精准5G承载网：未来移动网络的基石（英文版）.pdf

Precision 5G Transport TheFoundation of Future Mobile Network Published: February 2021 Precision 5G Transport The Foundation of Future Mobile Network February 2021 1 Contents1. Executive Summary.31.1. Key messages.32. Current Perspective of 5G Transport.42.1. 5G Transport Market State of Play. 42.2. Status of Standard Development. 42.3. Technologies Used and Projected.52.3.1. Fronthaul and Backhaul.52.3.2. L0-L1 Choices.53. 5G Transport Key Challenges.6 3.1. Growing Capacity Demand. 63.2. Stringent Requirements for Latency, Timing, Synchronization, Determinism.63.3. Network Criticality Increasing.73.4. Integration into the Current Networking Environment.83.5. Automation and Onevertheless, the developments in the marketplace have shown that: Current 5G networks have not generated massive demand for new transportnetwork capabilities this will however dramatically change with standalone 5G deployments. The list of quantitative and qualitative differences between legacy and future-state 5G transport is relatively long, and the complexity of new networks willbe, in most cases, significantly higher compared to legacy mobile transport. 5G mobile transport will need to set new standards in capacity, scalability, andprecision of service delivery, satisfying stringent timing and synchronizationrequirements, and tight system-wide latency budget. Additionally, it will needto be very versatile in terms of protocol support, and likely utilize a wider set ofconnectivity solutions than was the case with previous generations of mobiletechnology. The new transport network will need to be built relatively quickly, to supportmassive increase of subscriptions and new use cases that will underpindigitalization of industry verticals and society functions. Precision 5G Transport The Foundation of Future Mobile Network February 2021 4 2. Current Perspective of 5G Transport2.1. 5G Transport Market State of PlayIn planning, development and deployment of mobile networks, transport needs primarilystem from the capacity of deployed radio access. With that in mind, the current situation in5G transport corresponds with the state of the 5G radio access deployments: Operators are at present mostly using 5G as the carrier of high-speed data,using the non-standalone (NSA) mode of operation, utilizing 5G radio accessin mid-band spectrum, in conjunction with legacy (4G) transport and corenetworks. Additionally, these NSA deployments geographic footprint usuallycorresponds with existing 4G network deployments. The above characteristics of NSA deployments usually mean that thetransport needs of such networks scale mostly in capacity required per link. Consequently, mobile network operators therefore primarily choose toaugment capacity of transport links in accordance with the pace of 5Gdeployment, using legacy technologies at their disposal.However, the NSA deployments will likely be a relatively short and transitional phase of 5Gnetworks development. A wide industry consensus is that great majority of networks will,over time, transform into standalone (SA) 5G networks. This is because only an end-to-end,SA 5G network will be able to satisfy the capacity, availability, latency, and determinismneeds of advanced mobile connectivity use cases that are expected to be the primaryrevenue drivers for mobile operators in the future. With that in mind, operators and vendorsare at present evaluating and developing several types of solutions designed to contributeto SA 5G architecture and provide transport fit for future advanced use cases.2.2. Status of Standard Development The complexity of the future 5G radio landscape translates into complexity for 5G transportas well. Several standards bodies, or trade organizations, like CPRI Consortium, 3GPP,ORAN Alliance, IEEE, or TIP have already defined, or are working on defining. elements ofthe 5G transport. Other organizations, like ITU for example, are contributing standardsrelated to system-wide functions, like operations and management and network slicing.Finally, 5G transport also can use standards not specifically developed for this specificpurpose, but very useful for enabling some demanding 5G use cases, like FlexE(developed by OIF), and IETF-defined segment routing.Most of these technologies are standardized already, with the notable exception of xRANinterface promoted by ORAN Alliance. Precision 5G Transport The Foundation of Future Mobile Network February 2021 5 2.3. Technologies Used and ProjectedAs a rule of thumb, mobile transport network complexity grows with each new generation ofmobile radio technology. 5G is no exception to the rule quite the contrary, because thenumber of options for distributing physical and logical elements significantly increased in 5G,compared to previous mobile technology generations.2.3.1. Fronthaul and Backhaul5G Reference architecture divides transport domain into three subdomains, which may allbe used within one network. Virtually all 5G networks will need backhaul; the use offronthaul and midhaul depends on the chosen network architecture, namely on the level ofnetwork distribution used. As a rule of thumb, the more geographically distributed the radionetwork, the more complex transport network that serves it will become. SOURCE: ITU-T, 2018Figure 1: 5G Transport Reference Network3GPP standardized F1 interface, deployed over IP/Ethernet links, seems to be the mostprobable candidate for the midhaul standard interface; while backhaul will most likelyremain the domain of L3-capable, routed IP connectivity.2.3.2. L0-L1 ChoicesThe complexity in architecture and protocol selection translates into increasing complexityin physical and connection layer technology choices. Most vendors today offer a blend ofWDM and gray optical interfaces in their mobile transport portfolio; smaller number ofoperators offers PON (passive optical networking) based solutions in service of mobiletransport; proprietary solutions, such as point-to-multipoint WDM have also been proposed. In parallel with wired solutions, several microwave options stay in play, although in mature5G networks optical fiber will most likely be used as the preferred medium. Precision 5G Transport The Foundation of Future Mobile Network February 2021 6 3. 5G Transport Key Challenges3.1. Growing Capacity DemandTransport bandwidth requirements vary with the radio channel bandwidth; for example,fronthaul will require single-digit Gbps links for 10-20 MHz channel bandwidth, to tens ofGbps for 200 MHz of spectrum used, with future fronthaul links of up to 200 Gbpsconceivable for high-load scenarios with millimeter wave cells in dense urban areas.Notwithstanding the much higher efficiency of 5G fronthaul interfaces compared to CPRI,these requirements will mean a quantum leap of capacity required throughout the network,all the way to the metro and in some cases long-haul transport. The impact will, however,not be linear, due to statistical multiplexing gains in parts of the network where L3functionality exists.3.2. Stringent Requirements for Latency, Timing, Synchronization,Determinism 5G use cases especially the ones related to real-time IoT applications require very low-latency performance from the transport network, both in 5G-specific transport domains(fronthaul, midhaul, backhaul) and the transport network overall. 5G also introduces verystringent time synchronization standards into the transport network. Network-widesynchronization precision of +-1.5 s maximum time error has been adopted in ITU-Trecommendations and 3GPP 5G standards. In addition to time synchronization, frequencyand phase synchronization also need to be considered. Additionally, inter-band CA requires+/-130ns accuracy, while local area 5G high-precision positioning needs +/-10nssynchronization accuracy.Table 1: 5G Transport Latency RequirementsAt F1 interface 1.510 msecAt Fx interface 100, 125, 250 and 500 sec (a few hundred sec)Fronthaul 100 secUE-CU (eMBB) 4ms UE-CU (uRLLC) 0.5msSOURCE: ITU-T, 2018As is evident in the latency requirements, the importance of precision grows with moresophisticated use cases the ones that are supposed to be the biggest new revenuegenerators for future 5G networks. This set of requirements is also one of the mostimportant factors driving the redesign and innovation of mobile transport network elements,as legacy network elements cannot ensure adherence to these stringent qualityrequirements. Precision 5G Transport The Foundation of Future Mobile Network February 2021 7 Additional to these requirements is the capability of networks to provide deterministiccommunications meaning very little to no fluctuation in latency and synchronization (inaddition to strictly guaranteed bandwidth and superior availability and resilience). Thisnetwork characteristic may be vital for some existing and future use cases, most importantlyin energy utility sector, and industrial M2M use cases.3.3. Network Criticality IncreasingOne of the most important differentiating characteristics of 5G networks is their role as aplatform for vertical use cases, time-sensitive use cases, and critical communications.Whether these use cases belong to the category of ultra-reliable low latencycommunications (uRLLC) by their technological characteristics or not, the role of thesecommunication services in vertical and society will inevitably be increasingly critical. SOURCE: NGMN Figure 2: 5G Use Cases ScenariosDue to this increased criticality, operators deploying 5G transport need to focus on networkQoS guarantees, QoS differentiation, traffic isolation, traffic management, service resilienceand service reliability mechanisms much more than was the case with 4G. Precision 5G Transport The Foundation of Future Mobile Network February 2021 8 3.4. Integration into the Current Networking EnvironmentOperators deploying 5G already understand the complexities of deploying 5G NSAnetworks in conjunction with their existing 4G infrastructures; 5G SA deployments, andassociated new 5G transport will also need to be carefully integrated into the currentnetwork environments. This goes both for mobile network environments, but, due tonetwork convergence trends and greater capillarity of 5G compared to previous networkgenerations, increasingly for fixed access networks.3.5. Automation and O this requirement can be served through strict traffic isolation,achieved through hard slicing of 5G transport. Network Multi-Tenancy: Network slicing is also required for makingsupporting multiple clients and service types on the transport network fromsubsuming legacy 4G mobile broadband onto the 5G transport, to enablingprivate mobile network users who carry traffic over operators 5G network tomonitor and co-manage their network slice.Soft slicing, separating different network services and/or tenants traffic into logical slices, thus becomes a necessity for any 5G transport network. To enable soft slicing, 5G transport Precision 5G Transport The Foundation of Future Mobile Network February 2021 10 network elements must support segment routing (SR), coupled with MPLS and/or IPv6.However, use cases and clients with increased requirements for security and trafficisolation will require the use of hard slicing, where traffic in different slices is carried viaphysically separate interfaces. Hard slicing is technically more demanding, requiringnetwork elements to support additional technologies, like FlexE.4.4. Interface Support5G transport networks will mostly be built supporting L2 and L3 networking protocols, suchas Ethernet, MPLS, SR, and IPv6. However, in the fronthaul several specific protocols willbe used, mostly deployed over basic Ethernet connectivity. CPRI (for legacy transport andshort fronthaul connections) and eCPRI (for C-RAN) support is currently considered to bethe norm. In the future, however, it is conceivable that open interfaces, such as xRANpromoted by ORAN Alliance, will gain prominence.4.5. Timing and Synchronization Stringent timing and synchronization requirements are a necessity for precision 5Gtransport. Moreover, due to increased complexity of 5G transport deployments compared toprevious mobile technology generations, operators will need more robust and redundanttiming sources going forward.Most of todays network elements use GNSS (global navigation satellite system) clockinformation as a synchronization source; the use of these systems will continue, but willincreasingly include multi-band capability, to improve accuracy and resiliency of thesesystems. Additionally, ITU-T has defined a new set of enhanced network clocks (enhancedprimary reference clocks (ePRCs) and enhanced primary reference time clocks (ePRTCs),designed to enable greater timing accuracy with PTP and SyncE services. Combination ofGNSS receivers with network-based clocks will likely be necessary for most 5G transportnetworks, for resilience, accuracy, and support of indoor deployments.4.6. Platform and Technologies Choice Operator choice of platforms and technologies will be the result of several factors: Chosen radio network topology: Operators mostly deploying macro basestations in D-RAN mid-band or low-band deployments will primarily need toinvest into backhaul and put most emphasis on sophisticated backhaul routingplatforms; those operating highly centralized, small cell networks operating inmillimeter wave band will need to pay special attention to fronthaul andmidhaul. However, these clearly delineated use cases will be exceedinglyrare most likely, the operators will need to operate heterogeneous, diverse Precision 5G Transport The Foundation of Future Mobile Network February 2021 11 5G transport, consisting of several different platforms and transporttechnologies. Use Case Support: The most basic requirement for 5G mobile transport willbe the step change in capacity required. This is already a requirement forindustrialization and mass adoption of the first eMBB Operators aiming tosupport most sophisticated uRLLC and MMTC applications will need to investinto superior timing, synchronization, in addition to capacity. SOURCE: ITU-T, 2018 Figure 3: 5G Use Cases Requirements Business Case: Operators choice of platforms can and will depend on thebusiness case they choose for their 5G deployments as well. For example,operators can choose whether to deploy L2 or L3 based mobile transportarchitectures, build standalone 5G transport or share transport resourcesbetween 4G, 5G, or fixed access services. Similar can be said of hard slicing,which will come into play for specific use cases and specific operator andclient requirements. Precision 5G Transport The Foundation of Future Mobile Network February 2021 12 5. Future Outlook5.1. Projected 5G market developmentGlobalData market forecasts project fast growth for 5G subscriptions globally,encompassing consumer and business users. This projected growth will require relativelyquick scaling of 5G transport, in line with increasing 5G mobile data traffic as the domin