DISTRIBUTED CROSSBAR SWITCH

Abstract: The invention provides a locally distributed crossbar switch (46) to optically connect many computer processors and memories (48-62) in arbitrary permutation and without interruption. A processor and an associated memory can be packaged together, taking advantage of the fact that the distributed crossbar switch (46) employs local switching (104-118). At the same time, processor-memory pairs can be physically separated from other such processor-memory pairs, and by great distance.

Description: DISTRIBUTED CROSSBAR SWITCH The U.S. Government has rights in this application pursuant to Contract No. N 6601-86-C—0010 with the Department of the Navy. Field of Invention This invention relates to communication networks between computer processors and memories. Introduction to the Invention Computer architectures now commonly comprise a large number of processors executing programs concurrently. Such parallel computing architectures require a communi¬ cation network between processors and between processors and memories. In parallel processing systems, it is desirable to allow many processors to send information to the processors and/or memories simultaneously. The use of optics for the communication network is particularly effective, since optical systems can provide appropriate inherent parallelism, ' suitable bandwidths and low interaction between separate beams in a linear medium. One of the most important optical communication networks utilizes the well known crossbar network. In particular, the crossbar network allows all processors and/or memories to be dynamically interconnected in an arbitrary permutation, without moving any existing interconnections. Attention is now directed to FIG. 1 which exemplifies a "star" crossbar network 10 that utilizes opto-electronic detection components. The crossbar network 10 is disclosed in MacDonald et al, Electron Lett. 16 (1978) 402. The disclosure is incorporated herein by reference. The crossbar network 10 includes four transmitters, 12, 14, 16 and 18, each of which transmitters may be considered to be run by a processor. The transmitters provide input signals which are routed on optical fibers 20, 22, 24, and 26. The four separate inputs 20, 22, 24, 26, are in turn, fanned out four times and then made incident on 16 separate detectors 28. Each of the detectors 28 comprises a photodetector connected in series with a preamplifier, and each detector 28 provides the same electronic function, namely, to serve as a switching element. Briefly, each of the detectors 28 inputs light, and outputs an electrical signal on instruction from bias control circuitry (not shown). In operation, therefore, tne crossbar network 10 functions to allow selection of a particular transmitted input signal for rebroadcast to a desired output (a memory or processor unit), shown in FIG. 1 as receivers 30, 32, 34, and 36. This rebroadcasting further requires a conversion of the electrical signals to optical signals by way of optical repeaters 38, 40, 42, 44, and transmission over a return fiber line. SUMMARY OF THE INVENTION We have discovered that the crossbar network 10 may not be an optimal architecture in all cases. For example, we have observed that the FIG. 1 scheme precludes local, distributed switching. That is, a transmitter and an associated receiver are separated by the switch and an optical repeater. We have recognized, in contrast, that in many important, practical cases, it would be desirable and necessary to provide an arrangement so that (1) each of the transmitter-receivers that make up a pair, could be fabricated and packaged together and, (2) so that each of the pairs could be separated from other pairs by a great distance, e.g., at least 100 meters, preferably at least 1000 meters. In this way, accordingly, one could ascertain by visual inspection that a signal sent by a transmitter had been received by its associated, paired receiver, and further provide transmitter-receiver pairs in separate rooms or buildings. To this end, the invention provides in one aspect, a distributed switching assembly comprising: a) N transmitter - receiver pairs, wherein each receiver comprises a switched array of N detectors for detecting and selecting a particular transmitted signal; b) optical fiber routing means for linking a transmitter and a receiver; and' c) a fanout - reformatter which i) accepts a signal "output from each transmitter by way of the optical fiber routing means; and ii) fans out the signal output from each transmitter from 1 to N branches; and iii) reformats all of the Kth branches, where K is from 1 to N, from each transmitter for providing N inputs to the Kth receiver by way of the optical fiber routing means. The fanout-reformatter preferably comprises a passive optical structure. Preferably, the fanout- reformatter provides efficient throughput, easy alig¬ nment, and is suitable for large arrays, for example, where N is 64, 128 or 256. Preferred such fanout- reformatters include a fused half-star coupler, a planar waveguide structure, or crossed phase gratings. For all three identified 1 to N branching techniques, it is desirable that the fanout-reformatter: 1. exhibits low throughput loss e.g. , less than 2dB, particularly less .than 3db for N equal to 128; 2. provides a reconfiguration time of less than 10 microseconds, particularly less than 1 microseconds; and/or 3. processes asynchronous signals that may be outputted from the transmitter. Satisfactory fanout-reformatters are available from commercial vendors, for example, model 8010 Monolithic Splitter from Corning, NY . Reformatting involves gathering the Kth branches from each of the 1 to N fanouts and forming a suitable cable structure. In the cable structure, the identity of each branch can be established by labeling each cable or by the position of the branch in a ribbon of fibers. It is an advantage of the present invention that the assembly comprises receivers that include a suitable array of N detectors for detecting and selecting a particular transmitted signal to be processed by a particular receiver. Such detection electronics preferably comprise opto-electronics and includes conventional electrical signal summation/ amplification, optical signal regeneration, and a switch controller. Accordingly, using well known techniques, for example, those disclosed by MacDonald supra, turning on the bias on a particular switch controller provides selection for a particular desired transmitted signal for retransmission to a desired receiver. Note that it is in this sense that we call the present switch a distributed crossbar, since the switching elements are located art the transmitter/receiver nodes. This is different from the prior art "star" network which uses a centrally positioned switching unit. The invention provides, in a second aspect, a method, which method comprises: a) providing N transmitter-receiver pairs; each pair containing its transmitter and receiver in the same package and each pair being separated from every other pair by at least 10 meters; b) fanning out from 1 to N branches a signal output from each of said transmitters; and c) reformatting the 1 to N branches so that all of the Kth branches, where K is from 1 to N, are provided as inputs to the Kth receiver. Note that it is another advantage of all aspects of the invention that the optical repeaters 38-44 required in the prior art architecture are-eliminated in the present invention. BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawing, in which FIG. 1 is a circuit diagram of a prior art opto¬ electronic crossbar switch; FIG. 2 is a circuit diagram for the circuit used in the present invention; and ... FIGS. 3a, b, c provide schematics of a fanout- reformatter employed in the present invention. DETAILED DESCRIPTION OF THE INVENTION Attention is now directed to FIG. 2 which shows an assembly 46 of the present invention. The assembly 46 includes 8 pairs of transmitter-receiver units packaged together, numerals 48-62, and a fan-out refor atter 64. The fan-out reformatter 64 can accept a signal output from each of the transmitters that is routed along optical fibers 66-80. The fan-out reformatter 64 reformats the input optical fibers 66-80 in the following way. To start, the optical fiber 66 from the first transmitter is split into 8 branches. The splitting may be done in accordance with the splitting scheme shown in FIG. 3. For example, FIG. 3a shows that the optical fiber can be split from 1 to 8 branches using a - half-star coupler 82 structure. FIG. 3b shows that the optical fiber can be split from 1 to 8 branches using a planar network 84 structure. FIG. 3c shows that the optical fiber can be split from 1 to 8 branches using a cross-phase grating 86 structure. Thus, in all these cases, an exemplary structure is shown for splitting an input optical fiber into 8 branches. Continuing, the input optical fiber 68 from the second transmitter is split into 8 branches, again using, equivalently, any of the structures shown in FIG. 3. Further, and in the same manner, each of the input optical fibers 70, 72, 74, 76, 78 and 80 from the remaining 6 transmitters are split into 8 branches, again in accordance with the FIG. 3 structures. At this stage, therefore, there are 8 input groups of 8 branches each, or 64 total branches. As indicated above, reformatting involves gathering the 64 branches in an appropriate manner to form a cable for input to the receivers. This is done as follows. To start, the first branch from each of the 8 input groups is bundled together, using the reverse of any of the structural branching techniques shown in FIG. 3. This procedure therefore establishes a first cable comprising the first branch from all of the 8 input groups. The first cable, shown in FIG. 2 as optical fiber 88, is routed to the first receiver. Continuing, the second branch from each of the 8 input groups is bundled to¬ gether, again using the reverse of any of the structural branching techniques shown in FIG. 3. This procedure therefore establishes a second cable comprising the second branch from all of the 8 input groups. The second cable, shown in FIG. 2 as optical fiber 90, is routed to the second ' receiver. Further, and in the same manner, the third, fourth, fifth, sixth, seventh and eighth branches from each of the 8 input groups, are individually bundled together by way of the structural techniques shown in FIG. 3. This procedure, accordingly, establishes 6 more cables, shown in FIG. 2 as optical fibers 92-102. The optical fibers 92-102 are routed, respectively, to the third through eighth receivers. At this point, each of the 8 receivers has an input fiber optic cable comprising 8 separate branches bundled together. The fiber optic cable in each receiver is inputed to the opto-electronic detectors that form part of each receiver. The 8 opto-electronic detector units shown in FIG. 2 are indicated by numerals 104-118. Each of the 8 opto-electronic detector units 104-118, in turn, address the 8 separate branches that are bundled together and inputted to each receiver. Each of the 8 detector units 104-118 comprises conventional electrical signal summation/amplification, optical signal regeneration, and a switch controller., (not shown). As . indicated above, each of the 8 opto-electronic detector units 104-118 provides selection for a particular desired transmitter signal, one to eight, to be received by a particular desired receiver, one to eight.

Claims: CLAIMS What is Claimed is: 1. A distributed switching assembly, comprising: a) N transmitter - receiver pairs, wherein each receiver comprises a switched array of N detectors for detecting and selecting a particular transmitted signal; b) optical fiber routing means for linking a transmitter and a receiver; and c) a fanout - reformatter which i) accepts a signal output from each transmitter by way of the optical fiber routing means; and ii) fans out the signal output from each transmitter from 1 to N branches; and iii) reformats all of the Kth branches, where K is from 1 to N, from each transmitter for providing N inputs to the Kth receiver by way of the optical fiber routing means. 2. A switching assembly according to Claim 1, wherein the fanout reformatter is passive. 3. A switching assembly according to Claim 2, wherein the fanout-reformatter comprises a fused half-star coupler. 4. A switching assembly according to Claim 2, wherein the fanout-reformatter comprises a planar waveguide structure. 5. A switching assembly according to Claim 2, wherein the fanout-reformatter comprises crossed phase gratings. 6. A switching assembly according to Claim 1, wherein the Kth transmitter-receiver is packaged together. 7. A switching assembly according to Claim 1, wherein each of the transmitter-receiver pairs is separated from the other pairs by at least 100 meters, and each of the pairs, thus separated, comprise a transmitter and receiver packaged together. 8. A switching assembly according to Claim 1, wherein the Kth transmitter-receiver pair is physically separated from the Nth transmitter-receiver pair by at least 100 meters. 9. A switching assembly according to Claim 1, wherein at least one transmitter outputs an asynchronous signal. 10. A switching assembly according to Claim 1, wherein the fan-reformatter reconfigures a signal in less than 10 microseconds. 11. A switching assembly according to Claim 1, wherein N is 2 to 256. 12. A method comprising: a) providing N transmitter-receiver pairs; each pair containing its transmitter and receiver in the same package and each pair being separated from every other pair by at least 10 meters; b) fanning out from 1 to N branches a signal output from each of said transmitters; and c) reformatting the 1 to N branches so that all of the Kth branches where K is from 1 to N, are provided as inputs in one cable to the Kth receiver.

Inventors: TANGONAN GREGORY L, TANGONAN, GREGORY, L.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1988
Main classification: H04Q3
IPC codes: H04B10, H04Q3