b. Potential slaves make themselves discoverable by entering inquiry scan mode and listen for an inquiry from a master. c. On receiving an inquiry, the slave responds to the master with a Frequency Hop Synchronisation packet (FHS). The FHS contains information that is needed to create a connection to the device; this information includes its Bluetooth address and class of device. d. The master collects the FHS information from each device discovered.
To connect to one of these devices the master goes into page mode and will page the device using the corresponding Bluetooth address. e. The slave being paged by a master will need to be in page scan mode to be able to connect to a master This is a simplified overview of steps the link controller uses to make a connection. The specification describes various parameters that can be set, examples being the number of inquiry responses from slaves to the master, the length and number of inquiry scans the master issues. Devices do not need to carry out the full set of inquiry, inquiry scan, page and page scan.
For instance some devices may not scan for inquiries, thus making them invisible, or they may be set up to only page scan so they can be connected to only one specific master. Perhaps a master device may not go into page scan mode, as it may only need to page or inquire. As entering any mode necessitates the turning on of RF circuitry then limiting options reduces power consumption as this is a major influence on power consumption, so missing out a few of these modes has an advantage (see Bluetooth Security Review, Part I).
Once a connection is created between two devices, the application can use the Service Discovery Protocol (SDP) to find out what particular services a device supports. This is done via a L2CAP channel to the service discovery server. The potential Bluetooth Services available sit on top of the Bluetooth protocol stack. In real terms, before the service can be accessed and used, the lower levels must be connected, i. e. an ACL must be connected and configured (see Bluetooth Security Review, Part I). B. 2 Products
In the previous section the importance of profiles in the development of applications and how these applications get access to the service running on top of the Bluetooth protocol stack was explored. To truly appreciate the products that could be realised with Bluetooth technology, then a trip to the Bluetooth website is a good starting point. Look at the products that have been qualified already. As the technology matures more applications will emerge within the Personal Area Network using profiles as their building blocks.
We will certainly see the PDA that is placed in the vicinity of your desktop computer and files synchronised between them. This latter concept extending to your digital stills camera when downloading your holiday photos without the palaver of connecting the inevitable miniature cable (see Bluetooth Information). The wireless headset to mobile phone contains a speaker and microphone and connects over a Bluetooth link permitting voice and data communications. The RF power for this link is significantly lower than the power required by the mobile phone to connect to its base station.
Therefore from a health hazard, it is preferable to have the Bluetooth headset with its small RF power next to your head than the phone (see Bluetooth Information). C. Compatibility Compatibility covers various issues, whether it is compatibility with other Bluetooth devices, the specification or other systems that share the same frequency band. We will examine some of these aspects here. C. 1 Interference Bluetooth is subject to interference from two types of emitters, as well as environmental problems.
The emitters break down to intentional emitters and accidental emitters. Intentional emitters occupy the ISM band and are systems based on IEEE 802. 11b (Wi-Fi), Home RF, DECT variants, proprietary systems and of course other Bluetooth devices. Accidental emitters include microwave ovens and lighting. The third source of interference could be the environment, signals can fade due to distance or being blocked by walls, furniture and human bodies; device positioning could be critical (see Bluetooth. Wikipedia, the free encyclopaedia, Feb.
12, 2007). How is the affect of these forms of interference minimised? With respect to the intentional emitters, Bluetooth has a clear advantage over systems based on Direct Sequence Spread Spectrum (DSSS) systems such as IEEE802. 11b as it is based on a FHSS system. What makes the DSSS poor in comparison is that transmission power is often low in any specified band. The FHSS signal may have high enough power in its narrower band to effectively punch through the parts of the wider band occupied by the DSSS system.
Otherwise, if there is strong interference from the DSSS system on Bluetooth, its ability to hop utilises the rest of the band more effectively. Bluetooth allows for retries meaning data will eventually get through, so users will see a decrease in the throughput of the network but it will continue to operate. The major degradation is in audio quality as voice data transmitted on a SCO channel is not retransmitted, as it is time critical (see Bluetooth.
Wikipedia, the free encyclopaedia, Feb. 12, 2007). HomeRF is another FHSS system and you could be lucky that both systems could hop around and miss each other. But HomeRF with greater data rates has larger channel sizes and therefore Bluetooth has a greater chance of hitting these on a hop. HomeRF covers a larger distance potentially over powering the Bluetooth on the channels where they clash. With retry available eventually data will get through, although performance degrades.
HomeRF on the other hand has built features in its higher layers of protocol and it has MAC level retry mechanism both for data and isochronous voice connections (see Bluetooth. Wikipedia, the free encyclopaedia, Feb. 12, 2007). Against other intentional emitters and accidental emitters the techniques for co-existence with IEEE 802. 11b and HomeRF are just as applicable i. e. the ability to hop away from offending channels and retry are Bluetooths natural defence (see Bluetooth. Wikipedia, the free encyclopaedia, Feb. 12, 2007).