5G-oriented Optical Transport Network Solution - ZTE

Impact of 5G Network Architecture Changes on Bearer Networks. Fronthaul Network Solutions for the ... DU mainly processes physical layer functions and the.

5G-oriented Optical Transport Network Solution

1 Overview In recent years, the evolution of mobile networks into 5G has become the industry focus. 5G will penetrate into almost all areas of our future society.

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Document DEVICE REPORT5G-oriented Optical Transport Network Solution
5G-oriented Optical Transport Network Solution




5G Development Brings Challenges to Bearer Networks


Impact of 5G Network Architecture Changes on Bearer Networks


Fronthaul Network Solutions for the C-RAN Architecture


5G Fronthaul Network Changes and WDM/OTN Bearer Solution


Unified Backhaul of Fixed-Mobile Convergence and OTN Bearer Solution


13 SDN-Based Optical Networks Effectively Support the Slicing and Intelligent Operation of 5G Networks

OTN Key Technologies in 5G Bearer Network







In recent years, the evolution of mobile networks into 5G has become the industry focus. 5G will penetrate into almost all areas of our future society. The construction of the user-centric information ecosystem will provide users with extreme service experience. The ITU defined three major application scenarios for 5G: Enhanced Mobile Broadband (eMBB), Massive Machine Type Communication (mMTC), and Ultra Reliable Low Latency Communication (uRLLC). These scenarios no longer simply emphasize the peak

transmission rate, but consider the eight key capabilities: peak rate, user experience rate, spectrum efficiency, mobility, latency, connection density, network energy efficiency, and traffic density. Different application scenarios have different technical requirements. In general, the 5G technology is quite different from previous wireless communication technologies in the bandwidth, latency, number of connections, high-speed mobility and other aspects. Through 5G, performance indicators can be greatly improved.


ITU 5G key capabilities

Key capabilities in different scenarios

Figure 1 5G Application Scenarios and Key Capabilities

As the application scenarios are becoming gradually clear, standards development is accelerating, and breakthroughs are continuously made in technology research and development. The commercial use of 5G networks is just around the corner. 5G wireless

network construction requires the support and cooperation of bearer networks to meet the requirements of 5G application scenarios and key capabilities, and to continue to be evolved and developed.

5G Development Brings Challenges to Bearer Networks

5G will have a wider wireless spectrum and use massive MIMO, high-order QAM and other technologies to improve bandwidth of air interface. With a high frequency band, the bandwidth for 5G networks can even reach tens of Gbps. Compared with 4G networks, the peak bandwidth and user experience bandwidth of 5G networks is 10 times

higher, and eMBB services including HD video and VR/ AR can be provided more easily. However, 5G requires 10 times higher bandwidth for bearer networks.
In the 5G era, the Tactile Internet, automatic driving and other services will gradually be introduced and popularized. These uRLLC services require an end-to-


end latency shorter than 1 ms. The latency assigned to bearer networks should be even shorter. The existing bearer network equipment and networking modes must be optimized to reduce the latency and meet new service development requirements.
5G networks have the same requirements for frequency synchronization as LTE networks. However, 5G networks have put forward higher requirements for time synchronization. Compared with the +/- 1.5 microseconds required by LTE networks, the time synchronization precision required by 5G networks is improved by more than one order of magnitude. 5G networks must support

high-precision time synchronization.
The network slicing concept is put forward for 5G. To meet the different requirements of eMBB, mMTC, uRLLC and other services for the bandwidth and latency, different network resources should be allocated. This requires that 5G bearer networks provide the network slicing capability to flexibly and dynamically allocate and release the network resources required by different services, and dynamically optimize network connectivity, reduce the costs of the entire network, and enhance efficiency.

Impact of 5G Network Architecture Changes on Bearer Networks

Compared with the 4G network architecture, the 5G network architecture has the following changes:

1.The core network is based on cloud and deployment is virtualized. The control plane and user plane of the core network are separated. The user plane is moved downwards and has evolved from a centralized plane into a decentralized plane. Through the virtualization technology, the physical entities of the core network are separated into multiple virtual network elements, which are deployed based on cloud in the network. In this way, their geographical positions are closer to

terminals and the latency can be reduced.
2.There will be more C-RANs. In the 3G/4G era, C-RANs have showed advantages in overall cost reduction, wireless collaborative antiinterference, energy saving, O&M simplification and other aspects. However, C-RANs were not deployed on a large scale. The C-RAN architecture used in the 5G stage facilitates flexible wireless resource management, allows functions to be deployed flexibly to meet the


requirements of mobile edge computing, supports hardware and software decoupling, and enhances the software capabilities of wireless networks.
3.The base station density is higher. 5G uses a new spectrum. The 3.5 GHz, and 6 GHz+ frequency bands are higher than the existing 3G/4G frequency bands. Theoretically, the coverage range is smaller and more base stations are required. In hot spot areas with high capacity, ultra-dense base stations are used in networks

These changes of the 5G network architecture have also resulted in impact to bearer networks:
The service anchor point of the core network is moved downwards, and the backhaul network is flatter. The C-RAN architecture has resulted in more fronthaul networks, and they must meet the low cost and flexible networking requirements. Fibers are moved downwards, and more transmission nodes need to be deployed.

Figure 2 5G Network Architecture Has Impact on Bearer Networks

Fronthaul Network Solutions for the C-RAN Architecture

A variety of fronthaul network technologies can be used in the C-RAN architecture. Each of them has advantages and disadvantages.

1.Dark fiber This method eliminates the need for transmission equipment between the BBU and RRU, with the lowest latency and simplest deployment. However, this method uses a large number of fiber resources. When base station density is increased in the 5G stage, fiber resources will be insufficient. This solution refers to point-to-point direct connections, it has no network protection and cannot provide high reliability for the uRLLC services.
2.Passive WDM This method uses a passive optical multiplexer/ demultiplexer to multiplex many wavelengths to an optical fiber for transmission. This can save valuable fiber resources. The latency caused by optical component is very small. Passive equipment does not need to be powered on. Its maintenance is simple and the cost is low. However, the RRU and BBU must provide colored optical interfaces, which increase the wireless equipment cost. In a ring network or chain network, due to the accumulated insertion loss of multiple passive WDM components, the optical power budget is insufficient and the transmission distance is

limited. There is no OAM or fault management capability, line protection is not provided in most cases.
3.WDM/OTN This method uses WDM/OTN to achieve the multiplexing and transparent transmission of the fronthaul signals of multiple sites. It can save fiber resources, provides OAM functions such as optical layer and electrical layer performance management and fault detection, provides network protection, and ensures high service reliability. WDM/OTN is an L0/L1 transmission technology, naturally with high bandwidth and short latency features. This technology can achieve short-latency transmission for all services at the same time. The solution does not require wireless equipment to support colored optical interface, reducing the difficulties of wireless equipment deployment. In addition, during the migration of an established network from a non-C-RAN architecture to the C-RAN architecture, the optical interfaces of the wireless equipment do not need to be replaced. The disadvantage is that the equipment cost is relatively higher, and a low cost solution needs to be developed.


4.WDM PON This method uses star networks. The fiber resources deployed on the PON network access layer can be used, and the equipment cost is low. The current access rate can reach 10 Gbps, which is suitable for the access of small cells. The related technologies and standards are developing.
5.Ethernet At present, the industry is also discussing the Ethernet-based fronthaul solution. This method uses packet technologies, and uses the statistical multiplexing feature to achieve traffic convergence

and improve line bandwidth usage. It supports pointto-multipoint transmission and saves fiber resources. However, this solution needs to solve the problems including identification and fast forwarding of shortlatency services and high-precision synchronization, and needs to be compatible with CPRI signal transmission which is based on the TDM technology. The IEEE has set up the 802.1 TSN task group to study the latency-sensitive Ethernet forwarding technology, and set up the 1914 NGFI working group to study CPRI over Ethernet and new Ethernet-based next generation fronthaul interface.

Figure 3 Optional Fronthaul Network Technologies in the C-RAN Architecture

5G Fronthaul Network Changes and WDM/OTN Bearer Solution

5G RAN functions are re-split. The original BBU and RRU are reconstructed as three functional entities: CU, DU, and RRU/AAU. The CU mainly provides the non-real-time wireless high-level protocol processing function such as radio resource management and dual connection, it can use general hardware platform, and be deployed together with mobile edge computing. A DU mainly processes physical layer functions and the real-time HARQ flow through a dedicated equipment platform or a general+dedicated hybrid platform. For large-scale MIMO antennas, some physical layer functions can also be moved downwards to RRU/AAU to significantly reduce the transmission bandwidth

between RRU/AAU and DU and reduce transmission costs. The high-level function division solution (between the CU and DU) focuses on OPTION2 and OPTION3-1, which will be standardized in the near future with bandwidth features close to those of backhaul networks. The industry has not yet reached a consensus on the standardization of the underlying function division solution (between DU and RRU/ AAU). The standardization may be started when the 5G new radio interface protocols are mature and stable enough. At present, there are NGFI, eCPRI and other solutions in the industry. RRU with not too many antenna channels can continue to use CPRI.

Dual connection, seamless switch over, radio resource management. Non-real-time processing. Service oriented, ensure quality of services.

Real-time digital signal processing. Real-time HARQ processing. Radio oriented, ensure efficiency of spectrum.

Part of PHY processing move to RRU/AAU to reduce the BW of fronthaul interface.

Figure 4 RAN Function Re-Split

According to the different positions of the CU, DU, and RRU/AAU, there can be different fronthaul networking modes, see Figure 5. The specific mode is determined by the operator's fiber distribution, room conditions, O&M mode and other conditions.

Figure 5 Fronthaul Networking Modes

If DU and RRU/AAU are deployed on the same site, they are connected directly through dark fibers. If DU are centrally deployed, the connections between DU and RRU/AAU correspond to level-1 fronthaul. To meet the requirement of real-time DU processing for the latency, the level-1 fronthaul distance should be shorter than 10 km. In this case, you can use dark fibers for direct connections, or use WDM/OTN to save fibers and provide protection. The Muxponder of WDM/OTN multiplexes the 10 Gbps or 25 Gbps CPRI or eCPRI signals of multiple RRUs/AAUs into 100/200 Gbps high-speed signals and transfers them

to DU, meeting the high bandwidth transmission requirement. As fiber routes can flexibly establish point-to-point networks, chain networks, and ring networks, the single-fiber bidirectional technology can be used in a point-to-point network to save fibers. DU pooling saves wireless equipment investment while providing the best cooperative gain, see Figure 6. At present, the cost of level-1 fronthaul of the WDM/ OTN equipment is comparatively high. The reduction of the cost is the key to the successful commercial use of this scenario.


Figure 6 WDM/OTN Solution for Level-1 Fronthaul

The connection between the CU and DU correspond to level-2 fronthaul, which is based on a ring network in most cases. The WDM/OTN technology allows wavelengths to pass the intermediate site on the optical layer and achieve one-hop direct access, meeting the high bandwidth and low latency requirements. Optical channel protection can be configured to meet reliable service requirements. As the DU capacities of different transmission sites may be different, different rates can be configured for the wavelength of each transmission site to meet different DU capacity requirements. In addition, each access site can be individually expanded and upgraded without affecting other sites. If OTN integrates the packet function (Packet Enhanced OTN), service convergence and flexible forwarding can be achieved on the CU site, and the services of multiple DU can be converged on the DU site.

With the same E-OTN equipment, the 100 Gbps packet optical ring network solution can be provided. For multiple sites with a small number of DU and light traffic, ODUflex sub-wavelengths can be connected to form a packet ring. Through multi-site service statistics and multiplexing, bandwidth utilization ratio can be improved. For the sites (DU pool) with heavy traffic, ODUflex sub-wavelengths can be crossconnected on the intermediate site and directly access the CU site. Different types of services can use different ODUflex slices for transmission. For example, the eMBB service uses a packet ring network for hop-by-hop forwarding, and the uRLLC service uses L1 for direct access to reduce the latency. ODUflex bandwidth can be flexibly adjusted by step of 1.25 Gbps. The total 100Gbps ring network bandwidth can be flexibly allocated to multiple logical ring networks on different sites.


Figure 7 E-OTN Solution for Level-2 Fronthaul
Unified Backhaul of Fixed-Mobile Convergence and OTN Bearer Solution

When mobile networks are evolving into 5G networks, CO reconstruction is also in progress. Traditional Central Offices are gradually transformed into localized edge DCs. Based on the SDN/NFV technology, the dedicated equipment of traditional NEs are replaced with general hardware for cloud deployment. The user plane of vEPC in the 5G core network will be moved downwards, and deployed together with the vBNG, vCPE, and vCDN of the fixed network in the edge DCs. Through computing and storage resource sharing, the number of equipment rooms and maintenance cost can be significantly reduced.
In addition, the establishment and completion of the operator's integrated service access point

(Point of Presence) achieves the unified access and convergence of mobile services, fixed services, and dedicated line services. With the virtualization of the CU, MEC, OLT, CDN and other network elements, the future PoP will evolve into a mini DC.
Future MAN traffic will be the north-south flows from edge DCs to PoPs, and the east-west flows between edge DCs and between PoPs. The backhaul networks in the 5G stage will also be the DC interconnection networks carrying all types of services. All levels of DCs can be interconnected at high rates with OTN. Optical networks build bandwidth resource pools, configure and adjust bandwidth according to the required traffic between DCs.


Figure 8 Backhaul Networks in the 5G Stage Will Be the DC Interconnection Networks Carrying Fixed Services and Mobile Services.

A 5G backhaul network can be achieved through the collaboration between an IP network and an optical network. IP networks and optical networks are the most basic infrastructure of future bearer networks. The heavy traffic of IP services between routers are directly connected through optical layer channels, reducing the number of intermediate routing hops and the network latency and improving the throughput of routers. The collaboration between IP network and optical network achieves multilayer protection and recovery and enhances service security. With the IP+optical synergy solution, the

flexible service forwarding capabilities of routers and the large-capacity and low-latency transmission capabilities of optical networks can be maximized.
5G backhaul networks can also be achieved based on E-OTN. The packet enhanced OTN can achieve not only service convergence and flexible service forwarding on L2 and L3, but also large-capacity and low-latency service transmission on L0 and L1. Because integrated transmission equipment is used, the network construction and maintenance cost is the lowest.


Figure 9 OTN Solutions for Backhaul Network

The topology of backhaul networks is complex, and OTN node equipment uses optical cross-connect and electrical cross-connect for optical-electrical hybrid scheduling, which is the best way to meet highspeed transmission, flexible scheduling, and diversity networking. Large-granularity services are scheduled on the optical layer, while small- and mediumgranularity services are scheduled on the electrical layer. With optical-electrical hybrid scheduling, the overall power consumption to capacity ratio is the

lowest. Networks can be hierarchically constructed. Ring networks are the main network topology on the convergence layer. The single-wave rate on the line side reaches 100 Gbps or higher, using 4-dimensional mini ROADM and 10T electrical cross-connects. Mesh networks are the main network topology on the core layer. The single-wave rate on the line side can be beyond 100 Gbps, using 9-dimensional to 20dimensional ROADM and large-capacity electrical cross-connects. Based on the intelligent control


plane, end-to-end service deployment, dynamic path calculation, auto adjustment of network resources, and protection and restoration against multiple failures are achieved. This not only meets the

bandwidth requirements of service development, but also ensures the flexibility of service scheduling and network reliability.

Figure 10 E-OTN Node Implements L0/L1/L2/L3 Unified Scheduling

SDN-Based Optical Networks Effectively Support the Slicing and Intelligent Operation of 5G Networks

5G network slicing is implemented from end to end, including wireless access networks, core networks, and bearer networks. The OTN transmission plane can implement slicing not only in hard pipes such as wavelengths, ODU, and VCs but also in packet soft pipes. As a part of the bearer network, the OTN based

on the SDN can configure and adjust bandwidth on demand, use OVPN and other applications. Fast service provisioning can be achieved in cross-domain and cross-vendor large scale network to reduce operating manpower, IP+optical synergy can be implemented to reduce network construction and operating costs,


and interconnection bandwidth between DCs can be automatically scheduled. These have made preparations for the future integration in the network architecture, the support for end-to-end 5G network slicing, and intelligent operation.

Fast service provisioning E2E service management in large scale network

Better tenant experience Less maintenance task Fully utilize network resource

Release work load of router Reduce latency Minimize cost of network

Support network cloudization Dynamic flow volume & direction Automation

Figure 11 Software Defined Optical Networks Effectively Support the Slicing and Intelligent Operation of 5G Networks

OTN Key Technologies in 5G Bearer Network

1.Large-Capacity Optical-electrical Hybrid Scheduling

5G services require high bandwidth and high-speed transmission. In a MAN with complex topology, using the OTN equipment with the optical-electrical hybrid scheduling capability for networking is the ideal way. The ROADM optical cross-connect technology, together with the OTN electrical cross-connect technology, can provide larger cross-connect capacity and more flexible scheduling capability, while reducing system costs, power consumption, and space

occupation. Optical-electrical hybrid cross-connects introduced in the MAN core and convergence layer can achieve service convergence on the electrical layer and service scheduling on the optical layer. Using the optical-electrical hybrid scheduling in mesh network can achieve multi-path access, reduce the number of network layers and achieve flatten network, reduce the service forwarding latency, and improves network security.


2.Low-Latency Transmission and Forwarding The latency introduced by OTN equipment is much lower than that of other technologies, but 5G fronthaul networks have very strict requirements for the latency. The total latency of fibers and transmission equipment in the level-1 fronthaul must be shorter than 50 us. For the level-2 fronthaul and backhaul , the shorter the latency the better. The latency introduced by OTN equipment need to be reduced from tens of microseconds to less than than 10 us. ZTE optimizes the internal mapping and multiplexing processing, forwarding mechanism, interfaces, and other aspects of OTN equipment. With the measures such as the reduction of cache time, automatic adjustment of cache depth, changing serial processing to parallel processing, increasing the internal processing clock frequency, optimization of FEC processing and optical modules, the latency of OTN equipment introduction can be reduced to the microseconds level, to better support new types of services
3.OTN Lite Standard for Fronthaul For 5G fronthaul, the industry is also studying the new lightweight OTN standard to reduce equipment costs, reduce the latency, and achieve flexible bandwidth configuration. For example, the OTN frame structure can be optimized, n*25 Gbps interfaces may be used on the line side, and low cost optical components can be introduced. The error detection and correction mechanism is changed so that the cache time can be reduced.

Fronthaul networks are simple topology in most cases, so OTN overhead can be simplified to reduce equipment processing steps. The innovative frame structure should be compatible with CPRI used in 3G/4G fronthaul, eCPRI and NGFI in 5G fronthaul, and small cell backhaul.
4.High-Precision Time Synchronization To meet the high-precision time synchronization requirements of 5G, ZTE's OTN equipment adopts 1588V2 specification, and implements phase detection and synchronization on the basis of frequency synchronization optimization. In addition, timestamp accuracy is improved by modifying the triggering mechanism. Time source selection and time synchronization algorithm are optimized. Single-fiber bidirectional transmission is used to eliminate latency asymmetry. Through the comprehensive use of these technologies, time synchronization accuracy is greatly improved.
5.Lossless and Low-Latency Protection Switching Traditional protection switching is triggered by LOS, LOF, and error over-threshold. When protection switching occurs, the data stream is interrupted, the switching time is short (< 50 ms), and most services are not affected. However, some high-reliability services may be affected in the future 5G. ZTE is studying the lossless and shortlatency protection switching mechanism, the error rate after the correction of the data blocks is used as the reference to select the optimal data block. The data stream is not interrupted when protection switching


occurs. This mechanism is suitable for the mission-critical service scenarios in the future.
6.Software-Defined Optical Network (SDON) The programmable features of optical networks are fully used to achieve SDN-based optical networks. ZTE's

SDON research and development focus on efficient and intelligent routing computing capabilities, open northbound and southbound interfaces, cross-layer and cross-domain collaboration, integration of management and control, secure and scalable controller software platforms and hardware platforms.


5G can bring more diverse services and better business experience to people's work and life. 5G networks need to be based on bearer networks, and have put forward higher requirements for bearer networks. As a basic bearer technology, OTN provides high bandwidth, short latency, flexible slicing, high reliability, open and coordination capabilities. It is suitable for mobile

fronthaul and backhaul in the new 5G network architecture, it can also support the development of the operator's fixed network services and other services, meeting the continuous evolution of future networks. The combination of optical networks and wireless networks will create a ultrafast and extreme Internet of Everything.

L2 / L3
L0 / L1 / L2

Figure 12 Overall Architecture of the OTN Solution for 5G Bearer Network


Acronym eMBB uRLLC

Full name in English
Enhanced Mobile Broadband Ultra Reliable Low latency Communication Massive Machine Type of Communication Multiple Input Multiple Output
Quadrature Amplitude Modulation
Long Term Evolution


Centralized Processing, Collaborative Radio, Cloudization, Clean Radio Access Network


Wavelength Division Multiplexing Optical Transport Network Passive Optical Network Base Band Unit Radio Remote Unit Time Division Multiplexing Common Public Radio Interface Enhanced CPRI Next Generation Fronthaul Interface Centralized Unit

Acronym DU AAU ODUflex CO vEPC vBNG

Full name in English Distributed Unit Active Antenna Unit Flexible Optical Data Unit Central Office Virtualized Evolved Packet Core Virtualized Broadband Network Gateway Virtualized Content Distribution Network Virtualized Customer Premier Equipment Software Defined Network Network Function Virtualization Mobile Edge Computing Point of Presence Reconfigurable Optical Add Drop Multiplexing Loss of Signal Loss of Frame Phase Locked Loop Bandwidth on Demand Optical Virtual Private Network Software Defined Optical Network


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