JPH0519853B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a highly practical information transmission system that can reliably detect collisions in, for example, signal transmission via a carrier band transmission path.

[Technical background of the invention and its problems]

As a bus-type network that does not require a central control station and is highly expandable, it uses a fully distributed peer-to-peer protocol transmission method, namely CSMA/CD (Carrier Sense).
Multiple Access/Collision Detection) method is known. This method performs baseband transmission via a coaxial cable as a transmission path, but recently, in order to use the coaxial cable more efficiently, the baseband signal is converted to a carrier band signal to transmit the baseband signal. The development of a so-called broadband network that aims to make effective use of the Internet is underway.

By the way, on such a carrier band transmission path,
As a challenge in realizing the CSMA/CD method,
It is necessary to ensure the same level of collision detection characteristics as conventional baseband transmission paths, and to ensure compatibility with higher-level protocols. Among these, the collision detection method described above detects that when the levels of beat signals generated by overlapping signals sent from two modems are in the same phase, the peak value is doubled, and a collision occurs. A so-called signal level detection method that determines the presence or absence of a signal is attracting attention. Since this signal level detection method can normally detect signal collisions on a transmission path using only the received signal in the modem, it has the advantage of being simple in structure and reliable in detecting collisions.

However, when using a unidirectional transmission path, there is a delay on the transmission path, so it takes a certain amount of time for the beat signal generated by one signal collision to disappear from the transmission path. This causes the following problems. That is, when focusing on a specific point on the transmission path, a signal whose level intermittently increases will pass for a predetermined period of time in response to one signal collision. In the case of a signal level detection method that detects collisions only from the received signal, it will be judged as if multiple collisions have occurred, and each modem will intermittently send collision detection signals to the upper protocol. I decided to. In response to this, the upper level protocol incorrectly counts the number of collisions, resulting in the loss of packets and erroneous evaluation of transmission characteristics by the monitor.

[Purpose of the invention]

The present invention was made in consideration of these circumstances, and its purpose is to ensure that collision detection on the carrier band transmission path is performed in each modem, and to ensure compatibility with the upper protocol. The purpose of the present invention is to provide a highly practical information transmission method that can perform the following tasks.

[Summary of the invention]

The present invention is a network system based on a completely distributed peer-to-peer protocol transmission method, which detects collisions of signals on a transmission path by monitoring only received signals on a unidirectional transmission path, and detects collisions based on the results. A network system including collision detection means for outputting a display signal is characterized in that the collision display signal continues for a predetermined period of time. Here, the predetermined time is set to be longer than the time required for the collision state on the transmission path to disappear.

That is, it is necessary to set the delay time to be longer than the signal propagation delay time required for a signal to be transmitted from the end of the transmission line to the end of the reception line via the connection between the transmission line and the reception line. Note that it is preferable that this unidirectional transmission path performs signal transmission using a frequency-shifted signal.

Further, it is preferable that the collision detection means detects a beat signal on the transmission path. Furthermore, amplitude monitoring is preferred for detecting beat signals.

〔Effect of the invention〕

According to this invention, once a collision is detected by the collision detection means and a collision display signal is output,
Since the sending of new collision indication signals is prohibited for a predetermined period of time, even if the beat signal caused by a collision is interrupted, this effect will not be transmitted to equipment that processes the upper layer protocol, and this will prevent these equipment from malfunctioning. The danger of triggering is avoided. Therefore, compatibility with the case of using a baseband transmission path can be ensured, making it possible to standardize the hardware and software that processes upper-layer protocols, which can reduce costs and improve reliability. A tremendous effect can be produced.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of a network to which this system is applied, in which a carrier band transmission line 1 is composed of a transmission line 2, a reception line 3, and a head end 4 consisting of a directional coupler. This head end 4
As a result, the signal sent out on the transmission line 2 is transmitted via the reception line 3, forming a so-called double cable network. A plurality of modems 6, 6a, 6b to 6n are connected to such a transmission line 1. It goes without saying that each modem 6, 6a, 6b to 6n is connected to the transmission line 2 and reception line 3 of the transmission line 1 via a directional coupler. Each modem 6, 6a, 6b to 6n is
Each is connected via an interface line to a device that processes a higher-level protocol, such as an information processing device.

The modem 6 is configured as shown in FIG. 2, for example. That is, transmission data consisting of a baseband signal or a signal encoded from the baseband signal given via the interface line from a device that processes the upper protocol is input to the FM modulator 11, frequency modulated, and then bandpassed. The signal is sent to the transmission line 2 via the filter 12 and the transmission amplifier 13. That is, the transmission data is modulated into a carrier band signal with a frequency shift and is transmitted to the transmission path 1. On the other hand, a carrier band signal inputted from the receiving line 3 of the transmission line 1 is inputted via the receiving amplifier 14, and is given to the FM demodulator 16 through the bandpass filter 15. The FM demodulator 16 demodulates the carrier band signal and reproduces the received data of the base band.
This baseband received data is then given to a device that processes an upper level protocol via an interface line.

Further, in this modem 6, the output signal of the bandpass filter 15 is also provided to the collision detector 17. The collision detector 17, whose configuration is shown in FIG. 3, detects a collision by detecting the level of a received signal (carrier band signal), and in particular by detecting the unique level of a beat signal caused by a collision. Then, the detection information is given to the collision indication signal generation circuit 18. This collision display signal generation circuit 1
Reference numeral 8 is a pulse generator of, for example, 10 MHz, which is energized by the beat signal detection, that is, collision detection, by the collision detector 17, and generates a collision indicating signal consisting of 10 MHz pulses. This collision indication signal is given to the device processing the upper layer protocol, and is notified that a collision has occurred on the transmission path 1.

Incidentally, the collision detector 17 that performs such collision detection is constructed as shown in FIG. The signal given from the bandpass filter 15 is detected via an envelope detection circuit 20, and its signal level is determined. Then, the level signal is guided to the amplifier 22 via the low-pass filter 21, where it is amplified with a predetermined gain, and is also input to the first and second comparators 23 and 24, respectively. These first and second comparators 2
3 and 24 are first and second reference signal generators 2;
The signal level is determined by using the reference signals outputted by the reference signals 5 and 26 as threshold values. The first reference signal generator 25 provides a threshold value slightly higher than the received signal level, which is substantially constant over time, when a signal is transmitted from only one modem 6 and no collision occurs. The receiver 23 is configured to output a signal when the received signal level exceeds the threshold value. The second reference signal generator 26 provides a threshold value slightly lower than the temporally constant received signal level, and the second comparator 24 outputs a threshold value when the received signal level falls below the threshold value. It is designed to output a signal. As a result, two or more modems 6 transmit signals at the same time, and by utilizing the fact that the beat signal caused by the collision shows a change in amplitude that is more than twice that of each transmitted signal, the collision can be detected. A condition has been detected. The respective output signals of the first and second comparators 23 and 24 are logically summed via an OR circuit 27, inputted to an AND circuit 28, and are monostable only during the period in which the received signal exists. It is designed to be applied to a multivibrator 29. As a result, due to intermittent increases or decreases in the level of the received signal, the collision indication signal sent to the equipment processing the upper protocol becomes intermittent, causing the equipment processing the upper protocol to malfunction. It prevents you from doing it. The device processing the upper layer protocol detects a collision in each modem using the collision indication signal sent from each modem 6, and then connects the transmission line 1 after the delay time of the interface line has elapsed.
Detects that a collision has occurred on the After the collision is detected, a collision state reinforcement signal called a jam signal is sent for a maximum of 48 bits, and then the signal transmission is stopped. After a process of waiting for a random time called a back-off process, the transmission line 1 is returned to the Check if it is free and try sending again.
In the worst case, the time t ₁ required from the time when the device processing the upper protocol stops selecting signals until the signal in the collision state disappears on the transmission path 1 is equal to the maximum delay time of the interface line t _i , Assuming that the maximum transmission time of the jam signal is t _j and the maximum transmission delay time from the end of the transmission line 2 to the end of the reception line 3 via the head end 4 is t _c , t _i =2t _i +t _j +t _c . Therefore, the time width T of the output signal of the monostable multivibrator 29 is set so that T>2t _i +t _j +t _c . However, in this case, for example, a modem 6 connected to a device that processes the upper layer protocol that is currently transmitting data detects a beat signal on the transmission line 1, and transmits the beat signal to the device that processes the upper layer protocol via the interface line. After notifying the collision on transmission path 1, the device processing the upper protocol interrupts data transmission, performs the aforementioned back-off processing, and then attempts to send data again. By the time it receives the signal on transmission path 1, the Since the signal of the previous collision state does not exist on transmission path 1, the number of collisions may be miscounted in the equipment processing the upper layer protocol, which may cause loss of packets or cause erroneous evaluation of transmission characteristics. There is no danger of doing so.

According to this method, the modem has a mechanism that uses beat signals to detect collisions of carrier band signals sent from multiple modems, and sends a collision indication signal of a fixed time width to the upper layer protocol. The fixed time period is set to be equal to or longer than the time period from when the modem detects a collision on the transmission path until the collision state on the transmission path disappears. When a signal on the transmission path is received in an attempt to send data again after performing back-off processing, there is no risk of receiving a signal in the previous collision state. Full compatibility is maintained. Therefore, it is possible to standardize the hardware and software for processing higher-level protocols, and reduce their cost.
This has great practical effects, such as improved reliability.

By the way, the collision detector 17 used in this method can also be configured as shown in FIG. The signal given from the bandpass filter 15 is detected via an envelope test circuit 30, and its signal level is determined. The level signal is then led to an amplifier 32 via a low-pass filter 31, where it is amplified with a predetermined gain, and is also input to first and second comparators 33 and 34, respectively. These first and second comparators 3
3 and 34 are first and second reference signal generators 3;
The signal level is determined by using the reference signals outputted by the reference signals 5 and 36 as threshold values. The first reference signal generator 35 provides a threshold value that is slightly higher than the received signal level that is substantially constant over time when a signal is transmitted from only one modem 6 and no collision occurs. The receiver 33 is configured to output a signal when the received signal level exceeds the threshold value. The second reference signal generator 36 provides a threshold value slightly lower than the temporally constant received signal level, and the second comparator 34 outputs a threshold value when the received signal level falls below the threshold value. It is designed to output a signal. As a result, two or more modems 6 transmit signals at the same time, and by utilizing the fact that the beat signal caused by the collision shows a change in amplitude that is more than twice that of each transmitted signal, the collision can be detected. A condition has been detected. The respective output signals of the first and second comparators 33 and 34 are logically summed via an OR circuit 37, and inputted to an AND circuit 38. 39, the counter 40 is reset at the rising edge of the output of the AND circuit 38. The output of this counter 40 is applied to the collision display signal generation circuit 18 via a NOT circuit 41 in positive logic. The output of the counter 40 is also given to the AND circuit 38, and while the collision display signal is being sent, the output of the counter 40 is determined by the collision detection information.
0 will not be reset again. On the other hand, the output of the NOT circuit 41 is also input to the AND circuit 42, and the output of the NOT circuit 41 is also input to the AND circuit 42, which clocks the clock generator 4 during the sending of the collision indication signal.
The clock signal from the counter 3 is applied to the counting input of the counter 40. The output of this counter 40 is set after counting the clock signal from the clock generator 43 for a time slightly longer than the time width 2t _i +t _j +t _c . Because the clock signal from the clock generator 43 is not input to the counter 40 due to the action of the AND circuit 42, the collision detection information is again transmitted to the AND circuit 3.
8, this state is maintained until it is applied to the reset input of the counter 40 via the differentiating circuit 39. In this way, even if a counter is used in the collision detection circuit 17, it is possible to obtain an output signal with a time width slightly longer than the time width 2t _i +t _j +t _c after detecting an increase or decrease in the beat signal level. As in the embodiment, complete compatibility with the case where a baseband transmission line is used can be ensured.

[Other embodiments of the invention]

The above embodiments have focused on compatibility with higher-level protocols, but an embodiment that has more flexibility in relation to higher-level information processing devices will be described.

For example, in addition to an interface line for transmitting data between the modem 6n shown in FIG. 1 and an information processing device (not shown), a control line may be used. It is assumed that a control signal is sent to the information processing device via this control line.

That is, once a collision is detected in each modem 6n, a collision display signal with a predetermined time width is sent to the upper information processing device. At the same time, a prohibition signal is sent via the control line. The information processing device that receives this prohibition signal does not enter back-off for a predetermined period of time, and after the predetermined period of time has elapsed, moves to back-off. This predetermined time is also the time during which the beat signal disappears on the transmission path. Alternatively, the information processing device may be configured to send the prohibition signal for the predetermined time period described above and prohibit state changes while the prohibition signal is being supplied. In short, it is important to maintain an inactive state between the information processing device and the transmission path (or modem) for the time period during which the beat signal is thought to remain on the transmission path.

Although the system of the present invention has been described above, the present invention is not limited to these embodiments. For example, in the embodiment, a double-cable network was used as the network to which this method is applied, but a so-called single-cable network in which the transmit signal and the receive signal are frequency-divided using one cable and frequency-converted at the head end is used. Needless to say, the present invention can be applied to any network with one-way transmission, such as a network with a 1-way network. Furthermore, the distinction between a modem and an information processing device is not simply a structural distinction, but it goes without saying that the two may be combined.
Furthermore, although we have taken as an example a collision detection method that detects based on the level of the beat signal, there is a collision detection method that can detect signal collisions on the transmission path using only the received signal, and the detection is intermittent. It goes without saying that the transmission line 1 can be realized using twisted pair wires or coaxial cables that handle electrical information, but the transmission line 1 can also be realized using light as the information medium. It is also possible to configure it with an optical fiber cable. In short, the present invention can be implemented with various modifications without departing from the gist thereof.

[Brief explanation of drawings]

The figures show an embodiment of the present invention. Figure 1 is a network configuration diagram, Figure 2 is a modem configuration diagram,
FIG. 3 is a block diagram of the level detector, and FIG. 4 is another block diagram of the level detector. 1... Carrier band transmission path, 4... Head end,
6, 6a to 6n...Modem, 17...Level detector, 20,30...Short line detection circuit, 23,2
4, 33, 34... comparator, 25, 26, 35,
36...Reference signal generator, 39...Differentiating circuit, 4
0...Counter, 43...Clock generator.

Claims (1)

[Scope of Claims] 1. A unidirectional transmission line in which a transmission line and a reception line are connected at one end, a modem connected to the transmission line and reception line, and an information processing unit connected to this modem. In a network system comprising devices, the modem includes a receiving means for receiving signals on the unidirectional transmission path, and detecting whether or not there is a collision of signals on the unidirectional transmission path according to a reception result by the receiving means. and a signal sending means for sending a collision indication signal to the information processing device according to a detection result by the collision detection means, wherein the collision indication signal is generated at least on the unidirectional transmission path. An information transmission system characterized in that the signal is transmitted from the signal transmission means until the time required for the signal to be transmitted on the unidirectional transmission path after a collision is detected by the collision detection means. 2. The information transmission system according to claim 1, wherein the signal on the transmission path is subjected to a frequency shift. 3. The information transmission system according to claim 1, wherein the one-way transmission line consists of a transmission line and a reception line, and is connected at the head end. 4. The information transmission system according to claim 1, wherein the one-way transmission path is formed by frequency-dividing a single signal transmission path. 5. The information transmission system according to claim 1, wherein the collision detection means comprises means for comparing the amplitude of the received signal with a predetermined value.