There are many types of physical channels. All Bluetooth physical channels are characterized by a combination of frequency and time parameters at the physical layer, limited by spatial factors. Frequency hopping is used to change frequencies periodically to reduce the impact of interference and also for regulatory reasons, both for basic and adapted physical channels for Bluetooth networking (piconet).
In the BR/EDR core system, peer devices communicate using shared physical channels. To do this, their transceivers need to be tuned to the same physical layer frequency at the same time, and need to be within a suitable range of distance from each other. Due to the limited number of RF carriers and the fact that many Bluetooth devices may operate independently in the same space and time region, it is very likely that the transceivers of two independent Bluetooth devices are tuned to the same RF carrier, resulting in physical channel collisions. To reduce the adverse effects of such collisions, each transmission on a physical channel begins with an access code, which is set to be associated with the device's physical channel. The channel access code is an attribute of the physical channel. The access code is present at the beginning of each transmitted packet. Access codes are used for timing synchronization, offset compensation, paging and inquiries.
Multiple BR/EDR physical channels are defined. Each channel is optimized for a different purpose:
-
Two of the physical channels (the basic piconet channel and the adapted piconet channel) are used for communication between connected devices and are associated with a specific piconet.
-
Other BR/EDR physical channels are used for discovery (query scan channel) and connection (page scan channel) of Bluetooth devices.
-
Devices use synchronous scanning of physical channels to obtain timing and frequency information for connectionless slave broadcast physical links, or to recover the current piconet clock.
A Bluetooth device can only use one BR/EDR physical channel at any one time. To support multiple concurrent operations, the device uses time-division multiplexing between channels.
A number of types of physical channel are defined. All Bluetooth physical channels are characterized by a set of PHY frequencies combined with temporal parameters and restricted by spatial considerations. For the basic and adapted piconet physical channels frequency hopping is used to change frequency periodically to reduce the effects of interference and for regulatory reasons.
In the BR/EDR core system, peer devices use a shared physical channel for communication. To achieve this their transceivers need to be tuned to the same PHY frequency at the same time, and they need to be within a nominal range of each other. Given that the number of RF carriers is limited and that many Bluetooth devices may be operating independently within the same spatial and temporal area there is a strong likelihood of two independent Bluetooth devices having their transceivers tuned to the same RF carrier, resulting in a physical channel collision. To mitigate the unwanted effects of this collision each transmission on a physical channel starts with an access code that is used as a correlation code by devices tuned to the physical channel. This channel access code is a property of the physical channel. The access code is present at the start of every transmitted packet. Access code is used for timing synchronization, offset compensation, paging and inquiry.
Several BR/EDR physical channels are defined. Each is optimized and used for a different purpose.
* Two of these physical channels (the basic piconet channel and adapted piconet channel) are used for communication between connected devices and are associated with a specific piconet.
* Other BR/EDR physical channels are used for discovering (the inquiry scan channel) and connecting (the page scan channel) Bluetooth devices.
* The synchronization scan physical channel is used by devices to obtain timing and frequency information about the Connectionless Slave Broadcast physical link or to recover the current piconet clock.
A Bluetooth device can only use one BR/EDR physical channel at any given time. In order to support multiple concurrent operations the device uses timedivision multiplexing between the channels.
Basic piconet channel
During normal operation, basic piconet channels are used for communication between connected devices.
-
features
-
-
The basic piconet channel is characterized by pseudo-random sequence hopping through the PHY channel. The jump sequence is unique to the piconet and is determined by the Bluetooth device address of the master device. The phase of the skip sequence is determined by the Bluetooth clock of the master device. All Bluetooth devices participating in the piconet are time and frequency hop synchronized to the channel.
-
The channel is divided into time slots, and each time slot corresponds to a physical layer frequency hop. Continuous frequency hopping corresponds to different physical layer frequency hopping. The slots are numbered according to the bluetooth clock of the piconet master. Packets are transmitted by Bluetooth devices participating in a piconet and start at a slot boundary. Each packet starts with a channel access code which comes from the piconet master's bluetooth device address.
-
On basic piconet channels, the master device controls access to the channel. The master starts transmission only on even slots. The data packet transmitted by the master station is consistent with the starting point of the time slot, and determines the piconet timing. Data packets transmitted by the master can occupy up to five time slots, depending on the type of data packet.
-
-
Topology
A basic piconet channel can be shared by any number of Bluetooth devices, limited only by the resources available to the piconet master device. Only one device is piconet master, all other devices are piconet slaves. All communication is between master and slave. There is no direct communication between slave devices on a piconet channel.
However, there is a limit to the number of logical transfers that a piconet can support. This means that while there is theoretically no limit to the number of Bluetooth devices that can share a channel, there is a limit to the number of those devices that can actively exchange data with the master device.
Basic piconet channel
The basic piconet channel is used for communication between connected devices during normal operation.
Characteristics
* The basic piconet channel is characterized by a pseudo-random sequence hopping through the PHY channels. The hopping sequence is unique for the piconet and is determined by the Bluetooth device address of the master. The phase in the hopping sequence is determined by the Bluetooth clock of the master. All Bluetooth devices participating in the piconet are time- and hop synchronized to the channel.
* The channel is divided into time slots where each slot corresponds to an PHY hop frequency. Consecutive hops correspond to different PHY hop frequencies. The time slots are numbered according to the Bluetooth clock of the piconet master. Packets are transmitted by Bluetooth devices participating in the piconet aligned to start at a slot boundary. Each packet starts with the channel access code, which is derived from the Bluetooth device address of the piconet master.
* On the basic piconet channel the master controls access to the channel. The master starts its transmission in even-numbered time slots only. Packets transmitted by the master are aligned with the slot start and define the piconet timing. Packets transmitted by the master may occupy up to five time slots depending on the packet type.
Topology
A basic piconet channel may be shared by any number of Bluetooth devices, limited only by the resources available on the piconet master device. Only one device is the piconet master, all others being piconet slaves. All communication is between the master and slave devices. There is no direct communication between slave devices on the piconet channel.
There is, however, a limitation on the number of logical transports that can be supported within a piconet. This means that although there is no theoretical limit to the number of Bluetooth devices that share a channel there is a limit to the number of these devices that can be actively involved in exchanging data with the master.
Adapted piconet channel (Adapted piconet channel)
Basic piconet channels are used for normal communication between connected devices. Adapted piconet channels differ from basic piconet channels in two ways:
1, The slave device transmits at the same frequency as the master device used in the previous transmission. In other words, no frequency recalculation is required between the master and subsequent slave packets.
2, The adjusted piconet channel can be based on less than all 79 frequencies. Some frequencies can be marked as "unused" and thus excluded from frequency hopping patterns. The remainder of the 79 frequencies are included. The difference between these two sets of frequency sets is that when an unused frequency is selected by the basic pseudo-random frequency hopping sequence, a frequency replacement will be selected from the used frequencies. On the same adapted piconet channel, frequency sets used by different physical links may be different.
Adapting the piconet channel refers to a mechanism used in Bluetooth communication between connected devices. In a bluetooth device network piconet, adapting the piconet channel ensures that the slave transmits on the same frequency as the master's previous transmission. This mechanism helps to maintain synchronization and efficient communication within the piconet.
Adapted piconet channel
The basic piconet channel is used for communication between connected devices during normal operation. It differs from the basic piconet channel in two ways:
1. the frequency on which a slave transmits is the same as the frequency used by its master in the preceding transmission. In other words the frequency is not recomputed between master and subsequent slave packets.
2. the adapted piconet channel may be based on fewer than the full 79 frequencies. A number of frequencies may be excluded from the hopping pattern by being marked as “unused”. The remainder of the 79 frequencies are included. The two sequences are the same except that whenever the basic pseudo-random hopping sequence selects an unused frequency, it is replaced with an alternative chosen from the used set. The set of frequencies used may vary between different physical links on the same adapted piconet channel.
The adapted piconet channel refers to a mechanism used in Bluetooth communication between connected devices. In a piconet, which is a network of Bluetooth devices, the adapted piconet channel ensures that the frequency used by the slave device for transmission is the same as the frequency used by the preceding master device transmission. This mechanism helps maintain synchrony and efficient communication within the piconet.
Inquiry scan channel
-
In order for a device to be discovered, an inquiry scan channel is required. A discoverable device listens on its inquiry scan channel for inquiry requests and then sends a response to the request.
-
In order for a device to discover other devices, it iterates through (jumps) all possible inquiry scan channel frequencies in a pseudo-random fashion, sending inquiry requests on each frequency and listening for any responses.
Features:
-
Inquiry scan channels use a slower frequency hopping pattern and use access codes to distinguish between two co-located devices using different physical channels that occasionally occupy the same radio frequency.
-
The access codes used on the inquiry scan channel come from a set of reserved inquiry access codes shared by all Bluetooth devices. One of the access codes is for general inquiries and the other access codes are reserved for limited inquiries. Each device can access several different inquiry scan channels. Since all of these channels have the same hopping pattern, a device can simultaneously occupy more than one inquiry scan channel if it can associate multiple access codes simultaneously.
-
A device using one of the inquiry scanning channels remains passive on that channel until it receives an inquiry message from another Bluetooth device on that channel. This can be identified by the corresponding inquiry access code. Then, the query scanning device will return a response to the query device according to the query response procedure.
-
To discover other Bluetooth devices, a device sends an inquiry request using the inquiry scan channel. Since the devices to be discovered are not known in advance, the exact characteristics of the query scan channel cannot be known.
-
The device will take advantage of the characteristics of fewer frequency hopping times and slower frequency hopping speed of the query scanning channel. The query device sends a query request on each query scan hop and waits for a query response. Due to the fast transmission speed, the query device can cover all query scanning frequencies in a short time.
Topology:
The querying device and the discoverable device use a simple packet exchange to implement the query function. The topology formed during this process is a simple and ephemeral point-to-point connection.
Inquiry scan channel
* In order for a device to be discovered, an inquiry scan channel is used. A discoverable device listens for inquiry requests on its inquiry scan channel and then sends a response to that request.
* In order for a device to discover other devices, it iterates (hops) through all possible inquiry scan channel frequencies in a pseudo-random fashion, sending an inquiry request on each frequency and listening for any response.
Characteristics
* Inquiry scan channels follow a slower hopping pattern and use an access code to distinguish between occasional occupancy of the same radio frequency by two co-located devices using different physical channels.
* The access code used on the inquiry scan channel is taken from a reserved set of inquiry access codes that are shared by all Bluetooth devices. One access code is used for general inquiries, and a number of additional access codes are reserved for limited inquiries. Each device has access to a number of different inquiry scan channels. As all of these channels share an identical hopping pattern, a device may concurrently occupy more than one inquiry scan channel if it is capable of concurrently correlating more than one access code.
* A device using one of its inquiry scan channels remains passive on that channel until it receives an inquiry message on this channel from another Bluetooth device. This is identified by the appropriate inquiry access code. The inquiry scanning device will then follow the inquiry response procedure to return a response to the inquiring device.
* In order for a device to discover other Bluetooth devices it uses the inquiry scan channel to send inquiry requests. As it has no prior knowledge of the devices to discover, it cannot know the exact characteristics of the inquiry scan channel.
* The device takes advantage of the fact that inquiry scan channels have a reduced number of hop frequencies and a slower rate of hopping. The inquiring device transmits inquiry requests on each of the inquiry scan hop frequencies and listens for an inquiry response. Transmissions are done at a faster rate, allowing the inquiring device to cover all inquiry scan frequencies in a reasonably short time period.
Topology
Inquiring and discoverable devices use a simple exchange of packets to fulfill the inquiring function. The topology formed during this transaction is a simple and transient point-to-point connection.
Page scan channel ( Page scan channel )
-
A connectable device (a device that is ready to accept a connection) listens for paging requests on its page scan channel, and once received, a series of information exchanges with the device occur.
-
For a device to connect to another device, it traverses (hops) through all the page scan channel frequencies in a pseudo-random fashion, sending a page request on each frequency and listening for a response.
features
-
Page scan channels use an access code extracted from the scanning device's Bluetooth device address to identify communications on the channel. The page scan channel uses a slower hopping rate than the basic and adaptive piconet channels. The frequency hopping selection algorithm uses the Bluetooth device clock of the scanning device as input.
-
A device using the page scan channel is passive until it receives a page request from another Bluetooth device. This can be identified by page scanning the channel access code. The two devices will then follow the paging procedure to establish a connection. After successfully completing the paging procedure, both devices switch to the basic piconet channel, which is characterized by the paging device as the master.
-
To connect with another Bluetooth device, the device sends a page request using the target device's page scan channel. If the paging device does not know the phase of the paging scan channel of the target device, it cannot know the current hopping frequency of the target device. The paging device sends a page request on each page scan hop and waits for a page response. This allows for faster frequency hopping, allowing the paging device to cover all page scan frequencies in a reasonably short amount of time.
-
The paging device may have some knowledge of the target device's Bluetooth clock (either shown in a previous query transaction between the two devices, or as a result of previous participation in a piconet with the device), in which case it is able to predict the target device's paging time. Call the phase of the scan channel. It can use this information to optimize the synchronization of the paging and page-scanning process, speeding up connection establishment.
Topology
Paging and connectable devices implement paging functionality through simple data packet exchange. The topology formed during this process is a simple and ephemeral point-to-point connection.
Page scan channel
* A connectable device (one that is prepared to accept connections) listens for a page request on its page scan channel and, once received, enters into a sequence of exchanges with this device.
* In order for a device to connect to another device, it iterates (hops) through all page scan channel frequencies in a pseudorandom fashion, sending a page request on each frequency and listening for a response.
Characteristics
* The page scan channel uses an access code derived from the scanning device’s Bluetooth device address to identify communications on the channel. The page scan channel uses a slower hopping rate than the hop rate of the basic and adapted piconet channels. The hop selection algorithm uses the Bluetooth device clock of the scanning device as an input.
* A device using its page scan channel remains passive until it receives a page request from another Bluetooth device. This is identified by the page scan channel access code. The two devices will then follow the page procedure to form a connection. Following a successful conclusion of the page procedure both devices switch to the basic piconet channel that is characterized by having the paging device as master.
* In order for a device to connect to another Bluetooth device it uses the page scan channel of the target device in order to send page requests. If the paging device does not know the phase of the target device’s page scan channel it therefore does not know the current hop frequency of the target device. The paging device transmits page requests on each of the page scan hop frequencies and listens for a page response. This is done at a faster hop rate, allowing the paging device to cover all page scan frequencies in a reasonably short time period.
* The paging device may have some knowledge of the target device’s Bluetooth clock (indicated during a previous inquiry transaction between the two devices, or as a result of a previous involvement in a piconet with the device), in this case it is able to predict the phase of the target device’s page scan channel. It may use this information to optimize the synchronization of the paging and page scanning process and speed up the formation of the connection.
Topology
Paging and connectable devices use a simple exchange of packets to fulfill the paging function. The topology formed during this transaction is a simple and transient point-to-point connection.
Synchronization scan channel
In order to receive packets sent on the CSB logical transport, a device must first obtain information about the timing and frequency channel of these packets. If the device misses the Coarse Clock Adjustment notification, it needs to restore the current piconet clock. Synchronous scan channels are provided for this purpose. The scanning device listens for synchronous train data packets on the synchronous scanning channel. Once a sync train packet is received, the device can stop listening for sync train packets because it already has the timing and frequency information needed to start receiving packets sent on the CSB logical transport or to recover the piconet clock.
features
The Sync Scan Channel uses an access code derived from the Sync Train transmitter's Bluetooth device address to identify Sync Train packets on the channel. Once a sync train packet is received, the scanning BR/EDR controller can start receiving packets sent on the CSB logical transport, depending on the needs of the host and any applicable configuration files.
Topology
The topology formed during scanning is instantaneous and point-to-multipoint. There can be an unlimited number of scanning devices simultaneously receiving sync train packets from the same sync train transmitter.
Synchronization scan channel
In order to receive packets sent on the CSB logical transport, a device must first obtain information about the timing and frequency channels of those packets. If a device misses a Coarse Clock Adjustment notification, it needs to recover the current piconet clock. The synchronization scan channel is provided for these purposes. A scanning device listens for synchronization train packets on the synchronization scan channel. Once a synchronization train packet is received, the device may stop listening for synchronization train packets because it has the timing and frequency information necessary to start receiving packets sent on the CSB logical transport or to recover the piconet clock.
Characteristics
The synchronization scan channel uses an access code derived from the Bluetooth device address of the synchronization train transmitter to identify synchronization train packets on the channel. Once a synchronization train packet is received, the scanning BR/EDR Controller may start receiving packets sent on the CSB logical transport, depending on the needs of the Host and any applicable profile(s).
Topology
The topology formed during this scan is transient and point-to-multipoint. There can be an unlimited number of scanning devices simultaneously receiving synchronization train packets from the same synchronization train transmitter.
reference:
1，Overview of BR/EDR Physical Channels