DPT: Data transmission technology of optical Network

With the rapid growth of Internet users and web numbers, the Internet's demand for bandwidth has increased dramatically. According to statistics, every 6-9 months, the main ISP's Internet backbone link bandwidth will increase by one times, link rate will increase from oc-3/stm-1 to oc-48/stm-16 or oc-192/stm-64 or even higher. It is predicted that the data will be the most important information flow on the future network, it will bring the most profitable business opportunities, and now the Internet technology because of the small bandwidth, in many applications has shown its limitations, so the network provider to turn their attention to fiber-optic internet technology to meet their customers and business development needs.

SONET/SDH and the development of WDM technology

To ensure interoperability between different systems and the reliability of the entire optical network, the industry has introduced new technical standards: Synchronous optical Networks (SONET) and synchronous digital sequences (SDH). These standards define interoperability between systems and make it easier to convert electronic signals to fiber-optic signals. In addition, it makes the development of the annular structure possible, and the annular structure can provide the necessary redundancy to ensure the reliability of the system.

However, in recent years, the TDM (SONET/SDH) line rate has been growing slowly, it is expected to be unable to meet the demand of data traffic growth in a few years, and wavelength division multiplexing (WDM) is the newest new technology, which can increase the capacity of optical fiber. WDM systems use different wavelengths (near 1550nm) to host multiple access information, each with a speed of up to 2.5Gbps or 10Gbps. The first generation WDM systems support 4 to 16 wavelength, each wavelength path has a speed of 2.5Gbps, and second-generation WDM systems now support 32 to 40 wavelengths and are expected to reach 100 wavelengths. At present, there is a WDM experimental demonstration system which can support 1Tbps capacity (100 10Gbps paths). In addition, the use of WDM technology, reduce the data network SONET/SDH equipment, can greatly reduce the cost of network construction.

Second, IP over optical

Optical interconnection Network (optical internetworking) based on WDM is also called IP optimized interconnection network or IP over optical network. It is the data network, its bottom layer uses the optical transport network as the physical transmission network. In the data network, its components are mainly ATM switches, routers and so on, in the optical transmission network, its components are mainly WDM terminals, optical amplifiers and optical fiber. Optical interconnection networks are not dedicated to the study of the problems within the data network or optical network, but are committed to the interoperability or compatibility between data networks and optical networks.

In order to promote IP over optical technology and network development, by At&t, Bellcore, Ciena, Cisco, Hewlett-Packard, Qwest, Sprint and WorldCom and other companies to launch the establishment of the Optical Internet forum (optical internetworking Forum). Mr. Andrew Greenfield of Cisco served as chairman of OIF.

OIF aims to accelerate the development of optical interconnection network technology and promote the standardization process of optical interconnection network technology, so OIF developed a series of optical interconnection network should have the function and the technical level should be achieved. Optical interconnection networks, like TCP/IP, adopt layered models, including data network layer, optical network layer and layer adaptation and management functions. The data network layer provides data processing and transmission, the optical Network layer is responsible for providing the transmission channel, and the layer adaptation and management function is used to adapt the data network and the optical network, while ensuring the independent of the data network and the optical network.

Inter-layer management function between data network and optical Network layer is an important content of optical interconnection network, inter-layer management function is to exchange state and configuration information between data network layer and optical network layer, and to manage business and path through control interface.

OIF that, inter-tier management should have the following main functions:

(1) Protection and restoration;

(2) fault management;

(3) performance management;

(4) configuration management;

(5) connection management;

(6) Session management, etc.;

Iii. the realization of DPT technology and its great advantages

Based on the problems and requirements of IP over optical proposed by the optical Internet forum, Cisco has studied the IP over optical technology deeply, and developed the dynamic Packet transmission technology (dynamical Packet transport), Referred to as DPT, used directly in optical fiber on the efficient and reliable transmission of IP packets.

DPT is one of the most advanced optical transmission technologies for IP packet optimization, which solves the problem of IP over optical and has great superiority.

1. Unprecedented bandwidth for DPT ring

The DPT adopts the annular fiber structure, and the DPT ring is composed of two fiber rings which are transmitted in different directions respectively. In the SDH ring, you can connect up to 16 routers only, while in the DPT ring, you can connect up to 256 gigabit high-speed routers.

The physical interface rate of the DPT can be from 155Mbps, 622Mbps, 2.4Gbps to 10Gbps or even higher, while in a gigabit Ethernet, its interface rate is the maximum of 1Gbps. In IP over SDH fiber ring, because of the need to reserve bandwidth for fiber ring protection switch, IP over SDH only half of the bandwidth used to transfer IP data packet, bandwidth utilization is very low. In DPT, because all the bandwidth in the two-fiber ring can be used simultaneously to transmit IP data grouping and control groupings, DPT can maximize the utilization of fiber bandwidth.

2. DPT a high degree of flexibility and incredible compatibility

A key problem of IP over optical is to solve the problem of adaptation between IP packet layer and optical path layer, DPT uses the frame format of SDH and adds a new definition of the second layer (MAC layer) protocol, which is the partial reuse Protocol (SRP), as the adaptation between IP packet layer and optical path layer. The reason for using SDH frame format is to take into account the need for interconnection with the world's large-scale construction of SDH networks. The purpose of using the SRP protocol is to identify each IP packet in the cascaded net load of SDH and provide bandwidth reuse/reuse and other control functions.

It is precisely because DPT uses SDH in frame format that DPT can operate on all current fibre-optic transport infrastructures, including:

* Bare Optical fiber (Dark fiber);

* Wavelength Division multiplexing Equipment (DWDM);

* Sdh/sonet point-to-point path or self-healing loop.

DPT can also be transmitted on a facility that blends the three optical transmission pathways, for example, in a DPT ring where several nodes are connected to the bare fiber, while several other nodes are connected to the SDH/DWDM device.

This high degree of flexibility in DPT technology makes it applicable to the situation of a variety of operators. For example, for a new CATV ISP that wants to provide broadband IP services to users, it can directly use the DPT technology directly on the bare fiber they have built, that is, the router with the DPT interface runs directly on the bare fiber, which saves the cost of purchasing SDH equipment, At the same time can provide users with QoS-assured IP services, and because of DPT itself self-healing function, so that DPT network has a very fast protection switching time, so that the slightest impact on the provision of business to users, especially in the banking, public security, government and other key departments to provide business.

For the operators who have built SDH infrastructure on a large scale, DPT technology provides them with a very economical network evolution scheme. For example, in the early stage of IP network construction, due to the small number of users, IP traffic is also less, so at this time can use the existing SDH equipment spare capacity to form a DPT ring. When the user and the volume of business development to a certain scale, you can introduce DWDM or new bare fiber, and use DPT technology on it.

DPT can be run on multimode fibers, or on single-mode fibers. For example, for connections within a business access point (PoP), multimode fibers can be used to connect, and for a metropolitan or wide area network, a medium or long distance single-mode fiber can be used to connect.

DPT is based on more advanced WDM technology, but it can be compatible with today's widely used TDM based SONET/SDH, while you can connect the network directly to DPT without SONET/SDH. This wide range of compatibility provides manufacturers with ample choice and the possibility of scaling up, and is a good scalability, compatibility and redundancy of the optical Interconnection Network solution. 3. Bandwidth multiplexing Characteristics of DPT

In addition to the incredible compatibility mentioned above, the greatest benefit of DPT is that it utilizes the bandwidth of the fiber and the Fast Protection switching function (within 50ms). DPT uses the following technology to ensure its efficient use of bandwidth characteristics.

* Partial reuse (SRP) technology

In the previous data ring network technology, such as FDDI or token ring, the use of the source deletion technology. The obvious disadvantage is that when IP packets reach the destination, a portion of the bandwidth from the destination to the source will be consumed by an already insignificant IP packet, resulting in lower bandwidth utilization. The DPT SRP technology overcomes the defect of the source deletion technology, which stipulates that the destination will remove the IP packet from the DPT ring immediately after receiving the IP packet, so that no part of the bandwidth is wasted and its bandwidth utilization is increased by one times.

* DPT uses two rings at the same time

The DPT ring is a two-way fiber ring that can be used to transmit data simultaneously using two fiber loops. This greatly improves the situation of the SDH ring. In SDH, the corresponding bandwidth needs to be reserved for switching protection, while DPT can use all the bandwidth to transmit IP data grouping and control groupings.

* Statistical multiplexing

Unlike TDM loops such as SDH, DPT loops do not assign a specific slot or bandwidth to a node. DPT is similar to ATM, each node in the ring uses the bandwidth of DPT ring, so the utilization of bandwidth will be greatly improved.

4. DPT powerful protection switching and business recovery capabilities

The DPT Loop utilizes a complete set of technologies called Intelligent Protection Switching (IPS) to provide active link performance monitoring, rapid self-healing, and IP service recovery when a loop node or fiber failure occurs. IPs not only provides functionality similar to SDH's APS (Automatic protection Switching), but also provides some other IP optimization features, including:

* IPs can be independent of SDH overhead bytes, allowing IPs to be applied to non-SDH ring environments, such as bare fiber and WDM environments;

* When the network node or link failure, you can restore the IP service in 50ms time, so no need to converge the 3rd Layer routing protocol, so that does not interrupt the IP service; * Does not require dedicated protection bandwidth, so there is no need to keep 50% of the bandwidth for protection.

Iv. concluding remarks

The introduction of DPT makes optical interconnection network (IP over optical) have a solid foundation, by then, SDH Time Division Multiplexing network will be replaced by WDM wavelength Division multiplexing Network, so that IP optimized optical network has more superior performance: Better than SDH fast self-healing recovery capability and excellent compatibility, This will certainly lead to the formation of an advanced integrated network system.