Abstract:
A subscriber radio access system in which it becomes possible to facilitate communications for realizing high throughput and by which a radio LAN communication can be realized even if a radio link section is longer according to the CSMA protocol on Ethernet is provided. The subscriber radio access system includes plural transceiver terminals; and a base station linked to each of the plural transceiver terminals via a radio communication channel, wherein one of the plural transceiver terminals communicates to the other transceiver terminal with an Ethernet packet employing a carrier sense multiple access (CSMA) protocol, of which a packet length is corrected to correspond to a transmission delay time between the one and other transceiver.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a subscriber radio access system. More particularly, it relates to a subscriber radio access system on Ethernet for linking a radio base station and subscriber terminals through a radio transmission line. 
   2. Description of the Related Art 
   In recent years, it has been increased to have chances for using Internet in both of companies and each home because ISDN lines are now widespread among the general public, and providers appear to provide services for Internet with low costs. However, when considering infrastructures for communication in each home, only a telephone line is installed, and therefore, high-speed communications can not be facilitated in each home under the existing condition. 
   On the other hand, a conception of Fiber to the Home (FTTH) is held among enterprises related to a communication industry. However, a huge infrastructure investment is required, and therefore, it is considered that it will be in about 2010 to completely install the FTTH in each home. In this situation, a subscriber radio access system in which high speed transmission channels can be provided to each home by employing radio transmission channels has been expected much in recent years. 
   In here, a Point-multi point system for linking one radio base station and a plurality of subscriber terminals is generally employed as the subscriber radio access system. When considering the entire configuration, a cellular zone structure, as used in a mobile telephone system and a cellular phone system, is employed. 
   When forming a Point to multi-point connection by the use of radio communication, it is general to employ a Time Division Multiple Access (TDMA) system to transmit and receive communication data between the base station and the subscriber terminals with a predetermined frame signal. 
   In the case of using a TDMA system, it is possible to facilitate communications stably without collision of signals while each subscriber facilitates communications. However, when employing data communication, such as Internet communication in each home, there is much time when the subscriber does not facilitate communications. In this case, each subscriber terminal may keep a certain time interval in a predetermined frame so that transmission speed becomes constant in each subscriber terminal even if a few subscribers facilitate communications. 
   Accordingly, it is no use because there are many unemployed signal periods on the whole signal transmission. Further, even if the TDMA system can be realized, as it is not standardized currently, each subscriber terminal should be recognized as a special one, thereby increasing the cost and taking much time to expand the system. 
   To cover the shortage of the above-described problem, a wired LAN is standardized. On Ethernet, which is a bus type network for short distance transmission, i.e., a lower layer protocol in a stack of Transmission Control protocol/Internet protocol (TCP/IP), employed as a typical communication method on LAN, it is effective to employ a method for transmitting and receiving a packet only when facilitating communications according to Carrier Sense Multiple Access (CSMA) protocol. 
   However, as Ethernet follows a standard of the wired LAN, there is a limited transmission distance to make it possible to detect data collision even if the packet format has a minimum length. For example, in the case of 10 BaseT, i.e., a data communication speed of 10 Mbps employing a twist cable, the maximum length of the cable is limited to 100 m. Therefore, in the case where a radio transmission link section is longer than the limited length, it becomes impossible to detect data collision, thereby not facilitating the data communication. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a subscriber radio access system in which it becomes possible to facilitate communications for realizing high throughput. 
   It is an another object of the present invention to provide a subscriber radio access system by which a radio LAN communication can be realized even if a radio link section is longer than the required, according to the CSMA (Carrier Sensor Multiple Access) protocol on Ethernet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an explanatory diagram of a principle of a subscriber radio access system according to the present invention. 
       FIG. 2  shows a frame architecture of a packet transmitted on Ethernet. 
       FIG. 3  is a block diagram of a first embodiment according to the present invention. 
       FIG. 4  is an operational flowing chart corresponding to the first embodiment shown in  FIG. 3 . 
       FIG. 5  is a block diagram of a second embodiment according to the present invention. 
       FIG. 6  is an operational flowing chart corresponding to the second embodiment shown in  FIG. 5 . 
       FIG. 7  shows an other embodiment according to the present invention. 
       FIG. 8  shows an outline of a subscriber radio transmission system. 
       FIG. 9  is an explanatory diagram of data collision detection according to CSMA method on Ethernet. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments according to the present invention will be now explained in accompanying with the attached drawings. Throughout the following descriptions, the same reference numerals and symbols are used to denote and identify corresponding or identical components. 
   An outline of a subscriber radio transmission system is shown in  FIG. 8  for better understanding of the present invention before explanations of the present embodiments. A plurality of transceiver terminals (A to C)  30  to  32  within a radio link section are connected by radio transmission channels to a base station  2  linked to a center  1 . 
   In this structure, data collision detection according to the above-described CSMA method on Ethernet will be now considered in accompanying with  FIG. 9 . In  FIG. 9 , a transmitter TX  10  in a first transceiver terminal  30  transmits a communication request packet to a base station  2 . 
   In general, a receiver RX  20  of base station  2  receives the communication request packet and a transmitter TX  22  returns the packet through a repeater amplifying circuit  21  on base station  2 . Therefore, a receiver RX  11  receives the communication request packet returned from base station  2  on the first terminal  30 . 
   Then, a comparing circuit  12  compares the communication request packet transmitted from the own terminal and the communication request packet returned from base station  2  on the first transceiver terminal  30 . As the result, if the packet coincides with the returned packet, it is possible to facilitate communications between first transceiver terminal  30  and base terminal  2 . On the contrary, if it does not coincide with the returned packet, it is detected that the data collides with other data transmitted from other transceiver terminal or a wired terminal, not shown in  FIG. 9 , through base station  2 , which is illustrated with a dotted line in  FIG. 9 . 
   On Ethernet, a shortest packet length of 64 bytes is prescribed as an interval between forward and backward transmission via a transmission medium within a transmission distance to accord with a standard of a delay time for making it possible to detect packet data collision on the transmission medium according to the CSMA protocol. The delay time is an interval between forward and backward transmission in a distance 100 m on the case of employing 10BaseT as described above. 
   Therefore, in the case where the radio link section is shorter than 100 m, and the delay time of the packet follows the above-described standard, it is possible to detect data from transmitter  22  in base station  2  becomes large, and therefore, an own terminal or the other terminal transmits a next packet even if data collision occurs before detecting the data collision, thereby making it impossible to facilitate communications correctly. 
   The present invention overcomes such inconvenience as described above. A principle of the present invention will be now explained with  FIG. 1 . In  FIG. 1 , a packet communication is facilitated via radio transmission channels between a plurality of transceiver terminals  30  to  31  and base station  2  linked to a center  1 , similarly to the structure shown in  FIG. 8 . 
   In here, an interval between forward and backward transmission through a transmission medium is found between the end time of a packet transmitted from a transmitting terminal and the arrival time when a header of the packet reaches to a receiving terminal. 
   Accordingly, in the present invention, dummy bits I are added to a packet II of which length is prescribed according to the CSMA protocol on Ethernet to increase the packet length. Thereby, even if the radio link section is longer than the standard, the data collision can be correctly detected. 
     FIG. 2  shows a frame architecture of a packet transmitted on Ethernet which is applicable to the present invention. The frame length varies according to the data amount to be transmitted in octet. The frame includes a preamble section PA for establishing bit synchronization in its header. The frame is maximum 1518 octet and minimum 64 octet except a frame start delimiter section SFD. 
   A frame destination address DA and a source address SA are indicated in the frame. However, each node, i.e., each terminal, is distinguished by Medium Access Control (MAC) address, but not IP address. The MAC address is an ID number employed for transmitting data packet among network nodes according to a lower protocol (Ethernet and so on) of IP (internet protocol) address. 
     FIG. 3  is a block diagram of a first embodiment according to the present invention.  FIG. 4  shows an operational flowing chart corresponding to the block diagram of  FIG. 3 . Same reference numerals in ◯ are respectively attached to processing directions corresponding to  FIG. 3 . 
   In the embodiment shown in  FIG. 3 , when a packet is transmitted for long distance transmission according to CSMA protocol, dummy bits are added according to the distance between a transmitter and a receiver in order to detect data collision. 
   An operation I for starting up is performed as follows: An interface section  300  of terminal  30  reads out a MAC address from a LAN card of a personal computer  302  linked to terminal  30  when the power is ON. 
   Next, a radio transmitting section  301  transmits a distance measurement packet to find a delay time between transceiver terminal  30  and base station  2  (STEP S 1 ). The measured delay time is used as a memory delay time of the own terminal, and is accumulated in a memory of interface section  300 , not shown in  FIG. 3  (STEP S 2 ). 
   Further, the measured and accumulated value is registered to base station  2  as a delay time of the MAC address on the own terminal (STEP S 3 ). Base station  2  accumulates the value as the registered data in a delay memory, of the interface section  200 , not shown in  FIG. 3  (STEP S 4 ). 
   Accordingly, each delay time in each MAC address correspondingly registered from each terminal is stored in the delay memory. 
   As there is a possibility that dummy bits become longer than the packet data, it is required to make the length of the dummy bits I constant. In this case, it may be difficult to detect the data collision within the radio link section. 
   Therefore, even when the dummy bits I are added as described in  FIG. 1  on measuring the delay time of the own terminal, which is described above, an alarm is indicated on each terminal when it is difficult to detect the data collision. 
   Next, on an operation II for transmitting a packet, when a transceiver terminal  30  receives the Ethernet packet from personal computer PC  302 , transceiver terminal  30  searches the communication history from a built-in memory. If the terminal did not facilitate communications to the destination address in the past, transceiver terminal  30  requests base station  2  to notify a delay time information of transceiver terminal  31  to which communication is to be facilitated (STEP S 5 ). 
   On the contrary, base station  2  reads the delay time of transceiver terminal  31  having the corresponding MAC address from the delay memory and informs it to transceiver terminal  30  (STEP S 6 ). Interface section  300  of transceiver terminal  30  calculates a required delay time from the delay time information transmitted from the base station  2  and the delay time of the own terminal measured in advance according to the following equation:
 
Required delay time=(delay time of the own terminal)+(delay time between base station and each received terminal):
 
   Further, the length of the dummy bits I is calculated from the calculated required delay time, and the result is added to the Ethernet packet II (STEP S 7 ). Then, the dummy bit includes transmitted packet information and error correction information. The error correction information is for data of the Ethernet packet II. 
   On the other hand, when there is a history for facilitating communications, the dummy bits including the packet length information and the error correction information are added according to the bit length of the history. 
   The packet constituted in this way is transmitted through radio transmitting section  301  (STEP S 8 ). In response to this packet, the transmitted data is returned from base station  2  (STEP S 9 ). Therefore, transceiver terminal  30  compares the returned with the transmitted data (STEP S 10 ). As the result, if the returned coincides with the transmitted data, this transmission is successful. If it does not coincides with the transmitted data, it is judged that data collision occurs. 
   On the comparison of the transmitted data and the received data, it is possible to detect data collision by comparing either of both all bits and only one part of bits. 
   Next, on an operation III for receiving a packet, the receiving transceiver terminal  31  demultiplexes a transmission packet II and dummy bits I Error correction is executed according to the error correction information in the dummy bits I (STEP S 11 ). Then, the Ethernet packet after error correction is transmitted to the PC  312  (STEP S 14 ). 
   On the other hand, when the destination is a wired LAN  3  connected through base station  2 , base station  2  demultiplexes and removes the dummy bits I added within the radio link section (STEP S 12 ). Therefore, only the Ethernet packet II is transmitted to wired LAN  3 . 
   Further, when the terminals connected to wired LAN  3  transmit data to the radio transmission line, base station  2  adds the required dummy bits I to transmit to radio terminal  31  (STEP S 13 ). Then, the dummy bits I include the packet length information and the error correction information, similarly to the above described. 
     FIG. 5  shows a second embodiment according to the present invention. It shows an example on transmitting the packet by the radio transmission line for long distance transmission employing the CSMA method according to TCP/IP protocol, as the same as in the embodiment of  FIG. 3 . In this embodiment, a distance between the transmitting terminal and the receiving terminal is measured by employing a ping command. 
   The ping command is used for confirming whether or not the IP packet is correctly reached to the destination, which employs an echo function of Internet Control Message protocol (ICMP) to check whether or not the network protocol up to the IP layer works correctly. 
   The echo data includes an interval time from transmitting to returning the packet. Thereby, it is possible to obtain a delay time for transmitting the packet within the radio link section, as equivalent to the embodiment shown in  FIG. 3 . 
     FIG. 6  shows an operational flowing chart corresponding to  FIG. 5 . Same reference numerals in ◯ are respectively attached to processing directions corresponding to  FIG. 5 . 
   An interface section  300  of transmitting terminal  30  reads out the MAC address from the LAN card of PC  302  connected to transceiver terminal  30 , when the power is ON. 
   Next, transceiver terminal  30  checks whether or not there is any communication history (STEP S 20 ). When there is a communication history, the dummy bits I are added so that a packet has the packet length which is employed in the previous communication to make it possible to detect data collision according to the history information. Further, the error correction information is added to the packet section II as contents of the added dummy bits I (STEP S 22 ). 
   On the other hand, when it is detected that there is no communication history by checking the above-described ping command is transmitted to the designated transceiver terminal  31  through a radio transmission section  201  of base station  2  to execute it (STEP S 21 ). Thereby, it is possible to know the delay time of the packet according to the echo information transmitted from the designated transceiver terminal  31 . 
   Therefore, the dummy bits I to be added are calculated to be a packet length such that the packet can be reached within the delay time. Further, the error correction information of the data of the packet section II is added as contents of the added dummy bits I, which is similarly to the above-described case (STEP S 22 ). 
   The packet obtained in this way is transmitted through radio transmission section  301  (STEP S 23 ). In response to the packet, the data transmitted from base station  2  is returned from base station  2  (STEP S 24 ). Therefore, transceiver terminal  30  compares the returned data and the transmitted data (STEP S 25 ). As the result, if the returned back data coincides with the transmitted data, it is judged that the data transmission is successful. On the contrary, if the returned data does not coincide with the transmitted data, it is judged the data collision occurs. 
   On the comparison of the transmitted data and the received data, data collision can be detected by comparing either of all bits and only one part of the bits, similarly to the embodiment shown in  FIG. 3 . 
   Next, on an operation III for receiving a packet, receiving terminal  31  demultiplexes the transmitted packet II and the dummy bits I. Error correction is executed according to the error correction information in the dummy bits I (STEP S 26 ). Then, the Ethernet packet after error correction is transmitted to PC  312  (STEP S 27 ). 
   On the other hand, when the destination is the wired LAN  3  connected through base station  2 , base station  2  demultiplexes and removes the dummy bits I added in the radio link section (STEP S 28 ). Therefore, only the Ethernet packet II is transmitted to wired LAN  3 . 
   Further, when the data is transmitted from terminals connected to wired LAN  3  to the radio transmission line, base station  2  checks whether or not there is a communication history. If there is no communication history, base station  2  sends a ping command to transceiver terminal  30  and executes it (STEP S 29 ). 
   Then, the required dummy bits I are obtained, and the bits I are added to the Ethernet packet II and is transmitted, similarly to the step S 22  (STEP S 30 ). 
     FIG. 7  shows still another embodiment according to the present invention. In the above-described embodiments shown in  FIGS. 3 and 5 , the dummy bits corresponding to the calculated delay time are added. 
   In the embodiment of  FIG. 7 , no dummy bit is added. The shortest packet length corresponding to the communication is calculated from the calculated delay time to control a LAN card. That is, after transceiver terminal  30  calculates the delay time on the step S 7  of the operational flowing chart shown in  FIG. 4  or the step S 22  of the operational flowing chart shown in  FIG. 6 , interface section  300  of transmitting terminal  30  calculates the shortest packet length corresponding to the calculated delay time. 
   Next, the PC card of PC  302  is controlled such as the transmitted packet length to be the calculated shortest packet length. The length of transmitted packet sent at first from PC  302  is not controlled. Therefore, it is desired to add a preamble section to the packet in this embodiment as needed. 
   As the embodiments according to the present invention are explained in accompanying with the attached drawings, communications are facilitated only when there is data to be sent. Therefore, even when a few subscribers facilitates communications, it is possible to realize communicates with high throughput. 
   Further, even when subscriber terminals are added, it is not required to provide any special setting on a base station, as the same as in a LAN system. Therefore, it is easy to start the communication service and give flexibility for the system. A LAN card of the wired LAN can be employed on connecting to the personal computer of the subscriber terminal, and therefore, it is realized to provide a low-priced system. 
   The present invention may be embodied in other specific forms without departing from the sprit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.