In recent years, as accompanied with development of a communication technology, a wireless communications system using wireless communications is utilized so as to eliminate bothersome wiring for constructing a LAN (Local Area Network) and so as to attain communication connection between (i) an AV source, such as a tuner, and (ii) an AV playback device, such as a display, both of which are provided at a distance from each other.
Meanwhile, proposed as one means of attaining Internet connection is the use of a power line communications system utilizing an existing power line. Such a power line communication is a promising next generation communications system.
However, the wireless communications system suffers from such a problem that: the longer a transmission distance becomes in the wireless communications, the more easily the wireless communications system is influenced by multi-path phasing. On the other hand, the power line communications system is susceptible to noise generated from a home electric appliance. These problems apt to cause data loss in a case where real-time stream data such as video are transmitted under deteriorated communications environment, resulting in the occurrence of video disturbance. Accordingly, a user cannot enjoy viewing the stream data comfortably. Required for solving such a problem in each of the wireless communications system and the power line communications system is control for securing the QoS (Quality of Service) indicated by a transmission error rate, jitter (delay information), and the like.
One method for restraining such video disturbance is a video rate control technique by which a compression ratio of video to be transmitted is changed according to dynamically changing communications environment. The video rate control technique is a method for adjusting an amount of data to be actually transmitted. The adjustment is carried out in the following manner. When the communications environment deteriorates, the stream data to be transmitted is encoded at a high compression ratio (at a low encoding rate). On the other hand, when the communications environment becomes good, the stream data to be transmitted is encoded at a low compression ratio (at a high encoding rate).
The video rate control technique has (i) a translation technique of re-encoding stream data which has been already encoded at arbitrary compression ratio at another compression ratio and (ii) an encoding technique of encoding analogue-inputted stream data at arbitrary compression ratio.
The “deterioration of the communications environment” indicates such a state that an effective bandwidth of a transmission channel is narrow. As such, under deteriorated communications environment, reduction of the amount of data to be transmitted alleviates the video disturbance.
A specific example of a publicly known technique using such a video rate control technique is a communications control method disclosed in Patent document 1 which will be described later. The communications control method uses the RTP (Real-time Transport Protocol) and the RTCP (RTP Control Protocol), and is such a method as follows. That is, a receiving station measures the jitter and the packet loss rate, and transmits, to a transmitting station, information indicative of the jitter and the packet loss rate. The transmitting station thus receiving the information predicts the transmission channel conditions so as to adjust the bit rate of the data to be transmitted.
Further, later-described Patent document 2 discloses another communications control method. The communications control method is carried out in the following manner. That is, a receiving station measures a reception level so as to calculate a required transmission rate. The receiving station carries out this operation with respect to the n-number of samples so as to find an average value of the required transmission rate. Then, the receiving station gives a contents rate change instruction to the transmitting station in accordance with a past record. The transmitting station alters contents rate in accordance with the contents rate change instruction.
As such, each of Patent documents 1 and 2 discloses the technique for adjusting the rate of the stream data so as to restrain the video disturbance even when the effective bandwidth of the transmission channel is narrowed. Meanwhile, there are further alternative methods (1) and (2) for restraining such video disturbance: (1) a control method of securing a stable transmission channel, and (2) a control method of increasing the effective bandwidth that was narrowed due to the deterioration of the transmission channel conditions.
A specific example of a wireless LAN specification taking the QoS into account is the IEEE 802.11 specification specified by the TGe (Task Group E) (hereinafter, referred to as “IEEE 802.11e”). See Non-patent document 1.
The IEEE 802.11e employs (i) a distribution communication control method “EDCA (Enhanced Distributed Channel Access)”, and (ii) a central communication control method “HCCA (Hybrid Coordinator Function Controlled Channel Access)”. The communications control carried out in accordance with the EDCA method allows communication stations to be granted a transmission right at a priority-based probability. On the other hand, the communication control carried out in accordance with the HCCA method allows the communication stations to be accessible to the communication channel under control of an HC (Hybrid Coordinator). It is preferable to secure the bandwidth by communications control of the HCAA, in transmitting the stream data. In cases where the bandwidth is managed by the HC as such, a certain communication bandwidth is securely allocated to the transmitting station for a predetermined time period. This allows for stable transmissions.
However, even when the bandwidth is secured in accordance with the HCCA method, an error can occur frequently in wireless communications due to (i) the hidden node problem occurring in cases where the communication stations are too distant away from each other to communicate with each other; and (ii) limitation in a communication distance. In this case, such a transmission error can be reduced by altering error resilience to a higher one in the data transmission. This makes it possible to increase the effective bandwidth.
Examples of means for altering the error resilience include: (i) a method of applying, to the transmission data, an error correcting code (FEC) such as a Read Solomon code or a turbo code; (ii) a method of changing types of transmission rate in a physical layer (hereinafter, referred to as “PHY rate”). For example, in the IEEE802.11a that is a wireless LAN standard, the PHY rate is determined according to a combination of (i) a modulation scheme such as the BPSK, the QPSK, and the QAM, and (ii) a convolution encoding ratio.
However, strengthening the error resilience and increasing the communications bandwidth are in a trade-off relation. In other words, the communications bandwidth decreases when the error resilience is strengthened under good channel conditions.
(Patent Document 1)
Japanese Unexamined Patent Publication No. 204278/2002 (Tokukai 2002-204278; published on Jul. 19, 2002)
(Patent Document 2)
Japanese Unexamined Patent Publication No. 153610/2004 (Tokukai 2004-153610; published on May 27, 2004)
(Non-Patent Document 1)
IEEE STD 802.11e Draft10.0 September 2004
According to the method disclosed in each of Patent documents 1 and 2, the receiving station predicts the conditions of the band in accordance with (i) information on the jitter and the packet loss rate or (ii) the reception level. The receiving station gives, to the transmitting station, feedback with specially created packets including the information, and the transmitting section carries out the video rate control. This allows for high-quality video transmission. However, in this method, the receiving station gives, to the transmitting station, the feedback about the information on channel conditions measured by the receiving station. This causes delay until the transmitting station performs the video rate control after receipt of the feedback information. Therefore, in a case when the conditions of communications channel rapidly change due to an environmental change such as passage of obstacles including human beings and devices through the channel, there could occur a delay in tracking with the video rate control with respect to the deterioration of communications environment. In addition, there could occur such a state that the transmitting station is unable to grasp the communications environment because the feedback information itself does not reach the transmitting station due to the deterioration of communications environment.
Further, it is not easy for the transmitting station to predict the conditions of the band in accordance with (i) the jitter and packet loss rate measured by the receiving station or (ii) the reception level. Specifically, under deteriorated communications environment, it is easy to predict how much amount of data should be reduced from the amount of currently transmitting data in accordance with the proportion of the amount of data successfully received by the receiving station to the amount of data transmitted by the transmitting station. In other words, it is easy to predict how much the current video rate should be decreased.
However, in a case where the communications environment recovers and the amount of data transmitted by the transmitting station becomes equal to the amount of data successfully received by the receiving station, it is not easy to predict how much amount of data should be increased from the amount of currently transmitting data, i.e. how much the current video rate should be decreased. In this case, a technique of increasing the rate little by little in reference to a table or the like is generally adopted. Such a technique causes the problem of a delay in tracking with the video rate control.
This requires the receiving station to have (i) a mechanism which measures the channel conditions, and (ii) a mechanism which gives, to the transmitting station, the feedback about the channel conditions. Accordingly, a circuit structure of the receiving station becomes complex. This causes cost increase in the receiving station.
In the meanwhile, the HCCA communication control disclosed in Non-patent document 1 is effectively used to secure a stable communication bandwidth. Moreover, when an error occurs frequently due to the hidden node problem and/or the limitation in the communication, distance, the communication control by the HCCA is carried out more strictly, and the error resilience in the data transmission is strengthened, with the result that the effective bandwidth can be increased. However, as disclosed in each of Patent documents 1 and 2, a present technique of controlling the error resilience is generally the technique of controlling the error resilience in response to the feedback given from the receiving station to the transmitting station about the information on (i) the jitter and packet loss rate or (ii) the reception level, each of which is measured by the receiving station. Therefore, as is the case with the video rate control operation, it is not easy to predict the extent to which the error resilience can be lowered when the communications environment becomes good.
Further, at present, only a few communications devices adopt the HCCA communication control to secure the bandwidth for transmission. Predominant are communications devices performing communications without securing the communications bandwidth. For this reason, control needs to be performed in view of not only (i) the communications with a bandwidth secured under the HCCA communication control, but also (ii) the communications by the EDCA or the like, described in the IEEE802.11e, where a communications right is granted at a priority-based probability.
Further, the control in each of the aforementioned conventional methods is carried out in such a manner that the receiving station predicts the conditions of the band, and the transmitting station performs controls, tracking the change of the predicted conditions of the band. However, the radio channel is such a medium that the bandwidth is changed rapidly, so that it is not always expedient that the encoding rate (video rate) is altered according to the change of the bandwidth of the radio channel. For example, see the case where the encoding rate is drastically slowed down according to a sudden deterioration of the band of the radio channel. Such sudden slowing-down causes apparent video image deterioration. However, the deterioration in the radio channel is likely to be recovered to the original state after a certain time passes. For example, in cases where there is a relatively long time limit within which the stream data must be reproduced at a receiving station (valid period of the stream data), good control can be possibly carried out after observing for a while whether or not the conditions of the radio channel are recovered to the original state. In contrast, in cases where there is a short time limit within which the transmitted stream data must be reproduced at a receiving station (valid period of the stream data), the video image deterioration possibly occurs when the encoding rate is not drastically slowed down according to the deterioration of the radio channel.