Optical Burst Exchange Technology

Dense Wavelength Division Multiplexing (DWDM) technology provides a huge transmission capacity for communication networks and gradually becomes a mainstream transmission technology. With the maturity of DWDM technology and the rapid growth of transmission capacity, traditional electronic exchange systems are under increasing pressure, and the introduction of optical exchange technology is becoming increasingly urgent.

In contrast to the three optical signal division and multiplexing methods, optical switching can also be divided into air separation, Time Division and wavelength division, to complete the exchange of air separation channel, Time Division Channel and wavelength division channel.

Based on the supported business types, optical switching can be divided into two types: Circuit Switching (Wavelength Routing) and group switching. As early as the beginning of 1990s, people began to study photon exchange technology, and ATM optical exchange and packet optical exchange have become a hot research topic. It is expected that optical switching will break through the electronic speed limit and increase the throughput of switching units. However, these optical switches must be implemented by high-speed optical switches. In addition, the optical logic devices are not mature and cannot complete complex logic processing functions. Therefore, only electronically controlled optical switches can be implemented, that is, the optical switch is controlled by electrical signals to identify the signal head in the electrical field. Because the electronic control optical switch does not get rid of the electronic "bottleneck", it limits its development and application. So far, there are still no major breakthroughs in the technology of high-speed optical switches and Optical Logic Devices.

The wavelength division switching or wavelength routing-based optical network has developed considerably over the past few years and is gradually being applied in scale. The all-optical network is a transmission network that provides optical processing for signal at the customer layer, including optical transmission, multiplexing, routing selection, monitoring, and survival functions. The Optical Crossover Connector (OXC) and optical divider (OADM) are used to complete the switching function ). After several years of research and experiments, all-optical networks are now developing in an intelligent direction. Automatic Switching Optical Networks (ASON) is the mainstream direction for intelligent development.

Strictly speaking, wave splitting switching is different from wavelength routing. Wavelength converter is required for wavelength division switching networks, while wavelength routing networks use different wavelengths to achieve routing. The wavelength routing network is a circuit switching method. It uses bidirectional Resource Reservation to set optical connections. The intermediate node does not need optical caching and can provide guaranteed services. However, circuit switching is coarse-grained, with the wavelength or wavelength group as the switching granularity. The bandwidth utilization is low, and statistical multiplexing cannot be realized. It is not suitable for burst services such as IP addresses.

Optical Burst Switching (OBS) was proposed by Qiao Chunming and others [1]. It tries to combine the advantages of large-granularity wavelength (Circuit) switching and fine-grained optical packet switching, and overcome the shortcomings of the two exchange methods, with low requirements of photon devices, quick resource allocation and high resource utilization for IP-oriented burst services are realized. Therefore, it can effectively support burst services of upper-layer protocols or higher-level users.

1. OBS network structure and node Structure

There are two types of optical packet data streams in the OBS network: The burst control group (BCP) that contains route information and the burst data group (BDP) that carries the service ). The control group is transmitted in a specific channel on the WDM transmission link and must be electronically processed by network nodes in the OBS network; data packets are transmitted over another channel of different wavelengths. In OBS networks, they do not need to undergo photoelectric/electro-optic conversion or intermediate node electronic forwarding, ensuring end-to-end transparent transmission and exchange. The control group is transmitted in a specific DWDM (Dense Wavelength Division Multiplexing) channel before the data group, and network resources are reserved. The core switch node Reserves Resources for the corresponding data group based on the information in the control group and the current status of the Network to establish an all-optical path. After a delay, the Data Group is transparently transmitted directly in the pre-configured all-optical channel without confirmation. This one-way reserved Scheme Reduces the delay wait time for Channel establishment and improves bandwidth utilization.

This method separates the data channel from the control channel simplifies the handling of burst data exchange, and the length of the control group is very short, so that high-speed processing can be realized. Isolation of data groups and control groups, suitable switch granularity, low control overhead, and non-Time Window switching reduce the requirements for Photonic Devices and the complexity of intermediate switching nodes. In the OBS network, intermediate nodes can neither use the cache nor have time slot synchronization problems in the network.

An OBS network structure and node Structure example are provided. The OBS network can be based on WDM Optical Networks to realize the exchange of burst data groups between different links and different wavelength channels. In the OBS network, Data grouping and control groups are transmitted over different wavelength channels with a different offset time. Consider the time when the optical switch of the switching node takes effect, and the Protection time must be left before and after the Data grouping.

The OBS network consists of edge nodes, core nodes, and DWDM links. The entry edge node classifies, caches, and encapsulates data packets according to their destination addresses and service level (CoS), combines them into burst data groups, and generates control groups, then it is sent to the nearest adjacent OBS core node. The core node exchanges incoming burst data packets based on the routing information of the control group. The egress edge node disconnects BDP and sends it to another Subnet or end user.

OBS has the following advantages: Medium switching granularity. The length of a burst group can be changed from several groups to a short session, and only one control group is used, so that each data unit has a low control overhead. Burst groups from different origins to different destinations can use statistical multiplexing to effectively use the bandwidth of the same wavelength on the link, resulting in high bandwidth efficiency. The separation of BHP and BDP effectively reduces the complexity of the intermediate switching node and the requirements for optical devices. The intermediate node does not need to be cached and has low synchronization requirements. One-way reserved bandwidth, short wait latency.

2. MAC layer and encapsulation technology of OBS

In order to generate burst data groups, the layer of edge nodes must have a Media Access Control (MAC) layer. Figure 3 shows the WDM-based MAC function and the process of creating burst data groups. As shown in figure 3, the MAC layer at the input edge node needs to complete the following functions: encapsulate the input group into a burst group, and the burst group length can be equal or different. Send the burst group to the queue. When the burst packet is at the beginning of the queue, set a suitable offset time, A control group containing routing information, burst packet length, and offset time is sent. The data packet is sent to the optical layer after a suitable offset time.

On the egress edge node, the obs mac layer function is to simply disassemble burst data and extract IP data packets. The latency produced by the obs mac layer includes the packet encapsulation latency, queuing latency, And the offset time between the burst data packet and the control packet.

Burst encapsulation is an important topic in OBS networks. A common burst encapsulation technology is based on timer and threshold. In the timer-based burst encapsulation method, burst data is generated at a fixed interval and periodically sent to the optical network. The burst length is variable. In the threshold-based burst encapsulation method, the burst length is usually fixed.

A frame format example of burst data is provided. PT indicates the load type, PL indicates the Load Length, NOP indicates the number of IP data packets, and offset indicates that the data is filled with the first byte address and the receiving end synchronization information.

3 OBS Protocol

To enable the OBS network to efficiently coordinate the work between data groups and control groups, researchers have developed a large number of bandwidth access control protocols. To avoid all-optical extraction of control signaling, OBS adopts the out-of-band signaling method, which mainly represents the notification-sending (TAG) protocol and the reserved fixed time period (RFD) protocol.

The TAG protocol sends control groups to reserve bandwidth at the source end. After the burst data streams are transmitted, the group used to release the connection is sent to remove the connection. The RFD protocol is similar to the TAG protocol, but the control signal sent from the source end contains the burst length information, which is used to reserve the link holding time for the intermediate node. The data group can only send the data before the offset time is reached, to improve resource utilization, an outstanding representative of this method is just enough time (JET) protocol.

When the JET protocol is executed, the source node sends a control group to the destination node on the control channel before sending the burst data group. The control group is processed on the intermediate node, creates an all-optical channel for the Data Group to be sent. JET uses the delay reservation (DR) method to reserve bandwidth. Its control overhead signal contains the burst data length information and offset time information. After the Source sends the control overhead signal, wait for a bias time T before sending the burst data. The size of T is sufficient to compensate for the processing time of the overhead on each intermediate node, that is, T ≥ n x delta, where n is the number of intermediate nodes, and delta is the average overhead processing time at a node, so that all intermediate nodes no longer need fiber delay line (FDL ). In order to reduce the network end-to-end wait latency, we should set a small offset time as much as possible. However, a small offset time is not easy to solve the channel competition problem in multi-point communication, which may cause data loss or congestion. If the control group cannot reserve bandwidth for a certain node, it will have to discard the burst data group. The rejected confirmation information is sent back to the source end. The source end will send the control group and burst group again later. Discarding a burst group will waste part of the channel bandwidth and increase the transmission latency of the Data Group. Therefore, an important design problem in OBS networks is how to solve resource competition and provide Differentiated Services.

In the OBS network using the JET protocol, the priority of the service can be determined by adjusting the offset time T. Because increasing the offset time is not only conducive to loose routing, but also allows the corresponding control group to reserve bandwidth more successfully. Two priority schemes are associated with burst encapsulation. The 1st scheme encapsulates the OBS priority and the IP packet priority, set a long offset time for high-priority burst data groups. The 2nd scheme is a so-called hybrid encapsulation method, that is, multiple IP packets with different priorities can be encapsulated in one burst data group, but the order is the top priority of the package, and the lower priority of the package is the back. When there is competition, the lower-priority tail of the burst packet can be discarded, while the higher-priority burst packet is passed.

4. OBS and Automatically Switched Optical Networks

Looking at the Research and Development Trends of optical networks at home and abroad, we can see that ASON Based on wavelength division multiplexing technology has become the mainstream direction of next-generation optical networks. ASON adopts the extended universal Multi-Protocol Label Exchange (GMPLS) Protocol to achieve multi-granularity switching and multi-layer routing, so as to realize dynamic allocation of network resources and automatic connection establishment, it will become the main direction of the development and evolution of the transmission network.

The next-generation optical network is an integrated network. Whether OBS can be integrated into ASON or whether the OBS protocol can be combined with GMPLS [2] Makes OBS a technology supporting optical domain group switching in ASON, it has become a matter of concern and also concerns the application prospects of OBS. Through the above analysis, it is not difficult to see that the OBS and GMPLS-based networks have many similarities:

(1) Both nodes are composed of edge nodes, core nodes, and WDM links. The OBS network encapsulates input groups into burst data groups on edge nodes and generates control groups; GMPLS networks divide input groups into equivalent forwarding classes (FEC) on edge nodes and assign tags.

(2) Both adopt the out-of-band signaling transmission mode, and the load data stream can be transparently transmitted and exchanged in the network.

(3) The GMPLS network can use the label exchange technology to establish a semi-permanent data channel between the ingress and egress routers; OBS networks can establish transparent data channels for loads between entry-exit edge nodes through control groups.

(4) GMPLS adopts the nested labeled switching path (LSP) concept and supports multi-granularity switching, such as wavelength channel level, wavelength group level, optical fiber level, and SDH level. It is reasonable to consider Protocol Expansion and node structure improvement, and also support the exchange of Optical Burst groups.

Due to the profound impact of Optical Burst Switching and GMPLS technology on the future network networking technology, the MPLS Label Switching concept can be introduced into the optical burst switching to generate the Label Optical Burst Switching (LOBS ), 6. LOBS separates the control channel from the data exchange channel. The label information is included in the control packet, and the control packet should be integrated with the control plane of GMPLS. Although this solution still requires in-depth research on many key technologies, the solution will have a profound impact on the future development of optical networks.

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