JPH07202954A - Transmission control device - Google Patents

Abstract

(57) [Summary] [Purpose] Conventionally, it was not possible to construct a highly reliable digital communication system by using optical elements and parts that were economical but had deteriorated performance over time and were not highly reliable. Allow configuration. In a digital transmission system in which bidirectional regenerative repeaters using light as a transmission medium are connected in cascade, a pair of upstream and downstream transmission paths 3 and 4 between adjacent repeaters 1 and 2 is used. When a level fluctuation occurs in the signal transmission path from one transmission circuit to the reception circuit of the other repeater, the fluctuation amount information is sent to the repeater on the transmission side by using the other transmission path of the pair. Then, the repeater on the transmitting side controls the transmitting circuit based on the information and controls the output level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention comprises at least two bidirectional regenerative repeaters for transmitting information using light as an information transmission medium.
The present invention relates to a transmission control device that is formed by connecting in series.

More specifically, the present invention relates to a decrease in output level due to deterioration of element performance of a light emitting source, increase in loss in an optical fiber, a transmission line such as a space, an optical connector, etc. in information transmission in such a transmission control device. The present invention relates to such a transmission control device capable of preventing a signal transmission error and communication disconnection, providing accurate line operation, rationalizing maintenance, and configuring an optical signal transmission system having high reliability. .

[0003]

2. Description of the Related Art Conventionally, in a transmission control device, in order to cope with a received signal level due to output level fluctuations on the transmission side and loss fluctuations on the transmission line, the transmission level fluctuations and the loss fluctuations on the transmission line must be dealt with. Generally uses a so-called AGC (automatic gain adjustment method) for adjusting the receiving sensitivity on the receiving side. Also, if a transmission error occurs due to a reduction in the signal level, etc., it is detected by a method such as parity error check,
It is general to notify this result to the transmitting side and request retransmission of the signal.

[0004]

On the other hand, conventionally, in a digital signal transmission system using an inexpensive E / O (electrical / optical) and O / E (optical / electrical) conversion element of a short wavelength system, the conversion element Degradation may occur. Therefore, E /
A digital signal transmission system using O, O / E conversion elements,
There is a problem in applying it to a communication network that requires high reliability such as a local communication network.

Further, when a plastic optical cable is used as a transmission medium, the transmission loss increases due to the aging of the cable, and there is a problem in applying it to a communication network which also requires high reliability.

Further, in the method of monitoring the reception level decrease by detecting a transmission error such as a parity check, once a transmission error due to a change with time occurs, the error sharply increases. For this reason, the method of predicting element deterioration and temporal change by detecting a transmission error such as a parity check and taking countermeasures has a problem that even if a sufficient margin is taken, it is not possible to detect and respond to a failure occurrence in advance.

The present invention is to solve the above problems of the prior art. That is, according to the present invention, even if an element with insufficient reliability is used, a decrease or fluctuation in the level of a signal to be transmitted is detected before a failure occurs, and the emission intensity of the electro-optical conversion element is increased or decreased. By automatically increasing the light output by taking precautionary measures, maintaining communication function, displaying the place where a failure is predicted, and taking measures before the failure occurs. , A transmission that can solve the problem that it was not possible to construct a highly reliable digital communication system by using optical elements and parts that are economical but their performance deteriorates over time and are not sufficiently reliable. An object is to provide a control device.

[0008]

To achieve the above object, in the present invention, a transmission controller (bidirectional, bidirectional, comprising at least two bidirectional regenerators using light as a transmission medium is connected in cascade.
In the regenerative repeater type digital transmission system), in a signal transmission line in a section from one transmission circuit to a reception circuit of another relay, using a pair of upstream and downstream transmission lines of adjacent relays, When a level change occurs, the change amount information is sent back to the transmission side repeater using the other transmission path of the pair, and the transmission side repeater controls the transmission circuit based on the information. However, it was decided to control the output level. In addition, at that time, the generated level fluctuation is displayed by the display means.

[0009]

According to the present invention, in a two-way, regenerative repeater type digital transmission system using light as a transmission medium, a pair of upstream and downstream transmission lines between adjacent repeaters are used, and one transmission circuit is connected to another. When a level fluctuation occurs in the signal transmission line in the section up to the receiving circuit of one repeater, the fluctuation amount information is returned to the transmission side repeater using a pair of other transmission lines, and the transmission side repeater. Then, based on the information, the transmission circuit is controlled and the output level is controlled.

Therefore, when the luminous efficiency is lowered due to the deterioration of the electric / optical conversion element and the received signal level is lowered, this information is transmitted to the transmission side, and the bias current is changed and other electric / optical conversion elements are used. Measures have been taken to increase the output of, and stable communication can be continued by keeping the reception level constant.

At the same time, the fact that the control for improving the transmission output is executed is performed by the transmitting repeater, the receiving repeater, the device (main device) for controlling the entire system, and the like.
By displaying on the ED lamp or display, the deterioration of the element can be known in advance, so maintenance work such as element replacement can be performed before the system stops functioning due to a failure, and even if an element with low reliability is used. Therefore, a highly reliable communication system can be configured.

[0012]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, the repeater 1 and the repeater 2 are connected by upstream and downstream optical transmission lines 3 and 4, and constitute a transmission control device (bidirectional, regenerative repeater digital transmission system).

In FIG. 1, receivers 1-5-1, 1-5-2 and 2-5 from the preceding stage are provided in the respective relays 1 and 2.
-1,2-5-2, transmitters 1-6-1, 1-6-2, 2
-6-1, 2-6-2, The signal from the repeater in the previous stage is received by the receiver, the address is processed, the information is extracted to the repeater, and the information is superimposed on the transmission signal from the repeater. Transmission information storage circuits 1-7-1, 1-7-2, 2 for performing
In addition to incorporating -7-1, 2-7-2,

A transmission output level control circuit 8 for controlling the transmission output level of the transmitter, an output level setting information storage circuit 9 for giving output level setting information to the transmission output level control circuit 8, and reception of an optical signal receiver in the repeater 2. An optical reception signal level detection circuit 10 for detecting a signal level, an optical reception signal level quantization circuit 11 for quantizing a detection level by the optical reception signal level detection circuit 10, and an output from the optical reception signal level quantization circuit 11 are stored. Reception level information storage circuit 1
2 is built in.

Next, the operation of this circuit will be described. Repeater 1
Of the output level setting information storage circuit 9
Depending on a certain value of, via the transmission output level control circuit 8,
It is sending out a certain level of signal. This optical signal is received by the receiver 2-5-1 of the repeater 2 via the downlink transmission path 4. This received signal is transmitted from the transmitter 2-6-1 to the repeater in the subsequent stage via the transmission information storage circuit 2-7-1.

Here, when the level of the received signal in the receiver 2-5-1 is lowered due to deterioration of the electrical / optical conversion element or increase in loss of the transmission line, the optical received signal level detection circuit 10 lowers the received level. Is detected. The reduced reception level is converted into digital information by the optical reception signal level quantization circuit 11 and stored in the reception level information storage circuit 12.

Next, this information is transmitted to the transmission information storage circuit 2-7.
-2, it is superimposed on the transmission information transmitted from the repeater in the preceding stage, and is transmitted from the transmitter 2-6-2 to the repeater 1 via the upstream transmission path 3. This signal is received by the receiver 1-5-2 of the repeater 1, and the transmission information storage circuit 1-7-
Accumulated in 2.

The reception level information transmitted from the repeater 2 is transferred to and accumulated in the output level setting information storage circuit 9. The information of the output level setting information storage circuit 9 is given to the transmission output level control circuit 8 and the transmitter 1-
Adjust the output level of 6-1.

In this way, even if the received signal level at the receiver is lowered or fluctuated due to deterioration of the electro-optical conversion element or increase of loss in the transmission line, the output level of the transmitter can be adjusted and the loss is increased. As a result, it is possible to prevent in advance the occurrence of transmission errors and the situation in which communication is disabled. As a result, even if a component whose reliability is insufficient is used, a highly reliable communication system can be configured as a result.

The first embodiment described above has been shown as an embodiment using the transmission path on one side of the bidirectional transmission system.
In an actual transmission system, it is necessary to use a bidirectional transmission path, and an embodiment using a bidirectional transmission path will be described below as a second embodiment.

FIG. 2 shows another embodiment of the present invention (embodiment 2).
It is a block diagram showing. In the figure, the configuration and operation of automatically adjusting the levels of the transmitters 1-6-1 in the repeaters 1 and 2 are the same as those in the first embodiment (FIG. 1), and therefore description thereof is omitted. To do. Here, the configuration and operation for automatically adjusting the level of the transmitter 2-6-2 of the repeater 2 will be described.

Each of the repeaters 1 and 2 has a receiver 1-5-1, 1-5-2, 2-5-1, 2-5- from the preceding stage.
2, transmitters 1-6-1, 1-6-2, 2-6-1, 2-
6-2. Transmission information storage for receiving a signal from the repeater in the preceding stage, processing the address, extracting information to the repeater, and superimposing information on the transmission signal from the repeater Circuits 1-7-1, 1-7-2, 2-7-1, 2-
7-2 built-in,

Further, a transmission output level control circuit 1-8-1, 1-8-2, 2-for controlling the transmission output level of the transmitter.
8-1, 2-8-2, output level setting information storage circuits 1-9-1, 1-9-2, 2-9-1, 2 for giving output level setting information to the transmission output level control circuit (8) -9-2,
Optical reception signal level detection circuit 1-10-1, 1-10-2, 2-10- for detecting the reception signal level of the optical signal receiver
Built-in 1, 2-10-2,

Also, an optical reception signal level detection circuit (10)
Optical reception signal level quantization circuit 1-11-1, 1-11-2, 2-11-1, 2- for quantizing the detection level by
11-2, reception level information storage circuit 1-12-1 that stores the output from the optical reception signal level quantization circuit (11),
1-12-2, 2-12-1, 2-12-2 are built in.

Next, the operation of this circuit will be described. Repeater 2
The transmitter 2-6-2 has an output level setting information storage circuit 2
A transmission output level control circuit 2-by a certain value of -9-2
A signal of a certain level is transmitted via 8-2. This optical signal is transmitted via the upstream transmission line 3 to the receiver 1 of the repeater 1.
Received at -5-2. This received signal is transmitted from the transmitter 1-6-2 to the repeater in the subsequent stage via the transmission information storage circuit 1-7-2.

Here, if the level of the received signal in the receiver 1-5-2 is lowered due to deterioration of the electrical / optical conversion element, increase of loss in the transmission path, etc., the optical received signal level detection circuit 1-1.
A decrease in the reception level is detected by 0-2. This reduced reception level is the optical reception signal level quantization circuit 1-1.
It is converted into digital information in 1-2 and stored in the reception level information storage circuit 1-12-2.

Next, this information is transmitted to the transmission information storage circuit 1-7.
In -1, it is superimposed on the transmission information transmitted from the repeater in the preceding stage, and is transmitted from the transmitter 1-6-1 to the repeater 2 via the downlink transmission path 4. This signal is received by the receiver 2-5-1 of the repeater 2 and transmitted by the transmission information storage circuit 2-7.
It is stored in -1.

The reception level information transmitted from the repeater 1 is transferred to and accumulated in the output level setting information storage circuit 2-9-2. Output level setting information storage circuit 2
The information of -9-2 is the transmission output level control circuit 2-8-2.
To adjust the output level of the transmitter 2-6-2.

As described above, in the bidirectional relay transmission system, even if the received signal level at the receiver is lowered or fluctuated due to deterioration of the electrical / optical conversion element or increase of the loss of the transmission line, The output level can be adjusted, and it is possible to prevent in advance the occurrence of transmission errors due to increased loss and the situation in which communication is disabled. As a result, even if a component whose reliability is insufficient is used, a highly reliable communication system can be configured as a result.

As described above, in the first and second embodiments, it has been explained that even if the output of the transmitter is lowered due to the performance deterioration of the element, the output level can be automatically adjusted to maintain the communication function. However, in this state, even if the element deterioration progresses,
Since the output level is automatically adjusted, the maintenance person cannot recognize the degree of progress of element deterioration. For this reason,
In Example 3 described below, a configuration and a function for notifying a maintenance person of deterioration of the element will be described.

FIG. 3 is a block diagram showing still another embodiment (Embodiment 3) of the present invention. In the transmission control device shown in FIG. 1, a configuration for notifying a maintenance person of element deterioration is shown. It corresponds to a diagram showing functions.

In FIG. 3, the receiver 1-5-1 to the reception level information storage circuit 12 have the same structure and function as those shown in FIG. Reference numerals 1-13-1 and 2-13-1 are level fluctuation indicators, which are based on the fluctuation information of the reception level in the output level setting information storage circuit 9 and the reception level information storage circuit 12 and indicate the electric / optical conversion elements. This is a level fluctuation indicator for displaying that the received signal level at the receiver has dropped or fluctuated due to deterioration or increased loss in the transmission line, and that the output level of the transmitter has been adjusted.
A status display element such as a lamp or LCD is used.

Referring to FIG. When the received signal level at the receiver 2-5-1 fluctuates due to deterioration of the electrical / optical conversion element or increased loss of the transmission path, the optical received signal level detection circuit 10 detects a decrease in the received level. The reduced reception level is converted into digital information by the optical reception signal level quantization circuit 11, and the reception level information storage circuit 12
Memorized in.

This information is transmitted to the transmission information storage circuit 2-7-2.
, The transmission information transmitted from the repeater at the previous stage is superimposed on the transmission path 3 from the transmitter 2-6-2 to the repeater 1.
Be sent via. At the same time, this information is given to the level fluctuation indicator 1-13-1, and for example, the fact that the received signal level is below a certain value is displayed by the lighting state of the level fluctuation indicator. As a result, the repeater 2 can display the deterioration of the electrical / optical conversion element of the repeater 1 or the increase in the loss of the transmission path, and notify the maintenance person.

Further, the signal, which has been detected in the reception level information storage circuit 12 and has detected the decrease in the reception level, is superimposed on the transmission information transmitted from the repeater in the preceding stage in the transmission information storage circuit 2-7-2. And transmitted from the transmitter 2-6-2 to the relay 1 via the transmission line 3. Further, this signal is received by the receiver 1-5-2 of the repeater 1 and stored in the transmission information storage circuit 1-7-2.

The reception level information transmitted from the repeater 2 is transferred to and accumulated in the output level setting information storage circuit 9. The information of the output level setting information storage circuit 9 is given to the transmission output level control circuit 8 and the transmitter 1-
At the same time that the output level of 6-1 is adjusted, the level change indicator 1-13-1 indicates that the output level has changed.

As a result, the repeater 1 can display the change of its own transmission level, that is, the deterioration of the electro-optical conversion element and the increase of the loss of the transmission line, and can notify the maintenance person. Further, the level change indicators 1-13-1 and 2-13-1 can be used individually or simultaneously to display deterioration of the electro-optical conversion element and increase in loss of the transmission line.

Further, the outline of the operation is as shown in FIG.
Although the operation was explained using only one side of the bidirectional repeater, the second embodiment
As shown in (FIG. 2), each repeater actually constitutes a bidirectional repeater, so that the level fluctuation indicator 1-13-
1 and 2-13-1 are provided corresponding to the upstream and downstream transmission paths, respectively.

In this way, the present invention automatically adjusts when the received signal level at the receiver is lowered or fluctuated due to the deterioration of the electro-optical conversion element or the increase of the transmission line loss. By displaying the result on the display, it becomes possible for the maintenance person to easily read it, and it is possible to prevent the situation in which the communication is disabled in advance. As a result, even if a component whose reliability is insufficient is used, a highly reliable communication system can be configured as a result. Also, it goes without saying that a single notification method using an acoustic signal can be adopted instead of the visual display.

FIG. 4 is a block diagram showing a local communication system as an example to which the present invention is applied. In the figure,
20 is a main device, 22 is a control panel, 21-1, 21-2, 2
1-3, ... 21-n-1, 21-n are repeaters, T3, T (n-1), and Tn are communication terminals, 5
0 is an outside line connected to an external communication network.

In FIG. 4, terminals (mainly communication terminals) hang in each repeater, and transmission (communication) between the terminals is possible via the repeater, and an external line accommodated in the main unit 20. Communication between the terminal and an external communication network is also possible via 50. The main device 20 is a device for controlling the entire in-house communication system or collectively managing processing information that the system must have.

In the local communication system as shown in FIG. 4, since the reception level fluctuation information can be detected for each transmission section of each repeater, the reception level fluctuation information and the address of the repeater are included in the upstream signal to the main unit 20. By superimposing and transmitting the information or the position information of the transmission path, the main device 20 or the control panel 22 of the main device indicates that the electrical / optical conversion element is deteriorated or the loss of the transmission path is increased. It is possible, and if necessary, it is possible to notify by an alarm sound.

Therefore, the maintenance person can easily specify the faulty repeater and the transmission section, and can quickly replace the optical element and the transmission component, so that the service interruption due to the trouble can be prevented.
As a result, it is possible to construct a highly reliable communication system even if components having insufficient reliability are used.

[0044]

As described above, according to the present invention,
In a transmission control device that employs a bidirectional, regenerative repeater digital transmission method that uses light as a transmission medium, the transmission signal level is reduced and the variation of the reception signal level is detected, and the detection information is returned to the transmission side to determine the transmission signal level. Therefore, even if an electro-optical conversion element or transmission medium with insufficient reliability is used, there is an advantage / effect that the reception level can be automatically made constant and a highly reliable communication system can be configured. , Its utilization effect is great.

Further, by displaying the detection result of the fluctuation of the received signal level, it is possible to identify the repeater in which the element deterioration is occurring and the increasing transmission path, so that it can be prevented before the communication system stops functioning due to a failure. Even if an electro-optical conversion element or a transmission medium that does not have sufficient reliability can be maintained, it is possible to configure a highly reliable communication system by maintenance and operation, and the utilization effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing another embodiment of the present invention.

FIG. 3 is a block diagram showing another embodiment of the present invention.

FIG. 4 is a block diagram showing a local communication system as an example to which the present invention is applied.

[Explanation of symbols]

1, 2 ... Repeater, 3, 4 ... Optical transmission line, 1-5-1, 1-
5-2, 2-5-1, 2-5-2 ... Receiver, 1-6-
1, 1-6-2, 2-6-1, 2, 6-2 ... Transmitter, 1
-7-1, 1-7-2, 2-7-1, 2-7-2 ... Transmission information storage circuit, 8 ... Transmission output level control circuit, 9 ... Output level setting information storage circuit, 10 ... Optical reception signal Level detection circuit, 11 ... Optical reception signal level quantization circuit, 12 ... Reception level information storage circuit, 1-13-1, 2-13-1 ... Level fluctuation indicator.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H04B 10/08 (72) Inventor Keisuke Sugiyama 1-1-6 Uchiyuki-cho, Chiyoda-ku, Tokyo Nihon Telegraph Phone Co., Ltd.

Claims (4)

[Claims] 1. A first transmission information storage circuit (1-7-) that temporarily stores signals received by an upstream optical signal receiver (1-5-2) and an upstream optical signal receiver using light as a transmission medium. 2), an upstream optical transmitter (1-6-2) that transmits the signal stored in the transmission information storage circuit, and a downstream optical signal receiver (1-5)
1), a second transmission information storage circuit (1-7-1) that temporarily stores the signal received by the downlink optical signal receiver, and a downlink optical transmitter (1 that transmits the signal stored in the transmission information storage circuit −
6-1), a bidirectional regenerator having at least 2
When the reception level of the second repeater (2) fluctuates, the information is transferred from the second repeater to the first repeater ( Using the optical signal transmission path to 1), the first repeater is notified, and the output level of the transmission side of the first repeater is adjusted according to the information, so that the reception level of the second repeater is In a transmission control device of a digital transmission system for stabilizing and maintaining an appropriate reception level, a downlink transmitter (1-6-1 that sends an optical signal from a first repeater to a second repeater ) The transmission output level control means (8) for controlling the transmission output level, the output level setting information storage means (9) for giving the output level setting information to the transmission output level control means, and the second relay from the first repeater. Optical signal reception at the second repeater of the optical signal transmitted to the repeater Vessel (2
-5-1) optical reception signal level detecting means (10) for detecting the reception signal level, optical reception signal level quantizing means (11) for quantizing the detection level by the optical reception signal level detecting means, and Receiving level information storing means (12) for storing the output from the optical receiving signal level quantizing means is provided, and the output level fluctuation or transmission of the optical signal from the first repeater to the second repeater is provided. When the reception signal level of the reception side optical signal receiver (2-5-1) fluctuates due to signal attenuation in the path, this is detected by the optical reception signal level detection means (1) of the reception side second repeater.
0), the optical reception signal level quantization means (11) generates information according to the level fluctuation amount, the reception level information storage means (12) stores this information, and the second repeater The stored information is transferred to the second transmission information storage circuit (2-7-2) that temporarily stores the signal to be transmitted to the first repeater, and the second transmission information storage circuit has transmitted from the preceding stage. Information is mixed with the received level information which is the transferred storage information, and the information is converted into an optical signal from the optical transmitter (2-6-2) to the first repeater in the second repeater. Transmitting, and receiving the optical signal from the second repeater at the first repeater, receiving the signal at the receiver (1-5-2),
The second transmission information storage circuit (1-7 in the first repeater
-2) stores the received information, the second transmission information storage circuit extracts the reception level information transmitted from the second repeater, and extracts the extracted reception level information from the first repeater. In the first output level setting information storage means (9), and controls the transmission output level of the transmitter (1-6-1) which sends the optical signal from the first repeater to the second repeater. A transmitter for transmitting the reception level information to the transmission output level control means (8) and transmitting an optical signal from the first repeater to the second repeater based on the reception level information in the transmission output level control means ( 1-6-1) controlling the transmission output level to stabilize the reception level of the second repeater and maintain an appropriate reception level. 2. The transmission control device according to claim 1, wherein the second repeater transmits an optical signal from the second repeater to the first repeater, which is transmitted by a transmitter (2-6-2). Second transmission output level control means (2-8-2) for controlling the output level
And second output level setting information storage means (2-9-) for giving output level setting information to the transmission output level control means.
2), and second light for detecting the received signal level of the optical signal receiver (1-5-2) in the first repeater of the optical signal transmitted from the second repeater to the first repeater. A received signal level detecting means (1-10-2), a second optical received signal level quantizing means (1-11-2) for quantizing a detection level of the optical received signal level detecting means, and the optical receiving Second reception level information storage means (1-12-2) for storing the output from the signal level quantization means, and the output level of the optical signal from the second repeater to the first repeater. When the reception signal level of the reception side optical signal receiver (1-5-2) fluctuates due to fluctuations or signal attenuation in the transmission path, this is detected by the second optical reception signal level detection means (of the reception side first repeater ( 1-10-2), and second optical reception signal level quantization means (1-11-2) It generates information representing the level variation, the second reception level information storage means (1-
12-2) stores this information, and transfers the stored information to the first transmission information storage circuit (1-7-1) that temporarily stores the signal to be transmitted to the second relay in the first relay. Then, in the first transmission information storage circuit (1-7-1), the information transmitted from the preceding stage and the reception level information which is the transferred storage information are mixed, and the mixed information is transmitted from the first repeater. A receiver that transmits the mixed information as an optical signal from the optical transmitter (1-6-1) to the second repeater, and receives the optical signal from the first repeater in the second repeater ( 2-5-1) receives the signal, stores the received information in the first transmission information storage circuit (2-7-1) in the second repeater, and in the first transmission information storage circuit, , The reception level information transmitted from the first repeater, and the extracted reception level information Second and stored in the output level setting information storage means (2-9-2) in the repeater transmitter for transmitting the optical signal from the second repeater to the first repeater (2-6
2) Sending the reception level information to the second transmission output level control means (2-8-2) for controlling the transmission output level,
The second transmission output level control means controls the transmission output level of the transmitter (2-6-2) for transmitting an optical signal from the second repeater to the first repeater based on the reception level information. Thereby, when the reception level of the first repeater fluctuates, the information is notified to the second repeater using the optical signal transmission path from the first repeater to the second repeater, A transmission control device characterized in that the output level of the transmission side of the second repeater is adjusted according to the information to stabilize the reception level of the first repeater and maintain an appropriate reception level. . 3. The transmission control device according to claim 1, wherein when output level setting change information is output from the output level setting information storage means, the optical transmission is performed based on the output level setting change information. A transmission control device comprising level change display means for displaying that the output level of the device has been changed. 4. The transmission control device according to claim 1, wherein when the reception level information storage means outputs information indicating that the level of the reception signal has changed, the reception level is based on the information. The transmission control device is provided with a level change display means for displaying that the change has occurred.