The invention relates to a rearrangeably nonblocking, all-optical cross connect device to be used in wavelength routing Wavelength Division Multiplexing (WDM) systems. The algorithm followed by the control mechanism of cross connect is able to accommodate all possible combinations of connection requests on demand. The objective is to use switching in both space and wavelength domains inside the cross connect to achieve the desired rearrangeable non-blocking behaviour. Variable-input-fixed-output type wavelength converters having full-range - conversion-capability have been used for realizing switching in the wavelength domain. Any wavelength-independent space switching technology can be used for carrying out switching in space domain.
One switching block per wavelength is arranged in the developed device. A set of 'F' input (output) ports in every switching block is connected via DEMUXs (MUXs), respectively, to the "F" inbound (outbound) fiber links of the WDM system that are connected to the cross-connect. Therefore the device is similar to the WSXCS (Wavelength Selective Cross Connects) already proposed earlier. However, unlike WSXCs, device does not suffer from wavelength blocking. This is because, unlike WSXCs, there are an additional 'B-l' redundant input/output ports in every switching block for interconnecting the blocks among themselves and thereby eliminate wavelength blocking where B is number of switching blocks. Each of the redundant output ports of a given block is connected from the output of that block to an input port to every other block, without any repetition. Therefore, these redundant ports can also be called as recirculating ports. Interconnection of blocks among themselves also makes the unused output ports of a given block to become usable by all other blocks so that the desired rearrangeably nonblocking behavior can still be achieved with the individual blocks having minimum port count.
Devices of non-blocking ) either in strict or in wide sense) optical cross-connects that have been proposed so far either are of single-switching- components. Viz. optical filters or wavelength converters to achieve the desired non-blocking behaviour . Single-switching-block type devices are not modular. Therefore, scaling up of the
system with increase in demand is not possible. Further, the losses and dynamic range incurred by the channels passing through the cross connect become too high to be tolerated when only one switching block is used to support a very large system. For example, it quite possible that F=10 and number of wavelengths W=100 so that the size of the only block becomes (1000X1000) which is too large to be implementable with allowed ratings of loss and dynamic range. Device of WSXCs that are modular but do not use any tunable component introduce some blocking, e.g. wavelength blocking due to their inherent architectural limitations.
The developed device does not require the tenability feature of any of the above mentioned components and can still achieve the rearrangeably non-blocking behaviour. The device is modular. Use of fixed type of components only makes proposed device inherently more reliable and cost-effective.
The algorithm required to decide the output wavelengths, which the tunable optical filters must be tuned to for achieving the desired non-blocking behaviour is quite complex in the sense that it requires a lot of computation. However, the algorithm required for controlling present device is quite simple and consists of only a few steps. This is the advantage of the present device. Also, it is efficient in the sense that the number of rearrangements required for accommodating a new connection request has been kept at the minimum. Simultaneous arrival of connection requests does not cause any problem. Rather, it is advantageous in the sense that the probability of rearrangements decreases with more and more requests coming simultaneously.
The number of converters used in WSXCs and in the single-space-switching-block type strictly non-blocking cross-connect proposed earlier is 'FW. The number of wavelength converters used in the present device is still 'FW. The single-space-switching-block type strictly non-blocking cross-connect uses variable-input type wavelength converters. WSXCs have to use variable-input-fixed-output type wavelength converters to become non-blocking, which in turn, require tunable laser sources. However, present device uses fixed-output type wavelength converters and is still non-blocking. So the present device is comparable with the devices proposed
earlier in terms of the number and tunability requirements of wavelength converters and still does not suffer from the drawbacks of the other comparable devices. The device does not incur any additional cost on wavelength converters. Since wavelength converters in general and the tunable types in particular are very costly components, present device is cost-effective.
The number of recirculating ports in the individual switching blocks that are required for achieving the desired rearrangeably non-blocking behavior has been minimized in the device for a given number of 'F'.
The developed device is always capable of supporting as many number of wavelengths as there are blocks.
Accommodation of connection-requests in the present device may sometimes require the optical signals to pass through the switching blocks repeatedly upto a maximum of three times. But, smaller size of individual switching blocks makes the optical path lengths, corresponding to a single pass, to become much smaller than the non-modular or single-switching-block type case. Hence, the device is implementable with many different technologies like optical MEMS (both 2D and 3D) or with 2X2 building blocks that use teclmologies like electro-optic or thermo-optic. However, this device is better than the ones proposed earlier, as long as the insertion loss, BER etc. incurred by passing thrice through the switching blocks is less than the non-modular or single-switching-block type case. It is claimed that the above requirements will be met by systems that consist of large number of'F' and 'W\ Hence, it is advantageous to use this device in large systems.
Statement of the invention
According to the present invention there is provided a rearrangeably non-blocking all optical cross-connect device for use in wavelength routing wavelength division multiplexing system comprising:
a demultiplexer connected to a plurality of inbound fibers to divide the optical signal in each channel to specific wavelength;
a plurality of switching blocks connected to the demultiplexer for receiving signals of a specific wavelength;
a plurality of fixed output type wavelength converters connected to the switching block for providing a fixed output to the multiplexer;
a plurality of out bound fibers connected to the multiplexer for achieving desired rearrangeably nonblocking, all-optical cross connect behaviour;
wherein each of said switching blocks have a number of redundant recirculating input ports equal to output ports.
According to the another embodiment there is provided a method for rearrangeably non-blocking cross connect in wavelength routing comprising the steps:
receiving signal from the plurality of inbound fibers at the input port of the demultiplexer;
supply the output of the demultiplexers to the switching blocks;
passing the signals through the switching blocks repeatedly upto a max. of three times;
feeding the output of switching blocks to a fixed output wavelength converters;
feeding the output of the converters to the multiplexers;
feeding the output of multiplexers to the outbound fibers to achieve desired rearrangeably nonblocking, all-optical cross connect behaviour.
The invention is described in brief with reference to the accompanying drawings wherein:
Figure 1 shows a rearrangeably non blocking, all optical cross connect device.
Figure 2 shows output of switch block used to connect remaining switch blocks.
The technique of using recirculating ports to reduce the blocking probability and to increase the utilization of resources like wavelength converters is known the art. It has already been suggested in connection with single-switching-block type devices. The present invention uses a technique in connection with an device which is more or less like WSXCs. Also the minimum number of recirculating ports required for making the cross connect rearrangeably nonblocking have been proposed.
'W of the 'B' blocks used in the present cross connect correspond to the 'W wavelengths in the WDM system, just like we have 'W' blocks in a WSXC. Rest 'K=B-W' redundant blocks are required, additionally, for achieving the desired rearrangeably nonblocking behavior. However, when B=W, no block is redundant. Because, every block would correspond to a specific wavelength in that case. The inequalities that specify the desired relative magnitudes of the quantities 'B', 'W' and 'F' to achieve the rearrangeably nonblocking behavior have been summarized and given further.
ADD/DROP functions can be achieved by reserving one input/output fiber port for local use, in this cross connect. Filters and equalizers can also be placed at suitable positions to increase the spectral purity and limit the dynamic range, respectively, of the optical signals. A slightly modified version of the device given in Figure 1 can also be used by displacing all the wavelength converters from the output side to all the redundant input ports. This modified device will completely eliminate
interchannel cross talk. However, elimination of interchannel cross talk is achieved at the expense of increased inter channel crosstalk. Also, increase in the number of wavelength converters as well as increase in the insertion loss and BER for this modified device have to carefully be taken into account.
Significance of the Symbols Used in Figure-1:
F = Number of inbound/outbound fiber links of the WDM system that terminate at
cross connect. This can take one of the values 2,3,4 i.e., F> 2.
W=Number of wavelengths supported by the WDM system. This can take one of the
values 1,2,3....i.e. W>1.
B= Total number of switching blocks used for providing the desired rearrangeably
nonblocking behaviour. Its value depends on the magnitude of 'F'
K= Number of redundant switching blocks used. Its value depends on the relative
magnitudes of "B" and "W".
Individual blocks (3) are designated as i in Figure - 1. This means that a specific block corresponds to a specific wavelength. Input channels on a specific wavelength i from different inbound fibers (1) come to the top 'F' input ports of the block designated as i after being demultiplexed by a demultiplexer (2). Also, all wavelength converters (4) having the output wavelength i, are connected to the top 'F' output ports of the same block designed as i One redundant output port of every switching block is connected to one of the redundant input ports of the block designated as i (except from the block designated as i itself). Therefore, we can associate one of the redundant output ports in every block that is connected to the block designated as i, with the wavelength i and can them as the i ports of their respective blocks. The wavelengths received by each multiplexer are multiplexed and fed to outbound fiber (6).
According to one embodiment, the signal received from the plurality of inbound fibers (1) fed to demultiplexer (2). The output of demultiplexer is supplied to plurality of switching blocks (3). to enable non-blocking behaviour the signal can be passed through switching blocks repeatedly upto a maximum of three times. The
output of switching blocks is fed to a fixed-output wavelength converters (4). The output of the wavelength converter is fed to multiplexer where the signals are multiplexed and then fed to outbound fibers to achieve the desired rearrangeably non-blocking all optical cross connect.
Inequalities To Be Satisfied:
Certain inequalities must be satisfied if this architecture were to exhibit the desired rearrangeably nonblocking
behaviour and at the same time have minimum port-count in the individual switching blocks. These inequalities are
concerned with the relative magnitudes of the positive integers 'F', 'W, 'B' and 'K' and are summarized below.
(A) B>2F-1
(B) K = B-W => K > 2F-W-1
(C) W>2F-1=>K = 0
Fortunately, the inequality (C) is almost always satisfied in practical WDM systems, where 'F' is of the order of tens and 'W is of the order of hundreds. The inequalities (A) and (B) are within our lj|lrol and can always be satisfied by suitable choice of'B'and'K'.
Observations:
To minimize the number of redundant blocks, we shall do the following steps.
(a) Check if W>2F-1.
(b) If W > 2F-1 is satisfied, then we do not have to use any redundant switching blocks and should use as many switching blocks as there are wavelengths in theWDM system. In other words, we shall have B=W in this case.
(c) If W > 2F-1 is not satisfied then we have to use B=2F-1 number of switching blocks, out of which K=B-W=2F-W-1 are redundant.
(d) Each of these individual blocks will be of same size and will have equal number of input and output ports. The port count of each of them will be 'F+B-l'. Hence, apart from the 'F' ports that are required for connecting a given block to 'F' inbound/outbound fibers, we will have to use 'B-l' number of redundant recirculating input/output ports in every block.
Algorithm For Accommodating Connection Requests On Demand Basis To Achieve The Desired Rearrangeably Nonblocking Behavior:
STEP-1: (Initialization)
(a) Reserve storage for maintaining the status information of all redundant output ports. This status information consists of only one field and can assume one of the following two values; either 'BUSY' or 'FREE'. Hence, we need 'Bx(B-l)' units of memory locations for this purpose. The status of the redundant input ports are maintained automatically because of the existence of the one-to-one correspondence between the redundant input and output ports. Also, the status of the top 'F' input/output ports in every block need not be maintained. Because, care is taken by the Routing & Wavelength Assignment (RWA) algorithm as well as the associated Call Admission Control mechanism to avoid all conflicts by excluding the possibility of assigning the same output channel to two different input channels.
(b) Reserve storage for maintaining the status of all connection requests. The status of a connection request will consist of the following fields:

(i) set-up/take-down
A request can be meant for setting-up a new coonection or for releasing (or taking-down) an existing connection. Also, the fields (ii)-(v) given below assume a unique set of values for a given connection request.
(ii) inbound fiber number (iii) outbound fiber number (iv) input wavelength (v) output wavelength
(vi) pending/accommodated
The connection request associated with an input port of a given block remains pending until an appropriate output port is allocated to it. It becomes accommodated after an appropriate output port is allocated to it.
(vii)priority number
Significance of this field has been explained in STEP-2(a).
(viii) direct/indirect
Whenever the redundant output port desired by a connection request cannot be allocated to it because the output port has already been allocated to some other connection request which arrived earlier than the current one, then the current one requires indirect routing. We shall explain the meaning of the word indirect at the appropriate place.
Fields marked (i)-(v) are to be supplied in the form of a routing table by the RWA algorithm operating at a higher layer. Fields (vi)-(viii) are to be generated by the switch control mechanism of our cross connect . At most FW; connection requests can exist at any given time. Because, a connection request corresponds to one of the top F' input ports of a specific block and there are FW input ports in our cross connect. Therefore, we need FW units of memory locations for storing the status of these 'FW input ports.
(c) Reserve storage for a variable that keeps track of the connection request with the highest priority number among all the connection requests that are still pending at a given time. This variable acts like a pointer to the connection request currently under consideration.
(d) Initially, label all redundant output ports as FREE. Set the fields (vi) and (viii) of all connection requests to accommodated and direct, respectively.
STEP-2: (Priority Assignment)
(a) Check if any new connection request has arrived 7.
If any new connection request has arrived,
then set the field (vi) of all such new requests to pending and go to the next step, i.e., to STEP-2(b).
Else, repeat STEP-2(a).
(b) Assign First-Come-First-Serve priority numbers to the newly arrived connection requests based on their arrival
times *. Two or more new connection requests that arrive simultaneously should be assigned consecutive priority
numbers. However, which one of them will get higher priority and which one lower, will be decided in a random
fashion. Also, all take-down requests will be assigned priorities higher than all set-up requests that are pending at
the time of priority assignment. This will result in the updating of highest priority number still pending when
control is passed from STEP-5(a) to STEP-2(a). The priority assignment at this point can also suitably be
modified to incorporate QoS.
STEP-3: (Accommodation of 'take-down' Requests In
(a) Check if there is any request pending.
If there is no request pending, then go to STEP-2(a).
Else, go to the next step, i.e., to STEP-3(b).
(b) Consider the connection request still pending with the highest priority number. Check the value of field (viii) of
the request.
If the value of field (viii) of the request is indirect, then go to STEP-6(a).
Else, go to the next step, i.e., to STEPS(c).
(c) Check if the request is of take-down type.
If the request is of take-down type,
then label all redundant output ports that were previously in use by this take-down request to FREE. Set field (vi) of this request to accommodated. The connection request still pending and having priority immediately after the request just accommodated now is treated, hereafter, as the request with the highest priority number. Go to STEP-3(a).
Else, go to the next step, i.e., to STEP-4(a).
STEP-4: (Accommodation of 'direct' 'set-up' Requests In Order Of Priority)
(a) Check whether the current connection set-up requires wavelength conversion. This is done by comparing the
fields (iv) and (v) of the connection request.
If the current connection set-up does not require wavelength conversion,
then connection to the desired outbound fiber 'j' can be set up through the j" port of the top 'F' output ports of the same block, which the connection request belongs to. Set field (vi) of the current request to accommodated. The connection request still pending and having priority immediately after the request just accommodated now is treated, hereafter, as the request with the highest priority number. Go to STEP-3(a).
Else, go to the next step, i.e., to STEP-4(b).
(b) We shall assume that field (v) of the current connection request contains the generic value i i.e., it requires
conversion into the generic output wavelength Ai. Check if the redundant output port corresponding to i, i.e.,
the i; port of the block, which the current connection request belongs to, is already BUSY.
If the desired redundant output port corresponding to i is already BUSY, then go to STEP-5(a).
Else, go to the next step, i.e., to STEP-4(c).
(c) First change the status of the desired FREE redundant output port corresponding to i, i.e. the desired i, port of
the block, which the current connection request belongs to, into BUSY. Then, route the current connection
request to the input side of block i through this desired i output port. Connection to the desired j" outbound
fiber can then be set up through the jth of the top 'F' output ports of the block i. Set field (vi) of the current
connection request to accommodated. The connection request still pending and having priority immediately
after the request just accommodated now is treated, hereafter, as the request with the highest priority number.
Go to STEP-3(a).
STEP-5: (Reassignment Of Priority Numbers And Rearrangement Of Existing Connections)
(a) Check if the desired i port is BUSY because of a request belonging to the block, which the current request
under consideration belongs to.
If the desired i port is BUSY because of a request that belongs to the block, which the current request under consideration belongs to, then the current request under consideration is assigned a priority number that it is given priority
immediately after the connection request that has the lowest priority of all requests still pending at this
time. Set field (viii) of the current request to indirect. Go to STEP-2(a).
Else, go to the next step, i.e., to STEP-5(b).
(b) Identify the already accommodated connection request that caused the desired i port td become BUSY. Label
all redundant output ports that are in use by this connection to FREE. Set field (vi) of this identified connection
request as pending and assign it a priority number such that it is given priority immediately after the current
request. To accommodate the current request, repeat STEP-4(c).
STEP-6: (Accommodation Of 'indirect' connection Requests)
(a) Let's assume that the current request belongs to block k. Check the status of the  ports of all those blocks that are connected to block k through FREE redundant output ports of the block k. Select randomly one of those blocks whose  port is FREE. Identify the redundant output port of block Xk that is connected to the randomly selected block. First of all, route the current connection request to the input side of the randomly selected block through the identified redundant output port and set it to BUSY. Then route the current connection request to the input side of block , through the  port of the randomly selected block and set the  port of the randomly selected block to BUSY. Connection to the desired jth outbound fiber can then be set up through the jth of the top 'F' output ports of the block  Set field (vi) of the current connection request as accommodated. The connection request still pending and having priority immediately after the request just accommodated now is treated. hereafter,as the request with the highest priority number. Go to STEP-2(a).
The switch actuation mechanism then finally establish the connections. This is done by keeping track of the output ports allocated to given input ports as decided by the algorithm mentioned above.

WE CLAIM:
1. A rearrangeably non-blocking all optical cross-connect device for use in wavelength
routing wavelength division multiplexing system comprising :
a demultiplexer connected to a plurality of inbound fibers to divide the optical signal in each channel to specific wavelength;
a plurality of switching blocks connected to the demultiplexer for receiving signals of a specific wavelength;
a plurality fixed output type wavelength converters connected to the switching block for providing a fixed output to the multiplexer;
a plurality of out bound fibers connected to the multiplexer for achieving desired rearrangeably nonblocking, all-optical cross connect behaviour;
wherein each of said switching blocks have a number of redundant recirculating input ports equal to output ports.
2. A device as claimed in claim 1, wherein one of said redundant recirculating output ports of each switching block is connected to one of the redundant input ports of each of the remaining switching blocks and wherein each said redundant output port is connected to only to one redundant input port and each said redundant input port is connected to only one redundant output port.
3. A device as claimed in claim 2, wherein the number of each said redundant recirculating input and output ports in each said switching block is one less than number of said switching blocks.
4. A device as claimed in claim 1, wherein the number of said switching block is such that number of redundant blocks is governed by equation K = 2F-W-1, when the inequation W> 2F-1 is satisfied, where K is the number of redundant blocks, F is the number of the in bound/out bound fibers and W is the number of specific wavelengths.
5. A device as claimed in claim 1, wherein the number of said switching blocks is equal
to number of specific wavelengths if said inequation K = 2F-W-1 is satisfied.
6. A device as claimed in claim 1, wherein each output port other than redundant output
port of each switching block is connected to a wavelength converter.
7. A device as claimed in claim 1, wherein number of wavelength converter is equal to
square of number of switching blocks.
8. A method for rearrangeably non-blocking cross connect as claimed in claim 1,
comprising the steps :
receiving signal from the plurality of inbound fibers at the input port of the demultiplexer;
supplying the output of the demultiplexers to the switching blocks;
passing the signal through the switching blocks repeatedly upto a max. of three times;
feeding the output of switching blocks to a fixed output wavelength converters;
feeding the output of the converters to the multiplexers;
feeding the output of multiplexers to the outbound fibers to achieve desired rearrangeably nonblocking, all-optical cross connect behaviour.
9. A rearrangeably non-blocking all optical cross-connect device, substantially as
hereinbefore described with reference to the accompanying drawings.
10. A method for rearrangeably non-blocking cross connect, substantially as hereinbefore described with reference to the accompanying drawings.