JP2005064795A - Communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a communication system which doesn't bring about a transmission collision even in the case that respective terminals asynchronously start transmission by random access, and reduces an unnecessary non-transmission time resulting from a back-off time even in the case that the number of terminals where transmission requests have occurred is one. <P>SOLUTION: In the communication system, respective terminals detect the idleness of communication media and then asynchronously start transmission by random access after measuring a back-off times calculated on the basis of (back-off time)=(random()×N+M)×(slot time) wherein; the random () is a function generating random numbers of integral values generated from an uniform distribution in a range of [0, CW]; CW is an integral value within a prescribed range determining a range of the generation of random numbers; the slot time is a fixed unit time; N is the number of offsets used in all terminals in common; and M is an offset value individually used in each terminal. The number of offsets N and the offset value M are integral values satisfying relations 2≤N, 0≤M, and M<N. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an access control method in a communication system in which transmission is started asynchronously by random access in each terminal.

  For example, a wireless communication system includes a base station (access point) and a plurality of terminals (stations) that communicate with each other via a wireless communication medium (wireless communication channel) via the base station. .

  In a wireless communication system, for example, a wireless LAN (local area network) standard defined by IEEE 802.11 is used as a communication protocol. In this standard, CSMA / CA (carrier sense multiple access method with collision avoidance function) is adopted as an access control method, and transmission is started asynchronously by random access in each terminal. That is, in each terminal, the availability of media is monitored, and if it is available, transmission is started asynchronously after measuring the backoff time.

  The back-off time is provided in order to disperse the chance that transmissions started asynchronously from a plurality of terminals collide. In a wireless communication system in which transmission from each terminal is started asynchronously by random access, the same value of back-off time may be generated. When transmission is started from two or more terminals simultaneously, A transmission collision occurs. The probability of occurrence of transmission collision increases with an increase in the number of media and communication traffic (communication information amount). When a transmission collision occurs, there is a problem that communication throughput (transfer capability) decreases.

  For example, as shown in the timing chart of FIG. 3, a case will be described in which a transmission request is generated in terminal B and a transmission request is subsequently generated in terminal C while terminal A is transmitting. As shown in the flowchart of FIG. 4, in the terminals B and C, when a transmission request is generated (S1), the availability of the media is monitored (S2), and if the media is available (Yes in S2), transmission is started. However, when the terminal A is transmitting (No in S2) as in this example, the terminal A waits until the transmission of the terminal A is completed and the medium becomes free (S3), during which the back-off time Is calculated (S4).

The back-off time is calculated based on the following formula in the wireless LAN standard defined by IEEE 802.11.
Backoff time = random () × slot time Here, random () is a function that generates integer random numbers generated from a uniform distribution in the range of [0, CW], and CW (contention window) is An integer value and a slot time within a predetermined range (0 ≦ random number <CW) that define a range for generating a random number are a fixed unit time (fixed value) defined by the system.

  Subsequently, in the terminal B and the terminal C, the empty media is detected again (S5). If the medium is not free (No in S5), the detection is repeated until the medium becomes free. When the transmission of the terminal A is completed and the available media is detected (Yes in S5), the terminals B and C subtract the back-off time after a certain time DIFS (distribution control frame interval) has elapsed. (No in S6 and S7) When the value becomes 0, transmission is started (S8). Therefore, when the back-off time values of the terminals B and C are the same, that is, when the same random number is generated in the terminals B and C, a transmission collision occurs between the terminals B and C.

  The operation when a transmission request is generated at a plurality of terminals is also described in Patent Document 1 and the like. Patent Document 1 measures a waiting time from when a transmission request is generated until transmission is started, and generates a backoff time that is shorter as the waiting time becomes longer. It is to give. As described in Patent Document 1, in the conventional wireless communication system, the same calculation formula for the back-off time is used in all terminals regardless of the order in which transmission requests are generated. Accordingly, when a transmission request is generated at N terminals while a certain terminal is transmitting, a transmission collision occurs with a probability P expressed by the following equation.

  The probability of occurrence of a transmission collision increases as the number of all terminals, more precisely, the number of terminals for which transmission requests are generated while a certain terminal is transmitting. In order to reduce the probability of occurrence of a transmission collision, the value of the contention window CW may be increased. However, if the value of the contention window CW is increased, the average value of the backoff time increases, so that the terminal that has generated the transmission request Even if there is only one, there is a problem that the wasteful non-transmission time due to the back-off time becomes longer on average and the communication throughput decreases.

  On the other hand, in Patent Document 2, in a wireless communication system composed of a plurality of terminal devices each connected to a host, a carrier sense unit that detects the presence or absence of carriers emitted from other terminal devices on a communication medium. Based on the waiting time from reception of a data transmission command from the host being detected to detection of the absence of the carrier sense unit, and the priority set for each of the plurality of terminal devices, the longer the waiting time becomes, the higher the priority. It is described that the back-off time is generated so as to be shorter as the rank is higher.

  According to the wireless communication system of Patent Document 2, the back-off time of a specific terminal device is generated so as to become shorter as the standby time becomes longer and as the priority is higher, that is, a plurality of transmission requests that have occurred at the same time. Although the waiting time is the same for terminals, the priority is different, so the generated backoff time is different, so even if transmission requests occur simultaneously for multiple terminals, transmission efficiency is improved without causing transmission collisions It has the effect of being able to.

  However, in Patent Document 2, when a transmission request is generated at a plurality of terminals at the same time while a certain terminal is transmitting, a transmission collision does not occur because the back-off time is different. When a transmission request is generated at a terminal having a higher priority after the occurrence, there is a problem that the back-off time becomes the same and a transmission collision may occur. Further, Patent Document 2 does not solve the problem that the useless non-transmission time due to the back-off time becomes long even when there is one terminal that has made a transmission request.

Japanese Patent Laid-Open No. 9-46347 JP-A-11-55266

  An object of the present invention is to solve the problems based on the above-described conventional technology, and even when transmission from each terminal is started asynchronously by random access, transmission collision does not occur, and wasted due to back-off time An object of the present invention is to provide a communication system that can reduce the non-transmission time.

To achieve the above object, according to the present invention, after each terminal detects a vacant communication medium and measures a backoff time calculated based on the following equation, transmission is performed asynchronously by random access. A communication system to be started,
The back-off time = (random () × N + M) × slot time The random () is a function that generates random numbers of integer values generated from a uniform distribution in the range [0, CW], and the CW is An integer value within a predetermined range that defines a range for generating the random number, the slot time is a fixed unit time, the N is the number of offsets commonly used by all the terminals, and the M is each of the terminals A communication system, wherein the offset number N and the offset value M are integer values satisfying the relationship of 2 ≦ N, 0 ≦ M, and M <N. It is to provide.

  Here, it is preferable that the offset number N is a value greater than or equal to the number of all the terminals, and the offset value M is a different value for each of the terminals. Alternatively, the offset number N is a value equal to or greater than the number of terminals for which transmission requests are generated during transmission of a certain terminal, and the offset value M is a terminal for which transmission requests are generated at least during transmission of the certain terminal. Then, it is preferable that the value is different.

The communication system is a wireless communication system including a base station and a plurality of terminals that communicate with each other via the base station via wireless communication media,
The offset number N and the offset value M are preferably set for each of the terminals from the base station via the wireless communication medium.

  Further, the base station monitors the number of all the terminals or the number of terminals for which a transmission request is generated while the certain terminal is transmitting, and the number of all the terminals or the certain terminal is transmitting. In addition, it is preferable to dynamically change the number of offsets N and the offset value M used in each of the terminals according to the number of terminals for which transmission requests are generated.

  According to the communication system of the present invention, it is possible to prevent a transmission collision from occurring between terminals having different offset values M even when the offset number N is smaller than the total number of terminals. Further, the offset number N is set to be equal to or greater than the total number of terminals, and the offset value M is set to a different value for each terminal, or the offset number N is set to be equal to or greater than the number of terminals for which a transmission request is generated during transmission of a certain terminal. By setting M to a different value between terminals that generate a transmission request while a certain terminal is transmitting, it is possible to completely prevent a transmission collision from occurring regardless of the generation timing of the transmission request.

  Further, in the radio communication system to which the present invention is applied, the number of offsets N and the offset value M used when calculating the back-off time at each terminal are transmitted from the base station to all terminals or a certain terminal. In addition, by dynamically changing the setting according to the number of terminals for which transmission requests are generated, the average value of the back-off time is set according to the total number of terminals or the number of terminals for which transmission requests are generated while a certain terminal is transmitting. Since it can be made smaller, it is possible to prevent useless no transmission time due to the back-off time.

  Hereinafter, a communication system according to the present invention will be described in detail based on preferred embodiments shown in the accompanying drawings.

  INDUSTRIAL APPLICABILITY The present invention is applicable to a communication system in which transmission is started asynchronously by random access after measuring a predetermined back-off time after detecting a vacant communication medium (communication channel) in each terminal. It is. Hereinafter, the case where the present invention is applied to a wireless communication system will be described as an example so that the comparison with the prior art becomes easy.

  In a wireless communication system to which the present invention is applied, when a transmission request occurs in each terminal, the availability of media is monitored, and if it is available, transmission is performed after measuring the back-off time calculated based on the following formula. Asynchronously started.

Back-off time = (Random () × N + M) × Slot time Here, random () is a function that generates a random number of integer values generated from a uniform distribution in the range of [0, CW], CW (con (Tension window) is an integer value within a predetermined range (0 ≦ random number <CW) that defines a range in which random numbers are generated, and slot time is a fixed unit time (fixed value) defined by the system. N is the number of offsets commonly used in all terminals, M is an offset value used individually in each terminal, and these offset numbers N and offset values M are 2 ≦ N and 0 ≦ M. , M <N satisfying the relationship of integer.

  Random (), contention window CW and slot time are the same as in the conventional wireless communication system.

  The offset number N and the offset value M are set for each terminal from the base station via the wireless communication medium. For example, the offset number N is a beacon frame (common broadcast information) periodically transmitted from the base station to each terminal, and the offset value M is a management frame transmitted from the base station to each terminal. To be sent. In each terminal, the back-off time is calculated based on the above calculation formula using the set offset number N and offset value M, which are transmitted from the base station.

  For example, when the offset number N is set to a value equal to or greater than the total number of terminals and the offset value M is set to a different value in each terminal, the back-off time generated in each terminal is always different. In this case, transmission collision does not occur regardless of the generation timing of the transmission request.

  On the other hand, when the offset number N is set to a value smaller than the total number of terminals, there are a plurality of terminals having different offset values M and a plurality of terminals having the same offset value M. Transmission collisions do not occur between terminals having different offset values M, but transmission collisions may occur when terminals having the same offset value M have the same backoff time. However, even if terminals having the same offset value M are transmitted, a transmission collision does not occur unless a transmission request is generated at these terminals while a certain terminal is transmitting.

  Accordingly, when the offset number N is set to a value smaller than the total number of terminals, the offset number N is set to a value greater than or equal to the number of terminals for which a transmission request is generated while a certain terminal is transmitting, and the offset value M is transmitted from a certain terminal. Among these, it is preferable to set a different value at a terminal where a transmission request occurs. Even when the offset number N is set to a value smaller than the total number of terminals, there are a plurality of terminals having different offset values M, and these terminals generate different backoff times. The probability of transmission collision can be reduced as compared with the communication system.

  In a wireless communication system, the total number of terminals and the number of terminals for which transmission requests are generated while a certain terminal is transmitting dynamically change. In a wireless communication system to which the present invention is applied, the base station monitors the total number of terminals or the number of terminals for which a transmission request is generated while a certain terminal is transmitting, and is used by each terminal accordingly. The offset number N and the offset value M are dynamically changed. As a result, the average backoff time can be reduced according to the total number of terminals or the number of terminals for which a transmission request is generated while a certain terminal is transmitting, and the wasted no transmission time due to the backoff time. It can be prevented from occurring.

  Subsequently, referring to the timing chart of FIG. 1, when the total number of terminals is three terminals A, B, and C, the offset number N is 3, and the offset values M of the terminals A, B, and C are each 0. , 1 and 2 will be described as an example. Note that the operation of the terminals B and C will be described with reference to the flowchart of FIG.

  In order to facilitate comparison with the conventional communication system, a case will be described in which a transmission request is generated at terminal B and a transmission request is subsequently generated at terminal C while terminal A is transmitting. As shown in the flowchart of FIG. 4, when a transmission request occurs (S1), the terminals B and C monitor the availability of media (S2). If the media is available (Yes in S2), transmission is started. However, when the terminal A is transmitting as in this example, the terminal A waits until the transmission of the terminal A is completed and the medium becomes free (S3), and the back-off time is calculated during that time (S3) (S8). S4).

  Here, terminal B generates a backoff time calculated by (random () × 3 + 1) × slot time, and terminal C has a backoff time generated by (random () × 3 + 2) × slot time. Generated. The timing chart of FIG. 1 shows a case where the random numbers generated by random () are the same value for both terminals B and C. That is, in this example, the terminal C generates a back-off time longer than the terminal B by one slot time.

  Subsequently, in the terminals B and C, the empty media is detected again (S5). If the medium is not free (No in S5), the detection is repeated until the medium becomes free. When the transmission of the terminal A is completed and the vacant media is detected (Yes in S5), the terminals B and C subtract the back-off time after a certain time DIFS has passed (No in S6 and S7). When the value becomes 0 (Yes in S7), transmission is started (S8). In this example, as described above, since the back-off time of terminal B is shorter than the back-off time of terminal C, the back-off time of terminal B is first 0 and transmission is started.

  When the transmission from the terminal B is started, the subtraction of the back-off time is temporarily stopped in the terminal C, and the vacant media is detected again (S5). If the medium is not free (No in S5), the detection is repeated until the medium becomes free. When the transmission of the terminal B is completed and the available media is detected (Yes in S5), the terminal C resumes the subtraction of the back-off time after a certain time DIFS has elapsed (No in S6 and S7). When the value becomes 0 (Yes in S7), transmission is started (S8). As described above, in a wireless communication system to which the present invention is applied, even when a transmission request is generated at a plurality of terminals while a certain terminal is transmitting, a transmission collision occurs regardless of the generation timing of the transmission request. do not do.

  Next, the operation when the offset number N and the offset value M set for each terminal from the base station are changed will be described with reference to the conceptual diagram shown in FIG. Here, FIG. 2A shows an operation when the offset number N is increased, and FIG. 2B shows an operation when the offset number N is decreased.

  When increasing the number N of offsets, as shown in FIG. 2A, the number N of offsets of all terminals A, B, and C is changed first, and then the offset value M of each terminal A, B, C is sequentially changed. Be changed.

  That is, first, a beacon frame is transmitted from the base station to all terminals A, B, and C, and the offset number N included in this beacon frame is set in common to all terminals A, B, and C. The beacon frame is always transmitted periodically, and the offset number N is always included in the beacon frame. All terminals A, B, and C observe the beacon frame every time regardless of whether the offset number N is changed or not.

  Subsequently, a management frame requesting to change the offset value M is transmitted from the base station to the terminal A, and the offset value M included in the management frame is set in the terminal A. In response to this, a response message for confirming the change of the offset value M is returned from the terminal A to the base station. This response message confirms the completeness of the setting change of the offset value M in each of the terminals A, B, and C. Similarly, the offset value M is changed for the terminals B and C.

  On the other hand, when the number of offsets N is reduced, as shown in FIG. 2B, the offset values M of the terminals A, B, and C are sequentially changed first, and then the numbers of offsets of all the terminals A, B, and C are then changed. N is changed.

  That is, first, a management frame requesting to change the offset value M is transmitted from the base station to the terminal A, and the offset value M included in the management frame is set in the terminal A. In response to this, a response message for confirming the change of the offset value M is returned from the terminal A to the base station. Similarly, the offset value M is changed for the terminals B and C.

  Thereafter, a beacon frame is transmitted from the base station to all terminals A, B, and C, and the offset number N included in this beacon frame is set in common to all terminals A, B, and C. Similarly, the beacon frame is always transmitted periodically, and the offset number N is always included in the beacon frame. All terminals A, B, and C observe the beacon frame every time regardless of whether the offset number N is changed or not.

  As described above, the setting of the offset number N and the offset value M can be changed while satisfying the relationship of the offset number N> the offset value M.

  The present invention is not limited to a wireless communication system, and transmission is started asynchronously with random access in each terminal regardless of a wired communication system or a communication system in which a wireless communication system and a wired communication system are mixed. It can be applied to a communication system.

The present invention is basically as described above.
Although the communication system of the present invention has been described in detail above, the present invention is not limited to the above-described embodiment, and various improvements and modifications may be made without departing from the spirit of the present invention. .

5 is a timing chart of an embodiment showing the operation of a terminal in the communication system of the present invention. (A) And (b) is the conceptual diagram of one Embodiment showing operation | movement in the case of changing the number of offsets and offset value which are set to each terminal from a base station in the communication system of this invention. 6 is a timing chart illustrating an example of operation of a terminal in a conventional wireless communication system. It is an example flowchart showing operation | movement of a terminal in the conventional radio | wireless communications system.

Claims (5)

Each terminal is a communication system in which transmission is started asynchronously with random access after measuring the back-off time calculated based on the following formula after detecting the availability of communication media,
The back-off time = (random () × N + M) × slot time The random () is a function that generates random numbers of integer values generated from a uniform distribution in the range [0, CW], and the CW is An integer value within a predetermined range that defines a range for generating the random number, the slot time is a fixed unit time, the N is the number of offsets commonly used by all the terminals, and the M is each of the terminals In the communication system, the offset number N and the offset value M are integer values satisfying the relationship of 2 ≦ N, 0 ≦ M, and M <N.   The communication system according to claim 1, wherein the offset number N is a value that is equal to or greater than the number of all the terminals, and the offset value M is a value that is different for each of the terminals.   The offset number N is a value equal to or greater than the number of terminals for which a transmission request is generated during transmission of a certain terminal, and the offset value M is different at least for a terminal for which a transmission request is generated during transmission of the certain terminal. The communication system according to claim 1, wherein the communication system is a value. The communication system is a wireless communication system including a base station and a plurality of terminals that communicate with each other via the base station via wireless communication media,
The communication system according to any one of claims 1 to 3, wherein the offset number N and the offset value M are set to each of the terminals from the base station via the wireless communication medium.   The base station monitors the number of all the terminals, or the number of terminals for which a transmission request is generated while the certain terminal is transmitting, and the number of all the terminals or the certain terminal is transmitting. The communication system according to claim 4, wherein the number of offsets N and the offset value M used in each of the terminals are dynamically changed according to the number of terminals for which a transmission request is generated.

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