KR20040105459A - Method and system for retransmiting a multi-media data - Google Patents

Abstract

PURPOSE: A system for retransmitting multi-media data is provided to efficiently reproduce a moving picture having improved quality while minimizing a number of retransmission times and retransmission capacity by retransmitting only a minimum frame required for reproducing the moving picture. CONSTITUTION: A system for retransmitting multi-media data includes a reproducing station(20) selectively requesting retransmission of I-picture frame with error or loss from received packet data and a streaming server(10) selectively retransmitting a corresponding I-picture frame requested from the reproducing station to a retransmission buffer that separately stores I-picture frames. A method for retransmitting multi-media data includes the steps of transmitting a retransmission request signal to a streaming server after a reproducing station detects I-picture frame with error or loss from received packet data, and retransmitting a corresponding I-picture frame requested after the streaming server selects the I-picture frame from a retransmission buffer.

Description

METHOD AND SYSTEM FOR RETRANSMITING A MULTI-MEDIA DATA}

The present invention relates to a selective retransmission method for error data detected when transmitting multimedia data. The present invention more specifically requests only retransmission of major frames for errors or lost frames generated when real-time MPEG-2 multimedia data is transmitted over a wireless local area network / personal area network (WLAN / WPAN). Thereby, the present invention relates to a method for improving the efficiency and quality of retransmission.

In general, the Moving Picture Expert Group (MPEG-2) standard was developed as a high-quality video transmission standard for broadcast and video production, approved as an ISO standard in late 1994, and several companies are developing related chips and systems.

The MPEG system is a standard for transmitting or storing an MPEG video bit string and an MPEG audio bit string. When multiplexing into one bit string like this, it is necessary to have a format suitable for a protocol or a storage format of a communication channel or a storage medium. In addition, providing a means of synchronizing the video and audio (Lip sync) is an important role of the MPEG system.

MPEG systems include MPEG-1 systems used in a relatively narrow range of applications such as video CDs, and MPEG-2 systems used in a wide range of applications such as broadcasting, communication, and storage media.

There are two types of multiplexing schemes in the MPEG-2 system. One is called Program Stream (PS), which multiplexes a single program in an error-free channel environment and slightly improves the MPEG-1 system.

The other is a transport stream (TS), which multiplexes a plurality of programs in an error channel environment. Since multiple programs are multiplexed into a single bit stream, it is suitable for digital TV broadcasting in the multimedia era, and a scramble function for limited reception can be added (encrypting the bit stream so that only non-paying subscribers can watch it). have. In addition, directory information, individual bit string information, and the like can be loaded to facilitate random access.

When the multimedia streaming data is transmitted and received using the wireless LAN / fan network with the MPEG-2 frame format as described above, packet loss or errors may occur due to various internal and external problems. In this case, the WLAN / WPAN retransmits an error packet by specifying a retry bit in the header of the packet. The conventional system for transmitting / retransmitting MPEG-2 data is as shown in FIG. 1. 1 shows an error recovery method supported by the MAC layer / higher MAC layer.

First, with regard to MAC layer recovery and retransmission, the sender STA1 transmits a packet and checks whether an error has occurred in the receiver STA2. At this time, the receiver STA2 sends an acknowledgment (ACK) to the sender STA1 only for a packet without an error, and does not send an ACK when an error occurs. The sender STA1 starts the ACK timer immediately after transmitting the packet and checks whether the ACK arrives within a predetermined time. If the ACK does not arrive within the predetermined time, a management information base (MIB) relating to the management target is transmitted. Attempt to retransmit the packet up to the maximum retry count defined in the database of information).

The other relates to upper MAC layer recovery (TCP / Application) and retransmission. The sender STA1 transmits the packet, and the received receiver STA2 checks whether the received data packet is in error. At this time, an ACK is sent to the sender STA1 only for a packet without an error, and a retransmission request is sent if an error occurs. As soon as the sender STA1 transmits the packet, the sender STA1 starts an ACK timer to check whether the ACK arrives within a given time. If the sender STA1 does not arrive within the ACK timer, the sender STA1 attempts retransmission up to the maximum retry count, and retransmits only the packet to be retransmitted when the retransmission request signal is received.

As described above, the general retransmission mechanism applies the same principle to all packets, and when retransmission is required, retransmission requests for all frames of I-picture, B-picture, and P-picture of MPEG-2 are executed. Therefore, in case of retransmission in MPEG-2, even B-picture and P-picture having low utility value are treated the same as I-picture, so there is a problem that overall performance may be reduced in retransmission.

In addition, even if retransmission is caused by the real-time characteristics, it is often impossible to use it, and there is a problem in that all retransmitted packets are not used because the retransmitted packets may arrive after the replay time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to select only the minimum frame required for video playback for an error or lost frame generated during transmission of an MPEG-2 data stream over a wireless LAN / fan network. By requesting retransmission, it is to improve the efficiency and quality improvement of video playback while minimizing the number and capacity of retransmissions.

1 is a block diagram showing a conventional MPEG-2 data retransmission system on a wireless LAN / fan.

2 is a diagram showing the format of an MPEG-2 data frame on a WLAN / Pan according to the present invention.

3 is a main configuration diagram of an MPEG-2 data retransmission system according to the present invention.

4 is a view showing a time point to be screened for a retransmission request using the MPEG-2 data retransmission system according to the present invention.

5 is a flowchart illustrating a retransmission method in the MPEG-2 data retransmission system according to the present invention.

6 is a flowchart illustrating a retransmission request method in the MPEG-2 data retransmission system according to the present invention.

<Description of Symbols for Major Parts of Drawings>

10: streaming server 11: retransmission buffer

12: buffer controller 13: transmission control unit

20: playback station 21: playback buffer

22: playback buffer controller 23: reception control unit

24: Retransmission Decision / Request Algorithm

In order to achieve the above object, the present invention provides a playback station that selectively requests retransmission for an error or lost I-picture frame in a received packet, and a retransmission buffer in which the I-picture frame is separately stored. An MPEG-2 multimedia data retransmission system including a streaming server for selecting and retransmitting a corresponding I-picture frame requested from a reproduction station is provided.

As a method for implementing an embodiment of the present invention, the present invention provides a method for transmitting a retransmission request signal to a streaming server after a playback station detects an error or lost I-picture frame in a received data packet, and the streaming server. Provides a selective retransmission method of MPEG-2 multimedia data including selecting and retransmitting a corresponding I-picture frame that has received a retransmission request from a retransmission buffer.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is a format configuration diagram of an MPEG-2 data frame in the MPEG-2 system of the present invention. Detailed description thereof is as follows.

The MPEG-2 format adopts a packet multiplexing scheme used in time division multiplexing (TDM). At this time, each of the video and audio bit strings is first divided into bit strings of appropriate length called packets, and this bit string is called a packetized elementary stream (PES).

The PES is a packetized flow by adding header information to transmit an elementary stream (ES), which is a digitally compressed video or audio signal. In general, the PES refers to one frame of image information. The header includes various pieces of information about the PES content, which is useful for decoding at the receiver. For example, the header of the video PES includes information such as a video format, a color format, a coding scheme, an aspect ratio, and the like.

In MPEG-2, three types of frames are used, such as I- (Intra), P- (Predicted), and B- (Bidir-ectional) Picture, as in Elementary Stream (ES). The I-picture is a Discrete Cosine Transform (DCT), which is an orthogonal transform coding scheme, which is adopted in MPEG, the international standard for moving picture compression, and is currently applied to high efficiency encoding and compression techniques of video. It is encoded and compressed only by the processing. This I-picture contains almost all the information needed for playback. The P-picture is an image immediately before the end of encoding, and is produced by motion prediction from the preceding I-picture or P-picture. The B-picture is predicted by using successive front and rear adjacent images, and is generated by bidirectional interpolation from the front and rear I- and P-pictures. In addition, the number of I-pictures, P-pictures, and B-pictures that make up an MPEG bit stream can vary depending on the manufacturer of the encoding / decoding chip or the type of compressed video. In some applications, I-picture and Sometimes P -picture is used to provide video.

The upper limit of the length of the PES is up to 64 KB so as to support various applications, and the fixed length or the variable length can be taken for each packet. Variable rates are also allowed and discontinuous transmission is possible.

Each PES can be multiplexed into one bit string to create a transport stream (TS). The TS is a protocol for transmitting video, audio, and data in one stream at the same time in an MPEG system. The TS is composed of one or more programs, and the header of the TS includes program information constituting the entire stream, time information of the program, and the whole. Control information and the like for controlling the system are input. In addition, a retry bit may be designated in the header of the TS to retransmit a packet having an error.

The length of the TS packet depends greatly on the transmission channel or the medium. For example, in the ATM (Asynchronous Transfer Mode) protocol, which is a protocol in a broadband integrated information communication network, a packet (cell) of 53 bytes is used. Since the header containing the basic information about the double packet takes 5 bytes, the actual user information (Payload) is 48 bytes. TS packets can have a relatively short fixed length of 188 bytes when considering connectivity with ATM. The first 4 bytes of each TS packet are for headers, and the remaining 184 bytes carry user information carrying actual video or audio. This is the part.

As described above, video data can be compressed and transmitted in a WLAN / Fan network using the MPEG-2 frame format.

3 shows a system configuration for retransmitting MPEG-2 multimedia data through a WLAN / WPAN network.

The retransmission system of the present embodiment receives and reproduces a streaming server 10 for transmitting MPEG-2 data streams in a WLAN / WPAN network and data transmitted from the streaming server 10. And a playback station 20 which detects an error or lost data among the received data and requests the streaming server 10 to retransmit.

The streaming server 10 includes a re-transmission buffer 11, a buffer controller 12, a sender 13, and the like. In this configuration, in the streaming server 10, each frame is divided into PES units for multimedia data transmission through the buffer controller 12, and is then generated as a TS packet and transmitted to the playback station 20 through the transmission control unit 13. . In this case, the I-picture frame containing almost all the information necessary for playing a video during transmission is stored and managed separately in the retransmission buffer 11. Therefore, when the streaming server 10 receives a retransmission request from the playback station 20, the retransmission efficiency may be improved by selectively retransmitting only the corresponding I-picture frame in the retransmission buffer 11.

In order to improve the retransmission efficiency, the retransmission buffer 11 of the streaming server 10 separately stores and buffers only the I-picture which is important for reproduction among the I-, P-, and B-pictures. As a result, when a retransmission is requested for an error or lost I-picture frame from the playback station 20, the buffer controller 12 selects the corresponding I-picture stored in the retransmission buffer 11 and quickly. You can resend it.

The buffer controller 12 divides each frame into PES packets, and then divides the PES packets into TSs, as shown in FIG. 2, for frame transmission. In order to transmit the TS packet on the WLAN / Fan, each header is encapsulated (encapsulation: a technique of embedding upper protocol communication protocol information in a lower protocol protocol frame user information area in computer communication). At this time, only the I-Picture frame important for moving picture reproduction is stored in the retransmission buffer 11 to manage this. The buffer controller 12 compares and manages the I-picture frame order to be transmitted to the reproduction station 20 with the I-picture frame ID stored in the retransmission buffer 11, thereby managing the image from the reproduction station 20. When a request for retransmission of an error or lost I-picture frame is requested, retransmission is possible by easily selecting the corresponding I-picture frame in the retransmission buffer 21.

The transmission control unit 13 controls data transmission by using a general transmission application to transmit MPEG-2 multimedia data to the reproduction station 20.

The playback station 20 is a device that receives and decodes and plays multimedia data transmitted from the streaming server 10. The playback station 20 detects errors or missing frames in the received data and determines whether to request retransmission. Request to the streaming server 10.

The playback station 20 includes a playback buffer 21, a playback buffer controller 22, a reception control section 23, a retransmission decision / request algorithm 24, and the like.

The playback buffer 21 is a buffer for storing the received MPEG-2 frame for decoding and playback.

The playback buffer controller 22 controls the MPEG-2 frames stored in the playback buffer 21, and performs the sequence check (checking whether the data is in ascending or descending order). An error or a lost frame is detected in decoding the frames stored in the reproduction buffer 21. Here, a retransmission request is executed for the I-picture frame among the detected frames according to the retransmission determination / request algorithm 24.

The reception control unit 23 receives the multimedia data transmitted from the streaming server 10 using a general reception application and controls the playback buffer 21 to adjust video playback.

The retransmission determination / request algorithm 24 determines whether a retransmission request can be made to the streaming server 10 when an error or lost I-picture is detected by the replay buffer controller 22 to determine the replay buffer. The controller 22 is notified. That is, after calculating whether there is sufficient time for the data to be retransmitted from the streaming server 10 to be received and reproduced by the reproducing station 20, it is determined whether to retransmit. Specifically, as shown in FIG. 4, the retransmission determination / request algorithm 24 determines whether to request retransmission from the streaming server 10 as follows.

If retransmission is requested: ΔTRTT <ΔTPlay

If no retransmission is requested: ΔTRTT> = ΔTPlay

* ΔTRTT (Time Retransmission): Playback Station 20 → (Retransmission Request)

→ streaming server (10) → (retransmission) → playback station (20)

* ΔTPlay (time to play): A (time to play)-B (time to detect error)

5 and 6 are flowcharts showing the operation of the MPEG-2 data retransmission system of the present invention.

5 shows an operation of the streaming server 10 in the retransmission system of the present invention.

First, the streaming server 10 divides each frame into PES units through the buffer controller 12, makes a packet in TS (188 Byte), and prepares for transmission of an MPEG-2 data stream (S100). In this process, the streaming server 10 stores only the I-picture, which is an important frame for playing a video, in the retransmission buffer 11 (S110).

After the MPEG-2 data is transmitted to the playback station 20 through the transmission control unit 13, the buffer controller 12 is an I-picture of an error or missing data transmitted to the playback station 20. A retransmission request signal for a frame is received (S120). When the retransmission request signal is received, the buffer controller 12 selects the corresponding I-picture frame that has received the retransmission request from the retransmission buffer 11 (S13) and retransmits (S140).

Next, an operation process of the reproduction station 20 will be described with reference to FIG. 6.

When the playback station 20 receives the MPEG-2 streaming data packets transmitted from the streaming server 10 through the reception controller 23 (S200), the playback buffer controller 22 receives the received packets. Is stored in the reproduction buffer 21 and managed (S210).

In this process, the reproduction buffer controller 22 performs an error check whether there is an error or lost I-picture frame when decoding the packets stored in the reproduction buffer 21 (S220).

In this case, when an error or lost I-picture frame is found, the playback buffer controller 22 uses the retransmission decision / request algorithm 24 to determine whether to request a retransmission from the current time point to the time of reproduction. The time ΔTRTT is compared with the time ΔTPlay that can be retransmitted, that is, the retransmission time ΔTRTT is compared with the time ΔTPlay to be screened (S230).

If the replay buffer controller 22 has a retransmission time ΔTRTT <time to be shown ΔTPlay, the reproduction buffer controller 22 requests the streaming server 10 to retransmit the error or lost I-picture frame (S240). If ΔTRTT> = ΔTPlay, the retransmission request is not performed (S250).

In this way, in checking the error or lost frames in the reproduction buffer controller 22, even if a P- or B-picture is found, they do not have a large effect on the playback of the video compared to the I-picture, so that the retransmission request is made. Ignore and play in order. Therefore, by quickly determining whether to request only retransmission of the I-picture according to the retransmission decision / request algorithm 24, the retransmission speed of the frame in which the error occurs can be improved, thereby providing an improved image. .

Although the present invention has been described in detail above, those skilled in the art to which the present invention pertains may variously modify the present invention without departing from the spirit and scope of the present invention as defined in the appended claims. It is apparent that the present invention may be modified or modified. Therefore, a simple change according to an embodiment of the present invention will not be possible without departing from the technology of the present invention.

According to the present invention having the above-described configuration, when a packet loss or an error of data occurs when transmitting a large amount of MPEG-2 data in real time in a WLAN / WPAN environment, a predetermined frame among these errors or lost frames By selectively requesting retransmission for only these devices, it is possible to improve the retransmission efficiency and to provide an improved quality video.

Claims (10)

A playback station selectively requesting retransmission for an error or lost I-picture frame in the received packet; And And a streaming server for selecting and retransmitting the corresponding I-picture frame requested from the playback station in the retransmission buffer in which the I-picture frame is separately stored. A transmission controller for transmitting the multimedia data and controlling the multimedia data; A retransmission buffer in which I-picture frames are stored and buffered; And And a buffer controller for selecting and retransmitting the corresponding I-picture frame from the retransmission buffer. The streaming server of claim 2, wherein the buffer controller compares and manages the transmitted I-picture frame order with the I-picture frame ID stored in the retransmission buffer. A reception controller which receives the multimedia data and controls the multimedia data; A reproduction buffer for storing and reproducing the received data frame; And And a replay buffer controller for retransmitting a detected or missing I-picture frame in the replay buffer. The retransmission controller according to claim 4, wherein the retransmission controller requests retransmission for the detected error or lost I-picture frame when the time ΔTRTT for retransmission is smaller than the time ΔTPlay to be screened according to the retransmission decision / request algorithm. And do not request retransmission if greater than or equal to the time ΔTPlay to be shown. Transmitting, by the playing station, a retransmission request signal to the streaming server after detecting an error or lost I-picture frame in the received data packet; And And retransmitting, by the streaming server, a corresponding I-picture frame that has received a retransmission request from a retransmission buffer. Storing an I-picture frame in a retransmission buffer by a buffer controller during multimedia data transmission; Receiving, by the buffer controller, a retransmission request signal for an error or lost I-picture frame among data packets received by the reproduction station; And And retransmitting, by the buffer controller, a corresponding I-picture frame that has been requested to be retransmitted from the retransmission buffer and retransmitting. The streaming server of claim 7, wherein the buffer controller compares and manages the transmitted I-picture frame order with the I-picture frame ID stored in the retransmission buffer. Storing the data packet received through the reception controller in a reproduction buffer; Detecting, by a play buffer controller, an I-picture frame that is in error or lost in the play buffer; And requesting, by the playback buffer controller, a streaming server for retransmission of the detected I-picture frame to a streaming server. 10. The method of claim 9, wherein the retransmission controller requests retransmission for the detected error or lost I-picture frame when the time ΔTRTT for retransmission is smaller than the time ΔTPlay to be shown according to the retransmission decision / request algorithm. And do not request retransmission if greater than or equal to the time ΔTPlay to be shown.