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Claims  |
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What is claimed is:
1. A random access communication system comprising:
a central station; and
a plurality of user stations, each of the user stations transmitting a
packet to said central station on a randomly selected timeslot of a
time-division multiple access channel, the user station defining a
plurality of short-duration slots within said selected timeslot and
transmitting a burst on one of said short-duration slots,
said central station monitoring the defined short-duration slots to detect
said burst transmission, assigning as many timeslots as required if more
than one burst transmission is detected within a same timeslot, and
transmitting a slot assignment signal to said user stations containing a
negative acknowledgement of said transmitted packet,
said user station from which said packet was transmitted being responsive
to said negative acknowledgement for selecting one of the assigned
timeslots and retransmitting to said central station a copy of said packet
on the selected assigned timeslot,
wherein said central station transmits a status signal indicating a status
of each of said defined short-duration slots, the packet-sending user
station being responsive to said status signal for determining a relative
position of the short-duration slot on which the burst was transmitted and
selecting one of the assigned timeslots according to the determined
position.
2. A random access communication system as claimed in claim 1, wherein a
plurality of said timeslots are organized into successive frames, and each
of said user stations includes:
table means for holding a usage status record of the user station, said
record indicating presence and absence of a burst transmission of the user
station on each of the short-duration slots; and
control means for checking said status signal from said central station
with said usage status record to determine said relative position of the
short-duration slot on which said burst was transmitted.
3. A random access communication system as claimed in claim 1, wherein said
central station includes means for detecting an error in said packet, and
means for transmitting said negative acknowledgement under conditions that
(1) said burst transmission is not detected in any of said short-duration
slots, and that (2) said burst transmission is detected only in one of
said short-duration slots and said error is detected in said packet, the
packet-sending user station being responsive to said negative
acknowledgement transmitted under said conditions for retransmitting a
copy of the packet on a randomly selected timeslot.
4. A random access communication system as claimed in claim 3, each of said
user stations further includes:
table means for holding a reservation status record of all user stations of
the system, said reservation status record indicating assignment of a
timeslot to all user stations and for holding an assignment status record
of the user station, said assignment status record indicating unique
assignment of a timeslot to the user station; and
mode determining means for determining a packet transmission mode according
to the reservation status record and said assignment status record.
5. A random access communication system as claimed in claim 4, wherein each
of said user stations comprises:
a transmit buffer for holding packets waiting for initial transmission;
a retransmit buffer for holding copies of the packets initially transmitted
from said transmit buffer;
means for determining a count indicating the number of packets in said
retransmit buffer to which packets timeslots are not assigned by the
central station if said packet transmission mode is determined by said
mode determining means to be a random access mode;
means for transmitting a packet from said transmit buffer on a randomly
selected timeslot if said count is equal to zero;
means for determining an interval between successive packet transmissions
from said retransmit buffer if said count is equal to or greater than
unity; and
means for transmitting a packet from said retransmit buffer on a randomly
selected timeslot if said interval corresponds to a prescribed value.
6. A random access communication system as claimed in claim 3, wherein each
of said user stations comprises:
a transmit buffer for holding packets waiting for initial transmission;
a retransmit buffer for holding copies of the packets initially transmitted
from said transmit buffer;
means for determining a packet transmission mode during each successive
timeslot;
means for determining a count indicating the number of packets in said
retransmit buffer to which packets timeslots are not assigned by the
central station if said packet transmission mode is determined by said
mode determining means to be a random access mode;
means for transmitting a packet from said transmit buffer on a randomly
selected timeslot if said count is equal to zero;
means for determining an interval between successive packet transmissions
from said retransmit buffer if said count is equal to or greater than
unity; and
means for transmitting a packet from said retransmit buffer on a randomly
selected timeslot if said interval corresponds to a prescribed value.
7. A random access communication system as claimed in claim 1, wherein said
burst transmission from each of the user stations contains a predetermined
bit sequence.
8. A satellite communications system comprising:
a central station; and
a plurality of user stations, each of the user stations transmitting a
packet to said central station via a communications satellite on a
randomly selected timeslot of a time-division multiple access channel, the
user station defining a plurality of short-duration slots within said
selected timeslot and transmitting a burst on one of said short-duration
slots,
said central station monitoring the defined short-duration slots to detect
said burst transmission, assigning as many timeslots as required if more
than one burst transmission is detected within a same timeslot, and
transmitting a slot assignment signal to said user stations containing a
negative acknowledgement of said transmitted packet,
said user station from which said packet was transmitted being responsive
to said negative acknowledgment for selecting one of the assigned
timeslots and retransmitting to said central station a copy of said packet
on the selected assigned timeslot,
wherein said central station transmits a status signal indicating a status
of each of said defined short-duration slots, the packet-sending user
station being responsive to said status signal for determining a relative
position of the short-duration slot on which the burst was transmitted and
selecting one of the assigned timeslots according to the determined
position.
9. A satellite communications system as claimed in claim 8, wherein a
plurality of said timeslots are organized into successive frames, each of
said user stations includes:
table means for holding a usage status record of the user station, said
record indicating presence and absence of a burst transmission of the user
station on each of the short-duration slots; and
control means for checking said status signal from said central station
with said usage status record to determine said relative position of the
short-duration slot on which said burst was transmitted.
10. A satellite communications system as claimed in claim 8, wherein said
central station includes means for detecting an error in said packet, and
means for transmitting said negative acknowledgment under conditions that
(1) said burst transmission is not detected in any of said short-duration
slots, and that (2) said burst transmission is detected only in one of
said short-duration slots and said error is detected in said packet, the
packet-sending user station being responsive to said negative
acknowledgment transmitted under said conditions for retransmitting a copy
of the packet on a randomly selected timeslot.
11. A satellite communications system as claimed in claim 10, each of said
user stations further includes:
table means for holding a reservation status record of all user stations of
the system, said reservation status record indicating assignment of a
timeslot to all user stations and for holding an assignment status record
of the user station, said assignment status record indicating unique
assignment of a timeslot to the user station; and
mode determining means for determining a packet transmission mode according
to the reservation status record and said assignment status record.
12. A satellite communications system as claimed in claim 11, wherein each
of said user stations comprises:
a transmit buffer for holding packets waiting for initial transmission;
a retransmit buffer for holding copies of the packets initially transmitted
from said transmit buffer;
means for determining a count indicating the number of packets in said
retransmit buffer to which packets timeslots are not assigned by the
central station if said packet transmission mode is determined by said
mode determining means to be a random access mode;
means for transmitting a packet from said transmit buffer on a randomly
selected timeslot if said count is equal to zero;
means for determining an interval between successive packet transmissions
from said retransmit buffer if said count is equal to or greater than
unity; and
means for transmitting a packet from said retransmit buffer on a randomly
selected timeslot if said interval corresponds to a prescribed value.
13. A satellite communications system as claimed in claim 10, wherein each
of said user stations comprises:
a transmit buffer for holding packets waiting for initial transmission;
a retransmit buffer for holding copies of the packets initially transmitted
from said transmit buffer;
means for determining a packet transmission mode during each successive
timeslot;
means for determining a count indicating the number of packets in said
retransmit buffer to which packets timeslots are not assigned by the
central station if said packet transmission mode is determined by said
mode determining means to be a random access mode;
means for transmitting a packet from said transmit buffer on a randomly
selected timeslot if said count is equal to zero;
means for determining an interval between successive packet transmissions
from said retransmit buffer if said count is equal to or greater than
unity; and
means for transmitting a packet from said retransmit buffer on a randomly
selected timeslot if said interval corresponds to a prescribed value.
14. A satellite communications system as claimed in claim 8, wherein said
burst transmission from each of the user stations contains a predetermined
bit sequence. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to a time-division multiple access
communication system in which user packets are transmitted on a randomly
selected timeslot for accessing a central station. Specifically, this
invention relates to a contention technique associated with collision of
data due to simultaneous packet transmissions from contending users. The
present invention is particularly suited for satellite communications
systems.
With a random access satellite communication system using slotted time
intervals, known as the slotted ALOHA system, short-delay communication is
possible between a user terminal and a central host computer when the rate
of message occurrences is relatively low. However, with an increasing rate
of message transmissions, data collisions occur with an increasing rate.
Under such circumstances, it is likely that retransmissions encounter
collision with new packet transmissions, and the channel utilization
efficiency of the system is severely degraded. This problem arises from
the fact that, since each user station has no knowledge on the status of
every timeslot of the access channel, it is impossible for user stations
to control their packet transmissions when a data collision occurs between
other stations.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a random
access communication system which ensures high channel utilization
efficiency even when the system is carrying heavy user traffic.
This object is achieved by defining "minislots" in a timeslot which is
randomly selected by each user station for packet transmission to a
central station, randomly selecting one of the minislots for burst
transmission and monitoring the minislots at the central station for
assigning timeslots if more than one minislot is detected within a
timeslot.
According to the present invention, there is provided a random access
communication system comprising a central station and a plurality of user
stations. Each of the user stations transmits a packet to the central
station on a randomly selected timeslot of a time-division multiple access
channel and defines a plurality of "minislots" or short-duration slots
within the selected timeslot and transmits a burst on one of the
short-duration slots. The central station monitors the defined
short-duration slots to detect the burst transmission, and assigns as many
timeslots as required if more than one burst transmission is detected
within a same timeslot, and transmits a slot assignment signal to the user
stations containing a negative acknowledgment of the transmitted packet.
The user station from which the packet was transmitted is responsive to
the negative acknowledgment to select one of the assigned timeslots and
retransmit to the central station a copy of the packet on the selected
assigned timeslot.
Preferably, the central station transmits a status signal indicating the
status of each short-duration slot, and the packet-sending user responds
to the status signal by determining the relative position of the
short-duration slot on which the burst was transmitted and selecting one
of the assigned timeslots according to the determined position. In
addition, the central station detects an error in the packet and transmits
a negative acknowledgment under conditions that a burst transmission is
not detected in any of the short-duration slots, and that a burst
transmission is detected only in one of the short-duration slots and an
error is detected in the packet. In response to such a negative
acknowledgment, the sending user station retransmits a copy of the packet
on a randomly selected timeslot.
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 block diagram of a satellite communications system according to
the present invention, showing details of a central station;
FIG. 2 shows a packet format used by the terminal stations of the system;
FIG. 3 shows a frame format of the central station;
FIG. 4 is a flowchart showing programmed instructions executed by a
decision circuit of FIG. 1;
FIG. 5A shows details of the timeslot status table of FIG. 1, and FIG. 5B
showing a format of minislot status and slot assignment data;
FIG. 6 is a block diagram of the terminal stations of the system;
FIGS. 7A, 7B and 7C show details of the transmit slot table, reservation
table and assignment table, respectively, of FIG. 6, and FIG. 7D shows bit
combinations organized from the reservation and assignment tables to
determine the mode of transmission of each timeslot;
FIG. 8 is a message flow diagram useful for briefly describing the
operation of the system;
FIG. 9 is a flowchart showing programmed instructions executed by the slot
controller of FIG. 6 when the transmit slot table, reservation table and
assignment table are updated; and
FIG. 10 is a flowchart showing programmed instructions executed by the slot
controller when a packet is transmitted from a terminal station.
DETAILED DESCRIPTION
Referring now to FIG. 1, there is shown a TDMA (time-division multiple
access) satellite communications system according to the present
invention. The system comprises a central station 1 and a plurality of
remote terminal stations 2, shown at 2.sub.1, 2.sub.2 and 2.sub.3 for
simplicity. Each terminal station gains access to a host computer located
in the central station via a satellite 3 by transmitting a packet in a
randomly selected timeslot and, in response, the central station sends
back data obtained from the computer in a broadcast mode in a frame
sequence to the terminal station. When a collision occurs between packets
simultaneously transmitted from two terminal stations, the central station
proceeds to assign two timeslots and notifies the identifications of the
assigned timeslots to the sending stations to allow retransmission of the
corrupted packets. To permit the central station to detect such data
collisions, each terminal station defines "minislots" in a specified
portion of a randomly selected timeslot and a "dataslot" immediately
following the minislots, and randomly selects one of the minislots and
transmits a preamble and a unique word in the selected minislot followed
by the transmission of a packet in the dataslot in which a preamble and a
unique word are transmitted as well in a conventional manner.
As illustrated in FIG. 2, the packet format of each terminal station 2 is
such that each timeslot comprises a minislot interval 200 defining a
plurality of minislots and a dataslot 201. The number of minislots
contained in each timeslot is determined on a statistical analysis of the
traffic data of the system, and corresponds to a maximum number of
terminal stations likely to transmit packets simultaneously. In a typical
example, five minislots are indicated in FIG. 2. Each minislot is a burst
transmission and comprises a guard time 202, followed by a subfield 203
containing a preamble and a unique word subfield 204 signifying the
beginning of a burst transmission. Dataslot 201 is also a burst
transmission including a guard time 205, a preamble 206, a unique word 207
identical to that used in the minislot, a user data field 208 and a frame
check sequence 209.
As shown in FIG. 3, the frame format of the central station is such that
each frame comprises a frame sync that defines the start timing of the
frame, a control field and a plurality of data fields. The control field
comprises an ACK/NAK subfield 300 for transmitting a positive or negative
acknowledgment of a packet, a minislot data subfield 301 for indicating
the busy/idle status of minislots contained in a frame. The minislot
subfield 301 is followed by an assignment subfield 302 in which
identifiers of assigned timeslots are inserted. Other control data is
inserted in a subfield 303.
Returning to FIG. 1, the central station receives downlink signals from the
satellite and transmits an uplink signal through an antenna system 4
including a low-noise amplifier, a high power amplifier, and up-and
down-converters. The signal detected by the antenna is converted to an
intermediate frequency by the down-converter of the antenna system, and
further converted to a baseband frequency by a demodulator 5. The output
of demodulator 5 is coupled to a demultiplexer 6 in which the received
signal is decomposed by dataslot and minislot timing signals from a slot
timing circuit 19 and supplied to a dataslot unique word detector 7 and a
minislot unique word detector 8.
The unique word of the sending terminal station is detected by each of the
unique word detectors 7 and 8 if the packet from the station is properly
received by the central station, and the user packet data contained in the
data field 208 and the frame check sequence 209 are passed on to a CRC
(cyclic redundancy check) circuit 9 to detect and correct errors and
ascertain the validity of the contents of the user data. If the user data
is verified, the CRC circuit 9 produces an ACK signal, otherwise it
produces a NAK signal. The user data checked by CRC circuit 9 is applied
to a decision circuit 10 to which the output of UW detector 8 and the
error presence/absence data from CRC circuit 9 are also applied.
The user data stored in the receive buffer 11 is fed into a host computer
12 where it is processed and stored into a transmit buffer 13 where it is
kept until the central station is ready for transmission through a path
including a multiplexer 14 and a modulator 15 whose output si coupled to
the up-converter of the antenna system 4 for transmission to the
satellite.
A frame sync generator 18 is connected to the frame timing circuit 17 to
generate a frame synchronization bit sequence which is multiplexed by
multiplexer 14 to define the start timing of the frame.
If a burst transmission from a terminal station is properly received in the
minislot interval 200 of a timeslot, the unique word detector 8 produces
an output indicating that a minislot is used by a terminal station. If
more than one burst is detected in the minislot interval 200, UW detector
8 will produce two output signals in response thereto. If the same
minislot is used by two terminal stations, collision is said to occur and
the unique words contained in such minislot are corrupted and made to
appear as noise. Under such circumstances, the unique word detector 8
produces an output indicating the absence of a burst transmission in any
of the minislots of a timeslot.
Decision circuit 10 monitors the output of UW detector 8 to check to see if
it has detected more than one unique word from the minislots of each
timeslot. If this is the case, decision circuit 10 recognizes that a data
collision has occurred, makes an unfavorable decision and discards the
data that follows the minislots and supplies a signal indicating the
number of minislots detected in a timeslot and the busy/idle status of the
minislots of the timeslots to a slot assignment circuit 21. If only one
unique word is detected in a timeslot, decision circuit 10 determines that
a packet is properly received and allows the data part of the packet to be
stored into the receive buffer 11. As illustrated in FIG. 4, decision
circuit 10 checks the output of minislot UW detector 8 and determines the
number of detected minislots detected during a timeslot interval (step
40). If the number of detected minislots is 0, a NAK signal is generated
(step 41). If the number of detected minislots is equal to 1, the result
of error check by CRC circuit 9 is examined (step 42). If a data error is
present, a NAK signal is generated (step 41) and if no data error exists,
an ACK signal is generated (step 43). If the number of detected minislots
is equal to or greater than 2, then a NAK signal is generated (step 44)
and the number of timeslots required for retransmission is informed to the
slot assignment circuit 21.
The ACK/NAK data from decision circuit 10 is stored into a buffer 16 in a
location determined by an address pointer from a timeslot counter 20 using
a dataslot timing signal DS from timing circuit 19. ACK/NAK data for a
given frame is then output from buffer 16 in response to a timing signal
supplied from a frame timing circuit 17, and multiplexed into the ACK/NAK
subfield 300 of the frame and transmitted with the user data from transmit
buffer 13.
In response to the signal indicating the number of minislots being used,
slot assignment circuit 21 proceeds to assign as many timeslots as there
are minislots detected by decision circuit 10 by using data stored in a
timeslot status table 22. System simplification is achieved by assigning
timeslots to contending terminal stations on an en bloc basis, rather than
specifying one-to-one correspondence between the assigned timeslots and
the contending terminal stations.
As shown in FIG. 5A, timeslot status table 22 defines a map indicating
busy/idle status of each time slot for a series of frames #1 through #N.
Using the busy/idle data, slot assignment circuit 21 detects idle
timeslots and selects as many timeslots from the detected idle timeslots
as there are required timeslots informed by decision circuit 10 and
generates, for each frame, minislot status data indicating the busy/idle
status of all minislots contained in that frame and slot assignment data
containing frame and timeslot numbers of timeslots (FIG. 5B) assigned in
one lump (on an en bloc basis) to user stations.
The data generated by slot assignment circuit 21 is stored into a buffer 23
in a location determined by an address pointer supplied from slot counter
20 and is output to multiplexer 14 in response to a frame timing signal
from timing circuit 17 and multiplexed into the minislot data subfield 301
and timeslot assignment subfield 302 of an appropriate frame.
FIG. 6 is a block diagram of each terminal station. A downlink signal from
the satellite is received by an antenna system 50 identical to the central
station, the intermediate frequency version of the received signal being
applied to a demodulator 51 for baseband frequency conversion. The
baseband signal is applied to a frame sync detector 59 whose output is
coupled to a slot timing circuit 60 to produce dataslot and minislot
timing signals DS and MS. The output of demodulator 51 is also applied to
a demultiplexer 52 which responds to the timing signals DS and MS to
decompose each of the received frames into the control field and the data
fields. User data contained in a data field addressed to the terminal
station is applied to a receive buffer 53 where it is held until it is
called upon by a user data terminal 54. ACK/NAK data indicating the result
of transmission of a previous packet from the terminal station is applied
to a slot controller 55, and the minislot data and assigned timeslot ID
data of the frame are supplied to slot controller 55. As will be
described, slot controller 55 is associated with transmit slot table 56, a
timeslot reservation table 57 and a timeslot assignment table 58 to
control retransmission of a packet which was previously transmitted and
corrupted by collision with other packets.
User data supplied from data terminal 54 is packetized through a packet
assembler 63 and fed into a transmit buffer 64. Slot controller 55
determines the identifier of a timeslot on which the packet is sent and
stores it into a transmit slot table 56 by setting a 1 into a
corresponding timeslot position. For each transmission of a packet from
the transmit buffer 64, a copy of the transmitted packet is read out of
buffer 64 into a retransmit buffer 65. Slot controller 55 checks the
ACK/NAK data from the central station with a corresponding timeslot
identifier to determine the identify of the ACK/NAK data and discards a
corresponding packet from the retransmit buffer 65 if an ACK signal is
received. If a NAK is received, slot controller 55 causes a corresponding
packet to be transmitted from the retransmit buffer 65.
The packet from either of the buffers 64 and 65 is input to a multiplexer
66 where it is combined with outputs of a preamble generator 67 and a
unique word generator 68 which are respectively activated at appropriate
minislot and dataslot transmit timing under the control of slot controller
55 using timing signals from slot timing circuit 60. The multiplexed
signal is output through a modulator 69 to the antenna system 50 for
transmission to the satellite.
Details of the transmit slot table 56, reservation table 57 and assignment
table 58 are shown in FIGS. 7A, 7B and 7C, respectively. These tables are
constantly updated by slot controller 55. Transmit slot table 56 stores a
list of busy/idle status of each minislot (#1 through #5) of all timeslots
(#1 through #K) which are contained in frames #i and #(i+1). A binary 1 is
set into a minislot entry if the corresponding minislot is used by the
terminal station. Reservation table 57 stores a map indicating the
busy/idle status of each timeslot which is used in any of the terminal
stations for frames #1 through #N. Assignment table 58 indicates the
busy/idle status of each timeslot which is assigned only to the home
terminal station by which the table 58 is owned, and the list is
maintained for frames #1 through #N.
Before going into details of the functions and operations of slot
controller 55, the overall operation of the system will be given below
with reference to a message flow diagram shown in FIG. 8. Assume that
terminal stations 2.sub.1 and 2.sub.2 transmitted packets A.sub.1 and
B.sub.1 during timeslot #1 of frame #1, using minislots #1 and #3,
respectively. As they collide at the central station, the latter sends
back data containing a NAK, minislot data and slot assignment data
assigning #1 and #3 timeslots of frame #3 to terminal stations 2.sub.1 and
2.sub.2, respectively. The response signal from the central station
reaches the terminal stations during frame #2. Upon receipt of each
response, terminal station 2.sub.1 retransmits the packet as A'.sub.1 on
timeslot #1 using minislot #2, and terminal station 2.sub.2 retransmits
the packet as B'.sub.1 on timeslot #3 using minislot #4. If there is a
packet C.sub.1 to transmit from terminal station 2.sub.3 during timeslot
#2 of frame # 3, it looks up timeslot reservation table 57 to recognize
that timeslot #1 of the frame has been reserved for another station and
timeslot #2 can be used for random access, and waits until timeslot #2 to
transmit the packet C.sub.1 using minislot #3.
FIGS. 9 and 10 are flowcharts describing programmed instructions which are
executed by the slot controller 55. In FIG. 9, during a given frame
interval "i", the program execution begins with step 80 in which variable
S.sub.N, that indicates the identifier of a timeslot, is set equal to 1.
Exit then is to step 81 to check to see if a packet was sent on time slot
S.sub.N of the (i-1)th frame by examining the transmit slot table 56. If
the answer is affirmative, control branches at step 81 to step 82 to check
to see if there is an ACK or NAK response from the central station
regarding the transmitted packet. If the response is a positive
acknowledgment, control branches at step 82 to step 88 to increment
variable S.sub.N by one and returns to the starting point of the program
if the program execution is does not reach the end of the "1th" frame
(step 89). If the response from the central station is a negative
acknowledgment, control branches at step 82 to step 83 to access transmit
slot table 56 and read the minislot identifier which was previously used
by the terminal station. Control proceeds to step 84 to compare the
minislot identifier with received minislot data. Since the minislot data
indicates the busy/idle status of all minislots of the timeslot on which
the previous packet was sent, the terminal station determines the position
of the minislot previously used relative to the positions of other
minislots which are used by other, contending terminal stations. Using the
position of the minislot so determined, one of timeslots, which are
assigned to all the contending stations but not respectively identified
for one-to-one relationship, is now identified as one to be used for
retransmission.
The result of the timeslot determination is checked (step 85). If timeslots
are assigned and one of them is successively identified, control branches
at step 85 to step 86 to set a 1 into reservation and assignment tables 57
and 58 in positions corresponding to the timeslot successfully identified
by step 84. If the timeslot determination fails or after the execution of
step 86, control exits to step 87 to set a 1 into reservation table 57 in
positions corresponding to the timeslots assigned to the other terminal
stations.
If no packet was sent on timeslot S.sub.N of the (i-1)th frame, control
branches at step 81 to step 90 to check to see if received timeslot
assignment data indicates timeslots assigned to other terminal stations.
If the answer is affirmative, control branches to step 87 to set a 1 into
corresponding positions of the reservation table 57. Otherwise, control
branches to step 88. The process is repeated until all timeslots are
examined.
In FIG. 10, the program execution starts with decision step 100. If
appropriate transmit timing is reached for a given timeslot, control
branches to step 101 to determine the mode of transmission of a packet
during the given timeslot by examining the reservation and assignment
tables 57 and 58. Specifically, the busy/idle status bits of the current
frame are read out of corresponding frame entries of both tables 57 and 58
and organized column-by-column as shown in FIG. 7D. Then, a search is made
along the column entry of the current timeslot for a bit combination "11",
"00" or "10". The bit combination "11" indicates that the current timeslot
#j is one that is assigned to the home terminal station, and "00"
indicates that the current slot #(j+1) can be used for random packet
transmission, and "10" indicates that the current slot #(j+2) is one
assigned to another terminal station.
If the current timeslot is determined to be the one that can be used for
random transmission, control branches at step 101 to step 102 to determine
a count indicating the number of retransmit packets to which timeslots are
not assigned. Such unassigned retransmit packets occur when the terminal
station receives a NAK under condition that the central station detected
no burst transmission in any of the minislots (see block 41, FIG. 4).
If the retransmit packet count is equal to or greater than 1, control
branches to step 103 to read the count of a retransmit interval counter
RIC. If the RIC count is equal to 1, then a packet is output from
retransmit buffer 65 (step 104) and the RIC counter is decremented by one
(step 105) and the unassigned retransmit packet count is decremented (step
106). The number of retransmit packet is then determined (step 107) and if
it is equal to 0, control branches to step 113. If it is equal to or
greater than 1, control branches to step 108 to reset the RIC counter to a
random number equal to or greater than 1, and control proceeds to step
113.
If the RIC count is determined to be equal to or greater than 2, it is
decremented by one (step 109) and control proceeds to the end of the
program. If the RIC count is determined to equal to 0, it is reset to a
random number equal to or greater than 1 (step 110), and control proceeds
to the end of the program.
If the number of retransmit packets is determined to be equal to 0, control
branches at step 102 to step 111 to output a new packet from the transmit
buffer 64, and control proceeds to step 113 to generate a random number
and select a minislot corresponding to the random number. Exit then is to
step 115 to successively activate preamble generator 67 and UW generator
68 and transmit a preamble followed by a unique word on the selected
minislot of the current timeslot. Following the transmission of the
minislot, preamble generator 67 and UW generator 68 are activated again to
transmit a preamble and a unique word respectively in the subfields 206
and 207 of the subsequent dataslot of the current timeslot (step 116), and
control goes to the end of the program.
If the current timeslot is determined to be a slot for retransmission,
control branches at step 101 to step 112 to output a packet from the
retransmit buffer 65 and steps 113 to 116 are successively executed.
If the current timeslot is determined to be the one assigned to other
terminal station, control branches at step 101 to the end of the program.
* * * * *
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