JP2005136494A - Communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a wireless communication sufficient in transmission efficiency, by controlling a wireless link in a gateway 3 mounted with a TCP interconnecting a wired network 4 and a wireless network 5. <P>SOLUTION: The gateway 3 for interconnecting the wired network and the wireless network is provided with an ARQ function 3c, having an agent function located between a TCP function 3a and a W-TCP function 3b, transmits data whose errors are detected and which are coded by a first generating polynomial G1(x) to a wireless host 2 of the wireless network 5, retransmits data whose errors are detected and which are coded by a second generating polynomial G0(x) to a wireless host 2, when the wireless host 2 detects an error of the data whose errors are detected and which are coded by the first generating polynomial G1(x), and the wireless host 2 applies Viterbi decoding to the data whose errors are detected and which are coded by the first and second generating polynomials and the errors are corrected. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a communication system including a gateway that connects a wired network and a wireless network, and more particularly to a technique for improving the efficiency of data packet transmission.

With the spread of network communication, data communication using a wired network using a wired communication path or a wireless network using a wireless communication path is widely used. Such data communication is realized, for example, by transmitting packetized data through a personal computer having a communication function with a wired network or a mobile tool having a wireless communication function with a wireless network as a terminal. Is done.
In such a communication system, the network is expanded by connecting between networks by connecting with a gateway that connects a wired network and a wireless network.

  As described in the document “Detailed Network QoS Technology”, QoS technology means that multiple data flows can be performed simultaneously and economically while maintaining specified quality for each data flow on a network such as the Internet. It is a technology to transmit. In the Internet, line transmission rate, reliability (error rate), delay, and the like are all factors of quality of service (QoS). Parameters for evaluating QoS include the average rate, peak rate, packet delay time, jitter, and packet loss rate when an application on one host sends a series of packet flows to an application on another host.

The packet loss will be described in detail. In the case of a wired network such as a wired LAN, packet loss caused by hardware factors is extremely rare due to the development of optical communication technology and the like. Therefore, the main cause of packet loss is buffer overflow in the network. In the Internet, even if a packet overflow occurs, it can be recovered by packet retransmission. Furthermore, recently, an active queue control technique that actively uses packet discard has been adopted as means for notifying the sender of the occurrence of congestion.
For this reason, the Internet does not treat packet loss as an important quality parameter.

However, the situation is different from the above in a wireless communication network that is currently undergoing rapid growth. In the case of a general cell phone, the bit error rate of a bare wireless link (wireless network) is about 10 ⁻² . Note that the error rate in the case of an optical fiber is 10 ⁻¹² or less.
In the case of a communication path with many error occurrences such as a wireless network, retransmission control is indispensable. On the other hand, if retransmission occurs frequently, the link rate decreases and the packet delay increases, leading to a decrease in link QoS. In addition, since the bit error rate varies with time, the QoS characteristics of the link also vary with time.
Here, as an effective technique for error countermeasures in the wireless network, FEC (Forward Error Correction: a technique for automatically correcting an error on the receiving side using an error correction code) or ARQ (Automatic Repeat Request: a link frame of a wireless link) A technique for performing error correction by retransmission is known. Further, as described in the document “Introduction to Coding Theory”, Viterbi decoding technology is known as a decoding method having extremely strong error correction capability.
Toda Atsushi, “Detailed Network QoS Technology”, Ohm Co., Ltd., First Edition, First Printing May 25, 2001, p. 8-9 Yoshihiro Iwadura, “Introduction to Coding Theory”, Shosodo Co., Ltd., October 10, 1997, p. 135-147

As described above, in the case of a wired network, packet loss due to errors hardly occurs. For this reason, it is considered that the packet loss of the gateway on the route in the wired network is caused by the buffer overflow due to congestion, and when TCP (Transmission Control Protocol) detects the packet loss, it performs the congestion control operation to reduce the transmission rate. I am doing so.
However, when a technique similar to that of the wired network is used for the wireless network, there is a problem that packet discard due to bit errors increases and there is a high possibility that duplicate ACK (acknowledgment) transmissions occur frequently. . In addition, when error correction is performed by introducing ARQ into a wireless network, a problem arises that the RTT (Round Trip Time) of the TCP segment increases and the retransmission timer is likely to expire. In either case, the transmission side TCP mistakenly considers packet loss and retransmits the segment, and recognizes that congestion has occurred and starts a window reduction operation such as slow start, so the transmission rate of the network drops sharply. There is a problem.

  The present invention controls a wireless link in a gateway that implements TCP that connects a wired network and a wireless network, thereby discarding packets due to bit errors in the wireless network, frequent occurrences of duplicate ACKs, and rapid transmission rates. An object of the present invention is to provide a communication system that avoids problems such as degradation and realizes wireless communication with good transmission efficiency.

  The communication system according to the present invention is a communication system in which an automatic repeat request function (ARQ) is provided in a gateway that implements TCP for connecting a wired network and a wireless network, and the automatic repeat request function of the gateway is a first generator polynomial. A function of transmitting the error detection coded data in (G1 (x)) to the receiving side of the wireless network, and a second case when the receiving side detects an error in the data error coded with the first generator polynomial. Data that has been subjected to error detection coding with the generator polynomial (G0 (x))) of the first generation polynomial, and data that has been subjected to error detection coding with the first generator polynomial. And Viterbi decoding from the error-encoded data using the second generator polynomial and error correction.

  According to the present invention, if there is a data error in the wireless network, the gateway performs retransmission request control. In this case, the data encoded by the first generator polynomial and the second generator polynomial is transmitted, On the receiving side, Viterbi decoding is performed from the data error-encoded with two generator polynomials to perform error correction. Therefore, since the error can be corrected with the retransmitted encoded data, the number of retransmissions can be reduced, and in the gateway that implements TCP connecting the wired network and the wireless network, the packet discard due to the bit error in the wireless network, Wireless communication with good transmission efficiency can be realized by avoiding problems such as frequent occurrences of duplicate ACKs and a sudden decrease in transmission rate.

The present invention will be specifically described based on an embodiment.
In the communication system according to the present embodiment, TCP and W-TCP (Wireless-TCP) are mounted on a gateway that connects a wired network and a wireless network, and an agent (Snooping Agent) having a type 2 hybrid ARQ function therebetween. Is provided.
FIG. 1 shows an example of a communication system configuration to which the present embodiment is applied. As shown in FIG. 1, this system includes a wired host 1 having a TCP function 1a, a wireless host 2 having a TCP function 2a, and a TCP A gateway 3 having both a function 3a and a W-TCP function 3b to connect the wired network 4 and the wireless network 5, and the gateway 3 further includes a TCP function 3a and a W-TCP function 3b. And an automatic repeat request function (Snooping agent with type 2 hybrid ARQ) 3c having an agent function to be described later.

  The gateway 3 connects the wired network 4 on the wired host 1 side having the necessary communication function and the wireless network 5 on the wireless host 2 side having the necessary communication function, and the automatic retransmission request function 3c having the agent function is: The TCP segment correctly received from the wired network 4 side by the TCP function 3a is transferred to the W-TCP function 3b on the wireless network 5 side, and the TCP segment correctly received from the wireless network 5 side by the W-TCP function 3b is transferred to the wired network 4 A process of handing over to the TCP function 3a on the side is performed.

FIG. 2 shows an operation concept of the above agent (snooping agent) 3c using the OSI reference model.
The gateway 3 includes a physical layer (PHY), a data link layer (DLL), a network layer (IP), an application layer (APP), and a TCP function layer 3a or a W-TCP function layer 3b corresponding to a transport layer. The agent function) 3c cooperates with the data link layer (DLL) and the TCP function layer 3a or the W-TCP function layer 3b to monitor a retransmission request from the receiving side (the wireless host 2 in the figure). Processing and transmission processing of encoded data to the receiving side are performed. Note that a similar OSI reference model is configured on the receiving side (wireless host 2 in the figure), and the following processing is executed.

With reference to FIG. 3, the data transmission / reception processing in the present embodiment will be described by taking as an example the case of transmitting data from the wired host 1 to the wireless host 2.
When data is transmitted from the wired host 1 to the wireless host 2, the data transmitted from the wired host 1 to the wired network 4 is received by the gateway 3. In the gateway 3, the received data is transferred from the TCP function 3 a to the W-TCP function 3 b and transmitted to the wireless network 5 to the wireless host 2.

The data to be transmitted is data 1 to 8 in the example of the figure, and the order of the data 1 to 8 is determined by the confirmation response number as shown in FIG. 4 showing the header format of the TCP frame. For example, as shown in FIG. 3, if an error occurs during data 2 packet transmission, the receiving side (wireless host) cannot receive data 2. Therefore, the data 2 is sent as a NAK (negative response) in the confirmation response frame sent from the reception side (wireless host) to the transmission side (gateway), and is retransmitted in the next transmission frame.
The ARQ function 3c having an agent (snooping agent) function monitors the order of such data, and the W-TCP monitors the segment number received by 3b. The ARQ function 3c is used when there is a drop in received data in the wireless host 2 (that is, when an ACK (acknowledgement) is not returned within a predetermined time or when a NAK (negative response) is returned. ), The corresponding data is retransmitted.

The type 2 hybrid ARQ function 3c will be described in more detail with reference to FIG.
The type 2 hybrid ARQ function 3c is a function that realizes a scheme for mutually compensating for the drawbacks of ARQ and FEC (forward error correction). In the type 2 hybrid ARQ system, a convolutional code having a constraint length of 7 is used as an FEC error correction code.
FIG. 5 shows the configuration of the type 2 hybrid ARQ function 3c. A CRC code is added to each transmission unit by adding a CRC code to each transmission unit by a six-stage register 11 and an adder 12 for adding these register outputs. Convolutional encoding is performed with two generator polynomials G0 (x) and G1 (x).

In the first transmission from the gateway 3 to the wireless host 2, the code generated from the polynomial G1 (x) is transmitted, and the receiving side (wireless host 2) performs an inverse operation on the polynomial G1 (x) to perform data and CRC. Is extracted and error detection processing is performed. Then, ACK (acknowledgement) or NAK (negative acknowledgment) is returned to the gateway 3 as a result of detecting the error.
When receiving the ACK, the transmission side (gateway 3) transmits a code based on the generator polynomial G1 (x) for the next transmission unit. On the other hand, when a NAK is received, a code generated from the first transmission data and another generator polynomial G0 (x) is transmitted, and G1 (x) and G0 (x) are transmitted on the receiving side (wireless host 2). Error correction is performed by performing Viterbi decoding from the code of. If an error is detected in the error correction decoding result, NAK is returned again, and the same G1 (x) code as that at the time of the first transmission is transmitted on the transmission side.

  Therefore, since the receiving side can correct the error by using FEC transmitted as retransmission, the number of retransmissions by ARQ can be reduced, transmission efficiency can be improved, and it is extremely effective when the line condition deteriorates. It is. Further, in the above example, by performing retransmission at the TCP layer, there is an advantage that the overhead of the ARQ header in the data link layer is eliminated, and this also improves the transmission efficiency. In other words, by controlling the radio link by combining a type 2 hybrid ARQ with a snooping agent having the above functions, packet discard or duplicate ACK transmission due to bit errors in the radio link is performed. The occurrence of frequent packet loss, segment retransmission due to erroneous packet loss on the sending side TCP, and start of window reduction operations such as slow start by acknowledging that congestion has occurred can prevent the transmission rate in the wireless network from dropping sharply. It can be avoided.

It is a block diagram of the communication system which concerns on one Embodiment of this invention. It is a block diagram of the gateway which concerns on one Embodiment of this invention. It is a figure explaining the data communication process which concerns on one Embodiment of this invention. It is a figure which shows the header format of the TCP frame which concerns on one Embodiment of this invention. It is a figure explaining the structure of the type 2 hybrid ARQ function which concerns on one Embodiment of this invention.

Explanation of symbols

1: Wired host 1a: TCP function
2: wireless host, 2a: TCP function,
3: Gateway, 3a: TCP function,
3b: W-TCP function, 3c: AQR function,
4: Wired network, 5: Wireless network,

Claims (1)

In a communication system having an automatic repeat request function in a gateway that implements TCP for connecting a wired network and a wireless network,
The automatic retransmission request function of the gateway has a function of transmitting data error-encoded with the first generator polynomial to the receiving side of the wireless network, and an error at the receiver side with respect to data error-encoded with the first generator polynomial. And a function of transmitting the error detection encoded data with the second generator polynomial to the receiving side when detecting
The receiving side performs error correction by performing Viterbi decoding from data error-coded with the first generator polynomial and data error-coded with the second generator polynomial. Communications system.

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