assure the promise of Network slicing can delivered end to end. 5G CT RAN3 also investigating the implications of Network slice as 5G NR will open access to all access including 5G Wi-Fi, it is not well known how in such cases Network slice will be delivered specially with convergence with Wifi Networks but it is agreed in the planetary meeting that this function will be available for slicing. In 5G Networks the UE will keep track of all slices associated with it by connecting to unique AMF and hence one UE can be associated with 8 Unique Slice offering which is more than enough as per business requirements.
From RAN perspective idea of Network slicing involves that the Slice ID can be linked or transferred to correct NF in the Core Network. In the initial phase this slicing can be static but over long term this scnerio from UE till the DN must be automatic and dynamic using NFVO closed loop control methodology.
4. End to End Slice Management in 5G
As we are well aware the key characteristics of Network slicing are two which are to sell common platform to end users and tenants using Naas <Network as a service> and to monitor /manage it. From end user perspective the Slice ID consists of Slice Type and Slice Differentiator to explain further use case with in a slice type <eMBB, IoT, uRLLC>. For some unique enterprise use cases it is also possible to provision unique nonstandard slice values. These values will be recognized by Radio network in 5G and UE will carry it during service use to assure the dedicated resources are delivered end to end. In order to manage the slice end to end each Telco VNF starting from RAN must have a new logical entity names NSSF (Network Slice selection function) to assure mapping of request to correct slice is done correctly and that consistency is delivered end to end. Below you will find how the Network view of Slice will come in NFV. As we can see the Os-Ma Sol5 is the key to integrate Slice management functions in the NFV.
Figure 4 E2E Slice management in 5G
From a resource management viewpoint, NSI can be mapped to an instance of a simple or composite NS or to concatenation of such NS instances. From a resource management viewpoint, different NSIs can use instances of the same type of NS (i.e. they are instantiated from the same NSD) with the same or different deployment flavors. 3GPP SA5 also considering exposure of Slice management to third party using Rest API which means an operator can become virtual and that slices can be provisioned, managed and optimized by 3rd party. This model is key to enable industry verticals in this domain.
In addition to that Transport Network being a multi service network must support slicing between 5G and non 5G Network services. Since all the services are placed in the VN so it is mandatory to isolate the traffic between different VN’s. Further it is expected that the Slice manager will request the configuration of Managed Network Slice Subnet Instances (MNSSIs) to support the different 5G services (e.g., uRLLC, eMBB, etc.). A MNSSI will be supported by a VN. In the fronthaul network only one MNSSI is required since all services are carried between the RRU and DU in a common eCPRI encapsulation.
Figure 5 Slice Diversity in 5G
5. What is the Optimum way for NW slicing
There are many SDO’s recently working on the NW slicing like NGMN, ETSI, NFV, 3GPP, etc. obviously because there is a lot of traction and appeal together to create a new business case to sell new products and solutions to the verticals and industry alike. As industry has witnessed In both pre and post era of Y2K of the industry and business shift from voice/SMS to MBB/Internet Era where CSP’s work as Pipes only the new 5G direction sets the new umbrella about the definition of broadband to include all verticals. Obviously to sell such dream the NW slicing need to cater dedicated slice and its E2E management and fulfillment by the tenant itself. This is the true model of SaaS something Facebook, Dropbox, Google have been doing so successfully in the past.
Figure 6 Network Slicing Logical view
However this is just the one sided view of picture the business case of 5G combined with business model requirements make it a worth of dime to consider possible definitions of a new standard in the ETSI NFV architecture itself.
Figure 7 Network Slicing Model in Hybrid Networks
However, the PNF would be managed outside of MANO, as well as sub-network parts composed of connection of PNF. This would require the Network Slice Life Cycle Management to also interface with a non-virtualized life cycle management & operation environment, as shown on Figure 4, with an open Nsl-PN interface, with PN standing for “Physical Network.”
6. Key Findings from ETSI GR NFV-EVE 012 standard on Network Slicing
Each tenant manages the slices that are operative in its administrative domain by means of its NFVO, logically placed in the tenant domain. Tenants rely on their NFVOs to perform resource scheduling functions in the tenant domain. As these resources may be provided by different Infrastructure Providers, the NFVO should need to orchestrate resources. Across different administrative domains in the infrastructure. Slicing requires the partitioning and assignment of a set of resources that can be used in an isolated, disjunctive or shared manner. A set of such dedicated resources can be called a slice instance. Defining a new network slice is primarily configuring a new set of policies, access control, monitoring/SLA rules, usage/charging consolidation rules and maybe new management/orchestration entity, when network is deployed with a given set of resources. In addition the ability to differentiate network slices through their availability and reliability and the ability for the network operator to define a priority for a network slice in case of scarce resource situations (e.g. disaster recovery) requires a strong coordination between NFV and SDN domain for slice management
In Point#2 I have already explained what it means to deploy a slice in Pure NFV environment so let us enlist Network slice as it apply to SDN below
A network slice in NFV context may belong to multiple Sites a compelling idea especially when VNF are split across different NFVi POPs
Within the context of NFV/SDN the Security is of prime importance for Slice management reason being that the Slice requires a view like a graph involving interconnecting VNF’s. Ideally it means for each slice to have a separate VNF and possibly PNF which is not optimal way hence it is assumed slice management and NFVO capable to ensure separation of data flows for each slice. In order to meet this requirement the VNF redesign may be necessary to incorporate the HMEE <Hardware mediated Execution Enclave> as discussed in ETSI NFV SEC 009.
Figure 8 What are the Slice End Points
A part from above the NSM <Network slice management> is very important as Slice involve NFV, SDN, Cloud and possibly PNF’s also and ability of Os-Ma SOL5 as well as the NFVO to manage hybrid environment is key.
In the nutshell Network slicing should enable proper network control and logical separation in terms of dedicated resources, operations isolation, feature isolation, reserved radio resource, separate policy control, SLA control, security and service reliability control. This is a unique differentiator of NFV/SDN/5G networks and opens new possibility for operators.
In this paper Author tried to explain new possibilities and evolution path for network slicing, it is well known before 5G SA complete standardization the Slicing will not be available end to end at least the dynamic slicing and also the framework of NSM is not locked however there are certain features and functions of slicing that can be exploited using NFV/SDN networks in today’s networks. It is very clear Operator need to start now for Slicing trying to implement use cases of static slicing using NFV/SDN and MANO functions as available today and that to upgrade it as the 5G becomes main stream. I hope audience have like this content and will help them details understand the details about important concept and that it will allow architects to best plan their 2020 Networks.
- 3GPP TR22.891
- 3GPP TS23.501
- 3GPP TR 28.801
- NGMN 160113 Network Slicing v1 0
- NFV-EVE 012v3.1.1 – GR – Network Slicing report
- GSTR –TN5G Transport Network support of IMT 2020/5G