JPH0362062B2 - - 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 and factory 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 communication operation using the CSMA/CD method.

Each node usually intercepts communication between other nodes on bus # at the start of transmission (Carrier Sense),
Transmission starts 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 buses and multiple communication nodes separated from each other that commonly access the multiple communication buses, a specific communication node performs communication between other nodes on each bus at the start of transmission. , selects one of the buses with no communication, starts transmission, and detects that this transmission collides with communication between other nodes that happened to start communication at the same time via the same bus. If this occurs, transmission is immediately stopped, and after an arbitrary period of time has elapsed, one of the buses in the non-communication state, including this bus, is selected again and retransmission is started.

[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 relating to the first embodiment of the present invention, and FIG. 4 is a diagram for explaining its operation.

Figure 3 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 selection 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 selection 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 receiving data line RD. There is.

Next, to explain the operation of this device, when node A wants to start data transmission, it intercepts communication between other nodes on buses #0 and #1 and selects a bus that is in a non-communicating state. and start sending. Note that when both springs #0 and #1 are in a non-communication state, one of the buses is randomly selected using a random number.

FIG. 4 is a diagram showing an example of the operation of this device. The figure shows that after nodes A and B successfully send out transmission bursts A ₁ and B ₁ through bus #0, and node C successfully sends out transmission burst C ₀ through bus #1,
This shows a case where a collision occurs because nodes A and C try to start transmission via bus #0 at the same time.

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 #0 after a certain period of time has passed, whereas the present method Immediately or after an arbitrary period of time has elapsed, the bus selection circuit L shown in FIG. 3 again randomly selects one of the buses in the non-communication state, including this spring #0, and starts retransmission. In the example of FIG. 4, as a result, the transmission burst A ₂ of node A reselects bus #1, and the transmission burst C ₁ of node C reselects bus #0 and retransmits, and the communication is successfully completed. It shows what you did.

As can be seen from the above explanation, according to this method,
The probability that communications will collide on a certain bus is significantly reduced. Furthermore, even if a communication collision occurs, another bus 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, if N buses are used, much larger traffic can be carried than N times the conventional system. 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% maximum), and let η be the upper limit of individual bus 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 buses in this method, N
The probability that all the book buses are occupied is η ^N. 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 buses are prepared, the efficiency of the bus can be dramatically improved compared to when N systems are prepared in the conventional system.In other words, the amount of data that can be processed is 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.

FIG. 5 is a diagram showing the configuration of a second embodiment system of the present invention. The difference from the first embodiment is that the collision detection circuit D2 is not provided for each bus, but is reduced to one per node. For this purpose, the receiver output corresponding to each bus is sent to the collision detection circuit D2.
A switch SW2 has been added to selectively lead to. The operating principle of the system of this embodiment is the same as that explained in FIG. 4 regarding the system of the first embodiment.

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. 6 shows an example of application of the present invention. This application example attempts to perform something equivalent to the method of the previously described embodiments without increasing the number of physical buses, and the basic idea is the same as the present invention.

In the conventional system shown in FIG. 1 and the embodiments of the present invention shown in FIGS. 3 and 5, which have already been described, it is assumed that data is sent to the coaxial cable using a so-called baseband transmission system. However, if a carrier wave modulation transmission method (for example, FM modulation method) is adopted, N buses can be logically constructed on a single coaxial cable by preparing N carrier waves with different frequencies. , the present invention can be applied.

FIG. 6 is an example of this, in which the transmitter TX controls the voltage controlled oscillator VCO to supply carrier waves to it, that is, its oscillation output frequency is set to f ₀ , f ₁
It can be switched by changing . Since the receiving side may receive data from both the two frequencies f ₀ and f ₁ at the same time, the receivers RX0,
RX1 is prepared for each of carrier waves f ₀ and f ₁ .

The operation of this application example method is to connect buses #0 and #1 shown in Fig. 4 to channel CH using carrier waves of f ₀ and f ₁ .
If read as 0 and CH1, it becomes exactly the same as the first and second embodiment systems.

It should be noted that this application example method can of course be applied to transmission media other than coaxial cables as described above.
It is also applicable to a system using multiple satellites in a wireless satellite communication system, or to a satellite communication system equipped with multiple transponders.

[Explanation of effects]

As explained above, the present invention prepares N springs and applies the CSMA/CD method to them, so that the present invention can achieve better performance compared to the conventional CSMA/CD system, especially when N systems are prepared. It is possible to dramatically improve bus efficiency and reduce the probability of communication collisions, and it is possible to process large amounts of high-speed data with a simple system configuration.

[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 the second embodiment system of the present invention. 6th
The figure is a system configuration diagram of an applied 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, the communication bus being composed of a plurality of buses, Each communication node includes a transmitting means for communicating with one of the plurality of buses, a receiving means for intercepting communication on each of the buses, and a collision of communication on each of the buses based on the output of the receiving means. collision detection means, each communication node selects one of the buses in a non-communication state by intercepting communication between other nodes on each bus with the reception means when starting transmission, and transmits the transmission by selecting one of the buses in a non-communication state. and if the collision detection means detects that this transmission collides with communication between other nodes that happened to start communication at approximately the same time via the same bus, the transmission is immediately stopped. In a local network communication method, the means for starting retransmission after an arbitrary time has elapsed, the means for starting retransmission after an arbitrary time has elapsed from the bus that collided with the communication between other nodes. 1. A local network communication system comprising means for selecting one of arbitrary buses in a non-communication state and starting retransmission.