1. Bluetooth Radio Basics
Bluetooth Radio Basics
Features, Specifications,
Protocols, and How it Works

2. Bluetooth Radio Summary
Normal Range : 10 meters
Normal Xmit Power : 1 milliWatt
Receiver Sensitivity : -70 dB
Spectrum : GHz (ISM band)
Max Data Rate : 721 kbit + 56 kbit/3 voice ch.
Bluetooth operates in the 2.402GHz GHz band. This spectrum is recognized internationally as the Industrial, Scientific, and Medical (ISM) band and is unlicensed. This means that you do not need a permit or license to operate a device in this band like you do for a radio station in the AM or FM band.
While the ISM band is freely available for anyone to use, there are rules for devices designed to operate in it. Specifically they must be of limited power (~125 milliwatts max.) and they must use some form of Spread Spectrum technology (i.e. they cannot continuously transmit across the entire ISM spectrum).
As low power was a primary Bluetooth criteria it operates at only 1mW in the typical case. This low power capability is a key factor in limiting range to 10m and the requirement for a very sensitive receiver at -70dB. It should be noted however that BT has an optional 10mW transmission mode which should increase range into the 100m range, however most products are not using this option today as it is apparently problematic to support a mixed environment of 1mW and 10mW devices.
Finally, BT is a modest performance technology with a maximum data transfer rate of only 721Kbps. While modest, this is still more than adequate for most applications except streaming video as even DSL and Cable modems (the current broadband internet standards) are typically throttled to only 384Kbps or so.

3. Bluetooth Radio Frequency Band
ISM (Industrial, Scientific, Medical) Band
2.402GHz GHz (79MHz total bandwidth)
Advantages
Free
Open to everyone worldwide
Disadvantages
Noise sources everywhere
Cordless phones, microwave ovens, garage door openers, other wireless LAN technologies, baby monitors,...
Recap about the ISM band. This spectrum is freely available worldwide with only a few basic equipment characteristics regulated (i.e. must be spread spectrum and low power). The major disadvantage of this band, especially for wireless communications, is the great number of incompatible device types that share it. These include cordless phones, microwave ovens, garage door openers, low cost wireless video surveillance systems, and numerous wireless LAN technologies. This means that the ISM band is likely to be fairly noisy and will thus impact data communications performance.

4. Bluetooth’s Noise Solutions
Frequency Hopping Spread Spectrum technology
Divides the band into 79 separate 1MHz channels
Uses short packets and makes 1600 hops/second
Minimizes exposure to noisy channels
Enables bad voice packets to be discarded
Forward Error Correction (FEC) of data packets
Data often recoverable even on a noisy channel without retransmission
Bluetooth avoids the ISM band noise issue in a number of ways:
1.) It divides the 79MHz ISM spectrum into 79 separate 1MHz channels.
2.) Each Bluetooth network randomly hops among these channels, each consuming only 1MHz of bandwidth at any given time. This means that concentrated noise sources which occupy a small segment of the spectrum have minimal impact. In addition many, many, Bluetooth networks can operate simultaneously in the limited spectrum without undue contention amongst themselves.
3.) Bluetooth frequency hopping occurs 1600 times per second (3200 for certain select operations) which limits the exposure of each transmission to potential noise sources.
4.) Rapid frequency hopping makes voice packets small enough that corrupt transmissions can simply be discarded. This has an added benefit of significantly simplifying voice transmission management at the baseband level.
5.) Data packets incorporate Forward Error Correction codes. Thus, even in noisy environments many packets can still be recovered without retransmission. In the worst case retransmission will be required, but is highly likely to occur on another channel in the spectrum and thus likely to avoid the disrupting noise source.

5. Bluetooth Transmission Protocol
Frequency Hopping with Time Division Duplexing
Transmission rapidly hops among the available channels
Transactions are divided into dedicated time slots each for the Master and the Slave
Typically odd cycles for the Master and evens for the Slaves
Terminology
Frame = a complete transmit/receive cycle
Slot = a 625 microsecond segment within a frame
The transmission protocol that Bluetooth uses is Time Division Duplexing. This means that the available bandwidth is divided into 2 segments in the time domain, one for the network Master device transmissions and one for Slaves.
The major time domain construct in the Bluetooth protocol is the FRAME. Each frame contains 2 or more transmission SLOTS which are allocated in series to Master and Slave transmissions. Slots are each 625 microseconds in duration.
These and additional concepts are illustrated on the following pages.

6. Bluetooth Transmission Protocol Frequency Hopping & Time Division Duplexing
Complete packet transmission occurs during a Slot
Master
Slave1 fk
625 ms
Slot 1
Frequency hops from Slot to Slot to Slot
Frames define matched Master / Slave Slot transmissions
fk+1
Frame 1
Slot2
fk+3
Frame 2
Slot4
fk+2
625 ms
Slot 3 t
The diagram above illustrates a typical 2 Frame Master/Slave transmission sequence. As you can see the Master transmits to the Slave in Slot 1 using a unique frequency and the Slave replies in Slot 2 at a different frequency. This process is repeated in Frame 2 and all subsequent Frames.
It is important to note that the frequency series k, k+1, k+2, k+3, etc. above IS NOT numerically sequential. Rather, these frequencies are sequential elements of a random pattern of frequencies. Further explanation of the variations on this protocol and Frequency Hopping pattern generation are covered in depth in the ESP materials.

7. Multi-Slave Transmissionfk
fk+1
fk+2
fk+3
fk+4
fk+5
Master
Slave2
Slave1 t
The Bluetooth master interleaves traffic between multiple simultaneously active slaves
Each Master can support up to 7 simultaneously active slaves
Bluetooth Master’s can support up to 7 Active Members of a Piconet (the name for Bluetooth networks). Communication among multiple devices is interleaved as illustrated above, with the Master switching dialog from device to device as necessary, but always maintaining the same basic transmit/listen Framing model.
While up to 7 Active Slaves can be connected, only the Slave addressed in the Master’s transmission participates in that particular Frame. Slaves with a different ID will simply ignore the transmission.

8. Multi-Slot Framing fk+3 Master Slave1 fk fk fk t
Frame
fk+3
Slot4
Master
Slave1 fk
625 ms
Slot 1
Slot2 fk
Slot 3 fk t
To increase bandwidth Bluetooth can aggregate multiple slots in one direction of the transmission (i.e. asymmetric transmission)
Eliminates turnaround time and reduces packet overhead
Note that frequency DOES NOT change during the multi-slot transmission
Bluetooth supports 1/1, 3/1, and 5/1 framing (example above is 3/1)
5/1 framing supports up to 721Kbps, Bluetooth’s maximum capacity
To increase performance Bluetooth supports a feature called Multi-Slot Frames in which several sequential slots are aggregated together to support a larger packet size. The example above is a 3/1 Frame in which the Master is allocated 3 sequential Slots and the Slave a single reply Slot. Bluetooth also supports 5/1 Framing which provides its maximum performance of 721Kbps in the fat channel and 57Kbps in backchannel. Multi-Slot Frames can be asymmetric in either direction.
Multi-Slot Frames improve efficiency in several ways:
They eliminate all header overhead from the 2-n Slot Playloads
They double the raw bandwidth allocated to the fat channel by giving it consecutive Slots instead of interleaved Slots
Transmission across Slot boundaries continues where timing margins normally exist

9. Point to Multi-Point Transmissionfk
fk+1
fk+2
fk+3
fk+4
fk+5
Master
Slave1
Slave2
Slave3 t
The Bluetooth Master can also simultaneously transmit to all of its active Slaves at one time
In such transmissions there can be no reverse traffic from the Slaves
Bluetooth Master’s can also support Point to Mulit-point transmissions. In this case the Master addresses the transmission to ID 0. Slave’s receiving a transmission in the Master’s Slot with this ID know that this is a point to multi-point transmission and will process it. Note that no back channel traffic is allowed in point to multi-point transmissions.

10. More on Frequency Hopping How Devices Know Where and When to Hop
Bluetooth uses the Master’s device ID to algorithmically determine the Frequency Hopping (FH) pattern
This algorithm generates a unique pattern that is quite random and exhibits an extremely long repeat cycle
In addition Slaves utilize a clock offset parameter to synchronize their pattern into alignment with the Master ID
f FH
03,23,42,71,07,54,28,13,15,32,48,79,61,25,59,08,19,26...
Native Slave Pattern = 39,47,27,12,66,47,12, 03,23,42,71,07,54,28,13,15,32,48,79,61,25,59…
Master Pattern = 03,23,42,71,07,54,28,13,15,32,48,79,61,25,59,08,19,26, 51,35,46,63…
Offset Slave Pattern = 03,23,42,71,07,54,28,13,15,32,48,79,61,25,59,08,19,26, 51,35,46,63…
This slide explains how Bluetooth devices know what frequency to hop to and when to hop. The Frequency Hopping (FH) pattern is calculated independently by each Bluetooth device in a Piconet using the Master’s Bluetooth ID as the seed. Each Bluetooth device has a 48-bit ID that is 100% unique in the world. So, no 2 devices will ever generate the same FH pattern. The FH algorithm generates a psuedo random series from the ID seed that exhibits good random behavior and has an extremely long repeat cycle.
Once devices know what hopping pattern to use there remains the problem of synchronizing themselves within this pattern. This is accomplished through the use of a clock offset parameter that allows each Bluetooth Slave to adjust their index into the FH pattern to align with that of the Master.
To facilitate these mechanisms there is a specific Frequency Hopping Synchronization (FHS) packet type defined in Bluetooth. These packets are used to communicate device IDs and clock offsets among devices during device discovery and whenever logical connections are established. This facilitates low latency connections whenever this data is recent. Synchronization data is also part of every packet ensuring that active links remain aligned as the offset is continuously updated.

11. Advantages of Bluetooth’s Architecture and Protocol
Bluetooth can support a high density of devices all within range of each other without undue contention
Transmission efficiency degrades gracefully as device density increases
The baseline 10 meter range limitation further extends device capacity
Fast hopping and short packets minimize the impact of noise on performance
The Bluetooth architecture and protocol provide a number of key benefits, particularly with regard to operating in the noisy ISM band. These include:
Bluetooth can support a high density of devices in close proximity without undue bandwidth contention
Transmission efficiency degrades very gracefully as device density increases and as the the environment gets noisier
The short 10m nominal range minimizes bandwidth contention among devices due to the simple fact that most of the time not too many other devices will be around to compete with given such a short range
The fast hopping speed keeps packet sizes small and this minimizes the impact of noise induced corruption. Voice packets are small enough that bad ones can simply be discarded with a marginal impact on transmission quality. The small packet size also minimizes the transmission efficiency hit when data packets must be re-transmitted.
Data packets use Forward Error Correction which can often recover data even in a noisy environment

12. Frequency Hopping Graphically Illustrated
Each channel can carry a separate
Bluetooth transmission without contention
Transmission Channel (1 - 79)
Transmission Slot (time)
The following slides illustrate what the psuedo random frequency hopping of Bluetooth looks like over 100 Slots of transmission time with a variety of Piconet densities. The vertical axis in this graph represents the 79 available channels in the ISM band. The horizontal axis represents 100 transmission slots. A tic is plotted on the slide showing the channel each Piconet uses for each and every slot. Collisions where 2 or more Piconets use the same channel at the same time are plotted as circles.
It is important to note that each Piconet can support up to 8 devices each. Thus an environment with 6 active Piconets could actually represent an occasion where 48 different Bluetooth devices are operating in close proximity. This is probably an extreme assumption, but it is feasible.

13. Each Bluetooth Piconet Randomly Changes Frequency Slot by Slot by Slot
This slide illustrates a single active Piconet. In this example we can see that Piconet A transmits on channel 33 in Slot 1, channel 2 in Slot 2, channel 54 in Slot 3, channel 9 in Slot 4, channel 68 in Slot 5, etc. ending on channel 12 in Slot 100.
Obviously with only 1 active Piconet there is no contention and transmission efficiency is 100% assuming no extraneous ISM band noise. This is summarized across the bottom of the slide.

14. Frequency Hopping Minimizes Exposure to Data Loss Due to Noise7
~93%
5MHz noise source
5MHz noise source
This slide illustrates how noise does not have a catastrophic effect on Piconet performance. In this example the yellow band represents a 5MHz noise source in the middle of the Bluetooth spectrum as one might find in close proximity to an operating microwave oven. As you can see Bluetooth’s random frequency hopping allows most of the transmissions to simply avoid the noise. Further, Bluetooth’s Forward Error Correction protocol for data packets can often allow successful transmission despite the presence of noise on the channel. 7
~93%

15. Frequency Hopping With Multiple Piconets Each Piconet Uses a Unique Frequency Hopping Pattern
Four active piconets
400 transmission slots
10 collisions
20 slots corrupted
~95% net efficiency
This slide illustrates four simultaneously active Piconets in the same location. As you can see each Piconet’s independently random frequency hopping pattern allows such operation with little to no contention.

16. Bluetooth Piconets Degrade Gracefully with Density...
Ten active piconets
1000 transmission slots
56 collisions
112 slots corrupted
~89% net efficiency
As Piconet density increases Bluetooth transmission efficiency degrades gracefully. In this example we have expanded out to ten active Piconets and we’re still near 90% in throughput efficiency. It is important to note that this performance assumes 100% use of transmission capacity by each piconet, thus insuring that every collision would result in a potential error. In reality this is a worst case assumption and efficiency would likely be even higher.

17. ...And Maintain Reasonable Performance Even In High Densities
Twenty active piconets
2000 transmission slots
210 collisions
420 slots corrupted
~79% net efficiency
Finally, 20 simultaneous active Piconets bring us down to 80%. Again however, note that this is a worst case condition. If each net is only running at 50% of its maximum capacity for instance, than 50% of the contentions will also likely disappear. And, as some connections will be more robust than others (due to closer proximity and therefore better signal strength for instance), a significant portion of contended transactions will still be recoverable through FEC without retransmission. Thus even in a highly dense environment such as this high 80s to low 90s efficiencies are not necessarily out of the question.