 |
Claims  |
|
|
What is claimed is:
1. A multiple access data communication system having a communication
channel for interconnecting a central station and a plurality of remote
stations, wherein each of said remote stations transmits a message signal
on said channel to said central station which broadcasts a return message
signal to said remote stations, said communication channel being divided
into frames each having a predetermined number of time slots of equal
length,
wherein said central station comprises:
first means for broadcasting a reservation status signal indicating
reserved status of said time slots at frame intervals and a time-slot
assignment signal indicating a certain of said time slots to be assigned
to one of said remote stations in response to a reservation request signal
transmitted therefrom;
wherein each of said remote stations comprises:
second means for discriminating a message signal having a length shorter
than a time slot length as a single packet having the same length as each
time slot and dividing a message signal having a length longer than the
time slot length into a series of packets of the time slot length; and
third means for detecting an idle time slot from said time slots in
response to said reservation status signal when said message signal is
shorter than the time slot length and inserting said single packet to said
detected idle time slot and for transmitting said reservation request
signal indicating the number of time slots to be reserved for the packets
in said series when said message signal is longer than the time slot
length and inserting the packets of said series to the time slots
indicated by said time-slot assignment signal.
2. A multiple access data communication system as claimed in claim 1,
wherein said third means comprises means for inserting a foremost one of
said series of packets and said reservation request signal to said
detected idle time slot and inserting the remainder of said series of
packets to the time slots indicated by said time-slot assignment singal.
3. A multiple access data communication system as claimed in claim 1,
wherein said first means further comprises means for broadcasting a
negative acknowledgement signal when there is an error in a received
single packet, and said third means further comprises means for randomly
selecting an idle time slot from said time slots in response to said
reservation status signal when said negative acknowledgement is broadcast
and inserting a replica of a previously transmitted packet to the randomly
selected time slot.
4. A multiple access data communication system as claimed in claim 1,
wherein said first means further comprises:
means for detecting a traffic volume of packets received by the central
station exceeding a predetermined level and broadcasting a traffic
overflow signal in response to the detection of said exceeding traffic
volume,
wherein said second means further comprises:
means for causing the length of each time slot to be decreased in response
to said traffic overflow signal.
5. A multiple access data communication system as claimed in claim 1,
wherein each of said remote stations further comprises:
means for detecting a traffic volume of said single packets exceeding a
predetermined level, causing said reservation request signal to be
transmitted in response to the detection of said exceeding traffic volume
and inserting said single packet and said series of packets to the time
slots indicated by said time-slot assignment signal which is broadcast in
response to the last-mentioned reservation request signal.
6. A multiple access data communication system as claimed in claim 1,
wherein said first means further comprises means for broadcasting a
negative acknowledgement signal when there is an error in a received
single packet, and wherein said third means comprises means for inserting
said single packet and said series of packets to the time slots indicated
by said time-slot assignment signal in response to said negative
acknowledgement signal.
7. A multiple access data communication system as claimed in claim 1,
further comprising a satellite transponder through which said
communication channel is established between said central station and said
plurality of remote stations.
8. A multiple access data communication system having a communication
channel for interconnecting a plurality of stations, said channel being
divided into frames each having a predetermined number of time slots of
equal length, comprising:
first means for broadcasting a reservation status signal indicating
reserved status of said time slots at frame intervals and a time-slot
assignment signal indicating a certain of said time slots to be assigned
to one of said stations in response to a reservation request signal
transmitted therefrom;
second means for discriminating a message signal shorter than the time slot
length as a single packet having a time slot length and dividing a message
signal longer than the time slot length into a series of packets of the
time slot length; and
third means for detecting an idle time slot from said channel in response
to said reservation status signal when said message signal is shorter than
the time slot length and inserting said single packet to said detected
idle time slot and for transmitting said reservation request signal
indicating the number of time slots to be reserved for the packets in said
series when said message signal is longer than the time slot length and
inserting the packets of said series to the time slots indicated by said
time-slot assignment signal.
9. A multiple access data communication system as claimed in claim 8,
wherein said third means comprises means for inserting a foremost one of
said series of packets and said reservation request signal to said
detected idle time slot and inserting the remainder of said series of
packets to the time slots indicated by said time-slot assignment singal.
10. A multiple access data communication system as claimed in claim 8,
wherein said first means further comprises means for broadcasting a
negative acknowledgement signal when there is an error in a received
single packet, and said third means further comprises means for randomly
selecting an idle time slot from said channel in response to said
reservation status signal when said negative acknowledgement is broadcast
and inserting a replica of a previously transmitted packet to the randomly
selected time slot.
11. A multiple access data communication system as claimed in claim 8,
wherein said first means further comprises:
means for detecting a total traffic volume of packets in said system
exceeding a predetermined level and broadcasting a traffic overflow signal
in response to the detection of said exceeding traffic volume,
wherein said second means further comprises:
means for causing the length of each time slot to be decreased in response
to said traffic overflow signal.
12. A multiple access data communication system as claimed in claim 8,
further comprising:
means for detecting a traffic volume of said single packets exceeding a
predetermined level, causing said reservation request signal to be
transmitted in response to the detection of said exceeding traffic volume
and inserting said single packet and said series of packets to the time
slots indicated by said time-slot assignment signal which is broadcast in
response to the last-mentioned reservation request signal.
13. A multiple access data communication system as claimed in claim 8,
wherein said first means further comprises means for broadcasting a
negative acknowledgement signal when there is an error in a received
single packet, and wherein said third means comprises means for inserting
said single packet and said series of packets to the time slots indicated
by said time-slot assignment signal in response to said negative
acknowledgement signal.
14. A multiple access data communication system as claimed in claim 8,
further comprising a satellite transponder through which said
communication channel is established between said stations.
15. A method for assigning time slots in a multiple access data
communication system having a communication channel for transmitting a
message signal from each of a plurality of remote stations to a central
station which in turn broadcasts a return message signal to said remote
stations, said communication channel being divided into frames each having
a predetermined number of time slots of equal length, the method
comprising the steps of:
(a) broadcasting from said central station to said remote stations a
reservation status signal indicating reserved status of said time slots at
frame intervals and a time-slot assignment signal indicating a certain of
said time slots to be assigned to a remote station in response to a
reservation request signal transmitted therefrom;
(b) discriminating a message signal as a single packet having the same
length as each of said time slots when the message signal is smaller than
the time slot length;
(c) dividing the message signal into a series of packets of the time slot
length when the message signal has a length greater than the time slot
length;
(d) detecting an idle time slot from said time slots in response to said
reservation status signal when said message signal is smaller than the
time slot length and inserting said single packet to said idle time slot;
and
(e) transmitting said reservation request signal indicating the number of
time slots required for said series of packets when said message signal is
longer than the time slot length and inserting said plurality of packets
to the time slots indicated by said time-slot assignment signal.
16. A method for assigning time slots as claimed in claim 15, wherein the
step (e) comprises inserting a foremost one of said series of packets and
said reservation request signal to said detected idle time slot and
inserting the remainder of said series of packets to the time slots
indicated by the time-slot assignment signal which is received after the
transmission of the last-mentioned reservation request signal.
17. A method for assigning time slots as claimed in claim 15, further
comprising the steps of detecting a traffic volume of packets received by
the central station exceeding a predetermined level, broadcasting a
traffic overflow signal from the central station in response to the
detection of said exceeding traffic volume, and causing the length of each
time slot in said remote stations to be decreased in response to said
traffic overflow signal.
18. A method for assigning time slots as claimed in claim 15, further
comprising the steps of detecting a traffic volume of said single packets
in each remote station exceeding a predetermined level, causing said
reservation request signal to be transmitted in response to the detection
of said exceeding traffic volume, and inserting said single packet and
said series of packets to the time slots indicated by said time-slot
assignment signal which is broadcast in response to the last-mentioned
reservation request signal.
19. A method for assigning time slots as claimed in claim 15, further
comprising the steps of broadcasting a negative acknowledgement signal
from the central station when there is an error in a received single
packet and inserting said single packet and said series of packets to the
time slots indicated by said time-slot assignment signal in response to
said negative acknowledgement signal. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a time division multiple access data
communication system in which a plurality of stations which shares a
slotted communication channel through which message signals are
transmitted.
The ALOHA system is the first random access multipoint packet data
communication system. This system uses a single radio channel which is
shared by a plurality of stations or data terminals. Whenever a station
generates a packet, which is a message of a fixed length, in the ALOHA
system, it transmits the packet on the common radio channel. Since more
than one station may attempt to transmit a packet simultaneously, several
transmissions may overlap. These overlapping transmissions are said to
collide if any portion of two packets overlap. Whenever a collision
occurs, random numbers are used to specify a period of time each
conflicting station must wait before an attempt is made to gain access to
the channel. To reduce increase channel utilization, the slotted ALOHA
system was proposed in which the channel is partitioned into slots of time
equal to a packet length and each station only transmits a packet at the
beginning of a slot. In this way overlapping transmissions are forced to
completely overlap. This technique substantially doubles the maximum
channel utilization of the unslotted ALOHA system.
Since the slotted ALOHA system still operates on a random access basis, an
increase in traffic causes collisions to increase with a resultant
increase in retransmissions. Therefore, the total traffic increases
disproportionately due to the retransmissions and prevents the channel
utilization from reaching its maximum.
To reduce the effects of collisions in the slotted ALOHA system, a slot
reservation scheme has been proposed. In this system, the channel is
partitioned into frames each containing a reservation slot for
transmitting a reservation packet and data slots for transmitting data
packets. Each station transmits a reservation packet on a random access
basis requesting slots as many as required for data packets to be
transmitted. If the request is granted, data slots of a subsequent frame
are assigned to the requesting station, which in turn transmits data
packets on the assigned slots.
Because of the neccessity of the packet reservation system to transmit a
reservation packet prior to the transmission of data packets, there is a
delay before the data packets are actually sent. If short messages are
dominant in the traffic, the amount of delay would become substantial and
satisfactory channel utilization is not attained.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a multiple
access data communication system having a high degree of channel
utilization. Briefly described, the object of the present invention is
achieved by combining a random access technique for transmission of short
message signals with a slot reservation technique for transmission of long
message signals.
More specifically, the multiple access data communication system of the
present invention has a communication channel for interconnecting a
plurality of stations. The channel is divided into frames each having a
predetermined number of time slots of equal length for transmitting
message signals, the number of such time slots being smaller than the
number of stations of the system. In the data communication system, a
reservation status signal is constantly broadcast at frame intervals,
indicating reserved status of the time slots, to permit a message signal
to be transmitted on a random access basis. When a request for
transmission is made from a station, a message signal shorter than the
time slot length is discriminated as a single packet having a time slot
length and a message signal longer than the time slot length is divided
into a series of packets of the time slot length. When the message signal
is discriminated as a single packet the requesting station detects an idle
time slot from the communication channel using the reservation status
signal and inserts the discriminated single packet to the detected idle
time slot. When the message signal is longer than the time slot length,
the requesting station transmits a reservation request to a central
station to receive from it a time-slot assignment signal which specifies
time slots to be assigned to the requesting station and inserts the
long-message packets to the specified time slots.
To reduce the amount of delay involved in transmitting long message
signals, a foremost one of the series of packets and the reservation
request signal are inserted to a detected idle time slot and the remainder
packets of the series are inserted to the time slots specified by the
time-slot assignment singal.
A further improvement in channel utilization is achieved for a traffic
carrying a large volume of short message signals by detecting when the
total traffic volume of packets in the system exceeds a predetermined
level, broadcasting a traffic overflow signal in response to the detection
of the excessive traffic volume, and causing the length of each time slot
to be decreased in response to the traffic overflow signal. Alternatively,
channel utilization for dominant short messages can further be improved by
detecting a traffic volume of single packets of a station exceeding a
predetermined level, causing a reservation request signal to be
transmitted in response to the detection of the excessive traffic volume
and switching the operational mode of the station by inserting each single
packet and a series of packets to the time slots specified by the
time-slot assignment signal which is broadcast in response to the
reservation request signal. Alternatively, the same improvement can be
achieved by broadcasting a negative acknowledgement signal when there is
an error in a received single packet and inserting each single packet and
the series of packets to the time slots specified by a time-slot
assignment signal when the negative acknowledgement signal is broadcast.
The present invention is particularly advantageous for multipoint data
communication systems which use a satellite transponder as a transmission
medium and further advantageous for a system in which the communication
channel interconnects a central station which transmits signals on a
broadcast mode to a plurality of remote stations to broadcast reservation
status signals at constant intervals and a time-slot assignment signal in
response to a reservation request from a remote station.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described in further detail with reference to
the accompanying drawings, in which:
FIG. 1 is a schematic block diagram of a multiple access satellite
communications system suitable for application of the present invention;
FIG. 2A is an illustration of a data format employed by the central station
of FIG. 1 and FIG. 2B is an illustration of a data format employed by the
remote stations;
FIG. 3 is a block diagram of the central station;
FIG. 4 is a block diagram of a portion of each remote station according to
the present invention;
FIG. 5A is a block diagram of the remainder of each remote station
according to a first embodiment of the present invention;
FIG. 5B is a block diagram of the remainder of each remote station
according to a second embodiment of the invention;
FIG. 5C is a block diagram of the remainder of each remote station
according to a third embodiment of the present invention;
FIG. 5D is a block diagram of the remainder of each remote station
according to a fourth embodiment of the invention; and
FIGS. 6A, 6B, 6C and 6D are views illustrating the operations of the first,
second, third and fourth embodiments of the invention, respectively.
DETAILED DESCRIPTION
As schematically represented in FIG. 1, the multiple access satellite
communications system of the invention has a central earth station 1 in
which a central data terminal equipment is installed and a plurality of
remote earth stations 2. Each remote earth station serves one or more data
terminal equipments, not shown, from which it receives message signal and
sends it in short bursts on time slots defined by the central station
through a commonly shared channel to a satellite transponder 3. After
amplification, the transponder 3 sends it to the central station 1 where
it is processed by the central data terminal equipment and sent back to
the transponder 3 where it is broadcast to all the remote earth stations.
As described below, the signal from the central station is broken into
frames each comprising a control field and a data field in which the
processed signal is carried and continuously broadcast through the
transponder 3 to allow all remote stations to obtain necessary information
from the control field at any instant of time whenever it receives a
request for transmission of a message signal from the own data terminal
equipment.
According to the present invention, the remote earth stations 2 transmit
data to the central station 1 on a randomly selected idle time slot if the
length of the data is smaller than the length of time slot and divide
longer-than-a-time-slot data into a plurality of packets of time slot
length and transmit the foremost of the packets to the central station 1
in a random access mode with a reservation request subfield appended to it
to request reservation for the remainder packets. If the request is
granted by the central station, idle time slots are selected from
nonreserved time slots and assigned to the requesting station for
transmission of the remainder packets.
FIG. 2A shows a format of data transmitted by the central station 1. As
illustrated, signals are transmitted in a series of frames each containing
a control field and a data field. The control field comprises a plurality
of subfields including a frame sync subfield 4, positive or negative
acknowledgement subfield 5, a slot assignment subfield 6 indicating
assigned time slot numbers and a code identifying the remote station
requesting reservation to which the time slot numbers are to be assigned,
and a reserved-slots information subfield 7 which indicates the status of
all time slots. Acknowledgement subfield 5 comprises bits which
respectively correspond to different time slots. If no error occurs in
data received on a given time slot, the bit position corresponding to the
given time slot is set to "1" indicating positive acknowledgement (ACK).
If error occurs in the received data, the corresponding bit position is
set to "0" indicating negative acknowledgement (NAK). The data field
contains a number of data packets each having a flag subfield 8 indicating
the starting point of the data packet, an address subfield 9 which
identifies a receiving remote station, a control subfield 10, a data
subfield 11 in which the data to be sent to the receiving station, a
cyclic redundancy check subfield 12 and a flag subfield 13 indicating the
ending point of the data packet. If the data field is not occupied by data
packets, vacant spaces are filled with flag sequences. FIG. 2B is an
illustration of a data format for the remote stations. Each remote station
transmits a time-slot length packet including a control field containing a
sync subfield 14, an address subfield 15 identifying the transmitting
remote station, a reservation request subfield 16 indicating the number of
packets to be assigned, a control subfield 17, a data subfield 18
containing data to be processed by the central data terminal equipment of
the central station and a cyclic redundancy check subfield 19.
FIG. 3 shows a block diagram of the central earth station 1. Signals
transmitted from the satellite transponder 3 are received by an antenna 20
and passed through a PSK (Phase Shift Keying) demodulator 21. After
appropriate amplification and frequency conversion, the demodulated signal
is supplied to an error detect circuit 22. If no error is detected in the
received data, detector 22 passes it to a decoder 29 of a central
processing unit (CPU) 23 and to a receive buffer 33. The output of receive
buffer 33 is coupled to an input of a central data processing equipment,
not shown, which operates in response to the time-slot sync contained in
the time slot data received from the remote stations. Simultaneously,
detector 22 causes a positive acknowlegement (ACK) to be supplied to a
latch 30 from an ACK/NAK generator 27. Decoder 29 decodes the header field
of the received time slot data including the reservation request, the
address of a transmitting remote station and data contained in the control
subfield 17. Decoder 29 supplies the address of the transmitting remote
station to ACK/NAK generator 27 to permit it to distinguish the ACK/NAK
subfield addressed to it from those directed to other remote stations. The
CPU 23 includes a reservation status table 24 which stores reserved time
slot numbers. If a reservation request is received from a remote station,
a selector 25 reads the output of decoder 29 to select next available time
slots from the reservation status table 24 as many as requested by the
remote station and updates the table 24 with the newly selected time slot
numbers. Data indicating the selected time slot numbers and the address of
the remote station requesting the reservation are read out of the table 24
and a slot-assignment signal indicating the time slots to be assigned to
it and the station address information are supplied to latch 30. The
stored time slot data, thus updated by selector 25, is also supplied to
latch 30 as a signal indicating a list of all the reserved time slots. As
will be described later, the reserved time slot signal is used in each
remote earth station 2 to generate an idle-slot timing signal when a data
packet is to be sent to the central station in a random access mode. If an
error is detected in the received data, detector 22 disables its data
output to decoder 29 and buffer 33 and causes a negative acknowledgement
(NAK) to be supplied from ACK/NAK circuit 27 to latch 30.
A timing signal generator 31 generates a plurality of timing signals
including a frame sync, frame-header timing and slot-header timing
signals, and bit timing signal. A frame pattern generator 26 is responsive
to the frame sync to generate a frame sync pattern which is stored into
latch 30. Latch 30 responds to the frame-header timing signal by
transferring the stored data to a shift register 32 which is clocked out
in response to the bit timing signal from the timing generator 31, thus
generating a control field, FIG. 2A. This control field data is supplied
to a time division multiplexer 34. A data field is generated by a circuit
including a register 35, address and control subfield generators 36 and
37, a CRC (cyclic redundancy check) subfield generator 38, and a flag
inserter 39. Address and control subfield generators 36, 37 supply address
and control subfields of a receiving remote station to the register 35 and
CRC subfield generator 38 supply a CRC code to the register 35. Register
35 is clocked by the bit timing signal to interleave the stored header and
trailer subfield data with transmit data supplied through the transmit
buffer 40. Flag inserter 39 is coupled to the output of register 35 to
insert a flag sequence at the starting and closing ends of each data
packet whenever there is information to be sent. If there is no
information in the register 35, the flag inserter 39 operates to fill the
vacant spaces with flag sequences. Time division multiplexer 34 is timed
with the frame-header and slot-header timing signals to multiplex each
time slot data with the control field data. The multiplexed signal is
modulated upon a carrier by a PSK modulator 41 and sent to the antenna 20
after appropriate amplification and frequency conversion for transmission
to satellite transponder 3. The central station 1 may include a traffic
overflow detector 42 connected to the output of receive buffer 33 to
supply overflow data to latch 30 if the traffic of the time slot data
received from all the remote stations exceeds a predetermined level. In
such events, a traffic overflow subfield is included in the control field.
FIG. 4 is a block diagram illustrating a portion of each remote earth
station. In each remote station, the PSK-modulated signal broadcast by
transponder 3 is received by an antenna 50 and, after appropriate
amplification and frequency conversion, passed through a PSK demodulator
51 to a decoder 52. The output of PSK demodulator 51 is also applied to a
bit recovery unit 54 and a frame recovery unit 55. Decoder 52 responds to
a frame timing signal from the frame recovery unit 55 to decode the
control field data into individual subfields including ACK/NAK, reserved
time slots, slot assignment and traffic overflow. Time slot generators 56
and 57 are connected to the decoder 52 to respond respectively to the
subfield data indicating reserved time slots and the slot assignment
subfield data by generating an idle-slot timing signal and an
assigned-slot timing signal. A presettable counter 58 is connected to the
output of bit recovery unit 54 to start counting the recovered clock bits
following a clear pulse supplied from the frame timing signal until a
preset count is reached. Under normal traffic conditions, the counter 58
is preset to a higher value to generate a slot timing signal that occurs
at normal intervals. If a traffic overflow occurs, the counter 58 is
preset to a lower value to generate a slot timing signal at one-half the
normal intervals. The output of PSK demodulator 51 is further applied to a
receive buffer 59 to detect the data field from each frame for application
to the data terminal equipment of the remote station.
FIG. 5A is a block diagram of the remainder portion of the remote station
according to a first embodiment of the present invention. Incoming data
from the data terminal equipment of the own station is entered to a data
length detect logic 60 of the CPU 53 and to a data selector 61. Data
length detect logic 60 compares the length of the incoming message data
with a reference length, or one time-slot length, from a register 62 and
instructs the data selector 61 to pass the incoming data on bus 67 to a
random transmit buffer 63 of a memory unit 66 if it is equal to or smaller
than one time-slot length. Alternatively, the data length detect logic 60
instructs the data selector 61 to divide the incoming data into several
packets each having a length equal to or smaller than a time-slot length
and pass the foremost packet on bus 67 to the random transmit buffer 63
and the remainder packets on bus 68 to a reservation transmit buffer 64.
In the latter case, the data length detect logic 60 instructs a
reservation request control logic 69 to generate a reservation request
field to be appended to the foremost packet that has been stored into the
random transmit buffer 63. A retransmit control logic 70 is responsive to
an ACK/NAK signal from decoder 52 to effect the transfer of a packet from
random transmit buffer 63 on bus 71 to a retransmit buffer 65 or effect
the transfer of packets from reservation transmit buffer 64 on bus 72 to
retransmit buffer 65. The data stored in buffers 63, 64 and 65 are
selected by a time-slot select logic 73 and passed to a register 74 during
a time slot selected in a manner as will be described. Simultaneously with
the transfer of packets from buffers 63, 64 to buffer 65, the retransmit
control logic 70 controls a random number generator 75 to randomly select
an idle time slot. Specifically, random number generator 75 is connected
to the decoder 52 to receive the reserved-slot data to determine idle
slots and randomly select one from the determined idle slots. The
idle-slot timing and assigned-slot timing signals from the decoder 52,
FIG. 4, are supplied to the time-slot select logic 73 to determine a time
slot to which one of the outputs of memory 66 is inserted for coupling to
the register 74.
Retransmit control logic 70 essentially comprises a comparator and
memories. The comparator checks to see if the ACK/NAK signal from the
decoder 52 has a "0" (ACK) or "1" (NAK) in the bit position corresponding
to the time slot number by which the data packet has been sent. If ACK/NAK
is "1", the data packet which has been transferred to the retransmit
buffer 65 is cleared. If ACK/NAK is "0", the retransmit control logic 70
enables the random number generator 75 to randomly select an idle time
slot and retransmit a packet from the retransmit buffer 65.
The CPU 53 includes a control field generator 76A including sync, address,
reservation request and control subfields. The reservation request
subfield is controlled in response to an output from the reservation
request control logic 69. Further provided is a CRC subfield generator
76B. Register 74 is loaded with the contents of the subfield generators
76A and 76B to form a control field which is clocked out in response to
bit timing signal to a PSK modulator 77, the control field being followed
by a packet carried on a time slot selected by the time-slot select logic
73. The output of PSK modulator 77 is connected to the antenna 50 after
appropriate amplification and frequency conversion for transmission to the
satellite transponder 3.
The operation of the first embodiment of the present invention will be
described with reference to FIG. 6A. For convenience it is assumed that a
frame comprises five time slots. Central station 1 and all the remote
stations 2 operate synchronously as illustrated. Central station 1 is
constantly broadcasting control fields (CF) with subsequent data fields at
frame intervals to allow remote stations to transmit data packets on
synchronized time slots. For purposes of description, control fields which
are of interest are designated with numbers #1 through #4 in FIG. 6A. In
response to each control field, the counter 58, FIG. 3, of each remote
station generates a series of slot timing signals and a transmit frame is
formed by every five such slot timing signals. It is shown that #1, #2 and
#4 transmit frames are formed respectively in response to corresponding
control fields. Likewise, the central station generates a receive cycle in
which five time slots constitute a receive frame. Assume that a one-slot
length message and a four-slot length message are applied to data length
detect logic 60, FIG. 5A, in succession, so that the short length message
and the foremost packet of the long-length message are stored into the
random transmit buffer 63 in succession as #1 and #2 data packets,
respectively, and the remaining data packets following #2 data packet are
stored into the reservation transmit buffer 64 as #3, #4 and #5 data
packets. At the same time, reservation request control logic 69 is
instructed by data length detect logic 60 to update the contents of the
reservation request subfield of the time-slot control field 76A to append
to #2 data packet a request for reservation of three time slots for #3, #4
and #5 data packets. Time slot generator 56 receives reserved- slot data
from decoder 52 which obtains it from the reserved-slots subfield of #1
control field, detects idle time slots from those not reserved by any
remote stations and generates idle-slot timing signals corresponding to
the detected idle time slots. Assume that the time-slot select logic 73
selects a second time slot from the time slots indicated by the idle-slot
timing signals from time slot generator 56. Thus, #1 data packet is sent
through bus 78 on the selected #2 time slot to register 74 during #1
transmit frame and transmitted via the satellite transponder to the
central station where it is received on #2 time slot of #1 receive frame
of the central station. In the remote station, #1 data packet is
simultaneously transferred from random transmit buffer 63 to retransmit
buffer 65. If the transmitted PSK time slot data encounters no collision
with data packets from other remote stations, the ACK/NAK subfield
generator 27 of the central station writes a positive ackncwledgement into
an ACK/NAK subfield which will occur in #3 control field.
In response to #2 control field that occurs during #1 transmit frame of the
remote station, the time slot generator 56 receives reserved-slot data
from decoder 52 to generate timing signals indicative of idle time slots.
Time-slot select logic 73 selects #1 time slot, for example, within #2
transmit frame from those indicated by the idle-slot timing signals. Thus,
#2 data packet is transmitted on #1 time slot of #2 transmit frame to the
central station. Similar to #1 data packet, #2 data packet is also
transferred to the retransmit buffer 65. If #2 data packet is not
destroyed by collision, it will be received on #1 time slot of #2 receive
frame which is separated by one frame from #1 receive frame, and a
positive acknowledgement will be sent on #4 control field. If #1 and #2
data packets have been received correctly by the central station, the
retransmit control logic 70 of the transmitting remote station will check
the ACK signals with the slot numbers of the packets stored in the
retransmit buffer 65 and clears the latter. However, if an NAK signal is
contained in one of the #3 and #4 control fields, the retransmit control
logic 70 enables the random number generator 75 to randomly select an idle
time slot to insert #1 data packet from the retransmit buffer 65 to the
idle time slot to effect retransmission. This process is repeated with
respect to #2 data packet.
Upon receipt of #2 data packet which is appended with the reservation
packet requesting three time slots, the central station's decoder 29
supplies this reservation request to selector 25 to cause it to select
idle time slots from the reservation status memory 24 to be assigned to
#3, #4 and #5 data packets and generates a slot-assignment signal for
these data packets. It is assumed that #1 to #3 time slots are selected
and assigned by selector 25 to #3 to #5 data packets. Reservation status
memory 24 is then updated with the newly assigned time slots. A control
field #4 is generated and transmitted on a broadcast mode to all the
remote stations. This control frame includes an address subfield
indicating the remote station requesting the reservation, a
slot-assignment subfield indicating the time slot numbers 1, 2 and 3 and
the identification of the remote station requesting the reservation and a
reserved-slot subfield indicating the most recent status of all the time
slots.
Upon receipt of the #4 control field, the remote stations other than the
station requesting the reservation for #3 to #5 data packets, are now
informed of the fact that #1 through #3 time slots have been assigned to
that remote station. Thus, if there is any data packet to be sent on a
random access basis from such stations, such a packet is transmitted on a
time slot other than #1 through #3 time slots. On the other hand, the
identification code appended to the slot-assignment data enables the
remote station re | | |
