JP2007282289A - Communication modem and communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a communication modem which avoids more positively a signal collision on a transmission line, and prevents throughput degradation. <P>SOLUTION: A receiver 21 of the communication modem, when a pre-carrier/sense condition is satisfied which can be established earlier than a carrier/sense condition for deciding that a carrier on the transmission line is detected and constitutes a part of the carrier/sense condition asserts a pre-carrier/sense signal indicating that the pre-carrier/sense condition is established. When the pre-carrier/sense condition breaks down after the pre-carrier/sense signal is asserted, the receiver negates the pre-carrier/sense signal. A transmitter 22 of the communication modem, even when a carrier/sense signal indicating the establishment of the carrier/sense condition is not asserted, stops starting the sending of a transmission signal during asserting the pre-carrier/sense signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to shortening of communication establishment time in communication using a noisy line such as a household electric power line.

Power line communication that uses home power lines as transmission media can use existing transmission media, so there is no need to install new transmission media, and communication systems can be built at low cost. Can do.
Usually, a communication system using a power line forms a multi-access communication system because a plurality of devices (power line modems) for communication are connected to the same power line and communicate with each other. One of the communication methods used in such a multi-access communication system is a CSMA / CA (Carrier Sense Multiple Access with Collision Avidance) method.
In the CSMA / CA system, a carrier signal (hereinafter referred to as a carrier) on a transmission path is monitored, a reception operation is performed when a carrier is detected, and transmission is performed only when no carrier is detected. This reduces the probability of communication signal interference.
In the carrier detection process (carrier sense), a condition (carrier sense condition) in a predetermined period is compared with an input signal from the transmission path to determine whether the noise is on the transmission path or an actual communication signal. In general, if it takes time to perform carrier sense, the possibility that a noise is erroneously received as a communication signal is reduced accordingly.

  On the other hand, in power line communication, various household electrical appliances are connected on the transmission line, and there is a lot of noise generated from them, and it is unpredictable in terms of time, frequency, or position on the transmission line. Since it is possible, it can be said that the communication is in a poor environment from the viewpoint of noise on the transmission path. For this reason, there is a high possibility that a carrier is detected erroneously as a communication signal and reception processing is performed. Even if a carrier is erroneously detected due to noise, reception processing is performed according to the same processing procedure as in the case of a normal communication signal, so a certain amount of time is required before returning to carrier sense again after reception processing. Cost. Therefore, if the original communication signal flows on the transmission line during the reception process for noise, there is a possibility that the communication signal may be missed, or in a system where the transmission is synchronized with each other It becomes a factor of breakage. As described above, if the carrier detection conditions are made stricter and more time is spent on the carrier detection determination, the possibility of erroneously detecting the carrier is reduced, but on the other hand, the throughput is lowered. Also, for example, in a system in which transmission is started immediately if a carrier is not detected when a transmission request is generated, increasing the time required for carrier detection determination is the probability that interference will occur. Will increase.

The operation from the first AGC control to the payload reception in these conventional communication devices is shown in FIG. In the figure, the communication device is first waiting in the reception state and in the level detection state. When the level is detected, AGC (automatic gain control) is performed in S501. When a symbol is detected in S502, this symbol does not mean 0 in S504. In step S506, it is confirmed that 6 symbols have been received and carrier sense is detected. Thereafter, in step S507, field code (FC) is detected, and the payload receiving operation in step S509 is performed. When FC is detected, the time monitoring in S511 is performed until the end of the determined FC reception, and nothing is done until this end.
Further, once carrier sense is performed, a synchronization timer for prohibiting the transmission operation is reset.

As an example of taking measures against interference in a wireless LAN using the same CSMA / CA method, there is a method disclosed in Patent Document 1.
According to the above document, when transmitting, when the maximum packet length is received, carrier sense is performed for a time longer than the processing time required for reply, and transmission is erroneously performed in the gap between transmission and reception. Therefore, it discloses a method for preventing the occurrence of interference. In addition, when transmitting packets continuously, a method for efficiently transmitting and receiving is disclosed by making the carrier sense time shorter than the above processing time.
However, in this method, the transmission of the first packet is always delayed by the processing time, so that a certain amount of time is consumed even when there is no communication before and after, resulting in a decrease in throughput. Yes. In addition, no contribution is made to the interference between the first packets.
JP-A-11-331185

As described above, in power line communication, there is a lot of noise on the transmission path, so there is a high probability that a carrier will be detected by mistake. However, in order to reduce this probability, the conditions for carrier sense are strict and the time required for determination is lengthened. Doing so causes a problem of reducing the throughput and increasing the possibility of interference.
In addition, if a carrier is detected by mistake, there is a possibility that data that should be received will be missed as much as it takes time to recover, and the synchronization between the modems may be lost. There is also a problem of being.

The present invention has been made to solve the above-described problem, and shortens the return to the initial state when the carrier sense is erroneously detected, reduces the possibility of interference, and prevents the throughput from being lowered. The purpose is to obtain.
Similarly, it is possible to obtain a communication modem in which the time for returning to the re-reception operation due to reception failure is shortened or the transmission start time is shortened to prevent the throughput from being lowered.

The return procedure shortened communication modem according to the present invention is a communication modem that monitors signal collision in a transmission line and starts transmission when there is no collision.
After the carrier sense condition of the received signal being monitored is established, a control circuit is provided that outputs a reset signal that returns to the first phase for monitoring signal collision when the carrier sense condition is broken.

  Furthermore, the carrier sense condition is broken because the signal level to be monitored is below a predetermined value.

  Furthermore, the carrier sense condition is broken because the signal-to-noise ratio to be monitored is deteriorated to become a predetermined value or less.

  Furthermore, the control circuit outputs a condition change signal to the carrier sense detection side so as to change the carrier sense condition.

Furthermore, a time determination circuit for checking whether a specified time has elapsed after the carrier sense is provided.
After the carrier sense condition of the received signal being monitored is established, if this carrier sense condition breaks down, instead of returning to the first phase for monitoring signal collision, the time determination circuit has assumed that the specified time has elapsed In this case, the synchronization is reset, and if the specified time has not elapsed, the process returns to the first phase for monitoring the signal collision.

Or, in a communication modem that monitors signal collision in the transmission path and starts transmission when there is no collision,
A receiver that sets a precarrier sense signal when detecting a symbol to be monitored, and resets the precarrier sense signal when this symbol detection is interrupted,
A transmission unit that suppresses the start of transmission while the precarrier sense signal is set when there is a transmission request.

  The carrier sense signal is interrupted because the symbol to be monitored cannot be detected or the signal level becomes a predetermined value or less.

  Furthermore, the transmission unit checks the type of transmission frame, and in the case of a predetermined type, when there is a transmission request, the transmission unit confirms that the precarrier sense signal has been reset and starts transmission.

The return procedure shortening communication method according to the present invention is a communication method for monitoring signal collision in a transmission line and starting transmission when there is no collision.
An erasure detection step for detecting the disappearance of the carrier sense condition after the carrier sense condition of the received signal being monitored is established,
A step of returning to the first phase of monitoring for signal collision when the disappearance is detected.

Furthermore, a time determination step for checking whether a specified time has elapsed after the carrier sense is provided,
When the carrier sense condition for the received signal being monitored is satisfied, if the carrier sense condition disappears, instead of the step to return to the first phase for monitoring signal collision, the specified time has elapsed in the time determination step above Has steps to resync,
When the specified time has not elapsed, the step is to return to the first phase for monitoring the signal collision.

Or, in a communication method for monitoring signal collision in a transmission line and starting transmission when there is no collision,
Setting a pre-carrier sense signal when detecting a symbol to be monitored;
Resetting the pre-carrier sense signal when the symbol detection is interrupted;
A step of suppressing transmission start when a pre-carrier sense signal is set when there is a transmission request.

A communication modem according to the present invention includes:
When the carrier sense signal indicating that the carrier sense condition for determining that the carrier on the transmission line has been detected by monitoring the transmission line is not asserted, the transmission signal is transmitted in response to the transmission request. In the communication modem to start,
When detecting a symbol to be monitored, assert a precarrier sense signal indicating that a symbol has been detected, and when detecting a symbol is interrupted, a receiving unit that negates the precarrier sense signal;
A transmission unit that suppresses the start of transmission of a transmission signal while the precarrier sense signal is asserted even when the carrier sense signal is not asserted is provided.

The above communication modem
The symbol detection is interrupted because the symbol to be monitored cannot be detected or the signal level of the monitoring target becomes a predetermined value or less.

  The transmission unit starts transmission of a transmission signal in response to a transmission request when the type of the transmission signal is a predetermined type even while the precarrier sense signal is being asserted.

A communication method according to the present invention includes:
When the carrier sense signal indicating that the carrier sense condition for determining that the carrier on the transmission line has been detected by monitoring the transmission line is not asserted, the transmission signal is transmitted in response to the transmission request. In the communication method to start,
When the communication modem receiving unit detects a symbol to be monitored, asserting a precarrier sense signal indicating that the symbol has been detected;
The receiver of the communication modem negates the precarrier sense signal when the detection of the symbol is interrupted;
The transmitter of the communication modem includes a step of suppressing transmission of a transmission signal while the precarrier sense signal is asserted even when the carrier sense signal is not asserted. Features.

  According to the present invention, when the carrier sense condition is broken after the carrier sense condition is established, the control circuit that outputs the reset signal to return to the first phase for monitoring the signal collision is provided. In this case, there is an effect of shortening the return to the initial state and preventing a decrease in throughput as a result.

  In addition, since the time determination circuit for checking the lapse of the specified time after the carrier sense is provided, not only the transmission control can be started early, but also there is an effect of preventing unnecessary transmission in some cases.

  Also, a receiver that sets a precarrier sense signal in symbol detection and resets when the carrier sense is interrupted, and a transmitter that confirms resetting of the precarrier sense signal and starts transmission upon request for transmission, Therefore, there is an effect of avoiding a decrease in throughput by avoiding a collision.

Embodiment 1 FIG.
The configuration and operation of a communication modem in which the time required for returning to the re-reception operation by reset in the initial level monitoring state will be described.
FIG. 1 is a flowchart showing the operation of the communication device in the present embodiment, and FIG. 2 is a diagram showing the configuration thereof. FIG. 3 is a diagram for explaining the operation of the power line modem shown in FIG. In FIG. 2, 1 is a power line as a signal transmission path, 2 is an entire power line modem, and 3 is a control device. The power line modem 2 is composed of 21 receiving units and 22 transmitting units, and exchanges commands and responses with the control device 3 via 23 control device interfaces (I / F). The receiving unit 21 further includes the following elements. That is, 201 is an AGC (automatic gain control) circuit that amplifies the amplitude of the received signal, 202 is an AD converter that converts analog data into digital, 203 is a digital signal processing circuit that performs signal filtering and fast Fourier transform, 204 Is a demodulator that demodulates the received signal and outputs the demodulated result to the tone verification unit, 205 is a level detection unit that checks the level of the received signal, and 206 is whether the received signal of the demodulated result is logical data or 0 data. The tone verification unit that discriminates and outputs to the CS determination unit, etc., 207 checks the carrier sense, in this case the CS determination unit checks whether six 0 symbols continue, 208 is the entire operation from various states during signal reception A reception control unit that controls the 209, an FC determination unit that determines whether a field code has been received, and a 210 that has a field code, Header receiving section for extracting a header from the signal signal, payload reception unit for extracting and outputting data to the control device I / F based on 211 likewise header of the payload described, 212 is a control device interface.

The reception operation will be described with reference to the configuration diagram of FIG. 1, the flowchart of FIG. 2, and the operation explanatory diagram of FIG.
The power line modem 2 of the communication device always stands by in the reception state except for the transmission period and monitors the power line. Reception monitoring starts from level detection, but the basic operation from step S101 to payload reception is the same as the conventional one. That is, when a signal is detected, the AGC 201 amplifies the signal, and the demodulation result detects a symbol a predetermined number of times. In this case, six symbols 0 are detected in S106, and FC detection is determined in S107. From this symbol detection S102 to a predetermined number of symbol detections S106 is a carrier sense state. That is, after this flow, the carrier sense condition is established. The received communication frame is composed of a header in which the frame length, the source of the control device, the address, etc. are described, and a payload in which the content for the control device is described, following the preample for carrier detection. .
A new function of the power line modem of this embodiment is that priority is given to the operation of the level detection unit 205 in S110. In other words, even in the state where it has advanced to the reception detection state of the field code, upon receiving a level decrease notification from the level detection unit 205, the reception control unit 208 immediately activates the reset signal 221 and controls the AGC circuit 201 for reception. Move to initial state (first phase). That is, the priority is given to the notification from the level detection unit 205. Furthermore, the operation may always return to the first phase S101. Further, instead of the level detection, the first phase may be returned due to the deterioration of the signal-to-noise ratio (S / N).

  Referring to the explanatory diagram of FIG. 3, in FIG. 3 (a), after carrier sense, the operation shifts to FC detection, and when FC detection is not possible, it waits for FC timeout and returns to the initial state. In the configuration of the present embodiment shown in FIG. 5, even if the FC detection state is entered after the carrier sense, if the level detection unit 205 receives a level drop notification (S110), the state immediately moves to the initial state and the subsequent normal communication frame is received. In response to this, it is possible to shift to CS processing, FC detection processing, header reception / payload reception. That is, the chance of failing to receive a regular communication frame can be reduced.

In the configuration of FIG. 2, details of the level detection unit 205 and the reception control unit 208 are not shown, but noise is received immediately before and CS processing is erroneously performed. As a result, FC detection cannot be performed, that is, normal reception cannot be performed. May be stored in the reception control unit 208 and notified to the level detection unit 205 as a level shown in the figure, and the level detection unit 205 may change the reference value for detecting the level decrease based on this notification. . Alternatively, the levels at the time of failure may be stored, and then changed to a reference value higher than these levels at the time of failure. Thus, for example, if the level reference is raised when there are many failures, it can be expected to reduce false responses to noise.
Instead of changing the level reference value, a specified value for carrier detection, for example, 6 times, may be increased by a frequency signal to the illustrated CS determination unit.
Further, for example, a control program may be incorporated in the reception control unit, and the operation function shown in FIG. 1 may be provided so that a flow of monitoring the level of S110 after FC detection and returning to the initial state when the level is lowered may be provided.

Next, the reset operation of the synchronization timer will be described with reference to FIG. 1, FIG. 4 which is a detailed configuration diagram, and FIG. 5 which is an operation explanatory diagram.
FIG. 4 is a diagram showing details of the reception control unit 208 in FIG. 2, and includes a reception state control circuit 231, a symbol counter 232, and a synchronous timer reset control circuit 233. The synchronization timer 235 itself sets a transmission prohibition period of the communication device, and sets a period during which transmission is not performed for the purpose of reception priority, collision avoidance, and the like. In the connection of FIG. 4, when carrier sense is detected in S106, the reception state control circuit 231 resets the symbol counter 232 and prepares for the next carrier sense. This state is transmitted to the synchronous timer reset control circuit 235 and compared with the post-CS time determination circuit 234.
In terms of operation, there is provided a step for checking whether or not the specified time has elapsed after the carrier sense in S120 in the operation flow of FIG. That is, the time, that is, the number of symbols is counted, and if the count value of the number of symbols is equal to or less than a specified value (time), it is determined as noise and synchronization is not performed again. If the count value is equal to or greater than the specified value, it is determined that the frame is a collision frame or an unsupported frame, and synchronization is resumed.

In the conventional example of FIG. 5B, for example, the transmission prohibition period is set to 50 h symbols, and in the operation flow of FIG. 5C, the synchronization timer is reset after FC timeout or frame timeout (FIG. 5). (B)). On the other hand, in the configuration having the functions of FIG. 4 and FIG. 1, as shown in FIG. 5 (f), it is checked whether the specified time after CS of S120 has elapsed, and the synchronization timer is reset only when the time has elapsed. In this case, the synchronization timer is not reset. Therefore, when the terminal itself has a transmission request, the transmission period is entered after the prohibition period of 50 h symbols. This has the effect of preventing useless transmission such as when the opposite communication destination requests a response from the partner, the response is normally returned, and the transmission process is delayed and a retransmission message arrives from the other party.
Also in this case, for example, a control program is incorporated in the reception control unit, and the operation function shown in FIG. May be.

Embodiment 2. FIG.
Another case will be described in which the time from the standby state to the start of transmission is shortened to prevent a decrease in throughput.
FIG. 6 is a flowchart showing the operation of the communication modem in the present embodiment. In the present embodiment, the concept of pre-carrier sense is introduced, and even when there is a transmission request from the transmission unit at the time of collision detection with reception priority, first the pre-carrier sense state is detected to prepare for transmission. Wait for transmission until it is determined that the reception is not started. Conversely, if it is determined that the reception is not correct due to noise reception or the like, transmission is started immediately.
The transmission unit 22 that has received the transmission request from the control device 3 cancels the transmission request in order to avoid interference when the carrier is detected according to the determination result of whether or not the carrier is detected. No transmission is performed. On the other hand, when no carrier is detected, the transmission data sent from the control device 3 is sent to the transmission unit 22, and the transmission unit 22 modulates the transmission data, and then transmits the transmission line. Outputs transmission data for.

The carrier detection requires a certain time because, for example, the determination is made based on whether or not the number of consecutive 0s of the logical data is counted and the specified number is reached. When the number of times less than the specified number of times, that is, a condition (precarrier sense condition) that can be determined earlier in time than the detection of carrier detection is separately specified, and this precarrier sense condition is satisfied Even if a transmission request is generated from the control device 3, the transmission is not started immediately, but the transmission is waited for the time necessary to determine the carrier sense condition. When the carrier sense condition is satisfied while waiting (when 0 consecutive reaches the specified number), transmission is canceled and reception processing is performed, and when the carrier sense condition is not satisfied (the specified number is reached) When 0 continuation is interrupted before or when the signal level drops below a predetermined value), the precarrier sense signal is reset, and it is determined that noise has been input, and transmission processing is started at that time.
In this way, by providing the precarrier sense condition, even when a transmission request is generated when a carrier is detected in a short time in a state where the demodulation result is 0, transmission is not started immediately. Therefore, it is possible to avoid interference without changing the probability of erroneously detecting a carrier.

FIG. 7 is a timing chart for explaining the effect of precarrier sense. For comparison, the same operation according to the conventional flow of FIG. 8 will be described. That is, when there is a transmission request in S551 in FIG. 8, only carrier sense confirmation in S552 is performed, and transmission is started immediately in S553. As a result, as shown in FIG. The probability of causing a collision is increased.
On the other hand, in the communication modem according to the present embodiment, if there is a reception signal on the transmission line, the precarrier sense signal is set in S133 upon reception of one symbol, so transmission is awaited in S152 and S153, and Collisions are avoided.

In addition, a type is provided in the transmission frame, that is, type information is described in the header, and a specific type of communication frame according to this description ignores whether or not the precarrier sense condition is satisfied and transmits immediately. You may make it leave. In this case, for example, when there is information that only the modem specified in the destination address in the header returns a response as the type of frame, a transmission request is received from the control device 3 based on this information. Even if the pre-carrier sense condition is satisfied, there is a high possibility that the pre-carrier sense condition is caused by noise, and communication can be performed efficiently without inadvertently delaying transmission by starting transmission as it is. . Alternatively, a plurality of precarrier sense conditions may be provided, and the conditions may be selected according to the priority order of types. Alternatively, a predetermined transmission frame signal may be obtained and the apparatus may wait only when transmitting this transmission frame.
Furthermore, by changing the precarrier sense condition according to the number of precarrier detections relative to the number of receptions, communication can be performed efficiently without delaying the start of transmission in response to excessive noise in accordance with the characteristics of the transmission path. be able to.

As described above, the communication modem according to the present invention is
In a communication modem that monitors signal collision in the transmission path and starts transmission when there is no collision,
A control circuit is provided that outputs a reset signal that returns to the first phase for monitoring signal collision when the carrier sense condition breaks down after the carrier sense condition of the received signal being monitored is satisfied.

The above communication modem
The carrier sense condition is lost because the signal level to be monitored is equal to or lower than a predetermined value.

The above communication modem
The carrier sense condition is lost because the signal-to-noise ratio to be monitored is deteriorated to become a predetermined value or less.

  The control circuit outputs a condition change signal to the carrier sense detection side so as to change the carrier sense condition.

The above communication modem
With a time judgment circuit that checks whether a specified time has elapsed after carrier sense,
After the carrier sense condition of the received signal being monitored is satisfied, if the carrier sense condition breaks, instead of returning to the first phase for monitoring signal collision, the time determination circuit has passed the specified time In such a case, the synchronization is reset, and if the specified time has not elapsed, the process returns to the first phase for monitoring the signal collision.

The communication modem according to the present invention is
In a communication modem that monitors signal collision in the transmission path and starts transmission when there is no collision,
A receiver that sets a precarrier sense signal when a symbol to be monitored is detected, and resets the precarrier sense signal when the symbol detection is interrupted;
A transmission unit that suppresses the start of transmission while the pre-carrier sense signal is set when a transmission request is made.

The above communication modem
The carrier sense signal is interrupted because the symbol to be monitored cannot be detected or the signal level becomes a predetermined value or less.

  The transmission unit looks at the type of transmission frame, and in the case of a predetermined type, when there is a transmission request, it confirms that the precarrier sense signal has been reset and starts transmission. .

A communication method according to the present invention includes:
In a communication method for monitoring signal collision in a transmission line and starting transmission when there is no collision,
An erasure detection step for detecting disappearance of the carrier sense condition after the carrier sense condition of the received signal being monitored is established;
And a step of returning to the first phase of monitoring for signal collision when the disappearance is detected.

The above communication method is
Provide a time determination step to check whether the specified time has passed after carrier sense,
When the specified time has elapsed in the above time determination step instead of returning to the first phase of monitoring signal collision when the carrier sense condition disappears after the carrier sense condition of the received signal being monitored is satisfied Has steps to resync,
When the specified time has not elapsed, the step is to return to the first phase for monitoring the signal collision.

Moreover, the communication method according to the present invention includes
In a communication method for monitoring signal collision in a transmission line and starting transmission when there is no collision,
Setting a pre-carrier sense signal when detecting a symbol to be monitored;
Resetting the pre-carrier sense signal when the symbol detection is interrupted;
A step of suppressing transmission start when the pre-carrier sense signal is set when a transmission request is made.

It is a figure which shows the operation | movement flow of the communication modem in Embodiment 1 of this invention. 3 is a diagram illustrating a configuration of a communication modem according to Embodiment 1. FIG. It is a timing diagram explaining the effect of the level fall detection step addition after FC detection in FIG. 3 is a diagram showing a detailed configuration of a reception control unit in Embodiment 1. FIG. It is a timing diagram explaining the effect of CS regulation time progress judging step addition after FC detection in FIG. It is a figure which shows the operation | movement flow of the communication modem in Embodiment 2 of this invention. It is a timing diagram explaining the effect of the transmission after the pre-carrier sense confirmation in FIG. It is an operation | movement flowchart of the conventional communication modem.

Explanation of symbols

  21 reception unit, 23 control device interface, 205 level detection unit, 206 tone verification unit, 207 carrier sense determination unit, 208 reception control unit, 209 field code determination unit, 210 header reception unit, 211 payload reception unit, 231 reception state Control circuit, 233 synchronization timer reset control circuit, 235 synchronization timer circuit, S106 carrier sense determination step, S107 field code detection step, S110 level decrease detection step, S120 stipulated time elapse determination step after carrier sense, S132 precarrier sense signal Negate step, S133 Pre-carrier sense signal assert step, S152 Pre-carrier sense signal assert confirmation step, S153 Pre-carrier sense signal negation confirmation Step.

Claims (7)

When the carrier sense signal indicating that the carrier sense condition for determining that the carrier on the transmission line has been detected by monitoring the transmission line is not asserted, the transmission signal is transmitted in response to the transmission request. In the communication modem to start,
A pre-carrier sense indicating that the pre-carrier sense condition is satisfied if the pre-carrier sense condition that can be established earlier than the carrier sense condition and a part of the carrier sense condition is satisfied. When the pre-carrier sense condition collapses after asserting the signal and asserting the pre-carrier sense signal, a receiving unit that negates the pre-carrier sense signal;
A communication modem comprising: a transmission unit that suppresses transmission start of a transmission signal while the pre-carrier sense signal is asserted even when the carrier sense signal is not asserted. The carrier sense condition is satisfied when the receiving unit detects a symbol on the transmission line at least a prescribed number of times continuously,
2. The communication modem according to claim 1, wherein the pre-carrier sense condition is satisfied when the receiving unit detects symbols on a transmission line continuously for a predetermined number of times less than the specified number of times.   3. The communication modem according to claim 2, wherein the predetermined number of times is one.   The pre-carrier sense condition is broken when the receiving unit cannot detect a symbol on the transmission line or when a signal level on the transmission line becomes a predetermined value or less. The communication modem according to any one of 1 to 3.   The receiving unit asserts a carrier sense signal when the carrier sense condition is satisfied, and when a communication signal is received after asserting the carrier sense signal, the precarrier sense signal and the carrier sense signal are received. 5. The communication modem according to claim 1, wherein the communication modem is negated.   The transmission unit starts transmission of a transmission signal in response to a transmission request when the type of the transmission signal is a predetermined type even while the precarrier sense signal is asserted. Item 6. The communication modem according to any one of Items 1 to 5. When the carrier sense signal indicating that the carrier sense condition for determining that the carrier on the transmission line has been detected by monitoring the transmission line is not asserted, the transmission signal is transmitted in response to the transmission request. In the communication method to start,
The pre-carrier sense condition is satisfied when the receiving portion of the communication modem can be established earlier than the carrier sense condition and the pre-carrier sense condition forming a part of the carrier sense condition is satisfied. Asserting a pre-carrier sense signal indicating
If the precarrier sense condition breaks after the receiving unit asserts the precarrier sense signal, the step of negating the precarrier sense signal;
The transmitter of the communication modem includes a step of suppressing transmission of a transmission signal while the precarrier sense signal is asserted even when the carrier sense signal is not asserted. A characteristic communication method.

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