5G-oriented Optical Transport Network Solution

Contents

Overview

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

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

OTN Key Technologies in 5G Bearer Network

Summary

Appendix

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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.

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.

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5G Development Brings Challenges to Bearer Networks

ITU 5G key capabilities Key capabilities in different scenarios

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

5G wi l l have a wider wire less spec trum 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

Figure 1 5G Application Scenarios and Key Capabilities

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-

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.

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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

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

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 ser vices, and

dynamical ly opt imize net work connec t iv i t y,

reduce the costs of the ent i re net work , and

enhance efficiency.

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 anti-

interference, 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

Impact of 5G Network Architecture Changes on Bearer Networks

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

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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.

F i b e r s a r e m o v e d d o w n w a r d s , a n d m o r e

transmission nodes need to be deployed.

Figure 2 5G Network Architecture Has Impact on Bearer Networks

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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.

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.

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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 point-

to-multipoint transmission and saves fiber resources.

However, this solution needs to solve the problems

including identification and fast forwarding of short-

latency 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

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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.

5G Fronthaul Network Changes and WDM/OTN Bearer Solution

Figure 4 RAN Function Re-Split

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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.

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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 cross-

connected 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.

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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.

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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 multi-

layer 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.

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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 high-

speed transmission, flexible scheduling, and diversity

networking. Large-granularity services are scheduled

on the optical layer, while small- and medium-

granularity 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 20-

dimensional ROADM and large-capacity electrical

cross-connects. Based on the intelligent control

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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,

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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.

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

OTN Key Technologies in 5G Bearer Network

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.

1.Large-Capacity Optical-electrical Hybrid Scheduling

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

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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 short-

latency 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

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DCL4~L7

IPL2 / L3

OpticalL0 / L1 / L2

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

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

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.

Summaryfronthaul 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.

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

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Acronym Full name in English

eMBB Enhanced Mobile Broadband

uRLLCUltra Reliable Low latency

Communication

mMTCMassive Machine Type of

Communication

MIMO Multiple Input Multiple Output

QAM Quadrature Amplitude Modulation

LTE Long Term Evolution

C-RAN

Centralized Processing,

Collaborative Radio, Cloudization,

Clean Radio Access Network

WDM Wavelength Division Multiplexing

OTN Optical Transport Network

PON Passive Optical Network

BBU Base Band Unit

RRU Radio Remote Unit

TDM Time Division Multiplexing

CPRI Common Public Radio Interface

eCPRI Enhanced CPRI

NGFINext Generation Fronthaul

Interface

CU Centralized Unit

Appendix

Acronym Full name in English

DU Distributed Unit

AAU Active Antenna Unit

ODUflex Flexible Optical Data Unit

CO Central Office

vEPC Virtualized Evolved Packet Core

vBNGVirtualized Broadband Network

Gateway

vCDNVirtualized Content Distribution

Network

vCPEVirtualized Customer Premier

Equipment

SDN Software Defined Network

NFV Network Function Virtualization

MEC Mobile Edge Computing

PoP Point of Presence

ROADMReconfigurable Optical Add Drop

Multiplexing

LOS Loss of Signal

LOF Loss of Frame

PLL Phase Locked Loop

BoD Bandwidth on Demand

OVPN Optical Virtual Private Network

SDON Software Defined Optical Network