The principle of optical burst switching and the structure of network nodes

Source: Internet
Author: User
Tags exit resource switches

Dense wavelength division multiplexing (DWDM) technology provides a huge transmission capacity for communication networks, and gradually becomes the mainstream transmission technology. With the maturation of DWDM technology and the rapid growth of transmission capacity, the pressure of traditional electronic switching system is increasing, and the introduction of optical switching technology is urgent.

corresponding to the 3 kinds of segmentation multiplexing modes of optical signals, the optical switching is divided into 3 kinds of space, time and wavelength, respectively, and the exchange of the space-dividing channel, the time division channel and the wavelength-dividing channel are completed.

From the supported business types, optical switching can be divided into two ways: circuit switching (wavelength routing) and packet switching. As early as the middle of the early 1990s, people began to study the photon Exchange technology, ATM optical switching, packet light exchange has become a hot research topic. It is expected that the optical exchange can break through the limit of electronic rate and improve the throughput of the switching unit. However, these optical switching requires high-speed optical switch to achieve, coupled with optical logic devices are immature, can not complete the complex logic processing function, so can only be implemented electronically controlled optical exchange, that is, in the domain of the identification of the head, by electrical signals to control the action of light switch. Because the electronic control optical Exchange does not get rid of the electron "bottleneck" the restriction, thus limits its development and the application. So far, high speed optical switches and optical logic devices still have no major breakthroughs in technology.

All-optical networks based on wavelength-division switching or wave-length routing have made great progress in the past few years and are gradually being applied in scale. All optical network refers to the transmission network which provides the optical domain processing for the customer layer signal, including the optical domain transmission, the multiplex, the route selection, the monitoring and the survival function and so on. The main switching functions are optical crossover connectors (OXC) and optical splitter multiplexer (OADM). After several years of research and experiment, all optical networks are developing in the direction of Intellectualization, and automatic switched Optical Network (ASON) is the main direction of intelligent development.

In strict terms, wavelength-sharing is not the same as the wave-length routing. WDM networks must have wavelength converters, and wavelength routing networks use different wavelength to achieve alternative paths. Wavelength Routing network is a circuit switching mode, using bidirectional resource reservation mode to set up optical path, intermediate node does not need optical buffer, can provide guaranteed service. But the circuit exchange is coarse granularity, the wavelength or wavelength group for the exchange of granularity, bandwidth utilization is low, can not achieve statistical reuse, not suitable for such a sudden business like IP.

Optical burst Switching (OBS) is presented by Qiaochunming et. [1]. It tries to synthesize the advantages of large granularity wavelength (circuit) exchange and finer-grained optical packet switching, and overcomes the disadvantages of these two modes, and realizes the fast resource allocation and high resource utilization of IP-oriented burst service in the low demand of photonic devices. Therefore, it can effectively support the upper layer agreement or the burst business of high level users.

Network structure and node structure of 1 Obs

In OBS networks, there are two types of optical packet data streams: Burst control grouping (BCP) with routing information and burst data grouping (BDP) for hosting services. Control packets are transmitted in a particular channel in a wavelength division multiplexing (WDM) transmission link. In OBS networks, electronic processing of network nodes is required, and data packets are transmitted on another wavelength channel, without photoelectric/electro-optical conversion and intermediate node electronic forwarding in OBS networks, Maintains end-to-end transparent transmission and exchange. The control group is transmitted in a specific DWDM (dense wavelength division multiplexing) channel prior to data grouping, and the network resources are reserved. The core Exchange node reserves resources for the corresponding data grouping according to the information in the control group and the current situation of the network, and establishes the all-optical path. After a period of delay, data packets are transparently transmitted directly in a preset all-optical channel without confirmation. This one-way reservation scheme reduces the latency waiting time of the channel and improves the bandwidth utilization.

This method of separating the data channel from the control channel simplifies the processing of the burst data exchange, and the control packet length is very short, so the high-speed processing can be realized. The separation of data grouping and control groupings, the suitable exchange granularity, the lower control cost and the non time slot switching method reduce the requirements of photonic devices and the complexity of intermediate switching nodes. In OBS networks, intermediate nodes can not use caching, and there is no time slot synchronization problem in the network.

An example of OBS network structure and node structure is given. OBS network can realize the exchange of burst data packets between different links and different wavelength channels based on WDM Optical network. In OBS networks, data grouping and control groupings are transmitted on different wavelength channels and one offset time. Considering the switching time of switch-node optical switches, the protection time must be kept before and after data grouping.

OBS networks are mainly composed of edge nodes, core nodes and DWDM links. The entry Edge node classifies, caches and encapsulates the packet according to the data packet's target address and service level (CoS), combines the burst data grouping, produces the control grouping, and sends to the nearest OBS core node. The core nodes exchange the incoming burst packets according to the routing information of the control group. The exit Edge node will be removed by the BDP and sent to other subnets or end users

The main advantages of OBS are: have medium exchange granularity. The length of the burst packet can be grouped from several to a short session, with only one control grouped so that each data unit has a lower control overhead. Burst groups from the same source to different hosts can use the method of statistical multiplexing to effectively utilize the bandwidth of the same wavelength on the link, and the bandwidth use efficiency is high. The separation of BHP and BDP effectively reduces the complexity of intermediate switching nodes and the requirements of optical devices. Intermediate nodes do not require optical caching, and synchronization requirements are low. Bandwidth one-way reservation, waiting for short delay.

MAC layer and encapsulation technology for 2 OBS

In order to generate the burst data grouping, the hierarchical structure of the edge node needs a media access control (MAC) layer. Fig. 3 gives the Mac function and the formation process of burst data grouping based on WDM. As can be seen from Figure 3, the MAC layer at the input edge node needs to complete the following functions: the input packet packaging into burst packets, burst packet length can be equal, can also range. The burst packets are fed into the queue, and when the burst is at the head of the team, a suitable bias time is set up, and a control group containing information such as routing information, burst packet length and offset time is sent. The data is wrapped into frames and sent into the light layer after a suitable offset time.

At the exit Edge node, the function of the OBSMAC layer is simply to disassemble the burst data and extract the IP packets. The time delay caused by the OBSMAC layer includes the packet delay of burst packets, the queueing delay and the offset between the burst packet and the control packet.

Burst encapsulation is an important subject in OBS networks, and the common burst encapsulation technology is based on timer and the other is based on threshold. In the burst packaging method based on timer, burst data is generated at fixed interval and is sent into the optical network periodically, the burst length is variable, and the burst length is usually fixed in the burst encapsulation method based on threshold value.

An example of a frame format for burst data is given. The PT is the load type, PL is the payload length, the NOP is the IP packet number, and the offset indicates that the data fills the first byte address and the End-to-end synchronization information.

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