JPH0810864B2 - Collision detection method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-drop communication system and a collision detection system for a communication control device using CSMA / CD as a medium access system.

2. Description of the Related Art A so-called multi-drop communication system in which one communication medium is used by a plurality of communication terminals in time division or frequency division is put on the market as various LANs (Local Area Networks). However, the biggest problem in the multi-drop method is how to use the transmission medium efficiently, that is, how to reduce the empty state of the transmission medium.
It becomes important in operation. Also, since many communication terminals are connected, simultaneous transmission requests may occur, in which case transmission data collisions occur on the transmission medium,
There was a drawback that normal data communication was not possible. CSMA / CD has been proposed as an effective method for eliminating this drawback.

This method checks the empty state on the transmission medium before outputting the data on the transmission medium, outputs the data only when the empty state continues for a period of time determined by the communication protocol, and after outputting the data. , A method of monitoring data on a transmission medium in order to determine whether or not another communication terminal has started data transmission at the same time.

Although the conventional data monitoring timing differs depending on the data code method, as shown in Fig. 5, AMi50% D
In the case of UTY, collision is detected at 1/4 bit length.
Further, in the case of the NRZ code, it is usual to detect a collision at a 1/2 bit length.

Now consider the collision in the case of the AMi 50% DUTY code shown in the conventional example. In FIG. 6, let us consider a case where the communication terminal 9 first starts transmission, the communication terminal 10 detects the traffic, and then starts data transmission in synchronization with it.

In FIG. 7, (A) is the waveform at the location 11 of the transmission data of the communication terminal 9, (B) is the waveform at the location 12 of the communication terminal 9, and (C) is the location 12 of the transmission data of the communication terminal 10. Waveform (D) shows the waveform at the location 11 of the communication terminal 10. T1 in the figure is a delay of the transmission data of the communication terminal 9,
t2 is a processing time when the communication terminal 9 detects traffic on the transmission medium and then outputs its own data to the transmission medium, and t3 is a delay of the transmission data of the communication device 10. Also,
a is a timing at which the communication terminal 9 detects a collision and has a 1/4 bit length from the leading edge, and b is a timing at which the communication terminal 10 also performs a collision detection, at a location with a 1/4 bit length from the leading edge. is there.

Problems to be Solved by the Invention However, as is clear from the figure, the above method has a problem that the communication terminal 9 cannot normally detect collision when t1 + t2 + t3 exceeds 1/4 bit length. Was.

The present invention has been made in view of the above problems, and an object thereof is to provide a collision detection method capable of significantly tolerating a transmission delay due to a long distance of a transmission medium.

MEANS FOR SOLVING PROBLEMS The collision detection method according to the present invention detects a traffic on a transmission medium during counting by a timer of its own and outputs the same, and detects data on a transmission medium in synchronization therewith. The timing of collision detection is changed depending on the case of outputting on the transmission medium.

Effect The present invention provides a bit margin by the above method,
The transmission delay due to the long distance of the transmission medium can be greatly tolerated.

Embodiment Hereinafter, a collision detection method according to an embodiment of the present invention will be described in detail with reference to the drawings.

The operation of the communication system as shown in FIG. 1 will be described. A case where a plurality of terminals 1 to 5 are connected to the transmission medium 8 and data transmission occurs simultaneously from the terminal 1 to the terminal 3 and from the terminal 2 to the terminal 5 will be described. The case of NRZ and start / stop synchronization will be described as the data code and the simultaneous method. There are various data formats for start-stop synchronization, but here, as shown in FIG. 2, a start bit (ST) 1 bit, a data bit 8 bits,
Parity bit (P) 1 bit, stop bit (SP)
It has a 1-bit configuration.

When the terminals 1 and 2 which are communication control devices try to output data onto the transmission medium 8, first, it is confirmed whether or not data from another terminal has already been transmitted onto the transmission medium 8 for a certain period of time. If the data has not been transmitted, the method of outputting the data for the first time is used (CSMA).
In this case, a delay in data transmission on the transmission medium 8
Since the accurate time of the CSMA differs for each communication control, when a data bit from another terminal is detected during the constant time CSMA, the data is immediately output in synchronization with the data (causing collision). ), If data bits from other terminals are not detected during the fixed time CSMA, it is common to start data transmission at the same time as the fixed time elapses. In addition, as the timing of collision detection, it is common to detect at the point of 1/2 bit length in consideration of data delay, etc.In this case, the margin for the deviation of the data bit is 1/2 bit length. Become.

The collision detection method of this embodiment makes it possible to further widen this margin. 3 and 4 are transmission waveforms at the observation point 7 on the transmission medium in FIG.

FIG. 3 shows a case where a data bit from another terminal is detected during CSMA and data is transmitted in synchronization with it, and FIG. 3 (A) shows a waveform of data transmitted by the terminal 1 at an observation point 7, FIG. 3 (B) shows a waveform at the observation point 7 from the terminal 2 output in synchronization with this data. t1 is a delay time due to internal processing from the terminal 2 detecting the leading edge of the data shown in (A) to transmitting the data shown in FIG. 3 (B). In this case, the terminal 2 performs collision detection at a position delayed by Δt1 from the leading edge of the data transmitted by itself. In the case of the start-stop synchronization method, since the level of the start bit always changes, this leading edge can be detected. However, in the case of NRZ code, when the data of "0" or "1" is continuous, the data delimiter cannot be identified. Collision detection is performed by determining the sampling interval according to the speed.

FIG. 4 shows the case where the terminal 2 completes the CSMA and outputs the data, the waveform of (A) is the waveform at the observation point 7 of the data transmitted by the terminal 1, and (B) is the waveform transmitted by the terminal 2. This is the waveform of the observed data at observation point 7. t2 is a signal delay due to the transmission medium. In this case, the terminal 2 detects a collision Δt2 hours earlier than the trailing edge of the data bit output by itself.
It is desirable that Δt1 or Δt2 be as short as possible.

As described above, according to the present invention, by changing the timing of collision detection according to the method by which the communication control device outputs the data bit, a margin close to one bit length can be obtained, and the length of the transmission medium can be increased. It is possible to allow a wide range of transmission delay due to the distance.

[Brief description of drawings]

FIG. 1 is a block diagram showing a network configuration for explaining a collision detection method according to an embodiment of the present invention, and FIG.
The figure is a waveform diagram of a general asynchronous signal in the case of NRZ code, Fig. 3 is the timing chart when data is output in the middle of CSMA, Fig. 4 is the timing chart when data is output after completion of CSMA, and Fig. 5 Is a timing chart showing a collision detection point in the conventional method, FIG. 6 is a block diagram showing a network for explaining the collision detection in the conventional method, and FIG. 7 is a timing chart showing a data delay state on a transmission medium. . 1 to 5 ... Terminal, 8 ... Transmission medium.

Claims (2)

[Claims] 1. In a communication control device using CSMA / CD as a medium access method, after confirming that the transmission medium is empty, when outputting data onto the transmission medium, counting is performed by its own timer or the like. And when outputting traffic on the transmission medium while the timer is counting, and when detecting traffic on the transmission medium while outputting the data on the transmission medium in synchronization with the detection of the traffic. Collision detection method characterized by changing. 2. When counting and outputting by a self timer or the like as the collision detection timing, the collision is detected near the trailing edge of the bit length of the collision target, and the traffic on the transmission medium is detected while the self timer is being counted. The collision detection method according to claim 1, wherein the collision detection is performed near the front edge of the bit length of the collision target in the case of detecting and synchronously outputting.