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Optical Transport Network Optical Service Unit (OTN OSU) and Its Applications in the Electric Power Industry


Sun Fuyou, President of Huawei's Global Energy Business Unit

Chen Xiaozhou, IEEE P2893 OTN OSU Standard Group Chair, IEEE PLC Standards Committee Voting Member,IEC TC57 wg20 China Member


Smart grid's digital substation is the focus of State Grid Corporation of China (SGCC) in recent 10 years. From the first 220 kV smart substation built 10 years ago to the current goal of 8000 smart substations, the upgrade of optical fiber communication networks has played an important role in infrastructure. SDH supports the digital substation production service, with the characteristics of physical isolation and stable low delay. OTN and other pipe optical communication technologies provide high bandwidth and reliable transmission networks for industrial IoT, video, office, and other digital power services. The next-generation platform, a multi-service transmission platform, integrates the advantages of SDH and OTN, and depicts the network architecture of power development in the next 10 years.

2.Existing Power Communication Network Solutions

2.1 Current Transmission Network Architecture

During the existing power transmission and transformation communication, a transmission network is composed of two planes: SDH is used to transmit production services, and OTN is used on the backbone network to transmit integrated data services. As the main optical communication technology of power dispatching data network, SDH generally adopts the mode of dual SDH networks to ensure the safety of production dispatching services. However, due to the protocol impact, SDH does not have the transmission capacity of higher than 10G network bandwidth. Optical communication technologies such as OTN need to be used to meet the requirements of large bandwidth for the digitalization of power transmission and transformation networks, and comprehensively carry industrial IoT, video, and office services.

Figure 2-1 Substation interconnection transmission network

2.2 Substation Interconnection Transmission Network Problems and Solutions

2.2.1 Requirements for High Bandwidth

Smart substation mainly includes IoT monitoring, video surveillance, and digital office business. These services need a high bandwidth network as the basis, and therefore the 10G bandwidth capability of SDH cannot meet the digital bandwidth requirements of smart substation. OTN technology increases the port rate to 100G or even 800G, becoming the first choice for the backbone construction of smart substation. The high security characteristics of SDH transmission for automatic control services and the high bandwidth and physical isolation characteristics of OTN transmission for industrial IoT have become the preferred combination scheme for the digital construction of power smart grid.

2.2.2 Multi-service Bearing of Smart Substation

The power grid carries a variety of automatic control, IoT equipment monitoring, video, and office services. SDH provides traditional automatic control services in the dispatching data network of substations, including teleprotection, supervisory control and data acquisition (SCADA), EMS, PMU, dispatching telephone, and other services. The wide area optical network interconnection of these services requires the high reliability, low delay, and physical isolation characteristics of SDH to ensure the safety and reliability of service transmission. Then, the smart grid has large bandwidth data services, including IoT, intelligent monitoring, intelligent detection, office, and video call. The SDH network provides narrow-band automatic control private line, while OTN provides high-bandwidth physically isolated private line service as the network foundation of power digitalization.

3.Application of OTN in the Electric Power Industry

3.1 Introduction

OTN is a multi-service integrated optical transmission network, which can flexibly carry 2 Mbit/s to 100 Gbit/s services. 2 Mbit/s to 1 Gbit/s automatic control services of the power dispatching network can use SDH private lines in OTN channels. Power digital services higher than 1 Gbit/s can be transmitted on the OTN platform by MPLS-TP. In addition, OTN service encapsulation technology is used to increase the port bandwidth from 10 Gbit/s to 100 Gbit/s, 200 Gbit/s, or even 400 Gbit/s. At the optical layer, wavelength division multiplexing (WDM) converges multiple OTN channels to further improve the bandwidth and cover more transmission scenarios. In addition, OTN is compatible with IEEE 1588v2 and other time protocols to meet the requirements of time synchronization of IoT in the electric power industry.

3.2 Unified Switching of the OTN Platform

The current OTN communication platform generally supports unified cross-connections on the VC, PKT, and ODU/OSU planes. A group of multi-service OTN devices can be used as SDH, OTN, and PTN devices to support SDH PCM/E1/STM-N, Ethernet, and OTN services. 2M to 1G service transmission adopts the SDH plane, and EoO services provides 1G to 100G hard pipe transmission for Ethernet private lines. MPLS-TP adds flexibility to Ethernet private line network. In addition, the OTN multi-service transmission platform of some equipment manufacturers also provides intelligent OTN (SDH/PKT/OTN) hybrid board. A pair of optical fibers is used to transmit services on multiple planes, as shown in Figure 3-1.

Figure 3-1 Unified multi-service OTN platform

Figure 3-2 shows the multi-service OTN platform defined by the China Communications Standards Association (CCSA).

Figure 3-2 Centralized cross-connections defined in YD/T 2484-2013

3.3 Hard Pipe Isolation Capability of OTN

OTN inherits VC hard pipes from SDH. SDH improves the transmission rate of optical fiber lines through byte interleaving and multiplexing. It uses higher-order VC-4 and lower-order VC-12/VC-3 granularities and occupies fixed bandwidth. Multiple VC granularity bindings carry different Ethernet private line services. SDH switching uses a TDM circuit switching architecture to provide fixed service rates and network delay without congestion of different pipe services.

Figure 3-3 SDH architecture defined by ITU-T G.707

Multi-service OTN additionally inherits the ODUk/ODUflex hard pipe switching of OTN, in which byte-interleaved multiplexing is also used. The frame structure is fixed at 4 rows and 4080 columns, and the frame rate can be adjusted to improve the transmission rate. In terms of mapping and multiplexing, the multiplexed lower-order ODUk signals can be mapped into higher-order ones via byte interleaving (similar to VC), or ODUflex signals can be multiplexed into ODUCn signals (similar to STM-N). The latter approach cuts mapping paths and lowers the delay.

Figure 3-4 OTN multiplexing architecture defined in ITU-T G.709

3.4 Multi-Service OTN Architecture for All-Service Transmission

Multi-service OTN simplifies the architecture into two planes to carry all services in a power grid.

Figure 3-5 Multi-service OTN transmission network

Multi-service OTN combines the SDH planes (A and B) and OTN network, as used in the traditional power transmission network, into two planes, supporting all PCM, PDH, STM-N, and Ethernet services.

• Teleprotection/stability control over E1 private line: E1 electrical and optical interfaces are used. To deliver fixed and low delay, high security, and high reliability, the SDH plane of multi-service OTN is used.

• Dispatching data over Ethernet private line: Considering the required bandwidth (lower than 50 Mbit/s) and sensitivity on security and reliability, EOS hard pipe private lines on the SDH plane are used.

• Integrated data over Ethernet private line: Because the interface bandwidth exceeds 1 Gbit/s, n x 1.25G OSUflex hard pipes on the OTN plane are used. In addition, OTN encapsulation protects Ethernet packets to ensure high security in Ethernet private lines.

• IoT/5G: The MPLS-TP private line or network on the packet switching plane can be used to implement Ethernet bandwidth convergence and statistical multiplexing with high efficiency.

3.5 Power Communication Network Based on OTN OSU Technology

3.5.1 What Promotes the Application of OTN OSU Technology in the Electric Power Industry?

Power substation dispatching data network services, such as remote protection as well as security- and stability-sensitive services, are currently encapsulated with SDH/VC-12. There is no OTN device to directly add or delete these low bandwidth services.

There is an urgent need for the power industry to introduce the production service-oriented small container optical service unit (OSU) into the existing OTN architecture in order to efficiently transmit services at a rate of lower than 1 Gbit/s. The OTN OSU technology effectively carries the existing WDM and MSTP optical transmission networks, making it the best choice for the special WAN technology in the electric power industry. IEEE p2893 standard work group currently defines this new technology. Figure 3-6 shows the layered OTN architecture supporting the OSU technology.

Figure 3-6 OTN architecture that supports the OSU technology

Figure 3-7 OTN OSU network architecture

3.5.2 OTN OSU Efficiently Carries Power Production Services

In the OSU technology newly defined by IEEE p2893 standard work group, small-granularity 2M to 1G OSU containers can be directly multiplexed into higher-order ODUk/OTUk/OTUcn signals for transmission. Compared with the traditional ODUk, OSU is more refined and flexible, independent of the traditional OTN slot structure, realizes high scalability, and adapts to the future evolution of Ethernet-based services.

3.6 Teleprotection Network Solution for Smart Grid

3.6.1 Smart Grid Network Solution

Major power production facilities around the world currently use SDH for communication-layer transmission, as it has many advantages in carrying production services. While the TDM switching architecture has controllable delay and low jitter, TDM hardware with a switching capacity of more than 10G is limited. The full TDM switching scale is doubled and the hardware scale is increased sixfold. As a result, SDH services above 10G have no relevant protocols and cannot evolve to meet the needs of future digital services.

OTN OSU is the small-granularity standard of OTN. At present, OTN is mainly used for the transmission of power backbone services. In 2020, various standards organizations began to output small-granularity technical standards with OTN bandwidth of 2 Mbit/s to lower than 10 Gbit/s. ITU-T began to operate the G.OSU standard, and IEEE began to operate the OTN OSU standard. At present, OSU combines the frame structure and low delay characteristics of SDH networks. We compare the applications of matching teleprotection production services in terms of switching capacity.

Teleprotection for communication networks has a high degree of delay jitter stability. The main influencing factor is the delay jitter caused by the switching architecture of transmission products. We compare the delay caused by SDH and OTN OSU switching architectures and algorithms respectively.

3.6.2 SDH and OTN OSU Switching Algorithm for Teleprotection

SDH is a synchronous switching technology. All teleprotection services 2M are mapped to the SDH line side through the rate of 2.048M. In the SDH switching process, all 2M services have fixed timeslots for transmission, and different timeslots do not affect the mutual delay jitter. As a result, this is the best teleprotection transmission scheme. However, due to the time-division synchronous switching architecture, this scheme requires a three-level cross-connection switching matrix as the basis, consuming a significant amount of hardware resources.

Figure 3-8 Technical architecture of SDH

The traditional OTN uses timeslots to divide the frame structure. It supports up to 80 timeslots with a minimum granularity of 1.25 Gbit/s. This means that the maximum service access number of ODU is 80 and the minimum bandwidth of efficient customer service is 1.25 Gbit/s. OSU adopts a new division method, whereby ODU frames are divided into several payload blocks (PBs), and one OSU occupies one or more PBs.

The OTN OPU frame payload area is divided into 192-byte payload blocks (PBs) from row 1 and column 17. The OPU payload area of three consecutive frames can be divided into 238 x 192-byte PBs. There are PBs across frames at the boundary of adjacent OPU frames, as shown in Figure 3-9. In order to locate the boundary of a 192-byte PB, the PB pointer (PBP) field is defined.

Figure 3-9 PB structure division of OPU frames

OSU adopts an SDH-like TDM cross-matrix algorithm to ensure uniform bit rate. The M-channel OSU service is mapped and multiplexed into PB in each transmission cycle of the OPU by determining the PB location in advance. Based on the mapping and multiplexing scheduling mechanism, an example of OSU service distribution is provided below.

In each transmission cycle, there are p PBs, which can be sequentially numbered as #1... #p. The corresponding PB position indicates I, and I circulates sequentially from 1 to P. The number of PBs allocated to each OSU in each transmission cycle is marked as C, and j represents the PB counter of OSU service. The value #1... #p circulates from 1 to P, and one round of counting is equal to one transmission cycle window. The starting value is Δ, and different OSU services' Δ values can be different. For OSU services with the same C value, different methods are adopted Δ. The initial value can effectively avoid mapping opportunities at the same time, and then reduce the conflict probability of multiple OSU services competing simultaneously for the same PB. M represents the mapping opportunity counter of OSU service.

Assume that a service frame period p = 10, OSU #1 occupies 4 PBs, and each of OSU #2 and OSU #3 occupies 3 PBs.

According to the bandwidth, the three OSUs are sorted to form a set Q = {OSU #1, OSU #2, OSU #3}, and the distribution position of each OSU in the queue in the service layer is calculated in turn.

For OSU #1, P1 = P = 10, C1 = 4. According to the sigma-delta algorithm, the positions of 4 PBs occupied by OSU #1 in the service frame shall meet:

(j × C1) mod P1 < C1, j=1,…,10;

When J = 3,5,8,10, the condition is met, and OSU #1 is carried in the PBs at 3, 5, 8, and 10 positions in the service frame.

For OSU #2, P2 = p-p1 = 10-4 = 6, C2 = 3. According to the sigma-delta algorithm, the positions of 3 PBs occupied by OSU #2 in the service frame shall meet:

(j × C2) mod P2 < C2, j=1,…,6;

When J = 2,4,6, the conditions are met. These values are the natural order of the remaining 6 PBs, and the actual order of the corresponding service layer is 2,6,9.

OSU #2 is carried in the PBs at 2, 6, and 9 positions in the service frame.

For OSU #3, P3 = p-p1-p2 = 10-4-3 = 3, C3 = 3. According to the sigma-delta algorithm, the positions of 3 PBs occupied by OSU #3 in the service frame shall meet:

(j × C3) mod P3 < C3, j=1,…,3;

The conditions are met when J = 1,2,3. These values are the natural order of the remaining three PBs, and the actual order of the corresponding service layer is 1,4,7;

OSU #3 is carried in the PBs at 1, 4, and 6 positions in the service frame.

Figure 3-10 Results of the mapping and multiplexing mechanism

From the preceding results, it can be seen that the overall test delay results are consistent when OSU uses the TDM switching algorithm. This is because the switching principle is similar to that used by SDH. In order to fully meet the delay jitter requirements of teleprotection services, OSU can be used instead of SDH for carrying power production services.

3.6.3 Application of OSU Transmission Teleprotection Devices in State Grid Corporation of China (SGCC)

Teleprotection service requires high real-time performance, which is a typical TDM service. The cross-connect algorithm similar to MPLS-TP cannot guarantee the delay stability and reliability. Therefore, when OSU transmits teleprotection scenario services, the TDM algorithm mode is adopted to ensure the stability of transmission delay.

SGCC has made relevant applications in the teleprotection OSU network transmission. In the actual application of the current network, equipment 1, 2 and 3 are network equipment based on OTN OSU, and the teleprotection equipment passes 2M E1 and C37.94 and Ethernet port access.

According to the actual application environment in the figure above, there are two bandwidth transmission teleprotection services: OSU 2M service, using the 2M OSU -> ODU2 mapping path; OSU 10M service, using the 10M OSU - > ODU2 mapping path. In practical application, 2M OSU carries services with a single-station forwarding delay of 272.5 μs and a single-station pass-through delay of 21 μs.

Figure 3-11 Teleprotection service

According to the above results, the OSU switching algorithm has better performance when tested with an optical port of more than 10 Mbit/s because it is similar to the switching principle used in SDH. The delay jitter requirements of teleprotection services are fully met. In the future, OSU technology can replace SDH technology for carrying power production services. The OSU switching mode can replace SDH to implement service transmission in future industrial scenarios.

4 Conclusion

In the smart grid era, innovative communication technologies are developed to automate power generation, transmission, transformation, and distribution. Multi-service OTN will enable high-reliability, low-delay, and all-service communication in the electric power industry. Compared with IP networks, OTN OSU networks are more suitable for WAN interconnection of power production services and time-sensitive network services in the future.

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