Apparatus and method for transferring data bursts in optical burst switching network

In an apparatus and method for transferring data bursts in an optical burst switching network, a control module connected between demultiplexers and multiplexers obtains information in control packets of data bursts to sort and confirm service classes of the data bursts according to delay time and blocking rate. A switching unit switches the data bursts to an output port of a corresponding destination node. A first buffer delays data bursts from the control module once by a preset delay time according to output resource availability and sends them to the output port when the data bursts are sensitive to delay time. A second buffer circulates and delays the sorted and confirmed data bursts up to a preset limit according to output resource availability and then sends the data bursts to the output port when the data bursts are insensitive to delay time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for transferring data bursts in an optical burst switching network, and more particularly, to an apparatus and method for transferring data bursts in an optical burst switching network which are capable of efficiently transferring data bursts with a minimized delay time and a low loss rate by using a hybrid optical buffer having a feed-forward type buffer shared in each port and a feedback type buffer shared in each node, the hybrid optical buffer using fiber delay lines (FDLs) to provide class-differentiated service.

2. Description of the Related Art

An optical burst switching (OBS) technique generally involves a unidirectional reservation technique of first sending a control packet to a destination node for resource reservation and directly transferring burst data without confirming whether the resource is reserved. In this case, intermediate nodes directly transfer the burst data without performing optical-electrical conversion.

Since the data bursts are transferred without confirming whether resource is secured at all nodes on paths to the destination node, they are inevitably lost in the nodes.

To solve this problem, several loss avoidance methods have been suggested, such as a method employing a wavelength converter, a method employing an fiber delay line (FDL) based buffer, a method of discarding contending portions of data bursts, and a method using a deflection routing.

Among these methods, the method employing a fiber delay line (FDL) based buffer is the best because when contention between the data bursts occur, data bursts are delayed through fiber delay lines (FDLs) by a contention time and then transferred.

This method is similar with that used in the Internet. In the Internet, a random access memory (RAM) is used as a buffer to store data for an exact contention time and then send the data. On the other hand, the fiber delay line (FDL) based buffer can store data for a time corresponding to a fixed length of the line.

In addition, because the data bursts are transferred along an unnecessarily long path, physical loss such as signal attenuation may occur. The buffer is directly connected to a switch, and accordingly increase in a buffer size is closely related to increase in a switch size. Accordingly, determination of the buffer size is a critical issue.

Fiber delay line (FDL) based buffers may be classified into a feed-forward type buffer and a feedback type buffer. The feed-forward type buffer can be used only once. If data bursts contend with each other at an output port, a buffer having a fixed length is used. At this time, the data bursts are exited from the buffer regardless of success of burst transmission.

On the other hand, in the feedback type buffer, data bursts are circulated through the buffer until output resource is available or up to an allowed maximum number of times if the output resource is not available after one round of buffering. This provides more buffering chances and thus a higher transmission success rate, but causes excessive buffering delay and signal attenuation.

Meanwhile, in an optical burst switching network, buffers may be classified into a buffer exclusively used for each wavelength, a buffer shared in a port with a number of wavelengths multiplexed, and a buffer shared in one switching network, depending on a buffer location.

These buffers have merits and demerits according to the size of a switching fabric and a sharing degree of resource. That is, when the buffers are exclusively used for each wavelength, a transmission success rate may be high but the size of the switch increases due to there being a large number of buffers. On the other hand, when a buffer is shared in the optical burst switching network, the size of the switch structure may decrease but the transmission success rate may be low.

Meanwhile, it is critical to provide class-differentiated service in an optical burst switching network. As previously stated, once the data bursts are transferred, intermediate nodes send the data bursts through a switching operation without a process of obtaining control information through optical-electrical conversion for acquiring information about a destination node. Thus, the loss of the data bursts is a more important problem than delay time.

Accordingly, services are differentiated according to loss of data bursts. An extra offset time is conventionally introduced to provide differentiated services. In this case, a control packet is sent much earlier in order to reserve resources in advance and transfer data bursts having a high priority. In this manner, by securing output resources for data bursts having higher priority earlier than for data bursts having lower priority, class-differentiated loss rates are obtained. However, the higher the priority of a data burst, the longer it is delayed prior to transmission.

In another conventional method, when different classes are in contention for output resource, they are arbitrarily blocked at a preset rate. This enables loss rates for the classes to differ from one another in the optical burst switching network, but the loss rates are too high and resources of the optical burst switching network cannot be used efficiently.

In another conventional method, input packets of each class are laid at a pre-selected location in optical bursts. When a contention between data bursts occurs, a portion of the data burst containing a packet having lower priority is cut out and a portion of the data burst containing a packet having higher priority is transferred, thus increasing the transmission success rate of higher priority packets.

In another conventional method, a group of wavelengths that can be used by different classes is virtually divided and dynamically re-arranged according to a state of an optical burst switching network, so that when there are classes having a high priority, wavelengths are preoccupied so as not to be occupied by traffic having a low priority, thus providing class-differentiated service.

Since, in such conventional service differentiating schemes, an overall blocking rate in the optical burst switching network is divided for each class, classes having higher priority have a lower blocking rate and classes having lower priority are subject to a relatively higher blocking rate. Accordingly, there is need for a scheme capable of providing differentiated service with a low blocking rate for all classes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus and method for transferring data bursts in an optical burst switching network which are capable of efficiently transferring data bursts with a minimized delay time and a low loss rate and satisfying various service demands by using a hybrid optical buffer having a feed-forward type buffer shared in each port and a feedback type buffer shared in each node, the hybrid optical buffer using fiber delay lines (FDLs) to provide class-differentiated service in the optical burst switching network.

A first aspect of the present invention provides an apparatus for transferring data bursts in an optical burst switching network including a number of nodes, each node including demultiplexers and multiplexers connected to a plurality of input/output ports for transferring the data bursts through a number of wavelengths, the apparatus comprising: a control module connected between the demultiplexers and the multiplexers for obtaining information in a control packet to sort and confirm service classes of the data bursts according to delay time and blocking rate; a switching unit connected between the demultiplexers and the multiplexers for switching the data bursts to an output port of a corresponding destination node; a first buffer connected between the switching unit and the output port for delaying the sorted and confirmed data bursts from the control module once by a preset delay time according to output resource availability and sending them to the output port when the data bursts are sensitive to delay time; and a second buffer connected between an input and an output of the switching unit for circulating and delaying the sorted and confirmed data bursts up to a preset recirculation limit according to output resource availability and then sending the data bursts to the output port when the data bursts are insensitive to delay time.

Preferably, each of the first and second buffers comprises a plurality of fiber delay lines (FDLs), each FDL providing a preset unit delay time D.

Preferably, the output resource is an output wavelength.

A second aspect of the present invention provides a method for transferring data bursts in an optical burst switching network including a number of nodes, each node including a plurality of input/output ports, the method comprising: (a) sorting and confirming data bursts input via the input ports into a plurality of classes according to sensitivity to delay time and blocking rate; (b) delaying data bursts belonging to a class sensitive to delay time once by a preset delay time and then transmitting the data bursts to the output port; and (c) circulating and delaying data bursts belonging to a class insensitive to delay time up to a preset recirculation limit and then transmitting the data bursts to the output port.

Preferably, in step (b), data bursts belonging to a class sensitive to delay time and blocking rate among the sorted and confirmed classes have a higher priority than data bursts belonging to a class sensitive only to delay time.

Preferably, the data bursts belonging to the class sensitive to delay time and blocking rate are for video conference or video telephone service, and the data bursts belonging to the class sensitive only to delay time are for Voice over Internet Protocol (VoIP) service.

Preferably, in step (c), data bursts belonging to a class insensitive to delay time but sensitive to blocking rate among the sorted and confirmed classes have a higher priority than data bursts belonging to a class insensitive to both delay time and blocking rate.

Preferably, the data bursts belonging to the class insensitive to delay time but sensitive to blocking rate are for file transfer protocol (FTP) service, and the data bursts belonging to the class insensitive to delay time and blocking rate are for web or E-mail traffic service.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1is a block diagram illustrating a node having an apparatus for transferring data bursts in an optical burst switching network according to an exemplary embodiment of the present invention, andFIG. 2is a conceptual diagram illustrating a structure of first and second buffers according to an exemplary embodiment of the present invention

Referring toFIGS. 1 and 2, a node having an apparatus for transferring data bursts in an optical burst switching network according to an exemplary embodiment of the present invention includes demultiplexers100ato100n, wavelength converters150ato150nand200ato200n, a control module250, a first switching unit300, combiners350ato350n,400ato400n, multiplexers450ato450n, first buffers500ato500n, second switching units550ato550n, and a second buffer600.

Each of the demultiplexers100ato100nis connected to one of a number of input ports1to M for transferring optical bursts. The demultiplexers100ato100nreceive the optical bursts from the input ports1to M, separate the optical bursts into a control packet and a data burst, and output them through a number of wavelengths (1 to W, the total number of wavelengths per port being W).

The wavelengths in the ports are connected to a next node by the multiplexers450ato450nand the demultiplexers100ato100n. Further, the wavelengths are respectively connected to the wavelength converters150ato150nand200ato200n. The wavelength converters freely select available wavelengths and transmit the data bursts through the wavelengths, which need not be continuous.

The wavelength converters150ato150nand200ato200nare connected to the output of the demultiplexers100ato100nfor receiving the data bursts separated by the demultiplexers100ato100nand changing wavelengths of the data bursts so that each data burst has an independent wavelength.

The control module250is connected between the demultiplexers100ato100nand the multiplexers450ato450n. The control module250receives the control packet separated by the demultiplexers100ato100nand recognizes a destination node of the data bursts from information in the control packet. Further, the control module250sorts and confirms service classes of the data bursts according to sensitivity to delay time and blocking rate.

Particularly, an edge node determines the service classes in consideration of requirements regarding delay time and blocking rate in creating the control packet, and stores information about the service classes in the control packet. Accordingly, each node can recognize the service classes from the information in the control packet but not detailed information about target delay time and blocking rate.

The first switching unit300is connected to the outputs of the wavelength converters150ato150nand200ato200n, and between the demultiplexers100ato100nand the multiplexers450ato450n. The first switching unit300performs a function of switching each data burst to an output port of a suitable destination node.

The combiners350ato350nand400ato400nare connected between the first and second switching units300and550ato550nand the multiplexers450ato450nfor combining data bursts having the same wavelength output from the first and second switching units300and550ato550nand outputting a combined data burst.

The multiplexers450ato450nare connected between the combiners350ato350nand400ato400nand the output ports1to M for multiplexing the data burst output from the combiners350ato350nand400ato400nand outputting it to a next node via one output port.

The first buffers500ato500nare feed-forward type buffers each shared in each of the output ports1to M, i.e., connected between the first switching unit300and the output ports1to M. When the data bursts classified by the control module250are sensitive to delay time, the first buffers500ato500ndelay the data bursts once by a preset delay time depending on output resource availability (e.g., output wavelength availability), preferably, when the output resource is not available, and then sends the data burst to the destination output port.

Each of the first buffers500ato500nof the feed-forward type is composed of a plurality (BFF) of fiber delay lines (FDLs) A maximum buffer delay time due to the FDLs is equal to the product of the number of FDLs and a unit delay time D.

For example, if the number of FDLs is B, the maximum buffer delay time is B*D. The first buffers500ato500nare accessible only once as shown inFIG. 1. Accordingly, when the output resource and the first buffers500ato500nare not available, traffic, i.e., data bursts, are immediately lost.

The second switching units550ato550nare connected between the first buffers500ato500nand the combiners350ato350nand400ato400nfor switching the data bursts from the first buffers500ato500nto suitable output wavelengths to be sent to the destination node.

The second buffer600is a feedback type buffer shared in each node, i.e., connected between the input and the output of the first switching unit300. When the classified data burst from the control module250is insensitive to delay time, it is circulated through the second buffer600up to a preset recirculation limit depending on output resource availability (preferably, when the output resource is not available), and then sent to the output port.

The second buffer600of the feedback type is composed of a plurality (BFB) of fiber delay lines (FDLs). In this case, a recirculation limit is R. Accordingly, a maximum delay time in the second buffer600is B*D*R. Buffering in the second buffer600is performed by data bursts circulating through the second buffer up to the recirculation limit when the output resource is not available.

As shown inFIG. 2, the first and second buffers500ato500nand600all have the same structure. Each buffer includes B fiber delay lines (FDLs), which can store traffic, i.e., data bursts for a time corresponding to each length. Thus, the buffer can provide a maximum delay time corresponding to B*D.

FIG. 3is a flowchart illustrating a process of transferring data bursts belonging to a class sensitive to delay time in a method for transferring data bursts in an optical burst switching network according to an exemplary embodiment of the present invention. The process is performed by the control module250ofFIG. 1, unless stated otherwise.

Referring toFIG. 3, when data bursts are input via the input ports1to M ofFIG. 1(S100), availability of output resource (e.g., output wavelengths) is checked (S101). When the output resource is available, the data bursts are successfully transferred (S102). Otherwise, classes of the data bursts, which are variously classified depending on whether the data bursts are sensitive to delay time and blocking rate, is confirmed (S103). In this case, the classes of data bursts can be confirmed through information contained in a control packet.

When it is determined in step S103that the data burst belongs to class1(which is sensitive to both delay time and blocking rate), it is determined whether the first buffers500ato500nof a feed-forward type ofFIG. 1, shared at the output ports1to M ofFIG. 1, are available (S104).

When it is determined in step S104that the first buffers500ato500nare available, the data bursts are successfully transferred (S102) by using the first buffers500ato500nand reserving the output resource, i.e., performing buffering (S105).

When the first buffers500ato500nare not available, it is determined whether class2data bursts (which are sensitive only to delay time) are using the first buffers500ato500n(S106). When the class2data bursts are using the first buffers500ato500n, the class2data bursts are discarded, and new class1data bursts use the first buffers500ato500nand the output resource. That is, the class1data bursts preempt the first buffers500ato500nhaving class2(S107).

In this case, the class2data bursts are lost. In this manner, the class1data bursts are serviced with the highest priority with regard to delay time and blocking rate.

On the other hand, when it is determined in step S106that the first buffers500ato500nare all used by class1data bursts, the input class1data bursts are not successfully transferred (S108).

When it is determined in step S103that the input data bursts belongs to class2(which are sensitive only to delay time), it is determined whether the first buffers500ato500nof a feed-forward type are available (S109). When the first buffers500ato500nare not available, the process returns to step S108and data burst transmission fails.

When it is determined in step S109that the first buffers500ato500nare available, contention is avoided by using the first buffers500ato500nand the output resource is reserved. That is, buffering is performed (S110). It is also determined whether the first buffers500ato500nare preoccupied by class1prior to complete transmission to the output ports1to M, i.e., whether class1data bursts attempt to use the first buffers500ato500n(S111). When class1data bursts attempt to use the first buffers500ato500n, the buffered class2data bursts concede the buffers to the class1data bursts and are lost. That is, the process returns to step S108and data burst transmission fails. Otherwise, the process returns to step S102and the data bursts are successfully transferred.

Meanwhile, class1indicates data bursts sensitive to both delay time and blocking rate. Examples of application services corresponding to class1are video telephone and video conference services. Class2indicates data bursts sensitive only to delay time. An example of an application service corresponding to class2is Voice over Internet Protocol (VoIP).

FIG. 4is a flowchart illustrating a process of transferring data bursts belonging to a class insensitive to delay time in a method for transferring data bursts in an optical burst switching network according to an exemplary embodiment of the present invention. The process is performed by the control module250ofFIG. 1unless stated otherwise.

Referring toFIG. 4, when data bursts are input via the input ports1to M ofFIG. 1(S200), it is determined whether output resource (e.g., output wavelengths) is available (S201). When the output resource is available, the input data bursts are successfully transferred (S202). Otherwise, the classes of the data bursts, which indicate the data bursts' sensitivity to delay time and blocking rate, are confirmed (S203). In this case, the classes of the data bursts may be confirmed using information contained in the control packet.

When it is then determined in step S203that the input data burst belongs to class3(which are insensitive to delay time and sensitive to blocking rate), it is determined whether the second buffer600of a feedback type ofFIG. 1, shared in the node, is available (S204).

When it is determined in step S204that the second buffer600is available, it is determined whether a recirculation count is less than a preset recirculation limit (S205). When the recirculation count is more than the recirculation limit, transmission of the input data burst fails (S206). When the recirculation count is less than the recirculation limit, buffering is performed in the second buffer600(S207) and the process returns to step S201.

When it is determined in step S204that the second buffer600is not available, the class4data bursts all use the second buffer600(S208). When there are no buffered class4data bursts (which are insensitive to both delay time and blocking rate), the process returns to step S206and transmission of data bursts belonging to the input class3fails.

When it is determined in step S208that the class4data bursts are buffered in the second buffer600, the class4data bursts are discarded and the process proceeds to step S207where new class3data bursts are buffered in the second buffer600.

Particularly, a determination is made as to whether to determine output resource availability upon buffering completion, not buffering initiation, and then transmit the data bursts to the output port, or to keep the data burst in the second buffer600. That is, the data bursts kept in the second buffer600are finally blocked when the output resource is not available until the recirculation count reaches the preset recirculation limit.

Meanwhile, when it is determined in step S203that the input data bursts belong to class4, it is determined whether the second buffer600of a feedback type is available (S210). When the second buffer600is not available, the process returns to step S206and data burst transmission fails.

When it is determined in step210that the second buffer600is available, it is determined whether the recirculation count is less than the preset recirculation limit (S211). When the recirculation count is less than the recirculation limit, the process returns to step S206and transmission of the input data bursts fails. When the recirculation count is less than the recirculation limit, buffering is performed in the second buffer600(S212) and it is determined whether the second buffer600is preoccupied by class3, i.e., whether class3data bursts attempt to use the second buffer600(S213). If class3data bursts attempt to use the second buffer600, the process returns to step206and the buffered class4data bursts concede the buffer to the class3data bursts and are lost. Otherwise, the process returns to step S201where it is determined whether the output resource is available.

That is, newly input data bursts or class4data bursts repeatedly circulated in the second buffer600of a feedback type remain in the second buffer600until the output resource is available. If the class3data bursts attempt to use the second buffer600in which the class4data bursts are buffered due to insufficient buffering space, the second buffer600is preempted by the class3data bursts and the class4data bursts are blocked.

Meanwhile, class3indicates data bursts insensitive to delay time and sensitive to blocking rate. An example of an application service corresponding to class3is file transfer protocol (FTP). Class4indicates data bursts insensitive to both delay time and blocking rate. Examples of application services corresponding to class4are web traffic and E-mail traffic.

FIG. 5is a graph of blocking rate versus unit delay time of a fiber delay line (FDL) for different classes when a provided load per wavelength is 0.5 in a method for transferring data bursts in an optical burst switching network according to an exemplary embodiment of the present invention.

It can be seen fromFIG. 5that, in each class, as a unit delay time of a fiber delay line (FDL) increases, a blocking rate, i.e., loss rate, decreases. However, the loss rate increases at a certain point of time.

If the fiber delay line (FDL) is short, the buffer is insufficient to absorb a contention time at the output port and the loss rate increases. On the other hand, as the FDL gets longer, the buffer becomes large enough to absorb the contention time and the loss rate decreases.

However, if the FDL increases to a certain length, the loss rate increases again. This is because the data bursts occupying the first buffers500ato500nof a feed-forward type reserve the output resource for an unnecessarily long time and accordingly even though there is available output resource, it is not allocated to newly input data bursts.

Notwithstanding, it can be seen that the four classes have sufficiently different loss rates. InFIGS. 3 and 4, the unit delay time of the FDL, zero (0), indicates non-use of the buffer. It can be seen that a great gain is obtained by using the buffer.

FIG. 6is a graph of average delay time versus provided load per wavelength for different classes when a unit delay time of an FDL is 0.3 in a method for transferring data bursts in an optical burst switching network according to an exemplary embodiment of the present invention.

It can be seen fromFIG. 6that as a provided load per wavelength increases, the delay time increases. That is, as the provided load increases, the number of lost data bursts increases, such that an additional buffer is required and a buffering delay time increases.

That is, classes1and2which are sensitive to delay time exhibit a significantly short delay time compared to classes3and4which are insensitive to delay time. However, in the case of class4, delay time is short for large provided load. This is because most data bursts concede resources to data bursts belonging to another class that is sensitive to blocking rate in a high load state and do not frequently use the buffer.

According to the apparatus and method for transferring data bursts in an optical burst switching network as described above, it is possible to efficiently transfer data bursts with a minimized delay time and a low loss rate and satisfy various service demands, by using a hybrid optical buffer having a feed-forward type buffer shared in each port and a feedback type buffer shared in each node, the hybrid optical buffer using fiber delay lines (FDLs) to provide class-differentiated service in the optical burst switching network.

While the present invention has been described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in from and detail may be made therein without departing from the scope of the present invention as defined by the following claims.