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Description  |
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The invention relates to a method of ATD (Asynchronous Time Division)
switching of data packets, according to which each packet is assigned a
uniform identification code for each connection.
The invention further relates to an arrangement for implementing this
method.
Such a method is known from "Digital Communications", "New Directions in
Switching and Networks", Proceedings of the International Seminar, Zurich,
Switzerland, Mar. 11-13, 1986, the article by J. S. Turner entitled "New
Directions in Communications", pp. A 3.1-A 3.8, more specifically, page A
3.6.
In a packet-switched network users are connected to the network by means of
subscriber lines. Since fast statistical variations in the number of
packets to be processed may occur in a packet-switched network, measures
are to be taken to maintain an acceptable chance of blocking under
conditions of peak loading.
The above article by Turner describes a method of avoiding an overloading
of the network by a single user when transmitting too many packets per
given time interval. The user is then provided with an arrangement which
blocks the switching of a packet to be transmitted by him if he offers too
many packets during the time interval concerned. In order to carry out
this monitoring function the arrangement constantly compares the number of
packets transmitted by the user with a predetermined maximum number of
packets. Once the number of transmitted packets exceeds this maximum
number, the packet switching is blocked. After each time interval the
stored number of transmitted packets is decremented by a predetermined
value and the user may continue transmitting packets.
This method, the so-called "leaky bucket" method, monitors the number of
transmitted packets of a single user connected to a subscriber line. When
a plurality of users (for example, independently operating telephones,
computers etc.) are connected to the same subscriber line, the number of
packets each subscriber transmits over the subscriber line should also be
monitored. From the above article by Turner no indications can be
extracted for performing the monitoring function if there are more users
per subscriber line.
It is an object of the invention to provide a method of monitoring for each
user the number of packets to be transmitted by a plurality of users
connected to a single subscriber line.
For this purpose the method according to the invention is characterized in
that for each connection a time interval and a maximum number of packets
to be transmitted within this time interval are chosen which time interval
and which maximum number of packets to be transmitted within this time
interval are stored per identification code at first and second memory
locations respectively, for the duration of the connection, in that for
each connection a predetermined time limit is stored at a third memory
location, and in that a representation of a number of packets still
authorized to be transmitted is stored at a fourth memory location, whilst
a real-time count is compared with the time limit for each packet that is
presented to be transmitted, and
I. if the real-time count exceeds the time limit:
(a) the time limit at the third memory location is replaced by a new time
limit which is determined by adding the time interval to the real-time
count,
(b) the number of packets still authorized to be transmitted in the new
time interval is replaced by the maximum number of packets to be
transmitted, and
(c) the packet to be transmitted is switched; and
II. if the real-time count falls short of the time limit: it is established
whether the number of packets still authorized to to be transmitted in the
relevant time interval exceeds the value "zero", in which case the number
of packets still authorized to be transmitted is decremented by the value
"one" and the packet to be transmitted is switched, and if the number of
packets still authorized to be transmitted is equal to "zero", the
switching of the packet to be transmitted is blocked.
Using the fact that each packet that is to be transmitted over a specific
connection line from a user to a receiver comprises a same identification
code, the number of packets transmitted in each time interval is
additionally monitored for each identification code (and thus for each
user). For this purpose, for each identification code a time interval is
determined as well as a maximum number of packets to be transmitted in
this time interval, whilst the interval and the maximum quantity are
chosen by the user and affect the cost allocation, and a number of packets
still authorized to be transmitted in the course of the current interval
is defined. At the start of a new time interval (that is, upon the arrival
of a first packet to be transmitted carrying the relevant identification
code after a previous time interval), the number of packets still
authorized to be transmitted is equated with the maximum number of packets
and subsequently reduced by one unit with each packet to be transmitted
carrying the same identification code. Once the number of packets
authorized to be transmitted reaches the value "zero", the switching of
subsequent packets carrying the same identification code is blocked until
the time interval has passed and a new time interval begins.
The fact whether for each identification code the time interval has or has
not elapsed is detected by adding, at the start of a new time interval,
the value of the new time interval to the real-time count and thus
constantly comparing with the real-time count the time limit obtained from
this addition. Once the real-time count has exceeded the time limit, the
time interval has elapsed and a new time limit can be established by means
of the above addition.
Each connection from a user to a receiver has its own identification code.
Each (data) packet which is to be transmitted over this connection is
given this identification code. If a user again wants to have a connection
with the same receiver some time after the connection has been terminated,
this new connection will generally be identified by another identification
code, although it is not entirely impossible that the same identification
code happens to be issued for this next connection.
According to the "leaky bucket" method from the above article by Turner it
is possible that if a user resumes transmission after a period of one or
more time intervals without transmission, he is temporarily offered a
larger transmission capacity than the predetermined one, because in the
intervals during which he did not transmit, the stored number of
transmitted packets was constantly decremented by a predetermined value.
This undesired disposal of a larger transmission capacity than was
determined beforehand is impossible with the method according to the
invention.
A preferred embodiment of the method according to the invention is
characterized in that at each packet switching the total number of
transmitted packets is stored per identification code at the fifth memory
location.
For this purpose, the total number of packets transmitted over all time
intervals elapsed till then is stored for each user and for each
connection enabling thus a cost allocation, for example.
The invention will now be further explained with reference to the
embodiment shown in the Figures, in which:
FIG. 1 shows a diagrammatic representation of the packet monitoring
arrangement according to the invention,
FIG. 2A shows a detailed diagram of the start unit according to the
invention,
FIG. 2B shows a time diagram in explanation of the start unit according to
the invention,
FIG. 2C shows a further time diagram in explanation of the start unit
according to the invention,
FIG. 3 shows a further explanation of the control unit according to the
invention,
FIG. 4 shows a time diagram in explanation of the control unit according to
the invention.
The packet monitoring arrangement as shown in FIG. 1 comprises a packet bus
1 which is connected to users which are symbolically represented by 2. The
packet bus 1 connects the users 2 to the network which is symbolically
represented by 3 by means of a delay buffer 4 and an input and an output
of a switching multiplexer 5 which functions as a blocking device. Delay
buffer 4 is used for delaying the packets arriving from users 2 so as to
allow time for the packet monitoring arrangement to compute whether a
packet is to be switched or not to be switched.
Switching multiplexer 5 switches or does not switch this packet under the
control of a signal originating from the output of the monitoring unit 6.
Users 2 are further connected over packet bus 1 to an input of an address
multiplexer 7 via a buffer 8. The output of the address multiplexer 7 is
connected to an address input 9 of a memory 10. Memory 10 comprises five
fields 11; 12; 13; 14; 15 which can be written or read out through two
connections each 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, the data being
exchanged over a memory bus 26. Memory bus 26 is further connected to the
monitoring unit 6, an arithmetic unit 27 and an input-output buffer 28.
Monitoring unit 6 has a control input which is connected to the output of
the arithmetic unit 27. Input-output buffer 28 is further connected to an
external bus 29 which itself is further connected to the network 3 and the
further input of address multiplexer 7 via a buffer 30.
The packet monitoring arrangement as shown in FIG. 1 further includes a
control unit 40 generating the control signals via the connections 16-25
and a start unit 41. Control unit 40 is connected to a packet sync guide
42, start unit 41 and monitoring unit 6. Start unit 41 controls, in
addition to control unit 40, also address multiplexer 7 and input-output
buffer 28. The inputs of start unit 41 are connected to the external
control guides 43 and 44 for receiving start signals originating from the
network 3.
Arithmetic unit 27 comprises a comparator 31, a time generator 32, an adder
33, the connecting means 34 (to be called adding multiplexer 34
hereinafter) and an up-counter 39. One input of the comparator 31, adder
33 and adding multiplexer 34 respectively, is connected to memory bus 26.
The other input of comparator 31 and adder 33 is connected to time
generator 32. The output of adder 33 is connected to the other input of
adding multiplexer 34, whose output is connected to memory bus 26. The
output of comparator 31 is connected to the control input of adding
multiplexer 34 and the output of arithmetic unit 27. The up-counter 39 in
arithmetic unit 27 is also connected to memory bus 26.
Monitoring unit 6 comprises a down-counter 35, transfer means 36 and a zero
detector 37. The inputs of zero detector 37 and transfer means 36 and the
output of the down-counter 35 are connected to memory bus 26. The output
of transfer means 36 is connected to the input of the down-counter 35. The
output of zero detector 37 forms the output of the monitoring unit 6 and
is connected to the control input of the down-counter 35. The control
input of transfer means 36 forms the input to monitoring unit 6 which is
connected to the output of arithmetic unit 27.
If a user 2 intends to transmit packets, he will first transmit a set-up
packet. Such a set-up packet is featured by a fixedly chosen header
comprising n bits, for example, fifteen bits having the logic value "zero"
and one bit having the logic value "one". The set-up packet further
includes information about the maximum number of packets to be transmitted
during each specific time interval and about the destination.
The set-up packet coming from a user 2 enters the packet monitoring
arrangement as shown in FIG. 1 over packet bus 1. At the same time a
packet synchronizing signal arrives over packet sync guide 42 which signal
is applied to control unit 40. The header of the set-up packet is stored
in buffer 8. Address multiplexer 7 receives a signal from the start unit
41 so that the contents of buffer 8 (the header of the set-up packet) are
applied to address input 9 of memory 10 via address multiplexer 7 in a
manner to be described hereinafter.
Control unit 40 generates first a read signal through connection 18 (for
reading the fields 11, 13 and 14 simultaneously) in response to the packet
synchronizing signal (for reading field 12) and then simultaneous read
signals through connections 16, 20 and 22. The contents of these fields
are applied to arithmetic unit 27 and monitoring unit 6 over memory bus
26. In response to receiving these contents, arithmetic unit 27 applies a
signal to monitoring unit 6 in response to which the latter applies a
signal to switching multiplexer 5. The set-up packet, delayed in delay
buffer 4 during the above procedure, is now transmitted to the network 3
via switching multiplexer 5 and packet bus 1.
The network 3 receives this set-up packet and derives therefrom the
information of interest such as destination, packet rate (the maximum
number of packets to be transmitted during each time interval) and the
time interval. The network 3 generates a VCI (Virtual Circuit Identifier)
belonging to the destination and transmits this VCI to the user who is to
put this VCI in the headers of all data packets belonging to the same
message which still have to be transmitted and thus have the same
destination. All this has not been shown in FIG. 1 owing to the fact that
it is of no importance to this invention.
The VCI generated by the network 3 is applied to address multiplexer 7 over
external bus 29 and buffer 30. The maximum number of packets to be
transmitted per time interval as well as the time interval concerned are
applied to input-output buffer 28 over external bus 29. The network 3
simultaneously generates a start signal which is applied to start unit 41
over external control guide 43.
As a response, start unit 41 will generate three signals:
a first signal as a loading signal for input-output buffer 28, which, in
response thereto, stores the data available on the external data bus 29 as
the maximum number of packets to be transmitted per time interval;
a second signal as a selection signal for address multiplexer 7, which, in
response thereto, applies the VCI from buffer 30 to address input 9 of
memory 10; and
a third signal as an auxiliary signal for control unit 40, which, in
response thereto, applies control signals to memory 10 so as to load the
fields 11, 13, 14 belonging to the address indicated by the VCI with data
coming from the input-output buffer 28 and to load the further fields 12,
15 with the values "zero", over memory bus 26.
Subsequently, the user 2 receives a ready signal through a connection (not
shown in FIG. 1) as a sign for him to start transmitting data packets
whose headers comprise the VCI's generated by the network 3.
The transmitted data packet first enters the circuit over packet bus 1. The
header comprising the VCI addresses the memory 10 via buffer 8 and address
multiplexer 7. The packet synchronizing signal is applied to control unit
40 over packet sync guide 42. In response thereto control unit 40 first
generates a read signal through connection 18 for reading field 12 which
comprises the time limit. In the case of the first transmitted data packet
this time limit is zero. The time limit is applied, over memory bus 26, to
arithmetic unit 27 which compares the time with the real-time count of the
time generator included in the arithmetic unit 27. Comparator 31 receives
over memory bus 26 the contents of field 12 (the time limit) and compares
the latter with the real-time count.
Subsequently, control unit 40 generates three read signals through the
connections 16, 20 and 22 for reading field 11, comprising the number of
packets still authorized to be transmitted in the relevant time interval,
field 13, comprising the maximum number of packets to be transmitted per
time interval, and field 14, comprising the time interval. Transfer means
36 receive, over memory bus 26, both the maximum number of packets and the
number of packets still authorized to be transmitted. Depending on the
signal at the control input, one of the two values is transferred with the
aid of transfer means 36 and applied to the down-counter 35. In the case
of the first transmitted data packet, these two values are equal and,
because the time limit has the value "zero" and, consequently, falls short
of the real-time count, the maximum number of packets to be transmitted is
applied to the down-counter 35. The down-counter 35 decrements its count
by the value "one" in response to a signal coming from the zero detector
37 which signal indicates that the number of packets applied to the zero
detector 37 is unequal to the value "zero". Monitoring unit 6 applies this
new value of the number of packets which is still authorized to be
transmitted in the relevant time interval to memory bus 26. Switching
multiplexer 5 receives from monitoring unit 6 the same signal of zero
detector 37, in response to which switching multiplexer 5 switches through
to the network 3 the data packet delayed in delay buffer 4.
The contents of the read field 14, being the time interval which is still
valid, are simultaneously applied to up-counter 33 in arithmetic unit 27
over memory bus 26. In this unit the time interval is then added to the
real-time count. Depending on the result of the comparison, either this
sum or the original time limit will appear on memory bus 26 of arithmetic
unit 27 because the adding multiplexer 34 optionally connects to either
the output of up-counter 33 or memory bus 26 in response to the output
signal of comparator 31 in a manner to be described hereinafter. In the
case of the first transmitted data packet the time limit in field 12 is
zero and is thus smaller than the real-time count, so the sum of the
real-time count and the time interval from field 14 will appear on the
memory bus 26 of arithmetic unit 27 as the new value for the time limit.
Subsequently, control unit 40 generates, through connection 17, a write
signal for field 11 by means of which signal the new value of the number
of packets still authorized to be transmitted is written into field 11
from monitoring unit 6. Then, control unit 40 generates, through
connection 19, a write signal for field 12 by means of which signal the
new value of the time limit is written into field 12 from arithmetic unit
27 over memory bus 26.
Finally, control unit 40 generates, through connection 24, a read signal
for field 15 comprising the packet count, that is to say, the total number
of packets that has been transmitted since the set-up packet. For the
first data packet this is the packet count "zero". This value is applied,
over memory bus 26, to the up-counter 39 in arithmetic unit 27, which
up-counter 39 increments by one unit if switching multiplexer 5 has
received from monitoring unit 6 the signal to pass the data packet. This
new value of the packet count is then written into field 15 over memory
bus 26 and, after a write signal from control unit 40, through connection
25.
From an arbitrary incoming data packet the header comprising the VCI is
applied to address input 9 of memory 10 through buffer 8 and address
multiplexer 7. This header addresses the five fields 11-15 with the
following respective contents: the number of packets still authorized to
be transmitted, the time limit, the maximum number of packets to be
transmitted, the time interval and the packet count. Control unit 40
receives over packet sync guide 42 the packet synchronizing signal
belonging to the data packet and, in response thereto, generates a read
signal through connection 18 for reading the time limit from field 12.
Over memory bus 26 the time limit is applied to comparator 31 in
arithmetic unit 27 which comparator compares this time limit with the
real-time count.
Thereafter, control unit 40 generates three read signals through the
connections 16, 20 and 22 for reading the number of packets still
authorized to be transmitted, the maximum number of packets to be
transmitted and the time interval from the respective fields 11, 13 and
14. The number of packets still authorized to be transmitted as well as
the maximum number of packets to be transmitted are applied to monitoring
unit 6 over memory bus 26. The time interval is applied to arithmetic unit
27 over memory bus 26.
In arithmetic unit 27 a comparison is made between the time limit and the
real-time count. If the time limit is smaller than the real-time count
(the time interval has elapsed then) arithmetic unit 27 will apply a
signal to monitoring unit 6 in response to which the down-counter 35
comprised therein is loaded with the maximum number of packets to be
transmitted (at the beginning of a new time interval the maximum number of
packets is again allowed to be transmitted). A down-counter 35 decrements
its count by one unit and the new value obtained thus is written into
field 11 owing to a write signal through connection 17. Switching
multiplexer 5 receives a signal from monitoring unit 6 in response to
which signal switching-multiplexer 5 switches the data packet. Arithmetic
unit 27 simultaneously adds the real-time count to the time interval. This
sum becomes the new time limit and appears on the memory bus 26. After
field 11 has been written into, this new time limit is then written into
field 12.
However, if the time limit exceeds the real-time count (the time interval
has not yet elapsed then) the down-counter 35 is loaded with the number of
packets from field 11. If this number is unequal to zero, the down-counter
35 decrements its count by one unit and this new value will be written
into field 11. The data packet will then be switched. If the number of
packets loaded as a count into down-counter 35 is already equal to the
value "zero", the count of the second counter is not changed and this
value "zero" will be written into field 11 and the data packet will be
blocked. After field 11 has been written into, the previous time limit
from arithmetic unit 27 is again written into field 12.
Subsequently, field 15 comprising the packet count is read and the packet
count is applied to the up-counter 39 in arithmetic unit 27 over memory
bus 26. If the data packet has been switched through, the up-counter 39
will increment its count by one unit and the new packet count obtained
thus is again written into field 15 over memory bus 26.
If a user wishes to terminate the connection, he will send out a release
packet. Such a release packet, just like a set-up packet, is featured by a
fixedly chosen header comprising n bits, for example 15 bits having the
logic value "zero" and one bit having the logic value "one". Consequently,
a release packet is switched to the network 3 in the manner described
above with respect to a set-up packet. The network 3 borrows from this
release packet the information which is of interest, such as the sender
(the user) and the VCI (destination).
The VCI obtained through the network 3 is applied to address multiplexer 7
over external bus 29 and buffer 30. At the same time, network 3 provides a
start signal which is applied to start unit 41 over external control guide
44.
In response thereto start unit 41 will generate three signals:
a first selection signal for address multiplexer 7 which applies, in
response thereto, the VCI from buffer 30 to address input 9 of memory 10;
an auxiliary signal for control unit 40 which generates, in response
thereto, through the consecutive connections 17, 21 and 23, write signals
for writing the values "zero" into the fields 11, 13 and 14, generates
through connection 24 a read signal for reading field 15, and transfers
the packet count to memory bus 26 (which packet count will then be loaded
by input-output buffer 28), and generates through connections 19 and 25
write signals for writing the values "zero" into the fields 12 and 15, and
a loading signal for input-output buffer 28 which is loaded, in response
thereto, by data available on memory bus 26. The packet count loaded into
input-output buffer 28 is then applied to the network 3 over the external
bus 29. The user receives the message that his connection has been
terminated and his total number of transmitted packets, the packet count,
is stored by the network 3 in view of cost allocation.
The start unit 41 represented in FIG. 2A comprises an OR gate 60, three
delay elements (time T) 62, 63 and 64 and an OR gate 61. OR gate 60 is
connected to the external control guides 43 and 44. Its output applies the
selecting signal to address multiplexer 7 over guide 46. Delay element 62
is connected to external control guide 43. Its output is connected to an
input of OR gate 61 and applies a loading signal to input-output buffer 28
over guide 48. Delay element 63 is connected to external control guide 44.
Its output is connected to the further input of OR gate 61 and applies a
loading signal to input-output buffer 28 over guide 47. The output of OR
gate 61 is connected to delay element 64 whose output applies an auxiliary
signal to control unit 40 over guide 45.
A pulse over external control guide 43 appears on guide 46 through gate 60,
and on guide 48 through delay element 62 after a time interval T, and on
guide 45 through OR gate 61 and delay element 64 after a time interval 2T.
A pulse on external control guide 44 appears on guide 46 through OR gate
60, and on guide 47 through delay element 63 after a time interval T, and
on guide 45 through OR gate 61 and delay element 64 after a time interval
2T.
So, a pulse (43-1) over one of the two external control guides 42 and 44
over start unit 41 first causes a pulse (46-1) to appear at the control
input of address multiplexer 7 over guide 46. Consequently, no longer the
contents of buffer 8 but those of buffer 30 are transferred to address
input 9 of memory 10, in other words, memory 10 is addressed with the VCI
originating from a response packet over external bus 29. This is caused by
a pulse at one control input of start unit 41, over guide 43, or at the
other control input, over guide 44. In the former case input-output buffer
28 receives over guide 48 a pulse (48-1) delayed by T with respect to
pulse 43-1, in response to which input-output buffer 28 loads data from
external bus 29 and these data are transferred to the memory bus 26. In
the latter case input-output buffer 28 receives a pulse (47-1) over guide
47 delayed by T relative to pulse 44-1, in response to which input-output
buffer 28 loads data from data bus 26 and transfers these data to external
bus 29. Subsequently, start unit 41 generates a pulse (45-1) delayed by 2T
relative to pulse 43-1 or 11-1, which is applied as an auxiliary signal
over guide 45 to control unit 40 which will be further described
hereinafter.
FIGS. 3 and 4 show the control unit 40 and the relevant time diagram
respectively.
Control unit 40 comprises an inverter 80 whose input is connected to packet
sync guide 42. The output of inverter 80 is connected to the input of
respective delay element 82 (delay T with T equal to the duration of a
sync pulse on packet sync guide 42), delay element 83 (delay 2.5T), delay
element 84 (delay 4T), delay element 87 (delay 5.5T), delay element 89
(delay 7.5T) and delay element 90 (delay 9T). The output of delay element
82 is coupled to connection 18. The output of delay element 83 is coupled
to the connections 16, 20 and 22. The output of delay element 84 is
coupled to an input of an AND gate 91, whose output is coupled to
connection 17. The output of delay element 87 is coupled to an input of an
AND gate 92, whose output is coupled to connection 19. The output of delay
element 89 is connected to an input of an AND gate 93, whose output is
coupled to an input of an OR gate 95, whose output is coupled to
connection 24. The output of delay element 90 is coupled to an input of an
AND gate 94, whose output is coupled to an input of an OR gate 96 whose
output is coupled to connection 25.
The input of inverter 81 is coupled to connection 45 over which start unit
41 transmits the auxiliary signal. The output of inverter 81 is coupled to
the inputs of time delay elements 85 (delay T), 86 (delay 3.5T) and 88
(delay 2T). The output of delay element 85 is coupled to the further input
of AND gate 91 as well as connections 21 and 23. The output of delay
element 86 is coupled to the further input of AND gate 92 and the further
input of AND gate 94. The output of delay element 88 is coupled to the
further input of AND gate 93.
Control unit 40 further includes two positive-edge-triggered SR flip-flops
97 and 98. The SET input of flip-flop 97 is coupled to the output of delay
element 84 and the reset input is coupled to the packet sync guide 42. The
output of flip-flop 97 is coupled to the SET input of 98 and the network 3
to which a ready signal is applied. The RESET input of flip-flop 98 is
coupled to connection 50 through which zero detector 37 applies a signal
to switching multiplexer 5 so as to transmit or not to transmit the data
packet. The output of this flip-flop 98 is coupled to the further inputs
of OR gates 95 and 96.
FIG. 4 shows the operation of control unit 40 in the form of a time
diagram.
A positive pulse on packet sync guide 42, T in length, is inverted by the
inverter 80 so that:
after time period T a negative pulse appears at connection 18 (the time
limit is read out);
after time period 2.5T a negative pulse appears at connections 16, 20 and
22 (the number of packets authorized to be transmitted, the maximum number
of packets and the time interval are read out);
after time period 4T a negative pulse appears at connection 17 (the number
of packets authorized to be transmitted is written);
after time period 5.5T a negative pulse appears at connection 19 (the new
time limit is written);
after time period 7.5T a negative pulse appears at connection 24 (the
packet count is read out) if through connection 50 the zero detector 37
has supplied a signal in response to which switching multiplexer 5
switches through the data packet; and
after time period 9T a negative pulse appears at connection 25 (the packet
count incremented by one unit is written) if through connection 50 zero
detector 37 has supplied a signal in response to which switching
multiplexer 5 switches through the data packet.
A positive pulse as an auxiliary signal through connection 45, T in length,
originating from start unit 41 is inverted by inverter 81 so that:
after time period T a negative pulse appears at the connections 17, 21 and
23 (the number of packets authorized to be transmitted, the maximum number
of packets and the time interval are written from the input-output buffer
28);
after time period 2T a negative pulse appears at connection 24 (the packet
count is read and loaded into input-output buffer 28 under the control of
start unit 41) and
after time period 3.5T a negative pulse appears at connection 19 (at the
location of the time limit the value "zero" is written) and also at
connection 25 (at the location of the packet count the value "zero" is
written).
The above operations of reading of the packet count and writing of the
packet count incremented by one unit in response to a pulse on the packet
sync guide 42 are performed under the control of two flip-flops 97 and 98.
A pulse on packet sync guide 42 appears at the RESET input of flip-flop 97,
causing its output Q to assume the logic value "zero". After a period of
time 4T this pulse appears at the SET input and the output Q assumes the
logic value "one". This is the "ready" signal intended for the network 3.
This signal is also applied to the SET input of flip-flop 98 causing its
output Q to assume the logic value "one". This output signal is applied to
the two OR gates 95 and 96; their outputs also assume the logic value
"one" and no negative pulses can appear at the connections 24 and 25.
Consequently, the packet count can neither be read nor written. Only when
zero detector 37 applies a signal (positive pulse) to switching
multiplexer 5 through connection 50 (the packet switching may take place)
will this positive pulse appear at the RESET input of flip-flop 98 and
will the output Q assume the logic value "zero": the packet count can now
be read or written by means of pulses through the connections 24 and 25.
* * * * *
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