Dense Wavelength Division Multiplexing (DWDM) technology provides huge transmission capacity for communication networks and gradually becomes the mainstream transmission technology. With the maturity of DWDM technology and the rapid growth of transmission capacity, the pressure on traditional electronic switching systems is increasing day by day, and the introduction of optical switching technology is becoming increasingly urgent.
Corresponding to the three division multiplexing methods of optical signals, optical switching is also divided into three types: space division, time division and wavelength division, which respectively complete the exchange of space division channel, time division channel and wavelength division channel.
From the perspective of supported service types, optical switching can be divided into circuit switching (wavelength routing) and packet switching. As early as the early and mid-1990s, people began to study photonic switching technology, ATM optical switching, packet optical switching became a hot research topic. It is expected that optical exchange will break through the limitation of the electronic rate and increase the throughput of the exchange unit. However, these optical exchanges require high-speed optical switches to implement, and optical logic devices are still very immature and cannot complete complex logic processing functions, so they can only implement electronically controlled optical exchanges, that is, identifying the letterhead on the electrical domain, which is controlled by electrical signals To control the operation of the optical switch. Because the electronically controlled optical switch does not get rid of the limitation of the electronic "bottleneck", it limits its development and application. So far, high-speed optical switches and optical logic devices have not yet made major breakthroughs in technology.
All-optical networks based on wavelength division switching or wavelength routing have made considerable progress in the past few years and are gradually being applied on a large scale. All-optical network refers to a transmission network that provides optical domain processing for customer-level signals, including optical domain transmission, multiplexing, routing, monitoring, and survival functions. The main completion of the switching function is the optical cross connector (OXC) and optical add-drop multiplexer (OADM). After several years of research and experimentation, all-optical networks are now developing in the direction of intelligence, and automatic switching optical network (ASON) is the mainstream of its development in intelligence.
Strictly speaking, wavelength division optical switching is different from wavelength routing. The wavelength division switching network must have a wavelength converter, and the wavelength routing network uses different wavelengths to achieve routing. The wavelength routing network belongs to the circuit-switching method, and the two-way resource reservation method is used to set the optical path. The intermediate node does not require optical buffering and can provide guaranteed services. However, circuit switching is coarse-grained. The wavelength or wavelength group is used as the granularity of switching. The bandwidth utilization rate is low, statistical multiplexing cannot be achieved, and it is not suitable for burst services like IP.
Optical burst switching (OBS) was proposed by Qiao Chunming et al. [1]. It managed to integrate the advantages of both larger-grained wavelength (circuit) switching and finer-grained optical packet switching, and overcome the deficiencies of these two switching methods. With lower photonic device requirements, IP-oriented The rapid resource allocation and high resource utilization rate of burst services can effectively support the burst services of upper-layer protocols or high-level users.
1 OBS network structure and node structure
In an OBS network, there are two types of optical packet data flows: a burst control packet (BCP) containing routing information and a burst data packet (BDP) carrying services. Control packets are transmitted in a specific channel on the wavelength division multiplexing (WDM) transmission link, and are electronically processed by network nodes in the OBS network; and data packets are transmitted on another different wavelength channel in the OBS network There is no need to go through the photoelectric / electro-optical conversion and the electronic forwarding of intermediate nodes, to maintain end-to-end transparent transmission and exchange. Control packets are transmitted in specific DWDM (Dense Wavelength Division Multiplexing) channels before data packets to reserve network resources. The core switching node reserves resources for the corresponding data packets according to the information in the control packets and the current status of the network, and establishes an all-optical path. After a delay, the data packet is directly transmitted transparently in the pre-set all-optical channel without acknowledgment. This one-way reservation scheme reduces the latency of channel establishment and improves bandwidth utilization.
This method of separating the data channel from the control channel simplifies the processing of burst data exchange, and the control packet length is very short, thus enabling high-speed processing. The isolation of data packets and control packets, suitable switching granularity, lower control overhead and non-slotted switching methods reduce the requirements for photonic devices and the complexity of intermediate switching nodes. In the OBS network, intermediate nodes can use no cache, 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 be based on WDM optical network to realize the exchange of burst data packets between different links and different wavelength channels. In the OBS network, data packets and control packets are transmitted on different wavelength channels with an offset time difference. Considering the operation time of the optical switch of the switching node, there must be a guard time before and after the data packet.
The OBS network is mainly composed of edge nodes, core nodes and DWDM links. The ingress edge node classifies, caches and encapsulates the data packet according to the data packet's destination address and class of service (CoS) and other information, combines them into burst data packets, generates control packets, and sends them to the nearest OBS core node. The core node exchanges the arriving burst data packet according to the routing information of the control packet. The egress edge node disassembles the BDP and sends it to other subnets or end users
The main advantage of OBS is that it has a medium exchange granularity. The length of a burst packet can range from several packets to a short session, and only one control packet is used, so that each data unit has a lower control overhead. Burst packets from different source ends to different sink ends can use statistical multiplexing to effectively use the bandwidth of the same wavelength on the link, and the bandwidth usage efficiency is high. The separation of BHP and BDP effectively reduces the complexity of intermediate switching nodes and the requirements for optical devices. Intermediate nodes may not require optical buffering, and synchronization requirements are low. The bandwidth is reserved in one direction and the waiting time is short.
2 MAC layer and encapsulation technology of OBS
In order to complete the generation of burst data packets, a media access control (MAC) layer is required in the hierarchy of edge nodes. Figure 3 shows the WDM-based MAC function and burst data packet formation process. As can be seen from FIG. 3, the MAC layer at the input edge node needs to complete the following functions: encapsulate the input packet into a burst packet, and the length of the burst packet may be equal or unequal. Send the burst packet to the queuing queue. When the burst packet is at the head of the queue, set an appropriate offset time, and send a control packet that contains routing information, burst packet length, and offset time. Frame the data packet and send it into the optical layer after a suitable offset time.
At the egress edge node, the function of the OBSMAC layer is simply to disassemble the burst data and extract the IP data packets. The delay generated by the OBSMAC layer includes burst data packet encapsulation delay, queuing delay, and offset time between the burst data packet and the control packet.
Burst encapsulation is an important issue in OBS networks. One common burst encapsulation technology is based on timers and the other is based on thresholds. In the timer-based burst encapsulation method, burst data is generated at fixed intervals and is periodically sent into the optical network. The length of the burst is variable; in the threshold-based burst encapsulation method, the burst The length is usually fixed.
An example of a framing format for burst data is given. Where PT is the payload type, PL is the payload length, NOP is the number of IP packets, and the offset indicates that the data fills the address of the first byte and the synchronization information at the receiving end.
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