1.  Field of the Invention
The present invention relates to a communication device, a communication system, and a communication method, and more particularly to a communication device, a communication system, and a communication method for performing communication using signals having different frequencies. 
2.  Description of the Background Art
The xDSL (x Digital Subscriber Line) technique of performing high-speed data communication using an existing telephone line includes, for example, ADSL (Asymmetric DSL), VDSL (Very high-bit-rate DSL) and the like. 
The modulation scheme of xDSL includes DMT (Discrete Multi-Tone) modulation scheme in which communication is performed by dividing a transmission frequency bandwidth to be used into narrow bandwidths.  For example, in full-rate ADSL (8 Mbps/12 Mbps), the transmission frequency bandwidth of about 1 MHz is divided into 256 subchannels (a bandwidth of 4 kHz). 
In the multi-carrier communication system using the DMT modulation scheme, training is carried out to check the state of a line between mutually connected communication devices before data communication is started.  In this training, a signal-to-noise ratio (also referred to as SNR hereinafter) is observed for each divided subchannel, and the allocation of the number of bits to be allocated to a subcarrier that is a carrier wave of the subchannel is set according to the observation result.  Thus, the communication speed according to the line state is automatically set (best effort method).  After completion of the training, a link (connection) between the communication devices is established, and data communication is then started at the set communication speed.  Usually, a dynamic change of the communication speed is not made during data communication. 
Furthermore, in xDSL device, the number of bits is allocated to each subcarrier such that a communication signal satisfies prescribed reception quality under the condition of SNR obtained by subtracting a prescribed margin value (also referred to as SNR margin hereinafter) from the measured SNR.  In other words, the SNR margin is a margin value to prevent a transmission error.  Here, SNR differs for each subchannel, and the subchannel having a lower frequency has a larger SNR under the normal use conditions of xDSL.  The immunity of a communication channel against a transmission error can be adjusted by setting the SNR margin. 
In addition, in xDSL device, an interleaving process of transmitting transmission data in such a manner as to be dispersed in the direction of time-axis is employed in order to reduce the effect of a burst error in transmission.  In xDSL device, an encoding process using a combination of this interleaving process with FEC (Forward Error Correction) and CRC (Cyclic Redundancy Check) is performed.  Here, a parameter of the interleaving process includes an interleave depth which determines the degree of dispersion of transmission data in the time-axis.  The immunity of a communication signal against a burst error can be adjusted by setting this interleave depth.  In other words, the immunity of a communication signal against a burst error is improved with the increase in interleave depth.  On the other hand, the degree of dispersion of transmission data in the time axis is increased, so that the time until which final data in a transmission data set arrives at the receiver side increases, thereby increasing the transmission delay time.  It is noted that that an interleave depth sets at 1 means that an interleaving process is not performed. 
Here, since a general telephone line is housed in a telephone cable having lines bundled, crosstalk is caused by electromagnetic coupling between two telephone lines proximate to each other.  In view of QOS (Quality of Service), in downloading data via HTTP (Hyper Text Transfer Protocol) and FTP (File Transfer Protocol), a relatively small transmission speed is acceptable, and data resending is also permissible in the event of a transmission error.  However, in recent years, demands for IP telephones, TV phones, match games, video distribution using real-time transmission have been increasing.  For example, in video streaming distribution, bulk data is transmitted and the communication has to be stabilized with a reduced error rate without interruption of distribution.  In addition, in voice data, the transmission delay time has to be reduced. 
Here, G.992.1 recommended by International Telecommunication Union, Telecommunication Standardization Sector (ITU-T) (see “Asymmetric Digital Subscriber Line (ADSL) transceiver,” ITU-T Recommendations G.992.1 (referred to as Non-Patent Document 1 hereinafter)) defines that two logical data paths (also referred to as logic channels hereinafter) having different interleave depths can be used for one physical transmission path between two communication devices performing ADSL communication.  In such xDSL devices, the immunity of a communication signal against a transmission error can be adjusted by setting an interleave depth. 
Now, the error rate is decreased with the increased SNR margin.  However, the transmission speed is decreased since the number of bits allocated to a subcarrier is reduced. 
Here, in the communication device described in Non-Patent Document 1, the same SNR margin has to be set for all the subcarriers.  Therefore, it is necessary to set the SNR margin for subcarriers at a larger value in accordance with the logic channel requiring the smaller error rate.  Thus, the transmission speed of the other logic channel becomes smaller than necessary.  Therefore, the communication device disclosed in Non-Patent Document 1 is unable to transmit data appropriately in accordance with a data type, a purpose and the like.