Comprehensive Analysis of Optical Burst Switching Technology and Its Research

The most basic switching unit in optical burst switching technology is burst data. Meanwhile, BCP and burst data in OBS are separated on the physical channel, and each control group corresponds to a burst data. With the rapid growth of IP services around the world, the demand for transmission network bandwidth and switch system capacity is increasing at an unprecedented rate. Currently, DWDM technology is used in the optical layer. The available bandwidth of an optical fiber can reach about 10 Tbit/s, which can meet the requirements of the transmission network bandwidth for a long period of time.

However, the exchange rate with the usual Circuit Switching Technology (Time Division, air division, and wavelength switching) is much lower than this value. In this way, the mismatch between the two puts forward new requirements for the development of optical burst exchange technology. In the long run, OPS (OpticalPacketSwitching, optical packet switching) is the development direction of optical switching, but OPS has two insurmountable obstacles in the near future: first, the technology of the optical cache is not mature yet. Currently, the optical fiber delay Line (FDL and fiber delay Line) used in the experimental system is often bulky and inflexible, and the storage depth is limited; second, it is difficult to accurately synchronize Multiple Input groups at OPS nodes. Therefore, in a short period of time, the commercial application prospect of optical group exchange is not optimistic.

In this case, ChunmingQiao and JsTurnor proposed a new Optical Burst Switching technology-OBS (Optical Burst Switching, Optical Burst Switching), as a transitional technology for circuit Switching to group Switching. The bandwidth granularity used by 0BS is between circuit switching and group switching, which is more flexible than circuit switching and high bandwidth utilization, and closer to practical than optical group switching. It can be said that it combines the advantages of the two and overcomes some of the shortcomings of the two. It is a balanced choice between the two, which has gradually attracted the attention of many experts and scholars.

Principles of OBS Technology

An "burst" in OBS is an ultra-long IP packet consisting of an IP packet with the same egress edge router address and QoS requirements. These IP packets can come from different electrical IP routers in a traditional IP network. Burst data is the basic exchange unit in the light burst exchange technology. In OBS, BCP (BurstControlPacket) and burst data (Net Load) are separated on physical channels. Each control group corresponds to a burst data. For example, in a WDM system, the control group occupies one or several wavelengths, and the burst data occupies all other wavelengths.

In OBS, burst data is always in the optical field from the source node to the target node, and the control information needs O/E/O transformation and electrical processing on each node. The rate of the control channel (wavelength) and the burst data channel (wavelength) can be the same or different. The OBS network is composed of a core optical router and an edge router. The edge router is responsible for encapsulating data in a traditional IP network as optical burst data and reverse unpacking, the task of the core router is to forward and exchange burst data. No O/E or E/O transformations are performed on the 0BS network.

Key OBS Technologies

The edge router assembly part sorts these data packets into the corresponding Burst buffer heap based on the destination address and QoS of the data packets input from the port channel. The Assembly of burst packages generally requires two parameters: one is the assembly time and the other is the maximum length of the burst package. In addition, we should also consider whether the length of the burst package is fixed or changed. There are currently the following Assembly algorithms.

(1) fixed assembly time (FAT)

In this algorithm, burst packets are assembled at a fixed assembly time. When the network traffic is large, burst packets using this algorithm will be large.

(2) fixed Assembly length (FAZ)

It is assembled according to the fixed burst packet length. For burst packets that do not reach the length, some bytes need to be filled to reach the fixed Assembly length. When the network traffic is relatively small, it takes a long time to assemble this algorithm, which increases the network latency and reduces the network performance.

(3) Adaptive Assembly length (AAZ)

The burst assembly algorithm controlled by time and maximum packet length is simple and easy to implement, but they are not optimized for the burst traffic characteristics of IP services. When the network load is low, the package size of the assembly algorithm changes significantly compared with the high-load network, which leads to additional network latency and reduces the transmission efficiency of burst data packets. In addition, multiple edge node routers can easily form burst transmission synchronization under the time-counting-based assembly algorithm mechanism, leading to sustained resource competition.

To address these problems, an intelligent assembly algorithm that can adapt to the service traffic conditions and damage the burst transmission synchronization of each edge node has emerged. This Intelligent Assembly algorithm introduces a burst packet length threshold and the number of flowmeters based on the previous algorithm, and dynamically adjusts the Group Assembly according to the statistical results of the traffic. This Intelligent Assembly Algorithm is conducive to restraining the unfavorable factors of excessive Packet Length changes under the timed Assembly mechanism, and disrupting the synchronization of bursts of time on nodes with different traffic characteristics, it helps solve the problem of resource competition in OBS networks.

The essence of the OBS signaling protocol is to reserve Bandwidth Resources for the intermediate nodes passing through the burst packet based on the messages in the control group. Based on whether the connection is established and released explicitly, OBS signaling protocols can be divided into four categories: Explicit establishment and release; estimated establishment and release; Explicit establishment and release; estimated creation and explicit release.

(1) Explicit creation and release

That is, the connection establishment and release are both explicit. The source node sends and establishes a connection before sending the burst packet data. The intermediate node establishes a connection after receiving the established message to receive the incoming burst packets; the source node sends the release message after the packet burst is sent. The connection must be released. This type of protocol is easy to implement, but the efficiency is relatively low. A typical example is the JIT (JustInTime) protocol.

(2) estimated establishment and release

That is, the establishment and release of connections are estimated. The source node sends the established message before sending the burst packet data. The intermediate node estimates the creation and release time based on the information contained in the created message. These protocols are complex and difficult to implement, but very efficient. The typical example is the JET (JustEnouthTime) protocol.

(3) explicitly create and estimate release

That is, the connection is established explicitly, and the release is estimated. The source node sends and establishes a connection before sending the burst packet data. The intermediate node establishes a connection after receiving the established message to receive the incoming burst packet data; the intermediate node determines the connection release time based on the length of the burst packet data contained in the established message.

(4) estimated creation and explicit release

That is, the establishment of the connection is estimated, and the release is explicit. The source node sends the established message before sending the burst packet data. The intermediate node determines the connection establishment time based on the information contained in the established message. After the burst packet data is sent, the source node sends a release message and requests to release the connection.

In the OBS network, when multiple groups reach the same output port at the same time, competition will occur. Currently, the main methods to solve the competition include optical caching, wavelength conversion, biased routing, combined burst packet (OCBS)/burst packet segmentation (BS), and a combination of multiple technologies.

(1) FDL Configuration

The application FDL cache can delay the burst package to the end of the competition. Compared with the cache in the electric domain, the FDL cache can only provide a fixed delay, and the data leaves the FDL cache in the order they enter the delay line, this limits the flexibility of competitive solutions. In addition, there is also a major problem in the optical cache: power loss. In order to compensate for power loss, we have to introduce optical signal amplification or optical signal regeneration. The former will introduce noise, and the latter will be too costly. In general, the introduction of FDL will greatly increase the cost of optical burst switching technology.

(2) wavelength Transformation

With wavelength converter, burst packets can be sent at different wavelengths than the specified output line in the event of competition. This solution is the best in terms of latency of competitive groups, suitable for circuit switching, and also suitable for optical burst switching technology, but requires fast tunable converter. Recent research results show that wavelength switching is one of the most promising options in Packet Switched Optical Networks, which can effectively reduce packet loss rate in optical packets/bursts, it is especially used in the multi-wavelength DWDM system. Therefore, the fast adjustable wavelength converter is currently a hot topic.

(3) biased Routing

Biased routing is a method that uses idle links to resolve conflicts, that is, when the competition occurs, the burst switching technology to the correct output port, it will be routed to another optional output port, it is possible to reach the target node through another path. When the link resources are sufficient, the redirection routing has better performance. However, this method has some potential problems in the re-sorting and fairness of the egress nodes. In addition, the performance may deteriorate when the load is heavy, so it is only suitable for networks with low network load.

(4) OCBS (combined burst packet)/BS (burst packet segment)

OCBS and BS share the same idea. In the event of competition, they all divide burst packets into several parts, and forward as many parts as possible during the forwarding, to minimize data discarding.

(5) combination of multiple technologies

Since a single conflict resolution mechanism has limited performance improvement, and the above technologies do not affect or conflict with each other, the above technologies can be organically combined. The most effective combination scheme is to organically combine the cache with the full-wavelength conversion, and then cooperate with the spatial biased routing. The most economical solution is the smallest Light cache, which works with some wavelength transformations and then introduces a biased Routing Mechanism, which can greatly reduce costs, but slightly degrade performance.