To meet diverse, differentiated and strict quality requirements of 5G vertical industry applications, 5G transport network architecture needs to be more flexible and adaptable to services.
5G network slicing allows operators to segment multiple virtual end-to-end networks out of the same 5G infrastructure, namely, network slices. The 5G end-to-end network slices require the collaboration of RAN, core and transport networks. As the carrier that connects wireless and core network slices, the transport network is a major part of end-to-end network slices. Increasingly differentiated slice applications are in urgent need of the slicing technology that can accurately address the needs of differentiated applications.

Slicing Architecture for Transport Network 
ZTE's optimized transport network slice solution based on Flex Algo provides the precise transport network slicing ability, meeting the needs of precise slices, precise slice isolation, and flexible slice deployment for transport networks. Its overall architecture is shown in Fig. 1. The optimized slice solution can create various Flex Algo slices according to the SLA requirements of different users and direct services to Flex-Algo slices.

Precise Slicing Granularity 
ZTE's transport network slice solution provides transmission resources of different granularities such as physical ports, 5 Gbps FlexE subrate, and megabit-level channels.

Resource Granularity Based on FlexE
The FlexE technology decouples physical port bandwidth and service rate by dividing timeslots in IEEE standard rates above 50GE and provides N×5Gbps sub-rates or channelized interfaces. In order to meet the needs of different SLA requirements, a physical link can be divided into multiple FlexE VEIs, or multiple low-rate ports are bound into a FlexE group to flexibly match bandwidth resources needed by the services. 

Megabit-Level Resource Granularity Based on Fine-Grained Timeslots
To secure flexible bandwidth for vertical industry applications, the existing resource granularity can be further divided to Megabit-level granularity based on some 5 Gbps FlexE VEIs or sub-interfaces. Each slice is identified with the network topology and allocated with forwarding queue resources to deploy the targeted QoS scheduling policy and offer precise megabit-level resource granularity.
With small-granule resources, the transport network can be accurately sliced according to service needs. It accurately carries various services while improving network resource utilization.

Precise Slice Isolation
Slice isolation mode varies from service to service. The services requiring high security and isolation usually employ hard isolation slices, that is, the services are exclusive to the underlying resources and the slices have the characteristics of physical isolation. The services with latency and jitter requirements adopt soft slice isolation, that is, the services can share underlying network resources. In ZTE's transport network slice solution, Flex-Algo is completely decoupled from the underlying resources, and can be flexibly mapped with FlexE resources, thus supporting soft and hard isolation slices. The following describes the FlexE sub-port mode.

Hard Isolation Slice
In this scenario, FlexE, as the underlying network resource isolation technology, divides timeslots physically isolated from each other at the physical layer, and configures the affinity attributes of related VEIs and links. The upper Flex-Algo maps to underlying specific FlexE resources through the affinity attributes in FAD to exclusively occupy FlexE sub-port resources, thus enabling hard isolation slices based on tenants or services.

Soft Isolation Slice

In addition to the 1:1 mapping between Flex-Algo and underlying resources, ZTE's transport network slice solution also supports their N:1 mapping relationship, with multiple FAs sharing a FlexE sub-port resource. Based on the unified underlying resources, different FAs can have different metric types and different metric types represent different SLA goals, which can meet the need of transmitting diverse service slices.

 

Flexible Slice Deployment
ZTE's transport network slice solution provides good support for operators to deploy their industry applications and achieve a two-level slicing. First, operators can have a basic resource planning of their infrastructure networks, divide them into multiple isolated FlexE physical resources for ToB, ToC and third-party applications, and build level 1 virtual transport network (vNET) for enhanced lease service product competitiveness, greater value-added profits, and reduced operating costs. Second, customers can choose to further slice based on the networks leased by themselves, set up a new vNET based on the Flex Algo view to suit their own diverse needs or lease the vNET to secondary customers, and to plan and manage their slice networks according to the nature of services being carried. The typical slice deployment scenarios include operator's own ToC service, highly secure self-operated ToB service, and large industries or third-party operators (Fig. 2).

End-to-End Precise Slice Distribution
The transport network is a part of 5G end-to-end network. When deploying end-to-end slice services, it selects proper slice types according to the needs of 5G slice applications to achieve precise slice distribution through the 5G end-to-end network slice management function (NSMF) and the transport network sub-slice management function (NSSMF).
NSMF orchestrates end-to-end slice services in accordance with slice applications, and delivers the orchestration results to professional sub-slice management functions. Among them, the parameters sent to the transport sub-slice contain slice topology information, slice isolation requirements, and slice SLA parameters. The transport NSSMF chooses the appropriate slice type based on this information and creates the corresponding slice services.

Transport Network Slicing Applications
To study the applications of transport network slicing in 5G vertical industries, ZTE has extensively cooperated with many Chinese and overseas operators and vertical industry customers to jointly explore deployment policies and overall solutions for precise and flexible transport network slicing applications.
ZTE and China Telecom have commercialized the FlexE hard slice technology on a large scale in the existing network, and put forward the precise and flexible slicing application solution. The FlexE hard isolation slicing pipeline technology divides the physical network into two hard isolation slice vNets (ToB and ToC) according to different scenarios, which carry high-priority services for 5G industries and services for public users respectively. The ToB slice network is scheduled according to priority queuing (PQ). The solution isolates resources between network slices and makes statistical multiplexing of network resources within slices. It transmits high-priority services accurately and stably while increasing network resource utilization.
ZTE has also studied the applications of 5G smart grid and analyzed the requirements for the transport network in various scenarios, such as power distribution network automation, distributed energy, precise load control, line monitoring, and energy consumption detection. To meet the needs of power grid for safety isolation, latency and bandwidth, multiple slices can be flexibly planned to transport hard isolation exclusive slices and soft isolation shared ones. The production area adopts the end-to-end exclusive hard isolation slice mode. Through independent slice identification and queue resource slice allocation, resource-exclusive small-granule transport slices are created on FlexE hard slices to provide a highly reliable slice channel with zero packet loss.
ZTE will continue to study the applications of 5G precise and flexible slicing in vertical industries, analyze their development trends and communication needs, and promote the deep integration and common development of 5G transport network technologies and vertical industries.