As a new generation of mobile communication standards, 5G becomes a hotspot in the industry. The 5G network provides eMBB (enhanced Mobile Broadband), uRLLC (ultra Reliable & Low Latency Communication) and mMTC (massive Machine Type of Communication) services, and has new requirements of transport networks for bandwidth, capacity, latency and networking flexibility. At present, the main solutions to 5G transport are SPN over OTN, IPRAN over OTN, and packet-enhanced OTN. This paper focuses on the key requirements of 5G transport networks, 5G packet-enhanced OTN transport solution, key technologies and standard progress.
Key requirements for 5G transport networks
Referring to the progress of the 3GPP standard, the functions of the 5G RAN are re-architected into three functional entities: CU, DU and AAU. The transport network is divided into three parts: fronthaul, midhaul and backhaul. 5G has higher requirements of transport networks for bandwidth, ultra-low latency, time synchronization and flexible connection.
5G fronthaul and midhaul networks have high latency requirements. Table 1 is the preliminary evaluation result of the maximum latency value allowed for the fronthaul, midhaul and backhaul networks.
Due to the limitation of the latency, the uRLLC service needs to move down the CU/MEC and integrate CU and DU. Even if the eMBB service adopts the the CU/DU-separated
Ultra-large bandwidth and flexible connection requirements
The typical bandwidth (64T/R antenna, 100MHz spectrum bandwidth) between the AAU and the DU of the 5G fronthaul network is 25GE, which is several times higher than the bandwidth of fronthaul CPRI3~CPRI7 in the 4G era. The Sub 6GHz low-frequency 5G single-S111 base station has an average bandwidth of 3~5GHz, which is increased by more than 10 times compared with the 4G macro station.
The large bandwidth requirement of 5G poses a big challenge to the transport network. The coordination between 5G base stations and the load balancing and multi-homed backup requirements of the core network cloud-based deployment make the traffic more complex and dynamic, requiring the transport network to provide sufficient bandwidth extension and flexible service connection.
Ultra-high-precision time synchronization
The introduction of 5G ultra-short frame, carrier aggregation and COMP multi-point coordination technologies requires time synchronization accuracy to be further improved. China Mobile’s related standard proposals require to increase the precision from 4G ±1.5μs to ±130ns. The time synchronization accuracy assigned to the transport network is about ±100ns, so the time synchronization accuracy of the single-node device is expected to be less than ±5ns.
The core network and the RAN adopt the SDN/NFV-based cloud-based slicing architecture to slice according to application scenarios. Different functional slices have different requirements for bandwidth, latency, network function and reliability. The 5G transport network is part of the 5G end-to-end service path. It must meet the eMBB, uRLLC and mMTC multi-scenario requirements as well as the requirements of the vertical industry and leased services. This requires the transport network to support the service isolation and independent O&M requirements of the 5G slicing network. Different types of services are allocated with different types of transport network slices, and each transport network slice is like an independent physical network.
5G integrated transport solution based on OTN evolution
The OTN technology combines the advantages of optical domain transmission and electrical domain processing. It not only provides end-to-end rigid transparent pipeline connection, flexible packet forwarding soft pipeline and powerful networking capabilities, but also supports LH, large-capacity transport. The perfect OAM mechanism guarantees the quality of service transmission and makes the network easy to maintain and manage. The packet-enhanced OTN can meet the requirements of large bandwidth, low latency, high-precision clock, and high reliability proposed by the 5G transport. On this basis, the midhaul/backhaul SR-MPLS flexible route forwarding function is complemented, which is a cost-effective technology evolution path to achieve 5G efficient transport. Figure 1 is a schematic diagram of the 5G integrated transport solution based on OTN evolution.
The solution matches the OTN down-shifting trend and enables the unified transport of services such as 5G, fixed broadband, cloud, and government & enterprise private lines. In the aggregation equipment room where integrated services are accessed, the wireless centralized equipment (DU or CU+DU) can be deployed centrally. The packet OTN equipment in the aggregation equipment room can aggregate the fronthaul traffic to the node wireless equipment, and supports the route forwarding function to upload the midhaul/backhaul services to the upper-layer transport equipment. For the small centralized site of the access-layer C-RAN, the n×10GE of the CU&DU uplink bandwidth can reach the aggregation room directly through 10G or 25G wavelength. For the DRAN site with sparse services, the access layer adopts the 10G/25G packet ring supporting the point-by-point relay.
Mobile-Optimized OTN (M-OTN) is currently a research hotspot. Recently, China Telecom and ZTE have worked together to successfully promote the standardization of the M-OTN technology in the ITU-T. The M-OTN adopts a single-stage ODUflex over FlexO mapping multiplexing structure to optimize the FEC algorithm by simplifying the OAM overhead such as TCM, and employs the FlexO-FR/SR frame structure to match low-cost optical modules of 25G, 50G, and 100G. ZTE creatively proposes a Cell-based FlexO solution that is highly efficient in service mapping, eliminating slot granularity optimization and computational complexity due to mixed timeslot granularity.
Ultra-high-precision time synchronization technology
The 5G ultra-high-precision time synchronization involves the improvement of time source accuracy and OTN equipment transport mode. Currently, the time source accuracy improvement path is the evolution from single-frequency to dual-frequency, and then evolves to the common-mode common-view differential. The OTN equipment transport increases accuracy in in-band and out-of-band modes. The out-of-band mode transmits PTP through the OTN OSC wavelength. In particular, the single-fiber bidirectional mode can naturally eliminate the asymmetric latency difference, which is a preferred solution. In order to improve the timing accuracy, the ultra-high-precision PHY is adopted. The phase detection and alignment clock devices need to be supported to improve the accuracy of the time stamp.
The in-band solution involves the OSMC bytes in the management overhead AM of the FlexO. The framer supporting the FlexO needs to improve the accuracy of the time stamp and strictly control the latency change range of the optical module due to power-on/-off and plugging/unplugging.
Flexible slicing technology
The packet-enhanced OTN needs to support the slicing of the transport forwarding plane, the control plane, and the management plane. The forwarding plane supports the L0/L1-based (λ, ODUk and VC) hard pipeline slicing, and L2/L3-based (VLAN, LSP, PW and L2/L3 VPN) soft pipeline slicing. Each slice has its own independent logic topology. Different slices have their own control and management planes. Through cooperation with wireless and core network slicing networks, an end-to-end 5G slicing solution is provided to meet the multi-scenario, multi-tenant application requirements of 5G vertical industries.
Progress of 5G integrated transport standard based on OTN evolution
The research on 5G transport based on the OTN technology has always been a hotspot in standard organizations. At the ITU-T SG15 plenary meeting in September 2016, some vendors proposed to initiate the study of wireless signal transport over OTN in the Q11 group, and called on everyone to discuss this topic. In the subsequent Q11 intermediate meeting, many domestic and foreign vendors such as ZTE put forward the need and consideration of 5G signal transport over OTN. In the next ITU-T SG15 plenary session, more and more vendors participated in the research of 5G transport. For the OTN, multiple members suggested to start a standard project based on 5G service transport over OTN .
At the ITU-T SG15 Q11&Q12 joint meeting in October 2017, China Telecom, ZTE and other members proposed the M-OTN technology to solve 5G transport problems. After a period of discussion, at the ITU-T SG15 plenary meeting in February 2018, the M-OTN standard made substantial progress and two M-OTN related projects are approved to initiate: “G.sup.5gotn: Application of OTN To 5G Transport and “G.ctn5g: Characteristics of transport networks to support IMT-2020/5G”.
With deep understanding of 5G communication networks and massive technical accumulation, ZTE has made great progress in the R&D of the packet-enhanced OTN products supporting 5G transport. In the 5G OTN fronthaul transport test organized by the Beijing Research Institute of China Telecom in mid-February, the M721 board supports the client side to simultaneously access three or six 25G eCPRI 5G fronthaul signals and multi-channel CPRI 4G fronthaul signals or other service signals such as GE and 10GE. The line side supports 100Gbps or 200Gbps bandwidth. The device uses the latest M-OTN technology solution to optimize the service mapping structure path and FEC. For key performance indexes such as latency and latency jitter, the M721 support end-to-end 1μs transport latency and nanosecond-level latency jitter.
At present, ZTE is developing, testing and piloting the SR-MPLS function of the packet OTN products of the 5G OTN integrated transport solution. As a leader in the 5G era, ZTE will continue to innovate in 5G transport technologies, solutions and equipment research so as to provide operators with competitive and cost-effective solutions.