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Claims  |
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We claim:
1. A transmission control method in multimedia information communication
performed between two terminal stations connected to each other through a
packet switching network, each of said terminal stations having control
means for performing transmission control for transmitting a video packet
including coded video data and a voice packet including coded voice data
to said packet switching network and for performing supply control for
supplying said video packet and voice packet received from said packet
switching network to a video codec and a voice codec respectively, said
control method comprising the steps of:
one of said two terminal stations operating as a receiving side terminal
station transmitting a video data transmission mode change request to the
other terminal station operating as a multimedia information transmission
side in accordance with a status of delay of video packets and/or voice
packets received from said packet switching network; and
said transmission side terminal station changing the video data
transmission mode in response to said transmission mode change request.
2. A transmission control method according to claim 1, wherein
said receiving side terminal station observes delay of video packets in
said packet switching network so that said receiving side terminal station
issues, as said transmission mode change request, a rate command for
instructing changing of a video packet transmission interval in accordance
with a quantity of the observed delay when delay of video packets exceeds
a first threshold value and issues, as said transmission mode change
request, a stop command for instructing stopping of video packet
transmission when the delay of video packets exceeds a predetermined limit
which is larger than said first threshold value, and
said transmission side terminal station changes its communication mode from
a first communication mode to transmit both video data and voice data into
a second communication mode to transmit only voice data, in response to
reception of said stop command.
3. A transmission control method according to claim 2, wherein
said transmission side terminal station transmits each video packet with
its transmission time added thereto to said packet switching network, and
said receiving side terminal station calculates the quantity of delay of
video packets on the basis of an arrival interval of video packets from
said packet switching network and a transmission interval of video packets
calculated from transmission time added to each video packet.
4. A transmission control method according to claim 1 wherein
said receiving side terminal station observes delay of voice packets in
said packet switching network so that said receiving side terminal station
issues, as said transmission mode change request, a stop command for
instructing stopping of video packet transmission when delay of voice
packets becomes greater than or equal to a predetermined upper limit value
and issues, as said transmission mode change request, a restart command
for instructing restart of video packet transmission when delay of voice
packets become less than or equal to a predetermined lower limit value,
and
said transmission side terminal station changes the communication mode from
a first communication mode to transmit both video data and voice data to a
second communication mode to transmit only voice data, in response to
reception of said stop command, and changes from said second communication
mode to said first communication mode in response to reception of said
restart command.
5. A transmission control method according to claim 4, wherein
said receive side terminal station observes delay of video packets in said
packet switching network so that said receiving side terminal station
issues, as said transmission mode change request, a rate command for
instructing changing of a video packet transmission interval in accordance
with a quantity of the observed delay when delay of video packets exceeds
a predetermined threshold value, and
said transmission side terminal station controls the video packet
transmission interval in response to reception of said rate command.
6. A transmission control method according to claim 4, wherein
said transmission side terminal station transmits each video packet with
its transmission time added thereto and each voice packet with its
transmission time added thereto, and
said receive side terminal station calculates the quantity of delay of
voice packets on the basis of an arrival interval of voice packets from
said packet switching network and a transmission interval of voice packets
calculated from transmission time added to each voice packet.
7. A transmission control method according to claim 6, wherein
said receiving side terminal station calculates the quantity of delay of
voice packets on the basis of a difference between an average arrival
interval of voice packets and an average transmission interval of voice
packets for every group of a predetermined number of voice packets
received from said packet switching network.
8. A multimedia communication terminal station for performing multimedia
communication with another terminal station through a packet switching
network, comprising:
video data transmission control means for controlling transmission of a
video packet including coded video data to said packet switching network;
video data reception control means for controlling supply of a video packet
received from said packet switching network to a video decoding circuit;
voice data transmission control means for controlling transmission of a
voice packet including coded voice data to said packet switching network;
and
voice data reception control means for controlling supply of a voice packet
received from said packet switching network to a voice decoding circuit;
wherein said video reception control means includes:
first means for observing video packet delay time generated in said packet
switching network, and
second means for issuing a video data transmission mode change request to
another terminal station in accordance with the delay time of video
packets observed by said first means; and
wherein said video data transmission control means controls a video packet
transmission interval in response to a video data transmission mode change
request received from another terminal station.
9. A multimedia communication terminal station according to claim 8,
wherein
said second means issues, as said transmission mode change request, a rate
command for instructing changing of a video packet transmission interval
in accordance with a quantity of the observed delay when delay of video
packets exceeds a first threshold value and issues, as said transmission
mode change request, a stop command for instructing stopping of video
packet transmission when the delay of video packets exceeds a
predetermined limit which is larger than said first threshold value, and
said video data transmission control means controls the video packet
transmission interval in response to a rate command received from another
terminal station, and stops video packet transmission in response to a
stop command received from another terminal station.
10. A multimedia communication terminal station according to claim 8,
wherein
said video data transmission control means sends each video packet with its
transmission time added thereto to said packet switching network, and
said first means calculates the delay time of video packets on the basis of
an arrival interval of video packets from said packet switching network
and a transmission interval of video packets calculated from transmission
time added to each video packet.
11. A multimedia communication terminal station according to claim 8,
wherein
said voice data transmission control means includes third means for
observing delay time of voice packets in said packet switching network,
and fourth means for issuing, as said transmission mode change request, a
stop command for instructing stopping of video packet transmission when
the delay time of voice packets observed by said third means becomes
greater than or equal to a predetermined upper limit value and issues, as
said transmission mode change request, a restart command for instructing
restarting of video packet transmission when the delay time of voice
packets becomes less than or equal to a predetermined lower limit value;
and
said video data transmission control means stops video packet transmission
in response to reception of a stop command received from another terminal
station, and restarts video packet transmission in response to reception
of a restart command received from another terminal station.
12. A multimedia communication terminal station according to claim 11,
wherein
said video data transmission control means observes delay time of video
packets in said packet switching network to issue a rate command for
instructing changing of a video packet transmission interval in accordance
with a quantity of the observed delay when delay of video packets exceeds
a predetermined threshold value,
wherein further another terminal station controls the video packet send
interval in response to reception of said rate command.
13. A multimedia communication terminal station according to claim 11,
wherein
said voice data transmission control means includes means for adding
transmission time to each voice packet to be transmitted, and
said voice data reception control means calculates the quantity of delay of
voice packets on the basis of an arrival interval of voice packets from
said packet switching network and a transmission interval of voice packets
calculated from transmission time added to each voice packet.
14. A multimedia communication terminal station according to claim 13,
wherein
said voice data reception control means calculates the quantity of delay of
voice packets on the basis of a difference between an average arrival
interval of voice packets and an average transmission interval of voice
packets for every group of a predetermined number of voice packets
received from said packet switching network. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multimedia information communication
system, and particularly to a method and apparatus for controlling
multimedia communication so that the communication mode is automatically
changed over in accordance with a load condition of a packet switching
network in a system in which multimedia information communication is
performed between two terminal stations through the packet switching
network.
2. Description of the Related Art
Recently, TV phone systems and TV conference systems, in which terminal
stations at remote locations are connected through communication lines
have been developed. In video communication, the quantity of information
to be transmitted is large and it is therefore desirable that a
transmission bandwidth which is in accord with the information quantity
can be secured. Presently, in a local area network provided in an office
or the like, however, it is impossible for every user to secure necessary
transmission bandwidth because of limitations in transmission capacity.
In a network to which terminal stations which may generate a large quantity
of information such as video signals are connected, problems include that
the bandwidth available by the network is changed greatly or that jitter
is generated on delay time of information transmission in the network for
reasons such as the data suddenly bursting into the network from one of
the terminal stations.
Conventionally, for example, JP-A 3-270430 teaches a voice and video
communication system which is provided with a function that a TV
conference terminal station connected to an LAN measures the quantity of
traffic to be sent out from its own side to the LAN and the current
quantity of traffic on the LAN, and when both the measured traffic
quantities are not less than predetermined threshold values with respect
to video and voice signals, the terminal station gives a command to a
voice coder-decoder (hereinafter referred to as "codec") and a video codec
so as to reduce the information transmission quantity to the LAN.
Upon reception of such an information quantity reducing command, in the
voice and video communication system, the video codec lowers the upper
limit value of the information transmission quantity and the voice codec
operates to reduce the information transmission quantity so that the
increase of the end-to-end transmission delay is suppressed to thereby
prevent the video and voice signals quality from deteriorating in the
video and voice. In the above-mentioned technique, however, there is a
problem that when the transmission delay is large even if the video and
voice transmission quantity is reduced to its minimum value, it is
impossible to maintain the quality of both media in the terminal station
on the receiving side.
Further, JP-A 2-209043 teaches a data communication system in which an
information frame transmission side terminal station judges the traffic
state in a network on the basis of the receiving state of an arrival
confirm frame/re-send request frame transmitted from a receive side
terminal station, and when the transmission side terminal station
concludes that the traffic quantity is large, the transmission side
terminal station prolongs the send interval of the information frames to
thereby control the traffic which flows into the network. This system
however has a problem in that if this system is applied to a voice and
video communication system which takes a serious view of real time
property, the response operation of traffic control is slow because the
traffic state is judged on the basis of the reception state of an arrival
confirm frame/re-send request frame from the data receiving side.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
transmission control method in multimedia information communication and a
multimedia communication terminal station, in which, when traffic load of
a communication network becomes so high that end-to-end transmission delay
becomes too large to perform both video and voice communication, the
information transmission mode is automatically changed while high quality
communication is maintained at least with respect to voice data.
In order to achieve the above object, the present invention features a
terminal station, operating as a video and voice data transmission side,
controlling the multimedia data transmission mode in response to a
transmission mode change request which another terminal station operating
as a receiving side has issued in accordance with the traffic load of the
communication network.
More specifically, a terminal station on the receiving side requests
another terminal station on the transmission side to lower the video data
transmission rate when the receiving side terminal station detects that
the video packet transmission delay exceeds a predetermined threshold
value because of an increase of the load of the network or the like, while
the receiving side terminal station requests the transmission side
terminal station to stop video data transmission in order to ensure the
quality of voice communication when the receiving side terminal station
detects that the voice packet transmission delay exceeds a predetermined
limit value L1. In the state where the video is frozen on a display screen
of the receiving side terminal station because the transmission side
terminal station has stopped video data transmission, if the receiving
side terminal station detects that the voice data packet transmission
delay has decreased, the receiving side terminal station requests the
transmission side terminal station to resume the video data transmission.
In a preferred embodiment, the receiving side terminal station issues the
video data transmission restart request when it detects that the voice
data packet transmission delay becomes less than a predetermined lower
limit value L2 which is lower than the above-mentioned limit value L1.
The foregoing and other objects, advantages, manner of operation and novel
features of the present invention will be understood from the following
description when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an example of a network system
constituted by a plurality of communication terminal stations to which the
present invention is to be applied;
FIG. 2 is a block diagram showing the configuration of a communication
terminal station;
FIG. 3 is a block diagram showing a function of a program for multimedia
communication control, provided in the communication terminal station;
FIG. 4 is a view showing a format of a coded video data packet;
FIG. 5 is a block diagram showing the function of the voice communication
control unit 203 shown in FIG. 3;
FIG. 6 is a flowchart of the program constituting the video transmission
control unit 201 shown in FIG. 3;
FIG. 7 is a flowchart of the program constituting the video receive control
unit 202 shown in FIG. 3;
FIG. 8 is a flowchart showing an embodiment of the voice data receive unit
203B shown in FIG. 5;
FIG. 9 is a diagram for explaining the relationship between the
transmission mode change-over by the terminal station and the delay time
according to the present invention;
FIG. 10 is a flowchart showing a modification of the voice data receiving
unit 203B shown in FIG. 5; and
FIG. 11 is a flowchart showing an embodiment of the voice data transmission
unit 203A shown in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 through 8, a first embodiment of the present invention
will be described.
FIG. 1 shows an example of the network system for performing multimedia
information communication according to the present invention.
In the network, communication terminal stations 100A through 100J are
connected to a LAN 10 and communication terminal stations 100K through
100N are connected to another LAN 20. The LAN 10 and LAN 20 are connected
to a packet switching network 30 through interfaces 11 and 21
respectively. Each of the communication terminal stations 100 (100A
through 100N) has a function for processing multimedia information
including video and voice data in addition to an ordinary function of data
processing, so that, for example, not only multimedia communication C1
between the communication terminal stations 100A and 100J accommodated in
one and the same LAN 10, but also multimedia communication C2 between the
communication terminal stations 100A and 100N respectively accommodated in
the LANs 10 and 20 connected to each other through the network 30, can be
performed by means of packets. Alternatively, the communication terminal
stations 100 may be connected to the packet switching network 30, not
through the LAN 10 or 20, but directly.
FIG. 2 shows the configuration of each of the communication terminal
stations 100 having a function of multimedia information communication
control according to the present invention.
The communication terminal station 100 includes: a communication interface
102 for performing interface processing with respect to the LAN or
communication network; an internal bus 103; a CPU 104 for performing, by
using a software, video and voice data transmission control (hereinafter,
simply referred to as "multimedia communication control") which will be
described later; and a memory 105 for storing various programs and data
including a program 200 for performing the function of the multimedia
communication control.
The communication terminal station 100 further includes: a video coder 106
for converting an analog video signal supplied from a video input device
110, such as a camera or the like into a digital video signal and then for
compressing and coding the digital video signal; a video decoder 107 for
decompressing and decoding compressed and coded video data, and then for
converting the decompressed and decoded video data into an analog video
signal; a voice coder 108 for converting an analog voice signal supplied
from a voice input device 112, such as a microphone or the like, into
coded voice data; a voice decoder 109 for decoding coded voice data and
for converting the decoded voice data into an analog voice signal; a
display unit 111 provided with a screen for displaying video information
thereon; a voice output device 113 for outputting voice information; and a
keyboard 114 for entry of commands and data.
The display unit 111 is capable of displaying not only video data but also
data from an application program executed by the CPU 104 on one and the
same screen at the same time utilizing a multi-window function provided in
an ordinary workstation or communication terminal station. For the sake of
simplification, however, functional elements unrelated to video and voice
data, though related to the communication control according to the present
invention are omitted in FIG. 2.
FIG. 3 shows the configuration of the multimedia communication control
program 200 stored in the memory 105.
The multimedia communication control program 200 is constituted by: a video
transmission control unit 201 for controlling a reading rate (a coding
rate) for reading coded video data from the video coder 106 to thereby
control the packet transmission interval for sending video packets
including coded video data to the network; a video receiving control unit
202 for controlling a supplying rate (a decoding rate) for supplying coded
video data extracted from a video packet to the video decoder 107 in
accordance with the receiving state of video packets received from another
communication terminal station; a voice communication control unit 203 for
performing transmission/receive control of a voice packet including coded
voice data to/from the network; and a communication management unit 204
for making delivery of multimedia data between the communication
management unit 204 and the communication interface 102. The communication
management unit 204 controls the operations of the video transmission
control unit 201, the video receiving control unit 202, and the voice
transmission control unit 203.
The video transmission control unit 201 has a function for receiving,
through the communication management unit 204, a coding rate change
request (a rate change command) and a communication mode change request (a
video stop/resume command) for instructing stop/resume of video
transmission, the requests being issued from a mate communication terminal
station. The video transmission control unit 201 has a further function
for reading video data from the video coder 106 at a coding rate (a packet
transmission interval) designated by the rate change command and a video
packet transmission function for editing the coded video data read from
the video coder 106 into a video data packet having a predetermined format
shown in FIG. 5 and for transmitting the video data packet to the
communication management unit 204. Each of the video data packets has, as
a time stamp, packet transmission time obtained from a timer 120 which
indicates current time on the basis of internal clocks of the CPU.
The video receiving control unit 202 has: a function for controlling supply
of coded video data extracted from a video packet received from the
communication management unit 204 to the video decoder 107; a function for
calculating an average of the packet arrival intervals of video packets
and for calculating an average of the packet transmission intervals from
packet transmission time extracted from video packets; a function for
issuing a coding rate change request (a rate change command) to a terminal
station to receive the video data when delay time represented by a
difference value .DELTA.t between an average packet arrival interval
.DELTA.T and an average packet transmission interval .DELTA.R is larger
than a predetermined threshold value and a function for issuing a video
stop command to the transmission side terminal station when the average
packet arrival interval .DELTA.R has reached a predetermined limited value
RTmax. The video stop command may be controlled on the basis of the result
of judgment as to whether the voice communication control unit 203 has
issued a video stop request or not.
FIG. 4 shows an example of the format of a coded video data packet 500.
The coded video data packet 500 is constituted by: a packet ID field 501
which indicates whether the information contained in an information field
508 is video data or a control command; a field 502 which indicates
whether a video data is that of a head portion of one video frame or that
of a remainder portion of the same when the video data is contained in the
information field 508; a field 503 which indicates whether video data
contained in the packet is intra frame information or inter frame
information; a field 504 which indicates the size of a video frame; a
field 505 which indicates the packet number; a field 506 which indicates
the packet length; a field 507 which indicates the send time of the
packet; and the above-mentioned information field 508.
FIG. 5 shows the configuration of the voice communication control unit 203.
The voice communication control unit 203 is constituted by a voice data
transmission unit 203A and a voice data receive unit 203B.
The voice data transmission unit 203A is constituted by a data read unit
601 for reading coded voice data from the voice coder 108 and a voice
packet assembly unit 602 for generating a voice packet of a predetermined
format in which current time designated by the timer 120 is added to the
coded voice data read in the data read unit 601 and for sending the voice
packet to the communication management unit 204.
The voice data receive unit 203B is constituted by a voice packet
disassembly unit 603 for extracting coded voice data and other field items
from a voice packet received from the communication management unit 204,
and a data output unit 604 for supplying the coded voice data to the voice
decoder 109.
FIG. 6 shows a flowchart for executing the control operation of the video
transmission control unit 201.
When a video communication start command is received from the communication
management unit 204 of a corresponding communication terminal station
(step 701), a predetermined initial value is set as the video packet
transmission interval .DELTA.Ts (step 702), and then transmission of coded
video data is started with this packet transmission interval (the coding
rate).
First, judgment is made as to whether a coding rate change request (a rate
change command) has been received or not from a corresponding
communication terminal station (step 703). If the judgment proves that a
rate change request has been received in the step 703, the value of the
video packet transmission interval .DELTA.Ts is changed into a value
.DELTA.R designated by the rate change request (step 704) and coded video
data for one frame is read from the video coder 106 (step 705). Next,
current time t is obtained from the timer 120 (step 706), and a video
packet having transmission time Ts=t is generated (step 707) to be sent to
the communication management unit 204 (step 708). Then judgment is made as
to whether a communication end command has been received or not from the
communication management unit 204 (step 709), and if YES, the video data
transmission operation is ended. A communication end command is issued in
response to communication end operation of a user. If no communication end
command has been issued in step 709, on the contrary, current time t is
obtained from the timer 120 (step 710) so that standby is continued until
the time t-Ts=.DELTA.Ts is reached (step 711). At a point of time when the
packet send interval has reached .DELTA.Ts in step 711, the operation is
returned to step 703.
If no rate change command has been received in step 703, on the contrary,
the operation is shifted to step 1201 in which judgment is made as to
whether a video stop command has been issued or not from the corresponding
communication terminal station. If a video stop command has been received
in step 1201, the video data transmission operation is frozen (step 1202)
and arrival of a video resume command from the corresponding communication
terminal station or arrival of a communication end command from the
communication management unit 204 is awaited (step 1203 or 1204). If a
video resume command has been received (step 1203), the video data
transmission operation is performed by repetition of step 705 et seq. If a
communication end command has been received from the communication
management unit 204 (step 1204), on the contrary, the video data
transmission operation is ended.
FIG. 7 shows a flowchart showing the control operation of the video
receiving control unit 202.
When a communication start command is received from the communication
management unit 204 (step 801), the value of the count parameter n of an
observation period for calculating delay time which will be described
later, specifically, the number of packets which is used as a population
parameter when the packet transmission and arrival interval averages
.DELTA.T and .DELTA.R are obtained, is cleared to be zero (step 802).
When a video packet is received from the network (step 803), the count
parameter n is incremented (step 804) and current time indicated by the
timer 120 and time extracted from the field 507 of the received packet are
stored as the receive time Rn and transmission time Tn respectively in the
table area of the memory 105 (steps 805 and 806). Next, a difference value
.DELTA.Rn between the receive time Rn and the receive time Rn-1 of the
preceding packet stored in the table area in advance and a difference
value .DELTA.Tn between the transmission time Tn and the transmission time
Tn-1 of the preceding received packet are calculated and stored in the
table area as the receive and transmission intervals .DELTA.Rn and
.DELTA.Tn respectively (steps 807 and 808).
Judgment is made as to whether the value of the count parameter n has
reached a predetermined value m or not (step 809). If the count parameter
n is smaller than m in step 809, the operation is returned to step 803 and
the foregoing packet receiving operation is repeated.
If the judgment proves that n=m in step 809, on the contrary, an average
.DELTA.R of m values .DELTA.R1.about..DELTA.Rm representing the receive
intervals stored in the table area is calculated (step 810). The average
.DELTA.R of the receive intervals is compared with a predetermined limit
value Rmax (step 811), and if .DELTA.R<Rmax, the average .DELTA.T of m
values .DELTA.T1.about..DELTA.Tm representing the transmission intervals
stored in the table area is calculated (step 812), and a difference value
.DELTA.t(=.DELTA.R-.DELTA.T) between the average receive interval .DELTA.R
and the average transmission interval .DELTA.T is calculated (step 813).
The difference value .DELTA.t represents transmission delay time of the
network due to a traffic load. The value of .DELTA.t is compared with a
predetermined threshold value TH (step 814), and if the value .DELTA.t is
larger than the value TH, a rate change command is issued to the
corresponding communication terminal station so as to instruct the
corresponding terminal station to make the video packet transmission
interval .DELTA.Ts agree with the average receive interval .DELTA.R (step
815). Then, judgment is made as to whether a communication end command has
been received or not from the corresponding communication terminal station
(step 816). If a communication end command has been received in step 816,
the communication is ended, while if no communication end command has been
received, on the contrary, the operation is returned to step 802 so as to
repeat the foregoing packet receiving operation.
In the case of .DELTA.R.gtoreq.Rmax in the step 811, on the contrary,
judgment is made as to whether a video stop request has been issued from
the voice receive control unit 203 or not (step 1301). If no video stop
request has been issued in step 1301, the operation is shifted to step 812
and the foregoing calculation for obtaining the average transmission
interval .DELTA.T is performed. If a video stop request has been issued in
step 1301, on the contrary, a communication mode change request for
instructing stopping of video transmission is issued to the communication
management unit 204 (step 1302). Thereafter, issuance of a video resume
request from the voice communication control unit 203 or arrival of a
communication end command is waited for (step 1303 or 1304). If a video
resume request is received in step 1303, the operation is returned to step
802 and the foregoing video data packet receiving operation is resumed. If
a communication end command is received in step 1304, on the other hand,
the communication is ended.
In this embodiment, while two communication terminal stations are operating
to perform communication with video packets, the receiving side terminal
station observes the video packet receiving state (the difference value At
between the average receive interval .DELTA.R and the average transmission
interval .DELTA.T), and when a traffic load in the communication network
increases so that the packet transmission delay (the difference value
.DELTA.t) exceeds a threshold value, a command for reducing the video data
transmission rate is issued to the transmission side terminal station.
Accordingly, video communication according to the network load state can
be carried out. Further, when the average receive interval .DELTA.R of
video packets reaches a limit value, the communication mode is changed
over so as to stop video communication so that it is possible to carry out
multimedia communication in which high communication quality of voice data
is maintained.
Step 1301 may be omitted, though the configuration of FIG. 7 is such that
when the average receive interval .DELTA.R of video packets exceeds a
threshold value (step 811), a video stop command is issued to the
communication management unit when a video stop command is issued from the
voice receiving control unit (steps 1301 and 1302).
Next, the configuration of the voice communication control unit 203 in the
communication terminal station according to the present invention will be
described.
FIG. 8 is a flowchart showing an embodiment of the data output unit 604 of
the voice data receiving control unit 203B provided with a function of
absorbing jitter in video packet transmission delay generated in the
network and a function of issuing a video packet transmission mode change
request.
As for the voice packet transmission delay jitter, the state of
transmission delay is observed for every voice packet group composed of M
voice packets, and the waiting time Tw for absorbing jitter is determined
in accordance with the observed quantity of transmission delay, so that
the timing of supply from the forefront voice data buffer memory to the
voice decoder is adjusted in accordance with the above-mentioned waiting
time Tw. The initial value of the waiting time Tw is made to be zero in
advance.
When a voice communication start command is received from the communication
management unit 204 (step 801), first, the value of the count parameter n
for counting received voice packets is cleared to zero (step 1001).
When one voice packet is received into the buffer memory (step 1002), the
value of the count parameter n is incremented (step 1003), and the current
time indicated by the timer 120 and the time stamp added to the received
voice packet are stored into the table area as the arrival time VRn and
transmission time Vtn of the Nth voice packet respectively (steps 1004,
1005).
Next, the value of the count parameter n is judged (step 1006). If n=1,
predetermined waiting time Tw is set to the timer (step 1014), and at a
point of time when the timer has timed out (step 1016), the coded voice
data read out from the buffer memory is supplied to the voice decoder
(step 1017) and the operation is returned to step 1002.
If n.noteq.1 in step 1006, the coded voice data is immediately read out
from the buffer memory and supplied to the voice decoder 109 (step 1007).
A difference value .DELTA.VRn (packet arrival interval) between the
preceding arrival time VRn-1 and the current arrival time VRn stored in
the above-mentioned table areas and a difference value .DELTA.VTn (packet
transmission interval) between the preceding send time VTn-1 and the
current transmission time VTn are calculated and the calculated data are
stored in the table area (step 1008). Then, if n.noteq.M (step 1009), the
operation is returned to step 1002.
If the count parameter n becomes equal to M in step 1009, the distribution
of packet transmission delay time in the network is analyzed from the data
stored in the table area (step 1010), and the waiting time Tw to be
applied thereafter is determined in accordance with the state of the
analyzed distribution (step 1011). Next, the current multimedia
communication mode is judged (step 1401) and if the current communication
mode is a mode in which both the video and voice data are to be
transmitted, judgment is made as to whether the waiting time Tw (or the
transmission delay quantity) is not less than a predetermined upper limit
value L1 or not (step 1012).
If the waiting time Tw is not smaller than the limit value L1, that is, if
Tw.gtoreq.L1 in step 1012, a video data transmission stop command is
supplied to the communication management unit 204 (step 1402). If the
current communication mode is a mode in which only the voice data are to
be transmitted, on the contrary, judgment is made as to whether the
waiting time Tw is less than or equal to a predetermined lower limit value
L2 (where, L2<L1) or not (step 1403). If the waiting time Tw is not larger
than the limit value L2, that is, if Tw.ltoreq.L2 in the step 1403, a
video data transmission resume request is issued and the mode is changed
over to a communication mode in which communication of both the video data
and voice data is made (step 1404). In the case where the waiting time Tw
has a value between the limits L1 and L2, the communication mode is not
changed. After these operations, judgment is made as to whether a
communication end request has been received or not (step 1013). If a
communication end request has been received in step 1013, the
communication is ended. If no communication end request has been received
in step 1013, on the contrary, the operation is returned to step 1001 in
which the count parameter n is cleared to be zero, and then the foregoing
step 1002 et seq. are repeated.
FIG. 9 shows the relationship between the above-mentioned limit values L1
and L2 and the communication mode.
For example, assume that multimedia communication is started in a
communication mode (mode 1) in which both video and voice data are
transmitted at a point P0, then the waiting time (or delay time) Tw
becomes longer as the traffic of the network becomes larger. If the value
of Tw reaches the upper limit value L1, the video communication is
interrupted (video freeze state) at a point P2 and the mode is changed
over to another communication mode (mode 2) in which communication is made
with only voice data. This communication mode 2 is maintained in a period
where the waiting time Tw exceeds the upper limit value L1, or, unless the
waiting time Tw becomes lower than the lower limit value L2, even in the
case where the traffic of the network so decreases that the waiting time
Tw becomes equal to or lower than the upper limit value L1 at a point P3.
If the waiting time Tw reaches the lower limit value L2 at a point P1, the
communication mode is changed to the mode 1 and the video transmission is
resumed.
According to this embodiment, the receiving side communication terminal
station not only performs decoding of voice data while absorbing jitter in
delay time of voice data packets which varies in depending on the state of
traffic in the network but also changes over the communication mode in
accordance with the relation of size between the waiting time Tw for
packet delay time or for absorption of delay time jitter and the limit
values (L1 and L2). Accordingly, it is possible to control video data
transmission mode in accordance with the traffic of the network, for
maintaining the quality of communication.
FIG. 10 shows another embodiment of the data output unit 604 of the voice
data receive control unit 203B. According to this embodiment, it is made
possible that the receiving side terminal station can instruct the
transmission side terminal station to reduce the transmission quantity
with respective to not only the video data but also the voice data. The
flowchart of FIG. 10 is substituted for the part constituted by the steps
1403.about.1404 in the flowchart of FIG. 8.
That is, in the case where the current communication mode is the mode in
which communication is made with respect to only voice data (step 1401 in
FIG. 8), the average arrival interval .DELTA.VR and average send interval
.DELTA.VT are calculated from the arrival intervals
.DELTA.VRi(i=1.about.M) and send intervals .DELTA.VTi(i=1.about.M) stored
in the above-mentioned table area (steps 1901, 1902), and then the
difference value .DELTA.rt between .DELTA.VR and .DELTA.VT is obtained
(step 1903). The above-mentioned difference value .DELTA.vt corresponds to
the parameter indicating the packet transmission delay which varies in
accordance with the traffic in the network.
Next, the above-mentioned value .DELTA.vt is compared with a predetermined
threshold value THv (step 1904), and when the value .DELTA.vt is larger
than the threshold value THv, a voice data coding mode change request is
issued to the transmission side terminal station (step 1905). The voice
data coding mode change request contains the value of the above-mentioned
.DELTA.vt as a parameter for designating the coding mode. Steps 1403 and
1404 are the same as those in the case of FIG. 8.
FIG. 11 is a flowchart showing an example of the data read unit 601 of the
voice data transmission unit 203A provided with a function of coding voice
data in the mode in accordance with the above-mentioned coding mode change
request. When the traffic of the network is small (when the transmission
delay of voice packets is small), voice data is coded in a normal mode,
while when the traffic of the network is large (when the transmission
delay of voice packets is large), for exampl | | |
