JPH0918441A - Method and system for frequency/time division multiplex transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the use efficiency of a line from lowering and to efficiently perform the transmission of a discontinuous enabling signal by sharing a control line with the one for the discontinuous enabling signal when the number of times of collision shows a value less than a prescribed value. SOLUTION: A center station 1 judges the occurrence of the collision of a control circuit by the size of an envelope in a time slot, and reports information representing the feasibility of transmission of the discontinuous enabling signal to terminal station 3a-3d by using the control line when the frequency of the occurrence of collision whose the value less than a prescribed value. The terminal station 3 executes packet processing 10 the discontinuous enabling signal, and transmits it to an opposite party station by using the time slot of the control line. At this time, the center station 1 sets a division ratio so as to set the transmission quantities of both lines at prescribed values from the number of working time slots and that of the time slots in which the collision occurs, and sends out a change notification signal as to the use purpose of a frequency band area. The terminal station 3 uses changed frequency band area for communication channel and frequency band area for control line according to such notification.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency / time-division multiplex transmission method and system suitable for application to a communication system of a type for transmitting signals such as telephone, image and FAX together with computer data.

[0002]

2. Description of the Related Art TDMA communication (Time Division Multiple
Access) is widely known as a communication method in which a communication transmission line is divided in time to form a time slot for signal transmission, and each of the time slots is assigned to a plurality of stations (for example, the June 1992 corporate association). Published by Institute of Electronics, Information and Communication Engineers:
Heiichi Yamamoto and Shuzo Kato "TDMA Communication" pages 4 to 2
(See page 0). In this communication system, as shown in FIG. 7, a TDMA frame Tf, which is the basic cycle of each communication transmission line, is temporally divided into a large number of time slots TS1 to TSn, and each terminal station # 1 to #n. Transmits a signal to the partner station using any of these time slots. The partner station is
A signal is received by selecting a time slot destined for itself from a large number of time slots.

In TDMA communication, when a plurality of terminal stations simultaneously use the same time slot, a plurality of signals collide with each other and correct transmission cannot be performed. For this reason, it is necessary to prevent other terminal stations from using the time slot that is being used by any one terminal station.As a countermeasure for this, a pre-assignment method in which a time slot is assigned to each station in advance ( Pre-Assignment) and the demand assignment method (Demand Assignment) which selects and assigns an empty slot to each terminal station each time a call is made. When there is a large change in the amount of signal transmission (line usage), the latter (demand assignment method) is generally advantageous because the line can be used more effectively.

A conventional example of the demand assignment method is described in, for example, Japanese Patent Application Laid-Open No. 2-183644.
In the case of this conventional example, as shown in FIG. 8, different frequency bands are used as upstream and downstream communication transmission lines, and the TDMA frame Tf of each communication transmission line is
One time slot TSc used as a control line,
Multiple time slots TS1 to TSn used as communication lines
It is time-shared. When there is a request for line allocation via the control line time slot TSc in the uplink communication transmission line, the center station sends an empty slot to the terminal station via the control line time slot TSc in the downlink communication transmission line. Notice. The terminal station transmits the communication signal using the time slot assigned by the center station.

In the demand assignment method, when calls from a large number of terminal stations are concentrated, a plurality of line allocation request signals collide with each other on the control line. You will not be able to use it. Moreover, when the transmission capacity of the control line is increased in anticipation of the concentration of outgoing calls, there is a problem in that the transmission capacity of the communication line is limited by that amount and the utilization efficiency of the entire system is reduced.

There is a random access method as another method in which each terminal station uses a line, and a representative example thereof is a slotted ALOHA method. This method is a method proposed mainly for packet transmission.Each terminal station divides the transmission signal into one or more packets according to its length, and selects a slot for each packet. To send. The partner station receiving the signal returns a response signal to the transmitting station. If there is a packet for which there is no response from the partner station even after a certain period of time, the transmitting station determines that there is a signal collision in that time slot, and retransmits. Retransmission is performed at random time intervals and is repeated until the packet reaches the partner station.

As described above, the random access method is used when a plurality of terminal stations simultaneously select the same time slot.
Retransmission is performed to avoid loss of signals due to collisions, but it is possible to simplify time slot allocation control compared to the case of the demand assignment method, and to use communication lines for the entire system. There are advantages that can be shared with. However, the random access method cannot guarantee the temporal continuity of the transmission signal when the retransmission is repeated, and therefore, except when transmitting data such as a computer that does not require signal continuity. Cannot be used. Note that the line assignment request signal in the case of the demand assignment method may have some discontinuity without any problem, so that a control line of the slotted Aloha method can be used.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, prevent a decrease in line utilization efficiency, and allow a signal (hereinafter referred to as "non It is to provide a novel frequency / time division multiplexing transmission method for efficiently transmitting a "continuity allowable signal").

[0009]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The above object of the present invention is to provide a control line in the up direction (a demand assignment type communication line allocation request from a terminal station to a center station) as a transmission line for a discontinuity allowance signal. If the frequency of collisions on the same control line is less than a predetermined value, it is effective to transmit at least a part of the discontinuity allowance signal via the control line. Can be solved. Since it becomes possible to transmit the discontinuity allowance signal through the control line without imposing undesired restrictions on the transmission of the line allocation request signal that should originally be transmitted preferentially, the line utilization efficiency of the entire system improves. Is.

[0010]

When a collision occurs on the control line, the signal envelope in that time slot becomes large.
Therefore, if the envelope exceeds a predetermined value, it can be determined that a collision has occurred, and the frequency of collision can be measured by counting the collisions for a certain period. The center station enables transmission of the discontinuity permission signal when the occurrence frequency is less than or equal to a predetermined value, and informs the terminal station of the transmission enable information by using the downlink control line. When the transmission is possible, the terminal station packetizes the discontinuity allowance signal and transmits the packet using the time slot of the control line. If not, it sends the discontinuity allowance signal to the time slot of the communication line. Will be assigned and sent.

When a dedicated frequency band is used for the control line, the division ratio of the frequency band used as the communication line and the frequency band used as the control line can be changed according to the transmission amount of signals on both lines. desirable. The center station can grasp the number of used time slots on the communication line and the number of used time slots on the control line and the number of time slots that have collided. The transmission amount of both lines can be known from these quantities, and the center station
The division ratio can be set so that the transmission amount of both lines becomes a desired value. A signal notifying the change of the purpose of use of the frequency band is sent to the terminal station through the downlink control line, and the terminal station uses the changed frequency band for communication line and control line frequency band according to the notification. .

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The frequency / time division multiplex transmission method according to the present invention will be described below in more detail with reference to the embodiments shown in the drawings.

<First Embodiment> FIG. 1 shows the configuration of a communication system in which the frequency / time division multiplex transmission method of the present invention is implemented.
In the figure, 1 is a center device (center station), 2 is a bidirectional transmission line, 3a, 3b, 3c and 3d are communication devices (terminal stations), 4a and 4b are network terminators, and 5a and 5b are terminals. . The communication line and the control line of the bidirectional transmission line 2 are configured as shown in FIG. In the figure, Fu1, Fu2, Fu3 are
Frequency band for upstream communication line, Fu4 is frequency band for upstream control line (for communication line setting), Fd1, Fd
2, Fd3 is the frequency band for downlink communication line, Fd4
Indicates a frequency band for a downlink control line. Each communication line in the frequency bands Fu1, Fu2, Fu3, Fd1, Fd2, and Fd3 is configured using a frame Tf composed of m time slots TS1 to TSm. Frequency band Fu1, Fu2, Fu3
Communication lines are TDM based on the demand assignment method
Used by all terminals on the A communication line. Frequency band Fd1, Fd2,
The communication line of Fd3 is a TDM communication line used by the center device 1.

In the upstream control line, contention control by the slotted Aloha method was implemented. Each network termination device 4
Transmits a communication channel time slot allocation request signal and a discontinuity allowance signal using the same channel. The transmission method will be described in detail later. The center device 1 always receives a signal on the same line. The control line is also time slot TS1 ~ T
Although it is divided into Sn, its time slot length is set so that no unnecessary space is generated when transmitting a signal for allocation request. Since the signal for allocation request is composed of relatively simple data, the time slot length is shorter than that for communication lines. On the other hand, in the downlink control line, the center device 1 transmits to each network terminating device 4. Each network terminating device 4 always receives this. The downlink control line does not have to be divided into time slots. Since only the center device 1 performs the downlink transmission, the packets do not collide even if the variable-length packets are continuously transmitted. Since the packet indicates the destination, each network terminating device can select and receive the packet addressed to itself.

FIG. 3 shows the internal structure of the center device 1. In the figure, 11 is a controller, 12a, 12b, 12
c, 12d are frequency converters, 13a, 13b, 13c,
13d is a tuner, 14 is a mixer, and 15 is a collision measuring device. The tuners 13a, 13b, 13c receive signals in the frequency bands Fu1, Fu2, Fu3, respectively, convert the signals into intermediate frequency signals IF, and supply the signals to the control device 11.
The control device 11 extracts the control information from the supplied signal IF and carries out space-time exchange of the signal IF according to the control information. As a result, the downlink frequency band and the position of the time slot therein are assigned. Based on the allocation result, one of the frequency converters 12a, 12b, 12c is selected, and the signal IF is supplied to the selected frequency converter. Further, the tuner 13d receives the Fu4 signal, converts the signal into an intermediate frequency signal IF, and supplies the intermediate frequency signal IF to the control device 11. The control device 11 supplies the supplied signal IF to the frequency converter 12d as necessary. Frequency converter 12a,
The input signals 12b, 12c and 12d convert the input signals into frequency bands Fd1, Fd2, Fd3 and Fd4, respectively, and output them. These signals are supplied to the mixer 14. The mixer 14
The supplied signals are mixed into one signal, frequency-multiplexed, and output to the bidirectional transmission line 2. The arrangement of the multiplexed frequency bands is shown in FIG. In the figure, each frequency band is continuous, but it is not necessary to be continuous.

The collision measuring device 15 (lower part of FIG. 3) measures the size of the envelope of the signal IF (control line) of the tuner 13d for each time slot, and the size is a constant value (for example, the envelope when there is no collision). The number of time slots exceeding 1.5 times) is counted for each fixed period. The counting result indicates the collision frequency, and the control device 11 is informed of the result. When the collision frequency is less than or equal to a predetermined value, the control device 11 generates a signal indicating that the discontinuity allowance signal can be transmitted, and transmits the signal to each network terminating device 4 through the downlink control line. To do. Each network terminating device 4 has a terminal 5
When the transmission is possible, the non-continuity permission signal is packetized and sent to the control line, and if it is not possible, it is temporarily stored in the buffer memory, and after receiving the time slot allocation of the communication line, Send to a timeslot.

In addition to the above, the criterion for permitting the transmission of the discontinuity allowance signal is to count the number of time slots in which the line allocation request signal is transmitted during the collision frequency measurement period and add this to the result of counting the collision frequency. However, it can be determined from the addition result.

In addition, the transmission enable control is such that once the discontinuity allowance signal is transmitted, the discontinuity allowance signal is not transmitted for a predetermined period, and the frequency of collisions reaches a desired value. Can be performed by adjusting the predetermined period as described above.

The flow of communication executed using the frequency / time division multiplexing method of the present invention will be described below. First, a case where the terminal 5a transmits a discontinuity allowance signal to the center device 1 will be described (see FIG. 1). The terminal 5a is a personal computer that outputs data that is a discontinuity allowance signal. The terminal 5a transfers data to the network terminating device 4a when a call originates. When the transmission is possible, the network terminator 4a adds a necessary identifier (an identifier for distinguishing from the line request signal, an identifier for indicating a destination and a source) to the data to be transmitted, generates a packet, and Is transmitted using the time slot of the control line. The packet is received by the center device 1 unless another network terminating device transmits an allocation request signal or packet in the same time slot.
An example of the above results is shown in FIG. Packetized data is transmitted in time slot TS2 (Fu4). The line allocation request signal is transmitted in the immediately preceding time slot TS1 (Fu4). The center device 1 detects the destination identifier of the received packet, recognizes that it is the packet addressed to itself, takes out the data from the packet and executes the necessary processing, and at the same time, accepts the packet to the network termination device 4a. A signal indicating that (hereinafter, referred to as “ACK signal”) is transmitted via the downlink control line.

The network terminating device 4a is connected from the center device 1 to the AC.
When the K signal is received, the terminal 5a is notified that the packet transmission is successful, and the terminal 5a proceeds to the next processing. When the transmission of the network terminating device 4a collides with the transmission of another network terminating device, the center device 1 cannot receive the packet and therefore does not transmit the ACK signal to the network terminating device 4a. The network terminating device 4a waits for an ACK signal to be sent for a certain period of time after transmitting the packet, but determines that the packet is lost because the ACK signal cannot be received, and according to the slotted Aloha method contention control, a random time is set. Retransmit the packet at an interval. The retransmission is repeated until the packet is received by the center device 1 or a prescribed number of times.

Next, the case where the terminal 5a transmits the discontinuity allowance signal to the terminal 5b will be described. The network terminator 4a, when transmission is possible, is an identifier (identifier for distinguishing from a line request signal) necessary for data (discontinuity allowance signal) to be transmitted.
A packet is generated by adding an identifier indicating a destination and a sender, and the packet is transmitted to the center device 1 using a time slot of the control line. The packet will be retransmitted if there is a collision as before. Upon receiving the packet, the center device 1 recognizes that the packet is addressed to the terminal 5b by looking at the destination address of the packet, and transmits the packet to the downlink control line. Since the network terminating device 4b always receives the signal of the downlink control line, the network terminating device 4b
5b receives the packet addressed to the terminal 5b transmitted to the terminal 5b.
b. At the same time, the network terminating device 4b transmits an ACK signal indicating that the packet addressed to the terminal 5b has been received, to the time slot of the control line in the upstream direction. The ACK signal is also sent to the center device 1 under the above-mentioned contention control. When the center device 1 receives the ACK signal from the network terminating device 4b,
The ACK signal notifying that it has been received is transmitted to the network terminating device 4b through the downlink control line and also to the network terminating device 4a. Upon receiving the ACK signal, the network terminating device 4a informs the terminal 5a that the packet transmission to the terminal 5b was successful, and the terminal 5a executes the following process.

As described above, the control line is used not only for transmitting the signal for requesting the communication line allocation but also for transmitting the discontinuity allowance signal, and the line can be effectively used.

The frequency bands Fu1, Fu2,
Fu3, Fd1, Fd2, and Fd3 can be used for any purpose. Since the transmission amount of both lines is grasped in the center device from the number of used time slots of the communication line and the used time slot of the control line and the number of time slots in which a collision has occurred, the center device can determine the transmission amount of both lines as desired. The division ratio of the frequency band can be set so that the value becomes a value.

When it is understood that the transmission amount in the TDMA communication line is small and the transmission amount in the slotted Aloha control line is large, for example, the frequency band Fu3 is changed from the communication line for the control line to the control line. The change of purpose of use of the frequency band is notified to each network terminating device 4 (FIG. 1) via the downlink control line. According to the notification, the network terminating device 4 uses the changed communication line frequency band and control line frequency band.

<Embodiment 2> The present invention is applied to communication carried out in a cable television (hereinafter referred to as "CATV"). Figure 5 shows the CATV communication equipment. In the figure, 21 is a CATV head end (signal transmission device), 22 is a preamplifier, 23 is studio equipment, 24 is a server control circuit, 25 is a video server, 26 is a digital modulator, and 27a and 27b are frequency converters. , 28 is a mixer, and 29 is an antenna. Further, 30 is tap-off (subscriber branch distributor), 31 is CATV subscriber's house, 32
Is a set top terminal (subscriber receiving terminal), 33 is a television monitor, 34 is a telephone, and 35 is a personal computer. Although not shown in the figure, the bidirectional transmission line 2
Is composed of an optical fiber, a coaxial cable or the like, or a mixture thereof, an amplifier, a branching device and the like are present in the middle, and a plurality of subscriber homes 31 are connected to a plurality of tap-offs 30.

An upstream signal is input from the network terminating device 4 in the subscriber's house 31 to the center device 1 through the bidirectional transmission line 2. The signal from the center device 1 in the downward direction is mixed by the mixer 28.
The signal is input to the network terminating device 4 through the two-way transmission line 2 via. A telephone 34 and a personal computer 35 are connected to the network terminating device 4. Both serve as the terminal 5 (see FIG. 1).

FIG. 6 shows a frequency arrangement of a general CATV. The transmission band is divided into a low band of 10 MHz to 50 MHz and a high band of 70 MHz to an upper limit frequency (for example, 450 MHz), and a large number of channels with a bandwidth of 6 MHz for TV broadcasting are arranged in each. In the present embodiment, the frame structure shown in FIG. 2 is adopted, and the frequency bands Fu1, Fu2, Fu
The whole 3 and Fu 4 were accommodated in one free channel of the low band transmission band, and the whole frequency bands Fd 1, Fd 2, Fd 3 and Fd 4 were housed in one free channel of the high band transmission band. The frequency bands Fu1, Fu2, Fu3 are TDMA communication lines in the upward direction based on the demand assignment method, and the frequency bands
Fu4 is an upstream control line based on the slotted Aloha method. The frequency bands Fd1, Fd2, and Fd3 are T in the down direction.
In the DM communication line, the frequency band Fd4 is a downlink control line. In order to prevent the signals of the communication line and control line from interfering with the broadcast signal, the channel to be used is determined in consideration of cross modulation, etc., and the output level of the signal of both lines is further minimized in order to minimize the influence. Lowered.

The telephone 34 and the personal computer 35 communicate with each other using the above transmission path. Telephone 3
The signal No. 4 is transmitted using the time slot of the TDMA communication line. The data of the personal computer 35 is packetized and transmitted under control of contention through the control line when the discontinuity allowance signal can be transmitted.
When transmission is impossible, the time slot of the TDMA communication line is used for transmission.

A new service can be realized by using the communication system provided by the center device 1.
In this embodiment, video on demand is taken as an example of the service. In video-on-demand, when a subscriber uses the set-top terminal 32 to specify an arbitrary program at an arbitrary time, the subscriber can watch the program as specified.

When the subscriber designates a program, the server control circuit 24 in the headend 21 takes out the designated program from the video server 25. The extracted program is converted into a frequency of a predetermined channel by a frequency converter 27b through a digital modulator 26, mixed with a signal of another frequency band by a mixer 28, and transmitted through the bidirectional transmission path 2. It The transmitted signal is delivered to the set top terminal 32, and the designated program is watched on the television monitor 33 by setting the reception channel of the set top terminal 32 to a predetermined channel.

In order to realize this, the set top terminal 3
It is necessary to send commands such as program designation, start, stop, fast forward / fast reverse, etc. from 2 to the server control circuit 24. When a command is issued from the set top terminal 32 to the server control circuit 24, the set top terminal 32 transfers the command to the network terminating device 4. Network terminator 4
Generates a packet by adding to the received command an identifier for distinguishing it from the line request signal, a destination (server control circuit 24) and a source (set top terminal 32) identifier. Since many commands to be transmitted are simple ones such as start and stop, the length of the generated packet is shorter than the time slot length of the slotted Aloha system.
The generated packet is transmitted by the slotted Aloha method through the control line of the frequency band Fu4.

Upon receiving the packet, the center device 1 recognizes from the destination identifier that the command is addressed to the server control circuit 24, extracts the command from the packet and sends it to the server control circuit 24. Most of the commands transmitted from the set-top terminal 32 need to be transmitted without waiting time, but since the packet length is limited to short ones, even if transmitted by the slotted Aloha method, there is a probability of signal collision. Is low and the probability of retransmission with waiting time is low, so there is no practical problem.

Conventionally, a dedicated frequency band was assigned for command transmission, but if the present invention is applied,
Since the frequency band Fu4 can be shared with the allocation request signal, the limited frequency band in the upward direction of 10 to 50 MHz can be effectively used.

According to this embodiment, various communication services can be carried out on the CATV, and in addition to the video-on-demand command, the interactive communication carried out between the CATV headend 21 and the subscriber's home 31. It goes without saying that it can be applied to all services.

[0035]

As described above, according to the present invention, the vacancy of the control line is suppressed, so that the reduction of the line utilization efficiency is avoided. Also,
It becomes possible to transmit various types of data, which are non-continuity permission signals, having different transmission rates, through a control line based on the slotted Aloha system. Further, since the division ratio of the frequency band for communication lines and the frequency band for control lines corresponds to the amount of transmission, it is possible to prevent a decrease in line utilization efficiency.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a communication system used in a first embodiment of a frequency / time division multiplexing method according to the present invention.

FIG. 2 is a frame Tf of a communication transmission line used in the first embodiment.
FIG.

FIG. 3 is a block diagram showing an internal configuration of a center device 1 used in the first embodiment.

FIG. 4 is a configuration diagram showing a frequency arrangement of frequency bands used in the first embodiment.

FIG. 5 is a configuration diagram of communication equipment of a cable television used for implementing the second embodiment of the present invention.

FIG. 6 is a configuration diagram showing a frequency arrangement of a general CATV.

FIG. 7 is a block diagram showing the concept of TDMA communication.

FIG. 8 is a configuration diagram showing an example of a conventional time division frame Tf.

[Explanation of symbols]

 1 ... Center device 2 ... Bidirectional transmission line 3 ... Communication device 4 ... Network terminating device 5 ... Terminal 11 ... Control device 12 ... Frequency conversion circuit 13 ... Tuner 14 ... Mixer 15 ... Collision measuring device 21 …… CATV head end 24 …… Server control circuit 25 …… Video server 31 …… CATV subscriber home 32 …… Set top terminal 33 …… TV monitor 34 …… Phone 35 …… Personal computer Fu …… Upstream frequency band Fd …… Downstream frequency band TS …… Time slot

Claims (6)

[Claims] 1. In a frequency / time division multiplex transmission method adopting a demand assignment method, a signal is allowed to have a discontinuity in time (hereinafter referred to as "discontinuity allowable signal") as a transmission line, and an upstream direction is used. Also used as the control line (transmission line of the line allocation request signal from the terminal station to the center station),
A frequency / time division multiplex transmission method, wherein at least a part of the discontinuity allowable signal is transmitted through the control line when the frequency of collisions in the control line is less than a predetermined value. 2. A frequency / time-division multiplex transmission method adopting a demand assignment method, wherein a transmission line for a signal in which temporal discontinuity is allowed (hereinafter referred to as "discontinuity-allowed signal") is used in the upstream direction. Also used as the control line (transmission line of the line allocation request signal from the terminal station to the center station),
After the discontinuity allowance signal has been transmitted once, a period during which the discontinuity allowance signal is not transmitted is provided, and at least part of it is controlled during the synchronization so that the frequency of collisions on the control line becomes a predetermined value. 2. The frequency / time division multiplex transmission method of transmitting the non-continuity allowance signal of 1 above via the control line. 3. The frequency / time division multiplexing transmission method according to claim 1, wherein a frequency band different from a frequency band for a communication line is used for the control line. 4. The frequency / hour according to claim 1, wherein the control line is arranged separately from the communication line in a part of a frame configured in a frequency band for the communication line. Division multiplexing transmission method. 5. The frequency / hour according to claim 3, wherein a division ratio of the frequency band for the communication line and the frequency band for the control line is changed according to the transmission amount of signals of both lines. Division multiplexing transmission method. 6. A frequency / time division multiplex transmission system having a communication line adopting a demand assignment system, and a transmission line of a signal (hereinafter referred to as "non-continuity allowable signal") in which temporal discontinuity is allowed. Is an upstream control line (transmission line for the line allocation request signal from the terminal station to the center station)
And also has means for transmitting at least a part of the discontinuity allowance signal through the control line when the frequency of collision occurrence in the control line is less than or equal to a predetermined value. A frequency / time division multiplex transmission method characterized in that a measuring device is included in a center station.