Overview of Metro Optical Network technology

Metropolitan Area Network is the extension and coverage of the data backbone network and long-distance telephone network in the area. It undertakes the complex tasks such as the group user, the commercial building, the intelligent district and so on the service access and the access, and so on, needs to realize the voice, the data, the image, the multimedia, the IP access and each kind of value-added service and the intelligence service through each kind of gateway, And with the operators of long-distance network and backbone network to achieve interoperability. Metropolitan Area Network is not only a bridge between traditional long-distance network and access network, but also the base of convergence of traditional telecommunication network and emerging data network and the fusion of three networks in the future.

In recent years, the optical fiber transmission technology represented by 10G SDH and DWDM technology has made a breakthrough, the backbone network bandwidth is developing from GB/s to TB/S, and the network rate at enterprise and resident user side is developing to GB/s with the application of G-bit Ethernet technology into commercial applications. These two trends have created a huge bandwidth pressure and a variety of new functional requirements for metropolitan transport networks, including high bandwidth, a large number of user nodes and numerous types, flexible bandwidth allocation, multiple business support and protocol independence, protection and self-healing, and easy network management.

There is demand for innovation, to meet the requirements of the optical metropolitan Area Network technology flourish, the industry will be collectively known as MSPP or MSTP. Because of the different application technology, its main development has the following several directions:

Ethernet-based metropolitan Area transport network Technology

As the initial form of metropolitan transport network, Ethernet direct connection is regarded as the simplest way of metropolitan transmission network, its lower cost and convenient opening, operation mode was once favored by service providers.

GE, 10GE high-capacity Ethernet technology has brought the metropolitan area network to a new level, and IEEE has defined it explicitly in the 802.3 standard. This technology adapts to the growing needs of the dominant IP business in the metropolitan area Network, and supports a large bandwidth, High-cost metropolitan area core network that can be seamlessly connected to TDM or DWDM optical networks to meet the needs of larger capacity networking. The disadvantage of this technique is that it occupies more fiber, the bandwidth is statically allocated, the efficiency of transmitting IP data service is relatively low, the reliability of Ethernet service is not satisfied, QoS is not ensured, and the development of circuit business is not supported.

The way to solve the problem of Ethernet routing is to use optical Ethernet RPR (optical Ethernet RPR). RPR (resilient Packet rings) combines the advantages of Ethernet and SDH to enable the device to share all or part of the bandwidth on the ring; it defines an independent physical layer-resilient packet Ring media access control layer, providing interoperability for each vendor, thus increasing competition and reducing user investment It provides a dual-loop structure that transmits the signal in reverse direction on the ring and provides protection switching less than 50ms. In the business aspect, RPR can combine MPLS protocol and use the Cos byte identification of MPLS label in frame structure to provide four kinds of business: fast delivery service (with strict delay, jitter and protection, clock synchronization). such as real-time E1 voice service and image business), guarantee transmission Service 1 (no special requirements for delay and jitter, but there is a bandwidth commitment, no burst of image, data Services), the protection of Transmission Services 2 (with a bandwidth commitment, is a burst of data services, using the best transmission mechanism), and try to transport business. If the technology is applied in a large scale, it will solve the problem of multi service transmission in metropolitan area network.

One thing needs to be suggested is that, although the benefits of RPR technology are many, but it and related MPLS technology are temporarily no uniform international standards (in place), can not provide multiple manufacturers interoperability mechanism, so there is no large-scale commercial.

The MSTP technology of the metropolitan area with SDH as platform

Because of its reliable business protection capability, SDH technology is becoming a choice of metropolitan transmission network.　　But the thorny problem is: for fixed-rate business (such as traditional voice service), SDH is easy to adapt to the fixed-capacity channel, but for the variable rate VBR service and any rate of service, SDH is not flexible, especially the transmission efficiency is not high. SDH's high market share and the huge value-added potential of the metropolitan area Network make SDH advocates effort, on the basis of the original SDH to add to the data business layer processing, such as Ethernet two layer processing, ATM statistical multiplexing and other functions, make it more suitable for the transmission of data services.

For Ethernet business, it needs to undergo processing before mapping to VC: Two-layer exchange, Protocol encapsulation, mapping pretreatment, etc.

The Ethernet data is mapped to the SDH frame structure through the special protocol, there are currently three kinds of schemes:

(1) through Point-to-Point Protocol PPP converted to HDLC frame structure, and then mapped to SDH virtual container VC, referred to as POS.

(2) The conversion of packets into laps structure to the SDH virtual container VC, this is the Chinese own IP over SDH proposal, has been officially approved as the ITU standard, its standard number is x.85/y.1321 IP over SDH.

(3) The data packet is mapped into SDH virtual container VC by simplifying the Data link protocol SDL.

POS technology is more mature and suitable for multi-protocol environment, but because PPP is not designed for SDH delivery, the efficiency of POS is not ideal; laps in the HDLC net load to eliminate padding byte pad, so for short packets, laps more efficient than PPP, and the scrambling code as a mandatory requirement, Unlike PPP, which is optional, SDL technology is highly efficient for high-volume data packets and transport systems.

After receiving the data service, the Ethernet port needs two layer Exchange processing (optional) to ensure its efficient transmission. In addition, in order to enhance the flexibility of the hosting business, cascaded (concat-enation) technology is applied before the data business enters VC. Cascade technology is divided into continuous (contiguous) cascade or virtual (virtual) cascade two kinds. The VC-12 cascade of 100M Ethernet is used as an example to illustrate its principle: the technique binds n VC-12 together to form a whole vc-12-n, and a c-12-n (or laps) link is established in the net load HDLC supported by Vc-12-n to transmit in the SDH network. When n VC-12 are continuously arranged for continuous cascade, usually the Poh of the first VC-12 in Vc-12-n as the whole Poh, the disadvantage is that n VC-12 must be adjacent to the address, bandwidth allocation is not flexible.

The virtual Cascade method does not need VC-12 adjacent, only need the channel terminal equipment to provide cascading function. In this way, the channel business start and end ports are required to add corresponding processing functions, and the receiving end needs to introduce a buffer to increase the additional delay.

For ATM service, the system provides statistical multiplexing function, which can be used to extract non-idle elements from multiple ATM traffic streams, and reuse an ATM service stream to improve its utilization on SDH line and save the port number of ATM switch. In addition, an ATM virtual channel loop can be formed on the SDH loop, so that the ATM's business level can realize the ring protection.