5G is important for IoT, and supports two major applications: ToC mobile broadband applications for individuals and ToB applications for vertical industries. Vertical applications are becoming the focus of the 5G industry chain as they are key to revenue growth for operators and digital upgrade of various industries. Vertical applications also put new requirements on 5G transport networks and challenge the traditional packet transport technologies.

Challenges Facing Traditional Packet Transport Technology

Vertical applications provide services through 5G network slices. ToB slicing services differ greatly from ToC services in terms of the SLA requirements. Based on requirements for slicing resources and value-added functions, the industry proposes a hierarchical service model including premium, exclusive, and preferential services. The premium service requires that the network should provide exclusive physical forwarding bandwidth and high-reliability performance with zero packet loss, low jitter, and deterministic latency. The exclusive service requires dedicated LSPs to support forwarding with high quality service bandwidth and latency guarantee. The preferential service requires shared LSPs to guarantee high-priority services based on QoS. Among them, the premium service is designed to meet the requirements of the URLLC scenarios in the ToB industry. These include industrial Internet, power differential protection and VIP government & enterprise service that require guaranteed bandwidth and also high network reliability (zero packet loss) and low deterministic latency.

In terms of network resources, ToB services put stricter requirements on resource guarantee and security isolation. Dedicated network resources are required to guarantee the high security of important production services. Some vertical industry applications, such as wide area monitoring, control and protection for power grids and base station positioning services, require 5G networks to provide highly accurate time synchronization. Moreover, vertical applications drive the downward shift of MEC services to realize service localization, and the requirements of service function chaining (SFC) drive the cloud-network synergy. The service traffic and flow directions are getting more complicated, which put higher requirements on end-to-end slice programmability and intelligent service provisioning. The service-level real-time monitoring of network performance becomes essential to 5G slice OAM. 

These 5G ToB requirements pose great challenges to the traditional IP/MPLS packet transport technology. The traditional packet forwarding technology is based on the best-effort forwarding mechanism. Although it can provide QoS priority-based scheduling, queue congestion due to service bursts during the scheduling makes it difficult to precisely control the network latency. There are also service resource isolation problems. The caches and queues are shared by multiple flows so that the forwarding resources of ToB slicing services cannot be exclusively occupied.

Precision 5G Transport Solution for Industry Applications
The precision 5G transport solution addresses the above-mentioned challenges and can fundamentally solves vertical service transport problem. Its architecture is shown in Fig. 1.

Small Granularity Slicing

For latency-sensitive vertical services, the transport network needs to introduce the "zero packet loss" network with controllable end-to-end latency and guarantee the isolation of forwarding resources for latency-sensitive services. The FlexE-based small-granularity technology is developed to meet this requirement. It introduces IP hardened pipes to allow small-granularity services to use dedicated bandwidth, and provides the extended Slice+ protocol (draft-ietf-isis-segment-routing-extensions) to create the end-to-end hardened LSP service paths that allow the physical layer isolation of channels. The channel with the minimum granularity of 1 Mbps can flexibly provide the N×1M bandwidth that matches the granularities of latency-sensitive services. Each small-granularity channel has the corresponding physical time slot, and can implement resource guarantee and strict physical isolation in any scenario. The small-granularity channels can support current L2/L3 VPN services. 

SRv6
Targeting the programmable service chain applications brought by a downward shift of 5G MEC, such as cloud POP node transport requirements, SRv6 is considered a technology for cloud-network synergy. SRv6 uses a 128-bit network instruction to define network functions. Each instruction consists of three parts: network node ID, operation code and required parameters. Through the instruction stack, it can control the forwarding and service processing behaviors of network equipment. In this way, it enables service programming in the cloud-network synergy scenario and implements unified orchestration of SFCs based on cloud service and transport network forwarding paths. SRv6 greatly simplifies the network protocols in the cloud-network synergy scenario and provides seamless network orchestration capability. 

High-Precision Intelligent Time Synchronization 
Some applications in the vertical industry require the 5G network to provide high-precision time service. The latest 3GPP R16 standard also specifies the indicators for high-precision time service of the 5G network. The traditional ground time synchronization system of a 1588v2 transport network cannot meet the time synchronization precision and OAM requirements of large-scale networking scenarios. By introducing such technologies as equipment-level high-precision timestamp, single-fiber bidirectional optical module, base station time difference backhaul, time network domain division and AI-based intelligent time network management and control, the high-precision intelligent time synchronization technology solves the pain points of the traditional 1588v2 approach in large-scale network scenarios by improving the time synchronization precision of a single node to 5 ns from 100 ns. It can rapidly locate and isolate the faults in the time network, fully meeting the OAM requirements of 5G ToB applications for high-precision time synchronization in large-scale networking scenarios.

Intelligent E2E Slice OAM 
The number of connections and flow directions of of 5G end-to-end slice services are very complicated. For more efficient slice service provisioning and precise delivery, the intelligent E2E slice OAM is very important. After the slice service provisioning is initiated from an app, the communication service management function (CSMF) will perform user authentication, and the NSMF will perform slice orchestration across the radio access network, the transport network and the core network. The transport network needs to support identification of the slice service, and provide the slice topology resource mapping and service orchestration capability according to the access information of the slice endpoints and the SLA requirements delivered by the NSMF.
Intelligent OAM shortens the slice service provisioning time from days to minutes while ensuring precise service delivery. This fundamentally solves service misconnections caused by human errors and ensures the security of slice services. After the service is provisioned, the transport network can provide in-band OAM based on the service layer. In-band OAM detects the packet loss, latency and jitter on a per-flow basis and provides online SLA monitoring of the slice service. When a fault or congestion occurs, it can restore the real-time forwarding path of the slice service and provide hop-by-hop localization of faults, packet loss and threshold-crossing latency, thus realizing intelligent provisioning, visualization and OAM capabilities of slices services. 

Summary
The 5G ToB vertical industry applications drive the technology evolution for the precision 5G transport network. Evolution directions mentioned above enable the intelligent and precise delivery of ToB vertical slice services in the transport network, precise end-to-end SLA performance guarantee and precise service perception, solving the pain points in combining 5G communication networks with vertical industries. At present, the precision 5G transport technology has been piloted on operator networks. Differentiated services such as premium, exclusive and preferential services can be provided on one physical transport network according to the vertical slicing requirements. This greatly improves the resource utilization and service provisioning capability of the 5G transport network, and helps realize the goal of empowering thousands of industries with 5G.