Information transmitting method and information transmitting system

1. An information transmitting method for transmitting information from a master station through a plurality of base stations to a plurality of slave stations, where the plurality of base stations transmit the information to a plurality of areas, respectively, with each area including a subset of the plurality of slave stations, the method comprising, for each respective area:

determining, on a basis of an packet error rate of the respective area, a number of times of multicast transmission required to achieve delivery where: an expected value of a number of the subset of the plurality of slave stations, that would be expected to transmit a retransmission request, is equal to or smaller than a predetermined retransmission-request value, or an expected value of a number of retransmitted packets responding to the retransmission request is equal to or smaller than a predetermined retransmitted-packets value, or a probability that the subset of the plurality of slave stations all receive all packets is equal to or larger than a predetermined receipt probability;

notifying a determined number of times of multicast transmission as determined by the determining, to the subset of the plurality of slave stations in the respective area; and

transmitting, via multicast, to the subset of the plurality of slave stations in the respective area, same information formed of plural packets, for the determined number of times of multicast transmission.

2. The information transmitting method according to claim 1 , wherein,

in the determining, the number of times of multicast transmission is determined such that the expected value of the number of slave stations that transmit retransmission requests, is equal to or smaller than the predetermined retransmission-request value which is a result of multiplying a predetermined ratio by a total number of slave stations managed in the respective area.

3. The information transmitting method according to claim 2 , wherein,

in the determining, a minimum integer of a value x obtained in accordance with Expression below is set as the number of times of multicast transmission:

x ≧log P (1− N √{square root over (1 −C 1 )})  Expression 1

where x is the number of times of multicast transmission, P is a packet error rate, N is a number of packets of transmitted information, and C 1 is a coefficient set in advance.

4. The information transmitting method according to claim 1 , wherein,

in the determining, the number of times of multicast transmission is determined such that the expected value of the number of retransmitted packets responding to a retransmission request from one slave station, is equal to or smaller than the predetermined retransmitted-packets value which is a result of multiplying a predetermined ratio by the number of packets of transmitted information.

5. The information transmitting method according to claim 4 , wherein,

in the determining, a minimum integer of a value x obtained in accordance with Expression below is set as the number of times of multicast transmission:

x≧log P C 2   Expression 2

where x is the number of times of multicast transmission, P is a packet error rate, and C 2 is a coefficient set in advance.

6. The information transmitting method according to claim 1 , wherein,

in the determining, the number of times of multicast transmission is determined such that the probability that the subset of the plurality of slave stations all receive all packets of information to be received, is equal to or larger than the predetermined receipt probability set in advance.

7. The information transmitting method according to claim 6 , wherein,

in the determining, the number of times of multicast transmission is determined in accordance with Expression below:

x =log P (1− NM √{square root over ( C 3 )})  Expression 3

where x is the number of times of multicast transmission, P is a packet error rate, N is a number of packets of transmitted information, M is a number of slave stations, and C 3 is a target probability value set in advance.

8. A master station for transmitting information from the master station through a plurality of base stations to a plurality of slave stations, where the plurality of base stations transmit the information to a plurality of areas, respectively, with each area including a subset of the plurality of slave stations, comprising:

a transmitting unit for transmitting information to slave stations;

a receiving unit for receiving information from the slave stations; and

a control unit that controls information transmission to the slave stations,

wherein the control unit is configured to effect operations to:

determine, on a basis of an packet error rate of the respective area, a number of times of multicast transmission required to achieve delivery where: an expected value of a number of the subset of the plurality of slave stations, that would be expected to transmit a retransmission request, is equal to or smaller than a predetermined retransmission-request value, or an expected value of a number of retransmitted packets responding to the retransmission request is equal to or smaller than a predetermined retransmitted-packets value, or a probability that the subset of the plurality of slave stations all receive all packets is equal to or larger than a predetermined receipt probability;

notify a determined number of times of multicast transmission as determined by the determine operation, to the subset of the plurality of slave stations in the respective area; and

effect transmission of, via multicast, to the subset of the plurality of slave stations in the respective area, same information formed of plural packets, for the determined number of times of multicast transmission.

9. An information transmitting method for transmitting information from a master station through a plurality of base stations to a plurality of slave stations, where the plurality of base stations transmit the information to a plurality of areas, respectively, with each area including a subset of the plurality of slave stations, the method comprising, for each respective area:

determining, on a basis of an packet error rate of the respective area, an expected number of times of multicast transmission required to achieve delivery where: an expected value of a number of the subset of the plurality of slave stations, that would be expected to transmit a retransmission request, is equal to or smaller than a predetermined retransmission-request value, or an expected value of a number of retransmitted packets responding to the retransmission request is equal to or smaller than a predetermined retransmitted-packets value, or a probability that the subset of the plurality of slave stations all receive all packets is equal to or larger than a predetermined receipt probability;

notifying the expected number of times of multicast transmission as determined by the determining, to the subset of the plurality of slave stations in the respective area; and

transmitting, via multicast, to the subset of the plurality of slave stations in the respective area, same information formed of plural packets, for the expected number of times of multicast transmission.