JP4952651B2 - Communication system and jitter buffer control method - Google Patents

Description

  The present invention relates to a jitter buffer control technique in a conversion apparatus installed at the boundary between a TDM (Time Division Multiplexing) network and a packet network in order to transmit a signal of a constant bit rate through the packet network.

  Currently, the communication network is shifting from a TDM network in which a transmission path with a constant bit rate is set between two points to a packet network such as the Internet using IP (Internet Protocol). However, since communication devices for the TDM network are still used, it is necessary to transmit a signal with a constant bit rate through the packet network (see, for example, Patent Document 1).

  In a packet network, the transmission delay of each packet is not uniform, and the value varies from packet to packet. Therefore, a converter that receives a packet from the packet network and transmits a signal to the TDM network usually includes a jitter buffer. Yes. Increasing the buffer amount of the jitter buffer increases resistance to delay fluctuations occurring in the packet network, but also increases transmission delay. For this reason, it is desired to dynamically control the buffer amount of the jitter buffer according to the state of the packet network. Patent Document 2 discloses that a silence state is detected from a bit pattern in a received packet to detect the jitter buffer. A configuration for controlling the buffer amount is disclosed.

JP 2006-67040 A JP 2002-271391 A

  The method described in Patent Document 2 detects a silent part by a bit pattern in a packet, but this method has a problem that it cannot be applied to a signal including data.

  Accordingly, an object of the present invention is to provide a communication system and method capable of dynamically controlling the buffer amount of a jitter buffer regardless of the type of signal to be transmitted, that is, whether it is voice or data. .

According to the communication system of the present invention,
A TDM terminal, a first converter that receives a signal of a fixed bit rate having a frame structure from the TDM terminal, packetizes and transmits data included in the frame, and a second that receives a packet from the first converter The TDM terminal sets the maintenance operation bit of the frame to a predetermined value when the data included in the frame is idle data, and the first conversion device. Indicates that the packet is an idle packet by monitoring the maintenance operation bit of the frame and transmitting an idle packet containing only idle data by setting a predetermined bit in the packet header of the packet to a predetermined value. The second conversion device includes a jitter buffer, and when the received packet is an idle packet, The received idle packet is discarded, and the buffer amount of the jitter buffer is decreased. When the received packet is not an idle packet, the received packet is stored in the jitter buffer, and the accumulated amount of the jitter buffer becomes less than a predetermined threshold. In this case, an idle packet is generated, the generated idle packet is stored in a jitter buffer, and the buffer amount of the jitter buffer is increased.

According to another embodiment of the communication device of the present invention,
Preferably, the amount by which the jitter buffer is decreased is equal to the bit length of the discarded idle packet, and the amount by which the jitter buffer is increased is equal to the bit length of the generated idle packet.

According to the communication system of the present invention,
The TDM terminal transmits a fixed bit rate signal indicating whether or not the included data is idle data by a maintenance operation bit, and the first converter receives the fixed bit rate signal from the TDM terminal. A step in which the first conversion device packetizes data contained in the signal of the fixed bit rate and transmits the packet to the second conversion device, and the second conversion device transmits a packet from the first conversion device to the second conversion device. The first conversion device monitors the maintenance operation bit, and when transmitting an idle packet including only idle data, the first conversion device can predetermine a predetermined bit of the packet header of the packet. When the received packet is an idle packet, the second conversion device indicates that the packet is an idle packet. Discards received idle packets and reduces the buffer amount of the jitter buffer. If the received packet is not an idle packet, the received packet is stored in the jitter buffer, and the accumulated amount of jitter buffer becomes less than the predetermined threshold. In this case, an idle packet is generated, the generated idle packet is stored in a jitter buffer, and the buffer amount of the jitter buffer is increased.

  The TDM terminal notifies that the user data in the frame is idle by using a bit reserved for the maintenance operation of the frame, and based on this notification, the conversion device transmits an idle including only idle data. The packet is clearly specified in the packet header and transmitted to the packet network. Therefore, the buffer amount of the jitter buffer can be dynamically controlled regardless of the type of signal to be transmitted, that is, whether it is voice or data.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings.

  FIG. 1 is a block diagram of a communication system according to the present invention. According to FIG. 1, the communication system includes TDM terminals 21 and 22, TDM networks 31 and 32, conversion devices 11 and 12, and a packet network 40.

  The TDM terminals 21 and 22 are terminals that are connected to the TDM network and transmit / receive a fixed bit rate signal. The conversion apparatuses 11 and 21 are connected to both the TDM network and the packet network, and receive fixed bits from the TDM network. It has a function of packetizing a rate signal and transmitting it to the packet network, and a function of transmitting data contained in the payload of a packet received from the packet network to the TDM network as a fixed bit rate signal.

  The TDM networks 31 and 32 are networks that provide a fixed bit rate path or transmission path between two points. In FIG. 1, the TDM network 31 has a fixed bit rate between the TDM terminal 21 and the converter 11. The TDM network 32 sets a fixed bit rate path between the TDM terminal 22 and the converter 12. The packet network 40 is a network that distributes a packet including a header having destination information and a payload having data based on the destination information in the header. In FIG. 1, the conversion device 11 receives from the TDM terminal 21. The data to be transmitted is stored in the payload of the packet addressed to the converter 12 and transmitted to the packet network 40. The converter 12 stores the data received from the TDM terminal 22 in the payload of the packet addressed to the converter 11 It is transmitted to the network 40. Thereby, the TDM terminal 21 and the TDM terminal 22 can communicate as if they are connected by a path provided by a single TDM network.

  In the communication system according to the present invention, when the stored data is idle data, the TDM terminals 21 and 22 store idle data using overhead bits reserved for maintenance operation. Is specified. For example, ITU-T (International Telecommunication Union Telecommunication Standardization Sector) 704 defines a frame structure including 24 time slots (1 time slot is 8 bits) and 1 frame bit (Fbit in FIG. 4) shown in FIG. 4 for a 1.544 Mbps signal. It is recommended that the frame bits of a frame be used for maintenance operation information in units of 16 bits (that is, in units of 32 frames). An unused pattern in this recommendation, for example, “11111100111100” is used as the idle data. Used for notifications.

  FIG. 5 is a diagram showing a packet conforming to the TDMoIP specification transmitted by the conversion apparatuses 11 and 12, and FIG. 6 is a diagram showing details of the control word in FIG. The converters 11 and 12 monitor a frame of a fixed bit rate signal received from the TDM terminal via the TDM network, determine whether or not the data included in the fixed bit rate is idle data, and When transmitting a packet including only data in the payload, a packet (hereinafter referred to as an idle packet) in which the L bit of the control word is set to “1” and the M bit is set to “01” is transmitted to the packet network.

  For example, when the conversion apparatus receives a 1.544 Mbps signal and packetizes it in units of 8 frames, that is, in units of 192 bytes, the idle is notified by the 16 frame bits of the odd frame. Four idle packets are transmitted to the packet network.

  FIG. 2 is a block diagram of the conversion devices 11 and 21 according to the present invention. FIG. 2 shows only the part related to signal processing from the packet network to the TDM network. According to FIG. 2, the conversion device includes a packet interface 1, a jitter buffer 2, a conversion unit 3, a TDM interface 4, a packet generation unit 5, and a buffer amount control unit 6.

  The packet interface 1 stores a packet received from the packet network in the jitter buffer 2. However, when an idle packet is received, it is discarded without being stored in the jitter buffer, and the buffer amount control unit 6 is notified of the idle packet reception. The conversion unit 3 takes out the packet from the jitter buffer 2 and outputs the payload data as a fixed bit rate signal to the TDM interface 4. The TDM interface 4 transmits the signal from the conversion unit 3 to the TDM network. .

FIG. 3 is a diagram for explaining the details of the jitter buffer 2. As shown in FIG. 3, the jitter buffer 2 manages the write position and the read position, and the write position moves to the left in FIG. 3 every time a packet is stored and reaches the left end. Repeats moving to the right end and moving to the left side every time a packet is stored again. Similarly, the read position moves to the left in FIG. 3 every time a packet is read, moves to the right when it reaches the left end, and moves to the left every time a packet is read again. repeat. The shaded portion in FIG. 3 shows the range of packets stored in the jitter buffer. In the following description, half of the maximum accumulation amount of the jitter buffer 2, that is, half of the bit amount from the left end to the right end in FIG. 3 is referred to as a set jitter buffer amount T _jb .

Returning to FIG. 2, when receiving the notification of idle packet reception from the packet interface 1, the buffer amount control unit 6 decreases the set jitter buffer amount T _jb of the jitter buffer 2 by an amount corresponding to the bit length of the idle packet. . Further, the accumulated amount of the jitter buffer 2 is monitored, and when the accumulated amount becomes 0, for example, when it becomes less than a predetermined threshold value close to 0, the set jitter buffer amount T _jb of the jitter buffer 2 is increased, An idle packet storage command is issued to the generation unit 5, and the packet generation unit 5 that has received the idle packet storage command generates an idle packet and stores it in the jitter buffer 2. The amount to be increased is an amount corresponding to the bit length of the idle packet.

  FIG. 7 is a diagram for explaining the operation of the conversion device according to the present invention. In FIG. 7, as described above, a case where a 1.544 Mbps signal is packetized in units of 8 frames is taken as an example. Therefore, one packet is read out from the jitter buffer 2 in 1 ms. For simplicity, it is assumed that an idle packet is stored in the jitter buffer 2 at the moment when the accumulated amount of the jitter buffer 2 becomes zero.

According to FIG. 7B, ideally, packets A to I that arrive at an interval of 1 ms from time 1 ms are actually time 4.5, 5, 5.5, 6, 6, 6, 7, Arrived at 7.5 and 8 ms. Packets G, H, and I are idle packets. It is assumed that the set jitter buffer amount T _jb of the jitter buffer 2 at time 0 is 3 ms and the accumulation amount at time 0 is 3 ms.

FIG. 7A shows transition of the accumulation amount (solid line) of the jitter buffer 2 and the set jitter buffer amount T _jb (dotted line) in this case.

Originally, the arrival of the packet A that should arrive at the time 1 ms is delayed, so at the time 3 ms when the buffer underflow occurs, the buffer amount control unit 6 causes the packet generation unit 5 to store the idle packet in the jitter buffer 2, The set jitter buffer amount T _jb is increased by 1 ms corresponding to the added idle packet. However, since the packet A does not arrive, the buffer amount control unit 6 again stores the idle packet in the jitter buffer 2 in the jitter buffer 2 and adds the set jitter buffer amount T _jb at the time 4 ms. Is increased by 1 ms.

From time 4.5 ms to time 6 ms, packets A to F arrive, and the accumulated amount increases at each time. Thereafter, the packets G to I arrive. Since these are idle packets, the packet interface 1 does not store the packets G to I in the jitter buffer 2, and thus the accumulation amount does not increase. Further, the buffer amount control unit 6 decreases the set jitter buffer amount T _jb by 1 ms corresponding to the idle packet due to the arrival of the idle packet.

  As described above, the TDM terminal notifies that the user data in the frame is idle by using the bits reserved for the maintenance operation of the frame, and the conversion apparatus transmits only the idle data based on this notification. The idle packet that is included is clearly specified in the packet header and transmitted to the packet network. Therefore, the buffer amount of the jitter buffer can be dynamically controlled regardless of the type of signal to be transmitted, that is, whether it is voice or data.

It is a block diagram of the communication system by this invention. 1 is a block diagram of a conversion device according to the present invention. It is a figure explaining the detail of a jitter buffer. It is a figure which shows the frame structure of a 1.544Mbps signal. It is a figure which shows the structure of the packet according to the rule of TDMoIP. It is a figure which shows the detail of a control word. It is a figure explaining operation | movement of a converter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Packet interface 2 Jitter buffer 3 Conversion part 4 TDM interface 5 Packet generation part 6 Buffer amount control part 11, 12 Conversion apparatus 21, 22 TDM terminal 31, 32 TDM network 40 Packet network

Claims (3)

A TDM terminal, a first converter that receives a signal of a fixed bit rate having a frame structure from the TDM terminal, packetizes and transmits data included in the frame, and a second that receives a packet from the first converter A communication system comprising:
When the data included in the frame is idle data, the TDM terminal sets the maintenance operation bit of the frame to a predetermined value,
The first conversion device monitors a maintenance operation bit of a frame, and when transmitting an idle packet including only idle data, sets a predetermined bit of the packet header of the packet to a predetermined value, and sets the idle packet Indicating that
The second conversion device includes a jitter buffer. When the received packet is an idle packet, the second conversion device discards the received idle packet and reduces the buffer amount of the jitter buffer so that the received packet is not an idle packet. The received packet is stored in the jitter buffer, and when the accumulated amount of the jitter buffer becomes less than a predetermined threshold, an idle packet is generated, the generated idle packet is stored in the jitter buffer, and the buffer amount of the jitter buffer is stored. Increase the
Communications system. The amount to decrease the jitter buffer is equal to the bit length of the discarded idle packet, and the amount to increase the jitter buffer is equal to the bit length of the generated idle packet.
The communication system according to claim 1. A TDM terminal transmitting a fixed bit rate signal indicating whether or not the included data is idle data with a maintenance operation bit;
A first conversion device receiving the fixed bit rate signal from a TDM terminal;
A first conversion device packetizing data contained in the signal of the fixed bit rate and transmitting the packet to the second conversion device;
A second converter device receiving a packet from the first converter device;
With
When the first conversion device monitors the maintenance operation bit and transmits an idle packet including only idle data, the first conversion device sets a predetermined bit of the packet header of the packet to a predetermined value, Show that there is
When the received packet is an idle packet, the second conversion apparatus discards the received idle packet and reduces the buffer amount of the jitter buffer. When the received packet is not an idle packet, When stored in the buffer and the accumulated amount of the jitter buffer becomes less than a predetermined threshold, an idle packet is generated, the generated idle packet is stored in the jitter buffer, and the buffer amount of the jitter buffer is increased.
Jitter buffer control method.

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