The present and future of SDH Transmission Systems in Access Networks

With the advancement of optical communication technology, the access network has gradually evolved from a common analog user loop to an optical access network (OAN). On the other hand, due to the maturity and advancement of SDH technology, it has gradually changed from long-distance network to relay network, finally, it is widely used in access networks. 1 ). The transmission network is the platform of all business layers including the support layer, and SDH technology is the soul of this platform. In the access network, SDH technology is widely used between the user end and the local end to meet the flexibility of networking and Real-Time circuit allocation. It provides security assurance for the "Last Mile" with sound environmental protection functions. At present, the access network transmission system still provides TDM Service Transmission for both PSTN and mobile base stations.

From another perspective, since the built-in SDH155 in the access network undertakes the transmission device of the optical fiber access network, the current speed can not meet the needs of the narrowband access network, users need to increase the transmission bandwidth. At the same time, in order to meet a large number of introduced broadband services and broadband access means, it is necessary to improve the transmission rate and efficiency of the access network, and build a new generation of Metro/Access Network Multi-Service transmission platform. Although the access technology used by the access network is diverse, the user needs vary widely, and the network structure is varied, a highly reliable transmission network is always required for bearing. SDH still occupies the dominant position in metro networks with its powerful protection and recovery capabilities and fixed latency performance. Of course, the diversity of network services poses new challenges to the Metro Transmission Network. To avoid multiple overlapping business networks, reduce the investment cost of network equipment, and simplify the deployment and management of network services, the Metro optical transmission network is bound to develop towards multi-service. The new generation of optical access network transmission systems will also develop towards multi-service and intelligent development.

Transmission System in futuristic Metro Access Networks
As the backbone transmission capacity increases, the access capabilities of Metro transmission networks are also diversified. However, IP-based network services are still unpredictable, which requires better adaptive capabilities of the transmission network, which is not only the adaptability of network interfaces or network capacity, the adaptive capability of network connections is also required. In general, low-cost, flexible and fast business access and convergence from the carrier terminal to the user end are the main requirements for the future man access system.

Technically speaking, the access layer requires less bandwidth and needs to provide IP addresses, TDM, and ATM and other integrated services for transmission. Systems Based on SDH and capable of providing IP and ATM transmission and processing include TDM, IP and ATM interfaces, and even IP and ATM Switching modules) it will be the main method to solve the transfer at the access layer, which can provide access, transmission and protection for high-quality leased lines, ATM, IP addresses, and other services on a low-cost point POP service.

Simply put, this method of using SDH to transmit multiple services, such as Ethernet, is to map different network-level services to each time slot of SDH circuit through VC cascade, the SDH network provides a completely transparent transmission channel, which is an integrated whole from the device perspective of the physical layer. This solution can greatly reduce the investment scale, equipment footprint, power consumption, and operation costs of network operators. At the same time, providing multi-service capabilities also enables network operators to quickly deploy network services, increase business revenue, and enhance market competitiveness.

In terms of the network structure, the access layer transmission nodes have a wide distribution and a large number of nodes, which requires low costs and high environmental adaptability. Complex networking is required.

Fiber-optic direct connection usually refers to the use of simple networking technologies such as routers, ATM switches, and Ethernet switches that use exclusive fiber bandwidth, including star trees, loops, and grids, because it is a pure data access device with exclusive bandwidth, a large amount of optical fiber resources are wasted, especially tree-type and grid-type optical fiber resources. As nodes increase, it puts a lot of pressure on operators, unable to efficiently access TDM services with a large number of applications. If E1 circuit simulation is used, on the one hand, the cost is very expensive and the user cannot afford it; on the other hand, the performance is poor, and it cannot meet the network requirements of carriers such as mobile and Unicom. Therefore, this solution is only applicable to the new pure data network.

Therefore, in the new access network, according to the importance of business users, the Integrated Access SDH device is used for ring, chain, and tree networking. Because star Networking requires a large number of optical fibers, the protection capability is poor, we recommend that you select the Ring, ring, and extra points. SDH, PON, APON, and EPON can be used as the forking method.

In general, in addition to basic SDH functions, the new multi-service access transmission system also has multiple service access functions, supporting transparent transmission of data services, it also provides the business aggregation function between point-to-point and point-to-point. It not only provides the ability to optimize data transmission and upgrade, but also provides bandwidth management capabilities for businesses. It also provides the ability to smoothly upgrade various types of business intercommunication.

In terms of Ethernet transmission:
The 10/100 M Ethernet interface and the 155 M/622 m pos interface provide transparent transmission capabilities. Each network element must be at least two GbE downlink or uplink ports.

Layer-2 switching capability between Ethernet nodes and nodes based on VC. It supports VLAN and L3 switching and upgrading.

The Ethernet transmission capability in SDH subnet is no less than 1.8Gbit ∥ s.

With RPR flexible grouping ring) Board upgrade capability, supports two independent and integrated modes of RPR and SDH.

ATM:
Provides transparent transmission capability for ATM Interfaces Based on VC-4 and VC-4-4c.

Provides a smooth upgrade capability for ATM switches below 5 Gbps.

By integrating ZXA10 and E10 products, the next generation of multi-service access system networking 2 ).

Conclusion
So far, there have not been any new technology that can completely replace SDH, and some are only the development and supplement of the existing SDH technology, which proves the powerful vitality of SDH, SDH will continue to develop in man for the following reasons:

China's Circuit Switching Network will continue to develop in about five years;

The development potential of SDH is higher than 40 Gbit/s and lower than 155mbit/s )? SDH channel cascade function and support of multiple data business ing structures, enhance the support of ATM/IP capability, is being supported by the new ITU-T recommendations, it effectively supports multi-service transmission capabilities.

In the future, the core optical transmission network of ultra-large capacity will be monopolized by DWDM. Considering the bandwidth granularity and cost, SDH will be transferred to the network edge, and more SDH access devices will be required for access networks.

SDH is still the transport network technology with the highest reliability and survivability in the near future.

IETF and IEEE 802.17 have released and will soon launch standards, laying a solid foundation for efficient and reliable IP Address Transmission on SDH.

The SDH/SONET-Based Multi-Service Transmission Platform has two development trends:

In addition to interfaces such as TDM E1, SDH provides Ethernet ports, ATM interfaces, and POS interfaces using other bandwidths to provide transmission channels for broadband data devices, uses SDH's 50 ms self-healing capability to provide protection. This solution is a solution that is easier to implement than the original one. It is also the preferred solution for various operators in the early stages of broadband network construction. It is also a widely used solution.

The second solution is MSTP, a data-optimized multi-service transmission platform ). Its advantages are obvious. It can be compatible with TDM services with a large number of applications and meet the increasing data service IP address and ATM requirements; the Performance Simulation Report of SEGAM's dynamic bandwidth adjustment solution shows that the technology is eight times higher than the average bandwidth utilization of the first solution. MSTP adopts the most mature SDH networking and protection technology, but absorbs the traffic control and protection attributes of the ATM and IP, achieving efficient transmission of multiple services. Solutions Using Dynamic Time Slot Allocation and elastic grouping ring are becoming increasingly mature.

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