JPH022339B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a data communication method on a two-way CATV system.

The CATV system operates in such a way that coaxial cables are laid within a limited area, and each user selects and receives images supplied from a video center using some method.

Compared to normal broadcasting systems that use air waves, CATV systems use a fixed medium called coaxial cables, so they have a greater degree of freedom in operation. For example, in this case, airwaves are transmitted over adjacent channels where there is a risk of interference. This is possible by lowering the level of audio waves that have frequency components close to those of adjacent channels.

In particular, the most distinctive feature of the CATV system is that
Two-way data communication is possible.

For this reason, a system has been adopted that uses the CATV system to pay a fee for each viewing of a paid channel, and the means to achieve this is to use bands that are not used for video signals (empty channels).

Data communication for billing, video selection, scrambling, etc., so-called CATV system maintenance, is already widely used.
Data communication as a ``COMMON-CARRIER'' that does not limit the content of data or its usage is a future issue.

Applications of data communication using the CATV system include, in addition to data communication for maintaining the CATV system as described above, one-way communication methods using the VBI part of the video channel such as telex and teletext, There are communication systems for downloading data such as unidirectional software, but they lack flexibility in terms of free communication between users. Now, the technical field to which the present invention is intended lies exclusively in the above-mentioned "COMMON-CARRIER" communication system.

[Prior art] The configuration of the network adopted as a communication system that can be applied to the CATV system is a so-called bus type as shown in Figure 1, so it is similar to the communication form used in general telephone networks. Since this is not possible, packet communication is used.

Packet communication is a time division multiplex communication system in which many users are connected to one line, each data is divided into packets, and each user uses the line as if it were exclusive to the line.

The CSMA/CD method (Carrier Sense Multiple Access with Collision Detection) has been proposed as a time division multiplexing method with high line utilization efficiency. It is used. Traditionally used in LAN, etc.
The CSMA/CD method communicates using the so-called baseband, and requires various changes to be applied to CATV systems.

The following conditions are required to apply this CSMA/CD method to a CATV system. Figure 2 shows an example of the frequency allocation of a general CATV system. Here, the frequency bands that can be used are roughly 2
The lower group of frequency bands is used in the upstream direction (from the terminal to the headend) and the upper group is used in the downstream direction (from the headend to the terminal). Usually, many channels are allocated in the downstream direction to supply a large number of images. The methods of dividing this frequency band in the United States include the MID-SPLIT method and the SUB-SPLIT method.
There are signals etc. In this way, on the CATV system, the direction of data communication is defined by the frequency band, so when applying the CSMA/CD method to the system, it is necessary to have the following configuration.

1 Baseband communication cannot be performed, and communication is performed by modulating the carrier wave using an appropriate carrier wave.

2 Uplink data and downlink data use carrier waves with different frequencies.

3. A frequency conversion device is required at the end of the uplink and the start of the downlink in the CATV system to connect both lines.

If a communication system is built on a CATV system that satisfies the above conditions, it will look like the one shown in Figure 3. Here, a frequency converter (FC) 101 converts uplink frequency f ₁ to downlink frequency f ₂ .
In addition, the terminal device (hereinafter referred to as NIU) 102 is
One device is provided for each user, and it controls data transmission and reception. In other words, the data from the user's terminal connected to the NIU is packetized, modulated at the transmission frequency _f1 , and then sent to CSMA/
Transmission is performed according to the CD control method. Also,
This is a series of controls that constantly monitors the data on the line, captures packets addressed to its own terminal, analyzes the packet structure, and outputs the data to the user terminal if there are no errors. In addition,
Multiple NIUs are connected to one system,
Each NIU (that is, the user terminal connected to the NIU) can communicate with any NIU.

The operation of this NIU is as follows.
In the CSMA/CD method, data can be sent only when there is no data on the communication line, but each NIU performs the data detection on its own. In CATV system,
The transmit and receive frequencies of each terminal are separated, and the uplink frequency is used for transmission and the downlink frequency is used for reception, so the fact that other terminals are transmitting data packets does not depend on the frequency. This can be recognized by receiving the frequency-converted downlink with a conversion device. In other words, in order to transmit data packets on the uplink, it is necessary to receive the downlink and understand the status of the uplink. In addition, regarding "collision detection", which is a feature of the CSMA/CD method, collisions on the uplink are monitored on the downlink, and the NIU sends data to the uplink and at the same time the data is sent to the downlink. "Collisions" must be detected by comparing data. In other words, a “collision” occurs when data sent by oneself cannot be received correctly.
It is determined that the temperature is hot. From the above, it will be understood that the downlink needs to accurately reflect the situation of the opposite uplink.

When the NIU has data to send, it monitors the downlink, and after confirming that there is no other data, begins to send data packets to the uplink. It then monitors the downlink, receives the data it has received, and compares it with the data it is currently transmitting.If it detects an error during the comparison, it immediately interrupts the transmission even in the middle of transmission. This is important in order to prevent the line from being occupied by meaningless data and to improve the overall usage efficiency of the line.

Next, data formats for uplink and downlink will be explained. In addition to the difference in frequency, uplink and downlink have the following essential differences. That is, while the downlink has one transmitter and many receivers, the uplink has one receiver for many transmitters. This imposes restrictions on the format of data on the uplink. In other words, in order to perform free data communication, it was necessary to perform time-division communication on the uplink, and each terminal was required to turn on the carrier wave only when necessary, and turn off the carrier wave at other times. In other words, the carrier wave itself is turned on and off according to the length of the data to be transmitted. In this case, if there is some carrier wave leakage in each NIU in the off state, it becomes noise for the NIU that is the source, and appears as the sum of that noise at the input end of the receiver at the center. Line C/N
It will cause deterioration.

On the other hand, on the downlink, it is sufficient to reflect the data on the uplink, but the contention for the data line is already taking place on the uplink, so it is sufficient to simply transmit the data in a broadcast-like format. In other words, the system can be implemented by converting the frequency of data that has arrived at the end of the uplink and transmitting it directly to the downlink.

[Problems with conventional technology] In this way, in CATV systems,
When a CSMA/CD communication system is adopted, the uplink carrier wave is simply frequency-converted and transmitted to the downlink, which has the following drawbacks.

(1) In large-scale CATV networks, the uplink data reception level fluctuates greatly, so the downlink transmission level also fluctuates.

(2) Uplink noise is directly superimposed on the downlink.

(3) Packets that connect or disconnect the carrier wave are also transmitted on the downlink, so each NIU needs to receive data with the carrier wave disconnected.

[Object of the present invention] The present invention was made based on the above circumstances, and it maintains a stable transmission level on the downlink without being affected by fluctuations in the reception level of data on the uplink, and also maintains a stable transmission level on the downlink. Avoid introducing noise, and connect the carrier wave on the uplink.
A communication method in a two-way CATV system that maintains the continuity of carrier waves in the downlink by replacing "dropped" packets with control of whether or not to modulate them in the downlink, thereby improving the quality of NIU mutual communication. This is what we are trying to provide.

[Summary of the Invention] To this end, the present invention provides a bidirectional communication system in which upstream and downstream signals are divided into certain frequency bands.
In the CATV system, the center equipment converts the packet-type data transmitted by connecting and disconnecting carrier waves in the frequency band for the two-way CATV system, and converts the frequency in the section where the data exists on the uplink. In addition, in sections where there is no data on the uplink, the data is frequency modulated using a modulation pattern that cannot appear in the modulated signal on the above line and sent to the downlink. This is a characteristic feature.

Embodiments of the present invention will be specifically described below with reference to FIGS. 4 to 12. In Figure 4,
Reference numeral 401 indicates the state of the carrier wave transmitted to the uplink, and the carrier wave is made "connected" only when necessary. Reference numeral 402 indicates a downlink carrier wave, and modulation is applied only when the carrier wave is turned on in the uplink. Further, 403 is downlink modulation data, which matches the uplink carrier wave. For this kind of signal control, there is a fifth
The configuration shown in the figure is available. Here, uplink data is demodulated once by the receiver 501,
The next noise separation circuit 502 removes uplink noise. Then, this noise-separated data is applied to the modulation input terminal of transmitter 503, and the data is transmitted to the downlink. In contrast to the construction of such a communication system on the transmitting side, the 6th communication system on the terminal side is
A configuration as shown in the figure is adopted. Here, upstream data is transmitted from a transmitter 602 via a signal distributor 601 used for input/output distribution.
In this case, data modulation and carrier wave connection/disconnection are performed in response to commands from the central processing unit (CPU). Also, data from the downlink is sent to the receiver 6.
Demodulates with 03. In the figure, reference numeral 604 is a controller with a built-in microcomputer, and the transmitter 60
2, input of transmission data, input of data from receiver 603, analysis, address recognition, etc.
Controls CSMA/CD procedures and communication to the user terminal 605. The user terminal 605 is a device that performs interpersonal interface, and inputs commands to the NIU and displays received messages. Also,
A computer or the like may be used as the user terminal 605. Regarding the application of this system,
Although various forms can be taken depending on the configuration of the user terminal, the explanation here will be limited to the construction of a communication system.

The format of the data signal on the downlink is as shown in FIG. For downlink signals, in consideration of the stability of the operation of the receiver at the terminal, the carrier wave is not connected or disconnected, but whether or not it is modulated is controlled. The format of the modulated data is the same as the upstream signal and is shown in FIG. 7b. An example of the format of this data will be briefly explained. This data is
It is structured in a so-called packet format, and a preamble signal is inserted at the beginning. This signal is a signal for performing bit level synchronization on the receiving side. Next, a flag signal for synchronizing packet framing is inserted, and by correctly receiving this signal, it becomes possible to obtain synchronization at the framing level or byte unit. Next, so-called address information is included. This is usually inserted with the destination address and sender address of this packet-shaped data. Based on this address information, each terminal performs an operation to capture the corresponding packet. Data to be transmitted is inserted after the address information. This length is variable depending on the packet construction method or system design, and also depends on the amount of data required. Additionally, a control byte may be provided at the beginning of this data. Next, an error check code for detecting data transmission errors is inserted, and finally a frame end flag is added.

In this method of receiving packets, the valid data is between the flags, and immediately after the first flag is the address information, which is the only given address of the terminal in the system. Only matching packets are captured and processed. Here, the length of the data is not constant and the end of the packet is detected only by the final flag. As a result, the data immediately before the final flag is detected is the error check code. Further, the data up to just before that is data.

In this way, in packet communication, so-called flags play an important role, and data areas surrounded by flags are recognized. In particular, in the HDLC method (High Level Denta Link Control), which communicates in a similar packet format, the flag configuration is "01111110", and the data area included in the flag is set to 6 consecutive bits of "1". A "0" insertion function is provided to prevent false flag detection. If you mistakenly recognize this flag,
Care must be taken because it may become impossible to detect the normal end of data, and the next data may be framed incorrectly. In other words, correct recognition of the "flag" signal is a condition for accurate packet communication. Furthermore, even if a framing error is made, it is necessary to promptly restore the correct framing.

In addition, in the CSMA/CD communication method mentioned above,
Each terminal determines whether or not other data exists on the line, but for this purpose, when each terminal monitors the line, it is necessary to distinguish between the data communication state and the state with no data ( (hereinafter referred to as idle state) must be immediately determined. However, as mentioned above, the decision based on the "flag" signal comes at the cost of immediacy. Moreover, the result would be that the transmissions from each user would be concentrated at the end of one packet.

If the above problems are considered in a line having a modulation section 701 and a non-modulation section 702 as shown in FIG. 7, the following problems will arise. That is, in general, when an unmodulated signal is received and demodulated, the following two forms are obtained compared to data modulation.

(a) Form of data modulation by AC coupling (b) Form of data modulation by DC coupling The demodulated waveforms of each of these data are shown in Figures 8a and b. Figure 8a shows the case using AC coupling,
In the figure, the section A is unmodulated, and the section B is a section with data modulation. FIG. 8b shows the case where DC coupling is used, and the sections A and B are exactly the same as in case a.

In the case of a, it is common sense to judge the data modulation at the zero crossing point, which is the method with the least code errors, but in this case, only noise is reproduced as the data in section A. becomes. In other words, in section A, the potential of the judgment level itself is outputted, so if even a small amount of noise is superimposed, the output of the data judger will be inconsistent due to its influence. In such a situation, it is impossible to determine whether a significant data section or an idle section is present, so it is impossible to detect the above-mentioned idle state.

In case b, the difficulty of data modulation and demodulation generally increases (it is necessary to transmit up to DC), and the idle period is fixed at a certain level (for example, all ” or “0”). In this situation, it would be possible to determine the idle section if a condition is set that several bits are continuous. However, even in this case, in addition to the difficulty of direct current modulation as described above, there arises a technical problem of suppressing as much as possible the causes of fluctuations in the direct current component. For example, when transmitting using the FSK method (frequency shift keying), the frequency at the transmitter and receiver must be stable for all components.

Therefore, it is generally necessary to upgrade the hardware.

Therefore, when implementing the CSMA communication method on a CATV system, simply looping back uplink data will cause problems in the system configuration on the downlink.

In this regard, in the present invention, a configuration as shown in FIG. 5 is prepared (configuration of the frequency converter), and it is possible to separate level variations in the uplink and noise in the uplink. Furthermore, the downlink carrier wave is not "connected" or "disconnected", and downlink data is superimposed on the carrier wave depending on whether the data is modulated or not. However, simply the presence or absence of modulation cannot be used to detect an idle period. Therefore, the following method is used.

That is, the code format of data having a normal packet format is set to a code format that does not follow a specific pattern. This includes codes that do not include DC components, such as Bi-L, BPS (Bi-
PHASESPACE) or Delayed Modulation
etc., and by doing so, a series of levels longer than a certain length will not occur. This type of code setting is used when the transmission band is narrow, and is not lacking in generality.

For example, Delayed, which is especially effective when narrowing the band.
Suppose we adopt the Modulation (DM) code format. This is shown in Figure 9. This code format is created by inverting at the center of the bit length when it is "1".
"0" does not change; however, when "0" and "0" occur consecutively, they are inverted at the boundary of the bit length. In this code, the result of code formation (MARK/SPACE) has a maximum length of 2 bits each,
The minimum length is 1 bit and 3/2 bit length. Other mark lengths and space lengths cannot be used. Furthermore, in this DM code, data "1", "0", marks, and spaces do not correspond in terms of level, and is basically differential coding that transmits changes in data. Furthermore, since the fundamental frequency component matches the maximum repetition frequency (1010...) of data, it is a code format that allows efficient transmission.

Considering this in reverse, if the mark length or space length is, for example, 3 bits long, consecutive marks or spaces cannot be determined to be normal data, and will be detected as an "illegal" pattern. This determination is included in a part of the receiving circuit. Therefore, if this illegal pattern is used outside the regular data area, it becomes possible to physically distinguish between the data area surrounded by the aforementioned flags and the other idle states.

In other words, considering the entire system, in the uplink, each terminal activates a carrier wave and transmits data when necessary, but this data packet is demodulated once at the receiver at the center, separated from noise, and then retransmitted to the downlink. It will be done. However, the transmitter at the center determines whether or not a carrier exists on the uplink based on the output of the uplink carrier detection circuit, and if there is no carrier, it transmits an illegal pattern on the downlink and detects the carrier on the uplink. By switching the modulation input so that uplink data is retransmitted when a problem occurs, it is possible to identify inside and outside the data area. On the other hand, on the terminal side, when attempting to make a call, the idle detection operation can be determined based on whether an illegal pattern exists on the downlink. In other words, when an illegal pattern exists, it can be said to be in an idle state, and when there is no illegal pattern, it is a data area, indicating that transmission is not possible.

With the system configured as described above, the CSMA/CD communication method can be built on the CATV system efficiently and without any problems.

Next, the configuration of the center equipment suitable for the method of the present invention is shown in Figure 10, and the configuration of the terminal is shown in Figure 11.
This will be explained with reference to the figures. In FIG. 10, the center frequency conversion device includes an up channel receiver (which demodulates and outputs baseband data and the presence or absence of a received carrier signal) 1001 and a circuit 10 which performs data synchronous reproduction and waveform shaping.
02. Illegal pattern generator 1 for further transmitting illegal patterns to the downstream channel
003, a switching circuit 1004 for switching patterns illegal to this data, and a transmitter 1005 for the down channel.

The operation of this device will be explained using timing flags shown in FIG. 12. A in the figure indicates the state of carrier waves in the uplink. In section A, the carrier wave is completely disconnected, but in section B, one of the many connected terminals starts transmitting a signal, and the receiver at the center detects the carrier wave. demodulation starts. In addition, B in the figure shows the carrier wave detection signal (hereinafter referred to as CD) at the center receiver.
shows the state in which it operates according to the carrier wave from section B. Further, as shown in d in the figure, demodulated data of the uplink signal is also output from section B. This demodulated data has been subjected to the DM modulation described above. In contrast, Figure C shows the output of the illegal pattern generator located inside the center, which is a repetitive signal of marks and spaces with a length of "3" bits, which does not occur in DM modulation. shows the baseband waveform modulated by the downlink signal.Here, when there is no signal in section A, an illegal pattern of H is output, but after section B, the uplink signal is input, so The data of the uplink signal will be output.It is effective to use the CD signal shown in (b) for this illegal pattern and demodulation data switching.In other words, when there is no uplink signal, the illegal pattern is used as the modulation signal. , when an upstream signal is received, the received data is used as a modulation signal.

The configuration of the center equipment for performing the above operations is as shown in Figure 10 above.
An uplink signal input from the CATV line is received by receiver 1001 and demodulated into a baseband signal. This receiver 1001 also outputs a signal (the above-mentioned CD) indicating the presence or absence of a carrier wave. The demodulated baseband signal is input to a synchronous reproduction and waveform shaping circuit 1002 for noise separation on the uplink. This circuit reproduces data from which jitter and the like generated by a data waveform containing noise have been removed. Various proposals have been made to implement this circuit, including methods using digital PLL (DPLL),
There are two methods: an analog lock method and a simple counter reset method. Therefore, with this circuit, the data from the uplink is free of noise components and is operated with a correct lock.

On the other hand, the illegal pattern generation circuit always generates the above-mentioned illegal pattern. This output is driven by an accurate lock on the basis of the data to be sent downlink. This illegal pattern is based on the above-mentioned received data and CD
Switched depending on the polarity of the signal. This is the AND-OR circuit 1004 in FIG. In order to perform this switching, it is necessary that the received data is synchronized with the illegal pattern at the bit level, and the output of the clock regeneration and waveform shaping circuit 1002 is synchronized with the clock used in the illegal pattern generation circuit 1003. It is operated by the same clock as . This situation is shown in FIG.

The signal generated by the AND-OR circuit 1004 is applied to the modulation input terminal of the downlink transmitter 1005 via the buffer circuit 1006, and is sent out to the downlink. This signal is output to one line via a coupling circuit with the uplink.

As described above, uplink signals are reflected in downlink signals. When there is no signal, an illegal pattern is output to the downlink, and when there is an uplink signal, the data content is output as is.

The configuration of the terminal that accepts this downlink signal is
As shown in FIG. The terminal detects the illegal pattern on the downlink as described above, recognizes the idle state of the uplink, and starts transmitting data. Therefore, the CATV line is branched into two at branch 1101, and one is connected to receiver 1102 for the downlink. The output of this receiver 1102 is a signal containing an illegal pattern and data, and this pattern recognition can be performed in the DM state. This data pattern is as shown in FIG. Illegal patterns are extracted from this data by detecting 3-bit continuous marks or spaces. This determination is possible because normal data only includes marks or spaces up to a maximum length of 2 bits. This illegal pattern determination circuit 1103 can be easily implemented using a counter circuit. Next, the illegal pattern detection signal is sent to the DM-NRZ conversion circuit 110.
4 (NRZ means NON RETURN ZERO) and determines the operating state of DM-NRZ conversion.
In other words, when one illegal pattern is detected,
DM-NRZ conversion is not performed, and when no illegal pattern is detected, DM-NRZ conversion is performed, and the received data converted to NRZ is output to the CPU board 1105. The operation of the CPU board 1105 is determined by the illegal pattern detection signal.
Performs carrier sense (CS) for CSMA/CD and determines the data on the line. Also, when the illegal pattern detection signal is off, the data is decoded and the content of the data packet is determined. The content of the determination is whether or not the packet is addressed to the own terminal at the time of reception, and whether the data being transmitted is being sent correctly (whether or not a collision has occurred) at the time of transmission. If there is further data to be sent, after performing the above CS, send the data to be sent.
The data output to the NRZ-DM converter 1106 and converted to DM is sent to the uplink transmitter 110.
The signal is applied to the modulation input terminal of No. 7, and outputted in the form of a modulated uplink carrier wave. This transmitter 110
The output of No. 7 is applied to the other terminal of the above-mentioned branching device 1101, and output as an uplink signal to the CATV line.

The communication method described here is suitable for large-scale
This is a method for efficiently performing bucket-type communication on a CATV system, and is different from communication methods developed on conventional small-scale CATV systems. For example, installing a receiver and a transmitter in the center equipment can be considered to increase the system cost in a small-scale system, but in a large-scale system, the number of terminals will be on the order of several thousand, and the center equipment The slight increase in cost can be ignored. Also, as mentioned above, large-scale
In CATV systems, the uplink quality is poor, so installing a receiver and transmitter in the center equipment greatly increases data reliability.

On the other hand, in a method in which the center side generates an illegal pattern when the uplink line is vacant, and each terminal detects the idle state of the line based on that signal, the circuit configuration of the center equipment becomes somewhat complicated, but as shown above. This can be ignored in large-scale systems.

Furthermore, an illegal pattern detector is added as a circuit on the terminal side, but this circuit is not complicated and the cost is small.

[Effects of the present invention] As detailed above, the present invention provides the following advantages when adopting the CSMA/CD method in a CATV system.
Packet communication on a two-way CATV system can be carried out with high reliability, and the advantage is that only a minor circuit configuration is required at the terminal. In addition, the line utilization efficiency is also extremely high.

[Brief explanation of drawings]

Figure 1 is a diagram conceptually showing the network of a bidirectional CATV system, Figure 2 is a diagram showing an example of the frequency allocation of the CATV system, and Figure 3 is the communication established between the transmitting side and the receiving side. 4 is a diagram showing the signal level of data communication according to the present invention, FIG. 5 is a block diagram of the transmitting side, FIG. 6 is a block diagram of the receiving side, and FIG. 7 is a diagram showing the signal level of data communication according to the present invention. FIG. 8 is a diagram showing the demodulated data waveform. FIG. 9 is a diagram showing the DM code format. FIG. 10 is a block diagram of the transmitting side showing the essential part of the present invention. The figure is a block diagram of the receiving side, and FIG. 12 is a signal waveform diagram in the system operation of the present invention. 101...Frequency conversion device, 102...Terminal device, 401, 402...Carrier waveform, 501...Receiver, 502...Noise separation circuit, 503...Transmitter, 601...Distributor, 602...Transmission Machine, 60
3...Receiver, 604...Controller, 605...User terminal, 701...Modulation section, 702...
...Non-modulation section, 1001...Receiver, 1002...
...Synchronized playback/waveform shaping circuit, 1003...Illegal pattern generator, 1004...Switching circuit, 1
005...Transmitter, 1006...Buffer circuit,
1101... Turnout, 1102... Receiver, 11
03...Illegal pattern determination circuit, 1104
...DM-NRZ conversion circuit, 1105 ...CPU board, 1106 ...NRZ-DM conversion circuit, 11
07...Transmitter.

Claims (1)

[Claims] 1. In a two-way CATV system where uplink signals and downlink signals are divided into certain frequency bands, the center device transmits signals by connecting or disconnecting carrier waves in the frequency band for the two-way CATV system. Packet data is frequency-converted in sections where data exists on the uplink and sent to the downlink, and sections where no data exists on the uplink cannot appear in the modulated signal on the uplink. A bidirectional device characterized by being configured to frequency-modulate the modulation pattern and send it to the downlink.
Communication method in CATV system.