Abstract:
A communication system is composed of first and second communication unit. The first communication unit sends transmission data, a control sequence, and an extended control sequence. The control sequence is originally defined by a protocol and includes an abort sequence requesting for abortion of the transmission data. The extended control sequence is undefined by the protocol and is provided for requesting an extended operation. The second communication unit receives the transmission data, the control sequence, and the extended control sequence. The second communication unit executes the extended operation in response to the extended control sequence. The extended control sequence includes the abort sequence.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a communication system and a communication method. More particularly, the present invention relates to a communication system for and a communication method of carrying out a communication in accordance with an extended protocol standard in which a certain protocol standard is extended.  
           [0003]    2. Description of the Related Art  
           [0004]    A packet transmission is widely used in data communication systems.  
           [0005]    PPP (Point-to-Point Protocol) provides a standard method for transporting packets. In PPP, a packet is usually mapped into a frame, and PPP defines a format of the frame. PPP specifies that the transmitter inserts a control code into a frame for requesting a receiver to execute a function. The control code is referred to as a control sequence. The receiver refers to the control sequence and then executes the specified function.  
           [0006]    An abort sequence is one of the control sequences. The transmitter inserts an abort sequence into a frame to request the receiver to abort the frame. The receiver aborts the frame when detecting the abort sequence.  
           [0007]    [0007]FIG. 1 shows a frame format of a packet transmission system based on PPP. Each of packets  101  is provided with an FCS (Frame Check Sequence)  102 . The FCS holds a CRC (Cyclic Redundancy Check) code of the packet  101 , which is used for an error correction.  
           [0008]    Flag sequences  103  are filled between the packets  101 . The flag sequences  103  is constituted by one byte, and has a value “7Eh” of in the hexadecimal notation. Here, “h” is a symbol indicative of a representation based on the hexadecimal number. The flag sequence  103  indicates the separation position between the two packets  101 .  
           [0009]    An abort sequence  104  is added to a packet  101   a  when the transmitter requests the receiver to abort the packet  101   a.  The receiver aborts the packet  101   a  when detecting the abort sequence  104 .  
           [0010]    PPP defines other control sequences besides the abort sequence.  
           [0011]    However, a function undefined by the PPP is often desired to be executed in the packet transmission system. Therefore, extended control sequences are often defined for executing the undefined function. Hereinafter, a protocol defining a control sequence undefined by the PPP in addition to the control sequences defined by the PPP is referred to as an extended PPP.  
           [0012]    For example, a control sequence for a function of transiently stopping a frame transmission is not defined by PPP. The transiently stopping is useful in adjusting a transmission speed of a frame. Therefore, an extended PPP is widely used in which an extended control sequence is defined for providing a function of transiently stopping a frame.  
           [0013]    An interface apparatus operating on an extended PPP is commercially available. An advanced data sheet for the interface apparatus is published by Lucent Technologies in August 1999, entitled “TDAT042G5 SONET/SDH 155/622/2488 Mbits/s Data Interface”. The advanced data sheet discloses an extended PPP on page 67.  
           [0014]    [0014]FIG. 2 shows a frame format based on the extended PPP. The transmitter operating on the extended PPP divides one packet into a plurality of partial packets  101   b  and  101   c  and transmits them. Invalid sequences  105 , indicative of a transient stop of a frame transmission, are filled between the partial packets  101   b,  and  101   c.    
           [0015]    The invalid sequences  105  are constituted by two bytes, and have a value of “7Dh, 20h”. The invalid sequence is not defined by PPP.  
           [0016]    The receiver operating on the extended PPP refers to the invalid sequence  105 , and recognizes that the partial packets  101   b,  and  101   c  are the components of a single packet. The receiver reproduces the single packet from the partial packets  101   b,  and  101   c.    
           [0017]    However, the conventional packet transmission system has the following problem. In the packet transmission system, communications are carried out between unspecific transmitters and receivers. Thus, a receiver not operating on the extended PPP but on the original PPP may be used on the receiving side. The receiver may erroneously recognize the invalid sequences included in the received frame as a data to be received. This may result in that the receiver receives wrong data. That is, this may bring about the erroneous reception.  
           [0018]    It is desirable to attain a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which a certain standard is extended, wherein if a receiver complying with the extended standard receives the signal, a function is attained correspondingly to the extended standard, and wherein even if the signal is received by a receiver complying with only an original standard, the communication apparatus and the communication method do not bring about an erroneous reception.  
         SUMMARY OF THE INVENTION  
         [0019]    Therefore, an object of the present invention is to provide a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which a certain standard is extended, wherein if a receiver complying with the extended standard receives the signal, a function defined by the extended standard is attained, and wherein even if the signal is received by a receiver complying with only an original standard, the communication apparatus and the communication method do not bring about an erroneous reception.  
           [0020]    Another object of the present invention is to provide a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended, wherein if a receiver complying with the extended standard receives the signal, a function is attained correspondingly to the extended standard, and wherein even if the signal is received by a receiver complying with only an original PPP standard, the communication apparatus and the communication method do not bring about an erroneous reception.  
           [0021]    Still another object of the present invention is to provide a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended and a transient stop function of a packet transmission is carried out, wherein if a receiver complying with the extended standard receives the signal, the transient stop function of the packet transmission is carried out, and wherein even if the signal is received by a receiver complying with only the PPP, the communication apparatus and the communication method do not bring about an erroneous reception.  
           [0022]    Still another object of the present invention is to provide a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended and a transient stop function of a packet transmission is carried out when a transmission FIFO included in a transmitter becomes at an underflow state, wherein if a receiver complying with the extended standard receives the signal, the transient stop function of the packet transmission is carried out, and wherein even if the signal is received by a receiver complying with only the PPP, the communication apparatus and the communication method do not bring about an erroneous reception.  
           [0023]    Still another object of the present invention is to provide a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended and a transmission speed of a transmitter is reduced when a reception FIFO included in a receiver has a fear of an overflow state, wherein if a receiver complying with the extended standard receives the signal, the function of reducing the transmission speed is carried out, and wherein even if the signal is received by a receiver complying with only the PPP, the communication apparatus and the communication method do not bring about an erroneous reception.  
           [0024]    In order to achieve an aspect of the present invention, a communication system is composed of first and second communication units. The first communication unit sends transmission data, a control sequence, and an extended control sequence. The control sequence is originally defined by a protocol and includes an abort sequence requesting for abortion of the transmission data. The extended control sequence is undefined by the protocol and is provided for requesting an extended operation. The second communication unit receives the transmission data, the control sequence, and the extended control sequence. The second communication unit executes the extended operation in response to the extended control sequence. The extended control sequence includes the abort sequence.  
           [0025]    The protocol may be PPP (Point-to-Point Protocol), and the abort sequence may be represented by “7D, 7E” in the hexadecimal notation.  
           [0026]    Preferably, the extended control sequence consists of a plurality of the abort sequence.  
           [0027]    The first communicating unit may divide the transmission data into a plurality of divided data, and sequentially transmits the plurality of divided data to the second communication unit. In this case, the second communicating system preferably reproduces the transmission data from the plurality of divided data in response to the extended control sequence.  
           [0028]    The first communication unit may include a transmission high order layer, a transmission FIFO, a transmission data processor. In this case, the transmission high order layer generates the transmission data. The transmission FIFO transiently stores the transmission data, and sequentially outputs the transmission data. The transmission data processor reads the transmission data out of the transmission FIFO to transmit the transmission data to the second communication unit. The transmission data processor divides the transmission data into a plurality of divided data when an underflow occurs in the transmission FIFO, and sequentially transmits the plurality of divided data to the second communication unit. The second communication unit reproduces the transmission data from the plurality of divided data in response to the extended control sequence.  
           [0029]    The first communication unit may include a CRC indicating unit, and a CRC calculator. In this case, the CRC indicating unit outputs a CRC indication sequence as the extended control sequence for indicating a CRC (Cyclic Redundancy Check) method. The CRC calculator calculates a CRC code according to the CRC method and outputs the CRC code to the second communication unit. The second communication unit executes a CRC on the transmission data in response to the CRC indication sequence based on the CRC code according to the CRC method.  
           [0030]    The first communication unit may include an operation test indicating unit outputting an operation test indicating sequence as the extended control sequence, and outputting test data. In this case, the second communication unit preferably executes an operation test based on the test data in response to the operation test indicating sequence.  
           [0031]    The second communication unit may send a predetermined signal to the first communication unit in response to the extended control sequence.  
           [0032]    In order to achieve another aspect of the present invention, a communication system is composed of first and second communication units. The first communication unit transmits first transmission data. The second communication unit receives the first transmission data, and transmits second transmission data, a control sequence, and an extended control sequence to the first communication unit. The control sequence is originally defined by a protocol, and includes an abort sequence requesting for abortion of the second transmission data. The extended control sequence is undefined by the protocol and includes the abort sequence. The first communication unit adjusts a rate of transmitting the first transmission data in response to the extended control sequence.  
           [0033]    The second communication unit may include a reception FIFO. The reception FIFO receives and stores the first transmission data. The extended control sequence is generated in accordance with a state of the reception FIFO.  
           [0034]    The first communication unit may divide the first transmission data into a plurality of divided data, and transmit the plurality of divided data and another extended control sequence to the second communication unit. In this case, the second communicating system preferably reproduces the first transmission data from the plurality of divided data in response to the another extended control sequence.  
           [0035]    In order to achieve still another aspect of the present invention, a transmitter is composed of a data receiver and a sending unit. The data receiver receives a transmission data. The sending unit sends the transmission data, a control sequence and an extended control sequence. The control sequence is originally defined by a protocol, and includes an abort sequence requesting for abortion of the transmission data. The extended control sequence is undefined by the protocol and is provided for requesting an extended operation.  
           [0036]    In order to achieve still another aspect of the present invention, a receiver is composed of a receiving unit, and an outputting unit. The receiving unit receives a transmission data, a control sequence, and an extended control sequence. The control sequence is originally defined by a protocol, and includes an abort sequence requesting for abortion of the transmission data. The extended control sequence is undefined by the protocol and is provided for requesting an extended operation. The outputting unit outputs the received transmission data. The receiving unit executes the extended operation in response to the extended control sequence, and the extended control sequence includes the abort sequence.  
           [0037]    In order to achieve still another aspect of the present invention, a communication method comprising:  
           [0038]    sending transmission data;  
           [0039]    sending a control sequence originally defined by a protocol wherein the control sequence includes an abort sequence requesting for abortion of the transmission data;  
           [0040]    sending an extended control sequence, which is undefined by the protocol and is provided for requesting an extended operation;  
           [0041]    receiving the transmission data, the control sequence, and the extended control sequence; and  
           [0042]    executing the extended operation in response to the extended control sequence. The extended control sequence includes the abort sequence. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]    [0043]FIG. 1 shows a signal format of a signal in accordance with PPP;  
         [0044]    [0044]FIG. 2 shows a signal format of a signal used in a conventional communication system;  
         [0045]    [0045]FIG. 3 shows a configuration of a communication system of a first embodiment according to the present invention;  
         [0046]    [0046]FIG. 4 shows a configuration of a PPP frame signal  7   a;    
         [0047]    [0047]FIG. 5 shows a configuration of a PPP frame signal  7   a  into which an abort sequence  24  is inserted;  
         [0048]    [0048]FIG. 6 shows a configuration of a PPP frame signal  7   a  into which a transient stop sequence  25  and a packet completion sequence  26  are inserted;  
         [0049]    [0049]FIG. 7 shows another configuration of the PPP frame signal  7   a  into which the transient stop sequence  25  and the packet completion sequence  26  are inserted;  
         [0050]    [0050]FIG. 8 shows still another configuration of the PPP frame signal  7   a  into which the transient stop sequence  25  and the packet completion sequence  26  are inserted;  
         [0051]    [0051]FIG. 9 shows a configuration of a communication system of a second embodiment according to the present invention;  
         [0052]    [0052]FIG. 10 shows a content of a PPP frame signal  37   a   1  into which a speed adjustment packet  51  and a speed adjustment sequence  52  are inserted;  
         [0053]    [0053]FIG. 11 shows a content of a PPP frame signal  37   a   1  into which an extension function check sequence is inserted;  
         [0054]    [0054]FIG. 12 shows a configuration of a communication system of a third embodiment according to the present invention;  
         [0055]    [0055]FIG. 13 shows a content of a PPP frame signal  7   a  into which an FCS control packet  54  and a FCS control sequence  55  are inserted;  
         [0056]    [0056]FIG. 14 shows a configuration of a communication system of a fourth embodiment according to the present invention; and  
         [0057]    [0057]FIG. 15 shows a configuration of a PPP frame signal  7   a  into which a test packet  55  and a test sequence  56  are inserted. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]    A communication system according to the present invention will be described below with reference to the attached drawings.  
         [0059]    First Embodiment  
         [0060]    [0060]FIG. 3 shows a communication system in a first embodiment of the present invention. The communication system in the first embodiment is provided with a transmission high order layer  1 , a transmission framer  2 , a transmission path  3 , a reception framer  4  and a reception high order layer  5 .  
         [0061]    The transmission high order layer  1  provides packets to be transmitted. The transmission high order layer  1  transmits the packets to the transmission framer  2  on a transmission packet signal  1   a.  Moreover, the transmission high order layer  1 , if aborting the transmission of the packets, transmits an abort indication signal  1   b  indicative of the abort of the packet transmission, to the transmission framer  2 .  
         [0062]    The transmission framer  2  is composed of a transmission FIFO  6 , a transmission packet processor  7 , a scrambler  8  and a transmission overhead processor  9 .  
         [0063]    The transmission FIFO  6  transiently accumulates the packet transmitted from the transmission high order layer  1 . The transmission FIFO  6  outputs the accumulated packet to the transmission packet processor  7  on a transmission packet signal  6   a,  in the input order.  
         [0064]    Moreover, the abort indication signal  1   b  is inputted to the transmission FIFO  6  from the transmission high order layer  1 . The transmission FIFO  6  outputs an abort indication signal  6   b  indicative of an abort of the packet transmission to the transmission packet processor  7  when receiving the abort indication signal  1   b.    
         [0065]    Moreover, the transmission FIFO  6  outputs an underflow occurrence signal  6   c  when an underflow occurs therein. The transmission FIFO  6  may be at the underflow state if a speed at which the packets are inputted to the transmission FIFO  6  is smaller than a speed at which the packets are outputted from the transmission FIFO  6 . In such a case, the transmission FIFO  6  outputs the underflow occurrence signal  6   c  indicative of the occurrence of the underflow to the transmission packet processor  7 .  
         [0066]    The transmission packet processor  7  reads out the packets from the transmission FIFO  6 , one byte at a time, at a predetermined timing. The transmission packet processor  7  adds FCS&#39;s, flag sequences and control sequences to the read packets to generate a PPP frame signal  7   a.    
         [0067]    In detail, the transmission packet processor  7  calculates CRC codes of the packets, and generates the FCS&#39;s for holding the CRC codes, and then adds to the packets.  
         [0068]    Furthermore, the transmission packet processor  7  inserts flag sequences between the packets. The flag sequences are filled between the two packets, and indicate a separation between two packets. The flag sequences have a value of “7Eh” in the hexadecimal notation.  
         [0069]    The transmission packet processor  7  inserts an abort sequence into the PPP frame signal  7   a,  if the abort of the packet is indicated by the abort indication signal  6   b.  The abort sequence is the control sequence defined by the PPP, as mentioned above. The abort sequence is a code of two bytes, and its value is 7Dh, 7Eh.  
         [0070]    Moreover, the transmission packet processor  7  inserts a transient stop sequence and a packet completion sequence into the PPP frame signal  7   a,  if the underflow occurs in the transmission FIFO  6 . Here, the transient stop sequence is a code of four bytes, and its value is 7Dh, 7Eh, 7Dh and 7Eh in which the abort sequence is repeated twice. The packet completion sequence is a code of six bytes, and its value is 7Dh, 7Eh, 7Dh, 7Eh, 7Dh, and 7Eh in which the abort sequence is repeated three times.  
         [0071]    If the underflow occurs in the transmission FIFO  6 , the input of the packet to the transmission packet processor  7  is transiently stopped. The input of the packet to the transmission packet processor  7  is resumed after the escape of the transmission FIFO  6  from the underflow state. That is, the packet is divided into a plurality of portions and inputted to the transmission packet processor  7 . In this description, the plurality of portions generated by the division of the packet are referred to as partial packets, respectively. The transmission packet processor  7  inserts the transient stop sequence into a portion immediately after the partial packet, if the input of the packet is stopped. Moreover, the transmission packet processor  7  inserts the packet completion sequence into a portion immediately after the finally partial packet required to constitute the packet.  
         [0072]    [0072]FIG. 4 shows the content of the PPP frame signal  7   a  generated by the transmission packet processor  7 . FIG. 4 shows the content of the PPP frame signal  7   a  if the abort of the packet is not indicated and the underflow does not occur in the transmission FIFO  6 . The PPP frame signal  7   a  includes a packet  21 . The packet  21  is the packet read from the transmission FIFO  6 . An FCS  22  is added to the final portion of the packet  21 . A CRC code of the packet  21  is recorded in the FCS  22 . Flag sequences  23  are filled between the packets  21 .  
         [0073]    On the other hand, FIG. 5 shows the content of the PPP frame signal  7   a  if the abort of the packet is indicated by the abort indication signal  6   b.  As shown in FIG. 5, an abort sequence  24  is added to a rear portion of a packet  21   a  to be aborted.  
         [0074]    Moreover, FIG. 6 shows the content of the PPP frame signal  7   a  if the underflow occurs in the transmission FIFO  6 . FIG. 6 shows the content of the PPP frame signal  7   a  if one packet is divided into a plurality of partial packets  21   b,    21   c  and  21   d  and inputted to the transmission packet processor  7 .  
         [0075]    The transmission packet processor  7  inserts a transient stop sequence  25  into a portion immediately after the partial packets  21   b  when the underflow occurs in the transmission FIFO  6 . The transient stop sequence  25  shows that the transmission of the packet constituted by the plurality of partial packets  21   b,    21   c  and  21   d  is transiently stopped. The reception framer  4  recognizes that the partial packet  21   b  is a part of one packet and that the remaining portions are sent after that, from the transient stop sequence  25 .  
         [0076]    Similarly, the transmission packet processor  7  also inserts another transient stop sequence  25  into a portion immediately after the partial packet  21   c  if the underflow occurs in the transmission FIFO  6  when the partial packet  21   c  is inputted to the transmission packet processor  7 .  
         [0077]    The transmission packet processor  7  inserts an FCS  22   a  into a portion immediately after the final partial packet  21   d.  The FCS  22   a  records therein a CRC code of one packet constituted by the plurality of partial packets  21   b,    21   c  and  21   d.    
         [0078]    Moreover, the transmission packet processor  7  inserts a packet completion sequence  26  into a portion immediately after the FCS  22   a.  The packet completion sequence  26  shows that the plurality of partial packets  21   b,    21   c  and  21   d  are all transmitted. The reception framer  4  recognizes that the partial packet  21   d  is the final partial packet of the packet that is divided and transmitted, from the packet completion sequence  26 .  
         [0079]    Both of the transient stop sequence  25  and the packet completion sequence  26  are the control sequences undefined by the PPP. However, both of them include “7Dh, 7Eh” implying the abort sequence. As described later, the fact that both of the transient stop sequence  25  and the packet completion sequence  26  include the “7Dh, 7Eh” implying the abort sequence provides the effect that even if a signal having the transient stop sequence  25  and the packet completion sequence  26  is sent to a reception framer which does not expect the reception of the transient stop sequence  25  and the packet completion sequence  26 , an erroneous reception is never induced only if the reception framer aborts the received partial packet.  
         [0080]    As mentioned above, the transmission packet processor  7  generates the PPP frame signal  7   a  while inserting the transient stop sequence  25  or the packet completion sequence  26  into the portion immediately after each of the partial packets constituting the packet, if the underflow occurs in the transmission FIFO  6 .  
         [0081]    The PPP frame signal  7   a,  generated by the transmission packet processor  7 , is outputted to the scrambler  8 . The scrambler  8  performs a scramble process on the PPP frame signal  7   a  and generates a scramble signal  8   a.  The scrambler  8  outputs the scramble signal  8   a  to the transmission overhead processor  9 .  
         [0082]    The transmission overhead processor  9  adds an overhead to the scramble signal  8   a  and generates a SONET/SDH frame  9   a.  The SONET/SDH frame  9   a  has a form defined by a SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) standard. The overhead is a code to control an optical communication defined by the SONET/SDH standard. The transmission overhead processor  9  outputs the SONET/SDH frame  9   a  through the transmission path  3  to the reception framer  4 .  
         [0083]    The reception framer  4  is the reception framer for carrying out the operations corresponding to the transient stop sequence  25  and the packet completion sequence  26  that are the control sequences undefined by the PPP. The reception framer  4  has a reception overhead processor  10 , a de-scrambler  11 , a reception packet processor  12  and a reception FIFO  13 .  
         [0084]    The reception overhead processor  10  removes the overhead from the SONET/SDH frame  9   a  transmitted through the transmission path  3 , and then reproduces a scramble signal  10   a  substantially equal to the scramble signal  8   a.    
         [0085]    The de-scrambler  11  performs the scramble process on the scramble signal  10   a,  and reproduces a PPP frame signal  11   a  substantially equal to the PPP frame signal  7   a.    
         [0086]    The reception packet processor  12  reproduces the packet from the PPP frame signal  11   a,  and outputs to the reception FIFO  13  by using a reception packet signal  12   a.    
         [0087]    The reception packet processor  12  reproduces the packet as follows.  
         [0088]    If the PPP frame signal  11   a  does not transmit the packet, the PPP frame signal  11   a  has a value 7Eh implying the flag sequence. The reception packet processor  12  detects that the sequentially input PPP frame signal  11   a  has another value which is not 7Eh implying the flag sequence, and accordingly recognizes the start of the input of the packet.  
         [0089]    Moreover, the reception packet processor  12  detects that the PPP frame signal  11   a  again returns to the value 7Eh implying the flag sequence, and thereby recognizes that the packets are inputted to the end. The reception packet processor  12  fetches the packets from the PPP frame signal  11   a  and outputs to the reception FIFO  13 . The reception FIFO  13  outputs the packets to the reception high order layer  5  in turn.  
         [0090]    At this time, the reception packet processor  12  judges the two bytes immediately before the flag sequence as FCS, and then judges whether or not the packet is normally transmitted, in accordance with the CRC code included in the FCS.  
         [0091]    If it is judged that the packet is not normally transmitted, the reception packet processor  12  outputs an error packet report signal  12   b  indicative of an abort of the packet to the reception FIFO  13 . The reception FIFO  13  receiving the error packet report signal  12   b  outputs a packet abort signal  13   b  indicative of the abort of the packet to the reception high order layer  5 . The reception high order layer  5  aborts the received packet in accordance with the packet abort signal  13   b.    
         [0092]    The reception packet processor  12 , even if the abort sequence is included in the PPP frame signal  11   a,  outputs the error packet report signal  12   b  to the reception FIFO  13 , and commands to abort the packet.  
         [0093]    Moreover, the reception packet processor  12 , if detecting the transient stop sequence from the PPP frame signal  11   a,  recognizes that the received packet is constituted by a plurality of partial packets and its transmission is transiently stopped.  
         [0094]    If detecting the transient stop sequence, the reception packet processor  12  does not judge that the packets are inputted to the end, even if detecting the flag sequence in succession after the transient stop sequence. Thus, the calculation of the FCS is not carried out.  
         [0095]    At this time, the reception packet processor  12  outputs the input partial packets to the reception FIFO  13  in turn. Moreover, the reception packet processor  12  outputs an EOP hold indication signal  12   c  to the reception FIFO  13 . Usually, the reception packet processor  12  sequentially transfers the data of a packet to high order layers, and sends a report of EOP (End Of Packet) to the high order layers after the output of the final data of the packet. However, the EOP must not be outputted in receiving a non-final partial packet. Therefore, the reception packet processor  12  outputs to the reception FIFO  13  the EOP hold indication signal  12   c  intended so as not to output the EOP in the non-final partial packet, as for the divided packets. Since the EOP hold indication signal  12   c  is outputted to the reception FIFO  13 , the EOP is reported with regard to only the final partial packet.  
         [0096]    In succession, the reception packet processor  12  detecting the transient stop sequence monitors whether or not the PPP frame signal  11   a  includes the packet completion sequence. The detection of the packet completion sequence enables the reception packet processor  12  to recognize the completion of the input of the final partial packet among the plurality of partial packets into which the packet is divided. The reception packet processor  12  detecting the packet completion sequence judges the two bits immediately before the packet completion sequence as the FCS, and then judges whether or not the packet is normally transmitted.  
         [0097]    If it is judged that the packet is not normally transmitted, the reception packet processor  12  outputs to the reception FIFO  13  the error packet report signal  12   b  indicative of the abort of the packet.  
         [0098]    Moreover, the reception packet processor  12 , if detecting the packet completion sequence, stops the output of the EOP hold indication signal  12   c,  and allows the reception FIFO  13  to output the packet to the reception high order layer  5 .  
         [0099]    The reception FIFO  13  transfers in turn the packets received from the reception packet processor  12 , to the reception high order layer  5 . Moreover, the reception FIFO  13  responds to the error packet report signal  12   b,  and outputs to the reception high order layer  5  the packet abort signal  13   b  indicative of the abort of the packet.  
         [0100]    The reception high order layer  5  receives the packet from the reception FIFO  13 . Moreover, the reception high order layer  5  aborts the received packet in accordance with the packet abort signal  13   b.    
         [0101]    As mentioned above, the transmission framer  2 , if the transmission FIFO  6  included therein becomes at the underflow state, uses the transient stop sequence and the packet completion sequence, and transmits the packet while carrying out the transient stop. The reception framer  4  refers to the transient stop sequence and the packet completion sequence, and normally receives the packet transmitted while it is transiently stopped. In the communication system of this embodiment, the packet can be normally transmitted and received even if the transmission FIFO  6  becomes at the underflow state. Thus, in the communication system of this embodiment, it is not necessary to abort the packet even if the transmission FIFO  6  becomes at the underflow state. That is, even if the transmission FIFO  6  becomes at the underflow state, the transmission high order layer  1  need not re-transmit the packet. Hence, in the communication system of this embodiment, there is no case that the transmission efficiency is dropped because of the re-transmission of the packet since the transmission FIFO  6  becomes at the underflow state.  
         [0102]    Here, let us consider the case that the transmission framer  2  in this embodiment is connected to a reception framer which does not comply with the transient stop sequence and the packet completion sequence although it complies with the PPP. Even in this case, there is no case that the reception framer receives the erroneous data and then transfers to the reception high order layer. This is because both of the transient stop sequence and the packet completion sequence include the 7Dh, 7Eh implying the abort sequence. The reception framer, which does not expect the reception of the transient stop sequence and the packet completion sequence, recognizes the transient stop sequence and the packet completion sequence as the abort sequence, and merely aborts the transmitted packet. Thus, there is no case that the reception framer that does not comply with the transient stop sequence and the packet completion sequence receives the erroneous data.  
         [0103]    In this embodiment, the transient stop sequence and the packet completion sequence may have other values if they can satisfy the following three conditions.  
         [0104]    (1) They include the abort sequence 7Dh, 7Eh.  
         [0105]    (2) They do not include the control sequence defined by the PPP.  
         [0106]    (3) The transient stop sequence and the packet completion sequence are different from each other.  
         [0107]    For example, as shown in FIG. 7, the transient stop sequence can have “7Dh, 20h, 7Dh, 7Eh”, and the packet completion sequence can have “7Dh, 21h, 7Dh, 7Eh”.  
         [0108]    However, as described in the explanation of this embodiment, both of the transient stop sequence and the packet completion sequence are the repetitions of the abort sequence “7Dh, 7Eh”. So, it is desired that the numbers of respective repetitions are different from each other. This reason is as follows. That is, it is understood that the abort sequence “7Dh, 7Eh” always indicates the abort process in the case of the packet communication system complying with the PPP. Thus, at least an erroneous reception is never induced in the case of the packet communication system complying with the PPP.  
         [0109]    Also, in this embodiment, it is possible that the packet completion sequence is not added to a portion behind the finally transmitted partial packet  21   d  among the partial packets  21   b,    21   c  and  21   d  constituting one packet, as shown in FIG. 8. In this case, the reception packet processor  12  recognizes that the partial packet  21   d  is the finally transmitted partial packet among the partial packets  21   b,    21   c  and  21   d  constituting the one packet, since the transient stop sequence is not added to the portion behind the partial packet  21   d.  A signal to be transmitted can be reduced since the packet completion sequence is not added.  
         [0110]    On the other hand, if a reception framer, which does not comply with the transient stop sequence although it complies with the PPP, receives a signal having a signal format shown in FIG. 8, the reception framer does not calculate an entire CRC code of the partial packets  21   b,    21   c  and  21   d  constituting the one packet when calculating a CRC code, and it calculates a CRC code of only the partial packet  21   d.  The CRC code calculated on the basis of only the partial packet  21   d  does not typically agree with the CRC code calculated on the basis of the entire packet composed of the partial packets  21   b,    21   c  and  21   d.  The CRC code calculated on the basis of only the partial packet  21   d  does not agree with the CRC code noted in the FCS. Thus, the data of the partial packet  21   d  is aborted because of an FCS error. In this way, even if the reception framer, which does not comply with the transient stop sequence although complying with the PPP, receives the signal having the format shown in FIG. 8, the erroneous reception is not usually induced.  
         [0111]    However, if the CRC code of the partial packet  21   d  accidentally agrees with the CRC code noted in the FCS  22   a  although this probability is very low, the partial packet  21   d  is judged as a single packet. Thus, if a packet transmission system requiring high data reliability is established, it is desirable to add the packet completion sequence.  
         [0112]    Second Embodiment  
         [0113]    [0113]FIG. 9 shows a communication system in a second embodiment according to the present invention. The communication system in the second embodiment is provided with transmission high order layers  31   1 ,  31   2 , a first framer  32 , optical transmission paths  33   1 ,  33   2 , a second framer  34  and reception high order layers  35   1 ,  35   2 .  
         [0114]    The first framer  32  is composed of a transmission FIFO  36   1 , a transmission packet processor  37   1 , a communication speed controller  38   1 , a scrambler  39   1 , a transmission overhead processor  40   1 , a reception overhead processor  41   2 , a de-scrambler  42   2 , a reception packet processor  43   2  and a reception FIFO  44   2 .  
         [0115]    The second framer  34  is composed of a transmission FIFO  36   2 , a transmission packet processor  37   2 , a communication speed controller  38   2 , a scrambler  39   2 , a transmission overhead processor  40   2 , a reception overhead processor  41   1 , a de-scrambler  42   1 , a reception packet processor  43   1 , and a reception FIFO  44   1 .  
         [0116]    The communication system in this embodiment is configured such that if the underflow occurs in the transmission FIFO  36   1  and the transmission FIFO  36   2 , the packet can be normally transmitted and received. Moreover, the purpose of the communication system in this embodiment is to adjust the transmission speeds at which the transmission packet processors  37   1 ,  37   2  transmit the packet, and thereby protect the underflow from occurring in the reception FIFO  44   1 ,  44   2 . The communication system in this embodiment will be described below.  
         [0117]    Here, the operations of the transmission FIFO  36   2 , the transmission packet processor  37   2 , the communication speed controller  38   2 , the scrambler  39   2 , the transmission overhead processor  40   2 , the reception overhead processor  41   2 , the de-scrambler  42   2 , the reception packet processor  43   2  and the reception FIFO  44   2  that are mounted on the route through which the packet is transmitted to the reception high order layer  35   2  from the transmission high order layer  31   2  are respectively equal to the operations of the transmission FIFO  36   1 , the transmission packet processor  37   1 , the communication speed controller  38   1 , the scrambler  39   1 , the transmission overhead processor  40   1 , the reception overhead processor  41   1 , the de-scrambler  42   1 , the reception packet processor  43   1  and the reception FIFO  44   1  that are mounted on the route through which the packet is transmitted to the reception high order layer  35   1  from the transmission high order layer  31   1 . Therefore, only the operations of the transmission FIFO  36   1 ,the transmission packet processor  37   1 , the communication speed controller  38   1 , the scrambler  39   1 , the transmission overhead processor  40   1 , the reception overhead processor  41   1 , the de-scrambler  42   1 , the reception packet processor  43   1  and the reception FIFO  44   1  that are mounted on the route through which the packet is transmitted to the reception high order layer  35   1  from the transmission high order layer  31   1  will be described below.  
         [0118]    The operations of the transmission high order layer  31   1  and the transmission FIFO  36   1  are respectively equal to the operations of the transmission high order layer  1  and the transmission FIFO  36  in the first embodiment.  
         [0119]    The transmission high order layer  31   1  generates a packet to be transmitted. The transmission high order layer  31   1  transmits the generated packet to the transmission FIFO  36   1  on a transmission packet signal  31   a   1 . Moreover, the transmission high order layer  31   1 , if aborting the transmission in the course of the packet transmission, transmits an abort indication signal  31   b   1  indicative of the abort of the packet transmission, to the transmission FIFO  36   1 .  
         [0120]    The transmission FIFO  36   1  transiently accumulates the packets transmitted from the transmission high order layer  31   1 . The transmission FIFO  36   1  outputs the accumulated packets to the transmission packet processor  37   1  by using a transmission FIFO  36   a   1 , in the input order.  
         [0121]    Moreover, the abort indication signal  31   b   1  is respectively inputted to the transmission FIFO  36   1  from the transmission high order layer  31   1 . The transmission FIFO  36   1 , when receiving the abort indication signal  31   b   1 , outputs an abort indication signal  36   b   1  indicative of an abort of the packet transmission to the transmission packet processor  37   1 , respectively.  
         [0122]    Moreover, the transmission FIFO  36   1  outputs, when the underflow occurs therein, outputs an underflow occurrence signal  36   c   1  to the transmission packet processor  37   1 .  
         [0123]    The transmission packet processor  37   1  reads the packets from the transmission FIFO  36   1 , one byte at a time, at a predetermined timing. The transmission packet processor  37   1 , adds the FCS, the flag sequence and the control sequence to the read packet, and then generates a PPP frame signal  37   a   1 .  
         [0124]    The transmission packet processor  37   1  calculates the CRC code of the packet read from the transmission FIFO  36   1 , and generates the FCS for holding the CRC code, and then adds to the packet. The transmission packet processor  37   1  inserts the flag sequence between the packets. Moreover, the transmission packet processor  37   1  inserts the abort sequence into the PPP frame signal  37   a   1 , if the abort of the packet is indicated by the abort indication signal  36   b   1 .  
         [0125]    Moreover, the transmission packet processor  37   1  inserts the above-mentioned transient stop sequence and the packet completion sequence into the PPP frame signal  37   a   1 , if the underflow occurs in the transmission FIFO  36   1 .  
         [0126]    The above-described operations of the transmission packet processor  37   1  are same as that of the transmission packet processor  7  contained by the communication system in the first embodiment, and the detailed explanation is omitted.  
         [0127]    In addition to the above-mentioned operations, the transmission packet processor  37   1  monitors a remaining amount of a memory region of the reception FIFO  44   2 , and inserts a speed adjustment packet and a speed adjustment sequence into the PPP frame signal  37   a   1 , in response to the remaining amount.  
         [0128]    The reception FIFO  44   2  outputs the remaining amount of the memory region of the reception FIFO  44   2  to the transmission packet processor  37   1  on an overflow indication signal  44   a   2 . The transmission packet processor  37   1  generates a speed adjustment information with regard to a speed at which the second framer  34  transmits a packet, on the basis of the remaining amount. Then, the transmission packet processor  37   1  generates a speed adjustment packet including the speed adjustment information. That is, the transmission packet processor  37   1  inserts the speed adjustment packet and the speed adjustment sequence indicative of the transmission of the speed adjustment packet into the PPP frame signal  37   a   1 .  
         [0129]    [0129]FIG. 10 shows a speed adjustment packet  51  and a speed adjustment sequence  52  that are included in the PPP frame signal  37   a   1 . As mentioned above, the speed adjustment packet  51  has the speed adjustment information. The speed adjustment sequence  52  is added to a portion immediately after the speed adjustment packet  51 . The speed adjustment sequence  52  is a code of  8  bytes, and it is equal to that of the four repetitions of the abort sequence. The speed adjustment sequence  52  shows that the packet immediately before is the speed adjustment packet  51 . As described later, the speed adjustment information included in the speed adjustment packet  51  is sent to the second framer  34 . The second framer  34  adjusts a transmission speed of a packet to be transmitted to the first framer  32 , on the basis of the speed adjustment information.  
         [0130]    Here, the speed adjustment packet  51  and the speed adjustment sequence  52  are not always inserted between the packets to be transmitted. It is also possible to transiently stop the transmission of one packet, and then possible to insert the speed adjustment packet  51  and the speed adjustment sequence  52 .  
         [0131]    The transmission packet processor  37   1  for inserting the speed adjustment packet  51  and the speed adjustment sequence  52  further adjusts the transmission speed, in response to a transmission speed control signal  38   a   2  outputted by the communication speed controller  38   2 . Here, the transmission speed control signal  38   a   2  is the signal for adjusting the transmission speed of the transmission packet processor  37   1 . The generating process will be described later.  
         [0132]    Actually, the transmission packet processor  37   1  divides one packet to be transmitted, into a plurality of partial packets, similarly to the first embodiment. The transmission packet processor  37   1 , while inserting the transient stop sequence or the packet completion sequence into a portion immediately after the plurality of partial packets, transmits the PPP frame signal  37   a   1 . The plurality of partial packets are transmitted at a predetermined temporal interval. The transmission packet processor  37   1  adjusts the temporal interval to accordingly adjust an effective transmission speed.  
         [0133]    The PPP frame signal  37   a   1  generated by the above-mentioned processes in the transmission packet processor  37   1  is outputted to the scrambler  39   1 . The operation of the scrambler  39   1  is similar to that of the scrambler  8  in the first embodiment. The scrambler  39   1  performs the scramble process on the PPP frame signal  37   a   1 , and generates a scramble signal  38   a   1 . The scrambler  39   1  outputs the scramble signal  39   a   1  to the transmission overhead processor  39   1 .  
         [0134]    The transmission overhead processor  40   1  adds an overhead to the scramble signal  39   a   1  and generates a SONET/SDH frame  40   a   1 . The transmission overhead processor  40   1  outputs the SONET/SDH frame  40   a   1  through the transmission path  33   1  to the reception overhead processor  41   1  of the reception framer  34 .  
         [0135]    The reception overhead processor  41   1  removes the overhead from the SONET/SDH frame  40   a   1  transmitted through the transmission path  33   1 , and reproduces a scramble signal  41   a   1  substantially equal to the scramble signal  39   a   1 .  
         [0136]    The de-scrambler  42   1  performs the de-scramble process on the scramble signal  39   a   1  substantially equal to the scramble signal  39   a   1 .  
         [0137]    The reception packet processor  43   1 , while referring to the transient stop sequence and the packet completion sequence, reproduces a packet from the PPP frame signal  42   a   1  and outputs to the reception FIFO  44   1 . The reception FIFO  44   1  transfers the reproduced packet to the reception high order layer  35   1 . The process is similar to the operation of the reception packet processor  12  included in the communication system of the first embodiment, and the explanation is omitted.  
         [0138]    Moreover, the reception packet processor  43 , fetches the speed adjustment information from the speed adjustment packet generated by the transmission packet processor  37   1 , and outputs to the transmission speed controller  38   1  by using a speed adjustment information signal  43   a   1 . The transmission speed controller  38   1 , in accordance with the speed adjustment information fetched from the speed adjustment packet, generates a transmission speed control signal  38   a   1 , and adjusts a transmission speed of the transmission packet processor  37   2 .  
         [0139]    The communication system in this embodiment having the above-mentioned configuration protects the overflow from occurring in the reception FIFO  44   1 ,  44   2 . The process for adjusting the transmission speed of the packet transmitted to the second framer  34  from the first framer  32  and accordingly protecting the overflow from occurring in the reception FIFO  44   1  will be described below.  
         [0140]    The packet generated by the transmission high order layer  31   1  is transmitted to and accumulated in the reception FIFO  44   1  of the second framer  34 , while its transmission speed is adjusted by the transmission packet processor  37   1 . The remaining amount of the memory region of the reception FIFO  44   1  for accumulating the packet generated by the transmission high order layer  31   1  is sent to the transmission packet processor  37   2  by using an overflow indication signal  44   a   1 . The transmission packet processor  37   2 , if judging that the remaining amount of the memory region of the reception FIFO  44   1  is small, generates a speed adjustment information having a content indicative of a command for making the transmission speed slower.  
         [0141]    The transmission packet processor  37   2  transmits the speed adjustment sequence and the speed adjustment packet including the speed adjustment information to the reception packet processor  43   2  in the first framer  32 . The reception packet processor  43   2  fetches the speed adjustment information from the received speed adjustment packet, and outputs to a transmission speed controller  38   2  by using a speed adjustment information signal  43   a   2 . The transmission speed controller  38   2 , in accordance with this speed adjustment information, generates a transmission speed control signal  38   a   2 , and then reduces the transmission speed of the transmission packet processor  37   1 . Thus, the transmission speed of the packet transmitted to the reception FIFO  44   2  is reduced to thereby protect the overflow from occurring in the reception FIFO  44   1 .  
         [0142]    The similar process protects the overflow from occurring in the reception FIFO  44   2 .  
         [0143]    As mentioned above, in the communication system in this embodiment, the adjustment of the transmission speed enables the overflow from occurring in the reception FIFO  44   1 ,  44   2 .  
         [0144]    Here, let us suppose that a signal including the speed adjustment packet and the speed adjustment sequence is sent to a framer that does not comply with the speed adjustment packet and the speed adjustment sequence although it complies with the PPP. As mentioned above, the speed adjustment sequence includes the abort sequence “7Dh, 7Eh”. Thus, if the signal including the speed adjustment packet and the speed adjustment sequence is inputted to the framer that does not comply with the speed adjustment packet and the speed adjustment sequence, the framer aborts the speed adjustment packet and the speed adjustment sequence. Hence, the framer does not bring about the erroneous reception.  
         [0145]    In the above mentioned communication system, in which packets are transferred between the two framers, one of the framers can check whether or not the other framer operates on the basis of the extended standard.  
         [0146]    The transmission packet processor  37   1  of the first framer  32  inserts an extended standard check sequence into the PPP frame signal  37   a   1  and transmits. FIG. 11 shows an extended standard check sequence  53  inserted into the PPP frame signal  37   a   1 . The extended standard check sequence  53  is the control sequence in which the abort sequence “7Dh, 7Eh” is repeated five times, as shown in FIG. 11. The extended standard check sequence is transmitted to the second framer  34 . The reception packet processor  43   1  of the second framer  34  recognizes the transmission of the extended standard check sequence. The reception packet processor  43   1  recognizing the transmission of the extended standard check sequence commands the transmission packet processor  37   2  to transmit the extended standard check sequence to the first framer  32 .  
         [0147]    In the extended standard check sequence transmitted by the transmission packet processor  37   2 , the reception packet processor  43   2  of the first framer  32  recognizes the transmission of the extended standard check sequence. Since the reception packet processor  43   2  receives an extended standard sequence, the first framer  32  can check that the second framer  34  is the framer complying with the extended standard. In accordance with the similar processes, the second framer  34  can also check that the first framer  32  is the framer complying with the extended standard.  
         [0148]    Here, let us suppose that a signal including the extended standard sequence is sent to a framer that does not comply with the extended standard sequence although it complies with the PPP. As mentioned above, the extended standard sequence includes the abort sequence “7Dh, 7Eh”. Thus, if the signal including the extended standard sequence is inputted to the framer which does not comply with the extended standard sequence, the framer aborts the extended standard sequence. Hence, also, the framer does not bring about the erroneous reception.  
         [0149]    In the second embodiment, the transient stop sequence, the packet completion sequence, the speed adjustment sequence and the extended standard check sequence may have other values if they can satisfy the following three conditions.  
         [0150]    (1) They include the abort sequence 7Dh, 7Eh.  
         [0151]    (2) They do not include the control sequence defined by the PPP.  
         [0152]    (3) The transient stop sequence, the packet completion sequence, the speed adjustment sequence and the extended standard check sequence are different from each other.  
         [0153]    Third Embodiment  
         [0154]    [0154]FIG. 12 shows a communication system in a third embodiment according to the present invention. The configuration of the communication system in this embodiment is substantially similar to that of the communication system in the first embodiment. The communication system in the third embodiment has the configuration in which an FCS controller  14  is added to the communication system in the first embodiment. Associated with the addition of the FCS controller  14 , the operations of a transmission packet processor  7  and a reception packet processor  12  of the communication system in the third embodiment are different in the following points from those of the communication system in the first embodiment.  
         [0155]    The FCS controller  14  outputs an FCS control signal  14   a  to the transmission packet processor  7 . The transmission packet processor  7  changes a method of calculating an FCS to be inserted into the PPP frame signal  7   a,  in response to the FCS control signal  14   a.  Moreover, the transmission packet processor  7  inserts an FCS control packet indicative of the FCS calculation method, and an FCS control sequence into the PPP frame signal  7   a.    
         [0156]    [0156]FIG. 13 shows an FCS control sequence  55  and an FCS control packet  54  that are inserted into the PPP frame signal  7   a.  The FCS control packet  54  is the packet indicative of the FCS calculation method. The FCS control sequence  55  is inserted into a portion immediately after the FCS control packet  54 . The FCS control sequence  55  is the control sequence in which the abort sequences “7Dh, 7Eh” are repeated four times. The FCS control sequence  55  shows that the packet immediately before is the FCS control packet  54 .  
         [0157]    The FCS control sequence  55  and the FCS control packet  54  that are inserted into the PPP frame signal  7   a  are sent to the reception packet processor  12  of the reception framer  4 . The reception packet processor  12  detects the FCS control sequence  55 , and fetches the FCS control packet  54  received immediately before. The reception packet processor  12  changes the FCS calculation method for the FCS calculation method shown by the FCS control packet  54 . The reception packet processor  12  calculates a CRC code of the packet received by it, in accordance with the changed FCS calculation method. Moreover, the reception packet processor  12  judges whether or not the packet is normally received, from the CRC code stored in the FCS added to the packet and the CRC code of the received packet.  
         [0158]    As mentioned above, the communication system in this embodiment can change the FCS calculation method, which is the function undefined by the PPP.  
         [0159]    Here, let us suppose that a signal including the FCS control packet and the FCS control sequence is sent to a framer that does not comply with the FCS control packet and the FCS control sequence although it complies with the PPP. As mentioned above, the FCS control sequence includes the abort sequence 7Dh, 7Eh. Thus, if the signal including the FCS control packet and the FCS control sequence is inputted to the framer which does not comply with the FCS control packet and the FCS control sequence, the framer aborts the FCS control packet and the FCS control sequence. Hence, the framer does not bring about the erroneous reception.  
         [0160]    In the third embodiment, the FCS control sequence may have another value if it can satisfy the following three conditions.  
         [0161]    (1) It includes the abort sequence 7Dh, 7Eh.  
         [0162]    (2) It does not include the control sequence defined by the PPP.  
         [0163]    (3) The transient stop sequence, the packet completion sequence and the FCS control sequence are different from each other.  
         [0164]    Fourth Embodiment  
         [0165]    [0165]FIG. 14 shows a communication system in a fourth embodiment according to the present invention. The configuration of the communication system in this embodiment is substantially similar to that of the communication system in the first embodiment. The communication system in the fourth embodiment has the configuration in which a test packet generator  15  is added to the communication system in the first embodiment. Associated with the addition of the test packet generator  15 , the operations of the transmission packet processor  7  and the reception packet processor  12  of the communication system in the fourth embodiment are different in the following points from those of the communication system in the first embodiment.  
         [0166]    The test packet generator  15  generates a test packet and outputs the test packet to the transmission packet processor  7  by using a test packet signal  15   a.  The transmission packet processor  7  inserts the test packet included in the test packet signal  15   a  into the PPP frame signal  7   a.  Moreover, the transmission packet processor  7  inserts a test packet sequence into the PPP frame signal  7   a,  in a portion immediately after the test packet.  
         [0167]    [0167]FIG. 15 shows a test packet  56  and a test sequence  57  that are inserted into the PPP frame signal  7   a.  The test sequence  57  is inserted into a portion immediately after the test packet  56 . The test packet  56  is the control sequence in which the abort sequences “7Dh, 7Eh” are repeated four times. The test sequence  57  shows that the packet immediately before is the test packet  56 .  
         [0168]    The test packet  56  and the test sequence  57  that are inserted into the PPP frame signal  7   a  is transmitted to the reception packet processor  12  of the reception framer  4 . The reception packet processor  12  detects the test sequence  57  and fetches the test packet  56  transmitted immediately before. The reception packet processor  12  carries out an operation test in accordance with the test packet  56 .  
         [0169]    As described above, the communication system in this embodiment can carry out the operation test of the reception packet processor  12 , which is the function undefined by the PPP.  
         [0170]    Here, let us suppose that a signal including the test packet and the test sequence is sent to a framer that does not comply with the test packet and the test sequence although it complies with the PPP. As mentioned above, the test sequence includes the abort sequence 7Dh, 7Eh. Thus, if the signal including the test packet and the test sequence is inputted to the framer that does not comply with the test packet and the test sequence, the framer aborts the test packet and the test sequence. Hence, the framer does not bring about the erroneous reception.  
         [0171]    In the fourth embodiment, the test sequence may have another value if it can satisfy the following three conditions.  
         [0172]    (1) It includes the abort sequence 7Dh, 7Eh.  
         [0173]    (2) It does not include the control sequence defined by the PPP.  
         [0174]    (3) The transient stop sequence, the packet completion sequence and the test sequence are different from each other.  
         [0175]    The present invention provides a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which a certain standard is extended, wherein if a receiver complying with the extended standard receives the signal, a function defined by the extended standard is attained, and wherein even if the signal is received by a receiver complying with only an original standard, the communication apparatus and the communication method do not bring about an erroneous reception.  
         [0176]    The present invention provides a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended, wherein if a receiver complying with the extended standard receives the signal, a function is attained correspondingly to the extended standard, and wherein even if the signal is received by a receiver complying with only a PPP standard, the communication apparatus and the communication method do not bring about an erroneous reception.  
         [0177]    The present invention provides a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended and a transient stop function of a packet transmission is carried out, wherein if a receiver complying with the extended standard receives the signal, the transient stop function of the packet transmission is carried out, and wherein even if the signal is received by a receiver complying with only the PPP, the communication apparatus and the communication method do not bring about an erroneous reception.  
         [0178]    The present invention provides a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended and a transient stop function of a packet transmission is carried out when a transmission FIFO included in a transmitter becomes at an underflow state, wherein if a receiver complying with the extended standard receives the signal, the transient stop function of the packet transmission is carried out, and wherein even if the signal is received by a receiver complying with only the PPP, the communication apparatus and the communication method do not bring about an erroneous reception.  
         [0179]    The present invention provides a communication apparatus for and a communication method of transmitting a signal in accordance with an extended standard in which PPP is extended and a transmission speed is reduced when a reception FIFO included in a transmitter becomes at an overflow state, wherein if a receiver complying with the extended standard receives the signal, the function of reducing the transmission speed is carried out, and wherein even if the signal is received by a receiver complying with only the PPP, the communication apparatus and the communication method do not bring about an erroneous reception.  
         [0180]    Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.