Patent Publication Number: US-2006013144-A1

Title: Optical switch

Description:
This invention relates to an optical switch and in particular but not exclusively an optical switch for use in an optical burst switching (OBS) network.  
      In burst switching technology networks, such as, optical burst switching (OBS) networks, data bursts, each of which is made up of multiple packets, are switched optically at switches or routers in the OBS network. Each data burst has an associated control packet called the burst header packet. A burst header packet travels an offset time ahead of its data burst along the same route and configures the optical switches to route the data burst. Thus, each burst header packet travels an offset time ahead of its associated data burst and establishes a path for the data burst.  
      Each burst header packet contains the information, such as a label, the length of the burst, and the offset time, required to route the associated data burst through the optical core backbone. A burst header packet is sent via out-of-band in-fiber control channels and is processed electronically at the controller of a switch to make routing decisions, such as selection of an outgoing fibre and wavelength. The switch is configured accordingly to switch the data burst, which is expected to arrive after the designated offset time. When it arrives at the switch, the data burst is switched entirely in the optical domain.  
      OBS networks that support high data rates require correspondingly high switching speeds at the network switches. Indeed, a switching time on the order of nanoseconds or even picoseconds is desirable in some networks. In an OBS network, the switching time introduces the constraint on the minimum time difference between the transmission across a switch of two consecutive bursts on a wavelength channel. This time constraint is illustrated in  FIG. 1  which shows a first data burst la of transport wavelength λ 1T  and its corresponding header packet burst  1   b  of signalling wavelength λ 1S  travelling ahead of a second data burst  2   a  of transport wavelength λ 1T  and its corresponding header packet burst  2   b   1S  of signalling wavelength λ 1S . If the switching time of a switch used to route the first  1   a  and second  1   b  data bursts is t s , then once the first data burst  1   a  has been switched by the switch, the second data burst  1   b  can be switched only after a time t s  has elapsed. If the second data burst arrives at the switch before the switching time t s  has elapsed, it is blocked. Such blocking occurs naturally in an optical switch for if an input port is not connected to an output port, the power of the optical signal it carries is simply lost. During the switching time, no information at the transport wavelengths corresponding to the input and output ports involved in the switching operation can be sent across the switch, so wavelength capacity is wasted. In some networks a relatively long switching time for a wavelength supporting a relatively low data rate ought not to be problematical. However, a switching time on the order of milliseconds for a wavelength supporting a data rate of Giga Bits per Second, would result in the lost opportunity of Mega Bits of information being switched during the switching time.  
      Known cheap switching fabrics such as Microelectromechanical Systems (MEMS) provide switching times on the order of milliseconds, which is not short enough for the data rate demands of OBS and APON networks that operate with link capacities on the order of Giga bits per second. Roughly speaking, the switching time for DWDM systems should be of the same order as the time between the transmission of two bits, or in other words, the inverse of the link capacity.  
      There are some types of switching fabrics that have switching times on the order of nano or even pico seconds which are short enough for OBS and APON networks. However, the current price of such fabrics is relatively expensive, being upwards of ten times or more expensive than a MEMS switching fabric. In addition, switching matrixes having more than two ports are made from cascading several switching elements, which leads to the problem of insertion loss in large switching matrixes.  
      In general then, a choice exists between relatively cheap switches that have long switching times resulting in inefficient bandwidth usage and increased blocking probability, and switches have a short switching time but which are relatively expensive.  
      Embodiments of the present invention aim to alleviate the above mentioned problems.  
      According to the present invention there is provided an optical switch for switching optical data bursts in a communications network, the switch comprising: a plurality of first optical switching matrixes each comprising a plurality of input ports and a plurality of output ports; a plurality of second optical switching matrixes each comprising at least one input port and a plurality of output ports and wherein each of the plurality of second optical switching matrixes has a different output port connected to a respective input port of each of the plurality of first optical switching matrixes; a plurality of optical combiners each comprising at least one output port and a plurality of input ports and wherein each of the plurality of optical combiners has a different input port connected to a respective output port of each of the plurality of first optical switching matrixes; and a controller for controlling the optical switch to switch optical data bursts from the input ports of the second optical switching matrixes to the output ports of the optical combiners across the plurality of first optical switching matrixes, the controller controlling the switch such that the plurality of first optical switching matrixes operate in parallel to switch optical data bursts across the switch. 
    
    
      The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings, in which:  
       FIG. 1  which has already been described illustrates data bursts and corresponding headers in an optical network;  
       FIG. 2  illustrates an optical switch;  
       FIG. 3  illustrates data bursts and corresponding headers in an optical network. 
    
    
      Turning now to  FIG. 2  of the accompanying drawings, an optical switch  10  comprises first  11  and second  12  low speed large switching matrixes. Each of the switching matrixes  11  and  12  comprises N input ports IP 1  to IP N  and N output ports OP 1  to OP N . The switching matrixes  10  and  11  may thus be thought of as being N×N switches. Typically, the first  11  and second  12  switching matrixes have a switching speed of the order of milliseconds.  
      The switch  10  also comprises a plurality of sets of fast switching matrixes  13   1  to  13   M , of which the first set  13   1  and the M th  set  13   M  are illustrated. Each of the M sets of fast switching matrixes comprises K fast switching matrixes, designated as  13   m1  to  13   mK . Each of the fast switching matrixes  13   m1  to  13   mK  in a given set comprises a respective single input port which ports are designated as IP 13m1  to IP 13mK . Furthermore, each of the fast switching matrixes  13   m1  to  13   mK  comprises a respective pair of output ports, which pairs are designated as OP 13m1  to OP 13mK . Typically, each of the fast switching matrixes  13   m1  to  13   mK  within a given set has a switching speed on the order of nanoseconds or even picoseconds.  
      Each of the fast switching matrixes  13   m1  to  13   mK  within a given set has one of its output ports connected to a respective one of the input ports of the first low speed large switching matrix  11 , and the other of its output ports connected to a respective one of the input ports of the second low speed large switching matrix  12 .  
      The input ports of the fast switching matrixes  13   m1  to  13   mK  in any given set are connected to a respective one of a plurality of multi-mode input fibers designated  14   1  to  14   M . Each of the multi-mode input fibres  14   1  to  14   M  supports K discrete wavelength modes λ 1  to λ K  or channels on which data bursts can be transmitted. Each of the input ports of the fast switching matrixes  13   m1  to  13   mK  in a given set is connected to receive data bursts having a particular one of the wavelengths λ 1  to λ K  supported by the multi-mode input fiber  14   1  to  14   M  connected to that set.  
      On the output side, the optical switch  10  comprises a plurality of sets of optical combiners  15   1  to  15   M , of which the first set  15   1  and the M th  set  15   M  are illustrated. Each of the M sets of optical combiners comprises K optical combiners, designated as  15   m1  to  15   mK . Each of the optical combiners  15   m1  to  15   mK  in a given set comprises a respective pair of input ports, which pairs are designated as IP 15m1  to IP 15mK  and a respective single output port, which ports are designated as OP 15m1  to OP 15mK .  
      Each of the optical combiners  15   m1  to  15   mK  in a given set has one of its input ports connected to a respective one of the output ports of the first low speed large switching matrix  11 , and the other of its input ports connected to a respective one of the output ports of the second low speed large switching matrix  12 .  
      The output ports of the optical combiners  15   m1  to  15   mK  in any given set are all connected to a respective one of a plurality of multi-mode output fibers designated  16   1  to  16   M . Each of the multi-mode input fibres  16   1  to  16   M  supports K discrete wavelength modes λ 1  to λ K  or channels on which data bursts can be transmitted from the switch  10 . Each of the output ports of the optical combiners  15   m1  to  15   mK  in any given set is for transmitting from the switch data bursts having a particular one of the wavelengths λ 1  to λ K  supported by the multi-mode output fiber  16   1  to  16   M  connected to that set.  
      A control processor  17  processes information contained in header bursts to configure the slow switching matrixes  11  and  12  and the fast switching matrixes, to perform the switching of data bursts across the switch  10 .  
      The architecture of the optical switch  10  takes advantage of the fact that in an OBS network a header burst packet always precedes a data burst in order to reserve bandwidth for the burst. Consequently, the optical switch knows in advance of a data burst actually arriving at the switch at what time the data burst is due. The structure of the first  11  and second  12  switching matrixes exploits this fact to switch data burst across the switch  10  in parallel.  
      During a switching operation, the switch  10  uses only one of the first  11  and second  12  switching matrixes to connect a pair of input and output fibers to switch a current data burst across the switch  10 . When one of the first  11  and second  12  switching matrixes is active in transmitting a current data burst across the switch, if a header burst arrives to reserve bandwidth for a second subsequent data burst, the controller  17  begins to configure the currently non active of the first  11  and second  12  large switching matrixes to switch this subsequent data burst across the switch.  
      When the configuration of the currently non active large switching matrix is complete, the controller  17  configures the fast switching matrix having the input port at which the subsequent data burst is destined to arrive, so that its input port is connected to the currently non active large switching matrix. Thus when the second data burst arrives at the switch  10  it is switched across the switch via the currently non active large switching matrix.  
      Since the time between header and burst transmission (the so called offset time) is much bigger than the transmission time of a bit, the two large matrixes need not be comprised of an overly quick and hence expensive switching fabric. A switching fabric having a switching time on the order of milliseconds should be sufficient in most cases. The switching fabric used for the faster switches could be based on Holographic Switching technology or on Electro-absorption Modulator (EAM) technology. A key point is, that the above describes switch architecture, is cheaper than a switch architecture based on a fast N×N Holographic or EAM switch. This will be discussed in more detail below.  
      A specific example of the Switch  10  in operation will now be discussed with reference to  FIG. 3 .  FIG. 3  illustrates a first data burst  30   a  of transport wavelength λ 1T  and its corresponding header packet burst  30   b  of signalling wavelength λ 1S  travelling ahead of a second data burst  31   a  of transport wavelength λ 1T  and its corresponding header packet burst  31   b  of signalling wavelength λ 1S .  
      Assume that the first header packet burst  30   b  has previously arrived at the switch  10  which has processed the information contained in the header packet and is now in the process of transferring the first data burst  30  across the switch  10  from a given input port, say input port IP 1321  connected to the input fiber  14   1 , to a given output port, say output port OP 1531  connected to the output fiber  16   1 , via the first large switching matrix  11 .  
      When the header packet  31   b  of the second data burst  31   a  arrives at the switch  10 , the controller  17  processes the header packet to determine on which input port the second data burst  31   a  will arrive on and on which output port the second data burst will need to leave on. In this example assume that the second data burst  31   a  is to arrive on the same input port as the first data burst  30   a,  that is input port IP 1321  connected to the input fiber  14   1 , and is to leave on output port OP 1531  connected to the output fiber  16   1 .  
      The controller  17  configures the second switching matrix  12  to switch the second data burst  31   a  from the input port input port IP 1321  to the output port OP 1531 . When the transmission of the first data burst  30   a  across the switch is completed, the fast switching matrix  13   12  connected to the input fiber  14   1  switches its output connection from the first switching matrix  11  to the second switching matrix  12 .  
      Thus, when the second data burst  31   a  arrives at the switch  10  on the input fibre  14   1 , it is transferred across the switch  10  via the second switching matrix  12 , from the input port IP 1321  of the fast switching matrix  13   12  to the output port OP 1531  of the optical combiner  15   12 .to exit the switch on output fiber  16   1 .  
      Importantly, the switching time of the switch  10  is faster than the switching time of the individual fast switching matrixes. This can be understood by considering the following reasoning.  
      The basic actions performed by any switch when switching from an existing input port/output port through connection to a different input port/output port through connection are:  
      1) To process the control information that requests the initiation of the switching operation;  
      2) To check if there sufficient resources available to perform the desired switching operation;  
      3) To generate and coordinate the proper control messages needed to perform the switching operation;  
      4) To physically perform the switching operation.  
      Thus in the context of the above example, to switch the second data burst across the switch  10  via the second switching matrix  12 , steps 1 to 3 may be performed in parallel with the transmission of the first data burst across the switch  10  via the first switching matrix  11 . Therefore, it is only the time taken by step 4, i.e the time taken to physically perform the switching of the second data burst that defines the switching time of the whole switch.  
      For reasons of complexity, fast switching fabrics based on Holographic Switching or EAM can only be used in the manufacture of relatively small individual switching matrixes, for example, matrixes of dimension 2×2. Traditionally, to make larger switching matrixes, a plurality of small individual switching matrixes are cascaded together. Thus, for example, to produce a 16×16 switching matrix, thirty two 2×2 switching matrixes are cascaded together.  
      Table 1 illustrates the number of small switching matrixes required in order to make a N×N optical switch as N increase (column 1), for two different switch architectures, namely, the number of 2×2 switches in a traditional cascaded architecture (column 2); and the number of 1×2 switches in an architecture based on that of the switch  10 .  
      The price of a traditional N×N switch may be defined as: 
 
Price of traditional N×N switch=price of 2×2 switch×number of 2×2 cascaded switches. 
 
      The price of a new N×N switch constructed in accordance with the above described switch architecture may be defined as: 
 
Price of new N×N switch=2×price N×N slow switch+(price of 1×2 switch+price of multiplexer)×number of 1×2 switches used. 
 
      A N×N slow switch is relatively cheap, for example, currently around 5,000 Euros for a MEMS switch, thus the price of a N×N switch constructed in accordance with the above described switch architecture is mainly determined by the price of the fast switching fabric 1×2 switch. Assuming that for a given fast switching fabric, the price of a 1×2 switch is the same as a 2×2 switch (in fact it is even cheaper) it can be understood from table 1 that the new switch architecture is cheaper than the state of the art architecture for switching matrixes larger than 4×4. Moreover the price difference between the two architectures increases almost linearly with the size of the matrix.  
      The invention may be used in other types of optical networks, for example, Adaptive Path Switched Optical Networks of the type described in our co-pending application DE 10339039.1.  
      Having thus described the present invention by reference to preferred embodiments it is to be well understood that the embodiments in question are exemplary only and that modifications and variations such as will occur to those possessed of appropriate knowledge and skills may be made without departure from the scope of the invention as set forth in the appended claims.