What is Bluetooth Technology (IEEE 802.15 Standard) ?

In the last post we have seen the  architecture of IEEE 802.11 (WIFI wlan standard) in detail. Now in this post we will explore the architecture of IEEE 802.15 standard i.e the Bluetooth technology in detail. Bluetooth is the next commonly used wlan (wireless LAN) technology used nowadays.

Bluetooth technology is widely used to connect to different devices such as computer, printers, mobiles etc. It is originally started as the project by the  Erricson company . Let us understand this further .

What is Bluetooth Technology Standard ?

A Bluetooth LAN is an ad-hoc network (already covered in the previous post ), which means that the network is formed instantly, the devices then find each other and form a network called a piconet (network created by the bluetooth devices). 

A Bluetooth LAN can also be connected to the Internet if one of the gadgets has this capability. A Bluetooth LAN, by nature, cannot be very  large.

Technically , a bluetooth technology is based on the IEEE 802.15 standard. The technique used in this is FHSS (frequency hopping) . It operates in the frequency band of 2.4 Ghz. It uses the modulation technique called GFSK. The data rate supported is upto 700 Kbps. The max operating distance range is 10 meters.

Bluetooth technology is basically the implementation of a protocol defined by the IEEE 802.15 standard. This standard defined as a wireless personal-area network (PAN) operable in an area the size of a room or a hall.

Bluetooth Technology Architecture (IEEE 802.15 protocol) 

A Bluetooth network (formed by peer devices) is called a piconet, or a small net. A piconet can have max eight stations (upto 8 connected devices), one of which is called the primary , the rest are called secondaries.

All the secondary stations synchronize their clocks and hopping sequence in accordance with the primary. Further a piconet can have only one primary station. The communication between the primary and the secondary is a one-to-one or one-to-many relationship.

Piconets can be combined to form another network that is called a scatternet. A secondary station in any one piconet can be the primary in some other piconet. This station can receive messages from the primary in the first piconet (as a secondary) and, acting as a primary, deliver them to secondaries in the second piconet.

A Bluetooth device has a built-in very short-range radio transmitter. The current data rate is 1Mbps alongwith a 2.4-GHz bandwidth.

Bluetooth Layers 

Bluetooth uses several layers as shown below :

The radio layer is approximately similar to the physical layer of the Internet model. Bluetooth devices are low-power and have a range upto 10 m. Bluetooth uses a 2.4-GHz ISM band which is further divided into 79 channels of 1 MHz each.

Bluetooth implements the frequency-hopping spread spectrum (FHSS) method in the physical layer to avoid interference from other devices or other networks.

To transform bits to a signal, Bluetooth uses a modified version of FSK, called GFSK (FSK with Gaussian bandwidth filtering ) . GFSK has a carrier frequency.

The Baseband layer is roughly equivalent to the MAC sublayer present in LANs. The access method is TDMA (time division multiple access). The primary and secondary communicate with each other using time slots. The length of a time slot is exactly the same as the dwell time, 625 µs (microseconds) .

Bluetooth uses a type of TDMA that is called TDD-TDMA (timedivision duplex TDMA). TDD-TDMA is a variety of half-duplex communication in which the secondary and receiver send and receive data, but not at the same time.

Frame format in IEEE 802.15 (Bluetooth standard)

A frame in the baseband layer can be one of three types ,  one-slot, three-slot, or five-slot. A slot is mainly of 625 µs. However, in a one-slot frame exchange, 259 µs is needed for hopping and control mechanisms. This means that a one-slot frame can last only for  625 – 259, or 366 µs.

 

 

Access code : This 72 bit field normally contains synchronization bits and the identifier of the primary to distinguish the frame of one piconet from another.

Header :  This 54 bit field is a repeated 18-bit pattern. Each pattern has the following subfields:

Address : It is a 3 bit address subfield that can define up to seven secondaries (1 to 7). If the address is zero, it is then used for broadcast communication from the primary to all secondaries.

Type : It is a  4-bit type subfield that defines the type of data coming from the upper layers.

F :  This 1-bit subfield is for flow control. When set (I), it indicates that the device is unable to receive more frames (buffer is full).

A : This 1-bit subfield is for acknowledgment. Bluetooth uses Stop-and-Wait ARQ. 1 bit is sufficient for acknowledgment.

S : This 1-bit subfield that holds a sequence number. Bluetooth uses Stop-and-Wait ARQ. 1 bit is sufficient for sequence numbering.

HEC :  The 8-bit header error correction subfield is a checksum to detect errors in each 18-bit header section.

Payload :  This subfield can be upto  0 to 2740 bits long. It contains data/control information coming from the upper layers.

L2CAP in bluetooth frame 

The Logical Link Control and Adaptation Protocol or L2CAP (L2 here means LL), is mostly equivalent to the LLC sublayer in LANs. The 16-bit length field mainly defines the size of the data, in bytes, coming from the upper layers. Data can be up to 65,535 bytes. The channel ID (CID) defines a unique identifier for a specific virtual channel.

The L2CAP mainly performs specific duties i.e multiplexing, segmentation and reassembly, quality of service (QoS), and group management.

In the next post we will look into the other most important standard of IEEE i.e the Wimax technology in detail. So stay tuned.