Data terminal equipment and data transmission control method

A data terminal equipment which is coupled to an ISDN through an access line and uses a hierarchical protocol in conformity to an OSI reference standard model includes the following structural elements. A first data link protocol circuit provides a link access procedure balanced (LAPB) modulo-8 data link protocol. A second data link protocol circuit provides an LAPB modulo-128 data link protocol. A third data link protocol circuit provides a data link protocol based on a link access procedure for a data channel (LAPD). A protocol determining circuit determines a data link protocol to be used by referring to an address field and a control field of a cell set-up signal which is supplied from a second data terminal equipment. A data link protocol activating circuit selects and activates one of the first, second and third circuits on the basis of the result provided by the protocol determining circuit.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a data terminal equipment, and 
more particularly to a data terminal equipment which uses a hierarchical 
protocol based on an OSI standard reference model. The present invention 
further relates to a data transmission control method suitable for the 
above-mentioned data terminal equipment. 
Recently, there is a trend that a data terminal equipment which uses a 
digital line as a transmission line uses, as a transmission protocol, a 
hierarchical protocol based on an OSI (Open Systems Interconnection) 
standard reference model in order to establish data transmission between 
devices of different types. The hierarchical protocol consists of first to 
seventh layers arranged in this order from the lowermost layer. The first 
layer is a physical layer which controls a physical medium. The second 
layer is a data link layer that ensures communication up to an adjacency 
open type system. The third layer is a network layer which ensures 
communication up to an end open type system. The fourth layer is a 
transport layer which presents a transparent transmission line. The fifth 
layer is a session layer which controls conversation. The sixth layer is a 
presentation layer which unifies information presentation styles. The 
seventh layer is an application layer which presents system management and 
protocols for users. The OSI reference model is described in "DATA 
TRANSMISSION TECHNIQUES FOR PRACTITIONERS", NTT Suzuka Electric and 
Communication School Data Transmission Working Group, for example. 
Conventionally, a high level data link control (HDLC) procedure is widely 
used as a protocol for the second layer, i.e., the data link layer in the 
hierarchical protocol. Particularly, when a public digital network is used 
as a transmission line, link access procedure balanced data link protocols 
defined in the CCITT recommendation X.21 or X.75 are used as HDLC 
procedures. Hereinafter, the link access procedure balanced data link 
protocol is simply referred to as LAPB data link protocol. 
The LAPB data link protocol is used for data transmission of point-to-point 
type. In the LAPB data link protocol, data to be transmitted is arranged 
to a predetermined frame format, and is transmitted for every frame. A 
receiver determines whether a data error is contained in data for every 
frame. When a data error is found, the receiver can request the 
transmitter to retransmit the frame related to the found data error. Thus, 
the receiver can obtain data having no data error. 
The receiver can put together consecutive frames which have continuously 
been received and subject the consecutive frames to the error checking 
procedure. According to the LAPB data link protocol, a maximum number of 
frames which can continuously be received is limited. A maximum of frames 
is seven in a standard mode, and 127 in an extended mode. In the following 
description, an LAPB modulo-8 data link protocol indicates the case where 
a maximum of frames which can consecutively be received is seven, and an 
LAPB modulo-128 data link protocol indicates the case where a maximum of 
frames which can consecutively be received is 127. 
On the other hand, recently, integrated services digital networks 
(hereinafter simply referred to as ISDNs) are being built. In ISDNs, a 
link access procedure for D-channel is used as a data link protocol for a 
signal channel (D channel) for performing a line control such as a call 
setting up procedure. Hereinafter, this link access protocol for D-channel 
is simply referred to as LAPD data link protocol. The LAPD data link 
protocol is based on the LAPB data link protocol and is obtained by adding 
new functions necessary for ISDNs to execute the LAPB data link protocol. 
The LAPD data link protocol can be applied to D channel, and further will 
be applied to information channels of ISDNs (B channels) and data link 
protocols used in the existing public digital networks. 
From the above-mentioned viewpoints, there is a possibility that protocols 
of the three different types, that is, the LAPB modulo-8 data link 
protocol, the LAPB modulo-128 data link protocol and the LAPD data link 
protocol may be used together as data link protocols for information 
channels. In other words, data terminal equipments based on the different 
data link protocols may be used together. 
ISDNs are intended to integrate functions provided by conventional public 
switched telephone networks (PSTNs), circuit switched public data networks 
(CSPDN) and packet switched data networks (PSDNs) in the future and will 
be used in place of those networks. However, it is estimated that it takes 
long time the conventional networks to be replaced with IDSNs. As a 
result, ISDNs, PSTNs, CSPDNs and PSDNs by be used together until the 
replacement is completed. 
It is basically necessary for the data terminal equipment itself to have an 
ISDN-based transmission function in order to be coupled to ISDN. In 
actuality, it is possible to connect a conventional data terminal 
equipment to ISDN by use of a terminal adapter which realizes ISDN 
transmission functions. The terminal adapter realizes a signal control and 
a packet communication control which use the signal channel (D channel) of 
ISDN, and low-level layer functions on information channel thereof. 
Generally, a data terminal equipment has transmission functions provided by 
high-level layers, which are used when connected to existing networks. 
Therefore, even when the data terminal equipment is connected to ISDN 
through the terminal adapter, the high-level layer transmission functions 
are used. 
As data terminal equipments coupled to digital line networks, there is a 
facsimile machine or a telematic terminal such as a Japanese teletex 
device. When a group-4 facsimile machine uses, as a transmission line, 
CSPDN or PSDN, the transmission function defined in th CCITT 
recommendation T.70 is used. 
On the other hand, according to the CCITT recommendation T.90, the 
transmission function defined in the ISO standard ISO8208 is a standard 
function in ISDN's. The CCITT recommendation T.90 sanctions using the 
CCITT recommendation T.70 transmission function as an optional 
transmission function. 
As described above, there are plural protocols for each layer With respect 
to a data terminal equipment connected to ISDN. Therefore, it is 
impossible to perform data transmission between data terminal equipments 
which have the same terminal function but the different transmission 
functions. This is inconvenient. 
SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a novel and useful 
data terminal equipment in which the aforementioned disadvantages are 
eliminated. 
A more specific object of the present invention is to provide a data 
terminal equipment capable of communicating with other data terminal 
equipments based on various different protocols. 
The above-mentioned objects of the present invention can be achieved by a 
data terminal equipment which is coupled to an integrated services digital 
network (ISDN) through an access line and uses a hierarchical protocol in 
conformity to an OSI reference standard model, comprising first data link 
protocol means for providing a link access procedure balanced 
(LAPB)modulo-8 data link protocol, second data link protocol means for 
providing an LAPB modulo-128 data link protocol, and third data link 
protocol means for providing a data link protocol based on a link access 
procedure for a data channel (LAPD). The data terminal equipment further 
comprises protocol determining means for determining a data link protocol 
to be used by referring to an address field and a control field of a call 
set-up signal which is supplied from a second data terminal equipment, and 
data link protocol activating means, coupled to the first, second and 
third means and the protocol discriminating means, for selecting and 
activating one of the first, second and third means on the basis of the 
result provided by the protocol determining means. 
The aforementioned objects of the present invention can also be achieved by 
a data terminal equipment which is coupled to an integrated services 
digital network (ISDN) through an access line and uses a hierarchical 
protocol in conformity to an OSI reference standard model, comprising 
protocol control means for providing a plurality of protocols for each of 
at least a data link layer (layer 2) and a network layer (layer 3) of the 
hierarchical protocol, and protocol storage means for storing information 
on a protocol which is related to each of at least the data link layer and 
the network layer and which is available in a second data terminal 
equipment with which the data terminal equipment wants to communicate. The 
data terminal equipment further comprises protocol learning means for 
discriminating a transmission procedure in the second data terminal 
equipment for each of at least the data link layer and the network layer 
by referring to a procedure signal transferred between the data terminal 
equipment and the second data terminal equipment at the time of setting up 
a call and for storing the discriminated transmission procedure in the 
protocol storage means, and protocol setting means for setting one of the 
plurality of protocols for each of at least the data link layer and the 
network layer in the protocol control means by referring to the 
information stored in the protocol storage means. 
Another object of the present invention is to provide a transmission 
control method for controlling the data terminal equipment. 
This object can be achieved by a data transmission control method adapted 
to a data terminal equipment which uses a hierarchical protocol and has, 
as a transmission function on an information channel, a first transmission 
procedure in conformity to the ISO standard ISO8208 and a second 
transmission procedure in conformity to the CCITT recommendation T.70. The 
method comprises the steps of receiving a procedure signal related to a 
network layer (layer 3) supplied from a calling terminal when an 
information channel related to the network layer is established, the 
procedure signal having a layer-3 header preceding procedure information 
on the layer 3, extracting the layer-3 header from the received procedure 
signal, discriminating a transmission function on the information channel 
available in the calling terminal by referring to the extracted layer-3 
header, and selecting one of the first and second transmission procedures 
from the discrimination result. 
The aforementioned objects of the present invention can also be achieved by 
a data transmission control method adapted to a data terminal equipment 
which uses a hierarchical protocol and has, as a transmission function on 
an information channel, a first transmission procedure in conformity to 
the ISO standard ISO8208 and a second transmission procedure in conformity 
to the CCITT recommendation T.70, wherein the data terminal equipment 
further has a memory which registers information on a transmission 
function available in each of other terminals. The method comprises the 
steps of determining whether information on a destination terminal with 
which the data terminal equipment serving as a calling terminal wants to 
communicate has been registered in the memory, reading out the information 
related to the destination terminal stored in the memory when it is found 
that information on the destination terminal has been registered, 
determining whether the read-out information shows the first transmission 
procedure in conformity to the ISO standard ISO8208 or the second 
transmission procedure in conformity to the CCITT recommendation T.70, 
selecting one of the first and second transmission procedures so as to 
coincide with that of the destination terminal on the basis of the 
determined result, and setting up a call to the destination terminal on 
the basis of the selected one of the first and second transmission 
procedures. 
Additional objects, features and advantages of the present invention will 
become apparent form the following detailed description when read in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description is given of a preferred embodiment of the present invention 
with reference to FIG. 1A. 
Referring to FIG. 1A, there is illustrated an ISDN data terminal equipment 
according to a preferred embodiment of the present invention. A subscriber 
line or access line LN is of passive bus type suitable for the basic 
interface of ISDN, and is connected to a digital service unit DSU, which 
is also called a network terminator NT. It is possible to connect a 
maximum of eight data terminal equipments TM to the access line LN. As 
shown in FIG. 1B, the network terminator NT or digital service unit DSU is 
coupled to an ISDN. 
Each of the data terminal equipments TM includes an ISDN interface circuit 
1, which is physically connected to the access line LN and functions to 
separate and combine signals on the signal channel (D channel) and signals 
on the information channel (B channel) in ISDN. A signal on the D channel 
is exchanged between the ISDN interface circuit 1 and a D-channel 
transmission controller 2. A signal on the B channel is exchanged between 
the ISDN interface circuit 1 and a B-channel layer-2 (second layer) 
discrimination circuit 3. 
The D-channel transmission controller 2 executes a signal control procedure 
and a data transmission control procedure for D channel. A variety of 
information necessary for those procedures is exchanged between the 
D-channel transmission controller 2 and a system controller 4. 
The B-channel layer-2 discrimination circuit 3 executes, at the time of 
setting up a call, the following procedure on the basis of the contents of 
a link setting up signal which is transmitted from a calling terminal 
after line control on the D channel is completed. That is, the B-channel 
layer-2 discrimination circuit 3 discerns whether the protocol of the 
layer 2 related to the B channel is the aforementioned LAPB modulo-8 data 
link protocol, LAPB modulo-128 data link protocol, or LAPD data link 
protocol. When the discerned result indicates the LAPB modulo-8 data link 
protocol, the B-channel layer-2 discrimination circuit 3 activates an LAPB 
(modulo-8) controller 5, which controls data transmission after activated. 
When the discerned result indicates the LAPB modulo-128 data link 
protocol, the B-channel layer-2 discrimination circuit 3 activates an LAPB 
(modulo-128) controller 6, which controls data transmission after 
activated. When the discerned result indicates the LAPD data link 
protocol, the B-channel layer-2 discrimination circuit 3 activates an LAPD 
controller 7, which controls data transmission after activated. 
A B-channel high-order protocol transmission controller (hereinafter simply 
referred to as controller) 8 executes transmission procedures related to 
high-order protocol layers which consists of the third layer and the 
higher-order layers on the B channel. The controller 8 exchanges data with 
the ISDN (FIG. 1B) through one of the protocol controllers 5, 6 and 7 
which has been activated at this time. Further, the controller 4 exchanges 
data with a system controller 4. The system controller 4 controls an 
operation of the illustrated data terminal equipment TM. An input/output 
device 10, a console/display device 11 and an external storage device 12 
are connected to the system controller 4. The input/output device 10 
inputs and outputs transmission data. The console/operation device 11 
inputs necessary information into the data terminal equipment TM and 
displays necessary information. The external storage device 12 stores data 
including transmission data. Further, the system controller 4 executes a 
control procedure for the highest-order protocol for data transmission 
which uses the B and D channels. 
When the data terminal equipment TM sets up a call, the LAPB modulo-8 data 
link protocol is used as the data link protocol for B channel. 
A description is given of framed signals used in the LAPB and LAPD data 
link protocols. FIG. 2A is a diagram of the frame format of a signal based 
on the LAPB data link protocol. As illustrated, the LAPB frame format 
consists of data equal to one byte (1 octet) having a predetermined bit 
pattern. Details of the LAPB frame format are as follows. A flag F1 
indicates the beginning of the frame. An address field AF has one-byte 
data indicating the other equipment. A control field CF has one- or 
two-byte data indicating the kind of signal. An information field IF is of 
an arbitrary length. In the information field IF, parameters dependent on 
the kind of signal or transmission information are stored. A frame check 
sequence FCS has two-byte data which is obtained by applying a generator 
polynomial to data between the first bit of the address field AF and the 
last bit of the information field IF. A cyclic redundancy check (CRC) code 
is applicable to the frame check sequence FCS, for example. A flag F2 is 
formed by the same bit pattern as the flag F1, and indicates the end of 
the frame. 
FIG. 2B shows the frame format of a signal based on the LAPD data link 
protocol. As illustrated, the LAPD frame format is basically identical to 
the LAPB frame format shown in FIG. 2A. The LAPD frame format uses an 
address field AF' extended to two bytes. A service access point identifier 
SAPI for discriminating information transfer services presented to a 
higher-order layer by the layer 2 from one another is set by use of six 
high-order bits of the first one byte of the address field AF'. A C/R bit 
for discerning whether the content of the signal is a command or a 
response, is arranged in the seventh bit of the first one byte. An 
extended bit EA of a value of "0" which indicates that the address field 
AF is an extended field of LAPB is arranged in the eighth bit of the first 
one byte. A terminal end point identifier TEI for discriminating a 
plurality of terminals accommodated in the same access line from one 
another is arranged in seven high-order bits of the second one byte of the 
LAPD frame format. An extended bit of a value of "1" which indicates the 
end of the address field AF is arranged in the eighth bit in the second 
one byte. 
The LAPB frame format is used in transmission of point-to-point type. 
Therefore, the address of a calling (source) terminal is fixed to an 
address of 03h (h represents hexadecimal digit), and the address of a 
called (destination) terminal is fixed to an address of 01h. The address 
of the other terminal is set in the address field AF of signal to be 
transmitted. The standard procedure of the LAPB data link protocol is the 
LAPB modulo-8 data link protocol, and the extended procedure is the LAPB 
modulo-128 data link protocol. Therefore, when a data terminal equipment 
which uses the LAPB modulo-8 data link protocol sets up a call, a call 
set-up signal which is transmitted for setting a link after the 
establishment of B channel is a set asynchronous balanced mode signal 
(hereinafter simply referred to as SABM signal), which is set to a bit 
pattern of 3Fh. When a data terminal equipment which uses the LAPB 
modulo-128 data link protocol sets up a call, a call set-up signal which 
is transmitted for setting a link after the establishment of B channel is 
a set asynchronous balance mode extended signal (hereinafter simply 
referred to as SABME signal), which is set to a bit pattern of 7Fh. It is 
noted that no information field IF is added to the SABM signal or SABME 
signal. 
As a result, when the data terminal equipment which uses the LAPB modulo-8 
data link protocol sets up a call, as shown in FIG. 3A, the call set-up 
signal which is transmitted for setting a link after the establishment of 
B channel is such that data 01h and 3Fh are set in the address field AF 
and the control field CF, respectively, and there is no information field 
IF. When the data terminal equipment which uses the LAPB modulo-128 data 
link protocol sets up a call, as shown in FIG. 3B, the call set-up signal 
which is transmitted for setting a link after the establishment of B 
channel is such that data 01h and 7Fh are set in the address field AF and 
the control field CF, respectively, and there is no information field IF. 
On the other hand, in the LAPD data link protocol, a calling terminal sends 
the SABME signal as a call set-up signal. At this time, since the signal 
is for call set-up, the access point identifier SAPI is "000000", and the 
C/R bit is 0. Therefore, when the data terminal equipment which uses the 
LAPD data link protocol sets up a call, as shown in FIG. 3C, the call 
set-up signal which is transmitted for setting a link after the 
establishment of B channel is such that data 00h and 7Fh are set in the 
address field AF' and the control field CF, respectively, and there is no 
information field IF. The terminal end identifier TEI (1 in this case) 
which is set in the other terminal, is set in the terminal end identifier 
TEI. 
The B-channel layer-2 discrimination circuit 3 makes a decision on the kind 
of data link protocol related to the calling terminal in accordance with a 
procedure shown in FIG. 4. In the link setting and link resetting stages, 
when the first data is received (step 101), the B-channel layer-2 
discrimination circuit 3 draws data from the address field which is of the 
first byte thereof, and determines whether the drawn data is 01h (step 
102). When the result in step 102 is YES, the data link protocol being 
used is the LAPB data link protocol. Then, the B-channel layer-2 
discrimination circuit 3 determines whether the content of the control 
field CF which is the second byte of the data received in step 101 is 3Fh 
(step 103). When the result in step 103 is YES, the content of the call 
set-up signal which has been received is the SABM signal for the LAPB data 
link protocol, and therefore the calling terminal uses the LAPB modulo-8 
data link protocol. Thus, the B-channel layer-2 discrimination circuit 3 
activates the LAPB modulo-8 controller 5 (step 104). Then data 
transmission is carried out under control by the activated LAPB modulo-8 
controller 5. 
On the other hand, when the result in step 103 is NO, the B-channel layer-2 
discrimination circuit 3 discerns whether the content of the control field 
CF is 7Fh (step 105). When the result in step 105 is YES, the content of 
the call set-up signal which has been received is the SABME signal for the 
LAPB data link protocol, and therefore the calling terminal uses the LAPB 
modulo-128 data link protocol. Thus, the B-channel layer-2 discrimination 
circuit 3 activates the LAPB modulo-128 controller 6 (step 106). Then data 
is transmitted under control by the LAPB modulo-128 controller 6. 
When the result in step 102 is NO, the B-channel layer-2 discrimination 
circuit 3 discerns whether the content of the address field AF is 00h 
(step 107). When the result in step 107 is YES, the B-channel layer-2 
discrimination circuit determines whether the content of the control field 
CF which is the fourth byte data is 7Fh (step 108). When the result in 
step 108 is YES, the content of the call set-up signal which has been 
received is the SABME signal for the LAPD data link protocol and therefore 
the calling terminal uses the LAPD data link protocol. Thus, the B-channel 
layer-2 discrimination circuit 3 activates the LAPD controller 7 (step 
109). Then data is transmitted under control by the LAPD controller 7. 
When the result in step 105, 107 or 108, the received signal is ignored 
since it does not relate to the procedure for setting a data link. 
In the above-mentioned manner, according to the first embodiment, the 
B-channel layer-2 protocol to be used is determined on the basis of the 
contents of the address field and control field of the call set-up signal 
which has been received from the calling terminal when setting up a call 
related to the B-channel. Thus, it is possible to use a protocol suitable 
for the calling terminal and then suitably execute the data transmission 
procedure. 
That is, when the data terminal equipment TM is called, the D-channel 
transmission controller 2 executes the line control which uses the ISDN 
and the D-channel so that a communication path is established. At this 
time, when a B-channel is established as a communication path, the 
D-channel transmission controller 2 activates the B-channel layer-2 
discriminating circuit 3. Thereby, the B-channel layer-2 discrimination 
circuit 3 is switched to a waiting state where it waits for a call set-up 
signal supplied from a calling terminal. When a call set-up signal is 
received, the B-channel layer-2 discrimination circuit 3 executes the 
aforementioned procedure, and activates suitable one of the LAPB 
(modulo-8) controller 5, the LAPB (modulo-128) controller 6 and the LAPD 
controller 7 on the basis of the discrimination results. Then data is 
transmitted under control of the selected one of the data link protocol 
controllers. When the established data link is released and it is desired 
to establish a data link again, the data link protocol controller suitable 
for the data link protocol related to the calling terminal is activated in 
the same manner. 
When the data terminal equipment TM shown in FIG. 1A sets up a call, it can 
identify the kind of layer-2 protocol for the B-channel which is used in 
the other terminal by the following procedure. The data terminal equipment 
TM sends the destination terminal the SABM (SABME) signal which is the 
call set-up signal for the layer-2 protocol. When the called terminal can 
operate in the layer-2 protocol set in the calling terminal equipment TM, 
the called terminal sends back an unnumbered acknowledge signal UA, as 
shown in FIG. 5A. Thereby the layer 2 is established, and the function 
provided by a higher-order layer (layer 3) can be activated. On the other 
hand, when the called terminal cannot respond to the received SABM (SABME) 
signal, it sends back a signal DM, as shown in FIG. 5B, which indicates 
that the destination terminal is switched to a disconnect mode. In this 
state, the called terminal waits for the source terminal TM to send the 
SABM or SABME signal indicating a layer-2 protocol in which the called 
terminal can operate. When the source terminal TM sends the layer-2 
protocol to which the called terminal can respond, the layer 2 is 
established and then the function provided by the higher-order layer is 
activated. 
A description is given of an example of the above-mentioned layer-2 
discrimination procedure which is to be executed at the time of setting up 
a call, by referring to FIG. 6. 
The B-channel layer-2 controller 3 (FIG. 1A) activates the LAPB 
(modulo-128) controller 6 (step 201). The activated LAPB (modulo-128) 
controller 6 sends the SABME signal to the destination terminal through 
the B-channel layer-2 discrimination circuit 3 and the ISDN interface 
circuit 1 (step 202). Then the LAPB (modulo-128) controller 6 waits for 
the signal UA or DM sent from the called terminal (a NO loop consisting of 
steps 203 and 204). When the called terminal can use the LAPB (modulo-128) 
data link protocol as a layer-2 protocol and the called terminal sends 
back the signal UA, the result in step 203 is YES. The LAPB (modulo-128) 
controller 6 terminates the layer-2 protocol setting procedure (step 205) 
and activates the B-channel high-order protocol transmission controller 8. 
Thereby, protocols related to the higher-order layers are sequentially 
made active. 
When the called terminal cannot respond to the LAPB (modulo-128) data link 
protocol and therefore sends back the signal DM, the result in step 204 is 
YES. In this case, the B-channel layer-2 discrimination circuit 3 stops 
the LAPB (modulo-128) controller 6 (step 206), and alternatively activates 
the LAPB (modulo-8) controller 5 (step 207). Then the B-channel layer-2 
discrimination controller 3 has the LAPB (modulo-8) controller 5 send the 
SABM signal (step 208), and waits for the signal UA or DM sent from the 
called terminal (a NO loop consisting of steps 209 and 210). When the 
called terminal can use, as a layer-2 protocol, the LAPB (modulo-8) data 
link protocol and therefore sends back the signal UA, the result in step 
209 is YES. In this case, the B-channel layer-2 discrimination circuit 3 
terminates the layer-2 protocol setting procedure, and activates the 
B-channel high-order protocol transmission controller 8. 
When the called terminal cannot respond to the LAPB (modulo-8) data link 
protocol and therefore sends back the signal DM, the result in step 210 is 
YES. In this case, the B-channel layer-2 discrimination circuit 3 stops 
the LAPB (modulo-8) controller 5 (step 211), and activates the LAPD 
controller 7 (step 212). Then the LAPD controller 7 sends the SABME signal 
so that the layer-2 protocol is started (step 213). Then the B-channel 
layer-2 discrimination circuit 3 waits for the signal UA or DM sent from 
the called terminal (a NO loop consisting of steps 214 and 215). When the 
called terminal can use, as a layer-2 protocol, the LAPD data link 
protocol and therefore sends the signal UA, the result in step 214 is YES. 
In this case, the procedure returns to step 205 and is then terminated. 
Then protocols related to the high-order layers are sequentially started. 
On the other hand, when the called terminal cannot use, as a layer-2 
protocol, the LAPD data link protocol and sends the signal DM, the result 
in step 215 is YES. Then the B-channel layer-2 discrimination circuit 3 
stops the LAPD controller 7 (step 216). In this case, it is impossible to 
set the layer-2 protocol, and therefore the procedure is terminated as 
error (step 217). 
In the aforementioned manner, the source (calling) terminal identifies the 
layer-2 protocol which is available in the called terminal by referring to 
the signal which is sent back from the called terminal during a call 
set-up procedure for B-channel. Thereby, it is possible to use the 
protocol suitable for the called terminal and execute the data 
transmission procedure based on the determined data link protocol. 
That is, when the data terminal equipment TM sets up a call to a 
destination terminal, the D-channel transmission controller 2 executes the 
line control which uses the ISDN and the D-channel so that a communication 
path is established. At this time, when a B channel is established as a 
communication path, the D-channel transmission controller 2 activates the 
B-channel layer-2 discrimination circuit 3. The activated B-channel 
layer-2 discrimination circuit 3 discriminates the data link protocol to 
be used by executing the aforementioned procedure shown in FIG. 6. Then 
the B-channel layer-2 discrimination circuit 3 activates, based on the 
discrimination result, one of the LAPD (modulo-8) controller 5, the LAPB 
(modulo-128) controller 6 and the LAPD controller 7. Then, data is 
transferred through the selected data link protocol controller. Thus, the 
data link protocol controller suitable for the called terminal is 
activated so that data can suitably be transferred. 
A description is given of a second preferred embodiment of the present 
invention with reference to FIG. 7. In FIG. 7, those parts which are the 
same as those shown in the previous figures are given the same reference 
numerals. 
The subscriber line LN is of passive bus type suitable for the basic 
interface of ISDN, and is connected to the digital service unit DSU 
(network terminator NT). It is possible to connect a maximum of eight data 
terminal equipments TM to the access line LN. 
A group-4 facsimile machine FX serving as a data terminal equipment TM is 
constructed as follows. The ISDN interface circuit 1 is physically 
connected to the access line LN and functions to separate and combine 
signals on the signal channel (B channel) and signals on the information 
channel (B channel) in ISDN. A signal on the D channel is exchanged 
between the ISDN interface circuit 1 and the D-channel transmission 
controller 2. A signal on the B channel is exchanged between the ISDN 
interface circuit 1 and the B-channel layer-2 discrimination circuit 3. 
The D-channel transmission controller 2 executes a signal control procedure 
for D channel and a data transmission procedure. Information necessary for 
those procedures is exchanged between the D-channel transmission 
controller 2, a B-channel layer-2 controller 21, a B-channel layer-3 
controller 22, a B-channel high-order layer controller 23, and a system 
controller 24. 
The B-channel layer-2 controller 21 controls B-channel layer-2 
transmission. Signals related to B-channel are exchanged between the 
B-channel layer-2 controller 21 and the ISDN through the ISDN interface 
circuit 1. Further, signals related to a higher-order layer (third layer) 
are exchanged between the B-channel layer-2 controller 21 and the 
B-channel layer-3 controller 22. 
The B-channel layer-3 controller 22 controls B-channel layer-3 
transmission. Signals are exchanged between the B-channel layer-3 
controller 22 and the ISDN through the B-channel layer-2 controller 21. 
Signals related to a higher-order layer, that is, the layer 4, are 
exchanged between the B-channel layer-3 controller 22 and the B-channel 
high-order layer controller 23. 
The B-channel high-order layer controller 23 executes a B-channel 
transmission procedure related to high-order protocol layers equal to the 
fourth layer (layer 4) and the higher-order layers. The B-channel 
high-order layer controller 23 exchanges signals with the ISDN through the 
B-channel layer-3 controller 22, and further exchanges signals with a 
system controller 24. Each of the B-channel layer-3 controller 22 and the 
B-channel high-order layer controllers 23 has both the CCITT 
recommendation T.70 transmission function and the ISO standard ISO8208 
transmission function. 
The system controller 24 controls an operation of the group-4 facsimile 
machine FX. A scanner 25, a plotter 26 and a coder/decoder 27 are 
connected to the system controller 24. The scanner 25 reads a document to 
be scanned at a predetermined resolution level. The plotter 8 records an 
image on a recording medium (paper for example) at a predetermined 
resolution level. The coder/decoder 27 encodes and compresses an image 
signal derived from the scanner 25, and expands and decodes a coded and 
compressed image signal so that the original image signal can be 
generated. An external storage device 28 and a console/display device 29 
are connected to the system controller 24. The external storage device 28 
stores encoded and compressed data, and various data which the system 
controller 24 needs. The console/display device 29 inputs necessary data 
in the group-4 facsimile machine FX and displays necessary information. 
The system controller 24 controls a highest-order data transmission 
protocol which uses the B and D channels. 
A description is given of a fundamental data transmission procedure which 
is performed in the ISDN data terminal equipment TM such as the 
illustrated group-4 facsimile machine FX with reference to FIG. 8. 
First, a calling terminal sends a call set-up message SETUP to the ISDN and 
thus requests a call set-up to a destination terminal. The ISDN sends the 
call set-up message SETUP to the specified destination terminal and 
thereby calls out the destination terminal. Further, the ISDN sends the 
calling terminal a call proceeding message CALL PROC for informing the 
calling terminal of the status of call set-up. 
When the called terminal detects the call-in and is in a communication 
active state, it sends the ISDN an alerting message ALERT. Then, the ISDN 
sends the calling terminal the alerting message ALERT so that it lets the 
calling terminal know that calling (ringing) to the called terminal has 
been started. When the called terminal replies to the call, it sends the 
ISDN a connect message CONN. Then the ISDN sends the calling terminal the 
connect message CONN to thereby let the calling terminal know the called 
terminal has accepted the call. The ISDN sends the called terminal a 
connect acknowledge message CONN ACK. At this time, an information channel 
for data transmission between the calling terminal and called terminal is 
established. 
Thereby, data can be exchanged between the calling terminal and called 
terminal according to the respective transmission control procedures. For 
example, the group-4 facsimile machine FX executes image information 
transmission based on the group-4 facsimile transmission control 
procedure. 
When data transmission is completed, the calling terminal sends the ISDN a 
disconnect message DlSC so that the information channel is requested to be 
released from the connected state. Then the ISDN sends the called terminal 
the disconnect message DISC so that it lets the called terminal know the 
information channel is cleared. Thereby, the called terminal sends back 
the ISDN a release message REL which lets the ISDN know the completion of 
channel disconnection. Then, the ISDN sends the calling terminal the 
release message REL. When the information channel is cleared by the 
calling terminal, it sends the ISDN a release completion message REL COMP 
so that the release of the information channel has been effected. Thus, 
the information channel between the calling terminal and the called 
terminal is completely released. 
When the ISO standard ISO8208 is used for providing a transmission function 
on information channel, a transmission procedure to be executed when 
starting transmission is as shown in FIG. 9. Referring to FIG. 9, a 
calling terminal sends the ISDN the call set-up message SETUP. Then the 
ISDN sends a called terminal the call set-up SETUP. When the called 
terminal replies to the call, it sends the ISDN the connect message CONN. 
Then the ISDN sends the calling terminal the connect message CONN. Then 
the calling terminal sends the ISDN the connect acknowledge message CONN 
ACK. Then the ISDN sends the called terminal the connect acknowledge 
message CONN ACK. Thereby, an information channel between the calling 
terminal and the called terminal is established. 
The calling terminal sends the ISDN the call set-up signal SABM related to 
the layer 2 for setting a link through the ISDN. When the called terminal 
can accept this call, it sends the signal UA to the calling terminal 
through the ISDN. Thereby, the layer 2 is established. Thereafter in order 
to establish the end-to-end communication in the layer 3, the calling 
terminal sends the ISDN the restart request packet SQ, which is supplied 
to the called terminal as a signal SI. When the called terminal accepts 
the request, it sends back the request confirmation packet SF through the 
ISDN. Thereby, the layer 3 is established. Thereafter, the calling 
terminal sends the ISDN a signal CR, which is sent to the called terminal 
as a signal CN. When the called terminal accepts the request, the called 
terminal sends back the ISDN a signal CA, which is sent to the calling 
terminal CN. Thereafter, in order to establish the transport layer, i.e., 
the layer 4, the calling terminal sends a transport connect request signal 
TCR to the called terminal through the ISDN. When the called terminal 
accepts the request, it sends back the ISDN a transport connect accept 
signal TCA through the ISDN. Thereby, the layer 4 is established. After 
that, a conventional transmission procedure related to higher-order layers 
is executed so that data transmission is carried out. 
When the CCITT recommendation T.70 is used for providing a transmission 
function on information channel, the procedures indicated by (*) are 
omitted. That is, the calling terminal sends the call set-up signal SABM 
to the destination terminal through the ISDN. When the called terminal 
accepts this request, it sends back the calling terminal the 
acknowledgement signal ACK through the ISDN. Thereby, the layer 2 is 
established. It is noted that there is no recommendation with respect to 
the layer 3. For this reason, in order to establish the layer 4 which is 
the transport layer, the calling terminal sends the called terminal the 
transport connect request signal TCR through the ISDN. When the called 
terminal accepts the request, it sends back the calling terminal the 
transport connect acknowledge signal TCA through the ISDN. Thereby, the 
layer 4 is established. Thereafter, procedures related to higher-order 
layers are executed. 
It can be seen from the above-mentioned description that the ISO standard 
ISO8208 differs from the CCITT recommendation T.70 in procedures related 
to the layer 3 or higher-order layers. As a result of this difference, the 
called terminal can identify the transmission function being used by 
referring to the contents of the signal which has first been received 
after starting the transmission procedure for the layer 3. 
FIG. 10A shows a format of the signal TCR in conformity to the CCITT 
recommendation T.70, and FIG. 10B shows a format of the signal SQ in 
conformance with the ISO standard ISO8208. 
The signal TCR includes a flag field F, an address field A and a control 
field C in this sequence from the beginning thereof. The control field C 
is followed by 01h and 00h which form a header of the layer 3, which 
precedes a signal block related to the layer 4. The signal TCR ends with 
the flag check sequence FCS and the flag F related to the layer 2. The 
block related to the layer 4 includes an indicator field LI which 
indicates the entire length of the signal block related to the layer 4, 
and a block type field E0h which represents the signal TCR. Further, the 
signal block related to the layer 4 includes a function sign field which 
contains 00h, 00h, predetermined transmission source reference information 
(other than 00h) and 00h, and a parameter field which indicates 
information such as the block size. 
The signal SQ shown in FIG. 10B includes a flag field F which is a header 
of the layer 2, an address field A, and a control field C in this order 
from the beginning thereof. The control field C is followed by a general 
format identifier GFI which is a header related to the layer 3, a logical 
channel group number LCGN, a logical channel number LCN, a packet type 
identifier of FBh representing the signal SQ, a disconnect cause field of 
80h (DTE restart), and a diagnosis code field of 00h (no additional 
information). A flag check sequence FCS and a flag F are arranged in the 
end of the signal SQ. 
FIG. 11A illustrates a format of the general format identifiers GFI for 
modulo-8 and modulo-128. FIG. 11B illustrates a format of the logical 
channel group number LCGN which indicates group numbers of from 0 to 15. 
FIG. 11C illustrates a format of the logical channel number LCN which 
represents a number of between 0 and 255. FIG. 11D illustrates a format of 
the packet type identifier FBh. 
As described above, the signal TCR is different from the signal SQ in the 
contents of the header related to the layer 3. Therefore, it is possible 
to discriminate the type of the received signal by referring to the header 
related to the layer 3. 
When the call setting up procedure is started and the data link for the 
layer 2 is established by the B-channel layer-2 controller 21 of the 
called terminal, the B-channel layer-3 controller 22 executes a procedure 
shown in FIG. 12 so that it can identify transmission functions related to 
the layer 3 and higher-order layers, and sends the discrimination results 
to the B-channel high-order layer controller 23. 
Referring to FIG. 12, when the B-channel layer-3 controller 22 receives the 
first signal from the B-channel layer-2 controller 21, it extracts the 
header related to the layer 3 therefrom (step 301), and discerns whether 
the contents of the first two octets are 01h and 00h (step 302). When the 
result in step 302 is YES, the received signal is based on the CCITT 
recommendation T.70. Then the B-channel layer-3 controller 22 determines 
whether the content of the block type field located at the second octet of 
the layer-4 signal block is E0h which represents the signal TCR (step 
303). When the result in step 303 is YES, the B-channel layer-3 controller 
22 activates the procedure based on the CCITT recommendation T.70, and 
informs the high-order layer controller 23 of the activated procedure 
(step 304). Then an arbitrary conventional procedure is executed. On the 
other hand, when the result in step 303 is NO, the received signal is 
incorrect. Thus, a corresponding error procedure is executed (step 308). 
When the result in step 302 is NO, the received signal is based on the ISO 
standard ISO8208. Then the B-channel layer-3 controller 22 determines 
whether the packet type identifier in the layer-3 header is FBh which 
indicates the signal SQ (step 306). When the result in step 306 is YES, 
the B-channel layer-3 controller 22 activates the procedure based on the 
ISO standard ISO8208 and lets the B-channel high-order controller 23 know 
the activated procedure (step 307). Thereafter, an arbitrary conventional 
procedure is executed. When the result in step 306 is NO, the received 
signal is incorrect, and a corresponding error procedure is carried out 
(step 308). 
Thereby, before starting a procedure related to the high-order layer for B 
channel, it is possible to select the transmission procedures related to 
the layers 3 and 4 which are the same as those in the calling terminal As 
a result, a subsequent data transmission procedure can suitably be 
executed. 
In the second embodiment, the called terminal can know the B-channel 
transmission function provided by the calling terminal by analyzing the 
procedure signals supplied from the calling terminals, so that the called 
terminal can select the same transmission function as the calling 
terminal. It is however noted that the calling terminal can know the 
terminal function of the called terminal but cannot know the B-channel 
transmission function thereof in the call set-up procedure. 
A description is given of a third embodiment of the present invention in 
which the calling terminal selects the B-channel transmission function 
suitable for that of the called terminal. It is now assumed that the 
B-channel transmission function of the calling terminal is based on the 
CCITT recommendation T.70, and the B-channel function of the called 
terminal is based on the ISO standard ISO08208. In this case, a set-up 
call is cleared in the following manner. 
For example, as shown in FIG. 13A, it is assumed that the calling terminal 
sends the transport connect request signal TCR for the layer 4 after the 
link related to the layer 2 has been established and that the called 
terminal ingores the TCR because it is not an appropriate signal. In this 
case, the calling terminal cannot receive a response to the signal TCR 
within a predetermined time. Therefore, the calling terminal sends the 
disconnect signal DISC related to the layer 2 and thereby lets the called 
terminal know that call connection is cleared. In response to the 
disconnect signal DISC, the called terminal sends back the signal UA which 
is an acknowledgement of the disconnect signal DISC. Thereby, call 
connection on B channel is cleared. 
As shown in FIG. 13B, when the called terminal returns a diagnosis signal 
DIAG related to the layer 3 in response to the signal TCR sent from the 
calling terminal, the call terminal sends the disconnect signal DISC 
related to the layer 2 at this time so that it lets the called terminal 
know that call connection is cleared. In response to the disconnect signal 
DISC, the called terminal sends back the signal UA, so that call 
connection on B-channel is cleared. 
It is alternatively assumed that the B-channel transmission function of the 
call terminal is based on the ISO standard ISO8208 and the B-channel 
transmission function of the called terminal is based on the CCITT 
recommendation T.70. In this case, a set-up call is cleared as follows. As 
shown in FIG. 14A, when the link for the layer 2 is established and the 
calling terminal sends the signal SQ related to the layer 3, the called 
terminal sends back the calling terminal a transport block reject signal 
TBR which indicates that the signal SQ is ignored or an undefined signal 
related to the layer 4 is received. Thereby, the calling terminal sends 
the disconnect signal DISC related to the layer 2. The called terminal 
sends back the signal UA. Thus, the set-up call on B-channel is cleared. 
FIG. 11E is a diagram of a format of the transport block signal TBR. 
Alternatively, as shown in FIG. 14B, when the calling terminal sends the 
signal SQ, the called terminal sends back the disconnect signal DISC to 
clear the set-up call because it has received an unexpected signal. Then 
the calling terminal sends the signal UA. Thereby, the set-up call on 
B-channel is cleared. 
According to the third embodiment, as shown in FIG. 15, protocol management 
information is formed for each destination. Protocol management 
information is formed by a set of destination information and protocol 
identification information which indicates a protocol to be used as the 
B-channel transmission function in the destination terminal. The protocol 
management information is stored in a predetermined storage area in the 
external storage device 28 (FIG. 7). The B-channel transmission function 
is selected based on the contents of the external storage device 28. 
The procedure according to the third embodiment is described with reference 
to FIG. 16. When a call is generated, the system controller 24 of the 
calling terminal determines whether the destination address designated at 
that time has been registered in protocol management information stored in 
the external storage device 28 (step 401). When the result in step 401 is 
YES, the system controller 24 reads out the contents of the protocol 
identification information contained in the protocol management 
information, and determines whether the read-out contents are based on the 
CCITT recommendation T.70 (step 402). When the result in step 402 is YES, 
the system controller 24 controls the B-channel layer-3 controller 22 and 
the B-channel high-order layer controller 23 so as to be based on the 
CCITT recommendation T.70 (step 403), and starts setting up a call. On the 
other hand, when the result in step 402 is NO, the system controller 24 
controls the B-channel layer-3 controller 22 and the B-channel high-order 
layer controller 23 so as to be based on the ISO standard ISO8208 (step 
405). Then the procedure proceeds to step 404. 
Then the system controller 24 discerns whether the called terminal rejects 
call connection (step 406). When the result in step 406 is YES, the system 
controller 24 renews the protocol identification information being 
registered in the protocol management information (step 407), and starts 
up re-calling to the same destination terminal (step 408). On the other 
hand, when the result in step 406 is NO, a conventional procedure is 
subsequently executed (step 409). 
When the protocol management information having the same destination 
information as the designated destination information is not stored in the 
external storage device 28, protocol management information which has the 
designated destination information of concern and protocol identification 
information based on the CCITT recommendation T.70 is newly created and 
stored in the external storage device 28 (step 410). Then the procedure 
proceeds to step 403. Therefore, when the destination terminal is called 
by using protocol identification information which has been registered in 
protocol management information, if call connection is rejected, the 
registered contents related to the called terminal are renewed and 
re-calling is started. As a result, at the time of re-calling, call 
connection is not rejected on the ground of a difference of B-channel 
transmission function. Further, since the contents of the protocol 
identification information are renewed so that the transmission function 
available in the destination terminal is newly registered, the next 
request of call connection is not rejected on the ground of a difference 
of B-channel transmission function. As a result, an appropriate calling 
operation can be established. Moreover, when a destination terminal which 
has not yet been called is called, related protocol management information 
is newly generated and the CCITT recommendation T.70 is set as the initial 
transmission function. Therefore, when the same destination terminal is 
called again, call connection is not rejected. 
As described above, the calling terminal creates protocol management 
information related thereto when calling the same for the first time, and 
the B-channel transmission function is set by referring to the registered 
protocol management information. Consequently, call connection is not 
rejected on the ground of a difference of B-channel transmission function. 
In the aforementioned third embodiment, the case where the calling terminal 
fails to establish call connection to the called terminal is deemed to be 
based on a difference in B-channel transmission function. Alternatively, 
as shown in FIG. 13B, when the diagnosis signal DIAG is sent back from the 
called terminal so that a cause of clearance of call connection can be 
obtained, protocol management information is renewed based on the cause. 
For example, when it is found from the diagnosis signal DIAG that a fault 
occurs in the called terminal, protocol management information is not 
renewed because a cause of fault is not based on a difference in B-channel 
transmission function. 
In group-4 facsimile machines which use ISDNs as transmission lines, a line 
switching system or a packet exchange system can be used as an exchange 
system for lines. Further, one of the LAPB data link protocol and the LAPD 
data link protocol can be used as the data link layer (layer 2) protocol. 
In this case, the modulo size can be set equal to 8 or 128. Moreover, it 
is possible to use either the CCITT recommendation T.70 transmission 
function or the ISO standard ISO8208 transmission function when a line 
switching system is employed for the network layer (layer 3). As shown 
above, in group-4 facsimile machines, it is possible to selectively use a 
plurality of protocols. For this reason, there is a possibility that a 
protocol available in a calling terminal may be different from that in a 
called terminal. 
A fourth embodiment is configured taking account of the above-mentioned 
possibility. According to the fourth embodiment, kinds of protocols 
provided in a called terminal with which a call terminal may communicate 
are registered in the calling terminal. When a terminal discriminates the 
other terminal during call-out or call-in, transmission controllers 
related to the respective layers are provided with suitable protocols to 
be used. 
The fourth embodiment is further described in detail with reference to FIG. 
17, which is a block diagram of a facsimile machine according to the 
fourth embodiment of the present invention. In FIG. 17, those parts which 
are the same as those in the previous figures are given the same reference 
numerals. Referring to FIG. 17, a B-channel layer-2 transmission 
controller 31 processes a B-channel layer-2 transmission function and 
exchanges B-channel signals with the ISDN through the ISDN interface 
circuit 1. Further, the B-channel layer-2 transmission controller 31 
exchanges signals related to the higher-order layer (layer 3) with a 
B-channel layer-3 controller 32. 
The B-channel layer-3 transmission controller 32 processes a B-channel 
layer-3 transmission function, and exchanges signals with the ISDN through 
the B-channel layer-2 transmission controller 31. Further, the B-channel 
layer-3 transmission controller 32 exchanges signals related to the 
higher-order layer (layer 4) with a B-channel high-order layer 
transmission controller 33. 
The B-channel high-order layer controller 33 processes B-channel 
transmission functions related to the layer 4 and higher-order layers. The 
B-channel high-order layer controller 33 exchanges data with the ISDN 
through the B-channel layer-3 transmission controller 32, and exchanges 
transmission data with a system controller 34. 
The B-channel layer-2 transmission controller 31 can provide all layer-2 
protocols (including a packet mode protocol) which are available in the 
group-4 facsimile machine. The B-channel layer-3 transmission controller 
32 can provide all layer-3 protocols (including a packet mode protocol) 
which are available in the group-4 facsimile machine. 
The system controller 34 controls a facsimile operation of the group-4 
facsimile machine. Further, the system controller 34 controls data 
exchange with the D-channel transmission controller 2, protocol setting 
procedures through the B-channel layer-2 transmission controller 31 and 
the B-channel layer-3 transmission controller 32, and data exchange with 
the B-channel high-order layer transmission controller 33. 
A parameter memory 35 stores a variety of information peculiar to the 
group-4 facsimile machine, such as information on a one-touch dial table. 
A scanner 36 reads a document at a predetermined resolution level. A 
plotter 37 prints an image on a recording medium such as paper with a 
predetermined resolution level. 
A coder/decoder 38 encodes and compresses an image signal derived from the 
scanner 36, and expands and decodes an encoded and compressed image signal 
so as to reproduce the original image signal. An image memory 39 stores 
image signals which have been encoded and compressed. A console/display 
device 40 has operation keys such as one-touch dial keys through which 
telephone numbers of destination terminals can be input by one touch, and 
a liquid crystal display which presents operation guidances to the 
operator, for example. 
FIG. 18 shows an example of the one-touch dial table stored in the 
parameter memory 35. The one-touch dial table stores the following 
information for each of the one-touch dial keys. A one-touch dial number 
is assigned each of the one-touch dial keys. ISDN address information 
indicates a registered destination telephone number. Sub-address 
information indicates a destination sub-address. Exchange system 
information indicates an exchange system which is used when communicating 
with the destination terminal. Layer-3 protocol information indicates the 
type of layer-3 protocol related to the B-channel. Layer-3 modulo size 
information indicates a layer-3 modulo size related to the B channel. 
Layer-2 protocol-information represents the type of layer-2 protocol 
related to the B-channel. Layer-2 modulo size information represents a 
layer-2 modulo size related to the B-channel. 
At least ISDN address information, sub-address information and exchange 
system information among the above-mentioned table elements are input by 
the operator when registering one-touch dial number. The operator may 
input other information when registering one-touch dial number. 
Information which has not been registered is given a no registration code. 
When information which indicates the CCITT recommendation T.70 is given to 
the layer-3 protocol, in actuality the layer-3 protocol is omitted, and 
therefore a blank code is given as the content of layer-3 modulo size 
information. 
A description is given of a procedure for discriminating layer-2 protocols 
and a procedure for discriminating layer-3 protocols. When terminating a 
call, layer-2 protocols can be discriminated by the same procedure as that 
shown in FIG. 4. On the other hand, when setting up a call, layer-2 
protocols can be discriminated by the same procedure as that shown in FIG. 
6. When terminating a call, layer-3 protocols can be discriminated by the 
same procedure as that shown in FIG. 11. On the other hand, when setting 
up a call, layer-3 protocols can be discriminated by the same procedure as 
that shown in FIG. 16. 
When the other terminal is set in the packet mode, a call set-up message 
SETUP supplied from ISDN contains an information element which lets the 
called terminal know the occurrence of a call. Therefore, the called 
terminal can discern whether the calling terminal is in the packet mode or 
the line switching mode by analyzing the received call set-up message 
SETUP. 
FIGS. 19A and 19B are flowcharts of a procedure for protocol discrimination 
which is executed in the group-4 facsimile machine when setting up a call. 
Referring to FIG. 19A, the operator operates desired one of the one-touch 
dial keys on the console/display device 40 and inputs an instruction which 
starts image information transmission. The system controller 34 reads 
information related to the operated one-touch dial key from the one-touch 
dial table stored in the parameter memory 35 (step 501). Then the system 
controller 34 determines whether information indicative of line switching 
is set in the exchange system information read out of the one-touch dial 
table (step 502). 
When the result in step 502 is YES, the system controller 34 transfers the 
contents of the one-touch dial table to the corresponding transmission 
controllers 31, 32 and 33 (step 503). When valid information is set in the 
sub-address information, or the result in step 504 is YES, the system 
controller 34 lets the D-channel transmission controller 2 know the 
content of the sub-address information (step 505). Then, the D-channel 
transmission controller 2 executes the D-channel protocol procedure in the 
line switching mode (step 506). Thereby, the D-channel protocol between 
the group-4 facsimile machine of concern and the ISDN and the D-channel 
protocol between the other terminal and the ISDN are executed. Thereby, a 
B-channel is established. When the B-channel has been established, the 
B-channel layer-2 transmission controller 31 starts the following layer-2 
transmission control procedure. 
At this time, when the B-channel layer-2 transmission controller 31 has 
information on transmission mode to be used, or in other words, when the 
result in step 507 is YES, the B-channel layer-2 transmission controller 
31 starts the layer-2 transmission control procedure in the mode which has 
been set therein (step 508). Alternatively, when the result in step 507 is 
NO, the B-channel layer-2 controller 31 starts the layer-2 transmission 
control procedure in the mode which is set beforehand as a transmission 
mode in default (step 509). 
The B-channel layer-2 transmission controller 31 lets the system controller 
34 know the result of whether the transmission mode being used coincides 
with that of the other terminal. When the result shows that the system 
controller 34 sets another transmission mode in the B-channel layer-2 
transmission controller 31 in the aforementioned manner, and carries out 
the layer-2 transmission procedure again. When the transmission mode which 
is first set coincides with the transmission mode available in the other 
terminal (when the result in step 510 is YES), or when another 
transmission mode coincides with the transmission mode available in the 
other terminal (when the result in step 511 is YES), the B-channel layer-2 
transmission controller 31 stores information on transmission mode used at 
that time (step 512). Thereby the B-channel layer-2 transmission 
controller 31 suitably executes the layer-2 transmission control procedure 
with the other terminal. 
Then, the transmission procedure related to the layer 3 is started by the 
B-channel layer-3 transmission controller 32. When information on 
transmission mode to be used has been set in the B-channel layer-3 
transmission controller 32, the B-channel layer-3 transmission controller 
32 starts the layer-3 transmission control procedure in the mode being set 
(step 514). Alternatively, when information on transmission mode has not 
been set in the B-channel layer-3 transmission controller 32, that is, 
when the result in step 513 is NO, the B-channel transmission controller 
32 starts the layer-3 transmission procedure in the predetermined default 
mode (step 515). 
The B-channel layer-3 transmission controller 32 informs the system 
controller 34 of the result of whether the transmission mode being used 
coincides with the transmission mode of the other terminal. When the 
transmission mode related to the layer 3 which is first set coincides with 
the transmission mode available in the other terminal so that the result 
in step 516 is YES, the B-channel layer-3 transmission controller 32 
stores, as a transmission mode in the layer 3, the transmission mode used 
at that time (step 517). Then the system controller 34 compares 
transmission mode information related to the layers 2 and 3 stored in 
steps 512 and 517 with the contents of the one-touch dial table. When 
there is some information on which no coincidence is obtained, the 
information is replaced by transmission mode information stored in steps 
512 and 517 (step 518). Thereby, the B-channel layer-3 transmission 
controller 31 suitably executes the layer-3 transmission procedure with 
the other terminal. 
When the transmission mode in the calling terminal does not coincide with 
that in the called terminal and therefore the result in step 516 is NO, 
the called terminal rejects the call and data transmission is ended. 
Therefore, each of the transmission controllers clears call connection 
(step 519). Then information about the layer-3 protocol which has been 
registered in the one-touch dial table related to the destination of 
concern is renewed with another information (step 520). Then a calling is 
made active again (step 521), and the procedure is terminated. 
When the result in step 511 is NO and therefore the calling terminal fails 
to select the layer-2 transmission mode available in the called terminal, 
each of the transmission controllers 31 to 33 clears call connection (step 
522). Then the system controller 34 has the console/display device 40 
display a "communication inactive" message which represents that it cannot 
communicate with the designated destination terminal. At the same time, 
the system controller 34 has the plotter 37 output a report showing the 
communication inactive message (step 523). 
When the content of the exchange system registered in the one-touch dial 
table indicates packet exchange (when the result in step 502 is NO), the 
system controller 34 has the D-channel transmission controller 2 execute 
the D-channel protocol procedure in the packet mode (step 524). Then the 
system controller 34 activates, in the packet mode, the B-channel layer-2 
transmission controller 31, the B-channel layer-3 transmission controller 
32, and the B-channel high-order layer transmission controller 33 (step 
525). Then the procedure proceeds to step 507. 
As described above, when setting a call, the transmission modes related to 
the B-channel layer 2 and the B-channel layer 3 to be used are set based 
on the registered contents of the one-touch dial table. Thus, it is 
possible to exchange image information with all group-4 facsimile 
machines. 
When the transmission mode which is first set in the layer 2 or the layer 3 
does not coincide with that in the other terminal, the calling terminal 
changes its own transmission mode so as to coincide with the transmission 
mode of the other terminal, and renews the registered contents of the 
one-touch dial table with information on the changed transmission mode. 
Thus, even when the type of destination terminal is changed, it is 
possible to suitably communicate with the changed destination terminal 
after the contents of the one-touch dial table are renewed. 
FIGS. 20A and 20B are flowcharts of a protocol discrimination procedure 
which is executed in the group-4 facsimile machine when receiving a call. 
Referring to FIG. 20A, when the call set-up message SETUP sent from the 
ISDN is received, the system controller 34 analyzes the content of the 
received call set-up message (step 601). Then the system controller 34 
determines whether the requested exchange system relates to the line 
switching mode (step 602). When the result in step 602 is YES, the system 
controller 34 determines whether information on the other terminal has 
been registered in the one-touch dial table by referring to information on 
source number and source sub-address contained in the received call set-up 
message SETUP (step 603). 
When the result in step 603 is YES, the system controller 34 sets the 
registered contents in the corresponding transmission controllers 31 to 33 
(step 604). On the other hand, when the result in step 603 is NO, the 
transmission mode to be set in default is set in each of the transmission 
controllers 31 to 33 (step 605). Then the system controller 34 has the 
D-channel transmission controller 2 execute the D-channel protocol 
procedure in the line switching mode (step 606). 
Next, the B-channel layer-2 transmission controller 31 starts the 
transmission procedure for the layer 2 in the transmission mode which has 
been set (step 607). The B-channel layer 2 transmission controller 31 
informs the system controller 34 of the result of whether the transmission 
mode being used coincides with that of the other terminal When the 
transmission modes are mutually different, the system controller 34 sets 
another transmission mode in the B-channel layer-2 transmission controller 
31. Then the transmission control procedure related to the layer 2 is 
carried out again. On the other hand, when the transmission mode related 
to the layer 2 which is first set coincides with the transmission mode 
available in the other terminal (the result in step 608 is YES), or when 
the transmission mode set in step 609 coincides with the transmission mode 
available in the other terminal (the result in step 609 is YES), the 
B-channel layer-2 transmission controller 31 stores information on the 
transmission mode used at that time as the transmission mode related to 
the layer 2 (step 610). Thus, the B-channel layer-2 transmission 
controller 31 can suitably execute the transmission procedure related to 
the layer 2 with the other terminal. 
Next, the B-channel layer-3 transmission controller 32 starts the 
transmission control procedure in the transmission mode which has been set 
(step 611). The B-channel layer-3 transmission controller 32 informs the 
system controller 34 of the result of whether the transmission mode being 
used coincide with the transmission mode available in the other terminal. 
When the transmission mode in the calling terminal is not coincident with 
the transmission mode in the called terminal, the system controller 34 
sets another transmission mode in the B-channel layer-3 transmission 
controller 32. Then the call set-up signal is sent again. 
When the transmission mode related to the layer 3 which is first set 
coincides with the transmission mode available in the other terminal (when 
the result in step 612 is YES), and alternatively when the transmission 
mode set in step 613 coincides with the transmission mode available in the 
other terminal (when the result in step 613 is YES), the B-channel layer-3 
transmission controller 32 stores the transmission mode used at that time 
as the transmission mode related to the layer 3 of the destination 
terminal (step 614). Thus, the B-channel layer-3 transmission controller 
32 can suitably execute the transmission control procedure related to the 
layer 3. 
When the system controller 34 determines whether information on the 
destination terminal of concern has been stored in the one-touch dial 
table (step 615). When the result in step 615 is YES, the contents of the 
one-touch dial table are renewed with the contents of the transmission 
mode used at that time (step 616). On the other hand, when the result in 
step 615 is NO, the system controller 34 forms a report about 
communication with not-registered destination through the plotter 37 (step 
617). This report includes a list of the contents of the one-touch dial 
table. 
When the called terminal cannot select the transmission mode related to the 
layer 2 or 3 which is the same as the transmission mode available in the 
called terminal, or in other words, when the result in step 609 or 613 is 
NO, the system controller 43 stops each of the transmission controllers 31 
to 33 (step 618). When information on the other terminal has been 
registered in the one-touch dial table (the result in step 609 is YES), 
the system controller 34 has the console/display device 40 display a 
communication inactive message which shows that it is impossible to 
communicate with the calling terminal of concern (step 620). Further, the 
system controller 34 has the plotter 37 print a report showing that it is 
impossible to communicate with the calling terminal of concern (step 620). 
When the result in step 619 is NO, the set-up call is ignored (step 621). 
When the content of the exchange system information registered in the 
one-touch dial table indicates packet exchange (when the result in step 
602 is NO), the system controller 34 has the D-channel transmission 
controller 2 execute the D-channel protocol procedure in the packet mode 
(step 622). When the B-channel is established by the execution of step 
622, the system controller 34 activates, in the packet mode, the B-channel 
layer-2 transmission controller 31, the B-channel layer-3 transmission 
controller 32 and the high-order layer transmission controller 33 (step 
623). When information on the other terminal has been registered in the 
one-touch dial table (the result in step 624 is YES), the registered 
contents are stored in the corresponding transmission controllers (step 
625). On the other hand, when the result in step 624 is NO, the 
transmission mode to be set in default is set in each of the transmission 
controllers 31 to 33 (step 626). The procedure proceeds to step 607. 
As describe above, when receiving a call, if information on the other 
terminal has been registered in the one-touch dial table, the call-in 
procedure is started in the registered transmission mode. Thus, it is 
possible to exchange image information with all group-4 facsimile 
machines. 
Alternatively, when information on the other terminal has not been 
registered in the one-touch dial table, information on the transmission 
mode used at that time is printed as a report. Thus, it is convenient for 
the operator to register information on new terminal in the one-touch dial 
table. 
In the aforementioned manner, according to the fourth embodiment, since the 
B-channel transmission mode to be used is selected by referring to 
information registered in the one-touch dial table, it is possible to 
perform data transmission in conformity to the transmission mode of the 
other terminal. Thus, it is possible to exchange image with all group-4 
facsimile machines. 
When the transmission mode which is actually used is different from the 
registered contents of the one-touch dial table, the registered contents 
are renewed with new information on the actually used transmission mode. 
Thus, even if the type of group-4 facsimile machine related to an ISDN 
address is changed, the same transmission mode can be used in the second 
and subsequent transmission procedure. 
In the fourth embodiment, the transmission modes related to the layer 2 and 
layer 3 are registered in the one-touch dial table. It is particularly 
noted that the fourth embodiment has a protocol learning function whereby 
necessary information is registered in the one-touch dial table. 
Transmission modes related to higher-order layers may be registered through 
the one-touch dial table. The present invention is not limited to 
facsimile machines such as group-4 facsimile machines, and includes 
another data terminal equipment. 
The present invention is not limited to the aforementioned embodiments, and 
variations and modifications may be made without departing from the scope 
of the present invention.