Modes of Connection Bluetooth

Bluetooth is a technology that almost every device is using in today’s world, it was the most popular and safe way of sharing data in the early times. Bluetooth uses a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks. Bluetooth holds a transmission limit of up to 2.5 milliwatts, the reason for its short-range transmission.
The state of any given Bluetooth unit (having a required power supply) can broadly be categorized as standby or connection. A device in standby state is not active in any network. A Bluetooth-enabled device in the connection state is synchronized to a piconet. Such a device can be in one of the following four modes:

• Active mode.
• Sniff mode.
• Hold mode.
• Park mode.

Active Mode

When in Active mode, a Bluetooth device actively participates in the piconet by sending and receiving data/control packets. In order to distinguish between active Slaves and parked members, each active Slave in a piconet is given a distinct 3-bit AM_ADDR (Active Member address). 

• The MAC address, AM_ADDR, remains valid so long as the Slave is engaged in the channel. 
• A Master can address a specific Slave by using values between 001 and 111 in the AM_ADDR variable, or it can broadcast a message to all Slaves by using the value 000. 
• When sending packets to the Master, a Slave includes its own AM_ADDR, enabling the Master to recognize the Slave. 
• In addition to routine transmissions, the Master schedules data transmission based on traffic demands to and from various Slaves to keep Slaves synchronized with the channel. 
• An active slave only transmits in the odd-numbered Slave-to-Master slots and listens for packets in the even-numbered Master-to-Slave slots. 
• An active Slave may go to sleep (one of the power-saving modes) if it is not addressed until the subsequent Master transmission. 
• It should be noted that, according to the most recent Bluetooth specification, AM_ADDR determines the maximum number of active Slaves in a piconet to be 7. 
• The remaining three modes reduce their duty cycle during the Connection state in order to conserve power.

Sniff Mode

The Sniff mode on devices has the highest duty cycle of the three power-saving modes or the least power efficiency. By lowering the duty cycle, or increasing the listening interval, power efficiency is attained. 

• A Slave enters sniff mode by receiving a sniff command message at the LMP level from either the Master or another slave. 
• The programmable sniff interval parameter T_sniff is present in this message. Sniff intervals can last for a few seconds or longer. 
• When there is a known long communication lag between the slave and the master, sniff mode is appropriate. 
• However, there is no assurance that devices will receive maintenance at every interval. 
• Sniff mode might be useful for extending battery life by minimizing poll packet transmission which isn't necessary.\

Hold Mode

The power efficiency of devices in Hold mode is in the middle of the three power-saving modes. The device is briefly rendered inactive to increase power efficiency. Only an internal timer in the device is operational during a Hold. 

• A device can be placed into Hold mode by the Master by itself or by the Slave specifically asking the Master to do so. 
• Both devices agree on how long the Slave should be in Hold mode before the Hold mode is activated. 
• After the unit exits Hold mode, data transfer resumes. When a device knows there won't be any communication for a certain amount of time, it can enter this mode for the duration of that time in order to serve another task, such as participating in another piconet, or to conserve energy.

Park Mode

When a slave decides to remain inactive in the piconet for a period of time that is comparatively longer than the Sniff and Hold periods, it enters Park mode. The device maintains its synchronization in the piconet while remaining uninvolved in the traffic. In order to re-synchronize, parked devices surrender their MAC (AM_ADDR address) and occasionally listen to broadcast messages from the master. Of the three power-saving modes, it offers the highest power efficiency. Devices that occasionally need to communicate with a Master can be accommodated in park mode. Following a periodic beacon sent by the Master, a Slave exits Park mode by sending a packet to the Master requesting a transition from Park to Active mode. The beacon interval is programmable. Since parked members give up their MAC addresses, each of them is given two additional addresses to help Master keep track of them.

• PM_ADDR: Parked Member Address is an 8-bit long Master-local address used for the identification of parked slaves. The PM_ADDR is valid as long as the slave is parked. PM_ADDR being 8-bits long, allows as many as 256 devices to exist simultaneously in Park mode in a single piconet.
• AR_ADDR: Access Request Address is also an 8-bit long address that is used by parked slaves to determine the Slave-to-Master half slot within the access window in which it can transmit an access request