The present invention relates to a multi-directional multiplex communication system and an ISDN (Integrated Service Digital Network) servicing method therein and, more particularly, to an ISDN servicing method applied to a demand-assignment line setting system in which a base station and substations, each having a layer 2 terminating function, provide communication lines based on an ISDN basic interface (2B+D) by using transmission channels assigned to the respective subscribers in accordance with the generation/end of calls. 
A conventional multi-directional multiplex communication system will be described with reference to FIG.  9.  FIG.  9 shows the arrangement of a conventional multi-directional multiplex communication system for providing an ISDN basic interface.  Referring to FIG.  9, communication lines are always assigned between substations 3, 4, and 5 and a base station 2 regardless of whether communication is performed.  That is, a so-called pre-assign line setting scheme is used.  The ISDN basic interface is an interface which has two B channels (64 kbps) and one D channel (16 kpbs) on one user/network interface and can be commonly used for various types of telephone/non-telephone services.  In the conventional system, therefore, as shown in FIG.  9, communication lines equal in number to accommodated ISDN basic interfaces (N) are required between the base station and the substations. 
According to the prior art, communication lines equal in number to accommodated ISDN interfaces are required between the base station and the substations, and the communication lines are always assigned even when no communication is performed.  In addition, a user can use terminals such as a telephone set, a FAX unit, and a data terminal.  It is expected that the traffic in using these terminals is smaller than that in using a leased line.  For this reason, according to the conventional scheme of always assigning communication lines regardless of whether communication is performed, the number of subscribers accommodated is limited.  It is therefore difficult to quickly cope with an increase in the number of subscribers.  In addition, the communication line use efficiency becomes very low. 
It is an object of the present invention to provide a multi-directional multiplex communication system which improves transmission efficiency by effectively using communication lines using a demand-assignment scheme of assigning communication lines in accordance with the generation of calls, and an ISDN servicing method in the system. 
In order to achieve the above object, according to the present invention, there is provided a multi-directional multiplex communication system comprising a base station connected to an ISDN and at least one substation having a plurality of terminals connected thereto and serving to communicate with the base station through a radio channel, the substation including first layer terminating means for terminating layer 1 and layer 2 between the first layer terminating means and the terminal, and the base station including second layer terminating means for terminating layer 1 and layer 2 between the second layer terminating means and the ISDN, and channel control means for transmission channel assignment/release between the base station and the substation by a demand-assignment scheme in accordance with generation/end of a call.