JPH0362063B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field to which the Invention Pertains] The present invention relates to a local network communication system, and particularly to the CSMA/CD system (Carrier Sense
Related to communication methods using Multiple Access/Collision Detection methods).

[Description of prior art]

In recent years, local networks, which interconnect computers and data terminal equipment located in the same building, factory, or site, have been attracting attention as a basic means of promoting office automation. , various methods have been proposed and put into practical use. One of the most famous methods is the CSMA/CD bus-type communication method, which is attracting attention as a simple and low-cost method. A typical example of this method is a local network called Ethernet, which was jointly developed and announced by DEC, Intel, and Xerox.

FIG. 1 shows the basic configuration of this system, and shows an example in which nodes A and B, which are separated by a distance, are accessing a bus # using a coaxial cable. Although several dozen nodes generally access the same bus, the figure shows a case where there are only two nodes A and B for simplicity.

Each node has a transmitter TX that receives the transmitted data SD and sends a signal to the coaxial cable, and a transmitter TX that receives the transmitted data SD and sends a signal to the coaxial cable.
A receiver RX that sends out the RD to the node terminal, a collision detection circuit D that detects a transmission/reception collision and sends a collision detection output CD to the node terminal, and a transmitter.
It consists of a tap T, which is the means to physically connect the TX and receiver RX to the coaxial cable.

FIG. 2 is a diagram for explaining an example of the operation of the communication system based on the present CSMA/CD system.

Each node usually intercepts communications between other nodes on the bus (carrier sense) when starting transmission, and starts transmission only when it is confirmed that there is no communication. The transmission burst A ₁ from node A and the transmission burst B ₁ from node B in FIG. 2 are examples of cases in which each node transmits data in this manner and communication is successfully completed. However, even if a node starts transmitting after confirming that there is no communication, other nodes may also start transmitting in the same way, and in this case, collisions between transmission bursts occur on the same bus.

In the CSMA/CD system, a collision detection circuit D shown in FIG. 1 is provided for the purpose of detecting this collision. The specific collision detection method and its circuit configuration are well known to those skilled in the art, and further detailed description thereof will be omitted here.

When the collision detection output CD is output, the node that started transmitting immediately stops transmitting, and each node retransmits after a certain period of time, which is set by a random number for each node, has elapsed. Transmission burst A ₂ in Fig. 2,
C ₁ is an example of this.

As can be seen from the above explanation, the CSMA/CD method is characterized by a very simple system configuration, but on the other hand, due to its principle, the bus usage efficiency cannot be very high, and the CSMA/CD method cannot prevent interception and collisions. Detection (Carrier Sense & Collision
The disadvantage is that the bus speed itself cannot be very high (on the order of 10 Mbps) due to the inter-node propagation delay time in order to efficiently perform detection. Therefore, it can be said to be an extremely advantageous method when accommodating low-speed data terminals in a node.
This method is not suitable for accommodating high-speed data terminals, image terminals, or a large number of voice information terminals.

[Purpose of the invention]

The present invention provides a local network communication method that can process large amounts of high-speed data and improve bus usage efficiency while maintaining the simple system configuration of the CSMA/CD method. The purpose is to

[Key points of the invention]

In a bus communication system consisting of multiple communication nodes separated by a distance that commonly access a communication bus using carrier waves of different frequencies, a specific communication node performs communication between other nodes using each carrier wave at the start of transmission. Intercept, select one of the uncommunicated carrier waves, and start transmitting.
If this transmission happens to collide with communication between other nodes that started communication at the same time via the same carrier wave, the transmission will be stopped immediately, and after an arbitrary period of time, the bus will be in a state of no communication again. It is characterized by selecting one of arbitrary carrier waves and starting retransmission.

[Explanation based on examples]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 3 is a system configuration diagram showing one embodiment of the present invention, and FIG. 4 is a diagram for explaining its operation.

In FIG. 3, each node is connected to a bus # consisting of a coaxial cable by a tap T (only node A is shown in FIG. 3).

Node A has a transmitter TX0 and a receiver RX0,
RX1, voltage controlled oscillator VCO, collision detection circuit D, selection circuit L, buffer memory circuit BM
and a switching circuit SW0.

The transmitter TX0 is a carrier modulation transmission method (for example, FM modulation method) transmitter that receives transmission data SD and sends a signal to bus #, and transmits two frequencies f ₀ and f ₁ supplied from the voltage controlled oscillator VCO. is its carrier wave. On bus #, a channel CH0 is created by the carrier wave f ₀ and a channel CH1 is created by the carrier wave f ₁ . Receivers RX0 and RX1 are receivers that receive modulated signals of carrier waves with frequencies f ₀ and f _1, respectively, and take the information flowing on bus # into the node and convert it into received data.
Send it to the notebook terminal as RD. Selection circuit L
sends data SD to channel CH0, channel CH
This circuit determines which one of the oscillators should be transmitted through, and controls the voltage controlled oscillator VCO to change its oscillation frequency. The collision detection circuit D is a circuit that detects collisions of transmissions in each channel by each carrier wave and sends a collision detection output CD to the node terminal. The switching circuit SW0 connects the receivers RX0 and RX1 to detect the presence or absence of communication collision in each channel.
This is a switch for selectively guiding the output of the collision detection circuit D to the collision detection circuit D. The buffer memory circuit BM is a circuit for making it possible to send out data to one reception data line RD even when there is data arriving at the node from two channels at the same time.

Next, to explain the operation of this device, when node A wants to start data transmission, it intercepts communication between other nodes on channels CH0 and CH1, selects a channel with no communication, and transmits data. Start. Note that when both channels CH0 and CH1 are in a non-communication state, one of the channels is randomly selected using a random number.

FIG. 4 is a diagram showing an example of the operation of this device. In the figure, after nodes A and B have successfully sent out transmission bursts A ₁ and B ₁ through channel CH0, and node C has successfully sent out transmission burst C ₀ through channel CH1, nodes A and C are sent at the same time. Channel CH
This shows a case where an attempt to start transmission via 0 causes a collision.

In the conventional method shown in FIG. 1, in such a case, nodes A and C immediately stop transmission and attempt retransmission via the same bus # after a certain period of time has passed, whereas in the method of the present invention, Immediately or after an arbitrary period of time has elapsed, the channel selection circuit L shown in FIG. 3 again randomly selects one of the channels in the non-communication state, including this channel CH0, and starts retransmission. The example in Figure 4 shows that, as a result, node A's transmission burst _A2 is channel
CH1, and the transmission burst _C1 of node C reselects channel CH0 for retransmission, indicating that the communication has been successfully completed.

As can be seen from the above explanation, according to this method,
The probability that communications will collide on a certain channel is significantly reduced. Furthermore, even if a communication collision occurs, another channel is reselected, increasing the probability of successful retransmission. Furthermore, the waiting time (backoff processing time) until retransmission is attempted after a collision can be reduced. For these reasons, using N channels allows much greater traffic to be carried than using N conventional systems. This will be explained below using numerical examples.

Let the upper limit of bus usage efficiency in the conventional CSMA/CD method be η ₀ (said to be about 50% at most), and let η be the upper limit of individual channel efficiency in the case of this method.

In the conventional method, the probability that the bus is blocked when a node attempts to start communication is η ₀ . When using N channels in this method, the probability that all N channels are blocked is η ^N
It is. Therefore, if the service standards for both are set equally, η ₀ =η ^N . Now, assuming that η ₀ =50%, when N=2, η=71%, and when N=3, η=79%. In other words, according to this method, when N channels using carrier waves with different frequencies are prepared, the efficiency of the bus can be dramatically improved compared to when N channels are prepared using the conventional system. , the amount of data that can be processed can be dramatically increased.

The above comparison was based on the bus blockage rate, but it can also be proven that the probability of communication collisions occurring on a particular bus can be dramatically improved. However, details are omitted here.

The system of the present invention can of course be applied to transmission media other than coaxial cables, and can also be applied to systems using multiple satellites in wireless satellite communication systems, or to satellite communication systems equipped with multiple transponders.

[Application examples of the invention]

FIG. 5 is an example of application of the present invention. This application example attempts to do something equivalent to the method of this embodiment by increasing the number of physical buses, rather than providing multiple channels by frequency division as in the present invention, and the basic The idea is the same as that of the present invention.

In this method, like the conventional method shown in FIG. 1 described above, data is sent to a coaxial cable using a so-called baseband transmission method, and N buses are constructed using a plurality of coaxial cables.

Figure 5 is an example of using buses #0 and #1 with two coaxial cables, and each node has transmitters TX0 and TX1, receivers RX0 and RX1, and receivers corresponding to buses #0 and #1. , collision detection circuit D0, D
1 is available. Furthermore, each node includes a circuit L that determines whether to send the transmission data SD to bus #0 or bus #1, and a switching circuit SW that is controlled by this circuit L to switch the bus to which the transmission data SD is sent. Furthermore, a buffer memory circuit BM is provided so that even if there is incoming data to the node from two buses at the same time, it can be sent to one receive data line RD.
is provided.

The operation of this application example device is based on the frequency shown in Figure 4.
If channels CH0 and CH1 based on the carrier waves of f ₀ and f ₁ are read as bus #0 and bus #1, the system becomes exactly the same as the system of the embodiment of the present invention.

FIG. 5 is a diagram showing the configuration of the second application example system. The difference from the above-mentioned application example method is that the collision detection circuit D2 is not provided for each bus, but only one collision detection circuit D2 is provided for each node.
It was reduced to 1. For this purpose, a switch SW2 is added so that the receiver output corresponding to each bus can be selectively guided to the collision detection circuit D2.

The operating principle of this second applied example system is the same as that described for the applied example system described above.

Note that this application example system can of course be applied to transmission media other than coaxial cables.

[Explanation of effects]

As explained above, the present invention provides N channels using carrier waves of N different frequencies,
By applying the CSMA/CD method to this,
Compared to the conventional CSMA/CD system, especially when it is used as an N system, it can dramatically improve bus efficiency and reduce the probability of communication collisions, even though it has a simple system configuration. It becomes possible to process large amounts of data at high speed.

[Brief explanation of drawings]

Figure 1 is a system configuration diagram of the conventional CSMA/CD system. FIG. 2 is an explanatory diagram of the operation of the method shown in FIG. FIG. 3 is a system configuration diagram of the first embodiment system of the present invention. FIG. 4 is an explanatory diagram of the operation of the method shown in FIG. Fifth
The figure is a system configuration diagram of an applied example system of the present invention. FIG. 6 is a system configuration diagram of a second application example system of the present invention. TX0, TX1...Transmitter, RX0, RX1...
Receiver, D0, D1, D2... Collision detection circuit, L
...Selection circuit.

Claims (1)

[Claims] 1. A bus communication system comprising a communication bus and a plurality of communication nodes spaced apart from each other that commonly access the communication bus, and each of the communication nodes transmits a plurality of carrier waves having different frequencies. a transmitting means for transmitting using a carrier modulation transmission method, a receiving means for intercepting communications using each of the carrier waves, and a collision detecting means for detecting a collision of communications on each of the carrier waves based on the output of the receiving means, each communication When the node starts transmitting, means for selecting one of the carrier waves in a non-communication state by intercepting communication between other nodes using the respective carrier waves with the receiving means and starting transmission with the transmitting means; If the collision detection means detects that the collision detection means collides with communication between other nodes that happened to start communication at approximately the same time using the same carrier wave, means for immediately stopping transmission, and means for starting retransmission after a certain period of time has elapsed. In the local network communication system, the means for starting retransmission is configured to transmit one of the arbitrary carrier waves in a non-communication state, including the carrier wave that collided with the communication between the other nodes, after an arbitrary period of time has elapsed. A local network communication method characterized by comprising means for selecting one and starting retransmission.