next , a preferred embodiment of a network system , a communication device , and a communication routing method according to the present invention will be described in detail with reference to the accompanying drawings . a connection between a server 20 and a client 10 is established in the same manner as in a conventional connection ( fig1 ). connections among a telephone line , a satellite line , and the client 10 are established in the same manner as in conventional connections ( fig1 ). as shown in fig1 , the client 10 comprises a modem connecting section 16 to which a modem 32 is connected , a receiver connecting section 18 to which a satellite communication receiver 36 is connected . the modem connecting section 16 and the receiver connecting section 18 are connected to a mpu ( microprocessor unit ) 14 . mpu 14 requests the server 20 via the modem 32 for data transfer over a telephone line . mpu 14 specifies either one of the telephone line and the satellite line at the time of requesting for data transfer and receives data from the server 20 over the specified communication line . when data transfer is requested , the specified communication line is indicated by used communication line information stored in a memory 12 . mpu 14 can receive data transferred from the server 20 over a telephone line and data transferred over a satellite line . the received data is stored in the memory 12 . mpu 14 determines the speed of data transfer of a telephone line and of a satellite line to select one from the telephone line and the satellite line on the basis of the determined speed of data transfer . information of the selected communication line ( hereinafter referred to as used communication line information ) is stored in the memory 12 . a data transfer rate is judged by estimating total time taken to transfer required data . mpu 14 determines the total time taken to transfer the required data over the communication line , which is presently used for data transfer and also requests the server 20 to transfer the required data over the other communication line , which is not presently used for data transfer , to determine the total transfer time of the required data over the other communication line . the total transfer time is determined by adding transfer latency which is a time lag between the sending of a request to the server 20 for data transfer and the time the required data begins to be received by the client 10 . the transfer time taken to transfer the whole of the required data is determined by the transfer rate and the volume or size of the required data . the transfer rate is determined by the data transfer volume received within a given measurement time after the beginning of reception of the required data . mpu 14 compares a telephone line with a satellite line in data transfer rate to select a faster communication line . thus , the communication line whose total transfer time is shorter is selected . if the data transfer rate of the communication line which is not in use is faster , the communication line for data transfer is switched to the communication line which is not in use . more particularly , the specification of the communication line is switched by updating the used communication line information stored in the memory 12 . mpu 14 determines the data transfer rate of each communication line periodically at a predetermined time interval . next , a description will be given to the operation of selection of a communication line using such a network system , a communication device , and a communication routing method . it is assumed that the initial - value of the used communication line information is set to a “ telephone line .” when the used communication line information is a “ telephone line ”, as shown in fig2 ( a ), a request for data transfer over a telephone line is sent from the client 10 to the server 20 to transfer data over the telephone line . fig3 ( b ) shows an example of a data transfer procedure . the client 10 obtains the volume or size of data to be downloaded ( s 100 ), and requests the server 20 to transfer the data , whose volume has been confirmed , using a telephone line ( s 110 ), and then the server 20 transfers the data ( s 102 ). thereafter , data transfer , if any , is repeated in the same way until the connection is terminated ( s 104 ). a data transfer rate is determined for each communication line periodically at a predetermined time interval in the present invention . based on the determined data transfer rate , the communication line whose data transfer is faster is selected . fig3 ( a ) shows an example of a selection procedure of a communication line on the basis of the data transfer rate . the client 10 obtains the volume or size of data to be downloaded ( s 100 ), and requests the server 20 to transfer the data , whose volume has been confirmed , over a telephone line ( s 110 ), and then requests the server 20 to transfer identical data over a satellite line ( s 112 ). it is possible to give an instruction for partial data transfer in data transfer over a satellite line ( s 112 ). for example , as shown in fig6 ( a ), it is possible to request to transfer a head packet ( p 1 ) only because internet data is transferred in packets ( p 1 , p 2 , . . . , p 12 ) based on a communication protocol between the server 20 and the client 10 . mpu 14 calculates the total time taken to transfer the requested data for each communication line ( s 114 ). the total transfer time can be determined , for example , by the procedure shown in fig4 . measure transfer latency which is a time lag between the sending of a request to the server 20 for data transfer and the time the required data begins to be received by the client 10 ( s 122 ). determine data transfer rate from the data transfer volume received within a predetermined measurement time after the beginning of reception of the data ( s 124 ). that is , the following equation is calculated by mpu 14 : determine data transfer time from the data transfer rate and the data volume ( s 126 ). accordingly , the following equation is calculated by mpu 14 : total transfer time is determined by transfer latency and transfer time ( s 128 ). that is , the following equation is calculated by mpu 14 : on calculating transfer time for each communication line ( s 114 ), compare a telephone line with a satellite line in total transfer time ( s 116 ). when the total transfer time of the telephone line is shorter , keep using the telephone line ( s 118 ). when the total transfer time of a satellite line is shorter , update the used communication line information stored in the memory 12 to switch the line used for data transfer from the telephone line , to the satellite line ( s 120 ). fig5 shows an example of a switching procedure of the communication line . request the server 20 to transfer data in transit over a telephone line , using a satellite line ( s 130 ) after updating the used communication line information to “ a satellite line ” ( s 138 ). perform parallel data transfer over a telephone line and a satellite line ( s 132 ). since the transfer rate of the satellite line is faster , cancel the data transfer over the telephone line ( s 136 ) when the total data transfer volume of the satellite line dls exceeds the total data transfer volume dlt of the telephone line ( s 134 ). to cancel data transfer , a transfer cancellation request is sent from mpu 14 to the server 20 . as shown in fig6 ( b ), although a cancellation of data transfer over the telephone line can be made at the point that the data transfer over the satellite line catches up with data transfer over the telephone line , as shown in fig6 ( c ), the data transfer can also be canceled at the point of starting the data transfer over the satellite line . in fig6 ( c ), data is transferred over a satellite line from a packet p 7 , which is a packet in the process of being transferred over the telephone line at the time that transfer is requested . after the data transfer over the telephone line is canceled , as shown in fig2 ( b ), the data transfer is switched to the data transfer over a satellite line . as described above , switching from a telephone line to a satellite line has been described so far , the switching from the satellite line to the telephone line can be done , as well . if the data transfer rate of a satellite line is slower than that of a telephone line , data transfer over the satellite line ( fig2 ( b )) is switched to data transfer over the telephone line ( fig2 ( a )). because of a little variation of communication speed in a telephone line , it is also possible to store the data transfer rate over a telephone line determined at the time of switching from the telephone line to the satellite line in the memory 12 , and after that , this stored data transfer rate may be used as the data transfer rate of the telephone line . when using a measured value stored in the memory 12 , further measurement of the data transfer rate of the telephone line may be omitted . as shown in fig7 ( a ), the utilization factor of the satellite line is low when only the satellite line is used for data transmission regardless of its data transfer rate and data volume . on the other hand , when the communication line in data transfer is switched between the satellite line and telephone line according to the present invention such that data of transfer volume equal to or less than a maximum data transfer volume over the telephone line is transmitted over the telephone line and data of transfer volume more than the maximum data transfer volume over the telephone line is transmitted over the satellite line , as shown in fig7 ( b ), the utilization factor of the satellite line is improved . in fig7 ( a ) and 7 ( b ), data is transmitted to four users , user a , b , c and d by time division multiplexing . as mentioned above , one embodiment according to the present invention has been described so far , but the present invention is not limited to this embodiment . for example , as shown in fig8 ( a ), a modem board 42 and a satellite communication board 44 may be built into a client 40 . as shown in fig8 ( b ), the satellite communication receiver 36 may be connected to a parabolic antenna 38 via a satellite broadcasting receiver 46 . communication routing may be performed by a communication device connected to the client 10 . for example , as shown in fig9 , a communication device 50 comprises a modem unit 52 to which a telephone line is connected , a satellite communication receiving unit 54 to which a parabolic antenna 38 is connected , mpu 60 to which the modem unit 52 , the satellite communication receiving unit 54 and a connecting unit 58 are connected , and a memory 56 connected to mpu 60 . communication routing is performed in the same manner as in the above - mentioned embodiment using mpu 60 within the communication device 50 . a maximum data transfer rate depends on the performance of its modem and terminal adapter , so that the maximum data transfer rate of this modem or terminal adapter may be used as a data transfer rate of the telephone line . switching from a satellite line to a telephone line may be done in the case that monitored data transfer rate of the satellite line drops to the maximum data transfer rate of the telephone line or lower . the communication line may be switched by only updating the used communication line information ( s 138 ) instead of following all procedure shown in fig5 . the communication line may be switched not only based on its data transfer rate , but also based on , for example , its utility rate . when the line usage charge of a two - way communication line and a one - way communication line is proportional to hours of service , the line usage charge in the case of using each communication line is determined , whereby a communication line to be used for data transfer is selected . the two - way communication line is not limited to a telephone line , so that a line which is capable of carrying out arbitrary two - way communication can be used . the one - way communication line is not limited to a satellite line , accordingly , a line which is capable of carrying out arbitrary one - way communication can be used . as described above , specific embodiments of the present invention have been described so far , but the present invention is not limited to these embodiments . also , any modification , variations or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention . there have thus been shown and described a network system , a communication device , and a communication routing method which fulfill all the objects and advantages sought therefor . many changes , modifications , variations and other uses and applications of the subject invention will , however , become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof . all such changes , modifications , variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention , which is to be limited only by the claims which follow .