JP2995986B2 - Packet communication system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a packet communication system, and more particularly to a slotted Aloha type packet communication system in which three or more terminal stations share one communication channel for data communication.

[0002]

2. Description of the Related Art Packet communication and non-packet communication are conceivable as means for performing data communication by sharing a single communication channel among a large number of terminal stations when focusing on a data transmission mode.

In packet communication, control information such as destination, data attribute, data length, and serial number is added to data to be transmitted, and the data is transmitted as a set of burst data. In addition, each terminal station can transmit data at an arbitrary time or on a time slot in which a synchronized system clock is used as a minimum unit. in this case,
Access the communication channel only when there is a communication request.

On the other hand, non-packet communication has a finite number of communication channels, and an empty channel is allocated by control of the system only when a communication request is made from a terminal station. Therefore,
The terminal occupies the communication channel from the time when the communication request of the terminal is accepted to the time when the communication disconnection request is issued by the terminal.

[0005] In addition, many wireless systems using the packet communication system are applied to satellite communications, and the first to be put into practical use is a computer network called the Aloha System of the University of Hawaii, USA. The access protocol used for packet communication has been well studied in the last 15 years and many schemes have been proposed. Basically, Aloha system and CSMA (Carrier Sen)
se Multiple Access) system. Hereinafter, the slotted motor applied in this embodiment will be described.
The Aloha method will be described.

[0006] In the slotted Aloha system, each terminal station can transmit a packet only at the beginning of a predetermined time interval. As a result, packet collision occurs only when a plurality of terminal stations simultaneously transmit packets. Therefore, although the average transmission delay time of the packet is slightly longer than that of the pure Aloha system in which the packet can be transmitted at completely free time, the probability of collision of the packet is reduced and the channel capacity is doubled.

When each terminal station transmits a packet with a different signal strength, when a packet collision occurs, the packet having the stronger signal strength is correctly received. In this case, since there is only one retransmission packet caused by packet collision, an increase in channel capacity can be expected. Specifically, there is a method in which each terminal station randomly selects the signal strength of the transmission packet, a method in which the signal strength is changed according to the importance of the packet, and the like.

[0008]

The above-mentioned conventional packet communication system using the slotted Aloha system has the following problems.

FIG. 5 shows a wireless network composed of five terminal stations. Each terminal station can communicate with an arbitrary terminal station. For the sake of simplicity, it is assumed that the terminal stations are arranged at equal intervals on a straight line and transmit radio waves with equal transmission power.

Now, it is assumed that the terminal station 21 and the terminal station 24 have transmitted respective data to the terminal station 22 at the same time. Terminal 21
If the distance between the terminal stations 24 and 22 is 1, the distance between the terminal stations 24 and 22 is 2. Therefore, the reception input electric field at the terminal station 22 is higher at the terminal station 21 than at the terminal station 24. If each terminal station transmits a radio wave by modulation with a constant amplitude (for example, FSK modulation), the data of the terminal station 21 is correctly received even though both data of the terminal stations 21 and 24 collide. Probability is high. Such a phenomenon is called a power capture effect, and many studies have been made.

Although this power capture effect has a favorable result in terms of channel throughput, when applied to an actual system, there arises a problem that the throughput of each terminal station is not uniform due to the geographical conditions of each terminal station. .

For example, the packet generation rates (traffic of only new packets but not traffic of retransmitted packets) in the idle state (state in which the terminal station does not have a packet to wait for retransmission) of each terminal station are all the same,
Further, assuming that the new packet generation rates between the terminal stations are also the same, there is an unfair situation that the throughput of the terminal station 23 is maximum and the throughput of the terminal stations 21 and 25 at both ends is minimum.

This phenomenon will be further described as follows. If each terminal station has a communication request at an equal fixed rate to other terminal stations, when this communication request is issued,
If the communication terminal has an unprocessed retransmission waiting packet, this communication request is rejected, and if not, it is immediately sent out on the communication channel using the next time slot. At this time, if another terminal station is transmitting a packet at the same time, a packet collision occurs at the receiving terminal station, and the packet of the transmitting terminal station closest to the receiving terminal station is correctly received. The receiving terminal returns a reception response to the nearest transmitting terminal, and upon receiving this, the transmitting terminal recognizes that its own packet has been correctly received and waits for the next communication request. Return to the state.

On the other hand, the other transmitting end stations, after transmitting their own packets, know that their packet transmission has failed when a predetermined waiting time determined by the propagation delay and the hardware delay has elapsed. Initiate the retransmission procedure. Various methods have been proposed for this retransmission procedure. For example, as the simplest method, a terminal that has recognized retransmission of a packet,
A retransmission packet is transmitted with a probability of 1 / K to any one of the maximum K time slots counted from that time. Thereafter, the retransmission procedure is repeated until the own packet is correctly received by the other party.

Here, conventionally, the same value of the maximum retransmission interval K (time slot) has been assigned to each terminal station.
In general, if the value of K is gradually increased, the time (packet delay time) required from the transmission of a packet to the final correct reception increases, and theoretically when the value of K is infinite. This matches the case where retransmission is not performed.

When packet transmission is controlled by such a communication procedure, the following can be said with respect to the throughput, traffic, and packet delay time of each terminal station shown in FIG. In other words, the terminal station 23 located at the center of the network has the highest probability of successful transmission and can send new packets one after another. Therefore, the time ratio in which the communication channel is effectively used, that is, the throughput is the highest. high. Furthermore, since the average number of packet retransmissions is the smallest, the average delay time required for transmitting one packet is also the smallest. On the other hand, terminal stations 21 and 2 located at both ends of the network
Conversely, No. 5 has the lowest probability of successful packet transmission and has to retransmit the packet many times before successful transmission, so that the throughput is low and the average packet delay time is large. Also, the terminal stations 22 and 24 located in the middle take an intermediate value between these.

As described above, the conventional packet communication system has a problem that the throughput and the packet transmission delay of each terminal station are not uniform due to the power capture effect, and equal communication quality cannot be given to each terminal station. There is.

An object of the present invention is to provide a packet communication system capable of giving each terminal station equal communication quality.

[0019]

A packet communication system according to the present invention is a slotted Aloha type packet communication system in which three or more terminal stations share one communication channel to perform data communication. end office after sending a packet signal, the maximum retransmission interval K (K received response is not obtained if, assigned before Symbol advance for each terminal station from the remote end station within a predetermined time positive integer ), A uniform random number from “1” to “K” is generated, and the retransmission delay is determined by this value.
Means for determining the delay time and retransmitting the packet signal
The maximum retransmission interval K is assigned to each of the terminal stations based on the relative positional relationship between the terminal stations so that the throughput and the packet transmission delay of the terminal stations become equal. .

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention, and shows a case of a wireless communication network composed of five terminal stations 1 to 5 capable of bidirectional communication with the same configuration.

Each of the terminal stations 1 to 5 is provided with input / output devices 14 to 54 for input / output processing of user data and, after converting user data to be transmitted into burst data, adding information necessary for communication control. Packet control devices 12 to 52 for converting a received packet signal into user data, converting the packet signal into a radio frequency signal and transmitting the radio signal to the antenna, and converting the radio frequency signal received by the antenna into Transmission / reception device 1 for demodulating and outputting a packet signal
1 to 51, and packet retransmission control devices 13 to 53 for controlling retransmission of a packet whose transmission has failed when the transmitted packet is not received by the partner terminal station.

Here, the operation will be described by taking a case where the terminal station 1 and the terminal station 3 communicate with each other as an example.

Now, when the terminal station 1 transmits data to the terminal station 3, the terminal stations 2 or 4 close to the terminal station 3 do not transmit data at the same time, and the terminal station 3 itself transmits data. If not, the data of the terminal 1 is correctly received by the terminal 3, converted into user data by the packet control device 32 of the terminal 3, and transmitted to the input / output device 34. In this case, that is, when the data transmission from the terminal station 1 to the terminal station 3 is successful, the packet control device 32 sends a reception response to the terminal station 1 indicating that the data has been correctly received. On the other hand, after transmitting the packet, the terminal station 1 starts a timer in the packet control device 12, and if a reception response from the terminal station 3 is received within a predetermined waiting time, it is determined that the data transmission has been successful, and again The process returns to a state of waiting for input of user data from the input / output device 14.

If a reception response cannot be received within a predetermined waiting time (for example, because the terminal station 2 or 4 has transmitted radio waves simultaneously with the terminal station 1 or because the terminal station 3 itself has transmitted radio waves). In this case, when a packet collision occurs), the packet retransmission control device 13 of the terminal station 1 sets “1” in accordance with the maximum retransmission interval K (K is a positive integer) assigned in advance to each terminal station. Then, a uniform random number from “K” to “K” is generated, and the time for retransmitting the packet is determined based on the value. For example, if the random number is "5", the packet is transmitted again five time slots after counting from the time slot at that time, and thereafter, this operation is repeated until a reception response is received from the terminal station 3 within the waiting time.

A feature of the present invention is that a maximum retransmission interval Ki (i = 1, 2, 3, 3) assigned in advance to each terminal station is provided.
In this case, a uniform random number is generated according to (4, 5), and the time for retransmitting the packet is determined. Conventionally, the same maximum value of the uniform random number has been assigned to each terminal station. Hereinafter, the method of determining Ki will be described using a mathematical model proposed by the present invention. Now, pi: the probability that the terminal station i will retransmit the packet.

Qi: Probability that terminal station i has a retransmission packet.

Σi: Probability that a new packet is generated when the terminal station i has no retransmission packet.

Ti: Average round-trip propagation delay time from terminal station i sending a packet to receiving a reception response from the partner terminal station.

Ki: The maximum time slot interval until retransmission after packet transmission of terminal station i fails.

Si: Throughput of terminal station i.

Gi: Traffic of terminal station i. Then, pi = 1 / {Ti + (Ki + 1) / 2} (1) gi = (1-qi) σi + qipi (2)
It is. By the way, the terminal station i has to select any one of the Ki time slots after the packet transmission has failed.
Is assumed to be uniform. Therefore, considering Ti, terminal station i is represented by Ti + 1, Ti + 2,.
The packet is retransmitted in any one of the i + Ki slots. An object of the present invention is to determine the value of Ki so that the throughput and the average transmission delay time of each terminal station are all equal. Here, the average transmission delay time Di of the terminal station i is defined as follows. Di = τi [1+ ｛Ti + (Ki + 1) / 2｝ gi / si- (Ki + 1) / 2] (3) where τi represents the packet length of terminal station i.
In this embodiment, for the sake of simplicity of analysis, it is assumed that τi is constant for all terminal stations, and its value is one time slot. Therefore, assuming that τi = 1, equation (3) becomes: Di = 1 + {Ti + (Ki + 1) / 2} gi / si− (Ki + 1) / 2 (4)

Further, the equations (1) and (2) are converted into the equations (4)
Substituting into: Di = 1 + Ti + (gi / si-1) / pi
(5)

Next, consider a network having M terminal stations. Each terminal station is in one of the following three states. Idle state 1: State in which there is no new packet or retransmission packet. Idle state 2: the terminal station has a new packet with a probability σi,
The state that sends it. Backlog state: A state in which a packet collision has occurred in the past and is waiting for retransmission.

It is assumed that the transmission delay of each unretransmitted packet (backlog packet) follows a geometric distribution. That is, each unretransmitted packet is retransmitted in the current time slot with probability pi. Assuming that packets transmitted from each terminal station are burst-like, pi> σi holds. After generating a new packet, each terminal station cannot generate the next packet until the packet is correctly received. Now,
The probability ρi that a terminal station closer to the receiving terminal station j than the transmitting terminal station i will not transmit a packet at the same time as the time slot transmitted by the terminal station i is defined as follows.

[0036]

Here, M is the number of terminal stations included in the system, and Vi is a signal from the terminal station i to the terminal station j due to the occurrence of a packet collision when a packet is transmitted from the terminal station i to the terminal station j. Is defined as a dangerous area where there is a danger that packet transmission to the server will fail. Assuming that the probability that the terminal station is in the idle state 1 is U1, the probability that the terminal station is in the idle state 2 is U2, and the probability that the terminal station is in the backlog state is U3, U1 + U2 + U3
= 1, and U1, U2, and U3 are uniquely obtained. U1 = {pi (1-σi) ρi} / {pipi + σi (1-pi)} (7) U2 = piσipi / {pipi + σi (1-pi)} (8) U3 = σi (1-ρi) / {Piρi + σi (1-ρi)} (9) However, from the definitions of si and qi, it is easily understood that si = U2 (10) qi = U3 (11) Therefore, equations (10) and (1)
If ρi is eliminated from equations (8) and (9) using 1), then piσi ｛(1−qi) σi-si｝ = 0 (12) Here, pi ≠ 0, σi ≠ 0 without loss of generality
Thus, it can be assumed that si = (1−qi) σi (13) By substituting Expressions (13) and (2) into Expression (5), Di = 1 + Ti + qi / si (14) According to this equation, if each terminal has the same s
If Ki that gives i and qi is found, Di is the retransmission probability pi
, But simply depends on the average loop propagation delay time Ti. In a normal packet radio network, the propagation delay time is negligible compared to the packet length, so that it is possible to give equal throughput and transmission delay time to all the terminal stations, which is the object of the present invention.

Next, for a one-dimensional model in which the number of terminals is 5 and all the terminals are arranged on a straight line at equal intervals, the throughput when different Ki is given to each terminal is calculated.

Throughput Sij from terminal station i to terminal station j
(R) is defined by equation (15).

[0040]

Here, n is a terminal station other than the terminal station i in the dangerous area Vr. If the subtotal throughput of the terminal station i is Si (r),

[0042]

Here, Si (r) indicates that the terminal i is in the region Rr
Is defined as the sum of the throughputs possessed by all the terminal stations j belonging to. The region Rr is a region defined for facilitating the mathematical handling of the system model. Specifically, the region Rr extends from the position of the terminal i to the outer periphery of the region including all the terminals accommodated in the system. This is an area divided by bisecting points.

Based on the same concept, the total throughput Si that the terminal station i has over the entire area R is defined by the sum of the throughputs Si (r) that the terminal station i has for each area Rr obtained by dividing the entire area R. And

[0045]

Is represented by

Now, since the throughput of each terminal station is made equal, it is first assumed that all the terminal stations have the same input traffic, backlog probability and throughput. That is, σi = σo = const. qi = qo = const. si = si = const. And

Here, when equation (17) is solved for pi,

[0049]

Is obtained. Solving equation (18) for all i gives the distribution of pi, which should give equal throughput to all terminals.

Equation (18) is a simultaneous equation including M unknowns, and is also a transcendental equation. The first thing to consider is the condition for an expression to have physical meaning. Obviously, the input traffic σo must be greater than so. Otherwise, pi will have a negative value. However, it is known that in a network where the number of terminals that do not retransmit packets is infinite, the throughput of each terminal must be equal to the input traffic of each terminal. The mathematical model shown by the present embodiment has a solution equivalent to a solution for a network with an infinite number of terminals obtained by past research, and only the case where σo> so is considered.

Now, in order to compare with the conventional case,
The results of numerical calculations on the distribution of traffic, throughput, and transmission delay time under the same conditions are shown below. In addition,
In the calculation, each variable is set as σo = 0.2, so = 0.1,
M = 5 was set.

First, the distribution of the retransmission probability pi is obtained by solving the equation (18), and the value of Ki is assigned to each terminal station. Further, the value of K in the conventional method is set to be substantially equal to the average value of Ki in order to meet the operating conditions of the present embodiment.

As a result of the calculation, the optimal distribution of Ki is K1 = 1
2, K2 = 21, K3 = 23, K4 = 21, K5 = 12
It became. Since the average value of these Kis is 17.8, K = 18 was used as the value of K for the conventional method.

FIG. 2 shows the traffic distribution. The distribution state of the conventional method is upwardly convex. This indicates that a terminal located at the center can transmit a new packet more frequently than terminals located at both ends. On the other hand, the distribution state of this method is convex downward. This indicates that the terminal stations located at both ends retransmit the backlog packet more frequently than the terminal station located at the center, and maintain the same throughput as the terminal station at the center.

FIG. 3 shows the distribution of the throughput.
FIG. 4 shows the distribution of the transmission delay time. In the method according to the present invention, all have flat characteristics, and the throughput and the transmission delay time of all the terminal stations included in the network can be made equal.

[0057]

As described above, according to the present invention, when packet transmission has failed at each terminal station, the maximum retransmission interval K (K is a positive integer) assigned in advance to each terminal station is determined. A means for generating a uniform random number from "1" to "K", determining the time for retransmitting the packet based on the value, and retransmitting the packet, and optimizing the maximum retransmission interval K for each terminal station is provided. Since the difference between the throughput and the packet transmission delay of each terminal station caused by the power capture effect can be eliminated and equalized, the same communication quality can be given to each terminal station.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a diagram showing traffic distribution in the present embodiment and a conventional system.

FIG. 3 is a diagram showing throughput distributions of the present embodiment and a conventional system.

FIG. 4 is a diagram illustrating transmission delay time distributions of the present embodiment and a conventional system.

FIG. 5 is a diagram showing a wireless network including five terminal stations.

[Explanation of symbols]

 1-5 Terminal station 13-53 Packet retransmission controller

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04L 12/56 H04B 7/212 H04J 3/00-3/26

Claims (1)

(57) [Claims] 1. A slotted Aloha type packet communication system in which three or more terminal stations share one communication channel to perform data communication, wherein each of the terminal stations transmits a packet signal and then transmits a predetermined signal to a predetermined channel. reception response can not be obtained if from the remote end station in time, from the (K-positive integer) maximum retransmission interval K previous SL assigned in advance to each terminal station in accordance with the "1" and "K" The random number up to the packet is generated.
Means for retransmitting the maximum signal, wherein the maximum retransmission interval K is
Each terminal is based on a relative positional relationship between the terminal stations.
Station throughput and packet transmission delay are equal
The packet communication system characterized that you have allocated to each terminal station as.