The continuous growth of the Internet and the emergence of various new businesses, such as multimedia web pages, multimedia games, Multimedia conferences, and e-commerce, have led to exponential growth of Internet services. Due to the diversity and variability of services, the traditional core exchange network is no longer suitable for the needs of the continuous development of the Internet. This situation has prompted researchers to speed up in-depth research on WDM transmission and all-optical switching technologies to adapt to the explosive growth of Internet traffic, business diversity, and sudden business spikes.
At present, optical networks adopt static or dynamic wavelength routing (or optical circuit switching, OCS) mechanisms. The Protocol mechanism is relatively simple, and the technology is mature and easy to implement. However, it is similar to the circuit switching mechanism. It takes a certain period of time to establish and remove a channel, and this time is not related to its connection retention time, but mainly depends on the end-to-end signaling time. When the connection persistence time is relatively short, it will cause the channel utilization to deteriorate. Therefore, it is not suitable for continuously increasing and changing Internet traffic, such as web browsing, FTP file transmission, and email.
In optical switching, OPS provides better bandwidth utilization, latency, and adaptability. In the long run, OPS seems to be a promising technology, but its implementation is complicated and the current optical logic processing technology is not mature, there is no available optical random memory (ORAM), so it takes years of research to be applied.
In view of the existing problems of OCS and OPS, in recent years, people have proposed a new optical switching technology-OBS technology, which has quickly attracted the attention of scholars at home and abroad. OBS is notable because it has both the advantages of OCS and OPS while avoiding their shortcomings.
In the OBS network, the intermediate node does not need any optical RAM. The transmission of burst data is completed by the resources reserved by the corresponding control group (BCP). burst data groups are directly connected to the intermediate node, no storage required. In optical packet switching, burst data is stored and forwarded on the intermediate node. Compared with Optical Circuit Switching, OBS can achieve better bandwidth utilization because it allows Statistics reuse between burst data streams at each wavelength; otherwise, several wavelengths are required. In addition, the end-to-end (ETE) latency of burst groups is relatively small, because the offset time is much less than the time when the wavelength channel is established in the wavelength routing.
What is "light burst switching"
The concept of emergency exchange was put forward in early 1980s, and some papers were published one after another. The concept of Burst Switching was not as popular as circuit switching and group switching at the time, because the technology was mature at the time of proposal of burst switching, there is no need to deal with voice or data in units of bursts to change the entire network.
However, with the continuous development of communication technology, a profound change is that the increase in transmission rates greatly exceeds the growth in processing rates, network processing devices will be overloaded for a long time. Therefore, it is necessary to simplify the processing of network nodes. Improving the processing granularity through optical burst switching is a better solution. By sending control information in advance, the node can schedule resources at each node and then transmit burst data, data can always be stored in the optical field, and the group header can be processed one by one in group switching.
In the OBS network, the basic switching unit is burst ). Burst is composed of the same egress edge router address and IP groups with the same quality of service (QoS) requirements. Optical Burst Switching Nodes include core nodes and edge nodes. Edge Nodes are responsible for reorganizing and classifying burst data packets, and provide various business interfaces. The core node is responsible for forwarding and exchanging burst data. The core node Structure of optical burst switching is different from that of optical packet switching. It only needs to process and control signaling in the electrical domain.
Edge Nodes Group IP addresses with the same egress vro addresses and QoS requirements into burst packets to generate burst data groups and corresponding control groups. Transmission of burst data groups and control groups over physical channels (generally different wavelengths in the same optical fiber) and time (the control group is earlier than the burst data group for a period of time, that is, the offset time) is separated.
In the OBS system, one-way reservation is generally used, that is, the control group sends messages in advance of the data burst group, and the data burst group does not need to wait for a certain period of time before the response confirmation message, it is sent directly on a predetermined channel (wavelength. Each control group corresponds to a burst data group. It contains some basic information about the corresponding burst data group, such as burst length, offset time, wavelength ID, and route information, it is much shorter than sudden data bursts.
On the central core node, the control group Reserves Resources for the corresponding Optical Burst group after optical/electrical/optical conversion and information processing. Burst data groups do not require optical/electrical/optical processing, and are directly transmitted to the target node transparently (all-optical) through links configured in advance in the control group. In this case, a tour group (equivalent to a sudden trip) sends a person (equivalent to a control group) to book tickets and restaurants, and the subsequent tour groups do not have to consider such issues, travel based on the predefined information to complete the trip. Of course, if the previous control group reservation fails (that is, resource competition, or conflict), the corresponding light burst group may be lost, and its solution will be described later.
Control Protocol
Since 1980s, a variety of electrical burst switching technologies have been proposed in the research field, such as Tell-and-Go (TAG), in-band terminator (IBT), and reserved fixed period (RFD) protocols. The TAG technology is similar to fast Circuit Switching. It does not need to confirm that all bandwidth has been reserved, but directly sends burst data, and its bandwidth utilization is not high. The reserved bandwidth of the IBT Scheme starts from the time when the control group processing is complete to the time when the IBT detection is complete, but the IBT all-optical detection is relatively difficult. In RFD-based burst switching, only the bandwidth specified by its burst control group is reserved. This eliminates the impact of signaling overhead and provides an efficient Bandwidth Reservation Mechanism.
JET Protocol
The exactly-time (JET) protocol is based on the burst switching control protocol of RFD in the optical field. It adopts two unique features: Offset time and latency reservation. These features make JET more suitable for OBS than the TAG or other OBS protocols that do not use these two features. JET allows the data channel to be switched completely in the optical field. Its control is determined by the burst control group information processed in the electrical field. The control group must be sent before the burst data group. That is, the control group and its corresponding burst data group have an offset interval when sent at the source end. The separation and exchange of burst data from its header are easy to implement, and the requirements for core node processing and photoelectric processing capabilities are reduced. Additionally, by assigning additional offset times, JET can expand to support priority businesses in the optical field.
Figure 1 obs jet Protocol
The control group contains information about optical routing, burst length, and offset time of necessary burst data. Another important feature of JET is latency reservation, which only reserves the link bandwidth resources experienced by burst data. For example, if t1' is the time when the first control group arrives, t1 is the time when the burst data arrives at the node, and L is the data burst duration. After the control group is processed, the bandwidth from t1 to t1 + L will be reserved, which increases the bandwidth utilization and reduces the possibility of sudden packet loss. In the two situations in Figure 1, that is, the first case t2> t1 + L and the second case t2
Priority-based JET Protocol
There are multiple priority-based OBS reservation mechanisms. The most common one is the JET protocol based on the extra offset time, that is, pJET. In this protocol, the optical burst group with a higher priority is assigned a longer additional offset time, as long as its additional offset time is greater than the maximum group duration of the business with a lower priority, this ensures that high-priority businesses are not affected by low-priority businesses. Of course, the low-priority light burst grouping reduces the available resources, and the packet loss rate will inevitably be affected, however, the total average packet loss rate includes the packet loss rate of high-priority and low-priority Optical Burst groups, which is basically unaffected. This is like a normal person can only buy a train ticket for the day, while a person with a higher priority can buy a ticket a few days in advance. Of course, he is more likely to successfully buy a ticket, so that the priority is achieved.
There is a problem with the above Protocol, that is, although the packet loss rate performance of high-priority services has improved a lot, the cost is that it increases the delay of high-priority services. This technology is feasible as long as latency is controlled within the scope permitted by the business.
There is also a kind of proportional priority, which intentionally discards some low-priority light burst groups based on certain principles, to give more opportunities to reserve resources with higher priorities. In this way, although a certain priority can be completed and the delay with a higher priority will not be affected, it sacrifices the overall performance (the total average packet loss rate will increase a lot). If this ratio is required to increase, the overall performance will be worse.
Competing Solutions
To solve this problem, when multiple groups reach the same output port at the same time, a competitive solution is required. This is an inevitable problem for all group switching methods, that is, external blocking. A typical solution is to cache other conflicting groups and only allow one output. In OBS and OPS, the competitive solutions include optical caching, wavelength conversion, and biased routing, or the integration of multiple technologies. The following describes in detail.
Light cache
There is no available optical RAM in the optical field, so it is impossible to fully use the switching mechanism in the electrical field in the optical switching. An optional optical cache solution is the use of optical fiber delay line (FDL), which can reduce packet loss rate in a certain extent. However, one of the main problems of Light cache is its 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 switching.
Wavelength Conversion
The optical network has another domain, namely the wavelength domain. In a wavelength conversion system, if two or more optical groups/sudden competition occurs, one of the groups/sudden pass-through, another or several other groups/bursts are switched to the same output port, but different wavelengths are used. This solution is optimal in terms of latency of competing groups, and does not introduce additional latency. This method is suitable for circuit switching and optical packet/burst switching networks, but it needs to be quickly tunable. Recent research results show that it is one of the most promising options in group switched optical networks. It can effectively reduce packet loss rate of optical packets/bursts, especially when used in multi-wavelength DWDM systems. Therefore, fast adjustable wavelength converter is currently a hot topic.
Biased Routing
Because there are still several problems that are difficult to solve in the Light cache, it should be used less or not as much as possible. Biased routing is another solution when no cache is available. When a competition occurs, the Group/burst cannot switch to the correct output port, route it to another optional output port, and possibly reach the target node through another path. When the network size is small and its connectivity is good, that is, these nodes have many adjacent nodes, this method works well. However, if the connectivity of the network is poor, the attacked groups/bursts may not be able to reach the target node. These groups/groups suddenly consume a lot of resources in the network, but cannot reach the target node. Obviously, in this case, other solutions will achieve better results.
In addition, the biased routing scheme can only be used when the network load ratio is relatively low. If the average traffic load is heavy, the partial routing group can only reduce the network efficiency. The scheme of biased routing can be improved and only some ports can be used. If a reasonable route cannot be found in the group to reach the destination node, it will be blocked even if there is an idle port.
Integration of multiple technologies
The performance improvement of a single conflict resolution mechanism is limited, and the above three technologies do not affect each other or conflict with each other. Therefore, the above two or three technologies can be combined organically. Among them, the most effective combination solution is to use the cache and the full wavelength transform to combine with the appropriate spatial biased routing. The most economical solution is that the smallest Light cache works with some wavelength transformations and then the biased routing mechanism is introduced, which can greatly reduce costs, but slightly damage the performance.
OBS network structure
Use OBS to implement IP over WDM network topology 2. The edge router is connected to the service network, such as the IP network, and the other is connected to a core router. The core router can be connected to one or more edge routers, and a mesh network can be formed with other core routers. At the entry node, the edge router determines the data burst size and offset time based on the characteristics of the input IP stream.
Figure 2 OBS network topology
The control group contains information such as the egress address, offset time, data burst size, and QoS. The corresponding burst data group is sent at the separated control wavelength in advance, the corresponding burst data group follows the control group after a given offset time. These control groups are converted into electrical signals at the intermediate node for processing.
At the core node, the reserved bandwidth duration is the transmission duration of burst data. The core unit needs to monitor the basic elements of the traffic, including the blocking probability, delay, and processing time, which determine the optical path at the entry node. At the egress node, the data burst will be split into multiple IP packets. If necessary, re-order the nodes at the egress and re-Send the nodes with errors. Parameters such as the offset time, burst size, and QoS value are essential elements to be processed by the OBS network. These parameters must be assigned to the entry nodes of the OBS network.
OBS applications
The OBS technology grows to meet the needs of business growth. It features low latency (one-way reservation), high bandwidth utilization, flexible switching, data transparency, and large switching capacity (electronically controlled optical switching) and other advantages to achieve Tb/s switching capacity. Therefore, the OBS network is mainly used in the ever-evolving large-scale man and wan. It can support traditional services, such as telephone, SDH, IP, FDDI, and ATM, and also support services with high burstable and diverse features in the future, such as data file transmission, Web browsing, on-demand video, and video conferencing.
For large man networks, one or more universities, large office buildings, residential areas, enterprises and institutions, suburban counties or satellite cities, and densely populated public places, place an OBS edge router, complete the convergence of multiple local businesses to generate bursts. Its access services can be varied, such as Ethernet (10 M, 100 M, 1G or 10G), ATM, xDSL, etc. Then, some OBS core routers are placed in various major locations to reserve and exchange burst groups. In this way, an urban OBS network is built.
Although OBS is still immature in terms of standards and protocols, many technologies are still being studied. OBS is still a very promising optical exchange technology, it combines the advantages of Optical Circuit Switching and optical group switching, while avoiding their shortcomings. OBS is characterized by separation of control and data in time and space. Control groups are sent in advance, and the intermediate nodes are processed by electrical information to reserve resources for data groups; data groups are transmitted along with the control group. In the middle node, the reserved resources are used directly, without the need for optical/electrical/optical processing. The one-way reserved mechanism is used to ensure high bandwidth utilization and no light cache is required, easy to implement. With the maturity of fast wavelength conversion technology, the optical burst switching technology will develop rapidly and become the core technology of the next generation optical transmission and switching network.
