CA2310856A1

CA2310856A1 - Time-slotted optical space switch

Info

Publication number: CA2310856A1
Application number: CA002310856A
Authority: CA
Inventors: Alberto Leon-Garcia
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-06-06
Filing date: 2000-06-06
Publication date: 2001-12-06

Abstract

A method and apparatus for the switching synchronised bursts of optical signals arriving at a multiplicity of input ports and destined to distinct output ports is disclosed. A basic switching unit is disclosed that transfers n input optical signals to n distinct output signals under the control of digital electronic signals. Apparatus and methods for building n by n basic switching units that can be reconfigured at high rates are disclosed.
Methods are disclosed for building larger N input by N output optical switches by interconnecting and controlling arrays of basic switching units. Methods are disclosed for the allocation of transmission capacity across said optical switches.

Description

Time-Slotted Optical Space Switch Abstract A method and apparatus for the switching synchronised bursts lcal signals arnving at a multiplicity of input ports and destined to distinc ut ports is disclosed. A basic switching unit is disclosed that transfers n ' optical signals to n distinct output signals under the control of digital electr signals. Apparatus and methods for building n by n basic switching units be reconfigured at high rates are disclosed. Methods are disclosed for mg larger N input by N output optical switches by interconnecting and co mg arrays of basic switching units. Methods are disclosed for the allocation of transmission capacity across said optical switches.
Field of Invention The present invention in general relates to optical switches used in telecommunications and computer networks to switch and route optical signals arnving in one or a plurality of input ports to one or a plurality of output ports.
Optical transmission technologies have increased the information-carrying capacity of a single optical fiber to more than 1 Terabit per second. Future switches must therefore be able to transfer aggregate information rates in the many Terabits per second.
Electronic switches that can handle these information rates are extremely difficult to build because of the relatively-limited information-carrying capacity of electronic systems.
Optical switches that transfer information in optical form can avoid the bottleneck inherent in electronic switches.
Time slot exchange is a basic approach in the design of systems for transmitting and switching information. The time-division multiplexed telephone network and the SONET/SDH transport networks are both organized around a 125 microsecond time-slot.
ATM and related network technologies are designed around the notion of fixed-length units of information. The time-slot approach introduces modularity in the units of information that need to be handled and processed and through synchronization of time slots allows parallel and high-speed operation.
More specifically, the present invention relates to time-slotted optical switches that are:
1. modular in design and can be built from small to large number of port counts; 2.
flexible in the type of optical signals that can be carried, from single wavelengths, to bands of wavelengths, to broad regions of optical spectrum; 3. reconfigurable in terms of the allocation of transmission opportunities to different input-output port pairs. The present invention can be used to build optical and electronic switches and routers in telecommunications and computer networks.
Discussion of Previous Art The transmission of information over optical fiber systems provides the advantages of extremely high transmission rates (measured in bits per second) and extremely low bit error rates. The design of electronic switches to transfer information among optical fiber systems is very challenging because of the extremely large volumes of information that must be handled electronically. All-optical switches transfer information among optical fiber systems without converting the information streams into electronic form, and hence avoid the electronic bottleneck inherent in electronic switches.
Optical switches can categorized according to whether the transfer of information is controlled using the space, time, or wavelength domains. In the case of space switches, the optical signals that arrive in an input port are routed along disjoint paths through the switch to the desired output ports. Space switches are typically built from arrays of crosspoints that direct the signal across the switch. Multistage arrangements of small space switches can be used to produce space switches with a large number of parts.
Benes and Clos multistage switches are preferred because of their non-blocking properties, that is, the ability to provide connections from any idle input to any idle output.
Space switches can be used to provide long-term connections (of duration seconds or more) between input ports and output ports. During the term of the connection, the optical signal can flow continuously from input to output. The transfer of the optical signal is said to be transparent because the manner in which information is modulated and formatted is not relevant to the operation of the switch. Micro-electromechanical systems (MEMS) are an example of such space switches.
In wavelength switches, composite optical signals arrive at each input port.
Each composite signal consists of the combination of several optical signals, each using a distinct wavelength from some preselected set of allowable wavelengths. The wavelength switch separates the composite optical input signals according to wavelength, and it uses a separate space switch to transfer the set of inputs at each given wavelength to their desired output ports. The outputs from the various space switches directed to a given output port are then wavelength division multiplexed into a composite optical signal that exits the switch. Wavelength switches can be used to provide end-to-end transparent connections consisting of a single wavelength between two points in a network.
In time switches, the optical signals that arnve at each input port consist of a sequence of signal bursts, of some fixed duration called a "time slot". Each burst at an input port must be routed independently across the switch. A space switch that can be reconfigured rapidly can be used to build a time slot switch. At periodically recurring time instants, the switch is reconfigured so that it connects input ports to output ports according to some desired permutation arrangement. No valid signal information is allowed into the switch during the reconfiguration time, so a guard time is used to separate the bursts of optical signal that traverse the switch. The time slots that arrive at different input ports need to be synchronized so that they enter the switch at the same time, right after the end of the guard time. Time-slotted switches allow the transfer of optical information from any input port to any output port over relatively short time intervals, in the order of microseconds, and in effect support the simultaneous transfer of information among all ports in the switch.

The synchronization of the optical bursts that enter the time-slotted switch can be carried out in a straightforward fashion if the optical signals are generated (or regenerated) locally in the line cards that feed the switch. The synchronization of optical bursts that arrive from some distant system requires the use of burst alignment techniques such as those discussed in [Guillemot 1998, pg. 2130].
There are two fundamental approaches to the transfer of information in a network. The first approach involves the setting up of relatively long-lived circuits that enable the flow of information between two points in the network. The second approach involves the transfer of information in the form of packets. Both of these approaches can be supported by time-slotted switches.
In the case of long-term circuits, the time-slotted optical switch can be configured to provide a periodic transfer of bursts of optical signal from specific inputs to specific outputs according to some fixed schedule. The schedule ensures that each connection receives the desired rate of information transfer. A connection admission procedure is required to ensure that the switch is capable of supporting a new request for a circuit, and then to set up the desired transfer of time slots in the switch permutation schedule.
Time-slotted optical switches can also be operated to transfer individual packets or blocks of information that fit within a single time slot. In this case, the configuration of the space switch must be controlled dynamically according to the pattern of packet requests at the input to the switch.
Hybrid approaches that combine a fixed schedule to accommodate circuit connections and dynamic scheduling to accommodate short-lived packet traffic are highly desireable given the variability in volume and locality of Internet traffic. One hybrid approach involves the use of a frame structure in which some time slots are allocated on a longer-term basis to circuits while other time slots are allocated dynamically to on-demand traffic. Many allocation algorithms are possible according to what criteria is used in making the allocation and how quickly the adaptation to on-demand traffic is carried out.
The present invention discloses methods and apparatus for a time-slotted optical switch that can provide circuit or on-demand transfer of bursts of optical signals from input ports to output ports. The invention is based on a space switch disclosed in Patent Application # xyz, "A Modular, Expandable and Reconfigurable Optical Switch," by Leon-Garcia et al. Said space switch is composed of an array of basic space switching units that are constructed from electrooptic wafer beam deflection switch components. The time to reconfigure the basic space switching units and the overall space switch is limited essentially by the speed of the control voltage driver circuit, currently in the order of 100's of nanoseconds. The space switch can be operated in time-slotted fashion with time slots set to several microseconds.
Current optical transmission systems have an inherent information transmission capacity in excess of 1 Terabit per second, but the digital modulation systems currently available can only handle information in the tens of Gigabits per second. In order to use the large inherent capacity of optical fiber, wavelength division multiplexing (WDM) systems combine multiple independently modulated optical signals of different wavelengths into a single combined optical signal that can be transmitted in a single optical fiber. WDM
thus provides a means of packing extremely high volumes of information transfer in small regions in space.
The space switch disclosed in Patent Application # xyz, "A Modular, Expandable and Reconfigurable Optical Switch," by Leon-Garcia et al, has the property that it can transfer a broad range of wavelengths of an optical signal. Consequently, the space switch has a very large inherent information transmission capacity, much as in the case of optical fiber. The present invention discloses methods that use WDM to exploit this inherently high information transfer capability and to in effect provide a switch with a larger number of input and output ports.
Summary of Invention The present invention presents a method and apparatus for time-slotted optical switches that transfer bursts of optical signals arriving at a multiplicity of input ports and destined to distinct output ports. A basic switching unit is disclosed that transfers n input optical signals to n distinct output signals and that can be rapidly reconfigured under the control of digital electronic signals. A preferred embodiment of the basic switching unit using an electrooptic wafer beam deflector component is disclosed. Also disclosed is a modular approach for building large time-slotted optical space switches using basic switching units as building blocks. Methods for allocating the transmission capacity to different input-output flows as well as to circuit and on-demand traffic are also disclosed. Finally a method for expanding the number of switch ports using wavelength division multiplexing is disclosed.
Brief Description of the Drawings Figure 1 nxn basic switching unit with active splatters and active combiners Figure 2 nxn basic switching unit with active sputters and passive combiners Figure 3 Timing in time-slotted space switch operation Figure 4 Switch fabric and associated control unit Figure 5 16x16 Benes Switch using identical basic switching units Figure 6 16x16 Benes Switch using multiple size basic switching units Figure 7 Clos strictly non-blocking switch Figure 8 Time-slotted space switch Figure 9 TDM sequence of interconnection matrices & switch configurations Figure 10 Sequence of interconnection matrices for unbalanced and nonuniform traffic matrix Figure 11 Expanded optical switch using wavelength division multiplexing Detailed Description of the Invention Figure 1 shows a 4x4 example of an nxn basic switching unit 100 constructed from n 1 xn active sputters 20 and n nxl active combiners 21. A single output fiber 10 from each active splitter is connected to an input 11 of each of the active combiners.
The control voltage 12 in each active sputter directs the input optical signal to the desired output fiber and thereafter the optical signal propagates to the corresponding active combiner. The 10 active combiner directs the single arnving optical signal to the output fiber 15 under the control of a voltage signal 16.
A switch unit control 30 associated with the nxn basic switching unit in Figure 1 takes the control signal generate by a central controller (of the overall time-slotted space switch) and generates control signals c; that are distributed to the control circuitry associated with each splitter/combiner. Said control signal c; specifies one or more voltage levels which are applied in the splitter/combiner to produce the desired deflection in the beam and effect the desired routing from a given input fiber to a desired output fiber.
A consistent set of control voltage signals is required in the nxn basic switching system in Figure 1 to direct each of the n input optical signals to a distinct output port. The nxn basic switching unit is then equivalent to a crossbar switch in the sense that it can direct any of n input signals to any output port that is not already in use.
Figure 2 shows a 4x4 example of an nxn basic switching unit 200 constructed using n 1 xn active sputters 47 and n nxl passive combiners 48. A single output fiber 40 from each active splitter is connected to an input 41 of each of the active combiners.
'The control voltage 42 in each active splitter directs the input optical signal to the desired output fiber 43 and thereafter the optical signal propagates to the corresponding passive combiner.
The passive combiner combines all arnving optical signals and a portion of the energy in the arriving optical signal appears at the output fiber 45. The system in Figure 2provides an acceptable basic switching unit as long as the output signals have an adequate signal-to-noise ratio.
A preferred embodiment of the active splitter and active combiner in Figure 1 and in Figure 2 involves the use of electrooptic wafer beam deflectors as disclosed in U.S.
Patent #xxx and Patent Application # xyz, "A Modular, Expandable and Reconfigurable Optical Switch," by Leon-Garcia et al.
A preferred embodiment of the control signals c; in Figure l and in Figure 2 where c;
specifies whether a deflection voltage is on or off in each of the multiple prism segments in a substrate as disclosed in Patent Application # xyz, "A Modular, Expandable and Reconfigurable Optical Switch," by Leon-Garcia et al. The driver circuitry for producing the binary deflection voltages can effect the reconfiguration of the beam deflection in sub-microsecond time intervals [IR 2000].
The operation of the basic switching units of Figure 1 and Figure 2 as time-slotted optical space switches involves the repetition of a cycle of events as shown in Figure 3. Each cycle is T seconds in duration.
As shown in Figure 3, the first tco~g seconds in each cycle provide a guard time during which the control signals are distributed to the active splitters and combiners and the associated deflection voltages are applied. At the end of t~o~g seconds each optical beam is deflected from the specified input to the corresponding desired output.
During the guard time interval, any optical beams present at the inputs may propagate to various outputs in uncontrolled fashion producing a form of crosstalk. Indicator signals are available at the output of the switches to indicate that the optical signal at the output ports is not valid during the guard time interval.
The end of the guard time interval in Figure 3 is followed by the dwell-time interval of duration tdWeii seconds. At the beginning of the dwell-time interval, the input ports are given a signal indicating that switch is ready to switch the bursts of input optical signals.
During this interval, the space switch maintains a specific interconnection pattern directing optical signals from given input ports to specific corresponding output ports.
The bursts of input optical signals are then transferred to the desired output ports.
The time-slotted optical space switches of Figure 1 and Figure 2 can operate as in standalone mode and provide transfer of burst of optical signals from their n input ports to their n output ports. Time-slotted optical space switches of dimension nxm can be obtained by taking a basic switching of a given size and not using some of the input or output ports.
Physical constraints limit the maximum size n of an active sputter or an active combiner.
This in turns places a maximum size on the number of ports in a basic switching unit.
Traditional switching theory derived from telephony networks provides methods for the construction of large NxN space switches from modules consisting of smaller space switches [Stern 1999, pg. 39]. These constructions typically involve an array of multiple switches of various sizes. The outputs of the switches in a given stage are connected to inputs of switches in the next stage in order to provide a number of alternative paths from the inputs to the outputs of the NxN switch.
Figure 4 shows how the configuration of a large NxN switch is controlled by a switch fabric control unit. Figure 4 shows the multiple stages of modules that constitute the large NxN switch. (Note that the figure does not show the interconnections between the various stages.) A switch fabric control accepts requests for a given set of connections from given inputs to specific outputs. The switch fabric control executes an algorithm that is specific to the particular multistage construction to determine what internal configuration of connections within the modules will realize the requested set of connections. The switch fabric control then distributes digital control signals to the modules specifying their internal configuration. The desired set of connections becomes available when the modules complete the reconfiguration specified by the control signals.
Time-slotted optical space switches of large dimension NxN can be constructed from basic switch units of size nxm as modules and using multistage constructions.
The operation of said NxN switches must also operate in cycles of the form shown in Figure 3. Prior to the beginning of each reconfiguration interval, the fabric switch control must determine the set of internal connections in the basic switching units to achieve the required set of connections from the inputs to the outputs of the NxN switch.
During the reconfiguration interval, the control signals are distributed to the basic switching units which in turn reconfigure their internal set of connections. At the end of the reconfiguration interval, the NxN space switch is ready to transfer bursts of optical signals from given inputs to specified outputs.
The Benes and Clos multistage constructions are of particular interest because of their nonblocking properties. Figure 5 shows an example of how a 16x16 optical switching fabric 300 can be constructed from three stages of 4x4 basic optical switching units as disclosed in Patent Application # xyz, "A Modular, Expandable and Reconfigurable Optical Switch," by Leon-Garcia et al. More generally, given an nxn basic switching unit constructed as shown in Figure f or in Figure 2, it is possible to construct and n2 x n2 larger switching fabric using a three-stage construction using the interconnection approach described above. In general an n2 x n2 three-stage Benes construction requires 3n basic switching units. A five-stage n3 x n3 Benes construction for a large switch is also possible. In general said n3 x n3 five-stage Benes construction requires Sn2 basic switching units. More generally, an nk x nk (2k-1)-stage Benes construction requires (2k-l )nk-I basic switching units.
A preferred embodiment of the present invention involves the construction of n2 x n2 and n3 x n3 Benes constructions of optical switching fabrics using the basic nxn switching units shown in Figure l and in Figure 2. The corresponding three- and five-stage switches are feasible because of the low loss property of the basic switching units constructed using electrooptic wafer beam deflector components.
The Benes method also allows the construction of large optical switching fabrics from smaller basic switching units of several sizes. Figure 6 shows a three-stage 16x16 optical switch fabric 400 constructed from first and third stages consisting of 8 2x2 basic switching units 61 and a central stage consisting of 2 8x8 basic switching units 62. In general, an NxN switches can be constructed in three stages or in five stages if N can be factored as the product of two or three whole numbers, respectively.
A preferred embodiment of the present invention for a time-slotted optical space switch involves the construction of three and five stage Benes optical switching fabrics using basic switching units shown in Figure 1 and Figure 2. The corresponding three-and five-stage switches are feasible because of the low loss property of the basic switching units constructed using electrooptic wafer beam deflector components.

The Benes switch fabric constructions described above are "rearrangeably non-blocking"
in the sense that they can realize any interconnection pattern of any N inputs to any N
distinct outputs, but the addition of a new connection to an existing set of fewer than N
existing connections may require the re-arrangement of all connections. This non-blocking property of the Benes switch fabric constructions indicate that the switch fabric control in Figure 4, when implementing appropriate algorithms, can always find a set of internal connections for each constituent switching module to connect the inputs to an arbitrary set of distinct outputs.
[Clos 1953] developed a method for constructing non-blocking multi-stage fabrics that do not require rearrangement of existing connections when a new connection is set up. The basic Clos construction for an NxN switch consists of three-stages. The first and third stages consist of k rows of pxm basic switching units, and the central stage consists of m k x k basic switching units. It is well-known that if m=2p-1, then the Clos switch fabric is strictly non-blocking in the sense that existing connections do not need to be rearranged to establish a new connection from an available input to an available output.
Figure 1 shows an example of an 8x8 non-blocking Clos switch 800 constructed from 2x2 and 4x4 basic switching units. In this example, p=2, k=4 and m=2p-1=3. Clos switch fabrics can be operated in time-slotted mode using the control approach discussed with Figure 4.
A preferred embodiment of the present invention is a three-stage arrangement of a Clos switching fabric in which the basic switching units are constructed using electrooptic wafer beam deflector components.
When a large NxN time-slotted optical space switch shown in Figure 8 is built using a Clos or Benes construction, then the NxN switch operates as if it were an NxN
crossbar switch that can implement any permutation of connections from inputs to outputs. The configuration of said switch during cycle t is specified by a matrix P(t) _ {p;~(t)), where p;~(t) is equal to 1 if input i is connected to output j, and is equal to zero otherwise. P(t) has the property that each row has exactly one non-zero value, and each column has exactly one non-zero value. The sequence P(1), P(2), P(3), P(4), ... represent the sequence of interconnection patterns provided by the NxN switch. The number of times the ijth component equals 1 in the sequence P(1), P(2), ..., P(k) represents the number of time slots allocated to connection ij in k consecutive cycles. Hence the allocation of transmission opportunities ("bandwidth") among input-output pairs is determined by the sequence of configuration matrices.
The sequence of interconnection patterns P(1), P(2), P(3), P(4), ... can be selected to meet the bandwidth requirements of the traffic that traverses the switch. In the case where the same level of traffic flows between every input and output port and where the traffic flows are relatively steady, a suitable sequence consists of a repetitive interconnection pattern P( 1 ), P(2), . . . , P(N-1 ), P(N), P( 1 ), P(2), . .
., P(N-1 ), P(N), . . .
that provides every input-output pair with 1 transmission opportunity per repetition cycle.
We refer to said repetitive pattern as a Time-Division Multiplexing (TDM) schedule.
Figure 9 shows an example of such a repetitive pattern for a 4x4 switch: A
repetitive pattern of 4 permutation matrices and the associated switch configurations are shown.

Note in this example that the sequence uses only 4 of the 4!=24 possible permutation matrices. Note also that different sequences of permutation matrices can be used to produce TDM schedules.
In the case where different levels of traffic flow between different input and output ports and where the traffic flows are relatively steady, a suitable schedule consists of a repetitive interconnection pattern P(1), P(2), ..., P(N-1), P(N), P(1), P(2), ..., P(N-1) P(N), ... that provides an input-output pair with a number of transmission opportunities per repetition cycle that is proportional to the relative traffic flow of the input-output pair.
Figure 10 shows an example of a traffic matrix for a 4x4 switch and a corresponding repetitive interconnection pattern that satisfies the traffic demand. The ijth entry in the traffic matrix is the proportion of time information is available for transfer from input port i to output port j. The "x" in the permutation matrices in the figure denote "don't cares" for connections in the switch that have not been assigned. Various algorithms are available for synthesizing an repetitive interconnection pattern for a given traffic matrix [Algoxxx].
The interconnnection pattern can be modified over time to track variations in traffic levels and to deal with temporary surges in traffic. By keeping a running average of the traffic flow between each input-output pair, the variation in the traffic matrix can be tracked and adjustments in the interconnection pattern can be made. These adjustments may consist of small changes in the permutation matrices or in the repetitive pattern itself through the addition or deletion of one or more permutation matrices. Surges in traffic can be monitored through the backlog of information at the input to the switch. "Don't cares" in the permutation matrices can be set to help reduce the backlog for certain input-output pairs.
The electrooptic wafer beam deflector component can route optical signals and maintain high signal quality even when the optical signals are composite and consist of multiple wavelength signals. Consequently, the above disclosed optical switches constructed using electrooptic wafer beam deflector components have the capability of transferring composite optical signals. Figure 11 shows the use of WDM multiplexers and demultiplexers to concentrate multiple optical signals that occupy non-overlapping wavelengths into a single optical signal that can be switched across the NxN
optical switch. The structure of the switch constrains all components of the composite signal to be switched to the same output port. Each additional wavelength in the composite signal increases the transmission-carrying capability (measured in bits) in each time-slot. The transmission-carrying capability of the overall switch increases accordingly.

Claims

1. nxn time-slotted nxn basic switching unit with rapidly reconfigurable splitters and combiners

2. nxn time-slotted nxn basic switching unit with splitters and combiners built from electrooptic wafer beam deflector components

3. NxN multistage time-slotted arrangements of basic switching units with rapidly reconfigurable splitters and combiners

4. NxN multistage time-slotted arrangements of basic switching units with splitters and combiners built from electrooptic wafer beam deflector components

5. NxN multistage time-slotted Benes arrangements of basic switching units with rapidly reconfigurable splitters and combiners

6. NxN multistage time-slotted Benes arrangements of basic switching units with splitters and combiners built from electrooptic wafer beam deflector components

7. NxN multistage time-slotted Clos arrangements of basic switching units with rapidly reconfigurable splitters and combiners

8. NxN multistage time-slotted Clos arrangements of basic switching units with splitters and combiners built from electrooptic wafer beam deflector components

9. NxN multistage time-slotted nonblocking arrangements of basic switching units with rapidly reconfigurable splitters and combiners with TDM permutation schedule

10. NxN multistage time-slotted nonblocking arrangements of basic switching units with splitters and combiners built from electrooptic wafer beam deflector components TDM permutation schedule

11. NxN multistage time-slotted nonblocking arrangements of basic switching units with rapidly reconfigurable splitters and combiners with adaptive permutation schedule

12. NxN multistage time-slotted nonblocking arrangements of basic switching units with splitters and combiners built from electrooptic wafer beam deflector components adaptive permutation schedule