1. Field of the Invention
The present invention relates to techniques of transferring control rights of communication among a plurality of communication devices.
2. Description of the Related Art
IEEE802.15.3 (see IEEE Std. 802.15.3-2003, IEEE, 2003) is a Medium Access Control (MAC) standard for Wireless Personal Area Networks (WPANs) . This standard is specialized for WPANs and has an advantage in enabling simplification of the configuration of network devices, as compared with local area network (LAN) standards.
According to a wireless communication system conforming to the IEEE802.15.3 standard, in each piconet including one or more devices, one device serving as a control device is present and controls traffic in the piconet. However, because it is assumed that devices relatively frequently join and leave the piconet, the control device is not fixedly provided therein. A control rights transfer process of transferring control rights to a piconet device differing from the current control device is performed, for example, in a case where a piconet device, which is superior in functions and performance, such as the number of controllable devices, to a current control device, newly joins the piconet, or where the current control device leaves the piconet for some reason. The piconet can continuously be maintained by performing the control rights transfer process. The control rights transfer process is described in detail in Section 8.2.3 “PNC Handover” in IEEE Std. 802.15.3-2003, IEEE, 2003.
However, when performing the process of transferring control rights to a device, which is determined according to the function and the level of performance, as described above, in many instances the wireless communication system has encountered a problem that a piconet device may be unable to continue to join the piconet.
FIG. 1 illustrates a positional relationship among a piconet coordinator (PNC) 101, which is a piconet device enabled to serve as a control device, and other piconet devices (DEVs) 111 to 114, which do not serve as a control device, in a piconet. The PNC 101 controls/manages the DEVs 111 to 114 in the piconet.
In FIG. 1, a closed curve 150 represents a range in which the DEVs 111 to 114 can be located so that the PNC 101 can communicate with each of the DEVs 111 to 114. That is, in the range 150, each of the DEVs 111 to 114 can communicate directly with the PNC 101. However, it is not sure whether each of the DEVs 111 to 114 can communicate directly with any one of the other DEVs 111 to 114. For example, it is assumed that the DEVs 112 and 113 shown in FIG. 1 cannot communicate directly with each other because of signal attenuation due to the distance between them.
In a case where the PNC 101 is changed, by changing an operation mode, to a DEV, which is unable to control the other DEVs, it is necessary to transfer control rights (herein after referred to as “PNC handover”) to one of the DEVs 111 to 114. For example, it is assumed that the possibilities of communication between the devices provided in the piconet are classified according to levels as a security countermeasure. Because the PNC can communicate with all of the other devices in the piconet which the PNC joins, the PNC handover is needed in a case where it is necessary to reduce a security level of the PNC (e.g., a case where a person other than an owner of the PNC temporarily uses the PNC) . Also, in a case where the PNC finishes an operation and leaves the piconet, the PNC handover is needed. At that time, first, a DEV (hereinafter referred to as a PNC candidate) to be operated as a PNC, is selected as a DEV to which control is transferred. In a case where a plurality of PNC candidates is present, a DEV having the highest level function and performance is selected.
In a case where the DEVs 113 and 114 are PNC candidates and the DEV 113 is higher in function and performance than the DEV 114, the PNC 101 selects the DEV 113 as a DEV to which control rights are transferred. Subsequently, the PNC handover is performed. Thus, the DEV 113 and the PNC 101 change operation modes, so that the DEV 113 is changed to a PNC and the PNC 101 is changed to a DEV.
FIG. 2 illustrates a positional relationship among a PNC and DEVs after the PNC handover is performed. The PNC 201 and the DEV 215 correspond to the DEV 113 and the PNC 101 shown in FIG. 1, respectively. A closed curve 250 represents a communicatable range in which the DEVs can be located so that the PNC 201 can communicate with each of the DEVs. That is, the DEV 112 is out of the communicatable range of the PNC 201. Therefore, the DEV 112 is forcibly disconnected from the piconet, and cannot continue to join the piconet.