|
Piconet Topology
Any time a Bluetooth wireless link is formed, it is within the context of a piconet. A piconet consists of two or more devices that occupy the same physical channel (which means that they are synchronized to a common clock and hopping sequence). The common (piconet) clock is identical to the Bluetooth clock of one of the devices in the piconet, known as the master of the piconet, and the hopping sequence is derived from the master’s clock and the master’s Bluetooth device address. All other synchronized devices are referred to as slaves in the piconet. The terms master and slave are only used when describing these roles in a piconet. Within a common location a number of independent piconets may exist. Each piconet has a different physical channel (that is a different master device and an independent piconet clock and hopping sequence).
A Bluetooth enabled device may participate concurrently in two or more piconets. It does this on a time-division multiplexing basis. A Bluetooth enabled device can never be a master of more than one piconet. (Since the piconet is defined by synchronization to the master’s Bluetooth clock it is impossible to be the master of two or more piconets.) A Bluetooth enabled device may be a slave in many independent piconets.
A Bluetooth enabled device that is a member of two or more piconets is said to be involved in a scatternet. Involvement in a scatternet does not necessarily imply any network routing capability or function in the Bluetooth enabled device. The Bluetooth core protocols do not, and are not intended to offer such functionality, which is the responsibility of higher level protocols and is outside the scope of the Bluetooth core specification.
Logical transports, logical links and L2CAP channels are used to provide capabilities for the transport of data.
Operational Procedures and ModesThe typical operational mode of a Bluetooth enabled device is to be connected to other Bluetooth enabled devices (in a piconet) and exchanging data with that Bluetooth enabled device. As Bluetooth wireless technology is an ad-hoc wireless communications technology there are also a number of operational procedures that enable piconets to be formed so that the subsequent communications can take place. Procedures and modes are applied at different layers in the architecture and therefore a device may be engaged in a number of these procedures and modes concurrently.
Top
Inquiry (Discovering) ProcedureBluetooth enabled devices use the inquiry procedure to discover nearby devices, or to be discovered by devices in their locality.
The inquiry procedure is asymmetrical. A Bluetooth enabled device that tries to find other nearby devices is known as an inquiring device and actively sends inquiry requests. Bluetooth enabled devices that are available to be found are known as discoverable devices and listen for these inquiry requests and send responses. The inquiry procedure uses a special physical channel for the inquiry requests and responses.
Both inquiring and discoverable devices may already be connected to other Bluetooth enabled devices in a piconet. Any time spent inquiring or occupying the inquiry scan physical channel needs to be balanced with the demands of the QoS commitments on existing logical transports.
The inquiry procedure does not make use of any of the architectural layers above the physical channel, although a transient physical link may be considered to be present during the exchange of inquiry and inquiry response information.
Paging (Connecting) Procedure
The procedure for forming connections is asymmetrical and requires that one Bluetooth enabled device carry out the page (connection) procedure while the other Bluetooth enabled device is connectable (page scanning). The procedure is targeted, so that the page procedure is only responded to by one specified Bluetooth enabled device.
The connectable device uses a special physical channel to listen for connection request packets from the paging (connecting) device. This physical channel has attributes that are specific to the connectable device, hence only a paging device with knowledge of the connectable device is able to communicate on this channel.
Both paging and connectable devices may already be connected to other Bluetooth enabled devices in a piconet. Any time spent paging or occupying the page scan physical channel needs to be balanced with the demands of the QoS commitments on existing logical transports.
Connected ModeAfter a successful connection procedure, the devices are physically connected to each other within a piconet. This means that there is a piconet physical channel to which they are both connected, there is a physical link between the devices and there are default ACL-C and ACL-U logical links. When in the connected mode it is possible to create and release additional logical links, and to change the modes of the physical and logical links while remaining connected to the piconet physical channel. It is also possible for the device to carryout inquiry, paging or scanning procedures or to be connected to other piconets without needing to disconnect from the original piconet physical channel.
Additional logical links are created using the link manager that exchanges link manager protocol (LMP) messages with the remote Bluetooth enabled device to negotiate the creation and settings for these links. Default ACL-C and ACL-U logical links are always created during the connection process, and these are used for LMP messages and the L2CAP signaling channel respectively.
It is noted that two default logical links are created when two units are initially connected. One of these links (ACL-C) transports the LMP control protocol and is invisible to the layers above the link manager. The other link (ACL-U) transports the L2CAP signaling protocol and any multiplexed L2CAP best-effort channels. It is common to refer to a default ACL logical transport, which can be resolved by context, but typically refers to the default ACL-U logical link. Also note that these two logical links share a logical transport.
During the time that a slave device is actively connected to a piconet there is always a default ACL logical transport between the slave and the master device. There are two methods of deleting the default ACL logical transport. The first method is to detach the device from the piconet physical channel, at which time the entire hierarchy of L2CAP channels, logical links and logical transports between the devices is deleted.
The second method is to place the physical link to the slave device in the park state, at which time it gives up its default ACL logical transport. This is only allowed if all other logical transports have been deleted (except for the ASB logical transport that cannot be explicitly created or deleted). It is not allowed to park a device while it has any logical transports other than the default ACL and ASB logical transports.
When the slave device physical link is parked, its default ACL logical transport is released and the ASB logical transport is replaced with a PSB logical transport. The ACL-C and ACL-U logical links that are multiplexed onto the default ACL logical transport remain in existence but cannot be used for the transport of data. The link manager on the master device restricts itself to the use of LMP messages that are allowed to be transported over the PSB-C logical link. The channel manager and L2CAP resource manager ensure that no L2CAP unicast data traffic is submitted to the controller while the device is parked. The channel manager may decide to manage the parking and unparking of the device as necessary to allow data to be transported.
Top
Hold ModeHold mode is not a general device mode but applies to unreserved slots on the physical link. When in this mode, the physical link is only active during slots that are reserved for the operation of the synchronous link types SCO and eSCO. All asynchronous links are inactive. Hold modes operate once for each invocation and are then exited when complete, returning to the previous mode.
Sniff ModeSniff mode is not a general device mode but applies to the default ACL logical transports. When in this mode, the availability of these logical transports is modified by defining a duty cycle consisting of periods of presence and absence. Devices that have their default ACL logical transports in sniff mode may use the absent periods to engage in activity on another physical channel, or to enter reduced power mode. Sniff mode only affects the default ACL logical transports (i.e. their shared ACL logical transport), and does not apply to any additional SCO or eSCO logical transports that may be active. The periods of presence and absence of the physical link on the piconet physical channel is derived as a union of all logical transports that are built on the physical link.
Note that broadcast logical transports have no defined expectations for presence or absence. A master device should aim to schedule broadcasts to coincide with periods of physical link presence within the piconet physical channel, but this may not always be possible or practical. Repetition of broadcasts is defined to improve the possibilities for reaching multiple slaves without overlapping presence periods. However, broadcast logical transports cannot be considered to be reliable.
Parked StateA slave device may remain connected to a piconet but have its physical link in the parked state. In this state the device cannot support any logical links to the master with the exception of the PSB-C and PSB-U logical links that are used for all communication between the piconet master and the parked slave. When the physical link to a slave device is parked this means that there are restrictions on when the master and slave may communicate, defined by the PSB logical transport parameters. During times when the PSB logical transport is inactive (or absent) then the devices may engage in activity on other physical channels, or enter reduced power mode.
Role Switch ProcedureThe role switch procedure is a method for swapping the roles of two devices connected in a piconet. The procedure involves moving from the physical channel that is defined by the original master device to the physical channel that is defined by the new master device. In the process of swapping from one physical channel to the next, the hierarchy of physical links and logical transports are removed and rebuilt, with the exception of the ASB and PSB logical transports that are implied by the topology and are not preserved. After the role switch, the original piconet physical channel may cease to exist or may be continued if the original master had other slaves that are still connected to the channel.
The procedure only copies the default ACL logical links and supporting layers to the new physical channel. Any additional logical transports are not copied by this procedure, and if required this must be carried out by higher layers. The LT_ADDRs of any affected transports may not be preserved as the values may already be in use on the new physical channel.
If there are any QoS commitments or modes such as sniff mode on the original logical transports, then these are not preserved after a role switch. These must be renegotiated after the role switch has completed.
Enhanced Data RateEnhanced data rate (EDR) is a method of extending the capacity and types of Bluetooth packets for the purposes of increasing the maximum throughput, providing better support for multiple connections, and lowering power consumption, while the remainder of the architecture is unchanged.
EDR may be selected as a mode that operates independently on each logical transport. Once enabled, the packet type bits in the packet header are interpreted differently from their meaning in basic rate mode. This different interpretation is clarified in conjunction with the logical transport address field in the header. The result of this interpretation allows the packet payload header and payload to be received and demodulated according to the packet type. EDR can be enabled only for ACL-U, eSCO-S logical transports and cannot be enabled for ACL-C, SCO-S, and the broadcast logical transports.
Top
|