1. Field of the Invention
The present invention relates to a Bluetooth system and a communicating method thereof, and more particularly, to a Bluetooth system capable of communicating data with a plurality of different communication devices on a scatternet and a communicating method thereof.
2. Description of the Related Art
Bluetooth is the code name of a radio data communication technology in use for telecommunication, networking, computing and consumer product industries. Bluetooth technology allows wireless link to replace messy cables for connecting devices within a short range. For example, if Bluetooth radio technology is embodied in both of a portable telephone and a laptop computer, it is possible to use the portable telephone and the laptop computer as linked without cables. Devices which can be a part of a Bluetooth system include a printer, a Personal Digital Assistant (PDA), a desktop computer, a FAX, a keyboard and a joy stick together with all digital devices in practice.
In general, Bluetooth technology has the maximum data transmission rate of 1 Mbps and the maximum transmission distance of 10 m. 1 Mbps is the transmission rate in the range of 2.4 GHz Industrial Scientific Medical (ISM) frequency bandwidth which is available for a user without a license, and can be realized at low-cost. Further, the transmission distance of 10 m is determined sufficient as a transmission distance between a device carried by the user and a PC on a desk in an office.
Further, since Bluetooth is devised to operate in a radio frequency environment having a large quantity of noise, Bluetooth adopts the frequency hopping technique with a hopping rate up to 1600 hops/s so as to stably transmit/receive data in a radio frequency with a large quantity of noise. The frequency hopping technique is also called Frequency Hopping Spread Spectrum (FHSS) technique. The FHSS technique primarily divides a given frequency band into more number of hopping channels, and allocates (medium frequency) signals which are first modulated in a transmitting side to hopping channels different from one another according to a predetermined order when the signals are frequency-converted into a Radio Frequency (RF) bandwidth (2.4 GHz). The channels to which the signals are allocated are changed at a high rate to reduce effects of multi-channel interference and narrow-band impulse-based noise. A receiving side connects the signals received in several hopping channels as scattered into the same order as in the transmitting side to restore the original signals. The IEEE 802.11 uses 79 hopping channels, which are distanced from one another with an interval of 1 MHz. When the signals are allocated hopping the several channels, any two adjacent channels have an interval of at least 6 MHz for avoiding mutual interaction therebetween. The rate of changing the hopping channels, i.e. hopping rate, is regulated at least 2.5 hops/s.
The Bluetooth system supports not only one-to-one link but also one-to-multi link. In the Bluetooth system, as shown in FIG. 1, several communication devices can be organized and linked together about one communication device. The communication devices each are discriminated according to frequency hopping orders different from one another. The communication devices are linked as above to constitute a network so-called piconet 10. The piconet 10 means a component unit of a Bluetooth system which is constituted by connecting at least one slave 13 to one master 11. One piconet 10 can have one master 11 and the maximum 7 slaves 13. The master 11 determines the overall characteristics about channels in the piconet 10. The master 11 has a Bluetooth Device Address (BD_ADDR) for determining the frequency hopping sequence and the channel access code. The clock of the master 11 determines the phase of the hopping row and sets the timing of the same. Further, the master 11 controls traffic in the channels. Any digital devices can function as the master 11, and after the piconet 10 is constituted, the master 11 and the slave 13 can exchange their roles again.
The master 11 and the slave 13 carry out bilateral communication according to Time Division Duplex (TDD) in the unit of 1 hopping slot (625 μs=1/1600 second).
FIG. 2 shows communication between the master and the slaves via TDD. Referring to FIG. 2, the channels allocated to time slots each have a length of 625 μs. The number of the time slots is determined according to the Bluetooth clock of the piconet master. The master and the slaves can selectively transmit a packet via the time slots. The master transmits the packet only in time slots designated with even numbers, and the slaves transmit the packet only in time slots designated with odd numbers. Further, it is necessary to realize the packet transmitted by the master or the slaves in 5 or less time slots. Herein, the packet means the unit of data transmitted in the piconet channel.
When the Bluetooth system is linked, the master can be operated in several power saving modes such as a hold mode, a sniff mode and a park mode. The hold mode means a mode in which the master converts into a sleep state while possessing an Active Member Address (AM_ADDR) as linked to the slave. The sniff mode means a mode in which the interval of listen is prolonged while the master possesses the AM_ADDR as linked to the slave. The park mode means a mode in which the mater converts into the sleep state by opening the AM_ADDR as linked to the slave. Before transfer into the park mode, a Parked Member Address (PM_ADDR) or an Access Request Address (AR_ADDR) is given from the master.
The AM_ADDR is expressed as a member address and used for discriminating active members participating in the piconet. When at least two slaves link to one master, the master allocates a preliminary 3 bit address which will be used until the slaves each are activated in order to discriminate the slaves. Therefore, all packets exchanged between the master and the slaves carry the AM_ADDR. The AM_ADDR of the slave is used not only in a packet from the master to the slave but also in a packet from the slave to the master. A pre-allocated AM_ADDR is resigned when the slave is not linked to the master or the slave is in the park mode, and a new AM_ADDR is allocated when the slave is linked to the master again. The piconet is restricted into one master and seven slaves since the AM_ADDR allocated to the activated slaves by the master is designated 3 bit long according to Bluetooth standard. In other words, the piconet has the maximum eight addresses, but “000” address is used by the master for broadcasting the slaves, and the remaining seven addresses from “001” to “111” are used by the slaves.
A communication device in one piconet can carry out data communication with any communication devices in another piconet. A plurality of piconets are organically linked to constitute a network so-called scatternet.
The scatternet is comprised of at least two piconets which are gathered to constitute a new network where each of the piconets maintains its unique hopping frequency. Further, it is requested that communication devices participating in constitution of the scatternet can be inevitably converted between the scatternet communication and the piconet communication.
However, when a Bluetooth system belongs to the scatternet, the following situations may take place: The Bluetooth system may operate as a master while operating as a slave at the same time or operate as a slave to any master while operating as a slave to another master at the same time. In other words, the Bluetooth system is necessarily capable of transmitting/receiving data into/from at least two external devices respectively belonging to different piconets while preventing collision between the transmitted data.