Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
107894'7
The present invention relates to a packet exchange
system in a data communication system, and in particular re-
lates to the system for establishing the address of a receiving
terminal in a packet switching network.
In a data communication system, a process of data
communication consists of a sequence of phases, for example,
the phase of connection o~er a general switch network, the
phase of establishment of a data link, the phase of information
transfer, the phase of termination, and the phase of clearing
of general the switch network, as internationally standardized
in the bulletin of the International Organization for Standar- ;
dization, by International Standard ISO 1745-1975 (E).
In data exchange networks, there is an ever increasing
demand for higher speed, higher reliability, wider variety of
services, and broader service areas. To meet such a demand, at
l~ast three switching systems have been known, (a) a circuit
switching system, (b) a message switching system or a store-and-
forward system, and (c) a packet switching system. me circuit
switching system directly connects a pair of subscribers (a
calling subscriber and a called subscriber) and is utilized in
the world-wide telex system. In the message switching system,
a called subscriber is not directly connected to a calling sub-
scriber, but a message from a calling subscriber is temporarily
stored in a switching station and a stored message is forwarded
to a called subscriber. A packet switching system is a combi-
nation of a circuit switching system and a message switching
system. In a packet switching system, data or information from
a subscriber is arranged to form one or more data unit, called
"a packet", to be handled by the network. A called subscriber
is not connected directly to a calling subscriber, although the
former is logically connected to the latter. In a packet
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switching system, all data or packets from a calling subscriber
and/or a called subscriber are temporarily stored in a switch-
ing station and are forwarded to a receiving terminal.
The packet switching network originally used a
connection-control system, in which physical data links bet-
ween calling terminals and called terminals, such as those in
circuit switching networks, are not established, but call-
packets and response-packets that do not include information
are exchanged between the calling and called terminals, so as
to establish logical data links in the switching network through
which the information is transmitted. Once the logical data
links are established, the information alone can be transmitted
and received. The International Telegraph & Telephone Consul-
tative Committee's (C. C. I. T. T.) recommendations, X20 and
X21, are two international standards concerning a packet switch-
ing system.
According to the prior packet switching system in-
cluding a character terminal, the logical data link is estab-
lished in the following manner. First, a calling terminal
originates a call to a subscriber switching station, which re-
turns to the calling terminal a permission signal. Then, the
calling terminal sends the address information of the called
terminal to the switching network~ The format of the address
information is that the address information is inserted between
a pair of symbols "-" and ~+~O When the address information is
correctly received by the called subscriber, an acknowledgement
signal (connect-through (CT) signal) is sent to the calling
terminal. Then, the calling subscriber begins to send a data,
whose format is, for instance that a pair of symbols, STX
(Start of Text) and ETB (End of Transmission Block), sandwiches
a data.
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Later, another method was contrived for establishing
logical data links while disposing information within call-
packets and response packets, based on the fact that a connec-
tion-establishing phase and information-transferring phase can
be simultaneously executed in the packet switching network be-
cause the network takes part in the information-transferring
phase, too. This method can be easily applied to packet ter-
minals, but is difficult to apply to character terminals.
A packet terminal is defined as a terminal which can
dissolve and assemble packets, control the sequence of packets,
and handle a plurality of logical data links. A character
terminal is defined as a terminal which can handle only one
logical data link at a time as seen from the network and is
used in a conventional circuit switching system or a leased
circuit and operates in the conventional transmission control
procedure. At the character terminals, the simultaneous execu- `;~
tion of the connection-establishment phase and the data-trans-
ferring phase is not contemplated at the same time, since a
separate control unit (for instance, a dialing facility) is
provided in the character terminal for the connection-establish-
ment. Thus, a prior character terminal have the following
shortcomings.
(1) Since the control unit for connection-establish-
ment is necossary, the terminal is complicated. Said control
unit must at least function to send a dial signal inserted
between a pair of symbols "-" and "+", and to process a fixed
pattern of a permission signal, and a connect-through (CT)
signal.
(2) Call-establishing time is long, because the
information-transfer becomes possible only after a calling
signal and a selection signal are transmitted and a connect-
through (CT) signal is received from the switching network.
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(3) Due to the last mentioned shortcoming (2), when
a short telegram message is transmitted, the connection-
establishing process is more prominent than the data-transfer-
ring process in the processing at the switching network.
(4) A great deal of signal exchange takes place
between a character terminal executing communication and the
switching network, so that the operation of the character ter-
minal becomes complicated.
It is an object, therefore, of the present invention
to overcome the disadvantages and the limitations of a prior
packet switching system by providing a new and improved packet
switching system.
It is also an object of the present invention to
provide a new and improved character terminal and a subscriber
exchange for the present packet switching system.
The basic concept of the present invention is that,
in a character terminal, a prior means for connection-establish-
ment is removed, and the connection-establishment and/or the
line-release is performed according to the transmission control
procedure of an information transfer phase.
The above and other objects are attained by a packet
switching system having at least a character terminal and a
subscriber switching station and a receiving terminal, compri-
sing the steps of forwarding an information frame including the
address of a receiving station according to the transmission
control procedure of an information transfer phase from the
character terminal to the receiving terminal via the subscriber
switching station, forwarding a response packet from the re-
ceiving terminal to the switching station, and returning an
acknowledgement signal according to the transmission control
procedure of an information transmission phase from the sub-
scriber switching station to the character terminal when the
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switching station acknowledges receipt of the response packet
from the receiving terminal.
According to a further broad aspect of the present
invention, there is provided a method for controlling a packet
switching system having at least a character terminal, a sub-
scriber switching station and a receiving terminal. The method
comprises the steps of forwarding an information frame including
the address of a receiving station according to the transmission
control procedure of an information transfer phase from the
character terminal to the receiving terminal via the subscriber
switching station, forwarding a response packet from the receiv-
ing terminal to the switching station, and returning an acknow-
ledyement signal according to the transmission control procedure
of an information transfer phase from the subscriber switching
station to the character terminal when the switching station
acknowledges receipt of the response packet from the receiving
terminal.
According to a further broad aspect of the invention,
there is provided a subscriber switching station in a packet
switching system, including a line and a terminal. The switch-
ing station comprises a frame receiver, an address registration
circuit for storing a frame of address information, a packet -
transmitter for transmitting a packet to the line from one of
the address registration circuit and the frame receiver, a
packet receiver for forwarding a packet from the line to the
address registration circuit, an acknowledgement transmitter
for sending an acknowledgement signal to the terminal, further
characterized by comprising a first switching means responsive
to the address registration circuit for transferring a signal
from the frame receiver to one of the address registration cir-
cuit and the packet transmitter and a second switch means res-
ponsive to the address registration circuit for switching between
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the frame receiver and the acknowledgement transmitter.
The foregoing and other objects, features and atten-
dant advantages of the present invention will be appreciated
as the same become better understood by means of the following
description and the accompanying drawings wherein:
Fig. 1 is a block diagram, showing the structure of
a packet switching network,
Fig. 2 shows some examples of a format of a set of
frames sent from a character terminal according to the present
invention,
Fig. 3 is a diagrammatic illustration of an example,
in which connection-establishment is executed by using the
transmission-control procedure of an information transfer phase,
Fig. 4 is a diagrammatic illustration of an example,
which is similar to that of Fig. 3, except that the connection-
establishment and the information-transfer are simultaneously
executed,
Fig. 5 is a diagrammatic illustration of an example,
which is similar to that of Fig. 4, except that a response
signal includes information,
Fig. 6 is a diagrammatic illustration of an example,
in which operations from the connection-establishment until -~
the connection-release are simultaneously executed, and,
Fig. 7 is a block-diagram showing the structure of a
character terminal and a subscriber switching station.
Fig. 1 illustrates an example of packet switching net-
works, to which the present invention can be applied. The
packet switching network consists of subscriber switching
stations B (Bl, B2) to which subscriber terminals (character
terminals and packet terminals) A (Al, A2) are connected, and
relay stations C (Cl, C2, C3) relaying packets, for executing
the transfer through the network by selecting an optimal transfer
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path for each packet.
There are two types of packet switching networks,
namely, packet switching networks in which logical data links
are established, and other packet switching networks in which
no logical data links are established. An example of the
former type packet switching networks with the logical data
links will now be described.
It is assumed in this specification that the terminal
Al is a character terminal, the terminal A2 is a packet terminal,
and the call is originated at the terminal Al.
Fig. 2 shows some examples of the format of a set of
frames produced by the character terminal Al.
The information has in the present embodiment a
STX symbol (Start of Text), an ETB symbol (End of Transmission
Block), and a message frame, between the pair of symbols. The
message frame can be (A) a selection frame (SEL) which is the '
address of the called terminal, (B) a data frame (DATA) trans-
mitted between a pair of terminals, (C) a release frame (REL)
for releasing the line, (D) a selection frame and a data frame,
and (E~ a selection frame, a data frame, and a release frame.
Since each character terminal can have his own trans-
mission control procedure, the format of a frame depends upon
each terminal, and any format except Fig. 2 is of course
possible. For instance, a transmission control procedure which
has no STX or no ETB is possible. So Fig. 2 is the mere
embodiment for the simplicity of the explanation.
Fig. 3 illustrates the manner in which a selection
signal is transmitted by applying the transmission-control
procedure of an information-transfer phase to a connection-
establishing phase, as will be explained below in a step-by-
step fashion.
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(1) The character terminal Al initiates a call, as
indicated by the arrow (a).
(2) Upon detecting the initiated call (a), the
subscriber switching station Bl sends an input~permitting
signal (b) to the calling terminaL A .
(3) The thus call-acknowledged character terminal A
sends a selection frame SEL (the address of a called terminal)
to the subscriber switching station Bl, which selection frame
is in the same frame structure as that of the transmission-
control procedure of the information transfer phase. The format
of said selection frame is shown in Fig. 2(A).
(4) The subscriber switching station Bl sends the
address packet ADR representing the called terminal A2 to that
subscriber switching station B2 to which the called terminal
A2 is connected, through relay stations Cl and C2.
(5) The subscriber switching station B2 caLls up the
called terminal A2, and as the terminal A2 responds, a response
packet is sent to the Station B2, as indicated by the dotted
line, and a response packet (RES) is sent to the subscriber
switching station Bl.
(6) The subscriber switching station Bl sends an
acknowledging response signal (e.g., ACK) of the transmission-
control procedure, as a signal acknowledging the address regis-
tration. At this moment, a logical data link is established
between the calling terminal and the called terminal, for
facilitating the communication therebetween. It is a matter
of course that a busy signal instead of an acknowledging res-
ponse signal is sent to the character terminal when the called
terminal is busy.
Thus, it becomes possible to establish connection by
using the transmission-control procedure of the information
transfer phase provided in the character terminal. When the
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line is established, the pair of terminals Al and A2 begins
to send and receive a data packet or a frame DATA, whose format
is shown in Fig. 2(B).
It should be noted that the procedure shown in Fig.
3 is novel and different from a prior packet switching system.
In a prior character terminal of a prior packet switching
system, an address signal inserted between a pair of symbols,
+ and -, is sent from the terminal Al to the subscriber switch-
ing station Bl instead of the selection frame (SEL~ of Fig. 3,
and the connect-through (CT) signal is returned to the charac-
ter terminal. The format or the signal specification of "+",
"-" and "CT" is fixed and is determined by the switching net-
work, instead of a terminal. According to the present invention,
the format and/or the signal specification of the selection
frame and an acknowledgement signal which may correspond to the
prior +, - and CT depend upon the transmission control proce-
dure of each character terminal. While, in a prior character
terminal of a prior packet switching system although the ter-
minal sends a selection frame SEL, the subscriber switching
station Bl returns an acknowledgement signal to the calling
terminal Al just after the station Bl receives the selection
frame SEL. Said acknowledgement signal just indicates to the
terminal Al that the selection frame SEL is received by the
station Bl, but it does not indicate that the selection frame
SEL is correctly received by the called terminal A2.
Fig. 4 illustrates an example, in which connection-
establishment and information-transfer are executed by applying
the transmission-control procedure of an information transfer
phase to a connection-establishing phase. A call-acknowledged
character terminal Al sends a selection unit (the address of a
called terminal) and data to be transmitted (SEL + DATA) packed
in a single frame as shown in Fig. 2(D) to the subscriber
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switching station Bl, while forming both the selection unit and
the data to be transmitted in the same frame structure as that
of the transmission-control procedure of an information-transfer
phase. The switching network calls up a called terminal A2
through relay stations Cl and C2 and another subscriber switch-
ing station B2, so as to receive a response packet (RES). Such
a response packet (RES) is sent to the calling terminal's
switching station Bl, and the subscriber switching station B
sends an acknowledging response signal (e.g~, an ACK) of the
transmission-control procedure of the information transfer phase
to the character terminal Al, as a signal acknowledging the
address registration. Thus, the connection-establishment and
the information-transfer are simultaneously executed by using
the transmission-control procedure of the information transfer
phase~
Another example, as shown in Fig. 5, is similar to
that of Fig. 4, except that the response packet from a called
terminal A2 includes data in the form of RES + DATA) so that
the data included in the response packet are sent to a charac-
ter terminal Al from the subscriber switching station B2 insteadof a mere response packet (RES). In this case, the transmission-
control procedure of an information transfer phase is also used,
and the connection-establishment and the information-transfer
are simultaneously executed, too.
Fig. 6 shows an example, in which the transmission-
control procedure of an information transfer phase is applied
to a connection-establishing phase, so as to simultaneously
execute the connection-establishment, the information transfer,
and the connection-release. A character terminal Al, whose
call is acknowledged, sends a selection signal (the address of
a called terminal), the data to be transmitted, and an indica-
tion for releasing toward a subscriber switching s-tation Bl in
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the single frame structure shown in Fig. 2(E) as that of the
transmission-control procedure of an information transfer phase.
Upon detection of the presence of the indication for releasing,
the subscriber switching station Bl sends an address informa-
tion indicating the called terminal A2 and the indication for
releasing toward that subscriber switching station B2 to which
the called termianl A2 is connected. At the same time, the
switching station Bl sends out an acknowledging response signal
of the transmission-control procedure of an information transfer
phase toward the character terminal Al and then shifts into a
connection-releasing phase. Although the subscriber switching
station B2 calls up the called terminal A2 and sends out the
data, when the presence of the indication for releasing together
with the address and the data is detected, no response packet
is returned to the subscriber switching station Bl. Accordingly,
the connection-establishment, the information-transfer, and the
connection-release are simultaneously executed.
In the preceding examples, only packet terminals are
used as the called terminal A2, but similar operations can be
performed with called character terminals, too. The case of
the character terminals for the called terminals is different
from the case of the packet terminals therefor in that there
are a call from the subscriber switching station B2 to the
character terminal A2 and a response signal thereto.
Fig. 7 shows a block-diagram of the character ter-
minal Al and the subscriber switching station Bl.
The character terminal Al sends a calling signal
from the call-release unit (CAL/REL) as indicated (a) in Figs. 3
through 6. When the calling signal (a) is received by the call
detector (CAL) of the subscriber switching station Bl, station
Bl sends an input permitting signal, as indicated (b) in Figs. 3
through 6, to the terminal Al. Upon receipt of the input per-
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mitting signal, the terminal Al sends a frame through the frametransmitter (FRM TR) and the modulator-demodulator (MODEM) to
the line (LINE). The set of frames is in the format of Fig. 2
and may include a selection frame (SEL), a data frame (DATA),
and/or a release frame (REL). The subscriber switching station
Bl receives a frame through the frame receiver (FRM REC) and
the switch SWl and stores the address information in the frame
in the address registration circuit (ADR REG). The switch SWl
is in the position "a" in the initial state, and the switch
SW2 is in the position "off" in the initial state. When the
first frame includes only an address (selection), the address
registration circuit (ADR REG) sends the address information
to the packet transmitter (PCK TR), which sends, in turn, an
address packet (ADR in Fig. 3) to the line (LINE) through the
modulator-demodulator (MODEM). The address packet (ADR) is
received by the terminal A2 as a ringing packet or a calling
packet (CAL). When the first frame includes an address frame
and a data frame as shown in Fig. 2(D), the address registra-
tion circuit (ADR REG) sends both the address information and
the data to the packet transmitter (PCK TR). The address in-
formation stored in the address registration circuit (ADR REG)
is held until the switching station receives a release frame,
and said address information indicates the address of a data
packet.
When the switching station Bl receives a response
packet (RES) from the called terminal A2 through the subscriber
switching station B2 and the relay stations C2 and Cl, the
packet receiver (PCK REC) sends the response packet to the ad-
dress registration circuit (ADR REG). Then, the address regis-
tration circuit (ADR REG) causes the switch SWl to turn to theposition "b" so as to connect the frame receiver (FRM REC) to
the packet transmitter (PCK TR), and causes the switch SW2 to
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turn to the position "on" to connect the frame receiver (FRMREC) to the acknowledgement transmitter (ACK TR3. The switch-
ing on condition of the switch SW2 causes the ACK signal trans-
mission indication to be sent to the (ACK TR). As a result,
- the ACK TR sends the acknowledgement signal to the terminal A,
in accordance with the transmission control procedure. The
calling terminal Al, upon receipt of the acknowledgement signal
through the acknowledgement receiver (ACK REC), begins to trans-
mit a data frame by operating the keyboard (KEY). The acknow-
ledgement transmitter (ACK TR) functions merely as a paritycheck circuit after the second frame. Upon receipt of the re-
lease signal of character terminal Al, the call detector (CAL)
sends a signal to address registration circuit (ADR). Upon
receipt of said signal from the call detector (CA~), the address
registration circuit (ADR) controls the switches SWl and SW2
to turn back to the initial state.
It should be appreciated that the apparatus of Fig. 7
can accept a plurality of character terminals, each of which
can have his own transmission control procedure. To meet with
the above function, a program control of the subscriber switch-
ing station may be preferable.
In Fig. 7, it should be noted that the structure of
the character terminal Al may be the same as a conventional
terminal used in a leased line, and the presence of the switches
SWl and SW2 in the switching station is one of the important
features of the present invention.
As is apparent from the foregoing disclosure, the
following outstanding effects can be achieved, by using the
transmission-control procedure of an information transfer phase
provided in a character terminal for executing the connection-
establishment or executing the connection-establishment and the
information transfe~
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(1) The switching function between the conneetion-
establishing phase and the information transfer phase can be
dispensed with.
(2) Since the control device for connection-establish--
ment can be dispensed with, the terminal equipment can be sim-
plified.
(3) The waiting time from the call initiation until
the data-transmission can be reduced.
(4) The number of signals to be exchanged is small,
so that the operations at the terminals and the processing at
the switching stations are simplified, so that the processing
ability of a switching network can be enhanced.
In addition, if the operation from the connection-
establishment to the connection-release are simultaneously ~
executed, the following additional effects can be achieved. -
(5) The processing at the switching station can be
further simplified, as compared with the case of establishing
logieal circuits.
(6) The holding time of a call can be redueed.
From the foregoing, it will now be apparent that a ~-
new and improved packet switching system has been found. It
should be understood, of course, that the embodiments disclosed
are merely illustrative and are not intended to limit the scope
of the invention. Reference should, therefore, be made to the
appended claims, rather than to the specification as indicating
the scope of the invention.
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