OPTICAL COMMUNICATION NETWORK SYSTEM WITH COLLISION DETECTION

Abstract: An optical communication network system including a plurality of data terminal stations interconnected by an op­ tical communication medium for effecting data communi­ cation in accordance with a CSMA/CD system. For reliably detecting a collision between data signals, each terminal station delivers a data signal for transmission through the optical communication medium in the form of light signals of the wavelengths λ₁ and λ₂, corresponding to two value states thereof, and separates a light signal transmitted over the optical communication medium to demodulate the data signal. When a signal collision has occurred, an overlap between light signals of the wavelengths λ₁ and λ₂ is produc­ ed over a predetermined time period. The overlap between the light signals of the wavelengths λ₁ and λ₂ is detected and, upon expiration of the predetermined time period, a collision detection signal is produced.

Description: Optical communication network system with collision detection This invention relates to an optical communication network system and, more particularly, to an optical communication network system which is improved in detection of a signal collision. Various types of control systems have been proposed as a data bus access control system for a local area network (LAN). Above all, a carrier sense multiaccess/collision detection (CSMA/CD) system is well known as "ETHERNET" (trademark). The LAN employing this system uses a coaxial cable as its transmission medium to interconnect a plurality of stations. Various LANs which use the CSMA/CD system and a light transmission path as the transmission medium have hitherto been proposed. A technical problem arising from utilization of an optical transmission path resides in the difficulty of detecting a signal collision which occurs when signals are simultaneously transmitted from two or more stations. With the detection systems so far proposed, it is difficult to reliably detect such a signal collision. According to a typical detection system, a light signal is converted into an electric signal in a transmission path and a collision is detected on the basis of the amplitude level of the electric signal. An object of the present invention is to provide an optical communication network system based on the use of the CSMA/CD system which can reliably detect a collision of transmission signals from a plurality of data terminal stations. According to the present invention, each data terminal station involved in an optical communication network system is arranged to alternately generate and transmit light signals of wavelengths B1 and A2 in accordance with two value states of a data signal to be transmitted into a transmission medium, and to alternately receive light signals of the wavelengths X1 and B2 transmitted over the transmission medium to demodulate a data signal. In this optical communication network system, when two or more stations simultaneously transmit light signals, signals of the wavelengths X1 and X2 are received overlapping one another during a specified period of time or greater. Each data terminal station comprises means for detecting such an overlap of the light signals of the wavelengths X1 and X2 to produce a collision detection signal when an overlap is detected for the duration of the specified time period. According to the present invention, it is possible to provide a CSMA/CD local area network which enables a reliable detection of signal collisions by using light signals of two wavelengths. Stations which can be connected to the CSMA/CD LAN using coaxial cables can be connected to the optical CSMA/CD LAN using optical fiber cables without need of modification. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 schematically shows an arrangement of an optical network system according to the present invention; Fig. 2 schematically shows an optical star coupler; Fig. 3 shows an arrangement of a transmitting and receiving unit (medium attachment unit); Fig. 4 shows an optical multiplexer which is used in the transmitting and receiving unit; Fig. 5 shows an optical demultiplexer which is used in the transmitting and receiving unit; Fig. 6 shows waveforms of signals at parts of the transmitting and receiving unit in signal transmission; Fig. 7 shows waveforms of signals at parts of the transmitting and receiving unit in signal reception; and, Fig. 8 shows waveforms of signals at parts of each station when a plurality of stations transmit signals simultaneously. Referring to Fig. 1 which shows an optical star network according to an embodiment of the present invention, a plurality of stations are coupled to a star coupler 1 by way of light transmission paths. As shown in detail in Fig. 2, this star coupler is constructed such that an input light from an optical fiber at one input port is allowed to branch into optical fibers at all output ports. Each station has data terminal equipment (DTEs) 2a, 2b, ... 2n such as electronic computers, and medium attachment units (MAUs) 3a, 3b, ... 3n which are coupled to the corresponding DTEs. Each MAU is arranged to convert an electric signal from the corresponding DTE into a light transmission signal for transmission over an optical channel, convert a received light transmission signal into an electric signal for application to the DTE, and detect a collision of transmission signals from two or more stations on the optical channel. Light signals transmitted from MAUs 3a, 3b ... 3n are conveyed to star coupler 1 via light transmission paths (up links) 4a, 4b, ... 4n, respectively, and light transmission signals which appear at the output ports of star coupler 1 are led to MAUs 3a, 3b, ... 3n via light transmission paths (down links) 5a, 5b, ... 5n. DTEs 2a, 2b, ... 2n are coupled to MAUs 3a, 3b, 3n via tranceiver cables 6a, 6b, ... 6n, respectively. Fig. 3 shows an arrangement of MAU 3n. An optical multiplexer 11 introduces into light transmission path 4n output light signals from electric-to-optical converters 14 and 15, adapted to generate light signals of wavelengths X1 and B2 respectively, and an optical demultiplexer 12 separates, into light signals of the wavelengths X1 and A2 a light transmission signal which has been transmitted over light transmission path 5n. Transceiver cable 6n, coupled to DTE 2n, includes a transmission signal line 42, a reception signal line 43 and a collision detection signal line 44. A transmission signal is applied through transmission signal line 42 to a reception circuit 13 having complementary outputs 35 and 36 which drive electric-to-light converters 14 and 15 to generate light signals of the wavelengths X1 and X2, respectively. These light signals are led to optical multiplexer 11 via optical transmission lines 31 and 32, respectively. The light signals of the wavelengths X1 and X2, separated by optical demultiplexer 12, are supplied to optical-to-electric converters 16 and 17 through light transmission paths 33 and 34, respectively, to be converted into electric signals. The output signals 37 and 38 of optical-to-electric converters 16 and 17 are coupled to a demodulation circuit 18 to demodulate a received signal. Demodulation circuit 18 may be constituted by a set/reset type flip-flop circuit. Further, output signals 37 and 38 of optical-to-electric converters 16 and 17 are coupled to a signal collision detection circuit 20 via an AND circuit 19. An output demodulation signal 40 of demodulation circuit 18, and an output collision detection signal 41 of signal collision detection circuit 20 are coupled to reception signal line 43, and collision detection signal line 44 via driver circuits 21 and 22, respectively. Fig. 4 shows an arrangement of optical multiplexer 11. The light signal of the wavelength X1, transmitted over light transmission line 31, and the light signal of the wavelength X2, transmitted over light transmission line 32, are commonly introduced into light transmission path 4n. Fig. 5 shows an arrangement of optical demultiplexer 12. This optical demultiplexer 12 is provided with a half-mirror 50 through which the light signal of the wavelength X1 is transmitted to light transmission path 33, and by which the light signal of the wavelength X2 is refelected to light transmission path 34. The operation of the optical network system so far described will be described. It is assumed that signals are transmitted in the Manchester coded form. Fig. 6 shows waveforms of signals at parts of MAU 3n during the transmission of signals from DTE 2n. For example, when, as shown, a signal of "101011" is to be transmitted from DTE 2n, a signal having a waveform as shown in Fig. 6(A) is applied to receiving circuit 13 through signal line 42. Accordingly, complementary output signals 35 and 36, having waveforms as shown in Fig. 6(B) and (C), are taken from receiving circuit 13. Electric-to-optical converters 14 and 15 are constructed to produce a light signal in response to a high level input signal applied thereto. They are driven by complementary outputs 35 and 36 of receiving circuit 13 to generate, alternately, light signals of the wavelengths X1 and X2 as shown in Fig. 6(d) and (E), respectively. The light signals from electric-to-optical converters 14 and 15 are multiplexed by optical multiplexer 11 so that, as shown in Fig. 6(F), the light signals of the wavelengths X1 and X2 alternately pass through light transmission path 4n. Fig. 7 shows waveforms of receiving signals at parts of MAU 3n when only one DTE transmits a signal. As mentioned before and as shown in Fig. 7(A), a light transmission signal with the alternate wavelengths X1 and X2 is applied to optical demultiplexer 12 via light transmission path 5n. The light signals of the wavelengths X1 and X2, shown in Figs. 7(B) and 7(C), which have been separated by optical demultiplexer 12, are applied to optical-to-electric converters 16 and 17, respectively. From optical-to-electric converters 16 and 17 are taken output electric signals which have waveforms shown in Fig. 7(D) and (E), in accordance with the input light signals applied thereto. These electric signals are both applied to demodulation circuit 18. An output signal of demodulation circuit 18, as shown in Fig. 7(F), is coupled to receiving signal line 43 via driver circuit 21. The output signals of optical-to-electric converters 16 and 17 are applied to AND circuit 19 as well. This is intended to obtain logical product of the light signals of the wavelengths X1 and X2. Where signal transmission is performed by one station, the light signals of the wavelength X1 and X2 do not overlap, with the result that no signal is output from AND circuit 19. Actually, however, due to waveform distortions introduced in electric-to-optical converters 14 and 15, light propagation distortion in optical fibers, or waveform distortions made in optical-to-electric converters 16 and 17, edges of the output signals of optical-toelectric converters 16 and 17 are ANDed and, as a result, a spike-like pulse is developed at the output of AND circuit 19, as shown in Fig. 7(G). Collision detection circuit 20 is arranged to produce, at its output 41, a collision detection signal of 10 MHz in response to application of an input signal having a specified pulse width or greater. The collision detection circuit 20, however, is not responsive to the spike-like pulse signal and thus does not produce an output signal. Collision detection circuit 20 may be formed of an integration circuit connected to receive an output signal 39 of AND circuit 19, and an oscillator circuit responsive to a predetermined output voltage of the integration circuit. Next, a signal collision on the optical channel resulting from simultaneous transmission of signals from two stations will be described. Assume now that light signals have simultaneously reached star coupler 1 via optical fiber cables 4a and 4b. Fig. 8(A) shows a light signal of 101011 g on optical fiber cable 4a, while Fig. 8(B) shows a light signal of 101001 g- on optical fiber cable 4b. From all the output ports of star coupler 1 are taken a mixed light signal of the light signals which have been transmitted by way of optical fiber cables 4a and 4b. The mixture of two light signals remains identical in wavelength with respect to the same state of the signals. However, the mixture has a mixed portion of the wavelengths X1 and X2 over one bit length for different states of the signals. In MAU 3n, the light signal transmitted over optical fiber cable 5n is separated into light signals of the wavelengths X1 and X2 in optical demultiplexer 12 which are applied to optical-to-electric converters 16 and 17, respectively. In response to the mixed signal shown in Fig. 8(C), an output signal shown in Fig. 8(D) is taken out from optical-to-electric converter 16, and an output signal shown in Fig. 8(E) is taken out from optical-to-electric converter 17. In response to the application of these output signals, demodulation circuit 18 produces an output signal shown in Fig. 8(F). On the other hand, from AND circuit 19 is taken out an output signal, shown in Fig. 8(G), containing spike-like pulses and a one-bit length signal resulting from overlapping of the wavelengths X1 and X2. In response to this one-bit length signal, collision detection circuit 20 produces, on collision detection signal line 41, a collision detection signal of 10 MHz which indicates that a collision of signals has occurred on the light transmission paths. In Fig. 8(H), there is shown an envelope of the collision detection signal of 10 MHz. The collision detection signal is applied to DTE 2n via collision detection signal line 44 of tranceiver cable 6n. As has been described above, in the optical network system according to the present invention, a signal collision on the light transmission path can be detected by not only stations in the transmission state, but also by all other stations in the reception state. In the foregoing description, separate light emitting elements are employed for purposes of generating light signals of the wavelengths X1 and X2, but a single light emitting element capable of selectively generating light signals of two wavelengths may also be employed.

Claims: Claims: 1. An optical communication network system comprising: a plurality of data terminal stations (2a, 2b, 2n; 3a, 3b, ... 3n; and 6a, 6b, ... 6n); and, an optical communication medium (1; 4a, 4b, ... 4n; and 5a, 5b, ... 5n) for interconnecting said data terminal stations so as to transmit a light signal from each data terminal station to other stations to thereby effect communication among said stations; each data terminal station including; signal transmitting means (13, 14, 15, 11) for delivering a data signal for transmission through said optical communication medium in the form of light signals of the wavelengths X1 and X2 corresponding to two-value states thereof; signal separating means (12, 16, 17) connected to receive a light signal via said optical communication medium for separating the light signal into light signals of the wavelengths X1 and X2; and collision detecting means (19, 20) responsive to the separated light signals to detect an overlap on a time basis between the light signals of the wavelengths X1 and X2, said collision detecting means being arranged to produce, upon detecting an overlap between the wavelengths X1 and A2 over a specified time period, a collision detection signal indicating a collision of transmission signals from said data terminal stations. 2. An optical communication network system according to claim 1, characterized in that said light signal transmitting means includes means (13) for producing complementary signals of the data signal. to be transmitted, and electric-to-optical converter means (14, 15) responsive to the complementary signals and alternately generating the light signals of the wavelengths X1 and X2. 3. An optical communication network system according to claim 2, characterized by further comprising means (11) for multiplexing the light signals of the wavelengths X1 and X2, generated by said electric-to-optical converter means. 4. An optical communication network system according to claim 1, characterized by further comprising demodulation means (18) for demodulating the data signal in response to the separated light signals. 5. An optical communication network system according to claim 1, characterized by further comprising optical-to-electric converter means (16, 17) for converting the separated light signals of the wavelengths X1 and A2 into electric signals, and in which said collision detecting means includes means (19) for obtaining a logical product of the electric signals corresponding to the light signals of the wavelengths X1 and X2. 6. An optical communication network system according to claim 1, characterized in that each said data terminal station includes data terminal equipment (2a, 2b, ... or 2n) and a medium attachment unit (3a, 3b, ... or 3n) for operatively coupling said data terminal equipment to said optical communication medium. 7. In an optical communication network system comprising a plurality of data terminal stations (2a, 2b, ... 2n; and 3a, 3b, ... 3n) and an optical communication medium (1, 4a, 4b, ... Sa, Sb, ...) for interconnecting said data terminal stations to transmit a light signal from station to station, a method of detecting a collision of transmission signals from said and in said optical communication medium data terminal stations comprises the steps of: : converting a data signal to be transmitted into light signals of the wavelengths X1 and X2, corresponding to two value states thereof; alternately delivering the light signals of the wavelengths X1 and X2 into said optical communication medium; separating said light signals received via said optical communication medium into light signals of wavelengths X1 and X2; detecting an overlap, on a time basis, of the separated light signals of the wavelengths X1 and X2; and generating a collision detection signal when the overlap over a predetermined time period is detected.

Inventors: TOMIKAWA, MASATAKA C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: H04L12
IPC codes: H04L12, H04B10