Bluetooth (continuation) Piconets & Scatternets A set of Bluetooth device sharing a common channel is called a piconet. Device that is present in the center works as a master and all other devices operate asa slave and at max. Seven slaves can be active and served simultaneously by themaster. If the master needs to communicate with more than seven devices, then it willfirst be instructing active slave devices to switch to low-power park model and theninviting other parked slaves to become active in the piconet and their process will berepeated for making more & more connection (Scatternets). The idle communication achieves among small groups. When many groups of devicesneed to be active simultaneously then each group has to make its own Piconet. Theslave nodes in each piconet stay synchronized with the master clock and hopeaccording to a channel hopping sequence that is a function of the master's nodeaddress. Also probability of collision among piconet is small. But when the degree ofoverlapping is higher the performance of piconet, starts to degrade. A structure called Scatternet for inter piconet communication and it will form byinterconnecting multiple piconet. The connection is formed by bridge node that is the member of two or more piconetand it will be participating with each piconet on the time sharing basis by changing itshopping sequence it can move from one piconet to another piconet. By cycling throughall members piconet the bridge node can send and receive packet in each piconet andalso forward packet from one piconet to another piconet. A bridge node can be a slave in both piconet or be a slave in one and a master inanother. Inquiry and Paging For discovering other devices Bluetooth uses a procedure known as inquiry and then ituses paging to subsequently establish connection with them. Both inquiry and paying are a symmetric procedure. In other words, they involve theinquirer and the inquired devices to perform different actions that mean when twonodes want to establish a connection then they need to start from a different initialstate otherwise they would never discover each other. The inquiry and paging are simple operations but the frequency hopping nature of thephysical layer makes the low level detail complex. Two nodes cannot exchange messages until they agree to a common channel hopingsequence as well as the correct phase within the chosen sequence. Bluetooth solvethis problem by mandating the use of a specific inquiry-hopping sequence known toall devices.
During inquiry both nodes (listener and sender) hop using the same sequence. But thesender hops faster than the listener, transmitting a signal on each channel andlistening between transmission for an answer. When more than one listener is present their replies may collide. To avoid collisionlistener, defer their replies until expiration of a random back off timer. Eventually thesender backoff timer. Eventually the sender device collects some basic informationfrom the listeners such as device address & the clock offsets. The information issubsequently used to page the selected listener device. The communication steps during the paging procedure are similar, except that thepaging message is unicast to a selected listener, so the listener need not back offbefore replying. The sender also has a better estimate of the listener’s clock whichenables it to communicate with the listener almost instantaneously. Upon receiving anACK for the paging message. The sender become the master and the listener becomesthe slave of the newly formed piconet and both nodes switch to the piconet channelhopping sequence and further if necessary the master and slave roles can be swapped.Admitting a new slave into an existing piconet are more complex in this processmaster can either start discovering new nodes in its neighborhood and invite then tojoin the piconet or instead wait in listen state and be discovered by other nodes withboth options communication in the original piconet must be suspended for theduration of the inquiry and paging process. Low-power modes Due to limited battery Bluetooth uses different low power modes. Piconets are formedon demand when communication among devices is ready to take place. When apiconet is active, the slaves stay powered on to communicate with master. Thingpossible only a low power mode. Low power modes have been defined following: 1: Hold mode is used when a device should be put to sleep for a specified length of time. 2: Sniff mode it is used to put a slave in a low-duty cycle mode, whereby it wakes up periodically to communicate with the master. 3: Part mode it is basically used in stay synchronized with the master being an active member of the piconet. Park mode disconnect the master when its piconet have more than given slaves.