A bottom up approach is ideal for understanding BT, grasping the aspects of physical medium will clarify how data bits are transmitted over the air and how various abstractions can be enforced over this elementary framework. Two primary attributes which defines the physical layer are explained below:
1. Frequency Usage
BT employs frequency ranging from 2400MHz to 2483.4 MHz, the Unlicensed 2.4GHz ISM, in a typical wireless communication scenario there could be other devices in the vicinity operating with the same frequency and this can cause interference on the BT channel. To overcomes such noise BT articulates a frequency hopping scheme where the particular frequency of communication is switched every 625uS (but it can never switch outside of the 2.4GHz unlicensed spectrum), this unique pattern of hopping is mutually agreed by the devices during the connection establishment.
Across all the possible frequencies there are multiple modulation schemes, and this determines the speed of the transmission over the air. For example BT supports following types of modulations.
- BDR scheme (Basic Data Rate Scheme has over the air speed of 1MBPS) and
- EDR Scheme (Enhanced Data Rate Scheme has over the air speed of 2 and 3 MBPS).
Packets transmitted over the air could belong to any one of these two modulation schemes and the header tells the receiver about the scheme used, note that header is always send in BDR scheme.
Physical Channel Transmission
We know how frequency and modulation schemes are used for encoding data over the air but there has to be an efficient way for utilizing the medium too. With Ethernet we have CSMA but BT works in a slightly different manner.
- Similar to wired protocols like SPI and I2C we have a master slave relationship between the BT devices, its pure hegemony wherein the master controls the medium and decides when a slave can transmit.
- The physical medium is also divided into time slots of 625 uS and each slot is reserved for either the master or the slave, an illustration is done in the diagram below.
Master Tx always happens over a even numbered slot and slave Tx over an odd one. In the above graphical illustration we can see the following transmissions
- Master and slave exchange single slot packets in the first 3 slots.
- After which slave sends a three slot packet for which master sends a five slot packet response.
- Finally the slave sends another three slot packet.
Each BT device will have a 28 bit clock running in its controller which can identify the 625uS boundaries and initiate a Transmission (Tx) or a Reception (Rx). Data transmission happen using different packet structures, some of these packet types occupy more than one slot and needless to say every packet needs to be acknowledged by the receiving device. It is critical to note that the packet transmission will always occupy odd number of slots, this means that the slot immediately following the last packet will be reserved for the response from the receiver. Understanding the whole protocol stack is simpler if we start by studying its genetic constitution, in other words the physical layer which elaborates the strengths and the weakness in the most absolutely elementary form.