Abstract:
An optical packet switching system includes: an optical packet transmitter device configured to transmit an optical packet signal; and an optical packet switching device configured to route and output an input optical packet signal. The optical packet transmitter device is configured to adjust gap time between optical packets transmitted. The optical packet transmitter device adjusts the gap time to a fixed value defined by time required for switching in the optical packet switching device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japanese Patent Application No. 2010-280659, filed on Dec. 16, 2010. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical packet switching system in which packet-by-packet optical packet switching is enabled and to an optical packet transmitter device used in the optical packet switching system. 
     2. Description of the Related Art 
     The technology of switching the path depending on the wavelength in an optical transmission system based on wavelength division multiplexing (WDM) and by employing a wavelength selective switch (WSS) is in practical use. One of the next-generation technologies studied is an optical packet switching system in which the path is switched in smaller units, namely, IP packets (10 Gigabit Ethernet (registered trademark) signals, etc.). Each packet is converted in format into an optical packet and routed by using an ultrahigh-speed optical switch (see e.g., patent document No. 1). 
     When the transmission is based on IP packets, no significant information is transmitted absent any data so that the bandwidth is wasted accordingly. If the optical packet switching system is realized, however, any idle time in which data is absent can be occupied by another packet. Therefore, the optical packet switching system promises the possibility of dramatically increasing the bandwidth usage efficiency of the transmission path and is envisaged as a technology of the future.
     [patent document No. 1] JP 2008-235986   

     In the optical packet switching system, there is a need to secure gap time between optical packets commensurate with time required for the switching of optical packet signals. 
       FIG. 1  shows the gap time between optical packet signals.  FIG. 1  shows a client signal, optical packet signals at wavelengths λ 1 -λ 10  produced by causing the client signal to branch, and how the optical packet switching device is turned on or off. In  FIG. 1 , “PA” denotes a preamble and “IFG” denotes an inter-frame gap. 
     Referring to  FIG. 1 , optical packet signals are output at time intervals commensurate with the duration of the Ether frame of the client signal. As shown in  FIG. 1 , time required for switching in the optical packet switching device is secured and optical packet signals can be properly routed, by providing gap time between optical packet signals. 
     In addition to the client signal, an optical packet signal includes optical routing information, etc. For this reason, the length of an optical packet signal is not 1/N the frame length of the client signal even if the client signal is wavelength-divided by N (N is an integer equal to or greater than 2). The length of an optical packet signal is longer than 1/N the frame length. As the length of an optical packet signal at each wavelength grows larger than the length of a client signal, the gap time will be reduced with the result that sufficient time cannot be secured for switching.  FIG. 2  shows how an optical packet signal is discarded because of failure to secure sufficient time for optical switching. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the background as described above and a purpose thereof is to provide a technology capable of reducing the ratio of discarded optical packet signals. 
     The optical packet switching system according to an embodiment of the present invention comprises: an optical packet transmitter device configured to transmit an optical packet signal; and an optical packet switching device configured to route and output an input optical packet signal. The optical packet transmitter device is configured to adjust gap time between optical packets transmitted. 
     Another embodiment of the present invention relates to an optical packet transmitter device. The device is for use in an optical packet switching system including an optical packet switching device, and is configured to adjust gap time between optical packets transmitted. 
     Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, systems, computer programs, data structures, and recording mediums may also be practiced as additional modes of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  shows the gap time between optical packets; 
         FIG. 2  shows the gap time between optical packets; 
         FIG. 3  shows an optical packet switching system according to the first embodiment; 
         FIG. 4  illustrates the operation of the optical packet transmitter device in the optical packet switching system according to the first embodiment; 
         FIG. 5  illustrates the operation of the optical packet switching device in the optical packet switching system according to the second embodiment; and 
         FIG. 6  shows an optical packet switching system according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     A description will be given of an embodiment of the present invention with reference to the drawings. 
     (First Embodiment) 
       FIG. 3  shows an optical packet switching system according to the first embodiment of the present invention. As shown in  FIG. 3 , an optical packet switching system  100  comprises a 2-input×2-output optical packet switching device  10  and an optical packet transmitter device  12 . 
     A description will first be given of the optical packet switching device  10  The optical packet switching device  10  is provided with the function of switching the path of, i.e., routing, an input optical packet signal and outputting the signal accordingly. As shown in  FIG. 3 , the optical packet switching device  10  comprises an optical switch  14 , an optical switch control unit  16 , a first optical coupler  18 , a second optical coupler  20 , a first optical delay line  22 , and a second optical delay line  24 . 
     The optical packet signal input to the optical packet switching device  10  via an optical transmission path  17  is input to the first optical coupler  18 . The first optical coupler  18  causes the optical packet signal to branch into two signals. One of the optical packet signal from the branch is input to a first input port  14   a  of the optical switch  14  via the first optical delay line  22 . The other optical packet signal is input to the optical switch control unit  16 . 
     An optical packet signal input to the optical packet switching device  10  via another optical transmission path  19  is input to the second optical coupler  20 . In the first embodiment, the optical transmission path  19  is connected to the optical packet transmitter device  12 . The second optical coupler  20  causes the optical packet signal to branch into two signals. One of the optical packet signals from the branch is input to a second input port  14   b  of the optical switch  14  via the second optical delay line  24 . The other optical packet signal is input to the optical switch control unit  16 . 
     The optical switch control unit  16  extracts routing information from the input optical packet signal and outputs a control signal to the optical switch  14  in accordance with the routing information. As shown in  FIG. 3 , the optical switch control unit  16  is provided with a first optical/electrical converter unit  26 , a second optical/electrical converter unit  28 , a first analyzer unit  30 , a second analyzer unit  32 , and an output competition determination unit  34 . 
     The first optical/electrical converter unit  26  converts an optical packet signal received from the first optical coupler  18  into an electrical signal. The second optical/electrical converter unit  28  converts an optical packet signal received from the second optical coupler  20  into an electrical signal. 
     The first analyzer unit  30  analyzes the header of the packet signal received from the first optical/electrical converter unit  26  and detects the routing information. The second analyzer unit  32  analyzes the header of the packet signal received from the second optical/electrical converter unit  28  and detects the routing information. 
     The output competition determination unit  34  determines whether the optical packet should be transmitted or discarded and outputs an optical switch control signal based on the result of determination. For example, it will be assumed that optical packets are input to the first input port  14   a  and the second input port  14   b  and that the destination of output of the two optical packets is the first output port  14   c . In this case, the output competition determination unit  34  determines whether the two optical packets compete. In other words, the output competition determination unit  34  determines whether the two optical packets concur in time. If the two optical packets compete, the optical packet arriving first is transmitted and the following optical packet is discarded. 
     In this embodiment, the optical switch  14  is a 2×2 optical switch For example, the optical switch  14  may be implemented by a semiconductor optical amplifier. The optical switch  14  is controlled to be turned on or off by an optical switch control signal from the optical switch control unit  16  and routes and outputs the input optical packet signal. 
     The first optical delay line  22  and the second optical delay line  24  delay one of the optical packet signals from the branch for a duration required for the optical switch control unit  16  to generate an optical switch control signal. By providing the first optical delay line  22  and the second optical delay line  24 , on/off of the optical switch  14  can be controlled to be synchronized with the timing of arrival of the optical packet signals at the optical switch  14 . 
     The optical packet signal output from the first output port  14   c  of the optical switch  14  is output to a first optical transmission path  21 . The optical packet signal output from the second output port  14   d  of the optical switch  14  is output to another optical transmission path  23 . 
     A description will now be given of the optical packet transmitter device  12 . The optical packet transmitter device  12  is provided with the function of causing a client signal received from a client device (not shown) to branch into optical packet signals at a plurality of wavelengths and transmitting the signals. As shown in  FIG. 3 , the optical packet transmitter device  12  is provided with a client signal processing unit  36 , a packet generation unit  38 , an electrical/optical converter unit  40 , and an arrayed waveguide grating (AWG)  42 . 
     The client signal processing unit  36  subjects a received client signal to a predetermined process before outputting the signal to the packet generation unit  38 . The client signal processing unit  36  buffers the received client signal. 
     The packet generation unit  38  is provided with a signal length counting unit  44 , a signal branch processing unit  46 , a routing information addition unit  48 , a transmission processing unit  50 , and a delay processing unit  52 . 
     The signal length counting unit  44  counts the length of the received signal. The signal branch processing unit  46  causes the client signal to branch into a predetermined number of (10, in the case of the first embodiment) packet signals based on information on the length of the client signal from the signal length counting unit  44 . 
     The routing information addition unit  48  adds routing information to each of the packet signals from the branches. The transmission processing unit  50  adds a preamble to the head of each packet signal and outputs the signal to the electrical/optical converter unit  40 . The delay processing unit  52  controls the timing according to which the transmission processing unit  50  outputs the packet signal to the electrical/optical converter unit  40 . By controlling the timing of outputting the packet signal using the delay processing unit  52 , the gap time between the optical packet signals can be adjusted. 
     10 packet signals generated by the packet generation unit  38  are input to the electrical/optical converter unit  40 . The electrical/optical converter unit  40  comprises 10 E/O modules  40 - 1 - 40 - 10 . The E/O module convert the packet signals from the packet generation unit  38  into optical packet signals at wavelengths λ 1 -λ 10 . The optical packet signals output from the electrical/optical converter unit  40  are input to the AWG  42 . The AWG  42  subjects the optical packet signals at the wavelengths λ 1 -λ 10  to wavelength multiplexing and outputs the signals to the optical transmission path  19 . 
       FIG. 4  illustrates the operation of the optical packet transmitter device in the optical packet switching system according to the first embodiment. According to the first embodiment, the delay processing unit  52  of the optical packet transmitter device  12  controls the timing of outputting the packet signal so that the gap time between optical packet signals is a predetermined fixed value. The fixed value is determined in accordance with time required for the optical packet switching device  10  to perform a switching process (switching time) and is defined to be longer than the time required for optical switching. 
       FIG. 4  shows that the gap time of a desired length is secured between optical packet signals by causing the delay processing unit  52  to delay the start timing of the optical packet signals with respect to the start timing of the Ether frame. This secures sufficient switching time in the optical packet switching device  10  so that the ratio of discarded packets is reduced. 
     (Second Embodiment) 
       FIG. 5  shows the operation of the optical packet switching device in the optical packet switching system according to a second embodiment of the present invention. The components of the optical packet switching system according to the second embodiment are basically identical to the components of the optical packet switching system shown in  FIG. 3  and so are denoted by like symbols. A detailed description is omitted. 
     The operation of the delay processing unit  52  of the optical packet transmitter device  12  in the optical packet switching system  100  according to the second embodiment differs from that of the first embodiment. In the first embodiment, the delay processing unit  52  adjusts the timing of outputting an optical packet signal so that the gap time between packet signals has a fixed value. In the second embodiment, the delay processing unit  52  controls the timing of outputting a packet signal and adjusts the gap time in accordance with the length of a client signal (length of an Ethernet frame). Information on the length of the client signal is obtained from the signal length counting unit  44 . More specifically, the delay processing unit  52  adjusts the gap time so that transmission of an optical packet signal occurs substantially in the middle of the client signal, as shown in  FIG. 5 . 
       FIG. 5  shows that optical packet signals are located substantially in the middle of the Ether frame and sufficient gap time is secured between optical packet signals, by delaying the start timing of optical packet signals from the start timing of the Ether frame. According to the second embodiment, additional margin is created for switching control in the optical packet switching device  10  so that the ratio of discarded packets is further reduced. 
     (Third Embodiment) 
       FIG. 6  shows an optical packet switching system according to a third embodiment of the present invention. Those components of the optical packet switching system  100  shown in  FIG. 5  that are identical or corresponding to the components of the optical packet switching system shown in  FIG. 3  are denoted by like symbols and a detailed description is omitted. 
     In optical packet switching in the optical packet switching device  10 , optical packet signals from a specific input port could be more likely to be discarded than those from the other ports. The third embodiment addresses this by feeding back information on the ratio of discarded optical packets to the optical packet transmitter device  12  and adjusting the gap time so that the ratio of discarded packets is reduced. 
     The optical switch control unit  16  of the optical packet switching device  10  in the optical packet switching system  100  according to the third embodiment further comprises a discarded ratio calculation unit  54  in addition to the components shown in  FIG. 3  of the first optical/electrical converter unit  26 . 
     Information on the number of optical packets received at the respective ports of the optical switch  14  is input to the discarded ratio calculation unit  54  from the first analyzer unit  60  and the second analyzer unit  62 . Information on the number of discarded optical packets is input to the discarded ratio calculation unit  54  from the output competition determination unit  34 . The discarded ratio calculation unit  54  calculates the ratio of discarded optical packets for each input port, based on the information. The optical packet switching device  10  routes optical packets by reading transmission source information and destination information from the header of optical packet signals. The ratio of discarded packets can be calculated for each input port of the optical switch  14  by referring to the transmission source information of discarded optical packet signals. 
     Information on the ratio of discarded packets calculated in the discarded ratio calculation unit  54  is transmitted to the respective optical packet transmitter devices connected to the optical packet switching device  10 . Since the optical packet transmitter device  12  is connected to the second input port of the optical switch  14  according to this embodiment, the ratio of discarded optical packet signals received at the second input port  14   b  is transmitted to the optical packet transmitter device  12 . Meanwhile, the radio of discarded optical packet signals received at the first input port  14   a  is transmitted to the optical packet transmitter device (not shown) connected to the first input port  14   a.    
     As shown in  FIG. 6   a , information on the ratio of discarded packets is input to the delay processing unit  52  of the optical packet transmitter device  12 . The delay processing unit  52  adjusts the gap time between optical packet signals based on the information on the ratio of discarded packets. More specifically, if the ratio of discarded packets is equal to or greater than a predetermined reference value, the delay processing unit  52  delays the timing of outputting packet signals by, for example, 1 byte or several clocks in each step so as to extend the gap time. The delaying step is repeated until the ratio of discarded packets fed back from the discarded ratio calculation unit  54  of the optical packet switching device  10  is below the predetermined reference value. This can suitably reduce the ratio of discarded packets. 
     If the gap time is extended longer than a standard packet length of client signals, some optical packets cannot be transmitted. Therefore, it is desirable that the maximum value of the gap time is approximately the value obtained by subtracting the minimum necessary gap time from the standard packet length of client signals. In this embodiment, the optical packet switching device  10  is assumed to be a 2×2 device. In the case of an optical packet switching device configured for more inputs and more outputs, the likelihood of congestion of optical packet signals will be increased. The optical packet switching device  10  according to the embodiment is particularly suitable for such a case. 
     Described above is an explanation based on an exemplary embodiment. The embodiment is intended to be illustrative only and it will be obvious to those skilled in the art that various modifications to constituting elements and processes could be developed and that such modifications are also within the scope of the present invention.