JPS6322745B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an access method for efficiently communicating with a plurality of earth stations by sharing a communication satellite.

Figure 1 shows the configuration of the satellite communication system. In Figure 1, 1 is a communication satellite, 2a, 2b, 2c
indicates earth stations A, B, and C. Earth station A,
2a, B, 2b, C, and 2c use the same satellite line to divide data into units called packets of a certain length or less, add destination information to these, and transmit them over radio waves. The radio wave travels to the communication satellite 1 as a continuous burst of one packet, and the communication satellite 1 receives this burst, converts and amplifies the frequency, and sends it back to the ground. Earth station A, 2a, B, 2b, C, 2
C receives the burst, and according to the destination information, it takes in the burst addressed to itself and discards the burst addressed to other stations. In this way, communication is performed between the earth stations via the communication satellite 1, but when multiple earth stations transmit bursts at arbitrary timings, collisions between bursts occur, and the earth stations are not correctly It becomes impossible to receive data. Therefore, as one of the access methods for preventing such burst collisions, a reservation method, which will be described below, is conventionally used.

Figure 2 shows the operation of the conventional reservation system.
3 is the time of transmission from the earth station, 4 is the time of reception from the earth station,
5 is a cycle, 6 is a data area, 7 is a reserved area,
8 is a data slot, 9 is a reservation slot, 10 is a small slot, 11 is a round trip propagation delay time, 12 is a reference burst, and 13, 14, and 15 are reservations for earth stations A, 2a, B, 2b, C, and 2c, respectively. , 16, 17, 18, and 19 are packets a1, a2, a3, a4, and 2 of earth stations A and 2a, respectively.
0, 21, 22, 23 are earth stations B and 2b, respectively.
packets b1, b2, b3, b4, 24, 2
5 and 26 are packets c from earth stations C and 2c, respectively.
1, c2, and c3 are shown.

The satellite line is used on the communication satellite 1 by dividing the time into units called slots that are large enough to accommodate bursts, and each earth station transmits bursts so that they fit into these slots.

In the reservation method, a fixed number of consecutive slots are collected to form a cycle 5, and each cycle 5 is divided into a data area 6 and a reserved area 7 for use. Reference bursts are used to synchronize this cycle 5 globally, with a particular station transmitting a reference burst 12 at the beginning of each cycle 5, which is received by other earth stations and synchronized in cycle 5. Take your timing.

In the following explanation, the slot in the data area 6 will be referred to as a data slot 8, and the slot in the reserved area 7 will be referred to as a reserved slot 9. The reserved slot 9 is divided into shorter small slots 10 for use. Figure 2 shows a case in which one reserved slot is divided into two small slots 10, and each small slot 10 in the reserved area 7 within one cycle 5 is fixedly assigned to all earth stations in the system. It is being Each earth station uses this small slot 10 to transmit reservations.

Communication is performed by transmitting bursts containing packets using data slot 8 in cycle 5, but in the reservation system, data slot 8 cannot be used unless it is reserved. In FIG. 2, at the beginning of the second cycle, earth stations A, 2a, B, 2b,
If earth stations C and 2c each have three packets to transmit, in the reservation area 7 of the first cycle, these earth stations transmit reservations 13, 14, and 15 for three slots, respectively.

The reservation is received via the communication satellite 1, arriving at each earth station with a delay of round trip propagation delay time 11. Here, all reservations sent in the 1st cycle are received before the (+1)th cycle begins. Data slot 8 in the (+1)th cycle is allocated and used by each earth station according to the reservation received in the first cycle. This slot allocation is done independently by each earth station, but if all earth stations receive the same reservation and allocate according to the same algorithm, burst collisions and wasted slots due to allocation mismatches between earth stations can be avoided. Nothing will happen. This slot allocation algorithm must allocate equally to each earth station, but its details are not directly related to the present invention and will not be described here. In the (+1)th cycle, earth station A, 2a is assigned three data slots 8, and receives packets a1, 16, a2, 17,
A3,18 is being sent. Also earth station B, 2b
and earth stations C and 2c each have 2 data slots 8
b1, 20, b2, 2 respectively.
1, as well as c1, 24, c2, and 25.

Here, all earth stations cannot transmit all packets in the (+1)th cycle. This is known at the time of slot allocation processing performed at the start of the (+1)th cycle.

Also, in the second cycle, earth stations A, 2a, B, 2b
In each case, one new packet is generated to be transmitted. Therefore, earth stations A, 2a, B,
2b transmits reservations 13 and 14 for packets that were not sent in the first cycle and newly generated packets in the (+2)th cycle, respectively;
The earth stations C and 2c transmit reservations only for the packets that could not be sent in the first cycle.

Due to these reservations, the remaining packets b3, 22, c3, 26, a4,
19, b4, and 23 are transmitted.

In the conventional reservation method described above, each station performs slot allocation processing independently on the premise that all stations receive reservations equally. However, if there are many errors in the satellite link, each station may receive reservations incorrectly. This occurs independently, and the pattern of destination stations receiving reservations is no longer the same between stations. As a result, when the quality of the satellite link deteriorates with the conventional reservation method,
This had the disadvantage that slot assignments were inconsistent between stations, resulting in collisions of transmitted packets, resulting in a sudden drop in performance. This drawback was even more pronounced when the number of earth stations was large.

In order to eliminate the drawbacks of the conventional reservation method, the present invention performs slot assignment processing in a distributed manner at each earth station, but regarding the information to be assigned, each station uses reservations directly received, or Alternatively, one control station switches whether or not to use the accumulated reservation information depending on the status of the satellite link. The object of the present invention is to realize a reservation system that does not cause inconsistency in slot allocation between stations and packet collisions due to the slot allocation by performing slot allocation.

FIG. 3 shows an example of operation when the satellite line of the reservation method according to the present invention is in good condition, and 27 is reservation information.

In the reservation system according to the present invention, one control station is provided among all earth stations. Here, if the earth station with the largest antenna scale is used as the control station, the overall reliability will be improved. When the control station receives reservations, it accumulates them for each earth station. When slot allocation processing is performed, the number of slots allocated to each station is subtracted from the accumulated reservations to obtain the number of unallocated reservations for that cycle.

Furthermore, the control station monitors the overall slot allocation status. This is done by checking whether transmission is being performed according to the slot assignment made by the control station. That is, it is monitored whether the assignment of the control station matches the transmitting station of the actually received packet in a certain slot, or whether the packet is actually received. If a certain slot is assigned to a certain earth station by the control station, but the packet is not actually received at that slot, there is a possibility that the packet was not received due to a reception error, but the control station Determine that the packet was lost due to a collision. In this way, if transmissions with different slot assignments or collisions occur more than a predetermined number of times within a certain period of time, the control station determines that the slot assignment is in bad condition and decides that it is okay if the slot assignments do not occur more than the predetermined number of times. to decide. Based on this information, the control station transmits reservation information 27 once per cycle. In Figure 3, the control station receives 3 times in one cycle.
The slot 1 is fixedly assigned, and the reservation information 27 is transmitted through this slot. This reservation information 27 consists of the number of unallocated reservations and a flag, and the number of unallocated reservations includes reservations for each earth station that could not be sent out in that cycle. The flag indicates the status of slot allocation, and in this description, it is assumed to be 0 when the status is good and 1 when the status is bad. If the flag in the received reservation information 27 is 0, each earth station allocates slots from the directly received reservations and the number of unallocated reservations in the reservation information 27, and if the flag is 1, it allocates slots only from the number of unallocated reservations. conduct. FIG. 4 shows, for each cycle in the case of FIG. 3, the relationship between the reservations 13, 14, and 15 of each earth station transmitted in that cycle, the reservation information 27, and the number of allocated slots. In Figure 3, just before the start of cycle 5, A, 2a, B, 2b, C,
2c There are three packets to be transmitted to each earth station. Therefore, in the first cycle, each earth station transmits three packets of reservations 13, 14, and 15. These reservations 13, 14, 15 are received before the (+1)th cycle begins. Since the flag is 0 in the reservation information 27 of the 2nd cycle, each station receives directly received reservations 13, 14,
15 and the number of unallocated slots in the reservation information 27 for each earth station, slot allocation for the (+1)th cycle is performed. In this case, since the number of unallocated slots is 0 for all stations, slot allocation is essentially determined only from reservations 13, 14, and 15, and earth stations A, 2a, B, 2b, C, and 2c are assigned slots a1 and 2c, respectively. 16, a2, 17, b
1, 20, b2, 21, c1, 24, c2, 25
2 packets each are sent. At this point, each earth station has one packet that cannot be sent, but the control station transmits this as the number of unallocated reservations in the reservation information 27 in the (+1)th cycle, and each earth station re-reserves this shortfall. do not have to. Third
In the figure, before the start of the (+1)th cycle, A, 2
Since one packet is required to be transmitted to each earth station a, B, and 2b, one packet worth of reservations 13 and 14 is transmitted in the (+1)th cycle. Since the flag of the reservation information 27 is 0 in the (+2)th cycle as well, each earth station adds the reservations of all the stations directly received and the number of unallocated reservations of the reservation information 27 from the control station for each earth station, and uses the result as follows. Slot allocation is performed based on the As a result, the remaining packets a3, 18, b3, 22, 3, 26, a
4, 19, b4, 23 are transmitted.

As described above, when the overall satellite line is in good condition, slot allocation is mainly done based on reservations 13, 14, and 15 received directly by each earth station. packets can be sent. This is the same delay time as the conventional reservation method. On the other hand, if the condition of the satellite link deteriorates with this access control method, an error will occur in the reservation for direct reception, which will cause slot assignments to become inconsistent between earth stations. The control station monitors the resulting packet collision using the method described above, and when the slot allocation condition deteriorates, it sets the flag in the reservation information 27 to 1 and transmits it.

Fig. 5 shows an example of the operation of the reservation method according to the present invention when the satellite line is in poor condition, and Fig. 6 shows the reservations 13, 14, and 15 of each earth station transmitted in that cycle, and the reservations for each cycle in this case. It shows the relationship between the information 27 and the number of allocated slots.

As in the case of FIG. 3, each earth station transmits three packets of reservations 13, 14, and 15 in the third cycle. Further, the control station sets the flag of the reservation information 27 to 1 and transmits it. Since the flag of the reservation information 27 received by each earth station in the (+1)th cycle is 1,
Slot allocation is performed only from the number of unallocated reservations in the reservation information 27. In Figure 6, the number of unassigned reservations transmitted in the cycle is 0, so the slot is not assigned to any earth station in the (+1) cycle.
No packets are sent. In the (+1)th cycle, the control station makes reservation 1 for each earth station in the 1st cycle.
Accumulate 3, 14, 15. Also, since no slot was assigned to any earth station in this cycle, the received reservations 13, 14, and 15 are transmitted as they are as the number of unallocated reservations in the reservation information 27.

Also, since one new packet has been generated at the earth stations A, 2a, B, and 2b, they each transmit one packet worth of reservations 13 and 14. In the next (+2)th cycle, each earth station allocates slots based on the number of unallocated slots in the reservation information 27 received in the (+1)th cycle. As a result, each earth station secures two slots, and receives packets a1, 16, and b.
1,20, c1,24, a2,17, b2,2
1, c2, and 25 are transmitted. The control station stores the reservations that cannot be sent in the (+2) cycle, and adds the newly received reservations 13 and 14 in the (+1) cycle to this to create the number of unallocated reservations. (+2) In cycle 5, it is included in the reservation information 27 and transmitted. As a result, earth stations A, 2
A, B, 2b, C, and 2c secure 2 slots, 2 slots, and 1 slot, respectively, in the (+3)th cycle, and packets a3, 18, b3, 22, and c
3, 26, a4, 19, b4, 23 are transmitted.

As described above, in the reservation method according to the present invention, when the slot allocation condition deteriorates, the reservation information 2 of the control station is
Since slot allocation is performed only from the unallocated reservation number of 7, even if the satellite link condition deteriorates, there will be no discrepancy in the information of each station targeted for slot allocation, and packets will not collide. sent to. However, in this case, even if the satellite line is vacant, at least two cycles are required from the time the reservation is made until the packet is transmitted, resulting in a large delay. However, since packets do not collide, there is no large drop in performance as in the conventional reservation system.

In addition, in the above, the size of one cycle 5 is 9 slots, the size of the reserved area is 2 slots, one reserved slot 9 is divided into two small slots 10, and the reservation information 27 is set to the third slot in one cycle 5. Although the case of transmission has been described, the present invention is not limited to this, and may be used in cycles 5 and reserved areas 7 of other sizes. 1 more reserved slot 9
can also be divided into an arbitrary number of 10 small slots.
The reservation information 27 may also be transmitted at other slot positions. Further, in the explanation, the reserved area 7 has been described as being fixed, but the present invention is not limited to this, and may be used in a system in which the reserved area 7 is made variable using a vacant slot within one cycle 5.

As described above, in the reservation method according to the present invention, when the quality of the satellite link deteriorates, slot allocation is determined only by information from the control station, so packet collisions do not occur due to reservation discrepancies between earth stations, and This has the effect of providing more reliable communication than the previous reservation method.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a satellite communication system, Fig. 2 is a diagram showing the operation of the conventional reservation method, and Fig. 3 is a diagram showing an embodiment of the reservation method according to the present invention when the satellite line is in good condition. , FIG. 4 is a diagram showing the relationship between reservations, reservation information, and the number of allocated slots when the satellite line using the reservation method according to the present invention is in good condition, and FIG. FIG. 6 is a diagram illustrating the relationship between reservation, reservation information, and the number of allocated slots when the condition of the satellite line of the reservation method according to the present invention is bad. In the figure, 1 is a communication satellite; 2a, 2b, and 2c are earth stations A, B, and C, respectively; 3 is a transmission time point; 4 is a reception time point; 5 is a cycle; 6 is a data area; 7 is a reserved area; and 8 is a data slot. , 9 is the reservation slot,
10 is a small slot, 11 is a round trip propagation delay time, 12 is a reference burst, 13, 14, 1
5 are earth stations A, 2a, B, 2b, C, 2, respectively.
c reservation, 16, 17, 18, 19 are earth station A,
2a packets a1, a2, a3, a4, 20,
21, 22, 23 are packets b of earth station B, 2b
1, b2, b3, b4, 24, 25, and 26 are earth station C, and packets c1, c2, c3, and 27 of 2c are reservation information. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1 A single communication line is time-divided into slots of a certain length of time, a cycle is made up of a certain number of consecutive slots, the slots within one cycle are divided into reserved slots and data slots, and the communication station uses the reserved slots to control the data slots. A reservation requesting allocation is transmitted, all communication stations receive each other's reservations, and each communication station independently allocates data slots using the same method, and each communication station assigns data slots to its own station in units called packets. In a reservation method that transmits information divided into 2 segments in a fixed time cycle, a control station is provided for one of the communication stations, and this control station makes reservations for each communication station that was not sent in the previous cycle. A stage in which the reservation information is accumulated as reservation information, a stage in which the control station periodically notifies other communication stations of this reservation information, and a stage in which the number of data slot assignments made by the control station and the actual reception by each communication station in the cycle immediately after the assignment are The transmission status of each communication station is monitored by checking whether the number of data slots matches the number of data slots, and if they do not match, the control station determines the reservation from each communication station and the control station's reservation in the next cycle. There is an assignment stage in which data slots are assigned in the next cycle based on both of the reservation information up to now, and an assignment stage in which data slots in the next cycle are assigned only based on the reservation information of the control station if they match. a step in which the communication station receives the reservation information from the control station and transmits information instructing all communication stations which allocation step to use; and a step in which the communication station receives the reservation information from the control station and 1. A reservation method comprising a step of allocating data slots in the next cycle at the designated allocation step among the three allocation steps.