4G has only two parts: fronthaul and backhaul. With the reconstruction of the RAN (Radio Access Network) architecture, the 5G transport network has leaded core network downshifting and cloudification, split CU/DU architecture, and the transport network is divided into three parts, with fronthaul between an AAU and the DU, midhaul between the DU and the CU, and backhaul, a connection requirements between a CU and the core network.

On fronthaul networks, there is no requirement for a unified transport solution.

According to different access conditions and scenarios, there are some cab be selected: Direct fiber connection solution, passive WDM solution and active WDM/OTN solution. As for midhaul networks and backhaul networks, the requirements for the bearer network in terms of bandwidth, networking flexibility, and network slicing are basically the same. Therefore, a unified transport solution can be applied. This article mainly introduces the midhaul and backhaul bearer solutions based on OTN technology.

The metro OTN network architecture includes the backbone layer, aggregation layer, and access layer. The metro OTN architecture matches 5G midhaul / backhaul network. The backbone layer / aggregation layer corresponds to the 5G backhaul network. The access layer corresponds to the 5G midhaul / backhaul.

With the introduction of packet switching and processing capabilities such as Ethernet and multiprotocol label switching traffic policing (MPLS-TP), OTN has evolved into packet enhanced OTN to fully satisfy IP-based 5G transport requirements.

The OTN-based 5G midhaul / backhaul transport solution can maximize the powerful and efficient frame processing capabilities of packet enhanced OTN, and implement fast framing, compression, decompression and mapping functions through FPGA (Field Programmable Gate Array), dedicated chips, DSPs (Digital Signal Processors) and other dedicated hardware, as well as functions that are highly sensitive to the air interface MAC/PHY latency in DU transmission connections.

For CUs, on the one hand, the packet-enhanced OTN establishes a connection between CU and DU with ultra-high bandwidth and ultra-low latency, which effectively implements real-time, efficient and reliable PDCP processing, and supports fast signaling access. The WDM capability of the packet-enhanced OTN can implements long-haul transmission to suburban areas, and increase the bandwidth capacity of the transmission link as required.

In order to meet the requirements of flexible networking for midhaul / backhaul, the packet-enhanced OTN must enhance the route forwarding function based on the MPLS-TP technology.

The packet-enhanced OTN+IP RAN solution

In this solution, the packet-enhanced OTN device with enhanced routing and forwarding functions is deployed to form an midhaul network. The intermediate OTN device can be configured to work in ODUk pass-through mode as required to ensure 5G transport requirements for low latency and bandwidth assurance. The backhaul network inherits the existing IP RAN (IP Radio Access Network) transport architecture, as shown in the following figure. The packet-enhanced OTN and IP RAN exchange routing information through the BGP protocol.

To meet the 5G transport requirements for large capacity and network slicing, IP RAN must apply high-speed interface technologies such as 25GE, 50GE, and 100GE, and consider using new interface technologies such as FlexE (Flexible Ethernet) to implement physical isolation and provide better transport quality assurance.

End to end packet-enhanced OTN solution

This solution is implemented by using packet-enhanced OTN device with enhanced routing and forwarding functions, as shown in the figure below.

Compared with the packet-enhanced OTN+IP RAN solution, this solution can avoid the problems of interconnectivity and cross-domain coordination between packet-enhanced OTN and IP RAN. In this way, this solution maximizes the powerful networking capability and end-to-end maintenance and management capability of packet-enhanced OTN.

The OTN-based 5G transport solution is suitable for operators’ requirements for integrated service bearer. It provides low-cost bandwidth and high-quality bearer services for mobile, broadband, and private line architectures. It can simplify the network equipment and help reduce CPAEX and OPEX. At the same time, the large bandwidth, low latency, and ODU slicing capabilities provided by the OTN solution can well match the requirements of 5G transport.