JP2000196596A - Method for detecting collision in lan and terminal therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily and accurately detect a transmission collision in a LAN which is made to branch off in terms of high-frequency. SOLUTION: In the LAN branched off in terms of high-frequency, a terminal 30 is equipped with a collision decision AC signal generation part 35 which generates an AC signal and a collision detection part 34 which detects a beat signal. Prior to data transmission, the collision decision AC signal generation parts 35 of each terminal 30 send out collision decision AC signals which have slightly different frequencies. The signals pass through each branching device 20 and are transmitted to all the terminals 30 because they are AC signals. The collision detecting part 34 of each terminal 30 amplifies the collision decision AC signal by using a non-linear amplifier. If another collision decision AC signal is superposed on the collision decision AC signal, a beat signal is generated in the output thereof. If the output is the beat signal, a transmission collision is decided. Consequently, the transmission collision can easily and precisely be detected even in the LAN branched off in terms high-frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a network system for performing bidirectional data communication between terminal devices, and more particularly, to a high-frequency branched LAN using a coaxial cable.
And a terminal device thereof. The present invention relates to the C
The present invention can be applied to a LAN constructed using an ATV or TV coaxial cable.

[0002]

2. Description of the Related Art Conventionally, there is a system in which a plurality of terminal devices are connected to a transmission line within a predetermined area to communicate data bidirectionally. It is called a local area network (hereinafter, referred to as LAN), and includes a transceiver for monitoring data collision, a hub as a data distributor, and a plurality of terminals as data input / output devices. The transceivers are connected by a coaxial cable, and the transceiver, hub, and each terminal are connected by a twisted pair wire. As a typical example, Ethernet based on IEEE802 is well known.

[0003] The transmission system is a baseband system,
In this method, data of 0 and 1 is converted into a biphase voltage and transmitted to a transmission line. 10BASE5 depending on the transmission medium
It is divided into 10BASE2 and 10BASE-T, and its transmission speed is 10 Mbps. In this case, an access control method is CSMA / CD (Carrier Sense Multi).
This method is similar to ple Acess with Collection Detection (carrier sensing multiple access / collision detection). Before transmitting data, it checks whether the channel on the coaxial cable is occupied, that is, there is no transmission collision, and if there is no transmission collision. In this method, data is transmitted with a destination address and a source address.

Specifically, there is a network system shown in FIG. A plurality of transceivers 200 and 220 are attached to the coaxial cable 300 which is the main line, and the hub 100 and the hub 120
However, a computer device 101 or the like, which is a terminal device, is further connected thereunder. Transceiver 200, 2
Reference numeral 20 plays a role of relaying data and monitoring transmission collision on the coaxial cable 300. For example, the computer device 10
When there is a request for data transmission from 1 through the hub 100,
The transceiver 200 itself sends a DC constant current from the current source 201 to the terminating resistor of the coaxial cable 300. Then, the voltage between the lines is compared with the reference voltage Vref by the comparator 202. When a DC constant current for collision detection is further applied when another terminal device is already using the network, a DC current twice as large as that in a case where transmission is normally performed is passed through the coaxial cable 300. . At this time, the voltage between the lines of the coaxial cable 300 exceeds the reference voltage Vref. Thus, it is detected that transmission collision occurs before transmission starts. In this case, no data signal is transmitted. Thus, the data signal already transmitted on the coaxial cable 300 has been protected.

[0005]

However, a conventional LAN using a hub and a twisted pair line is a so-called bus type, which is a multi-drop system in which many hubs are connected to a trunk coaxial cable. The access control method is a method in which the transceiver supplies a DC constant current to a terminating resistor connected to the end of the transmission line, determines transmission collision from the voltage at both ends, and determines whether access is possible. Therefore, the terminal device does not directly detect a transmission collision, but indirectly detects a transmission and transmits the result to each terminal device. In addition, the above access control method is 2
Voltage detection by DC current using two termination resistors,
This was not applicable if the LAN was extended in an alternating fashion with a branch or distributor. In addition, the number of terminals connected to the hub is limited, and a LAN with a high degree of freedom is not necessarily constructed over a wide range.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a conventional system in which the problems concerning the access speed and the number of terminals or the degree of freedom of the arrangement are in the above access control system. Pay attention to
By enabling each terminal device to detect a transmission collision with an AC signal at the time of transmission, a local area network system having a high degree of freedom in a wide area that is branched in an alternating manner is enabled, and its access speed is increased. And to reduce the system cost. Another purpose is to use a CATV or TV coaxial cable line that has already been deployed in the city or at each office / home, and to maintain data between terminals at each office / home while maintaining the TV function. The purpose of this is to construct a LAN capable of transmitting and receiving data and to lower the construction cost.

[0007]

According to a first aspect of the present invention, there is provided a method for detecting a collision in a LAN, the method comprising: modulating a high-frequency carrier with data of Ethernet specifications to form a data signal; LA in which a plurality of terminal devices perform bidirectional communication by transmitting to
N, which is applied to a LAN in which a coaxial cable is branched at a high frequency using a branching device or a distributor, and is applied to a LAN. Each terminal device transmits a data signal to a network before the data signal. Transmitting a collision determination AC signal for collision determination to the network until the transmission is completed, and the terminal device that has transmitted the collision determination AC signal detects a transmission collision based on the presence or absence of a beat signal on the network. It is characterized by.

The method for detecting a collision in a LAN according to the present invention is applied to an extended network in which a coaxial cable is branched at a high frequency using a branching device or a distributor.
The splitter or distributor does not transmit DC,
The alternating current is a transformer coupling element that transmits.

Before each terminal device transmits a data signal,
In order to detect a transmission collision, a collision determination AC signal is transmitted to the network until the transmission of the data signal is completed. The collision determination AC signal passes through a branching device or a distributor and is transmitted to the entire network. Then, for example, when a plurality of terminal devices simultaneously transmit the collision determination alternating current, a beat signal is detected from the transmission path of the network. It is obtained by, for example, amplifying a signal on a transmission line in a non-linear region of a VI characteristic of a transistor or the like and extracting a low frequency.

The terminal device that has transmitted the collision determination AC signal is
A transmission collision is determined based on whether or not the beat signal is generated, and a data signal is transmitted. If the frequency of the collision determination AC signals from the plurality of terminal devices is different, the beat signal can be detected even if the amplitude is different. Therefore, even if the collision determination AC signal is randomly attenuated by transmission, a beat signal can be reliably detected and a transmission collision can be detected. Therefore, transmission collision detection accuracy can be improved as compared with the conventional DC method.

According to a second aspect of the present invention, the presence / absence of a beat signal indicating a transmission collision is detected by frequency discriminating means provided in the terminal device. The beat signal is a signal having a frequency difference between the two collision determination AC signals. The product of the two collision determination AC signals is composed of the sum and difference frequencies of the two collision determination AC signal frequencies on the frequency axis. For example, the sum frequency can be several tens MHZ, and the difference frequency can be several tens KHZ. The frequency discriminating means discriminates the two. For example, a low-frequency component that is a beat signal is extracted by a low-pass filter, and the low-frequency component is counted. If a numerical value equal to or greater than a predetermined value is counted, it is determined that a transmission collision has occurred. Thereby, transmission collision can be detected more accurately.

According to a third aspect of the present invention, a collision detection AC signal having a different frequency is transmitted from each terminal device provided in the network. Since the collision determination AC signals transmitted from the plurality of terminal devices have different frequencies, a beat signal is reliably generated. Therefore, when a plurality of terminal devices simultaneously transmit a collision determination AC signal, the collision detection method surely detects a transmission collision.

According to a collision detection method in a LAN according to a fourth aspect of the present invention, the terminal device transmits a collision determination AC signal phase-modulated or frequency-modulated by a random function. As a result, if a plurality of collision determination AC signals are present on the network, the terminal device that transmitted the collision determination AC signal demodulates the signal on the network with the square characteristic. Or a signal having a time-varying phase difference is detected. That is, a time-varying low-frequency beat signal is detected. When the collision determination AC signals having different frequencies are used, when the two collision determination AC signal frequencies are extremely close, the beat signal frequency becomes extremely low, and it takes time to detect the beat signal frequency. However, according to the present invention, by randomly changing the phase or frequency of the collision determination AC signal, the frequency of the beat signal changes randomly with time, and the transmission collision can be reliably performed in a shorter time. A determination can be made.

According to the collision detection method for a LAN according to the fifth aspect, during the initial period of the collision judgment AC signal, the terminal device transmits the collision judgment AC signal phase-modulated or frequency-modulated by a random function. I have. As a result, if a collision determination AC signal is transmitted from another terminal device in a state where a collision determination AC signal having a constant phase or frequency already exists on the network, two collisions occur in this initial period. The phase or frequency of the beat signal resulting from the synthesis of the judgment AC signal changes as a function of time. As a result, the terminal device that has transmitted the collision determination AC signal can detect a beat signal whose phase or frequency changes in a short time in the initial period. Therefore, it is possible to reliably determine the transmission collision in a shorter time. Also,
In a state where there is no other collision determination AC signal on the network, after the initial period, the collision determination AC signal can be fixed and used as a data carrier, for example.
Therefore, the frequency band can be used effectively.

According to a sixth aspect of the present invention, there is provided a terminal device for use in the method for detecting a collision in a LAN according to any one of the first to fifth aspects, wherein the terminal device transmits a collision determination AC signal and outputs a predetermined initial value. If the beat signal is not detected from the network during the period, the data signal is sent to the network.If the beat signal is detected, the sending of the collision determination AC signal is stopped without sending the data signal. It is characterized by. The predetermined initial period is 1
The collision determination AC signal transmitted from one terminal device is set to be equal to or longer than the minimum time for reaching all the terminal devices. Therefore, when the beat signal is not detected within the predetermined initial period, data can be reliably transmitted. Also,
If a beat signal is detected within this predetermined initial period, no data signal is sent, so that transmission collision does not occur. Therefore, the terminal device can reliably perform data communication.

According to a seventh aspect of the present invention, the terminal device in the LAN is connected to another terminal device in a tree structure by a coaxial cable and a branch or a distributor.
The collision determination AC signal transmitted from one terminal device is transmitted to all terminal devices via a coaxial cable and a splitter or a distributor. Therefore, even in a tree-structured network, a transmission collision is similarly detected, and the terminal device performs reliable data communication. In addition, by installing a distributor or a branching device as needed, the number of terminal devices can be increased as needed. Therefore, the terminal device realizes a local area network having a high degree of freedom.

The terminal device in the LAN according to the present invention is applied to a LAN formed in a CATV network, and the collision determination AC signal is transmitted using an unused channel of the CATV. This provides a highly convenient collision detection device that can be applied to a LAN using a CATV network.

The collision detection method in a LAN according to the present invention is applied to a TV wiring network provided in a predetermined area such as one office or one house, and the collision determination AC signal and the data signal are used when the TV is empty. Sent using the channel. The ordinary TV wiring network is formed by branching a coaxial cable by a branching device or a distributor, as in the CATV network. Therefore, if an empty channel of the TV wiring network is applied to the collision determination AC signal and the data signal, the existing T
In addition to V reception, a LAN can be formed within a predetermined area. Therefore, it is used for the existing TV wiring network,
This is a collision detection method that reduces the construction cost.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following examples. (First Embodiment) FIG. 1 shows a tree-like LAN connected at high frequencies using a coaxial cable and a branching device. The figure is a schematic configuration diagram. The collision detection method of the present invention is applied to this LAN. This LAN has a large coaxial cable 10,
It is composed of a branching unit 20, a terminal device 30, and a computer device 40 as an input / output device. All components except the terminal device 30 and the computer device 40 are connected by the coaxial cable 10. Further, the terminal device 30 and the computer device 4
0 are connected by a twisted pair line. Note that the coaxial cable 10 and the transmission path described later have the same meaning.

The terminal device 30 has a collision determination AC signal generator 35 for generating an AC signal for detection, similarly to the collision detector 34 for detecting transmission collision. The terminal device 30 is connected to the connected computer device 40.
When a transmission request is received from, a collision determination AC signal Fs is transmitted from the collision determination AC signal generation unit 35, and the collision detection unit 34 determines the transmission collision, as described later in detail. The splitter 20 is a transformer-coupled bidirectional coupler, and the collision determination AC signal Fs is transmitted to all terminal devices 30 from upstream to downstream or in the opposite direction. The same applies to the data signal transmitted from the terminal device 30. For the data signal, the same carrier frequency is used for transmission and reception, and packet communication is performed between the terminal devices 30. Note that each terminal device transmits a collision determination AC signal Fs having a slightly different frequency. Its frequency is
For example, based on 26 MHz, the difference frequency is about 10 kHz to 1
It is set for each 00 kHz. These are obtained by a crystal oscillator or the like.

For example, if one computer device 40 is LA
When a transmission request is made in N, the terminal device 30 connected to the N first reads the current transmission path state from a collision detection unit or a demodulation unit described later in order to avoid transmission collision. If another carrier or data is detected, data transmission is not performed. That is, if the transmission path is already used by another terminal device 30, data transmission is not performed.

Also, for example, a computer device 40
But when the transmission request to the LAN alone from the collision determining the AC signal generating unit 35 of the terminal device 30 connected to it, for example, the collision determining AC signal Fs frequency fs ₁ is sent on the network. After waiting for a predetermined initial period, immediately after that, the state of the transmission path is detected by the collision detection unit 34. The detected signal, since it is the terminal device 30 itself collision determination AC signal Fs frequency fs ₁ that sent, transmit collision is determined not. As a result, the terminal device 30 continues to transmit the collision determination AC signal Fs,
Send data signal. The data signal output from the terminal device 30 is transmitted to the network branched by the branching device or the distributor, and is read by another terminal device 30 having the same address.

On the other hand, a plurality of computer devices
40 may request transmission to the LAN. This simultaneous action
Using the time chart shown in Fig. 2 to avoid transmission collision during access
Will be described. For example, if the terminal devices 1 and 2₁, T
_Two(T₁≒ t_Two), The terminal device 1,
2 generate trigger signals 1 and 2, respectively (FIG. 2
(A), (c)). Then, each of the collision detection units 34
The trigger timing τ₁, Τ _TwoIn the transmission path state (Figure
Read 2 (e)). Both are states A and B with no signal
Therefore, each collision determination AC signal generation unit 35
For example, frequency fs₁Collision determination AC signal Fs₁And frequency f
s_TwoCollision determination AC signal Fs_Two(Figure 2)
(B), (d)). At this time, first, the collision determination AC signal F
s₁Is transmitted, and the collision determination AC signal Fs is slightly delayed._Two
Is sent. In this case, initially the transmission path
State is the frequency fs₁Collision determination AC signal Fs₁only
And then the frequency fs_TwoCollision determination AC signal Fs
_TwoAre superimposed (FIG. 2E).

Subsequently, the collision detection unit 34 of the terminal device 1
Trigger timing τ after an initial period, for example, 1 msec
_{At 3} , the transmission path state is read again. At this time, since two collision determination AC signals are superimposed on the transmission path, a beat signal of frequency | fs ₁ −fs ₂ | is detected (state C). Since beat signal, determines a transmission collision, the terminal apparatus 1 immediately stops the transmission of the collision determination AC signal Fs _1. Accordingly, the transmission path state is determined by the collision determination AC signal Fs _1.
Is removed and the collision determination AC signal Fs ₂ having the frequency fs ₂ is obtained.
Is left.

Subsequently, similarly, the collision detection unit 3 of the terminal device 2
4 is the trigger timing τ after the initial period_Foursmell
And read the transmission path state again. At this time, the transmission line
fs _TwoCollision determination AC signal Fs_TwoIs left,
It is detected (state D). Not a beat signal
It is determined that there is no transmission collision, and then data (not shown)
Data signal is transmitted. This allows the computer to be
Even if there is a transmission request from the
Data communication without fail.

At this time, the two collision determination AC signals Fs
Is very close, the beat frequency becomes extremely low, and it may take time until transmission collision is determined. In such a case, for example, it is desirable to randomly modulate the phase or frequency of each collision determination AC signal Fs. Randomly modulated collision determination AC signal Fs
Can be obtained by using a voltage-controlled crystal oscillator VCO and inputting a random function to it. Since the signal is randomly modulated, a beat signal having a frequency difference between a plurality of collision determination AC signals can be reliably detected in a short time. As a result, transmission collision can be determined in a short time.

The period in which the frequency or phase is randomly modulated may be a predetermined initial period, and the frequency or phase may not be modulated after the initial period has elapsed. In a predetermined initial period, the presence or absence of a beat signal can be confirmed in a shorter time. After the initial period, a constant frequency collision determination AC signal Fs may be used as a data carrier. Thereby, the oscillator can be used effectively. or,
The frequency or phase may be modulated by a monotonically increasing function or a monotonically decreasing function only during the initial period. A beat signal having a frequency corresponding to the function difference is detected.

As described above, when transmitting a data signal, detection of another carrier or data signal and collision determination AC signal F
Transmission collision is determined by two-step detection of beat signal detection by s. Therefore, according to the collision detection method in the LAN of the present embodiment, transmission collision can be reliably avoided, and reliable data communication is guaranteed.

(Second Embodiment) FIG. 3 shows a terminal device 3 of the present invention.
FIG. The figure is a block diagram. This terminal device 30 is applied to the collision detection method in the LAN of the first embodiment. The terminal device 30 of the present invention includes an Ethernet interface 31, a modulation unit 32 for modulating a carrier with data of Ethernet specifications, a demodulation unit 33 for extracting data from the modulated signal, and a collision determination AC signal Fs for collision detection. A collision determination AC signal generation unit 35, a collision detection unit 34 that detects a plurality of collision determination AC signals Fs on a transmission path as a beat signal, and a demultiplexer that bidirectionally multiplexes and combines a high frequency band and a low frequency band. It comprises a vessel 36.

The modulation of the data signal includes amplitude modulation, frequency modulation, and phase modulation. In this embodiment, the amplitude modulation is used. The transmission method uses a broadband method in which frequencies are multiplexed, and a frequency band of 5 MHz to 112 MHz is used, for example. In the normal broadband system, 150 MHz to 450 MHz is allocated for the downstream signal, but is not used in this embodiment. Also here
For convenience, the output carrier is denoted by Fu and the input carrier is denoted by Fd.
Both have the same frequency. Input / output data is input / output by amplitude-modulating / demodulating this carrier wave, for example, 100 MHz, according to the data. Also, the input / output carrier F
If u and Fd are set to the high frequency band and the collision determination AC signal Fs is set to the low frequency band, the signals are split and multiplexed in the respective directions by the splitter 36.

Next, each function will be described according to the signal flow. The data signal input from the computer device 40 is sent out over a twisted pair line in a predetermined communication format (10BASE-T). The data signal transmitted to the twisted pair line is transmitted via the Ethernet interface 31 to the modulation unit 3.
2 is sent. In the modulator 32, for example, the output carrier F
u is amplitude-modulated based on this data signal and sent to the network. Prior to this, in order to avoid data signal transmission collision, first, the collision detector 34 or the demodulator 33 checks the current use status of the transmission path. That is, the presence or absence of another carrier or the presence or absence of data is checked.
If another carrier or data is detected, data transmission is not performed.

When no other carrier is detected, in order to avoid simultaneous transmission collision, the Ethernet interface 31 triggers the collision determination AC signal generation unit 35 from the RS terminal to transmit the collision determination AC signal Fs. This is transmitted to all the terminal devices 30 on the network via the duplexer 36.

Subsequently, the Ethernet interface 3
1 reads the transmission path state from the collision detection unit 34 after a predetermined initial period. If a collision determination AC signal Fs of a different frequency exists in the transmission path, the collision detection unit 34 detects a beat signal of the difference frequency. If it is a beat signal, it is determined that a transmission collision has occurred. The terminal device 30 that has detected the transmission collision stops transmitting the collision determination AC signal Fs and also stops transmitting the data signal.

If the beat signal is not detected after the predetermined initial time, the terminal device 30 that transmitted the collision determination AC signal Fs determines that there is no transmission collision, and continues the collision determination AC signal Fs. And a data signal. In this way, transmission collision is avoided and data transmission is performed.

FIG. 4 is a block diagram showing the configuration of the collision detection unit 34. The collision detecting unit 34 is a frequency discriminating unit, and includes a nonlinear amplifier 34a, a low-pass filter (hereinafter, LPF) 3
4b, a comparator 34c, a frequency counter 34d, and a digital comparator 34e for comparing with a predetermined value.

For example, when only one-wave collision determination AC signal Fs ₁ is input to the non-linear amplifier 34a, it is a high-frequency signal, and after being amplified, is cut off by the LPF 34b. Therefore, the count is not counted by the frequency counter 34d, the output of the digital comparator 34e also becomes "L", and the transmission collision is not determined. Thereby, the terminal device 30 starts data transmission.

On the other hand, when two waves of the collision determination AC signals Fs ₁ and Fs ₂ are input to the nonlinear amplifier 34a, the LPF
34b, the beat signal A ₁ cos 2π (f) of the frequency difference | fs ₁ −fs ₂ | of the collision determination AC signals Fs ₁ and Fs ₂ |
s ₁ -fs ₂ ) is output.

This low frequency signal A ₁ cos 2π (fs ₁ −
fs ₂ ) is converted into a binary wave by the comparator 34c with reference to a certain threshold value, and is input to the frequency counter 34d. Frequency counter 34
In d, the signal is counted for a predetermined time in synchronization with the rise of the input signal, and the counted value A is calculated by the digital comparator 3.
4e is compared with a predetermined set value B. If it exceeds the set value B, it is determined that a transmission collision has occurred. For example, if the predetermined value B is set to 5 in consideration of the noise component and the beat signal of 100 kHz or more is counted for 100 μsec, the value A of the frequency counter 34d becomes 10 or more, and A> B is obtained by the digital comparator 34e. 'H' is output, and transmission collision is determined.

As described above, in the terminal device of this embodiment, the transmission collision is determined not by the signal level of the collision determination AC signal Fs but by the frequency of the collision determination AC signal Fs. Therefore, even if the signal amplitudes of the plurality of arriving collision determination AC signals Fs are different,
The transmission collision is reliably determined without being influenced by the transmission collision. Accordingly, various types of L having different attenuation rates and wiring lengths are used.
This terminal device can also be applied to an AN. Further, if a LAN is constructed using the terminal device of the present embodiment, the expensive transceiver of the conventional example becomes unnecessary. Therefore, the construction cost of the LAN can be reduced.

(Modifications) Although one embodiment of the present invention has been described above, various other modifications are conceivable. FIG.
Next, a modified example of the present invention will be described. This makes the invention a CATV
It is applied to a network to build a small LAN. This LAN comprises a CATV coaxial cable 10, a branching device 20 for branching a TV signal and a data signal, a terminal device 30, a computer device 40, and a TV device 80. This CATV network is an existing CATV system deployed in each home or office using the coaxial cable 10.

The frequency used for CATV transmission is separated into two bands. One is the upstream band in the station direction from 10 MHz to 55 MHz, and the other is 70 MHz to 770 MHz.
Is a downstream band to the terminal device side. The upstream band is mainly used for data communication, and the downstream band is used for television signals such as video signals (hereinafter referred to as TV signals). Therefore, the frequency band of 10 to 55 MHz in the upstream band is used, for example, the 55 MHz band is assigned to the carrier wave of the bidirectional data communication, and 10 MHz is assigned to the collision determination AC signal Fs. If a terminal device is adopted, a LAN can be easily formed. Further, any one of the 55 MHz to 70 MHz band, which is a guard band, may be assigned to the collision determination AC signal Fs. These LANs are compatible with existing CAT
Since the V network is used, it is not necessary to newly provide a LAN cable. Therefore, if this terminal device 70 is applied, an inexpensive and highly convenient small-scale LAN can be realized.

In the first embodiment, the collision determination AC signal F
Although a voltage-controlled crystal oscillator VCO is used to generate s, a voltage waveform based on a random function is given to the VCO, and a collision-determining AC signal Fs that is randomly frequency-modulated has been proposed. In the case of a triangular wave, the inclination may be different for each terminal device. Also, any type of voltage waveform can be used as long as it can reliably extract the beat signal by sweeping the phase.

In a modification applied to a CATV network,
Although an unused band is used for the collision determination AC signal Fs for collision detection, an arbitrary frequency band can be used as long as the band does not hinder the CATV system. For example, a vacant channel of a TV signal in a downlink signal band can be used.

Further, in the above-mentioned modification, the collision detection method and the terminal device in the LAN according to the present invention and the terminal device are applied to the existing CATV network. However, the present invention is not limited to the CATV network. The present invention may be applied to a TV network deployed in a small area.
Also in this case, a small-scale LAN is constructed in addition to ordinary TV reception.

Although various other modifications are conceivable, the coaxial cable is branched at a high frequency, and the terminal device has a tree shape.
In a star-structured network, a collision detection AC signal is transmitted to a transmission line prior to transmission of a data signal, and a terminal device detects a transmission collision based on the presence or absence of a beat signal on the transmission line. Any kind.

[Brief description of the drawings]

FIG. 1 is an LA to which a collision detection method according to a first embodiment is applied;
FIG.

FIG. 2 is a time chart for explaining simultaneous transmission avoidance according to the first embodiment;

FIG. 3 is a block circuit diagram of a terminal device according to a second embodiment.

FIG. 4 is a block circuit diagram of a collision detection unit according to a second embodiment.

FIG. 5 is a configuration diagram of a LAN using a terminal device according to a modification.

FIG. 6 is a configuration diagram of a LAN according to a conventional collision detection method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Coaxial cable 20 Branch device 30 Terminal device 34 Collision detection part 35 Collision determination AC signal generation part 40 Computer device Fs Collision determination AC signal

Claims (9)

[Claims] In a LAN in which a plurality of terminal devices perform two-way communication by modulating a high-frequency carrier wave with data of Ethernet (registered trademark) specification to generate a data signal and transmitting the data signal to a network, The present invention is applied to an extended LAN in which a coaxial cable is branched at a high frequency using a distributor, and the terminal device transmits the data signal to a network before the data signal and until the transmission of the data signal is completed. Period, sending a collision determination AC signal for collision determination to the network, the terminal device that has sent the collision determination AC signal, detects a transmission collision based on the presence or absence of a beat signal on the network. Collision detection method in a LAN. 2. The method according to claim 1, wherein the presence or absence of the beat signal is detected by frequency discriminating means provided in the terminal device. 3. The method according to claim 1, wherein the terminal device transmits the collision determination AC signal having a different frequency for each terminal device. 4. The method for detecting a collision in a LAN according to claim 1, wherein said terminal device transmits said collision determination AC signal which is phase-modulated or frequency-modulated by a random function. 5. The terminal according to claim 1, wherein the terminal device transmits the collision determination AC signal phase-modulated or frequency-modulated by a random function in an initial period of the collision determination AC signal. Collision detection method in LAN. 6. A terminal device used for the collision detection method in a LAN according to claim 1, wherein the terminal device transmits the collision determination AC signal within a predetermined initial period after transmitting the collision determination AC signal. When the beat signal is not detected, the data signal is transmitted to the network, and when the beat signal is detected, the transmission of the collision determination AC signal is stopped without transmitting the data signal. A LAN terminal device characterized by the above-mentioned. 7. The LA according to claim 6, wherein the terminal device is connected to another terminal device in a tree structure by the coaxial cable and the splitter or the distributor.
N terminal devices. 8. The terminal device according to claim 6, wherein the terminal device is applied to a LAN formed on a CATV network, and the collision determination AC signal is transmitted using a free channel of the CATV. A terminal device in a LAN according to item 1. 9. The method according to claim 1, wherein the collision detection method in the LAN is applied to a TV wiring network provided in a predetermined area, and the LAN is formed by using a TV free channel. 3. The collision detection method in a LAN described in 1. above.