JP4503408B2 - Data communication device - Google Patents

Description

  The present invention relates to a data communication apparatus that forms a data link that performs retransmission control using an automatic retransmission request method, and more particularly to a data communication apparatus that improves throughput.

  Conventionally, wireless data communication means using a wireless terminal such as a mobile phone and having an average transmission speed of 600 kbps and a maximum of 2.4 Mbps are becoming widespread. In a wireless communication path, highly reliable communication is performed by performing retransmission control using an automatic retransmission request between a base station and a wireless terminal. On the other hand, the time from when the retransmission request is made until the data is retransmitted is a delay time, and in the receiving device of the wireless terminal, data without error arrives one after another and is stored in the memory. As the transmission rate increases, the amount of data that is temporarily delayed increases. For example, when communication at a transmission rate of 600 kbps is delayed by 500 ms, 37500 bytes of data are temporarily stored.

  The reason for waiting for the arrival of retransmitted data even though it is received without error is considered to be because the transmission control protocol for data communication is TCP (Transmission Control Protocol). In TCP, data is divided into segments, and each segment is given a sequence number for transmission. If the receiving side receives the signal with the sequence number replaced, it can be replaced (reordering), but the allowable amount is usually 3 segments, or depending on the TCP implementation, it does not have the replacement capability. There are also things.

  In TCP error control, ACK (ACK acknowledgment: acknowledgment, confirmation response) is used, ACK is transmitted from the receiving side that received data to the transmitting side, and the transmitting side retransmits data when ACK is not returned . On the receiving side, when the capacity of the received data exceeds the permissible amount of exchange with the exchange of the sequence number, the receiving side judges that it is a segment loss and returns an ACK without increasing the ACK sequence number. . When the transmitting side receives three duplicate ACKs (ACKs with overlapping ACK numbers) from the receiving side, it starts high-speed retransmission of data.

  As described above, the reason that the permissible amount of replacement on the receiving side of TCP is small is that, in a wired data link such as IEEE 802.3, the error rate is low and the order of received packets is mostly changed. If the sequence number of the data segment is missing, almost all segment losses are assumed. Here, it is assumed that the data communication device performs communication with an external device via a network including a wireless communication path and outputs data received from the external device to other downstream devices. If the data communication apparatus outputs data downstream without maintaining the sequence number of the received segment based on the TCP as described above, the retransmission of the data based on the TCP is performed, resulting in a decrease in throughput. For this reason, in the data communication apparatus on the receiving side of the wireless communication path, while waiting for the arrival of the data requested to be retransmitted, the normally received data is temporarily delayed so as to preserve the sequence number order of the segments. This will improve the TCP throughput.

  However, even if a retransmission request is made, the requested data may not arrive and retransmission may time out. At this time, error-free data temporarily stored in the data communication device on the receiving side is output to other devices at once, causing a phenomenon called delay spike. The delay spike phenomenon causes TCP spurious timeouts. The spurious timeout means that when a data link existing in the middle of a wireless communication path causes a long delay, the TCP transmission side causes a retransmission timeout (RTO), and many data segments spike into downstream devices. This is a phenomenon in which many ACKs occur due to arrival, and the sender performs many unnecessary data retransmissions for the ACKs. It is known that the Eifel algorithm should be used to suppress unnecessary retransmission at the time of a spurious timeout.

  When the delay spike phenomenon occurs, the speed of data flowing downstream from the data communication device can be increased thereby, and an effect of avoiding a decrease in throughput can be expected, but the downstream of the data communication device can be expected. There are cases where congestion occurs in the downlink (a state in which effective communication cannot be performed due to an increase in network traffic). Hereinafter, this will be described with reference to FIG. FIG. 5 is a time sequence graph showing a data reception situation at the time of a delay spike when a conventional data communication apparatus is used. Normally, the time sequence graph shows the situation on the TCP transmission side, but FIG. 5 shows the situation on the reception side.

  In FIG. 5, the horizontal axis represents time, and the vertical axis represents segment sequence numbers. A circle indicates a segment that has arrived at a device downstream of the data communication device, a vertically long rectangle indicates the occurrence of an ACK transmitted to the transmission side, and a cross indicates a loss of the segment. Reception is proceeding smoothly up to the segment 40, and ACK is transmitted almost simultaneously with the arrival of data. The ACK corresponding to the segment 40 is ACK41. Due to the occurrence of a retransmission request in the wireless communication path, the arrival of the segment next to the segment 40 is delayed, and thereafter, reception is interrupted for 500 ms until the retransmission request times out.

  The conventional data communication apparatus outputs the data temporarily stored in a RAM (Random Access Memory) and delayed to the downstream side at once after the retransmission request times out. As a result, a delay spike occurs. A sharp rise after segment 42 in FIG. 5 is a delay spike. The downstream device that receives these segments generates a large number of ACKs (ACKs after ACK 43) based on TCP and transmits them to the transmission side of the segments. However, since one segment is lost, the sequence number of ACK does not increase in TCP, resulting in duplicate ACK.

  Toward the tail of the delay spike, congestion occurs in the downlink downstream of the conventional data communication device, which is indicated by the segment loss 44 and subsequent crosses. On the transmission side, based on TCP, retransmission times out or a segment notified of loss by duplicate ACK is retransmitted. In FIG. 5, the retransmitted segment is received by the data communication apparatus without being affected by the congestion, and is output to the downstream apparatus. The segment received at the downstream device is indicated by segment 46.

  An ACK 47 is transmitted for the segment 46. This ACK 47 designates a segment that has not arrived due to congestion. The operation of transmitting a segment specified by ACK is possible in TCP called NewReno or SACK (Selective ACK). A segment lost due to congestion is notified to the transmission side by the sequence number of ACK 47, and the transmission side retransmits the segment. After that, the normal transmission mode is restored.

  Even if congestion does not occur in the downstream downlink, congestion may occur in the uplink when the receiving side generates ACKs in bursts based on TCP. Hereinafter, this will be described with reference to FIG. The communication proceeds smoothly until the segment 50 is received by a device downstream of the data communication device and the ACK 51 corresponding to the segment 50 is transmitted to the transmission side. After the segment 52 is delayed in the wireless communication path, as in FIG. 5, many segments are output to the downstream apparatus, and the delay spike phenomenon occurs.

  Since the segment 52 is not lost, the ACK sequence number increases rapidly, and the ACK is transmitted in bursts. This situation is similar to the spurious timeout phenomenon, but here the TCP on the transmission side is not timed out. The ACK for segment 54 is lost due to uplink congestion. For this reason, the segment 56 is transmitted after a retransmission timeout has occurred on the transmission side based on TCP. However, since a long time is required until a retransmission timeout occurs, and a slow start is executed thereafter, the throughput is significantly reduced.

  The slow start means that the transmission side performs transmission with a data amount (congestion window (cwnd)) that can be transmitted at a time set to a small value. Further, even if uplink congestion as shown in FIG. 6 does not occur, burst-like ACK arrives at the transmission side, subsequently causes burst-like transmission, and congestion occurs in the downlink upstream from the data communication apparatus. May also cause.

Note that Patent Document 1 describes a line interface device based on TCP / IP.
JP-A-7-67088

  When downlink congestion as described above occurs, there is a problem that throughput is reduced due to segment retransmission based on TCP and reduction of the congestion window. In addition, when uplink congestion occurs, ACK packets are lost, which causes a retransmission timeout in TCP on the transmission side, a reduction in the congestion window (cwnd), and a significant decrease in throughput due to slow start. There was a problem. Note that according to the Eifel algorithm described above, unnecessary segment retransmission by TCP when spurious timeout occurs is suppressed, but the problem of throughput reduction caused by the delay spike phenomenon itself is not solved.

  The present invention has been made in view of the above-described problems, and provides a data communication apparatus that can suppress the occurrence of congestion and avoid a decrease in throughput even when a delay spike phenomenon occurs. With the goal.

The present invention has been made to solve the above-described problems, and has adopted the following means.
A data communication apparatus according to the present invention includes a transmission / reception unit that performs data transmission / reception, and a retransmission request unit that performs a check on an error in received data received by the transmission / reception unit and requests retransmission of the data when an error is detected. And storage means for storing received data received without error until the data requested for retransmission is received, and if the data requested for retransmission is not received within a predetermined time, the data is stored in the storage means. And a control means for outputting the data so that the speed does not exceed a preset maximum value .

A data communication apparatus according to the present invention includes a transmission / reception unit that performs data transmission / reception, and a retransmission request unit that performs a check on an error in received data received by the transmission / reception unit and requests retransmission of the data when an error is detected. And storage means for storing received data received without error until the data requested for retransmission is received, and if the data requested for retransmission is not received within a predetermined time, the data is stored in the storage means. Control means for outputting the received data at a predetermined speed and congestion detection means for detecting congestion based on the received data received by the transmission / reception means , the control means detecting the congestion by the congestion detection means In this case, the predetermined speed is reduced.

A data communication apparatus according to the present invention includes a transmission / reception unit that performs data transmission / reception, and a retransmission request unit that performs a check on an error in received data received by the transmission / reception unit and requests retransmission of the data when an error is detected. And storage means for storing received data received without error until the data requested for retransmission is received, and if the data requested for retransmission is not received within a predetermined time, the data is stored in the storage means. Control means for outputting the processed data at a predetermined speed and congestion detection means for detecting congestion based on the data transmitted by the transmission / reception means , wherein the control means detects the congestion by the congestion detection means In this case, the predetermined speed is reduced.

  According to the present invention, even when a delay spike phenomenon occurs, it is possible to suppress the occurrence of congestion and to avoid a decrease in throughput.

  The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a data communication apparatus according to the first embodiment of the present invention. This data communication apparatus performs wireless communication with a first apparatus which is a base station (not shown) by means of wireless data transmission / reception means 1 and retransmission control means 2. A TDMA (time division multiple access) technique is used for the forward link (reception) in this communication, and the transmission speed is 2.4 Mbps at maximum and 600 kbps on average. Further, CDMA (Code Division Multiple Access) technology is used for uplink (transmission), and the maximum transmission rate is 153.6 kbps. In addition, this data communication apparatus inputs and outputs data to and from other apparatuses by using a serial interface unit 11 of USB (Universal Serial Bus) standard as a data interface. The external device is a personal computer, a bridge, a router, or the like.

  Hereinafter, each component in the figure will be described. The wireless data transmission / reception means 1 is means for transmitting / receiving information signals transmitted / received to / from a physical communication path as electromagnetic energy through modulation and demodulation, and a part of the physical layer and the data link layer. including. The wireless data transmission / reception means 1 receives radio waves from the base station, demodulates the modulated signal, creates reception data, and outputs it to the retransmission control means 2. Further, the wireless data transmitting / receiving unit 1 modulates the transmission data output from the retransmission control unit 2, performs code spreading by the CDMA technique, and transmits a radio wave based on the transmission data.

  Data transmitted from the base station is divided into small frames, and an error check code is attached to each frame. Whether the data communication path is wired or wireless, the communication path has a relatively high error rate, and it takes a long time to complete retransmission at the time of a retransmission request, or a long time for retransmission to timeout and the sender to give up retransmission. It is preferable to apply the data communication apparatus of this embodiment to the above. In the present embodiment, a wireless communication path using a mobile phone or the like is assumed as such a communication path.

  The retransmission control means 2 performs error control, retransmission control, etc. regarding data transmission / reception. The RAM 3 is information storage means capable of writing and reading information for temporarily storing data frames that have been received and have no errors. The retransmission control means 2 realizes highly reliable data communication according to the automatic retransmission request protocol in the data link layer as follows. When the reception data is output from the wireless data transmission / reception means 1, the retransmission control means 2 checks the error of the reception data, and if there is an error, makes a data retransmission request to the transmitting base station.

  If there is no error in the received data, the retransmission control means 2 outputs the received data directly to the serial interface means 11 or temporarily stores the received data in the RAM 3 according to the data storage status in the RAM 3. Received data is read from the RAM 3 at a predetermined timing and output to the maximum speed limiting means 4. When transmission data is input from the serial interface unit 11 to the retransmission control unit 2, the retransmission control unit 2 outputs the transmission data to the wireless data transmission / reception unit 1.

  The retransmission control means 2 uses a queue for managing data stored in the RAM 3. When the received data is stored in the RAM 3, the retransmission control means 2 adds the information of the received data to the queue. When the received data is read from the RAM 3 and output to the maximum speed limiting means 4, the queue is sent from the queue. The received data information is deleted.

  The maximum rate limiting unit 4 limits the output rate of the data frame output from the retransmission control unit 2 to the serial interface unit 11 so as not to exceed a preset maximum value. The maximum speed limiting means 4 includes a timer that can change the time-out interval by setting, and a FIFO (First In First Out) in which data frame output to the serial interface means 11 is executed based on the timer event. It is a kind of scheduler composed of buffers.

  If the timer interval is shortened, the maximum speed increases, and if the timer interval is lengthened, the maximum speed decreases. As the maximum value of the output speed set in the maximum speed limiting means 4, a value stored in a non-volatile memory (not shown) is set at the time of power-on or reset operation. The user can adjust the maximum value of the output speed set in the nonvolatile memory depending on whether the communication path on which the data communication apparatus is installed is likely to cause congestion. Note that the maximum speed limiting means 4 may be means for varying the length of data output in one event while executing output in a constant timer event.

  The serial interface unit 11 is a USB standard communication unit, and performs data input / output with a downstream device. The serial interface unit 11 outputs the reception data output from the retransmission control unit 2 or the maximum speed limiting unit 4 to the downstream device, receives the transmission data output from the downstream device, and outputs it to the retransmission control unit 2 To do.

  Next, a communication control operation by the data communication apparatus having the above-described configuration will be described. The wireless data transmission / reception means 1 demodulates the signal received from the base station, creates reception data, and outputs it to the retransmission control means 2. When a retransmission request has not occurred, the retransmission control means 2 checks whether there is an error in the received data. If there is no error and no received data is stored in the RAM 3, the received data is sent to the serial interface means. 11 to output. For the received data frame, the header used in the wireless data link protocol is removed, and the content data is output to the serial interface unit 11.

  When there is an error in the received data, the retransmission control means 2 generates retransmission request information for requesting retransmission of data to the transmission side, and sends data to be transmitted to the transmission side and a header such as retransmission request information. The stored data frame is created, attached with an error check code, and output to the wireless data transmitting / receiving means 1. The wireless data transmitting / receiving means 1 performs processing such as modulation and code spreading on the data frame and then transmits the data frame to the base station. The retransmission control means 2 inspects the received data received until the data frame requested to be retransmitted is received, and if there is no error, stores the received data in the RAM 3.

  When the data frame requested to be retransmitted is received by the wireless data transmitting / receiving unit 1 and output to the retransmission control unit 2, the retransmission control unit 2 checks the error of this data frame. When the data frame requested to be retransmitted is received without error, the retransmission control means 2 compares the sequence number of the data frame with the sequence number of the data frame stored in the RAM 3.

  When the retransmitted data frame is one frame older than the oldest data frame stored in the RAM 3, the retransmission control means 2 includes the oldest data frame and the data frames that are in sequential order. Are output from the RAM 3 and the retransmitted data frame and the data frame read from the RAM 3 are output to the maximum speed limiting means 4 in accordance with the data order. Of the data frames in the RAM 3, those output to the maximum speed limiting means 4 are deleted by the retransmission control means 2. Note that this deletion may be to delete the data body and free the memory, or to keep only the data body and delete only the entry information in the queue managed by the retransmission control means 2. May be.

  When the retransmitted data frame is not one frame older than the oldest data frame stored in the RAM 3, the retransmission control means 2 stores the retransmitted data frame in the RAM 3. At this time, the retransmission control means 2 performs management using a queue so that the data frames can be output according to the data order, not the data reception order. This management can be realized by using, for example, a linear list, and a cell representing newly stored data is inserted at a position according to the order of the data in the linear list.

  When the retransmitted data frame is output to the maximum rate limiting means 4, the data frame is stored at the end of the FIFO buffer. The maximum speed limiting means 4 executes a timer event that occurs by time-out every predetermined time set in advance. Each time the timer event is executed, the data frame stored at the head of the FIFO buffer is serialized. Output to the interface means 11.

  FIG. 2 is a time sequence graph showing a state during a delay spike when the data communication apparatus according to the present embodiment is used. This figure shows the state of data reception on the receiving side based on TCP, the horizontal axis represents time, and the vertical axis represents the sequence number of the segment. In this figure, a circle represents a segment that has arrived at a device downstream of the data communication device, a bar-shaped square represents the occurrence of an ACK transmitted to the transmission side, and a cross indicates a segment loss. Since this figure shows the situation on the TCP reception side, an ACK is transmitted almost simultaneously with the arrival of data.

  The ACK corresponding to the segment 30 is ACK31. Although communication before the segment 30 is proceeding smoothly, the segment following the segment 30 is delayed due to the occurrence of a retransmission request for the wireless communication path. If 500 ms elapses from the occurrence of the retransmission request and the timeout occurs, the retransmission control means 2 gives up receiving the retransmission data. The data received without error before the timeout is temporarily stored in the RAM 3, read out by the retransmission control means 2 after the timeout, and output to the maximum speed limiting means 4.

  In FIG. 2, one segment is lost because a time-out occurs without being retransmitted, and a delay spike is generated by the next segment 32 and the subsequent segments. However, in the data communication device, the data output speed does not exceed the maximum value by the maximum speed limiting means 4. As a result, the segment 32 and the subsequent segments are received by the downstream apparatus at a time interval, and congestion does not occur. The ACK for the segment 32 is ACK 33, and the ACK is transmitted almost simultaneously with the arrival of the segment at the downstream apparatus, and the ACK transmission interval is maintained.

  On the transmission side, based on TCP, retransmission times out or a segment notified of loss by duplicate ACK is retransmitted. The retransmitted segment is received as segment 34. The ACK for segment 34 is ACK 35. After ACK 35, the normal transmission mode is restored. In FIG. 5, congestion occurs in the downlink direction downstream of the data communication device and segment loss 44 occurs. In FIG. 2, segment loss occurs in the downlink direction downstream of the data communication device. Thus, since the segment is not retransmitted, it can be seen that the throughput is improved.

  As described above, in this embodiment, the maximum rate limiting unit 4 limits the output rate of the data frame output from the retransmission control unit 2 to the serial interface unit 11 to a predetermined value (maximum value) or less. Thereby, even when the delay spike phenomenon occurs, it is possible to suppress the occurrence of congestion and avoid a decrease in throughput.

  Next, a second embodiment of the present invention will be described. FIG. 3 is a block diagram showing the configuration of the data communication apparatus according to the present embodiment. The same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 3, an IP layer ECN detection means 5 is provided. The IP layer ECN detecting means 5 is means for detecting a congestion notification used in an upper layer protocol higher than the network layer. For example, an ECN (Explicit Connection Notification) flag is prepared in an IP (Internet Protocol) packet header or TCP header (see RFC 3168), and whether or not congestion experience is notified by checking this flag. Can be detected.

  The IP layer ECN detection means 5 according to the present embodiment recognizes the structure of the IP packet, examines the ECN flag bit existing therein, and determines whether a flag indicating congestion experience is set. The IP layer ECN detection unit 5 performs ECN detection on the reception data output directly from the retransmission control unit 2 or via the maximum speed limiting unit 4 and outputs the reception data to the serial interface unit 11. Further, ECN detection is performed on the transmission data output from the serial interface unit 11, and the transmission data is output to the retransmission control unit 2. This detection is done in both the downlink and uplink directions.

  However, even if this detection is performed only for the uplink, it is effective. This is because the ECN flag indicating the occurrence of congestion due to a delay spike can be observed in the uplink in many cases. If the ECN flag indicating the congestion experience is detected, the IP layer ECN detection means 5 decreases the maximum value of the data output speed set in the maximum speed limit means 4 and resets it. As a result, the maximum value of the data output speed that does not cause congestion can be automatically set in the maximum speed limiting means 4 to suppress the occurrence of congestion.

  Next, a third embodiment of the present invention will be described. FIG. 4 is a block diagram showing the configuration of the data communication apparatus according to the present embodiment. The same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 4, transmission side congestion detection means 6 is provided. The transmission side congestion detection unit 6 is provided between the retransmission control unit 2 and the wireless data transmission / reception unit 1 in the uplink direction, receives the data frame output from the retransmission control unit 2, and transmits the data frame to the wireless data transmission / reception unit. Output to means 1. The transmission side congestion detection means 6 is means for detecting congestion in transmission on the wireless communication path. The transmission side congestion detection means 6 detects that congestion has occurred when a large number of data transmission requests are output from the retransmission control means 2 while transmission of data in the uplink direction is delayed.

  The transmission side congestion detection means 6 is composed of a buffer and a queue using the buffer, and determines that congestion has occurred when it is detected that the length of the queue has increased to exceed a predetermined value. . When the congestion detection unit 6 detects congestion, the transmission side congestion detection unit 6 decreases and resets the maximum value of the data output rate set in the maximum rate limiting unit 4. As a result, the maximum value of the data output speed that does not cause congestion can be automatically set in the maximum speed limiting means 4 to suppress the occurrence of congestion. The congestion may be determined when the queue reaches the limit of the storage capacity in the buffer and any data to be transmitted by the transmission side congestion detection means 6 is discarded.

  Note that the data communication device according to the above-described embodiment may be connected to a PC (Personal Computer Assistant), a PDA (Personal Digital Assistance), or the like according to a standard such as USB, or the data communication device is built in the PC, PDA, or the like. It may be.

  As described above, the embodiments of the present invention have been described in detail with reference to the drawings, but the specific configuration is not limited to these embodiments, and includes design changes and the like within a scope not departing from the gist of the present invention. It is.

1 is a block diagram showing a configuration of a data communication device according to a first embodiment of the present invention. It is a time sequence graph which shows the data reception condition at the time of the delay spike in the data communication apparatus by the embodiment. It is a block diagram which shows the structure of the data communication apparatus by the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the data communication apparatus by the 3rd Embodiment of this invention. It is a time sequence graph which shows the data reception condition at the time of the delay spike in the conventional data communication apparatus (when congestion occurs in the downlink). It is a time sequence graph which shows the data reception condition at the time of the delay spike in the conventional data communication apparatus (when congestion occurs in the uplink).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Wireless data transmission / reception means, 2 ... Retransmission control means, 3 ... RAM, 4 ... Maximum speed limiting means, 5 ... IP layer ECN detection means, 6 ... Transmission side congestion detection Means, 11... Serial interface means.

Claims (3)

Transmitting / receiving means for transmitting / receiving data; and
A check for error in the received data received by the transmission / reception means, and when an error is detected, a retransmission request means for requesting retransmission of the data;
Storage means for storing received data without error received until the data requested to be retransmitted is received;
Control means for outputting the data stored in the storage means so that the speed does not exceed a preset maximum value when the data requested for retransmission is not received within a predetermined time;
A data communication apparatus comprising: Transmitting / receiving means for transmitting / receiving data; and
A check for error in the received data received by the transmitting / receiving means, and a retransmission request means for requesting retransmission of the data when an error is detected;
Storage means for storing received data received without error until the data requested to be retransmitted is received;
Control means for outputting the data stored in the storage means at a predetermined speed when the data requested for retransmission is not received within a predetermined time;
Congestion detection means for detecting congestion based on the received data received by the transmission / reception means,
The data communication apparatus according to claim 1, wherein the control means reduces the predetermined speed when the congestion detection means detects congestion. Transmitting / receiving means for transmitting / receiving data; and
A check for error in the received data received by the transmitting / receiving means, and a retransmission request means for requesting retransmission of the data when an error is detected;
Storage means for storing received data received without error until the data requested to be retransmitted is received;
Control means for outputting the data stored in the storage means at a predetermined speed when the data requested for retransmission is not received within a predetermined time;
Congestion detection means for detecting congestion based on data transmitted by the transmission / reception means,
The data communication apparatus according to claim 1, wherein the control unit decreases the predetermined speed when the congestion detection unit detects congestion.

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