A communication system such as this is described, for example, in German Patent Application DE 101 25 342 A1.
The Bluetooth system is based on a communication standard which was recently developed and allows data transmission based on TDD (Time Division Duplex). The individual subscribers in this communication system define a communication network, a so-called piconetwork, within which the data packets to be transmitted are transmitted by radio and over a short distance.
One special feature of a Bluetooth communication system is that the subscribers which communicate with one another do not all have equal authorization. A Bluetooth communication system comprises one, and only one, master subscriber and at least one slave subscriber. The number of active slave subscribers, that is to say slave subscribers which are involved in the data interchange is typically limited to seven. In this case, the master subscriber acts as the control instance for the active slave subscribers.
Each subscriber (host) in the Bluetooth communication system has a transmission assembly (device). In the Bluetooth communication system, communication takes place between the host and the Bluetooth device via an interface, the so-called HCI interface (Host Controller Interface), which controls the data transmission via appropriate HCI commands.
The timeslot method is normally used for data transmission, with a Bluetooth communication channel having a bit rate of 1 Mbit/s, and a communication channel being subdivided into timeslots (slots) whose length is 625 μs. Two successive slots form a Bluetooth frame, whose length is 1250 μs. Only data from one transmitter (master or slave) may be transmitted in each timeslot, and an active slave subscriber can in this case transmit data in the form of data packets to the master subscriber only when it has already received a data packet from the master subscriber.
Each subscriber in the Bluetooth communication system has its own clock supply, with data communication being governed by the clock supply for the master subscriber. Each of the active slave subscribers attempts to receive the data packets which are intended for it and, if necessary, to respond to the master subscriber in those slots in which the master subscriber is transmitting.
A further special feature of the Bluetooth communications standard is that, in principle, there are two types of communication, a synchronous data link for transmission of speech data, and an asynchronous data link for pure data transmission. The synchronous data link is referred to in the Bluetooth system as an SCO link (Synchronous Connection Orientated link), and the asynchronous data link is referred to as an ACL link (Asynchronous Connection Less link). In the case of an ACL link, the amount of data which a slave subscriber can send back as the response to a data packet received from the master subscriber is limited such that it can send back only a single data packet in response to each received data packet.
There are three different data packets, which differ from one another in the information that they contain, for transmission of speech data using SCO links. The gross amount of data is the same in all data packets (240 bits). The net amount of data in each frame differs owing to the different codings and may be 80 bits, 160 bits and 240 bits. In order to make it possible to transmit the required amount of data bidirectionally, every frame must be allocated in the case of 80-bit data packets, every alternate frame must be allocated in the case of 160-bit data packets, and every third frame must be allocated in the case of 240-bit data packets.
The described SCO links and ACL links, which are operated in the sniff mode or in the park mode, are used by the master subscriber to periodically address the active slave subscribers, in each case using constant, very short repetition time intervals, and to fill the corresponding slots. This addressing process is also referred to in the following text as allocation. The sniff mode is an operating mode of a slave subscriber within a Bluetooth communication system with a reduced operating cycle, that is to say a state with reduced activity. In particular, the slave subscriber which is in the sniff mode is active only for a certain number of frames or slots within a fixed period. For the rest of the time, the slave subscriber is not involved in the communication in the Bluetooth communication system. Like the sniff mode, the park mode is likewise an operating mode with a reduced operating cycle. Furthermore, when the slave subscriber is in the park mode, it gives up its address, thus making it possible for the master subscriber to operate a far greater number of slave subscribers in the park mode via a separate access protocol, in contrast to the sniff mode or the active operating mode.
The park and sniff operating modes can be activated once an ACL link has been set up between the master subscriber and the slave subscriber. An ACL link which is in the active operating mode or in the sniff mode is used to set up and clear an SCO link.
Parameters are defined for the sniff operating mode and for the park operating mode which govern when the state with a reduced operating cycle may be assumed, and thus when the slave subscriber is in the active state. These parameters are the period T and the phase angle D within a sniff or park period. These parameters define the start and the interval of the respective data transmission. In addition, further parameters NAttempt, NTimeout, exist for the sniff mode, which determine the number of frames after the start of a respective period T during which the slave subscriber will actively participate in the data interchange within the communication system. Parameters T, d which govern how the speech processing is embedded in a frame grid likewise exist for an SCO link, in a comparable manner to the sniff operating mode and park operating mode.
An SCO link is set up and the sniff mode and park mode are started by synchronizing the link to the respective frame grid with the aid of the parameters which have been mentioned and using an appropriate initialization method.
An SCO link is set up, and the park or sniff mode is activated, in response to a command from the HCI interface, for example. The link master receives this command and controls its implementation. In a situation where an SCO link has been activated, the period T is also supplied in the HCI command. For the park and sniff operating modes, the corresponding HCI commands do not define any value for the period T. The HCI command provides only a lower limit and an upper limit for this period T, which must not be undershot or exceeded, respectively. The subscriber never defines any requirement for the phase angle D, so that the times at which allocation takes place are undefined, within the given degree of freedom.
The above description of a Bluetooth communication system has been described, for example, in the Bluetooth specification “The Specification of the Bluetooth System V1.1”, 22 Feb. 2001. The content of this specification is also included in its entirety in the present patent application in particular with regard to the hardware-specific and software-specific configuration, the various types of link and the operating modes, as well as their parameters.
If a link to the master subscriber is now intended to be set up in addition to an already existing link between the master subscriber and a first slave subscriber for a second slave subscriber, then, in the situation where the first link is an SCO link or a link in the sniff or park mode, the link manager has a number of configuration options for setting up the second link, which it can select without contravening the various requirements which are defined by the subscriber by means of HCI commands. Degrees of freedom therefore exist for setting up a new link, although these are not used at the moment by the respective subscribers.
This problem will be explained in more detail in the following using two examples with reference to FIGS. 1 and 2: