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Bluetooth Bit Rate: Raw versus Actual
Original Post : how is it the bit rate? (SIG
Forum) Date: 2000-03-28
In the spec, the modulation rate is defined as 1M symbol per second
[1Ms/s]. The symbol rate equals the bitrate since the modulation within
Bluetooth is Gaussian FSK. Therefore the raw bitrate available is
1Mbit/s .
If the BT protocol is used, the highest net (actual ) rate
obtainable in asymmetrical mode is 723kbit/s (DH5 packet)
This max obtainable bitrate is lower than the raw bitrate because of
several factors:
- Bluetooth maximum of a 5 slot packet, which can carry 339 bytes of
payload (2712 cycles).
- However, this also requires a 72 cycle access code, and a 54 cycle
packet header, and a 16 cycle CRC code. This is a total of 2854 cycles for
just 339 bytes of information.
- Then we have another hop which takes up 625 cycles, before we can send
another packet.
- So for each 3750 cycles, we are transmitting only 339 useful bytes of
data.
Given that we have 1Mbit/s to play with, that's 266 packets / second, or
723,200 bits per second. This means we are wasting over 27% of our bandwidth
for acknowledgement and for frequency hopping.
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