Osi Lower Layer Functions Implemented in Bluetooth Hardware

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1.4 The protocol stack

A key feature of the Bluetooth specification is that it aims to let devices from lots of different manufacturers to work with one another. To this end, Bluetooth does not merely define a radio arrangement, it also defines a software stack to enable applications to find other Bluetooth devices in the area, discover what services they can offer, and apply those services.

The Bluetooth stack is defined as a series of layers, though at that place are some features which cross several layers.

Every block in Figure 1–2 corresponds to a chapter in the core Bluetooth specification. The core specification also has three chapters on test and qualification:

• Bluetooth Test Mode.
• Bluetooth Compliance Requirements.
• Test Command Interface.

Effigy 1–2 The Bluetooth protocol stack.

The Bluetooth specification encompasses more than just the core specification. There are also profiles which give details of how applications should use the Bluetooth protocol stack, and a make volume which explains how the Bluetooth brand should be used.

1.4.1 The OSI Reference Model

Figure 1–3 shows the familiar Open Systems Interconnect (OSI) standard reference model for communications protocol stacks. Although Bluetooth does not exactly match the model, it is a useful practise to relate the different parts of the Bluetooth stack to the various parts of the model. Since the reference model is an ideal, well-partitioned stack, the comparison serves to highlight the segmentation of responsibility in the Bluetooth stack.

Figure i–three OSI reference model and Bluetooth.

The Physical Layer is responsible for the electrical interface to the communications media, including modulation and aqueduct coding. It thus covers the radio and part of the baseband.

The Data Link Layer is responsible for transmission, framing, and mistake control over a particular link, and as such, overlaps the link controller task and the command end of the baseband, including error checking and correction.

From now on, information technology gets a little less clear. The Network Layer is responsible for information transfer across the network, independent of the media and specific topology of the network. This encompasses the higher end of the link controller, setting upward and maintaining multiple links, and likewise covering most of the Link Manager (LM) job. The Transport Layer is responsible for the reliability and multiplexing of data transfer across the network to the level provided by the awarding, and thus overlaps at the high end of the LM and covers the Host Controller Interface (HCI), which provides the actual data ship mechanisms.

The Session Layer provides the direction and data flow control services, which are covered by L2CAP and the lower ends of RFCOMM/SDP. The Presentation Layer provides a common representation for Application Layer information by adding service structure to the units of information, which is the main task of RFCOMM/SDP. Finally, the Awarding Layer is responsible for managing communications between host applications.

1.4.ii The Concrete Layer

Bluetooth devices operate at 2.iv GHz in the globally available, licence-gratuitous ISM ring. This band is reserved for general utilize by Industrial, Scientific, and Medical (ISM) applications, which obey a basic set of ability and spectral emission and interference specifications. This means that Bluetooth has to be very robust, equally there are a great many existing users and polluters of this shared spectrum.

The operating ring is divided into one MHz-spaced channels, each signalling data at 1 Megasymbol per second so equally to obtain the maximum available channel bandwidth. With the chosen modulation scheme of GFSK (Gaussian Frequency Shift Keying), this equates to i Mb/south. Using GFSK, a binary one gives ascent to a positive frequency divergence from the nominal carrier frequency, while a binary 0 gives rise to a negative frequency divergence.

After each packet, both devices retune their radio to a different frequency, effectively hopping from radio channel to radio aqueduct (FHSS—frequency hopping spread spectrum). In this manner, Bluetooth devices use the whole of the available ISM ring and if a manual is compromised by interference on 1 aqueduct, the retransmission will ever be on a dissimilar (hopefully articulate) channel. Each Bluetooth time slot lasts 625 microseconds, and, generally, devices hop in one case per packet, which will be every slot, every iii slots, or every 5 slots.

Designed for low-powered portable applications, the radio ability must be minimised. Three different ability classes are defined, which provide operation ranges of approximately ten m, twenty m, and 100 g; the lowest ability gives upward to ten m range, the highest upwardly to 100 one thousand.

1.iv.three Masters, Slaves, Slots, and Frequency Hopping

If devices are to hop to new frequencies after each packet, they must all hold on the sequence of frequencies they will utilise. Bluetooth devices can operate in two modes: equally a Primary or equally a Slave. It is the Master that sets the frequency hopping sequence. Slaves synchronise to the Master in fourth dimension and frequency past post-obit the Master'south hopping sequence.

Every Bluetooth device has a unique Bluetooth device address and a Bluetooth clock. The baseband part of the Bluetooth specification describes an algorithm which can calculate a frequency hop sequence from a Bluetooth device address and a Bluetooth clock. When Slaves connect to a Primary, they are told the Bluetooth device address and clock of the Master. They then use this to calculate the frequency hop sequence. Because all Slaves use the Master's clock and address, all are synchronised to the Master's frequency hop sequence.

In add-on to decision-making the frequency hop sequence, the Master controls when devices are immune to transmit. The Primary allows Slaves to transmit past allocating slots for voice traffic or information traffic. In data traffic slots, the Slaves are merely allowed to transmit when replying to a transmission to them by the Main. In voice traffic slots, Slaves are required to transmit regularly in reserved slots whether or not they are replying to the Master.

The Chief controls how the total bachelor bandwidth is divided amongst the Slaves by deciding when and how often to communicate with each Slave. The number of time slots each device gets depends on its data transfer requirements. The arrangement of dividing fourth dimension slots among multiple devices is chosen Time Division Multiplexing (TDM).

i.4.iv Piconets and Scatternets

A collection of Slave devices operating together with one common Main is referred to equally a piconet (see Figure one–4). All devices on a piconet follow the frequency hopping sequence and timing of the Master.

Figure 1–iv Bespeak to point and indicate to multipoint piconets.

In Figure 1–4, the piconet on the left with only one Slave illustrates a point to point connection. The piconet on the right with three Slaves talking to the Master illustrates a point to multipoint connection. The Slaves in a piconet only have links to the Master; there are no directly links between Slaves in a piconet.

The specification limits the number of Slaves in a piconet to vii, with each Slave only communicating with the shared Master. Even so, a larger coverage area or a greater number of network members may be realized past linking piconets into a scatternet, where some devices are members of more than one piconet (see Figure ane–5).

Figure 1–v Scatternets.

When a device is present in more than than one piconet, it must time-share, spending a few slots on ane piconet and a few slots on the other. On the left in Figure 1–5 is a scatternet where ane device is a Slave in one piconet and a Master in another. On the right is a scatternet where one device is a Slave in two piconets. It is not possible to accept a device which is a Main of two different piconets, since all Slaves in a piconet are synchronised to the Master's hop sequence. By definition, all devices with the same Master must be on the same piconet.

In addition to the diverse sources of interference mentioned already, a major source of interference for Bluetooth devices will clearly be other Bluetooth devices. Although devices sharing a piconet will be synchronised to avoid each other, other unsynchronised piconets in the expanse will randomly collide on the same frequency. If in that location is a collision on a particular channel, those packets will exist lost and later retransmitted, or if voice, ignored. So, the more piconets in an surface area, the more retransmissions will be needed, causing data rates to fall. This is similar having a conversation in a noisy room: the more people talking, the noisier it gets, and you have to first repeating yourself to get the point across.

This effect will happen if there are many independent piconets in i area, and it will also happen to scatternets, since the piconets making up the scatternet practise non coordinate their frequency hopping.

1.4.5 Radio Power Classes

The Bluetooth specification allows for iii dissimilar types of radio powers:

• Course 1 = 100mW (20 dBm).
• Course 2 = 2.5mW (four dBm).
• Class 3 = 1mW (0 dBm).

These power classes allow Bluetooth devices to connect at different ranges. At the time of writing, almost manufacturers are producing Class 3, low power, i mW radios. These can communicate for a maximum of effectually xxx feet (10 k). All the same, because things like bodies and article of furniture absorb microwaves, reception may non be reliable at the limit of this range. And so, when using 1 mW radios, a more realistic figure for reliable operation in a normal room will probably be 5 1000. This provides a low cost, low ability communications solution which has plenty of range for a cablevision replacement technology.

Patently, college ability radios have longer ranges. The maximum range for a Grade one, 100 mW radio is about 100 metres. In that location is also a minimum range for a Bluetooth connexion. If radios are put besides close together, some receivers may saturate, so a few Bluetooth radios may be unreliable on brusk link lengths (nether 10 cm).

A 100 1000 link needs a high power Class 1 device at both ends, but it is possible to create piconets with a mixture of high and low ability devices at dissimilar ranges. Effigy 1–6 shows a mixture of loftier and low power devices in different piconets occupying an area.

Figure 1–six Piconets fabricated upward of different ability class devices.

This figure shows piconets which overlap each other. This is possible because each Primary has its own frequency hopping sequence, so 2 piconets are unlikely to be on the same frequency at the same fourth dimension. If they do run across on the same frequency, after the next frequency hop they will not still be on the same frequency, and then the data which may take been lost when the ii piconets were on the same frequency can be resent.

i.4.half-dozen Phonation and Data Links

Bluetooth allows both time critical information communication such equally that required for voice or sound, equally well equally high speed, time insensitive packet information advice. To behave such data, two different types of links are defined between any two devices. These are SCO (Synchronous Connexion Oriented) links for voice communication and ACL (Asynchronous Connectionless) links for data communication.

ACL data packets are synthetic from a 72-bit access code, a 54-chip parcel header and a 16-fleck CRC code, in addition to the payload data. There are a variety of package types allowing dissimilar amounts of information to exist sent. The package which carries the largest data payload is a DH5 packet, which stretches over five slots. A DH5 package can deport 339 bytes, or ii,712 bits of information. Then, 2,858 bits are sent on air for 2,712 bits of information.

A DH5 bundle uses up five slots, and the minimum length respond is one slot. Thus, the maximum baseband data rate in one direction is 723.2 kb/southward. In this case, with 5-slot packets sent in one direction, the 1-slot packets sent in the other direction volition only carry 57.six kb/s, so this would exist an asymmetric link with more data going in the direction using 5-slot packets. If 5-slot packets were sent in both directions, the information charge per unit obtained would be 433.9 kb/southward, quite a reduction from the i Mb/due south data rate on air.

This overhead in both data encoding and frequency hopping is necessary mainly to provide a robust link since the ISM band is a shared resources with many devices, and indeed other communications standards and even noise sources, cohabiting in the same spectrum. In improver, to further reduce the interference trouble in the spectrum, national radio regulations limit the ability emission per unit fourth dimension in the ISM band, making a frequency hopping scheme necessary to spread transmissions over the spectrum and over time.

The college layers of the protocol stack likewise apply up some of the bandwidth, so at the application level, the maximum data charge per unit could be around 650 kb/s.

The SCO links work at 64 kb/southward, and it is possible to have up to three total-duplex voice links at in one case or to mix vocalisation and data. These voice channels give audio communication of a quality one would look from a modern mobile cellular phone organization such as GSM. Every bit such, SCO links are non actually suitable for delivering audio of a quality required for music listening.

I culling to support music delivery is to use an ACL aqueduct to carry audio. Raw CD-quality sound requires 1411.2 kb/s, but with suitable compression, such as MP3, which reduces this bit rate to around 128 kb/s, near CD-quality audio could easily be carried providing the time criticality of the audio was maintained.

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