Collision/detection single node controlled local area network

A single node communication network control system which uses a plurality of transmit/receive terminals connected to the node, are controlled by the single node. A connection control device is installed in the node for controlling the connection of plurality of input channels and output channels which correspond to the input channels. This control means connects only the input channel on which first forward information has arrived the earliest. The connection control means then disconnects the output channels from the input channels except for the connected input channel which has not been used for other communication.

BACKGROUND OF THE INVENTION 
The present invention relates to a communication network control system 
and, more particularly, to a control system for a local area network 
(LAN). 
Communication network control systems know in the art include (a) a carrier 
sense multiple access (CSMA) baseband LAN, (b) a broadband LAN, (c) time 
division multiple access (TDMA) baseband LAN and digital PBX, (d) a system 
disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 57-104339, 
(e) a system disclosed in Jpanese Laid-Open Patent Pubication (Kokai) No. 
58-139543, (f) a system disclosed in Japanese Patent Application No. 
60-170428, (g) a system disclosed in Japanese Patent Application No. 
60-170429, and (h) a system disclosed in Japanese Patent Application No. 
60-170427. 
The system (a) proves effective when packets are short and occur in bursts 
like data information and text information. However, a problem with the 
system (a) is that when packets are potentially infinite in length and are 
generated continuously and real time processings of the packets are 
required, such as in multi-media communications, signals conflict very 
frequently to limit the throughput attainable. The words "multi-media 
communications" as used in the art of LANs covers not only the traditional 
data and text communications but also the interchange of image 
information, audio information and video information. The system (b) is 
rather short in capacity when applied to multi-media communications and, 
in addition, it is not fully acceptable in the aspect of cost performance 
and expansibility. As regards the system (c), although it has superior 
applicability to multi-media communications to any of the others, it also 
has problems left unsolved with regard to costs and expansibility; 
especially the costs are prohibitive when the system is applied to 
multi-media communications. 
The systems (d) and (e), on which the present invention is based, are the 
most feasible with respect to multi-media communications. Nevertheless, 
because both of the systems (d) and (e) are operated with a 
first-come-first-served logic and on a multiple-input one-output basis, 
i.e., a node which has received call requests from a plurality of 
terminals accepts only the first request to transfer a call from a single 
terminal which has originated the request, connection control means which 
is installed in the node cannot allow a plurality of different 
communications to be effected in parallel. 
The system (f) is a solution to the problems particular to the systems (d) 
and (e) as stated above. Specifically, in accordance with the system (f), 
a single node is capable of dealing with a plurality of communications at 
the same time, that is, a path with a particular pattern is fixed to 
prevent one communication from interfering with others. While such a 
scheme successfully promotes effective use of links and, thereby, 
interchange of massive data which is desirable for a large-scale LAN 
having a plurality of nodes, the system (f) is not applicable to a 
small-scale LAN in which the number of terminals is small. 
Meanwhile, it is usually necessary for a call packet to be propagated 
through a plurality of nodes before reaching a terminal called. A preamble 
which heads a packet is cut little by little at each of the nodes to bring 
about a propagation delay, which in turn correspondingly increases the 
period of time during which a path is fixed to thereby increase the 
probability of conflict of the packet with others. The system (g) is 
elaborated to elminate the propagation delay so that the period of time 
during which a path is fixed may be shortened. Further, the system (h) 
sets up semi-duplex communication to allow a single node to perform 
communications over a plurality of channels, while the system (g) sets up 
semi-duplex communication which is directed to the increase of 
communication capacity. However, a drawback with such systems on which the 
present invention is based is that they are incapable of detecting 
conflicts or collisions of packets. Although the probability and the 
influence of collisions are less in such systems than in the others, it is 
desirable that even the faults with least frequency be eliminated when it 
comes to highly reliable systems. 
As regards the reliability of communication systems, countermeasures have 
been proposed against (a) disconnection of a link, (b) down of a terminal 
or that of a node, and (c) collision, as will be described hereinafter. 
To begin with, in a carrier sense multiple access with collision detection 
(CSMA/CD) coaxial cable baseband LAN as typified by the Ethernet system 
(Xerox), upon disconnection of a link a communication with a station on 
the other side of the point of disconnection is disable, and event a 
communication with a station on this side of the same point cannot be 
effected normally since a signal is reflected at that point. Concerning a 
failure of a terminal (tap, transmitter/receiver, NTU, etc.), it would not 
affect communications so long as only the functions of the terminal were 
disabled. However, if the failure was of the kind transmitting unexpected 
signals, it would destruct signals of all the communications. Further, 
this system is apt to cause collision frequently since it is based on a 
contention-for-single-bus principle; every time a collision occurs, 
information has to be retransmitted at the sacrifice of traffic. 
In a TDMA optical fiber loop LAN, all the communications are shut off when 
a link is disconnected. To avoid this, such a LAN is usually provided a 
double-link configuration so that a communication may be returned over a 
new loop before it reaches a point of disconnection. Also, all the 
communications are disabled when a node which does not join in a 
communication under way has failed (functions disabled). Usually, such an 
occurence is handled with by doubling the link so that a communication may 
be returned over a new loop before it reaches the node in failure. 
Furthermore, when a node which is not taking part in a communication has 
failed and sent an unexpected signal, signals of all the communications 
are destroyed. Various controls adapted to settle the above sitations 
increase the total cost of the network. While a single supervisor node is 
installed in the network for the control of the entire network, it brings 
about another problem that if the supervising node fails, all the 
communcations are shut off. As regards collisions, because the capacity is 
substantially distributed to the nodes, there is no chance of a collision 
to occur although the capacity is limited. 
As stated above, in the prior art LAN systems, the contention for a common 
bus type scheme cannot avoid frequent collisions and, due to 
retransmission which is required at each time of collision, lowers the 
traffic. In addition, in the case of a system which cannot detect a 
collision, a fault would occur to down the entire system. 
SUMMARY OF THE INVENTION 
It is therefore a first object of the present invention to provide a 
communication network control system which is capable of handling a 
plurality of communications at the same time, effectively applicable to a 
small-scale network which is implemented with a single node, and 
expansible to a large-scale LAN by use of the same protocol. 
It is a second object of the present invention to provide a high throughput 
communication network control system which is capable of detecting a 
collision of packets so that communications may be effected efficiently 
despite congestion of communications. 
It is a third object of the present invention to provide a communication 
network control system which is capable of detecting a collision of 
packets and supplying a transmit terminal with collision information to 
allow it to perform retransmission. 
It is another object of the present invention to provide a generally 
improved communication network control system. 
A communication network control system to which the present invention 
pertains has a single node, a plurality of transmit/receive terminals 
connected to the node, and a connection control device installed in the 
node to control connection of a plurality of input channels and output 
channels which correspond to the input channels, the connection control 
device connecting only one of the input channels on which first forward 
information has arrived earliest to all of the output channels which have 
not been used for other communications or to all of the output channels 
except for the output channel, which corresponds to the input channel, 
which have not been used for other communications, and disconnecting all 
of the input channels except for the input channel which have not been 
used for other communications from the output channels, and, when 
transmission of the first forward information is completed, connecting to 
one of the output channels corresponding to the input channel any of the 
input channels which corresponds to any of the output channels over which 
the first forward information has been outputted, thereby passing first 
return information through the control device, and upon completion of the 
first return information, connecting any of the input channels 
corresponding to the output channel, over which the first return 
information has been outputted, to any of the output channels 
corresponding to any of the input channels on which the first return 
information has been received, and repeating the above sequence of steps. 
In accordance with the present invention, the connection control device 
repeats a procedure of detecting any of the input channels on which a 
first input signal has appeared earliest and storing the input channel 
first; when a second input signal is present on any of the input channels 
corresponding to any of the output channels over which the first input 
signal has been outputted, detecting the input channel and storing the 
input channel second; outputting the input signal over any of the output 
channels corresponding to the input channel which has been stored first 
and, when third input signals are present on any of the input channels 
other than the input channels which have been stored first and second, 
connecting any of the input channels on which the third input signal has 
appeared earliest to all of the output channels except for any of the 
output channels which correspond to the input channels stored first and 
second or which corresponds to the input channels, and storing the input 
channel on which the earliest one of the third input signals has appeared 
third; when a fourth input signal has appeared on any of the input 
channels corresponding to any of the output channels over which the third 
input signal has been outputted, detecting the input channel and storing 
the input channel fourth; and outputting the fourth input signal over any 
of the output channels which corresponds to the input channel stored 
third. 
A communication network control system to which the present invention also 
pertains has a single node, a plurality of transmit/receive terminals 
connected to the node, and a connection control device installed in the 
node for controlling connection of a plurality of input channels and 
output channels which correspond to the input channels, the connection 
control device connecting only one of the input channels on which first 
forward information has arrived earliest to all of the output channels 
which have not been used for other communications or to all of the output 
channels except for any of the output channels, which corresponds to the 
input channel, which have not been used for other communications, and 
disconnecting from the output channels the input channels except for the 
input channel which have not been used for other communications, and upon 
completion of transmission of the first forward information, connecting to 
any of the output channels corresponding to the input channel any of the 
input channels which corresponds to any of the output channels over which 
the first forward information has been outputted, and connecting any of 
the input channels corresponding to the output channel over which the 
first return information has been outputted to any of the output channels 
corresponding to the input channel on which the first return information 
has appeared, and repeating the above sequence of operations. In 
accordance with the present invention, the connection control device 
connects all of the input channels which have not been used for other 
communications, or are in a fixed path condition, to all of the output 
channels which are not in a fixed path condition or all of the output 
channels in the condition except for any of the output channels which 
corresponds to the input channel; when input signals have appeared on a 
plurality of the input channels, once outputs the input signals over the 
output channels and, then, detects and stores first any of the input 
channels on which first forward information of a first communication 
appeared earliest, and disconnects from the output channels the input 
channels except for the input channel stored first; and upon completion of 
the forward information, connects any of the input channels corresponding 
to any of the output channels over which the forward information has been 
outputted to any of the output channels corresponding to the input channel 
stored first and, then, stores second any of the input channels on which 
the return information of the first communication has arrived, thereby 
fixing a path. 
A communication network control system to which the present invention also 
pertains has a single node, a plurality of transmit/receive terminals 
connected to the node, and a connection control device installed in the 
node for controlling connection of a plurality of input channels and 
output channels which correspond to the input channels, the connection 
control device connection only one of the input channels on which first 
forward information has arrived earliest to all of the output channels 
which have not been used for other communications or to all of the output 
channels except for any of the output channels, which corresponds to the 
input channel, which have not been used for other communications, and 
disconnecting from the output channels the input channels except for the 
input channel which have not been used for the other communications, and 
upon completion of transmission of the first forward information, 
connecting to any of the output channels corresponding to the input 
channel any of the input channels which corresponds to any of the output 
channels over which the first forward information has been outputted, 
thereby passing the first return information through the connection 
control device, and upon completion of the first return information, 
connecting any of the input channels corresponding to any of the output 
channels over which the return information has been outputted to any of 
the output channels corresponding to any of the input channels on which 
the first return information has appeared, and repeating the above 
sequence of operations. In accordance with the present invention, the 
connection control device comprises a switching matrix unit constituted by 
switching elements for connecting any of the input channels to any of the 
output channels simultaneously in a plurality of combinations, a first 
input signal detecting unit for detecting any of the input channels on 
which an input signal has appeared first, an input signal monitoring unit 
for, when information on presence/absence of an input signal on any of the 
input channels has been changed, producing an output which is indicative 
of the change to the outside, a control gate unit constituted by switching 
elements for connecting any of a plurality of input channels to the first 
input signal detecting unit, and a switching control unit for controlling 
the switching elements of the switching matrix unit and the switching 
elements of the control gate unit by reading information on an input 
signal out of the first input signal detecting unit and input signal 
monitoring unit. 
In a communication network control system to which the present invention 
also pertains has a plurality of node connected by links, a plurality of 
transmit/receive terminals connected to the nodes, and a connection 
control device installed in each of the nodes for controlling connection 
of a plurality of input channels and output channels which correspond to 
the input channels, the connection control device connecting any one of 
the input channels on which first forward information has arrived earliest 
to all of the output channels which have not been used for other 
communications or to all of the input channels except for any of the 
output channels corresponding to the input channel, and disconnecting the 
input channels except for the input channels which have not been used for 
other communications from the output channels. In accordance with the 
present invention, each of the nodes compares a phase of an input signal 
arrived first on any of the input channels and a phase of an input signal 
arrived on another of the input channels later than the signal arrived 
first so as to detect any of the input channels on which a differential 
resulting from the comparison is greater than a predetermined value, 
thereby detecting that a collision has occurred between the node and 
another node or any of the terminals to which the input channel detected 
is connected. 
In a communication network control system to which the present invention 
also pertains has a plurality of nodes which are interconnected by links, 
a plurality of transmit/receive terminals connected to the nodes, and a 
connection control device installed in each of the nodes for controlling 
connection of a plurality of input channels and output channels which 
correspond to the input channels, the connecition control device 
controlling connection of any of the input channels on which first forward 
information has arrived earliest and all of the output channels which have 
not been used for other communications, connecting only the input channel 
on which the first forward information has arrived earliest to all of the 
output channels which have not been used for other communications or to 
all of the output channels except for any of the output channels which 
corresponds to the input channel, and disconnecting from the output 
channels the input channels except for the input channel which have not 
been used for other communications. In accordance with the present 
invention, each of the nodes compares and input signal appeared first on 
any of the input channels and an input signal appeared on another of the 
input channels later than the input signal and, assuming that a maximum 
delay time between input and output of a signal to and from the node is Tn 
and a maximum link propagation delay time at a maximum node-to-node 
distance is T1, determines that a collision has occurred between the node 
and another node or any of the terminals to which the input channel, on 
which the input signal has appeared later, is connected when a signal 
representative of a differential between the first signal and the later 
signal remains at a high level more than a predetermined period of time Td 
of 2 (Tn+T1). 
In a communication network control system to which the present invention 
also pertains has a plurality of nodes interconnected by links, a 
plurality of transmit/receive terminals connected to the nodes, and a 
connection control device installed in each of the nodes for controlling 
connection of a plurality of input channels and output channels which 
correspond to the input channels, the connection control device connecting 
only one of the input channels on which first forward information has 
arrived earliest to all of the output channels which have not been used 
for other communications or to all of the output channels except for any 
of the output channels which corresponds to the input channel, and 
disconnecting the input channels except for any of the input channels 
which have not been used for other communications from the output 
channels. In accordance with the present invention, each of at least a 
part of the terminals compares an output signal which the terminal has 
sent and an input signal which has been sent by any of the nodes to which 
the terminal is connected and, when detected that a differential resulting 
from the comparison is greater than a predetermined value, decides that a 
collision has occurred between the terminal and the node. 
In a communication network control system to which the present invention 
also pertains has a plurality of nodes interconnected by links, a 
plurality of transmit/receive terminals connected to the nodes, and a 
connection control device installed in each of the nodes for controlling 
connection of a plurality of input channels and output channels which 
correspond to the input channels, the connection control device connecting 
only one of the input channels on which first forward information has 
arrived earliest to all of the output channels which have not been used 
for other communications or to all of the output channels except for any 
of the output channels which corresponds to the input channel, and 
disconnecting the input channels except for any of the input channels 
which have not been used for other communications from the output 
channels. In accordance with the present invention, each of the terminals 
compares an output signal thereof and an input signal thereto and, when a 
signal has been outputted over any of the output channels of the terminal, 
compares the output signal on the output channel and an input signal on 
any of the input channels which corresponds to the output channel and, if 
a differential signal resulting from the comparison remains at a high 
level more than a predetermined period of time T'd which is equal to 
Tn+2T'1, where Tn is a maximum delay time between input and output of a 
signal to and from a node and T'1 is a maximum link propagation delay time 
at a maximum node-to-terminal distance, decides that a collision has 
occurred between the terminal and any of the nodes to which the terminal 
is connected. 
In a communication network control system to which the present invention 
also pertains has a plurality of nodes interconnected by links, a 
plurality of transmit/receive terminals connected to the nodes, and a 
connection control device installed in each of the nodes for controlling 
connection of a plurality of input channels and output channels which 
correspond to the input channels, the connection control device connecting 
any one of the input channels on which first forward information has 
arrived earliest to all of the output channels which have not been used 
for other communications or any of the output channels corresponding to 
the one input channel, and disconnecting from the output channels the 
input channels except for the input channel which have not been used for 
other communications. In accordance with the present invention, each of 
the nodes compares in phase an input signal which has arrived on any of 
the input channels first and an input signal which has arrived on another 
of the input channels later than the input signal and, if a differential 
resulting from the comparison is greater than a predetermined value, 
produces the difference as a differential signal and sends the 
differential signal as a collision detection signal over any of the output 
channels corresponding to the input channel on which the input signal 
arrived first. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following detailed 
description taken with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To better understand the present invention, a a brief reference will be 
made to prior art network control systems. 
Referring to FIG. 1, the construction of the network (d) as previously 
discussed and disclosed in Japanese Laid-Open Patent Publication No. 
57-104339 is shown. The system of FIG. 1 is characterized by a 
first-come-first-served logic, multi-connection structure, and fixing of a 
path by a multi-input single-output element. As shown, a number of 
terminals 12a, 12b, 12c and so on are interconnected by communication 
paths 10 to complete a network. Terminals which are to hold a 
communication interchange four basic packets, i.e., a call packet, a 
call-back packet, a message packet and an acknowledge or ack packet as 
shown in FIG. 2, which are adapted to control nodes 14a, 14b, 14c and so 
on. Having a multi-connection structure, each of the nodes gates only one 
communication according to the order of arrival of packets while causing 
the others to wait, whereby a multi-input single-output operation is 
accomplished. The node fixes a single communication path by changing the 
direction of transmission such that the sequence of packets of FIG. 2 are 
sequentially reversed in the direction of transfer. Further, the node has 
a function of identifying a destination address so that it may detect a 
destination address which is added to a packet and, if it is that of any 
terminal which is connected to itselt, delivers the packet to the 
terminal. If the packet is not meant for a teminal connected to the node, 
the destination address is not used for any control. 
As shown in FIG. 2, the call packet is made up of a preamble, an address 
which follows the preamble, and a message which follows the address. 
Usually, the message fragment of a call packet has no content since the 
function of a call packet is not more than calling a destination. Added to 
the call-back packet is only a preamable. A terminal called, or call-back 
end, sends the call-back packet to a terminal calling or call end, when 
the destination address is coincident with its own address. Because any of 
the nodes through which the call packet has been passed reverses the 
direction of transfer upon the lapse of a predetermined period of time, 
the call-back packet is allowed to safely reach the call end along the 
route through which the call packet had been propagated, but in the 
opposite direction. The message packet which is sent from the call end to 
the call-back end includes a preamble and a message. Because any of the 
nodes through which the call-back packet has been propagated reverses the 
direction of transfer upon the lapse of a predetermined period of time, 
the message packet is surely delivered to the call-back end even if it 
lacks an address. The ack packet is returned from the call-back end to the 
call end when the former has successfully received the message. Again, the 
ack packet can surely reach the call end even if it lacks an address. In 
this manner, a path is fixed simply by adding an address to the first 
packet, whereby a packet is interchanged by two full rounds. 
The system (e) disclosed in Japanese Laid-Open Patent Publication No. 
58-139543 is essentially the same as the above-described system except 
that it causes all the addresses to be identified by terminals and not by 
nodes so as to cut down the system cost. 
Any of the two prior art systems as discussed above is operated such that 
when received call requests from a plurality of terminals, a node accepts 
only the first request to transfer a call from a single terminal. It 
follows that connection control means installed in each node cannot 
accommodate a plurality of communications simultaneously, and this is 
quite inconvenient. Specifically, assume that a path is fixed in a 
particular pattern between the terminals 12a and 12b, as indicated by 
hatching in FIG. 1. Then, despite that each of the three nodes 14a, 14b 
and 14c through which the path extends has other available links, the 
links except for the one which has been occupied are disconnected with the 
result that the network is cut in halves. Under this condition, the 
terminals 12c and 12d, for example, cannot hold a communication with each 
other until the communication between the terminals 12a and 12d is 
completed. 
The system (f) disclosed in Japanese Patent Application No. 60-170428 as 
previously stated is a solution to the above problem. In accodance with 
this system (f), a plurality of communications may be handled 
simultaneously so as to prevent a path set up in a particular pattern from 
interfering with other communications. Specifically, as shown in FIG. 3, 
because each node is capable of dealing with a plurality of communications 
at the same time, there can be held a communication between, for example, 
the terminals 12d and 12e, a communication between the terminals 12c and 
12f, and a communication between the terminals 12g and 12h without being 
effected by the fixed path pattern. As soon as the first forward 
information and the first return information are fully interchanged 
through any of the nodes, all the other channels assigned to the node are 
brought into connection to output only that channel on which an input 
signal arrived earliest is present. As previously discussed above, this 
kind of system is limited in applicable range concerning the scale. 
Hereinafter will be described preferred embodiments of the present 
invention which are free from the drawbacks inherent in the prior art 
systems as described above. 
First embodiment 
This embodiment is directed to the first object as previously stated. 
First, the principle of this embodiment and that of a prior art system 
will be described to compare them. 
Referring to FIG. 4A, a prior art optical star network is shown which 
includes terminals 16a to 16d and a star coupler 18. Because a network in 
accordance with this embodiment has a star type physical topology, it will 
be described in conjunction with a prior art optical star type network for 
comparison purpose. While the physical configuration of an optical star 
network is of a star type, its connection will be understood more easily 
when represented in a side view, as shown in FIG. 4A. An input port IN and 
an output port OUT of each of the terminals 16a to 16d are respectively 
connected to an output port OUT and an input port IN of the star coupler 
18. FIG. 4A shows an access method which is used with the network of FIG. 
4A. As shown, a packet which is sent from the transmit terminal 16a is 
propagated to the input port IN of the star coupler 18 and, then, fed from 
the output port OUT of the start coupler 18 to the channels which are 
connected to the terminals 16a to 16d. It will be seen from FIG. 4B that 
the logical topology is of a bus type. Because contention usually occurs 
for the single bus, the CSMA/CD system is used to implement accesses. 
Referring to FIGS. 5 and 6, there are shown different specific 
constructions of the star coupler of FIG. 4A. The node shown in FIG. 5 is 
provided with the same number of optical-to-electronic converting sections 
(O/Es) and electronic-to-optical converting section (E/Os) as the input 
and output channels. The node of FIG. 6, on the other hand, is provided 
with the same number of O/Es as the input and output channels, and a 
single E/O which has great output energy. Alternatively, use may be made 
of an O/E having high sensitivity or a directional coupler which bundles 
up optical fibers at the input channels in order to reduce the number of 
O/Es to one. In FIGS. 5 and 6, the reference numeral 20 designates optical 
fibers, 22 designates photodiodes/transistors, 24 designates an inverter, 
26 designates metal oxide semiconductor (MOS) transistors, 28 designates 
light-emitting diodes (LEDs), and 30 designates optical fibers. Packets 
from four terminals, for example, are propagated through the optical 
fibers 20 to the input port of the star coupler 18. The packets are 
converted by the four diodes 22 associated therewith into electric signals 
which are directed toward a common line. At the output port as shown in 
FIG. 5, the electric signals cause the inverter 24 to drive the 
transistors 26 which are adapted to drive the LEDs 28, whereby light 
issuing from each LED 28 is sent to all the terminals 16a to 16d over 
their associated optical fibers 30. At the output port as shown in FIG. 6, 
on the other hand, the inverter 24 operates a single transistor 26 so that 
light issuing from a single LED 28 is propagated through the four optical 
fibers 30 to the respective terminals 16a to 16d. 
Referring to FIG. 7, the functions of this particular embodiment are 
schematically shown. In this embodiment, a single node 32 is installed in 
the network in order to control a plurality of communications. In FIG. 7, 
a first to a fifth terminals are labeled 34a to 34e, respectively, and may 
be implemented with a keyboard display, a word processor with a 
communication function, a personal computer, etc. Designated by the 
reference numeral 36 is a disk server, 38 an optical disk server, and 40 a 
print server. In this embodiment, the first terminal 34a outputs data to 
the printer server 40, and the second terminal 34b reads file data out of 
the optical disk server 38. In this manner, the single node 32 is capable 
of controlling a plurality of communications at the same time and in a 
parallel relationship. 
Referring to FIGS. 8A to 8E, a sequence of controlling the connection of 
the node as shown in FIG. 7 is demonstrated. In FIGS. 8A to 8E, the same 
devices as those shown in FIG. 7 are designated by like reference 
numerals. The node 32 does not discriminate a node-to-node connection and 
a node-to-terminal connection from each other, while allowing the number 
of nodes to be increased without any restriction. Further, even when the 
system is expanded by increasing the number of nodes, it is needless for 
the protocol to be modified. FIG. 8A shows a condition wherein a fixed 
link is set up between the first terminal 34a and the print server 40 by 
the interchange of information via the node 32. In this condition, assume 
that a call packet from the second terminal 34b has arrived at the node 32 
for the first time. Then, the node 32 delivers the call packet from the 
terminal 34b to the other terminals 34c to 34e, disk server 104, and 
optical disk server 38, as shown in FIG. 8B. Then, as shown in FIG. 8C, 
the node 32 reverses the direction of communication upon the lapse of a 
predetermined period of time after the passage of the first forward 
information (call packet). In FIG. 8D, the node 32 is shown as passing 
therethrough only one first return information (call-back packet from the 
destination terminal 36). This return information surely reaches the 
terminal 34b even if no destination address is added thereto. Finally, as 
shown in FIG. 8E, a fixed link is set up for the second communication 
(between the terminal 34b and the disk server 36) in addition to the fixed 
link for the first communication (between the terminal 34a and the print 
server 40). That is, two independent communications are held via the 
single node 32. 
Referring to FIG. 9, a specific internal arrangement of the node 32 of FIG. 
7 is shown. As shown, the node 32 comprises input ports I.sub.1 to 
I.sub.7, output ports O.sub.0 to O.sub.7, a switching matrix unit 42 
adapted to interconnect input channels and output channels which are 
associated with the ports I.sub.0 to I.sub.7 and O.sub.0 to O.sub.7, a 
first input signal detecting unit 44 for detecting a particular input 
channel on which an input signal arrived earliest has appeard, an input 
signal monitoring unit 46 for constantly monitoring input signals, a 
control gate unit 48 for selectively connecting the input channels to the 
unit 44, and a switching control unit 50 for controlling the entire node. 
Assuming that the node 32 has eight intput channels, the switching matrix 
unit 42 comprises eight modules which are assigned one-to-one to the 
output channels, while each of the units 44, 46 and 48 comprises a single 
module. These modules are connected to the switching control unit 50 by a 
select bus 52, a gate set bus 54, and a data bus (output signal lines of 
the modules) 56. Receiving outputs of the units 44 and 46 over the data 
bus 56, the switching control unit 50 controls the selection of a module 
of the switching matrix unit 42 via the module select bus 52 as well as 
the operation of the units 44 and 46 while, at the same time, controlling 
the units 42 and 48. 
Referring to FIGS. 10A to 10H, the operating sequence of each of the units 
as shown in FIG. 9 is shown. As shown in FIG. 10A, in the initial 
condition of the node 32, the switching control unit 50 connects all the 
input channels of the control gate unit 48 to the first input signal 
detecting unit 44 by a control signal C.sub.1 and, at the same time, 
connects the input channels I.sub.0 to I.sub.7 of the switching matrix 
unit 42 to the output channels O.sub.0 to O.sub.7 by a control signal 
C.sub.2. When an input signal has appeared on, for example, the input 
channel I.sub.2, as shown in FIG. 10B, the input signal is sent out over 
all the output channels O.sub.0 to O.sub.7. The input signal on the input 
channel I.sub.2 which is applied to the control gate unit 48 is 
transferred to the first input signal detecting unit 44. Further, the 
input signal is routed to the input signal monitoring unit 46. 
FIG. 10C shows a condition, wherein input signals have arrived over the 
input channels other than the channel I.sub.2, too. The unit 44 detects 
that an input signal has appeared on the input channel I.sub.2 first, 
while the switching control unit 50 reads it out by the control signals, 
C.sub.1 and C.sub.2 to perform first storage. Subsequently, when input 
signals have appeared on other channel, they are also outputted over the 
output channels O.sub.0 to O.sub.7. At this instant, an interference 
occurs in the outupt signals. FIG. 10D shows a condition wherein the input 
channels other than a one on which an input signal has arrived earlies are 
disconnected from the output channels. Specifically, the unit 50 cuts off 
the connection of the input channels of the unit 42 other than I.sub.2, 
i.e., the input channels I.sub.0, I.sub.1 and I.sub.3 to I.sub.7 to the 
output channels O.sub.0 to O.sub.7 by the control signal C.sub.1. 
FIG. 10E shows a modification to the operation as shown in FIG. 10D. The 
modified sequence of FIG. 10E makes it possible to disconnect an input 
channel with an input signal arrived first from a particular one of the 
output channels which corresponds to the input channel or any other output 
channel which is associated with a link which has failed. For this 
purpose, the terminal, like the node, is constructed such that when it has 
received the first forward information on an input channel thereof, it 
sends out the information from its channel. Specifically, although the 
input channel I.sub.2 of the unit 42 is connected to all the output 
channels to transmit the input signal, the signal applied to the output 
channel O.sub.2 associated with the input channel I.sub.2 will simply be 
returned to the call and to serve no purpose but confirmation and, for 
this reason, the particular input channel is disconnected from the output 
channel associated therewith. Meanwhile, assume that a signal sent from 
this node to an adjacent node or the first forward information sent to a 
terminal having the special construction as described above is not 
returned to any of the input channels on which the signal is expected to 
appear immediately, e.g. input channel I.sub.6. In this condition, it is 
possible to inhibit the connection to the output channel O.sub.6 which 
corresponds to the input channel I.sub.6, considering that the a fault has 
occurred in that link or that the adjacent node or the terminal is not 
powered. This is effected by a control signal C.sub.3 which is generated 
by the switching control unit 50. 
FIG. 10F shows a condition wherein after the completion of the forward 
information the direction of transfer is to be switched to prepare for 
return information. Specifically, when the input signal monitoring unit 46 
has detected the end of the input signal, the switching control unit 50 
senses it with the control signals C.sub.1 and C.sub.2 and, then, controls 
the switching matrix unit 42 by the control signal C.sub.3 to thereby 
connect the input channels I.sub.0 to I.sub.7, the input channels I.sub.0, 
I.sub.1 and I.sub.3 to I.sub.7, or the input channels I.sub.0, (I.sub.2), 
I.sub.3 to I.sub.5 and I.sub.7 to the output channel O.sub.2. This allows 
return information to be surely delivered to the sender of the forward 
information with no regard to the input channel. FIG. 10G shows a 
condition whrerein return information has arrived at the node. 
Specifically, as return information which is a response to the previously 
delivered forward information arrives over a certain input channel, e.g., 
input channel I.sub.0 within a predetermined period of time, it is routed 
to the output channel O.sub.2 which has already been connected to the 
input channel I.sub.0. Where the first communication is constituted by the 
previously delivered forward information and the return information to be 
delivered this time, the monitoring unit 46 detects the arrival of the 
first return information of the first communication on the input channel 
I.sub.0, while the control unit 50 reads it out with the control signals 
C.sub.1 and C.sub.2 to perform second storage. The unit 50 connects the 
input channels I.sub.1 and I.sub.3 to I.sub.7 of the control gate unit 48 
to the signal detecting unit 44 by the control signal C.sub.3, resets the 
unit 44 by a control signal C.sub.4, and connects the input channels 
I.sub.1 and I.sub.3 to I.sub.7 to the outut channels O.sub.1, O.sub.3 to 
O.sub.7. Under this condition, the node waits for input signals over all 
the channels except for the input channels I.sub.0 and I.sub.2 and output 
channels O.sub.0 and O.sub.2 which are being occupied. 
Further, FIG. 10H shows a condition wherein a plurality of different 
communications are handled at the same time. Assume that while an input 
signal on the input channel I.sub.0 is coupled to the output channel 
O.sub.2, an input signal appears on another input channel, e.g. input 
channel I.sub.4. Then, the input signal on the input channel I.sub.4 is 
outputted over all the output channels except for the channels O.sub.0 and 
O.sub.2 being occupied, as previously stated. Naturally, the input signal 
may be coupled to all the output channels other than the output channels 
in communication and the output channel which is associated with the input 
channel on which the input signal is inputted. Thereafter, when the first 
forward information of the second communication has ended, the input 
channel corresponding to the particular output channel is coupled to the 
output channel (O.sub.4) corresponding to the input channel (I.sub.4) on 
which the input signal has appearded, the node then waiting for an input 
signal. If the signal of the first communication has completed before, the 
node switches the direction of transfer of the input and output channels. 
Referring to FIG. 11, a specific construction of the switching matrix unit 
42 of FIG. 9 is shown. Assuming that eight input channels and eight output 
channels are provided, the unit 42 is made up of eight modules which are 
assigned one-to-one to the output channels. Each of the modules comprises 
eight switching gates 58, eight latches 60 connected one-to-one to the 
gates 58, and a single eight-input OR gate 62 to which outputs 58c of the 
gates 58 are coupled. The modules share eight input signal lines which 
extend from an input port, and the gate set bus 54. A D terminal 60a of 
each latch 60 is connected to the gate set bus 54, a Q terminal to an 
input terminal of the switching gate 58, and a G terminal to the module 
select bus 52 by a common enable line 64. An input terminal 58a of the 
switching gate 58 is connected to the input signal line, and the output 
terminal 58c to the input of the OR gate 62. 
Referring to FIG. 12, a specific construction of the control gate unit 48 
of FIG. 9 is shown. Again, assuming that eight input channels are 
provided, the unit 48 is made up of eight gates 66 and eight latches 68 
which are connected one-to-one to the gates 66. The unit 48 is constructed 
to control the connection of eight input signal lines which extend from 
the input port and eight output signal lines which are associated with the 
input signal lines and led to the first input signal detecting unit 44. A 
D terminal 68a of each latch 68 is connected to the gate set bus 54, a Q 
terminal 68c to an input terminal 66a of the gate 66, and a G terminal 66b 
to the module select bus 52 by a common enable line 70. The input terminal 
66a of the gate 66 is connected to the input signal line, and an output 
terminal 66c to the input of the unit 44. 
Referring to FIG. 13, there is shown a logic structure of the latches which 
are installed in the switching matrix unit 42 of FIG. 11 and the control 
gate unit 48 of FIG. 12. As shown, the structure includes two preceding 
NAND gates 72 and 74 having a switching function and two following NAND 
gates 76 and 78 having a storing function. 
It is to be noted that the arrangements shown in FIGS. 11 to 13 are not 
restrictive and may be modified in various manners. The modification will 
mainly depend upon the ratio of softwares and hardwares which implement 
the controls as previously described. 
In this particular embodiment, because the switching control unit 50 is 
implemented with a microprocessor, a control over this section relies on 
hardware and, accordingly, the switching matrix unit 42 and the control 
gate unit 48 are independent of each other with regard to function. Should 
the control mentioned above be implemented with hardware only, such units 
would be constructed inseparably with respect to function. 
Referring to FIG. 14, a network which is an expanded version of this 
embodiment is shown. Specifically although the network of this embodiment 
is in principle a small-scale LAN which includes only one node 32, the 
number of nodes may be increased to construct a large-scale network as 
desired. The expanded network may have any suitable configuration such as 
a linear one, a loop, a two-dimensional lattice as shown in FIG. 14, a 
thee-dimensional lattice, or a combination thereof. Any desired nodes 80 
and the nodes 80 and terminals 82 may be connected together by links 84 
which include a plurlality of channels. While the embodiment has been 
shown and described as including at least one input channel and at least 
one output channel in one link 84, it will be apparent to those skilled in 
the art that a plurality of channels or a single input/output channel may 
be accommodated. 
Turning back to FIG. 2, the packets which are applicable to this embodiment 
will be described. As regards the packets of FIG. 2, prerequisites with a 
terminal and other transmit/receive stations are as follows: 
(a) the first forward information (call packet) is provided with a preamble 
having a length (time) equal to or greater than a predetermined one, and 
an address of a destination (test address); 
(b) Each transmit/receive terminal receives the first forward information 
(call packet) meant thereof and, upon completion of that information, 
sends the first return information (call-back packet) immediately after 
the first predetermined period of time (T.sub.1) has expired. The period 
of time T.sub.1 is a period of time necessary for connection control means 
at a node to complete a control for the entry of the first return 
information (call-back packet) and generally referred to as a node time 
constant, or simply as a node constant; and 
(c) When the transmit/receive terminal has received information which has 
not been meant therefor (only the first forward information has been 
received), it must not sent any information until a second predetermined 
period of time T.sub.2 expires since the end of the information. The time 
period T.sub.2 is a time period necessary for a packet to be propagated 
into a network and generally referred to as a network time constant, or 
simply as network constant. This ensures that even if a plurality of nodes 
are present in the network, a node which is closet to a call end receives 
the first return information (call-back packet) within the second period 
of time T.sub.2 after the end of the first forward information (call 
packet). 
So long as the prerequisites associated with the communication procedure as 
described above are met, the freedom in the other aspects is ample enough 
to provide the following possibilities: 
(a) The packet length is limitless: 
(b) Any number of forward and return information may be interchanged by, of 
desired, occupying the channel; and 
(c) The data rate may be selected as desired between transmit/receive 
terminals insofar as it is less than the maximum data rate which depends 
upon the hardware constituting the network. 
The packets shown in FIG. 2 are the most typical ones and representative of 
two forward informations and two return informations. The first forward 
information and the first return information are adapted to secure a 
communication path on the network while opening needless parts to the 
other communications and, therefore, none of them contains a message. 
Referring to FIG. 15, a specific construction of the first input signal 
detecting unit 44 of FIG. 9 is shown. Again, assuming that eight input 
channels are provided, the unit 44 comprises eight latches 86 and eight 
gates 88 which follow the latches 86. When an input signal 86a is applied 
to any one of the latches 86, the gates 86b of all the latches 86 are 
disabled so that the signal 86a is delivered through an output 86d. The 
latches 86 are recovered by a clear signal 86c. D terminals 86a of the 
latches 86 are connected to the output of the control gate unit 48, and 
CLR terminals 86c to the module select bus 52 by a single common clear 
line 90. Further, Q terminals 86d are each connected to one input terminal 
88a of the gate 88 which follows the latch 86. The other input terminal 
88b of each gate 88 is connected to the module select bus 52 by a single 
common enable line 92, while an output terminal 88c is connected to the 
data bus (module output signal line) 56. 
Referring to FIG. 16, a logic structure of the latch 86 of FIG. 15 is 
shown. In FIG. 16, two preceding NAND gates 94 and 96 have a switching 
function, and two following NAND gates 98 and 100 have a storing function. 
Referring to FIG. 17, a specific construction of the input signal 
monitoring unit 46 of FIG. 9 is shown which is assumed to have eight input 
channels. As shown, the unit 46 comprises eight latches 102 and eight 
gates 104. A D terminal 102a of each latch 102 is connected to the input 
port, and a Q terminal 102d a G terminal 102b via inverter 104 and to one 
input terminal 104a of the gate 104. An output terminal 104c of the gate 
104 is connected to the data bus 56. A CLR terminal 102c of the latch 102 
is connected to the module select bus 52 by a common clear line 106, and 
the other input terminal 104b of the gate 104 is connected to the module 
select bus 52 by a common enable line 108. 
The switching control unit 50 selects one of the modules of the switching 
matrix unit 42 via the module select bus 52 while setting up connections 
of the gates via the gate set bus 54. Further, the unit 50 reads 
information out of the first input signal detecting unit 44 or the input 
signal monitoring unit 46 via the module select bus 52 while clearing the 
latches thereof. Still another role which the unit 50 plays is selectively 
connecting the gates of the control gate unit 48 via the module select bus 
52 and gate set bus 54. 
Referring of FIGS. 18A to 18K, the operation of a node which is 
representative of a modification to the embodiment as described above is 
shown. In this modification, the matrix of the unit 42 is partly modified 
such that the points of intersection on a diagonal are not interconnected 
to prevent an input channel from being connected to its associated output 
channel. Generally, when the unit 42 is made up of N.times.N switching 
elements, it is possible to connect an input channel to all the output 
channels except for its associated output channel before an input signal 
is applied to the node. Further, when the unit 42 is made up N.times.(N-1) 
switching elements, an input channel inherently is not connected to an 
output channel which is associated therewith. Hence, when the unit 42 is 
provided with such a structure, the operating sequence as shown in FIGS. 
10A to 10H is replaced with the sequence as shown as shown in FIGS. 18A to 
18K. It is to be noted that although the unit 42 of FIGS. 18A to 19K are 
provided with an 8.times.8 matrix so that only the points of intersection 
on a diagonal are not connected, use may be made of an 8.times.7 matrix. 
FIG. 18A is representative of an initial condition which corresponds to 
FIG. 10A. The switching control unit 50 controls the switching matrix unit 
42 and control gate unit 48 by the control signals C.sub.1 and C.sub.2 so 
that all the input channels are connected to the first input signal 
detecting unit 44, and the input channels I.sub.0 to I.sub.7 of the unit 
42 to all the output channels O.sub.0 to O.sub.7 except on the diagonal. 
FIG. 18B corresponds to FIG. 10B in which an input signal has arrived. 
Assuming that an input signal has appeared on the channel I.sub.2, it is 
delivered over the output channels O.sub.0, O.sub.1 and O.sub.3 to 
O.sub.7. The signal is also applied to the units 44 and 46. FIG. 18C 
corresponds to FIG. 10C in which other input signals have arrived. When 
input signals have appeared on the input channels I.sub.1 and I.sub.3 
after the input signal on the input channel I.sub.2, they, too, are 
delivered over all the output channels except for those which correspond 
to the two input channels I.sub.1 and I.sub.3. At this instant, an 
interference occurs in the output signals as well as in the input signals. 
FIG. 18D shows a condition corresponding to that to FIG. 10C, i.e., a 
condition in which one of a plurality of signals which arrived first is 
detected. Specifically, assuming that an input signal has appeared on the 
input channel I.sub.2 first and, then, input signals have appeared on 
input channels I.sub.0, I.sub.4, I.sub.5 and I.sub.7, the switching matrix 
unit 42 once connects those input chnnels to all the output channels 
except for those which are associated with those input channels. Because 
the input channels I.sub.0, I.sub.4, I.sub.5 and I.sub.7 are not connected 
to their associated output channels, the input signals on those channels 
are not outputted. The first input signal detecting unit 44 has already 
detected the input channel I.sub.2 on which an input signal has appeared 
first, and the switching control unit 50 reads it out to perform the first 
storage. FIGS. 18E and 18F show a condition corresponding to that of FIG. 
18D, i.e., a condition in which the input channels other than the one on 
which an input signal has appeared first are disconnected. Specifically, 
the unit 50 reads the first input channel I.sub.2 out of the unit 44 to 
store it and controls the unit 42 to disconnect the input channels 
I.sub.0, I.sub.1, I.sub.3 to I.sub.5 and I.sub.7 from the output channels 
O.sub.0 to O.sub.7. FIGS. 18G and 18H is representative of a condition 
corresponsding to that of FIG. 10F, i.e., a condition in which the 
direction of transfer is reversed upon completion of the first input 
signal. In FIGS. 18G and 18H, as the input signal monitoring unit 46 
detects the end of the signal on the input channel I.sub.2, the unit 50 
reads it out and controls the unit 42 by the control signal C.sub.1 to 
connect the input channel I.sub.0, I.sub.1 and I.sub.3 to I.sub.7 to the 
output channel O.sub.2. That is, the input channels which served 
communications are switched into connection with the output channels so 
that return information which may appear on any of the input channels may 
be returned to a terminal which originated it. 
FIG. 18I and 18J show a condition corresponding to that of FIG. 10G, i.g., 
a condition in which return information is inputted. When the previous 
input signal was the forward information which forms a part of the first 
communication, return information of the first communication arrives at 
any of the input channels within the predetermined period of time T.sub.1. 
In the example shown in FIGS. 18I and 18J, return information has appeared 
on the input channel I.sub.0 and, therefore, it is delivered over the 
output channel O.sub.2. While the monitoring unit 46 detects the arrival 
of the input signal at the input channel I.sub.0, the control unit 50 
reads it out to perform the second storage (control signals C.sub.1 and 
C.sub.2). Next, the control unit 50 controls the control gate unit 48 to 
connect the input channels I.sub.1 and I.sub.3 to I.sub.7 of the unit 48 
to the detecting unit 44 and resets the unit 44 to erase the previous 
first input channel. This prepares the node for the entry of the second 
and third communications, i.e. simultaneous handling of a plurality of 
communications. FIG. 18K shows a condition corresponding to that of FIG. 
10H, i.e., a condition in which a plurality of communications are dealt 
with at the same time. As regards the second communication, signals are 
delivered over the output channels O.sub.0 and O.sub.2 which are in 
communication and the output channels corresponding to the channels on 
which inputs have appeared. 
Referring to FIG. 19, a modification to the node construction of FIG. 9 is 
shown. The node of FIG. 9 is constructed on the assumption that each link 
84 includes a plurality of channels which in turn include at least one 
input channel and one output channel. The alternative construction of FIG. 
19 is applicable to a case wherein the link 84 includes only one 
input/output channel. Specifically, a bidirection driver 112 is connected 
to a single input/output channel 110 which is in turn connected to an 
input and an output ports of the switching control unit 50 of the node. 
Where eight links are connected to one node, eight drivers 112 will be 
connected. 
Referring to FIG. 20, a specific construction of the bidirectional driver 
112 (RS 422) is shown. As shown, the driver 112 has terminals A and B 
which are connected to the link, and terminals D (driver), R (receiver), 
DE (driver enable) and RE (receiver enable) which are connected to the 
node. A positive signal outputted by a driver D is coupled to the terminal 
A, and a negative signal to the terminal B. At a terminal, a signal is 
picked up on the basis of a voltage differential between the two terminals 
A and B. On the other hand, the terminal D is connected to an output port, 
and the terminal R to an input port. Ther terminals DE and RE are 
controlled by the control unit 50 so that the transmission and the 
reception may be prevented from being enabled at the same time. 
As described above, in accordance with the present invention, a single node 
is capable of handling a plurality of communications at the same time 
without resorting to substantial extra costs. In addition, the system is 
expansible from a small scale which includes a single node to a large 
scale without modifying the protocol. 
SECOND EMBODIMENT 
A second embodiment of the present invention which is directed to the 
second object as previously stated will be described, beginning with its 
principle. 
All the systems (d) to (h) previously discussed have a lattice structure 
and fix a path every time a communication is originated. Hence, when a 
link is cut off or when a terminal or a node is in a fault (down of 
functions), they suffer from only partial trouble and allows the other 
parts to perform usual communications. This redundancy is of significant 
inportance and becomes greater as the network is scaled up. A problem with 
the systems (d) to (h), however, is that they cannot detect the cut-off of 
a link and the fault of the node or that of a terminal. When a node which 
is not engaged in a communication encounters a fault, all the 
communications are effected after paths have been fixed and, therefore, no 
trouble occurs. Nevertheless, when a path is fixed, a trouble occurs and, 
therefore, a certain period of time is needed. 
Now, collisions may be classified into two different kinds, i.e., a first 
kind of collision which occurs when a plurality of terminals have sent 
their first forward information at the same time, and a second kind of 
collision which is such that in the event when a terminal which has 
received the first forward information sends the second return 
information, a network which has been divided is recovered and another 
terminal having been located at the other side of the division transmits 
the first forward information. The first kind of collision may occur only 
when the first forward information is sent. The probability of the first 
kind of collision is comparatively low since the transmssion of the first 
forward information is not performed frequently. When the first kind of 
collision is caused by two different first forward informations, each 
section of the network is filled with either one of the first informations 
with the result that a border is developed within the network. If a 
terminal to receive the information is located on this side of the border, 
the path is safely fixed; if each receive terminal is located on this side 
of the border, both of the paths are safely fixed. For this reason, the 
first kind of collision itself does not always entail a trouble. 
Further, assume that the first kind of collision has occurred between two 
different first forward informations one of which is longer in packet 
length than the other. Then, even though the shorter forward information 
may reach a receive terminal, the latter half of the longer information is 
apt to enter the path which is assigned to the first return information 
associated with the shorter information. In this condition, a terminal 
transmitted the shorter forward information cannot see why the path could 
not be set up, i.e. by a collision or any other cause. This makes it 
impossible for the transmit terminal to take any measure except for an 
inadequate control which would affect the other communications to thereby 
lower the throughput of the network. The condition for the second kind of 
collision of take place is further limited and, therefore, its probability 
is even lower than that of the first kind of collision. Stated another 
way, the probability of the second kind of collision will be reduced 
substantially to zero if all the input and output channels of a node can 
be connected. The second kind of collision is not related to this 
embodiment and, therefore, will not be described. It is to be noted that 
the first kind of trouble as stated above will simply be referred to as a 
collision hereinafter. 
In accordance with this embodiment, the reliability heretofore achieved is 
further enhanced by (a) allowing a node and a transmit and a receive 
terminals to detect a collision individually, and (b) preventing a 
collision from entailing a trouble in the subsequent communications. In 
this particular embodiment, a connection control device which is installed 
in each node generally comprises a detecting section for detecting a 
collision, and a control section for inhibiting input to an input channel 
which has encountered a collision, output from an output channel which 
corresponds to that input channel, or both of them. On the other hand, a 
communication control device installed in each terminal generally 
comprises a detecting section for detecting the first return information, 
and a first backoff control section adapted to prepare for a condition 
wherein the first return stroke is not received. 
The detecting section of each node compares an input signal which is 
selected on the first-come-first-served basis (a sigle input signal 
selected which may not always be a one arrived earliest) and another input 
signal so as to produce a differential therebetween. Such another input 
signal may be a one which is generated by the same terminal as the other 
and routed through a different path, or a one which is generated by a 
different terminal. While the former involves only a minimum of delay in 
phase since it has been selected on the first-come-first-served basis by a 
node on the route, the latter allows a sufficient differential to be 
produced only if a fraction, or area, of a packet to be compared is 
selected adequately. The backoff control section, when it has decided that 
the first return information has not been received within a predetermined 
period of time, determines whether the failure is attributable to a 
collision or to any other cause and, then, executes a backoff function (a 
control for retransmission). It is to be noted that the backoff is not 
particular to this embodiment and may be implemented, for example, by the 
binary exponential backoff algorithm (Xerox). 
Referring to FIG. 21, a connection control device built in each node or a 
communication control device built in each terminal in accordance with the 
present invention is shown in a block diagram. As shown, the device 
includes a switching matrix unit 120 whose capacity is great enough to 
interconnect all of input ports I.sub.0 to I.sub.7 and output ports 
O.sub.0 to O.sub.7 and their associated input channels I.sub.0 to I.sub.7 
and output channels O.sub.0 to I.sub.7 at the same time. An input signal 
detecting unit 122 has a function of locating a particular one of the 
input channels on which an input signal appeared first, and a function of 
detecting a collision and informing the unit 120 of it. An input signal 
monitoring unit 124 is adapted to constantly monitor the presence/absence 
of an input signal. A control gate unit 126 serves to connect any of the 
input channels and the input channel on which an input signal appeared 
first to the unit 122. The entire node including such various units are 
controlled by a switching control unit 128. Assuming that eight input 
channels are provided, the unit 120 is made up of nine modules (greater in 
number than the channels by one) which are assigned one-to-one to the 
input channels. Each of the units 122, 124 and 126 comprises a single 
module. The unit 128 is connected to those modules by a module select bus 
130, a gate select bus 132, and a data bus (module output signal lines) 
134. 
Referring to FIG. 22, a specific construction of the switching matrix unit 
120 is shown. As shown, the unit 120 is made up of nine modules (one 
collision signal transfer channel being added) which are assigned 
one-to-one to the input channels. Each of the modules consists of eight 
switching gates 136, and eight latches 138 (each being provided with D, G 
and Q terminals). Outputs 136c of the switching gates 136 of each module 
are connected to the input of a single nine-input OR gate 140 on an output 
channel basis. The eight modules are connected to the input channels. The 
collision signal transfer channel is connected to a collision signal line 
142 via the input signal detecting unit 122. The statuses of the switching 
gates 136 are selected via the select bus 130 and set up via the gate set 
bus 132 module by module. 
Referring to FIG. 23, a specific construction of the input signal detecting 
unit 122 is shown. As shown, the unit 122 is made up of eight latches 144 
(each being provided with D, G, CLR and Q terminals), eight gates 146 
which follow the latches 144, and eight gates 148 which precede the 
latches 144. The output of each latch 144 (output 144d on Q terminal) is 
coupled to an input 146a of the associated gate 146 and to an input of an 
eight-input NOR gate 150, while the output of the opposite polarity 
(output 144e on Q terminal) is connected to an input 152b of the 
associated AND gate 152. An output 152c of the AND gate 152 is connected 
to the gate (G terminal 144b) of the latch 144. The output of the NOR gate 
150 branches off and leads to an input 154a of an OR gate 154 on one hand 
and to an input 156a of a gate 156 on the other hand. The output 154c of 
the OR gate 154 is connected to the inputs 152a of the AND gates 152. The 
CLR inputs of the latches 144 are connected to the module select bus 130 
as a common clear signal line 158. Likewise, an input 154b of the OR gate 
154 and an input 156b of the gate 156 are connected to the bus 130 as a 
collision control signal line 160. Inputs 146b of the gates 146 are 
connected to the module select bus 130 as an enable signal line 162. The 
output of the gate 156 is connected to the switching matrix unit 120 as a 
collision signal line 164. Outputs 146c of the gates 146 are connected to 
the data bus 134. Inputs 148a of the gates 148 are connected to the 
control gate unit 126, and inputs 148b to a reference input signal line 
166 which extends out from the unit 126. Inputs 148c of each gates 148 
branches off and leads to an input 168a of the and gate 168 on one hand 
and to an input 168b of the gate 168 via a single delay element 170 on the 
other hand. An output 168c of the AND gate 168 is connected to the input 
(input 144a on D terminal) of the latch 144. 
In a first input signal detection mode, when the collision control signal 
line 160 is low level and an input signal is applied to any of the latches 
144 (input 144a on D terminal), all the gates (144b on G terminal) are 
disabled via the NOR gate 150 and AND gate 152 so that an output signal 
(output 144d on Q terminal) is outputted and read out by the gate 146 at a 
suitable timing. At this instant, because the input 148b of the gate 148 
is high level, the input signal on the input 148a is delivered as it is. 
In the meantime, the AND gate 168 and delay element 170 serve to prevent 
the latch 144 from latching up due to impulse-like noise. Hence, the delay 
element 170 has a time constant which is adequate for such a function and 
may be equal to a time constant of a collision detection mode, which will 
be described. 
In a collision detection mode for detecting collision (first kind as 
previously stated), an inverted version of an input signal arrived first 
is fed to the input 148b of the gate 148 over the reference signal line 
166, so that a signal representative of a differential between the first 
signal and the signal on the input 148a appears on the output 148c. That 
is, only a time differential between the two input signals in conflict 
(before or after the first input signal or both) is delivered via the AND 
gate 148. If the differential signal is shorter than a predetermined 
length, the delay element 170 and AND gate 168 erases it; if the 
differential signal is longer than the predetermined length, they produce 
it after subtracting a predetermined length from the signal. The 
predetermined length which is to be subtracted from the signal is 
determined in consideration of a phase deviation which occurs when the 
same packet originated by the same terminal is routed through different 
paths. In the above condition, because the collision control signal line 
160 is high level, any of the latches 144 to which an input signal is 
applied (input 144a on D terminal) disables its gate (144b on G terminal). 
As a result, the output of the latch 144 is read out by the gate 146 at a 
suitable timing and, then, delivered to the switching matrix unit 120 at a 
suitable timing via the NOR gate 150 and gate 156. The time constant of 
the delay element 170 is selected accordingly. The statuses of the latches 
144, gates 146, AND gate 1552 and gate 156 are selected via the clear 
signal line 158 of the module select bus 130, collision control signal 
line 160, and enable signal line 162. In this manner, in accordance with 
this embodiment, a signal indicative of the detection of the first kind of 
collision is fed to the switching matrix unit 120 via the gate 156. 
Referring to FIG. 24, a specific construction of the input signal 
monitoring unit 124 is shown. As shown, the unit 124 comprises eight 
latches 172 (each being provided with D, G, CLR and Q terminals), and 
eight gates 174 which follow the latches 172. The output of each latch 172 
(output 172d on Q output) is connected to the input of an inverter 176 the 
output of which is connected to an input of the latch (input 172b on G 
terminal). The unit 124 is capable of reading out the statuses of the 
input channels at a suitable timing by controlling the clear signal line 
178 and enable signal line 180 of the module select bus 130. Input signals 
on the input channels are fed to the data bus 134. 
Referring to FIG. 25, a specific construction of the control gate unit 126 
is shown. The unit 126 consists of eight gates 182, eight latches 184 
(each having D, G and Q terminals) which are connected to the gates 182, 
eight gates 186, eight latches 188 (each having D, G and Q terminals) 
connected to the inputs of the gates 186, and an eight-input NOR gate 190 
which is connected to the output of the gates 186. Each gate 182 controls 
the connection of the input channel and the input of the input signal 
detecting unit 122 associated with the input channel, while the gates 186 
serve to select a single input channel and are connected to a reference 
signal line 192 of the unit 122 via the NOR gate 190. The unit 126 is 
controlled via the enable signal lines 194 and 196 of the module select 
bus 130 and the gate set bus 132 to in turn control the above-mentioned 
various elements. 
It is to be noted that this embodiment may be modified in various manners 
in relation mainly to the ratio of softwares and hardwares which implement 
the controls as described above. The switching control unit 128 is 
implemented with a microprocessor and, therefore, controlled by software, 
so that the other units 120 and 126 are independent with respect to 
function. Should such controls be implemented with hardware only, the 
various units would be constructed inseparably with respect to the 
function. 
Referring to FIGS. 26A to 26L, there is shown a network control method in 
accordance with this embodiment. Assume that a path for the first 
communication, e.g., channels 4 and 7 have already been fixed and 
occupied, and that information is being transferred from the input channel 
4 to the output channel 7. 
In FIG. 26A, the switching control unit 128 conditions the input signal 
detecting unit 122 for a first input signal detection mode, disconnects 
the channels 4 and 7 of the control gate 1 unit 26 which have been 
occupied, and connects the other channels 0 to 3, 5 and 6). Further, the 
unit 128 clears the unit 122 and, to prepare for the second communication, 
connects the input channels I.sub.0 to I.sub.3, I.sub.5 and I.sub.6 to the 
output channels O.sub.0 to O.sub.3, O.sub.5 and O.sub.6. The unit 128 has 
already connected the input channel I.sub.4 to the output channel O.sub.7 
and the input channel I.sub.7 to the output channel O.sub.4 to accommodate 
the first communication (Step 1). 
As shown in FIG. 26B, when an input signal arrived first is present on any 
of the input channels I.sub.0 to I.sub.3, I.sub.5 and I.sub.6 (channel 
I.sub.2 in this example), it is outputted over the output channels O.sub.0 
to O.sub.3, O.sub.5 and O.sub.6 (Step 2). 
In FIG. 26C, the input signal detecting unit 122 detects the particular 
input channel I.sub.2. If input signals are present on other channels, 
they are also delivered over the output channels O.sub.0 to O.sub.3, 
O.sub.5 and O.sub.6 as in the Step 2. At this instant, an interference 
occurs in the output signals (Step 3). 
In FIG. 26D, the switching control unit 128 reads information out of the 
input signal detecting unit 122 to perform the first storage. If input 
signals appear on farther input channels, they are also delivered over the 
output channels as in the Step 1. That is, other nodes, too, receive the 
packets by performing the control as shown in the Step 3. In this 
instance, the input signal monitoring unit 124 may be caused to detect any 
of the input channels I.sub.0 to I.sub.3, I.sub.5 and I.sub.6 on which no 
input signal has appeared (Step 4). 
In FIG. 26E, the unit 128 disconnects the input channels of the switching 
matrix unit 120 except for I.sub.2, i.e., input channels I.sub.0, I.sub.1 
and I.sub.3 to I.sub.7 from all the output channels, while disconnecting 
the input channel I.sub.2 from its associated output channel O.sub.2. If 
desired, the connection to any output channel (channel 6 in this example) 
corresponding to an input channel on which no input signal has appeared 
may be cancelled (Step 5). 
In FIG. 26F, the unit 128 connects the input channel (channel I.sub.2) of 
the control gate unit 126 on which the first input has arrived to the 
reference signal line, conditions the input signal detecting unit 122 for 
a first collision detection mode, and detects and stores any of the input 
channels on which a collision has occurred. If desired, any of the input 
channels of the monitoring unit 124 on which an input signal has 
disappeared may be detected. Specifically, the input signal has 
disappeared since other nodes have performed the control as shown in the 
Step 5; a signal sent over the output channel which is associated with 
that input channel implies that the signal has been detected as a signal 
arrived first (Step 6). 
In FIG. 26G, the unit 128 connects the collision signal transfer channel of 
the switching matrix unit 120 to an output channel associated with the 
first storage (channel O.sub.2) so as to deliver the collision signal over 
the channel O.sub.2. Simultaneously, the unit 128 disconnects those input 
channels on which a collision has occurred from their associated output 
channels (Step 7). 
In FIG. 26H, if a collision signal appears on any of the input channels 
(channel I.sub.3 in this example), a new collision signal is produced as 
in the case of the first kind of collision. However, the unit 128 does not 
determine it as the first kind of collision (Step 8). 
In FIG. 26I, the input signal monitoring unit 124 detects the end of the 
input signal, while the switching control unit 128 reads it out (Step 9). 
In FIG. 26J, the unit 128 disconnects the collision signal transfer channel 
of the switching matrix unit 120 from the output channel O.sub.2 and, 
instead, connects the input channels I.sub.0, I.sub.1, I.sub.3 and I.sub.5 
to the output channel O.sub.2 (Step 10). 
In FIG. 26K, because the previously mentioned input signal is the first 
forward information of the second communication, the first return 
information of the second communication appears within a predetermined 
period of time on any of the input channels (channel I.sub.0 in this 
example) which has been connected at the Step 10, the return information 
being sent over the output channel O.sub.2. The monitoring unit 124 
detects the arrival of the first return information of the second 
communication on the input channel I.sub.0, while the switching control 
unit 128 reads it out and performs the second storage (Step 11). 
In FIG. 26L, the unit 128 connects the input channels I.sub.1, I.sub.3, 
I.sub.5 and I.sub.6 of the control gate unit 126 to the input signal 
detecting unit 122, conditions the unit 122 for a first input signal 
detection mode, connects the input channel I.sub.0 of the switching matrix 
unit 120 to the output channel O.sub.2 and the input channel I.sub.2 to 
the output channel O.sub.0 for the second communication, and connects the 
input channels I.sub.1, I.sub.3, I.sub.5 and I.sub.6 of the unit 120 to 
the output channels O.sub.1, O.sub.3, O.sub.5 and O.sub.6 in order to 
prepare for the third communication which may be generated next. By the 
first communication, the input channels I.sub.4 and I.sub.7 are held in 
connection with the output channels O.sub.7 and O.sub.4, respectively 
(Step 12). 
Referring to FIGS. 27A to 27E, the principle of detection of the first kind 
of collision in accordance with this embodiment is schematically shown. In 
order that a signal arrived first may be compared with another signal with 
the former constituting a reference, it is a prerequisite that the same 
signal which is routed through a different path from the reference signal 
(phase difference existing therebetween) be discriminated from a different 
signal which was send from a different terminal. For this purpose, factors 
on which the maximum value of the phase difference depends will be 
examined. First, (a) a packet A arrives at a node A (Step 1). 
Subsequently, (b) the packet A is delayed by a time Tn by the node A and, 
then, delivered therefrom (Step 2). Thereafter, (c) the packet A is 
delayed by a time T1 by a link and, then, inputted to a node B while, at 
the same time, a packet B is inputted to the node B. At this instant, the 
node B detects the packet B as the first input (Step 3). Then, the packet 
B is delivered from the node B after being delayed by a time Tn (Step 4). 
Finally, the packet B is received by the node A after being delayed by a 
time T1 by the link (Step 5). 
As a result, at the node A, a time gap Td is developed between the input of 
the packet A and that of the packet B by, at maximum, 
EQU Td=2 (Tn+T1) (1) 
For example, assuming that 
EQU Tn=50 nSec and 
EQU T1=500 nSec 
then 
EQU Td=1.1 .mu.Sec 
That is, so long as the packets A and B are the same as each other, a 
deviation in phase as shown above occurs. The deviation depends solely on 
the delay time Tn inside of each node and the longest distance between 
nodes (on which the delay time T1 of the link depends) and not on the 
configuration and scale of the network. This also holds true with a 
transmit terminal except that the reference signal is constituted by its 
own output signal. Specifically, in the case of a transmit terminal, a 
phase difference occurs which is equal to a time gap 
EQU T'd=Tn+2T'1 (2) 
Because the time gap T'd is generally shorter than the time gap Td, the 
latter may be used as a reference. 
Referring to FIGS. 28A to 28C, there is shown a method of comparing a 
reference signal and a signal to be compared. As shown in FIG. 28A, a 
reference signal A and a signal B/B' to be compared are applied to an 
Exclusive-OR (EXOR) gate 148A which then produces an output C/C'. The 
signal C/C' is branched to follow two different paths one of which leads 
to an AND gate 268A directly and the other by way of a delay element 170A 
having a delay time Td. The AND gate 168A produces an output E/E' in 
response to the output C/C' of the EXOR 148A and an output D/D' of the 
delay element 170A. Assume that the delay time is expressed as 
EQU T.gtoreq.Td (3) 
and that a minimum unit time Ts which defines the high level or the low 
level of a signal is 
EQU Ts.gtoreq.(TD+Td) (4) 
Then, if the reference signal and the signal to be compared are the same as 
each other, there will appear waveforms C, D and E as shown in FIG. 28B; 
if they are different from each other, waveforms C', D' and E' will appear 
as shown in FIG. 28C which are clearly distinguishable from the waveforms 
C, D and E. 
Referring to FIGS. 29A to 29C, an actual method of comparing a reference 
signal and another signal in accordance with this embodiment is shown. As 
seen from FIGS. 23 and 25, too, the actual method is implemented with an 
eight-input NOR gate such as shown in FIG. 29A (see 190 in FIG. 25) which 
replaces the EXOR 148A of FIG. 228A, and an AND gate (see 148 in FIG. 23). 
If the EXOR 148A is used, a decision that the first collision has occurred 
will be made even when a signal to be compared is absent. To prevent this, 
a decision has to be aided by the detection of presence/absence of a 
signal to be compared. Outputs C and C' of an AND gate 148B, output D and 
D' of a delay element 170B and outputs E and E' of an AND gate 168B are 
shown in FIGS. 29B and 29C, respectively. In this instance, the Eq. (4) is 
modified as 
EQU Ts.gtoreq.(TD+Td)/2 (5) 
Referring to FIG. 30, there is shown another example of the method of 
comparing a reference signal and a signal to be compared. As shown, a 
compare signal line 198 and a reference signal line 166 assigned to each 
channel are connected to a G input terminal and a load input terminal of a 
presettable synchronous up-down binary counter 200 (e.g. 74LS191 available 
from TI), respectively. The ripple clock (RC) output terminals of the 
counters 200 are connected to 1CK and 2CK input terminals of dual JK 
flip-flops 202 (with clear terminals; e.g. 74LS107 available from TI). 
Each of 1Q and 2Q output terminals of each flip-flops 202 is branched to 
be connected to an A-H input terminal of a thirteen-input NAND gate 204 
(e.g. 74LS133 available of TI) and to a 1A1-2A4 input terminal of an octal 
three-state bus buffer 206 (e.g. 74LS244 available from TI). 
Because an A-D and a U/D input terminals of the counter 200 are constantly 
maintained high level, when the reference signal line 166 (load input 
terminal) is high level (high active), data held thereby is always "16 
(four bits)". When the reference signal line 166 is low level and the 
compare signal line 198 (G input terminal) is low level (low active), the 
counter 200 is decremented by clock pulses which are applied to its CK 
input terminal. As the counter 200 reaches zero, it produces a signal from 
the RC terminal and, therefore, it is possible to locate a particular 
input channel on which the first kind of collision has occurred. 
Hereinafter will be described the constructin of the network in accordance 
with this embodment. The network in this embodiment may have subtantially 
any suitable configuration such as a linear configuration, a loop 
configuration, a two-dimensional lattice configuration made up of the 
nodes 80 and terminals 82 as shown in FIG. 14, a three-dimensional lattice 
configuration, or a combination thereof. In addition, desired ones of the 
nodes 80 may be interconnected by the links 84 having a plurality of 
channels, and so may be done the nodes 80 and the terminals 82. 
The packet formats as shown in FIG. 2 are applicable to this embodiment, 
too. 
As regards a communication procedure, prerequisites with the 
transmit/receive stations 82 such as terminals are as follows: 
(a) a transmit station sends the first forward information (call packet) 
which includes a preamable which is longer than a predetermined length 
(time) and a destination address; 
(b) the preamble generated by the transmit station includes a fraction or 
area which is adapted for the detection of the first kind of collision; 
(c) a transmit/receive station receives the first forward information (call 
packet) which is meant therefor and, upon completion of the information, 
transmits the first return information (call-back packet). A first period 
of time T.sub.1 is a period of time necessary for the connection control 
section of a node to complete a control which is performed to prepare for 
the entry of the next packet, i.e., the first return information 
(call-back packet) in this case, the time period T.sub.1 being referred to 
as a node time constant; and 
(d) when a transmit/receive station has received information which has not 
been meant therefor (only the first forward information or call packet has 
been received), it must not send any information until a second 
predetermined period of time T.sub.2 expires since the end of the 
information. The second period of time T.sub.2 is a period of time 
necessary for a packet to be propagated into the network and generally 
referred to as a network time constant. Thus, at a node closest to a 
transmit station, it is ensured that the first return information 
(call-back packet) be received within the period of time T.sub.2 after the 
end of the first forward information (call packet). 
In this particular embodiment, the fraction of the preamble of the first 
forward information (call-back packet) adapted for the detection of the 
first kind of collision plays an important role. Further, what is 
important is that the phase deviation of the first forward information 
(call packet) which is routed through a different path as previously 
stated be distinguished from the first kind of collision. It is necessary, 
therefore, for the bit pattern of the exclusive collision detection area 
of the preamble to be clearly distinguishable from one first forward 
information (call packet to another) to another. This requirement may be 
met by any of the following implementations: 
(a) filling the area with a bit pattern which is provided by coding the 
address of a transmit station (source address); and 
(b) filling the area with a bit pattern which is provided by coding a 
random number. 
So long as the prerequisites as mentioned above are met, a substantial 
degree of freedom is guaranteed in the other aspects and gives the 
following possibilities: 
(a) the minimum and maximum packet lengths are limitless; 
(b) forward and return informations may be repeated any number of times and 
may even occupy the channels; and 
(c) any desired data rate may be selected between a transmit and a receive 
stations insofar as it is smaller than a maximum data rate, which is 
determined by hardware. 
In accordance with this embodiment, the area of the packet which is 
assigned to the detection of the first kind of collision has a practical 
size. For example, because Ts is equal to or greater than 1.1 to 2.2 
.mu.S, the area for detection needs only to be about 20 .mu.S and this 
corresponds to not more than 20 bits for a data rate of 1 Mbps. 
While this embodiment has been described in relation to FIG. 21 which shows 
a node constructed to detect the first kind of collision, the node may be 
replaced with a transmit/receive terminal which is connected to the node. 
Specifically, each or a desired one of the terminals may be provided with 
the construction as shown in FIG. 21 so as to perform not only the 
transmission and reception of signals but also the detection of the first 
kind of collision, i.e. collision of a signal of the own station and a 
signal which is sent from another terminal or from a node. 
As described above, in accordance with this embodiment, the first kind of 
collision depends solely on the distance between nodes and not on the 
scale of a network, so that the system is very practical. In addition, 
even if communications overlap each other, the system throughput is 
enhanced. 
THIRD EMBODIMENT 
This embodiment is directed to the third object of the present invention as 
previously stated and is essentially similar to the second embodiment. The 
following description will concentrate on the differences between this 
embodiment and the second embodiment. 
In accordance with this embodiment, to further enhance the reliability 
attainable with the prior art systems (a) a node is capable of detecting 
the first kind of collision, (b) a first collision is prevented from 
entailing a trouble in the subsequent communications, and (c) a terminal 
is informed of the occurrence of a collision to perform adequate backoff 
processing. In this embodiment, a connection control device installed in a 
node generally comprises a detecting section for detecting the first kind 
of collision, a control section for inhibiting one or both of input and 
output to and from an input channel on which the first kind of collision 
has occurred and an output channel associated therewith, and a control 
section for producing a collision signal indicative of the occurrence of 
the first kind of collision and informing a transmit terminal of it. A 
communication control device installed in a transmit/receive terminal 
generally comprises a detecting section for detecting the first return 
information and the collision signal, and a backoff control section for, 
when the first return information has not been received, performing 
retransmission depending upon the presence/absence of a collision signal. 
The collision detecting section compares an input signal selected on the 
first-come-first-served basis (not limited to a signal arrived eariliest) 
and another input signal so as to produce a differential therebetween. The 
another signal mentioned above may be a one which is originated by the 
same terminal and routed through a different path, or a one which is 
originated by a different terminal. The former involves a minimum of delay 
(in phase) because it has been selected by the same logic by a node on the 
route. As regards the latter, a sufficient differential is attainable by 
adequately selecting an area of a packet which is assigned to the 
comparison. The backoff control section, when the first return information 
has not been received within a predetermined period of time after the 
detection of a collision signal, determines that the first kind of 
collision has occurred to prevent the first forward information from 
reaching a receive terminal and performs a backoff procedure. When the 
first return information has been received within the predetermined period 
of time after the detection of a collision signal, the backoff control 
section does not perform the backoff procedure and neglects the collision 
signal. It may occur that the first return information is not received 
within the predetermined period of time despite that a collision signal is 
absent, such as when the traffic is excessively high and when a receive 
terminal is unable to receive. In such a condition, the backoff control 
section may perform transmission upon the lapse of a longer period of time 
than in the case of the backoff against the first collision, or it may 
interrupt the communication and alert an operator at the transmit 
terminal. Again, the backoff itself is not particular to this embodiment 
and may be implemented by, for example, the binary exponential backoff 
algorithim which is applied to Ether net (Xerox). 
The packet formats as shown in FIG. 2 are applicable to this embodiment, 
too. 
As regards the communication procedure, conditions required of the 
transmit/receive stations 82 such as terminals are as follows: 
(a) a transmit station sends the first forward information (call packet) 
which includes a preamble which is longer than a predetermined length 
(time) and a destination address; 
(b) a transmit station defines an area for the detection of the first kind 
of collision in the preamble; 
(c) when a transmit station has received the first return information 
(call-back packet), it transmits the second forward information (message 
packet) with no regard to the presence/absence of a collision signal and 
immediately after the lapse of a first predetermined period of time 
T.sub.1. This period of time T.sub.1 is necessary for a connection control 
device at a node to perform a control for the entry of the next packet and 
is generally referred to a node time constant or node constant. What kind 
of communication should be performed thereafter is free to choose at a 
transmit/receive station or on a system basis and not limited by the 
network at all. Upon completion of the communication, it is only necessary 
for the transmission to be interrupted for a longer period of time than a 
second predetermined period of time, which allows a packet to be 
propagated into a network and is generally referred to as a network time 
constant or network constant; 
(d) When a transmit station has not received the first return information 
after the reception of a collision signal, it performs a predetermined 
backoff control. The system of this control is open to choice on a station 
basis or on a system basis; 
(e) The processing which a transmit station is to effect when neither a 
collision signal nor the first return information (call-back has been 
detected is also open to choice on a station basis or on a system basis; 
(f) When a receive station has received the first forward information (call 
packet), it transmits the first return information (call-back packet) as 
soon as the period of time T.sub.1 expires since the end of the forward 
information; and 
(g) When a transmit/receive station has received information which is not 
meant for this station (only the first forward information (call packet) 
has been received), it must not transmit information until the period of 
time T.sub.2 expires since the end of the received information. It is 
guaranteed that the first return information (call-back packet) be entered 
within the period of time T.sub.2. 
As described above, in accordance with this embodiment, the detection of 
the first kind of collision depends solely on the distance between nodes 
and not on the scale of a network, whereby the practicality is enhanced. 
Because collision information is delivered to a transmit station even when 
the transmit station is transmitting the first forward information (call 
packet), the transmit station is capable of retransmitting and, yet, the 
communication efficiency remains substantially the same as in the case 
without a collision. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure