Satellite communication system with variable number of satellite communication channels

A central station is connected to a plurality of remote stations through in-bound channels and out-bound channels. Data is transmitted from the respective remote stations to the central station through the in-bound channels and data is transmitted from the central station to all of the remote stations through the out-bound channels. Possibility of data transmission delay is reduced by changing the number of either or both of the in-bound channels and the out-bound channels according to a variation of amount of data transmitted/received between the remote statios and the central station.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a satellite communication system for 
performing a communication between a central earth station and a plurality 
of remote earth stations through satellite communication channels and, 
particularly, to a satellite communication system capable of performing a 
smooth communication by reducing transmission delay of data by changing 
the number of satellite communication channels correspondingly to a 
variation of amount of data transmitted through the channels. 
2. Description of the Related Art 
In general, a worldwide computer-use communication system such as an 
automatic cash handling system for banks, a stock trading system for stock 
companies and a financial credit-card authentication system includes 
networks of communication channels between a central earth station 
(referred to as "central station", hereinafter) and a plurality of remote 
earth stations (referred to as "remote stations", hereinafter) as 
infrastructure. In such system, a communication is performed between a 
plurality of terminals connected to these remote stations and a host 
computer connected to the central station. The respective remote stations 
transmit data to only the central station on in-bound channels through a 
satellite. A reply to the transmission of data from the remote stations to 
the central station is transmitted from the central station to all of the 
remote stations on out-bound channels. The respective remote stations 
extract only data designated to their terminals from the reply data from 
the central station and transfer them to their terminals. 
When a communication from the remote stations to the central station is 
performed, each remote station transmits data to the central station by 
using at least one of a plurality of time slots obtained by dividing one 
frame time. Access of the respective remote stations to the time slots is 
generally performed by the fixed access system, the random access system 
or the time slot reservation system. These three systems have merits and 
demerits in views of transmission delay of transmitted data and effective 
utilization of the satellite communication channel. A system which is a 
combination of the random access system and the time slot reservation 
system is disclosed in U.S. Pat. No. 4,736,371, assigned to the same 
assignee of the present application. 
The system disclosed in U.S. Pat. No. 4,736,371 restricts the frequency of 
occurrence of data collision by inhibiting data transmission of the random 
access system when a amount of short data transmission from the respective 
remote stations to the central station is increased. Further, this system 
can reduce transmission delay caused by re-transmission, etc., of short 
data collided in a same time slot. Therefore, this system is suitable when 
an amount of data transmitted from the respective remote stations to the 
central station varies substantially. 
Further, in this system, when the amount variation of data transmitted from 
respective remote stations substantially exceeds a predictable variation 
range of data for some accidental reason, the frequency of data collision 
may increase, resulting in a problem that a very large data transmission 
delay due to re-transmission of collided data occurs. 
An conventional satellite communication system which employs the 
combination of the above-mentioned random access system and the time slot 
reservation system is disclosed in NEC Research & Development, No. 89, 
"VSAT System (2): AA/TDMA (Adaptive Assignment TDMA) for the VSAT 
Networks", (April 1988). 
In this system, congestion of data transmission through in-bound channels 
is avoided by expanding an interval between times in which the remote 
stations send transmission signals to a central station through the 
in-bound channels. 
In this system, however, when an amount of data is so large that the number 
of in-bound channels which was set according to a data amount predicted 
initially is not enough, data transmitted from the respective remote 
stations may collide repeatedly. Therefore, in such case, there is a very 
large data transmission delay due to re-transmission of collided data. In 
this conventional satellite communication system, it is impossible to 
employ other measures than the expansion of transmission time interval of 
data from the remote stations and, consequently, there may be a case where 
data transmission from the remote stations has to be stopped. 
Further, in the same satellite communication system, the number of in-bound 
channels and the number of out-bound channels are preset by predicting the 
maximum data amount on a communication line. Since, therefore, the 
communication is performed through the preset number of in-bound channels 
and the preset number of out-bound channels even under condition that an 
amount of data under transmission is very small in such as night time, 
there are many useless channels, causing the efficiency of utilization of 
the satellite channels to be very low. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a communication system 
capable of reducing data transmission delay by changing the number of 
communication channels between a plurality of remote stations and a 
central station corresponding to a large variation of amount of data 
transmitted from the remote stations to the central station, which exceeds 
an estimated variation. 
Particularly, in a communication system in which a plurality of remote 
stations are connected to a central station through at least one in-bound 
channel and at least one out-bound channel data is transmitted from the 
respective remote stations to the central station through the in-bound 
channel and data is transmitted from the central station to the respective 
remote stations through the out-bound channel, an object of the present 
invention is to provide a satellite communication system capable of 
reducing data transmission delay and realizing a smooth communication by 
changing the number of communication circuits between the remote stations 
and the central station corresponding to a large variation of amount of 
data transmitted from the remote stations to the central station, which 
exceeds the estimated variation. 
Another object of the present invention is to provide a satellite 
communication system capable of utilizing satellite communication channels 
effectively by changing the number of the channels according to 
utilization time and/or utilization state thereof. 
A further object of the present invention is to provide a satellite 
communication system capable of changing the number of in-bound channels 
and the number of out-bound channels and setting carrier frequencies of 
these channels. 
In order to achieve the above-mentioned objects, a satellite communication 
system according to the present invention is composed of a plurality of 
remote stations and a central station connected to the respective remote 
stations through satellite channels. The remote stations transmit data to 
the central station through at least one of first channel of the satellite 
channels. The central station transmits same data to all of the remote 
stations through at least one second channel of the satellite channels. In 
an aspect of the present invention, the central station comprises a 
detector for detecting an amount of data transmitted from the remote 
stations to the central station and a changing device for changing the 
number of the first circuits according to the detected data amount. 
In another aspect of the present invention, the central station comprises a 
detector for detecting an amount of data transmitted from the remote 
stations to the central station and a changing device for changing the 
number of the second channels according to the detected data amount. 
Particularly, the central station of the satellite communication system 
according to the present invention comprises a receiver for obtaining a 
first data by demodulating signals from the remote stations and a first 
monitor for monitoring a data amount of the first data thus received. The 
central station further comprises a first circuit setting device for 
determining the number of the first channels and carrier frequencies of 
the respective channels and outputting a first control data indicative of 
an information of the channel number and carrier frequencies and a 
transmitter for transmitting the first control data to the remote 
stations. On the other hand, each remote station comprises a receiver for 
obtaining a second data by demodulating the signal transmitted from the 
central station and a separator for separating the first control data from 
the second data. Further, each remote station comprises a transmitter for 
transmitting data transmitted from at least one of first terminals 
connected to the respective remote stations to the central station and a 
first channel controller for controlling the first channel used by the 
transmitter on the basis of the first control data separated by the 
separator. 
With such construction as mentioned above, it is possible to restrict 
occurrence of transmission delay correspondingly to a variation of amount 
of the data transmitted/received through the satellite communication 
channel. 
Further, the present invention can be applied to a satellite communication 
system having a plurality of very small aperture terminals (VSAT's) and a 
central earth station (HUB) connected thereto through at least one 
in-bound channel and at least one out-bound channel. Data is transmitted 
from each of the VSAT's to the HUB through the in-bound channel and data 
is transmitted from the HUB to all of the VSAT's through the out-bound 
channels. The number of either or both of the in-bound channels and the 
out-bound channels and their carrier frequencies are set according to a 
variation of amount of data transmitted/received between the VSAT's and 
the HUB. The HUB and the VSAT's change the current number of either or 
both of the in-bound channels and the out-bound channels which are used to 
transmit/receive the data and their frequencies to the number of channels 
and their frequencies of either or both of the in-bound channel and the 
out-bound channel thus set. 
Detection of the amount of data transmitted/received between the plurality 
of VSAT's and the HUB is performed by detecting the number of collisions 
of the data transmitted from one VSAT to the HUB with the data transmitted 
from other VSAT's to the HUB within the same time slot and then detecting 
the number of data collisions occurred within a predetermined time. 
Another detection of the data amount is to detect an accumulation of length 
of data transmitted/received within a predetermined time. 
When the data amount thus detected is larger than a predetermined value, 
the number of the in-bound or out-bound channels is increased and, when 
the data amount thus detected is smaller than the predetermined value, the 
number of the in-bound or out-bound channels is reduced.

DETAILED DESCRIPTION 
A preferred embodiment of the present invention will be described with 
reference to FIGS. 1 to 6. 
The embodiment relates to a satellite communication system wherein a 
plurality of remote earth stations, that is, very small aperture terminals 
(VSAT's), are connected to a central earth station (HUB) through in-bound 
channels and an out-bound channel, each VSAT transmits data to the HUB 
through the in-bound channels and the HUB transmits data to all of the 
VSAT's through the out-bound channel. Particularly, the embodiment relates 
to a satellite communication system capable of reducing the frequency of 
occurrence of data transmission delay by changing the predetermined number 
of either one or both of the in-bound and out-bound channels according to 
a variation of amount of data transmitted/received between the VSAT's and 
HUB. 
Referring to FIG. 1, the satellite communication system of the present 
invention includes a single HUB 300 and a plurality of VSAT's 400-1 to 
400-12 connected to the HUB 300 through a satellite communication 
channels, that is, in-bound channels 501 and an out-bound channel 502, 
using a communication satellite 500 as a transponder. The respective 
VAST's 400-1 to 400-12 transmit data to only the HUB 300 in time division 
multiple access (TDMA) through the in-bound channels having carrier 
frequencies f1, f2 and f3. The HUB 300 transmits an identical data to all 
of the VSAT's 400-1 to 400-12 through the out-bound channel 502 having 
carrier frequency F1. Terminals 401-1 to 401-12 are connected to the 
respective VSAT's 400-1 to 400-12. The terminals 401-1 to 401-12 generate 
data to be transmitted in a time slot through the in-bound channels by the 
time slot reservation system or the random access system according to data 
length of the data to be transmitted. On the basis of the data length of 
the data generated by the terminals 401-1 to 401-12, the access system for 
sending the transmission data is determined within each of the VSAT's 
400-1 to 400-12. Further, depending upon utilization state of the 
satellite communication system, it is possible to connect terminals which 
generate data to be transmitted within time slots of the in-bound channels 
by the fixed assignment access system to the respective VSAT's 400-1 to 
400-12. 
Referring to FIG. 2A which shows a construction of the HUB 300, data 
transmitted from each of the VSAT's 400-1 to 400-12 is input to a receiver 
303 of the HUB 300 through an antenna 301 and demodulated thereby. A 
receiving data identifying section 304 determines the VSAT which transmits 
the data demodulated by the receiver 303. When the receiver 303 of the HUB 
300 receives the data from any of the VSAT's 400-1 to 400-12 normally, the 
HUB 300 sends a positive acknowledgement (ACK) signal to the VSAT through 
a transmitter 302 and, when the receiver 303 of the HUB 300 can not 
receive the data from any of the VSAT's 400-1 to 400-12, the HUB 300 sends 
a negative acknowledgement (NAK) signal to the VSAT through the 
transmitter 302. A receiving data monitor 305 of the HUB 300 detects the 
number of data collisions occurred in each in-bound channel having a 
specific frequency, on the basis of the identification supplied from the 
receiving data identifying section 304 and NAK signal transmission 
requests output from the receiver 303. The receiving data monitor 305 
supplies the number of collisions to an in-bound channel controller 306 as 
a data amount of the receiving data. The in-bound channel controller 306 
determines generation of traffic congestion in every in-bound channel on 
the basis of the data amount supplied from the receiving data monitor 305. 
When traffic congestion occurs in the in-bound channel, the in-bound 
channel controller 306 sets the number of in-bound channels necessary to 
avoid the congestion. 
Now, the operations of the receiving data monitor 305 and the in-bound 
channel controller 306 will be described in detail with reference to FIG. 
2A together with FIG. 2B which shows details of the receiving data monitor 
305 and the in-bound channel controller 306. 
The receiving data monitor 305 includes a counter 601 and a timer 602. The 
NAK signal transmission requests from the receiver 303 are counted by the 
counter 601. That is, the counter 601 counts the number of NAK signals to 
be transmitted from the HUB 300 to the VSATs 400-1 to 400-12. The timer 
602 outputs a reset signal to the counter 601 every predetermined time, 
for example, every 5 seconds. The counter 601 supplies the number of the 
NAK signals counted within the predetermined time to the in-bound channel 
controller 306 every reception of the reset signal and resets the count. 
As mentioned, the receiving data monitor 305 detects the number of the NAK 
signal transmission requests generated by the receiver 303 within a preset 
time, that is, the number of data re-transmission from the VSAT's 400-1 to 
400-12 within the constant time as the amount of data transmitted from the 
VSAT's 400-1 to 400-12 to the HUB 300. 
The in-bound channel controller 306 includes 603, an ROM 604, a comparator 
605, an RAM 606 and a setting section 607. The read circuit 603 converts 
the data amount transmitted from the receiving data monitor 305 into a 
predetermined address information and reads data stored in that address of 
the ROM 604. The ROM 604 stores, in every predetermined address thereof, a 
data indicative of the number of in-bound channels which is optimum when a 
communication is performed with a data amount corresponding to that 
address as shown in FIG. 2C. Incidentally, the optimum number of in-bound 
channels corresponding to the data amount is empirically obtained. In this 
manner, the read circuit 603 reads the optimum number of in-bound channels 
corresponding to the amount of data transmitted from the receiving data 
monitor 305. The optimum number of in-bound channels thus read out from 
the ROM 604 by the read circuit 603 is supplied to the comparator 605. The 
comparator 605 compares the current number of in-bound channels stored in 
the RAM 606 with the optimum number of in-bound channels which is supplied 
from the read circuit 603. When these are different from each other, it is 
judged that the current number of in-bound channels is inadequate. On the 
other hand, these numbers are the same, it is judged that the current 
number of in-bound channels is adequate. The setting section 607 sets the 
number of in-bound channels according to the comparison result of the 
comparator 605. That is, the setting section 607 changes the current 
number of in-bound channels to the optimum number of in-bound channels 
supplied from the read circuit 603 when the comparator 605 decides that 
the current number of in-bound channels is inadequate. When the comparator 
605 decides that the current number of in-bound channels is adequate, the 
setting section 607 keeps the current number of in-bound channels 
unchanged. Furthermore, the setting section 607 stores the number of 
in-bound channels in the RAM 606, when the number of in-bound channels is 
changed. The in-bound channel controller 306 supplies the number of 
in-bound channels thus set by the setting section 607 to an in-bound 
channel frequency controller 307. 
Returning to FIG. 2A, the in-bound channel frequency controller 307 sets 
frequencies of in-bound channels to be added, according to the number of 
in-bound channels informed by the in-bound channel controller 306. 
Further, the in-bound channel frequency controller 307 determines 
frequencies of the in-bound channels to be used by the respective VSAT's 
400-1 to 400-12 and outputs to a multiplexing section 314 a first control 
data containing an information as to in-bound channel frequencies to be 
used by the respective VSAT's 400-1 to 400-12. 
When an interface circuit (INTFC) 308 receives data transmitted from the 
respective VSAT's 400-1 to 400-12, the INTFC 308 outputs the data which is 
demodulated by the receiver 303 to a terminal 315. Further, when data is 
to be transmitted from the terminal 315 to all of the VSAT's 400-1 to 
400-12, the INTFC 308 outputs the data from the terminal 315 to a 
multiplexing section 314 and to a transmission data identifying section 
309. The transmission data identifying section 309 identifies at least one 
VSAT's 400-1 to 400-12 to which the data is to be transmitted. A 
transmission data monitor 310 monitors an amount of the data. That is, the 
transmission data monitor 310 detects a total length of data generated in 
the terminal 315 within a unit time as the amount of data. An out-bound 
channel controller 311 judges whether or not the current number of 
out-bound channels is adequate for the amount of the transmission data 
from the terminal 315 which is detected by the transmission data monitor 
310. When it is judged by the transmission data monitor 310 that the 
current number of out-bound channels is inadequate, the out-bound channel 
controller 311 determines the number of out-bound channels necessary for 
avoidance of traffic congestion in the out-bound channels and notifies the 
number to an out-bound channel frequency controller 312. The out-bound 
channel controller.311 may be realized by using a similar circuit 
construction to that of the in-bound channel controller 306 shown in FIG. 
2B. 
The out-bound channel frequency controller 312 sets frequencies of the 
out-bound channels to be added, according to the number of out-bound 
channels notified by the out-bound channel controller 311. Further, the 
out-bound channel frequency controller 312 determines frequencies of the 
out-bound channels to be used by the respective VSAT's 400-1 to 400-12 and 
outputs to the multiplexing section 314 a second control data containing 
an information as to in-bound channel frequencies to be used by the 
respective VSAT's 400-1 to 400-12. 
The multiplexing section 314 multiplexes the transmission data from the 
INTFC 308, the first control data from the in-bound channel frequency 
controller 307, the second control data from the out-bound channel 
frequency controller 312 and a frame timing signal supplied from a frame 
timing signal generator 313 and sends a resultant multiplexed signal to 
the transmitter 302. A format of the multiplexed signal is constructed 
with, for example, 9 blocks 700A to 700I as shown in FIG. 2D. A UNIQUE 
WORD block 700A is a reference indicating a position of a time slot of the 
in-bound channel. A block 700B is a frame timing signal portion. A first 
congestion judge block 700C is filled with "1" or "0" to indicate a 
traffic condition of the circuit judged by the in-bound channel controller 
306. That is, when the current number of in-bound channels is inadequate 
for the detected amount of data transmitted from the respective VSAT's 
400-1 to 400-12 to the HUB 300, which is the number of NAK signal 
transmission requests and hence the number of data collisions, the first 
congestion judge block 700C is filled with "1". When there is no need of 
changing the current number of in-bound channels, it is filled with "0". A 
data indicative of the number of in-bound channels set by the in-bound 
channel controller 306 is put in an in-bound channel number transmission 
block 700D. The first control data which indicates carrier frequencies of 
the in-bound channels assigned by the in-bound channel frequency 
controller 307 to the respective VSAT's 400-1 to 400-12 is put in an 
in-bound frequency transmission block 700E. A second congestion judge 
block 700F is filled with "1" when a congestion is detected by the 
out-bound channel controller 311 and otherwise with "0". A data indicative 
of the number of out-bound channels set by the out-bound channel 
controller 311 is put in an out-bound channel number transmission block 
700G. The second control data which indicates frequencies of the out-bound 
channels assigned by the out-bound channel frequency controller 312 to the 
respective VSAT's 400-1 to 400-12 is put in an out-bound frequency 
transmission block 700H. A block 700I is for a user data. 
Returning to FIG. 2A, the transmitter 302 modulates the multiplexed signal 
from the multiplexing section 314 and transmits it to all of the VSAT's 
400-1 to 400-12 on the out-bound channels through the antenna 301. 
Although, in this case, the receiving data monitor 305 detects the amount 
of data transmitted from the VSAT through the in-bound channel by 
monitoring the number of occurrence of NAK signal transmission requests 
supplied from the receiver 303, it is possible to monitor the length of 
data identified by the receiving data identifying section 304 similarly to 
the processing in the transmission data monitor 310. 
FIG. 3 shows a construction of the VSAT 400 of the satellite communication 
system of the present invention. The VSAT 400 includes a terminal 401 
connected thereto through an interface circuit (INTFC) 409. The terminal 
401 generates data to be transmitted in the time slot of the in-bound 
channel by the time slot reservation system or the random access system 
according to the length of data to be transmitted. Although not shown, it 
is possible to connect a terminal for generating data to be transmitted in 
the time slot of the in-bound channel by the fixed assignment access 
system to the VSAT 400. 
A receiver 405 extracts data from the out-bound signal supplied from the 
HUB 300 through an antenna 402, an out door unit (ODU) 403 and a 
multiplexer (MPX) 404 and demodulates the data. A separator 406 separates, 
from the demodulated data, a communication data from the terminal 315 of 
the HUB 300, the first control data from the in-bound channel frequency 
controller 307 of the HUB 300 and the second control data from the 
out-bound channel frequency controller 312 of the HUB 300. 
The INTFC 409 supplies the communication data extracted by the separator 
406 to the terminal 401. The data sent from the terminal 401 through the 
INTFC 409 is supplied to a data length judge section 410. The data length 
judge section 410 detects the length of data sent from the terminal 401 
and judges, on the basis of the detected data length, whether the data 
from the terminal 401 is to be transmitted in the time slot of the 
in-bound channel by the time slot reservation system or the random access 
system. The multiplexing section 412 multiplexes a frame timing signal 
output from a frame timing signal generator 411 and the data sent from the 
terminal 401 in a reserved time slot or an arbitrary time slot on the 
basis of the result of judgement in the data length judge section 410 and 
supplies a resultant multiplexed signal to a transmitter 413. 
An in-bound frequency controller 407 transmits the frequency of the 
in-bound circuit assigned by the HUB 300 to the transmitter 413 according 
to the first control data from the separator 406. An out-bound circuit 
frequency controller 408 transmits the frequency of the out-bound circuit 
assigned by the HUB 300 to the receiver 405 according to the second 
control data from the separator 406. 
The transmitter 413 modulates the multiplexed signal from the multiplexing 
section 412 according to a predetermined modulation system and sets the 
in-bound frequency to the frequency supplied from the in-bound channel 
frequency controller 407. The transmitter 413 transmits the modulated 
multiplexed signal on the in-bound channel having newly set frequency 
through the MPX 404, the ODU 403 and the antenna 402 to the HUB 300 as the 
in-bound signal. 
The receiver 405 sets the out-bound channel frequency to the frequency 
supplied from the out-bound frequency controller 408 and receives the 
out-bound signal having the newly set frequency among out-bound signals 
received through the antenna 402, the out door unit (ODU) 403 and the 
multiplexer (MPX) 404. 
The MPX 404 selectively outputs the out-bound signal from the ODU 403 to 
the receiver 405 and selectively outputs the signal from the transmitter 
413 to the ODU 404. 
Now, the operation of this embodiment will be described with reference to 
the drawings. 
First, the operation in a case where the traffic of the satellite 
communication channels is normal will be described. 
Referring to FIGS. 1 and 4A, the VSAT's 400-1 to 400-4 which belong to a 
group G1A transmit data in time division multiple access (TDMA) to only 
the HUB 300 through an in-bound channel 501 having frequency f1. The 
VSAT's 400-5 to 400-8 which belong to a group G2A transmit data in TDMA to 
only the HUB 300 through an in-bound channel 501 having frequency f2 and 
the VSAT's 400-9 to 400-12 which belong to a group G3A transmit data in 
TDMA to only the HUB 300 through an in-bound channel 501 having frequency 
f3. On the other hand, HUB 300 transmits an identical data to all of the 
VSAT's 400-1 to 400-12 through the out-bound channel 502 having frequency 
of F1. It is assumed here that the VSAT 400-1 transmits data 1, data 2 and 
data 3 to the HUB 300 through the in-bound channel 501 having frequency f1 
sequentially. In FIG. 4A, it is assumed that the HUB 300 can receive the 
data 1 and data 2 normally and can not receive the data 3 normally due to 
collision with other data from other VSAT's. In such case, the HUB 300 
sends ACK signals for the respective data 1 and 2 back to the VSAT 400-1 
and sends an NAK signal for the data 3 to the VSAT 400-1. In response to 
the NAK signal from the HUB 300, the VSAT 400-1 re-transmits the data 3 to 
the HUB 300. The HUB 300 transmits an ACK signal to the VSAT 400-1 when it 
receives the re-transmitted data 3 normally. In the HUB 300, the number of 
NAK signals generated within a constant time is detected (S101) and, when 
the detected number of NAK signals is small, it is decided that the data 
transmission reception can be performed smoothly with the current number 
of in-bound channels (S102). That is, since the number of data collisions 
is small, the HUB 300 decides that there is no congestion in the in-bound 
channel having frequency f1. 
Now, the operation of this embodiment when there is the congestion in the 
in-bound channel will be described. 
Referring to FIG. 4B, the VSAT 400-1 transmits data 4 to the HUB 300. 
Assuming that the transmitted data 4 collides with data transmitted from 
other VSAT in the same time slot, the HUB 300 can not receive the data 4 
normally and so it transmits an NAK signal to the VSAT 400-1. In response 
to the NAK signal, the VSAT 400-1 re-transmits the data 4 to the HUB 300. 
Assuming that the re-transmitted data 4 collides again with data 
transmitted from other VSAT in the same time slot, the HUB 300 can not 
receive the re-transmitted data 4 normally and transmits an NAK signal to 
the VSAT 400-1 again. This procedure shall be repeated until the HUB 300 
can receive the data 4 normally, causing large communication delay to 
occur. 
According to the present invention, the HUB 300 detects the number of NAK 
signals generated within a constant time (S201). Since, in this case, the 
number of NAK signals is large, the HUB 300 decides that the current 
number of in-bound channels is not enough for a smooth data communication 
(S202). That is, the HUB 300 decides that the congestion occurs in the 
in-bound channel having frequency f1. Then, the HUB 300 determines the 
total number of in-bound channels necessary to avoid the congestion 
(S203). Then, the HUB 300 determines frequencies of the in-bound channels 
to be added. In this example, the number of in-bound channels to be added 
is assumed as one and thus the frequency of the added in-bound channel is 
determined as f4. Thus, the HUB 300 assigns the in-bound channels having 
frequencies f1 to f4 as to be used by the respective VSAT's (S204). The 
HUB 300 notifies the newly determined number of in-bound channels and a 
frequency information of the in-bound channels to be used by the 
respective VSAT's to all of the VSAT's 400-1 to 400-12 through the 
out-bound channel having frequency F1 (S205). Further, the HUB 300 changes 
its characteristics so that it can also receive a signal transmitted 
through the in-bound channel having the newly set frequency f4 (S206). 
All of the VSAT's 400-1 to 400-12 change the transmission frequencies to be 
used thereby to new frequencies, respectively, according to the notice 
from the HUB 300 (S301). 
Incidentally, when the HUB 300 detects a congestion in the out-bound 
channel, the HUB 300 also notifies a newly set number of out-bound 
channels and a frequency information of the out-bound channels to be used 
by the respective VSAT's 400-1 to 400-12. In response to the notice, the 
VSAT's 400-1 to 400-12 change receiving frequencies to be used thereby. 
In an example shown in FIG. 5, in order to avoid the congestion of the 
satellite communication channel, the in-bound channel 501 having frequency 
f4 and the out-bound channel 502 having frequency F2 are newly added. That 
is, the VSATs 400-1 to 400-3 which belong to the group G1B use the 
in-bound channel 501 having frequency f1 and the out-bound channel 502 
having frequency F1 and the VSAT's 400-4 to 400-6 which belong to the 
group G2B use the in-bound channel 501 having frequency f2 and the 
out-bound channel 502 having frequency F1. Further, the VSAT's 400-7 to 
400-9 which belong to the group G3B use the in-bound channel 501 having 
frequency f3 and the out-bound channel 502 having frequency F2 and the 
VSAT's 400-10 to 400-12 which belong to the group G4B use the in-bound 
channel 501 having frequency f4 and the out-bound channel 502 having 
frequency F2. 
Thus, the number of VSAT's which receive the out-bound signals by using one 
out-bound channel is reduced to 1/2 and the number of the VSAT's which 
transmit data to the HUB by using one in-bound channel is reduced to 3/4. 
Therefore, the possibility of congestion can be substantially reduced. 
In a case where the amount of communication data is substantially reduced 
in such as night time, the number of in-bound channels 501 is reduced to 2 
channels such as shown in FIG. 6. This can be realized by changing the 
system such that the number of in-bound channels is changed when the HUB 
300 transmits no NAK signal to the VSAT's 400-1 to 400-12 within a 
predetermined time, say, 10 minutes. That is, the system is changed such 
that the VSAT's 400-1 to 400-6 are grouped as a group G1C and use the 
in-bound circuit 501 having frequency f1 and the out-bound channel 502 
having frequency F1 and the VSAT's 400-7 to 400-12 are grouped as a group 
G2C and use the in-bound channel 501 having frequency f2 and the out-bound 
channel 502 having frequency F1. With this scheme, it is possible to 
effectively utilize the frequency band of the satellite channel and power 
of the satellite communication in conformity with utilization time and 
utilization state of the satellite channel. 
As described, even when the amount of data from the terminals connected to 
the respective VSAT's is changed substantially beyond predictable range, 
the satellite communication system of the present invention can respond 
thereto by changing the number of out-bound channels and in-bound channels 
which are used by the respective VSAT's. Therefore, the present satellite 
communication can respond to a variation of data amount flexibly.