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Claims  |
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I claim:
1. A D channel packet communication system in a private branch exchange
having a plurality of line circuits, each one of said plurality of line
circuits connecting a plurality of subscriber lines for packet terminals
to said private branch exchange, a plurality of trunk circuits, each one
of said plurality of trunk circuits connected to a plurality of subscriber
lines of a public ISDN switching system in a network, a time division
switch for providing connection paths between each one of said plurality
of line circuits and each one of said plurality of trunk circuits and a
call processor for controlling overall operations of said private branch
exchange, said D channel packet communication system comprising:
a conversion control unit comprising
means for receiving first conversion information including first connection
control information allocated to a D channel packet transferred between
one of said packet terminals and said private branch exchange, and second
connection control information allocated to D channel packet transferred
between said private branch exchange and said network;
means for deriving second conversion information having correspondences
between said first connection control information and said second
connection control information at a beginning of D channel packet
communication based on said first conversion information, said deriving
means cooperating with said receiving means; and
converting means for converting said first connection control information
in said D channel packet received form one of said packet terminals to
said second connection control information for a D channel packet to be
sent to said network, converting said second connection control
information in said D channel packet received from said network to said
first connection control information for said D channel packet to be sent
to one of said packet terminals, based on said second conversion
information, said converting means cooperating with said receiving means;
a plurality of first switching control units, each one of said plurality of
first switching control units connecting to one of said plurality of
subscriber lines for one of said packet terminals, identifying D channel
information received from one of said packet terminals, establishing a
path to said conversion control unit where a D channel packet is being
received, and establishing a path to said call processor where information
other than said D channel packet is being received;
a plurality of second switching control units; each one of said plurality
of second switching control units connected to one of said plurality of
subscriber lines of said public ISDN switching system, identifying D
channel information from said public ISDN switching system, establishing a
path to said time division switch where a D channel packet is being
received and establishing a path to said call processor where information
other than said D channel packet is being received; and
a plurality of permanent connection paths in said time division switch,
each making a permanent connection between said conversion control unit
and one of said second switching control units.
2. The D channel packet communication system according to claim 1, wherein
one of said plurality of line circuits comprises said conversion control
unit and said plurality of first switching control units.
3. The D channel packet communication system according to claim 1, wherein
one of said plurality of first switching control units and one of said
plurality of second switching control units comprise D channel packet
discriminating means for discriminating a D channel packet by a service
access point identifier contained in D channel information.
4. The D channel packet communication system according to claim 1, wherein
said conversion control unit further comprises:
means for receiving third conversion information having correspondences
between a part of said first connection control information contained in D
channel information sent from one of said packet terminals and a part of
said second connection control information contained in D channel
information sent from said network, sent from said call processor at a
beginning of D channel communication performed by said network.
5. The D channel packet communication system according to claim 4, wherein
said conversion control unit further comprises:
packet type identification means for identifying an initial stage D channel
packet and a final stage D channel packet; and
second conversion information deleting means for deleting said second
conversion information at a final stage of D channel packet communication.
6. The D channel packet communication system according to claim 4, wherein
said call processor comprises:
first conversion information management means for deriving said first
conversion information based on information of service conditions of one
of said packet terminals and service conditions of said private branch
exchange, and for sending said derived first conversion information to
said conversion control unit and one of said plurality of trunk circuits;
and
third conversion information management means for deriving said third
conversion information based on first D channel information sent from said
network and send D channel information sent from one of said packet
terminals in response to said first D channel information at a beginning
of D channel communication performed by said network, and for sending said
derived third conversion information to said conversion control unit.
7. The D channel packet communication system according to claim 6, wherein
said cell processor further comprises permanent path establishing means
for establishing said plurality of permanent connection paths in
accordance with predetermined connection information.
8. A method of D channel packet communication in a private branch exchange
having a plurality of line circuits, each one of said plurality of line
circuits connecting a plurality of subscriber lines for packet terminals
to said private branch exchange, a plurality of trunk circuits, each one
of said plurality of trunk circuits connected to a plurality of subscriber
lines of a public ISDN switching system in a network, a time division
switch for providing connection paths between each one of said plurality
of line circuits and each one of said plurality of trunk circuits and a
call processor for controlling overall operations of said private branch
exchange, said method comprising the steps of:
generating first conversion information including first connection control
information allocated to a D channel packet transferred between one of
said packet terminals and said private branch exchange, and second
connection control information allocated to a D channel packet transferred
between said private branch exchange and said network;
receiving D channel information sent from one of said packet terminals and
identifying a D channel packet;
deriving second conversion information having correspondence between said
first connection control information and said second connection control
information based on said first conversion information and connection
control information contained in said discriminated D channel packet at a
beginning of D channel packet communication; and
converting said first connection control information in a D channel packet
received from one of said packet terminals to said second connection
control information for a D channel packet to be sent to said network, and
converting said second connection control information in a D channel
packet received from said network to said first connection control
information fair a D channel packet to be sent to one of said terminals,
based on said second conversion information.
9. A method of D channel packet communication in a private branch exchange
having a plurality of line circuits, each one of said plurality of line
circuits connecting a plurality of subscriber lines for packet terminals
to said private branch exchange, a plurality of trunk circuits, each one
of said plurality of trunk circuits connected to a plurality of subscriber
lines of a public ISDN switching system in a network, a time division
switch for providing connection paths between each one of said plurality
of line circuits and each one of said plurality of trunk circuits and a
call processor for controlling overall operations of said private branch
exchange, said method comprising the steps of:
generating first conversion information including first connection control
information allocated to a D channel packet transferred between one of
said packet terminals and said private branch exchange, and second
connection control information allocated to a D channel packet transferred
between said private branch exchange and said network;
generating third conversion information having correspondences between a
part of said first connection control information contained in D channel
information sent from one of said packet terminals and a part of said
second connection control information contained in D channel information
sent from said network at a beginning of D channel communication performed
by said network;
receiving D channel information and identifying a D channel packet;
deriving second conversion information having correspondences between said
first connection control information and said second connection control
information based on said first conversion information, said third
conversion information and connection control information contained in
said discriminated D channel packet at a beginning of D channel packet
communication; and
converting said second connection control information in a D channel packet
received from said network to said first connection control information
for a D channel packet to be sent to one of said packet terminals, and
converting said first connection control information in a D channel packet
received from one of said packet terminals to said second connection
control information for a D channel packet to be sent to said network,
based on said second conversion information. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a D channel packet communication system
and a method of D channel packet communication, and more particularly, to
a D channel packet communication system and a method of D channel packet
communication for an ISDN (Integrated Services Digital Network) if a
private branch exchange in which a D channel packet is discriminated by a
line circuit, and connection control information contained in the D
channel packet is converted and transferred to an ISDN network through a
permanent connection path established in a time division switch without
passing through a call processor.
2. Description of Related Art
A private branch exchange (hereafter referred to as PBX) is used in
corporations, schools, hotels and some other places for providing an
internal private communications network. The PBX accommodates its own
terminals of telephones, data communication terminals and other kind of
terminals for internal communications and communications to a public
communication network. Therefore, the PBX performs switching operations
for calls to and from those terminals, and establishes communication paths
not only within the private communications network, but also to public
communications networks.
On the other hand, a PBS may also be regarded as a subscriber terminal of a
public switching system in a public communications network. That is,
although the PBX itself accommodates an unspecified number of terminals,
the PBS is treated as a subscriber terminal in control operations of the
public switching system as far as the public communications network is
concerned. Therefore, where the public communications network provides the
ISDN, then a TEI (Terminal Equipment Identifier) and some other service
parameters to be allocated for subscriber terminals of a public ISDN
switching system are also allocated to the PBX. Those parameters are
allocated to each subscriber line which interconnects a central office
trunk (hereafter referred to as trunk) of the PBX and a line circuit of
the public ISDN switching system depending on service conditions of the
PBX as a subscriber terminal, but are not allocated to each terminal
accommodated in the PBX.
FIG. 1 is a block diagram illustrating a configuration of a conventional D
channel packet communication system in a PBX performing D channel packet
communication without the D channel packet passing through a call
processor (Japanese Laid Open Patent Application No. HEI 2-226844).
In this figure, the PBX 2 is provided with a line circuit 4 accommodating a
packet terminal 1 as an internal terminal, a trunk circuit 5 connected to
a public-ISDN switching system (not shown) in a public communications
network 3 (hereafter referred to as network), a time division switch 7 for
establishing a communication path between the line circuit 4 and the trunk
circuit 5, and a call processor 6 for controlling the overall operations
of the PBX 2. A switching control unit 41 is provided in the line circuit
4 and a switching control unit 51 is provided in the trunk circuit 5.
Those switching control units 41 and 51 perform connecting switching for
the flow of received D channel information to either side of the division
switch 7 or the call processor 6.
FIG. 2 is a diagram showing the sequence of D channel packet communication
performed by the conventional D channel packet communication system in the
PBX 2 of FIG. 1.
When the packet terminal 1 at the sending side requests D channel packet
communication, "link set up" information of D channel information is
transmitted from the packet terminal 1 to the network 3 based on
subscriber protocols of packet communications defined by CCITT
(International Telegraph and Telephone Consultative Committee, at present
ITU-T: International Telecommunication Union Telecommunication
Standardization Sector) recommendations Q.931.
The "link set up" information is the information for requesting the network
3 to establish a communication path, and this information includes a SAPI
(Service Access Point Identifier) of the layer 2 protocol information for
indicating the mode of data communication to be followed. As the D channel
packet communication based on CCITT recommendations X.25 protocol is
followed in this case, a value of "16" is set in the SAPI, i.e., the "link
set up" information with SAPI-15 means requesting a communication path for
the communication of D channel packet based on CCITT recommendation X.25
protocol.
The switching control unit 41 of the line circuit 4 is normally connected
to the call processor side, so the "link set up" information is
transferred to the call processor 6 from the line circuit 4 which has
received the "link set up" information and terminated the layer 2
protocol. The call processor 6 analyzes the "link set up" information and
recognizes that the mode of communication requested is D channel packet
communication by recognizing the value of SAPI which has been set to "16".
Then, the "link set up" information is transferred from the call processor
6 to the network 3 through the trunk circuit 5.
When the network 3 responds to the "link set up" information by
transmitting the "link set up confirmation" information, since the
switching control unit 51 of the trunk circuit 5 is normally connected to
the call processor side, the "link set up confirmation" information from
the network 3 is transferred to the call processor 6 from the trunk
circuit 5. Then, the call processor 6 identifies the information as a
response to the "link set up" information and transfers the information to
the packet terminal 1 through the switching control unit 41 of the line
circuit 4.
Once the "link set up confirmation" information has been acknowledged by
the circuit 4, the trunk circuit 5 and the call process 6, each of the
switching control units 41 and 51 switches its connection path from the
call processor side to the time division switch side, and the call
processor 6 establishes a communication path between the line circuit 4
and the trunk circuit 5 in the time division switch 7 using predetermined
connection information.
Throughout the procedures described above, the packet terminal 1 which has
received the "link set up confirmation" information is connected to the
network 3 through the line circuit, 4 the time division switch 7 and the
trunk circuit 5.
Next, the packet terminal 1 transfers the "X.25 call request" information
to a packet terminal at the receiving side (not shown) in the network 3
for requesting commencement of a packet communication with the X.25
protocol. When a link set up at the packet terminal of the receiving side
has been completed, the "X. 25 call termination completion" information in
response to the "X.25 call request" information is sent back from the
packet terminal of the receiving side to the packet terminal 1 at the
sending side, whereafter D channel packet communication commences.
At the end of communication, the "X.25 disconnect request" information is
transferred from the packet terminal 1. When the line circuit 4 and the
trunk circuit 5 recognize that the "X.25 disconnect request" information
has passed therethrough, each of the switch control units 41 and 51
switches its connection path from the time division switch side to the
call processor side. Therefore, the "X.25 disconnect confirmation" signal
in response to the "X.25 disconnect request" signal sent from the network
3, is received by the call processor 6. The call processor 6, having
received the "X.25 disconnect confirmation" signal, transfers this
information to the packet terminal 1 through the switch control unit 41 of
the line circuit 4. Thereafter, the call processor 6 disconnects the
communication path between the line circuit 4 and the trunk circuit 5 in
the time division switch 7 when the "link disconnection" information is
transmitted from the packet terminal 1 based on the Q.(31 protocol.
The conventional D channel packet communication system and the method of D
channel packet communication in the PBX as described above, however, have
the following problems:
(1) The call processor must bear the processing load each time D channel
packet communication is performed, because the call processor is required
for processing the D channel information based on the Q.931 protocol;
(2) The efficiency of the time division switch is degraded because the
communication path between the line circuit and the trunk circuit is
established for each D channel packet communication and connection paths
on the time division switch are occupied by particular packet terminals
such as the PVC (Permanent Virtual Circuit) type terminal or a terminal
which does not release a communication path even at the end of
communication; and (3) Only a limited range of services are available for
packet terminals which meet the service conditions allocated to the PBX as
a subscriber terminal of the public ISDN switching system in the network,
because the D channel information transmitted from the packet terminal is
only passed through to the network without any conversions.
SUMMARY OF THE INVENTION
The present invention solves the above problems, by providing a D channel
packet communication system and a method of D channel packet
communication, which are capable of identifying a D channel packet and
converting the connection control information contained in the D channel
packet sent from one side of the PX (e.g., the packet terminal) to a
suitable value of connection control information to be sent to the other
side of the PBX (e.g., the network), without passing through the call
processor.
To do this, the D channel packet communication system according to this
invention is implemented in a PBX that includes a plurality of line
circuits, each connecting a plurality of subscriber lines at terminals of
the PBX, a plurality of trunk circuits, each connecting a plurality of
subscriber lines of a public ISDN switching system in a network, a time
division switch for providing connection paths between the line circuit
and the trunk circuit and a call processor for controlling the overall
operations of the PBX. In particular, the inventive system comprises a
conversion control unit, a plurality of first switching control units, a
plurality of second switching control units and a plurality of permanent
connection paths in the time division switch.
Each first switching control unit is a part of the line circuit and
connects to a packet terminal via a subscriber line of the PBX. These
first switching units discriminate a D channel packet from received D
channel information, and transfers the D channel packet to the conversion
control unit. If the D channel information is other than a D channel
packet, the D channel information is transferred to the call processor.
Each second switching control unit is a part of the trunk circuit and
connects to a subscriber line of the public ISDN switching system in the
network. These second switching units discriminate a D channel packet from
received D channel information, and transfers the D channel packet to the
conversion control unit via the permanent connection path. If the D
channel information is other than a D channel packet, the D channel
information is transferred to the call processor.
The conversion control unit is a part of the line circuit, and comprises
means for receiving first conversion information, means for deriving
second conversion information and converting means. The first conversion
information includes first connection control information which is
allocated to the D channel packet transferred between the packet terminal
and the PBX, and second connection control information which is allocated
to the D channel packet transferred between the PBX and the network. The
first conversion information is generated and managed in the call
processor, and sent to the conversion control unit before starting D
channel communication. The second conversion information deriving means
derives second conversion information which has correspondences between
the first connection control information and the second connection control
information at the beginning of the sequence of D channel packet
communication based on the first conversion information. The converting
means converts the first connection control information in the D channel
packet received from the packet terminal to the second connection control
information for the D channel packet to be sent to the network, and also
converts the second connection control information in the D channel packet
received from the network to the first connection control information for
the D channel packet to be sent to the packet terminal, based on the
second conversion information.
The converted D channel packet from the packet terminal is transferred to
the network through a permanent connection path which permanently connects
the conversion control unit to one of the second switching control units.
If the D channel packet communications is performed by the network, third
conversion information is generated in the call processor at the beginning
of the sequence of the D channel communication, and transferred to the
conversion control unit. The third conversion information has
correspondences between a part of the first connection control information
contained in D channel information sent from the packet terminal and a
part of the second connection control information contained in D channel
information sent from the network. The third conversion information is
used together with the first conversion information to derive the second
conversion information in the conversion control unit when the PBX is at
the receiving side of the D channel packet communication.
A method of D channel packet communication according to the invention
comprises the steps of generating first conversion information, receiving
D channel information sent from the packet terminal and identifying a D
channel packet, driving second conversion information based on the first
conversion information and connection control information contained in the
identified D channel packet upon beginning a sequence of D channel packet
communication, and converting first connection control information in a D
channel packet received from the packet terminal to second connection
control information for a D channel packet to be sent to the network or
received therefrom.
A more complete understanding of the present invention and many of its
attendant advantages will be more readily attained from the following
detailed description when considered in connection with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram illustrating a convention D channel
packet communication system in a private branch exchange;
FIG. 2 is a sequence diagram showing the operation of a conventional D
channel packet communication system in the private branch exchange of FIG.
1;
FIG. 3 is a schematic block diagram illustrating the D channel packet
communication system according to an embodiment of the present invention;
FIG. 4 is a sequence diagram showing the operation of the D channel packet
communication system of FIG. 3;
FIG. 5 is a data table showing an example of the configuration of first
conversion data for a line side according to the present invention;
FIG. 6 is a data table showing an example of the configuration of first
conversion data for a trunk side according to the present invention;
FIG. 7 is a flow chart illustrating data conversion for the network side
information at the conversion control unit according to the present
invention; and
FIG. 8 is a flow chart illustrating data conversion for the line side
information at the conversion control unit according to the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 3 is a block diagram illustrating a D channel packet communication
system according to the present invention in a PBX which has ISDN
capability. The PBX 2 comprises a plurality of line circuits 4.sub.l to 1
4.sub.n, a plurality of trunk circuits (only one trunk circuit 5 is
shown), a time division switch 7 and a call processor 6. A packet terminal
1 is connected to a line circuit 4.sub.l by subscriber line 40.sub.l,
having a B channel and a D channel, and the PBX 2 is connected to the
network 3 (a public communications Network) by subscriber lines 50.sub.l
to 50.sub.m, each having a B channel and a D channel, of a public ISDN
switching system (not shown) through the trunk circuit 5 of the PBX 2.
Each line circuit such as a line circuit 4.sub.l accommodates a plurality
of subscriber lines 40.sub.l to 40.sub.m of the PBX 2 for connecting
packet terminals, and multiplexes signals from those lines by a conversion
control unit 42 for transmission to a time division switch 7. The opposite
direction signals from the time division switch 7 are also demultiplexed
by the conversion control unit 42 and distributed to a plurality of
subscriber lines 40.sub.l to 40.sub.m.
The conversion control unit 42 has another function of converting different
types of connection control information for D channel packet
communication. In particular, there are two types of connection control
information for the D channel packet communication. They are connection
control information for the line side which is contained in the D channel
packet information transferred between the packet terminal 1 and the PBX
2, and for the trunk side which is contained in the D channel packet
information transferred between the PBX 2 and the network 3. Details of
the connection control information will be described later.
First switching control units 41.sub.l to 41.sub.m are also provided in the
line circuit 4.sub.l. Each first switching control unit corresponds to a
subscriber line of the PBX 2, and controls switching of D channel
information to either the call processor or the conversion control unit
(time division switch side).
Each trunk circuit such as trunk circuit 5 accommodates subscriber lines
50.sub.l to 50.sub.m, each having a B channel and a D channel, of a public
ISDN switching system (not shown) and includes second switching control
units 51.sub.l to 51.sub.m. Each second switching control unit corresponds
to a subscriber line of the public ISDN switching system (not shown) in
the network 3, and controls switching of D channel information to either
the call processor or the time division switch.
The time division switch 7 provides permanent connection paths, 7.sub.l to
7.sub.n, which connects each of the conversion control units 42 of the
line circuits 4.sub.l to 4.sub.n to a respective one of the second
switching control units 51.sub.l to 51.sub.m of the trunk circuit 5.
A call processor 6 controls the overall operations of the PBX 2.
In the ISDN user-network interface, generally, the following connection
management is performed between a subscriber terminal (user) and an ISDN
switching system (network).
In the layer 2 of the ISDN user-network interface protocol, a TEI (Terminal
Equipment Identifier) is used to identify each subscriber terminal which
transmits and receives D channel information, and the TEI is stored in the
address part of the D channel information and transferred between the
subscriber terminal and the network. The TEI, in general, is allocated to
each subscriber terminal by the ISDN switching system (network) which
accommodates the subscriber terminal. However, the PBX itself is also an
ISDN switching system and it also can allocate TEIs to terminals which are
accommodated in the PBX.
The X.25 protocol which defines the protocol of packet communications
includes a LCGN (Logical Channel Group Number) and a LCN (Logical Channel
Number) which are used as control information for packet multiplexing. The
LCGN and the LCN are used to identify each of a plurality of communication
links multiplexed on a transmission path, and it allows multiplexing of
packets having a different destination. The packet multiplexing control
information of the LCGN and the LCN can also be allocated independently
between the line side (between the packet terminal and the PBX) and the
network side (between the PBX and the network).
The TEI, LCGN and LCN information constitutes the connection control
information, and there are two types of connection control information
(the line side information and the network side information) as described
above to provide flexible use of terminals accommodated in the PBX.
Although connection control information transferred between the PBX and
the terminal of the PBX can be freely defined within the PBX, the
connection control information transferred between the PBX and the network
should comply with service conditions contracted to the network as far as
the PBX is one of many subscriber terminals of the network.
Therefore, it is necessary for the PBX to manage correspondences between
the connection control information of the line side and the network side,
and three types of conversion information (first to third conversion
information) are provided in the PBX for this purpose.
The first conversion information is managed by the call processor 6 based
on service conditions of terminals accommodated in the PBX and service
conditions of the PBX contracted to the network 3. The first conversion
information is transferred to the line circuit 4.sub.l and the trunk
circuit 5 from the call processor 6 before D channel packet communication
is executed, and it is stored in the line circuit 4.sub.l and the trunk
circuit 5. The first conversion information is used to drive the second
conversion information which converts the connection control information
contained in the D channel packet information transferred at the network
side and the line side appropriately for allowing flexible service
allocation to terminals accommodated in the PBX 2 when the D channel
packet communication is initiated.
FIGS 5 and 6 show the first conversion information of the line side and the
trunk side (network side), respectively. The first conversion information
of both the line side of FIG. 5 and the trunk side of FIG. 6 are
transferred to line circuit 4.sub.l, whereas only the first conversion
information of the trunk side of FIG. 6 is transferred to trunk circuit 4.
The first conversion information of the line side of FIG. 5 shows one block
of information which includes the subscriber line number, the TEI of the
subscriber terminals (the TEI allocated to the subscriber terminal by the
PBX 2), the TEI of the network side (the TEI allocated to the PBX 2 of the
network 3), the LCGN of the terminal side, the type of connection whether
a VC (Virtual Circuit) or a PVC (Permanent Virtual Circuit) and the number
of multiplexible LCNs. The type of connection (VC/PVC) and the number of
multiplexible LCNs are provided for each LCGN (LCGN=00 to LCGN=15) of the
terminal side. There are the same number of blocks of information as the
number of TEIs used in a corresponding subscriber line of the PBX 2.
The first conversion information of the trunk side of FIG. 6 shows one
block of information which includes the TEI of the network side (the TEI
allocated to the PBX 2 by the network 3), the LCGN of the network side,
the type of connection whether a VC or a PVC and the number of
multiplexible LCNs. The type of connection (VC/PVC) and the number of
multiplexible LCNs are provided for each LCGN (LCGN=00 to LCGN=15) of the
network side. There are the same number of blocks of information as the
number of TEIs allocated to the PBX 2.
The second conversion information is derived when the D channel packet
communication is commenced. The second conversion information has
correspondences between the connection control information of the line
side and the network side, and is used to convert the connection control
information of the line side contained in the D channel packet information
of the network side to be contained in the D channel packet information
sent to the network 3 for the D channel packet communication, or received
therefrom.
The D channel packet communication procedures of the PBX on the sending
side will now be described referring to FIG. 4 to explain how the two
types of conversion information of the first and second conversion
information are used.
In particular, FIG. 4 is a diagram showing the operation of a D channel
packet communication system in a PBX according to the embodiment of the
present invention.
The packet terminal 1 of the sending side transmits the "link set up"
information based on the Q.931 protocol to the line circuit 4.sub.l and D
channel information, and this information includes SAPI set to "16" so as
to request D channel packet communication based on the X.25 protocol.
The line circuit 4.sub.l receives the D channel information from the packet
terminal 1 and terminates the protocol of layer 2. As the result of
termination of the layer 2 protocol, the SAPI=16 signal is identified and
the "link set up confirmation" information is sent back to the packet
terminal 1. At this time the first switching control unit 41.sub.l
establishes a path to the trunk circuit 5 through the conversion control
unit 42 and the permanent connection path 7.sub.l of the time division
switch 7, and the second switching control unit 51.sub.l in the trunk
circuit 5 also establishes a path to the permanent connection path 7.sub.l
of the time division switch 7. The permanent connection path 7.sub.l
between the line circuit 4.sub.l and the trunk circuit 5 has been
established by the call processor 6 based on permanent connection path
management information.
The trunk circuit 5 transmits the "link set up" information based on the
Q.931 protocol to the network 3. In this "link set up" information, the
value of "16" is set to indicate that D channel packet communication based
on X.25 protocol is being requested and the TEI allocated to the PBX 2 by
the network 3 is used which is written in the first conversion information
of the trunk side of FIG. 6.
The network 3 receives the "link set up" information and terminates the
layer 2 protocol, and sends back the "link set up confirmation"
information to the trunk circuit 5.
After the link set up has completed, the packet terminal 1 transfers the
"X.25 call request" information, and this request information is received
by the line circuit 4.sub.l. The value set in the SAPI is "16". This
information is transferred to the conversion control unit 42 through the
first switching unit 41.sub.l.
The conversion control unit 42 recognizes that this information is the
"X.25 call request" information by a packet identifier contained in the D
channel packet information, and obtains the LCGN and LCN contained in the
"X.25 call request" information. With the first conversion information,
the conversion control unit 42 derives second conversion information
comprising a TEI, a LCGN and a LCN of the network side which corresponds
to the TEI of the packet terminal 1, the subscriber line number and the
LCGN and LCN obtained from the "X.25 call request" information. Then, the
conversion control unit 42 modifies the connection control information of
the "X.25 call request" information based on the derived second conversion
information to match the connection control information in the network
side, and transmits the modified "X.25 call request" information to the
network 3 through the permanent connection path 7.sub.l of the time
division switch 7 and the trunk circuit 5. At the same time the second
conversion information is stored in the conversion control unit 42.
A method of deriving the second conversion information will now be
described with reference to FIG. 7.
FIG. 7 is a flow chart illustrating the operation of data conversion for
the network side information (the second conversion information) at the
conversion control unit 42 according to the present invention.
A packet identifier contained in the D channel packet information sent from
the packet terminal 1 is read out (step 101). If the packet identifier
indicates a call request, the TEI, LCGN and LCN of the packet terminal 1
is read out from the D channel packet information (step 102). An
information block having the same TEI and LCGN as those of the packet
terminal 1 is retrieved from the first conversion information of the line
side of FIG. 5 which has the same subscriber line number as an input
circuit number (step 103), and the TEI of the network side (the TEI
allocated to the PBX 2 by the network 3) in the retrieved information
block is read out (step 104).
Next, an information block having the same TEI as the network side is
retrieved from the first conversion information of the trunk side of the
FIG. 6 (step 105), and then, the number of multiplexible LCNs is read out
from a sub-clock, whose LCGN coincides with the LCGN of the packet
terminal 1, in the retrieval information block (step 106). Next, an LCN is
allocated, which is less than the number of LCNs read out and which has
not been used so far, as the LCN at the network side (step 107). A larger
number is to be selected as the LCN here. For example, if the number of
multiplexible LCNs read out from the first conversion information is "10"
and so far LCN=0, 1, 4, 6 and 9 have been used, the LCN to be allocated at
the network side is "8". The TEI, LCGN, LCN and the subscriber line number
at the lien side, and the TEI and LCN at the network side are stored as
the second conversion information (step 108). The LCGN is not converted
and the same LCGN is used at both line side and network side.
If the packet identifier read out at step 101 is other than a call request,
it means that the D channel packet information sent from the packet
terminal 1 is information sent to the line circuit 4.sub.l at a later
stage of packet communication, and the second conversion information has
already been obtained and stored. Therefore, the TEI, LCGN and LCN of the
packet terminal 1 is read out from the D channel packet information (step
109), and the stored second conversion information is retrieved using this
information to obtain second conversion information which has the same
TEI, LCGN, LCN and subscriber line number as those of the packet terminal
(step 110).
Then, the TEI and LCN are converted to those of the network side for
transmitting the D channel packet information to the network 3 (step 111).
The packet identifier is again read out (step 112), and if the packet
identifier indicates a disconnect confirmation, then the D channel packet
information sent from the packet terminal 1 is at the final stage of
packet communication and the second conversion information is no longer
required for this packet communication, and so it is deleted (step 113).
Thereafter, the converted "X.25 call request" information sent to the
network 3 is received by a PBX at the receiving side, and link set up
procedures based on the Q.931 protocol and call termination procedures
based on the X.25 protocol are performed at the receiving side PBX and a
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