Saturday, 7 May 2011

Basic Bluetooth functions

Basic Bluetooth functions

The Bluetooth technology is quite complex. This is not so surprising, considering the task it has to handle. It is mainly based on the IEEE 802.11 standard, briefly described at right. Of the 2 network modes described, Bluetooth uses the ad-hoc mode. This means that each station must observe "netiqette" and give all other units fair access to the wireless media.

The IEEE 802.11 standard

The IEEE 802.11communications standard defines the protocol for two types of networks; Ad-hoc and client/server.

The Ad-hoc network is a simple network where communications are established between multiple stations in a given coverage area without the use of an access point or server. The 802.11- standard specifies the etiquette that each station must observe so that all units have fair access to the wireless media. It provides methods for arbitrating requests to use the media to ensure that throughput is maximized for all of the users in the base service set.

The client/server network uses an access point that controls the allocation of transmit time for all stations and allows mobile stations to roam from cell to cell. The access point is used to handle traffic from the mobile radio to the wired or wireless backbone of the client/server network. This arrangement allows for point coordination of all the stations in the basic service area and ensures proper handling of the data traffic. The access point routes data between the stations and other wireless stations or to and from the network server.

Typically, WLANs (i.e. wireless LANs) controlled by a central access point will provide better throughput performance than ad-hoc networks.

The above diagram shows the main building blocks. With today´s technology, the transmitter/receiver-part for Bluetooth´s requirements could be made as small as a thumbnail (!!), and the antenna could be more or less hidden in the unit, much as it is in mobile telephones. Thus, the connectors in corresponding older units would not be replaced by something of similar dimensions; the transceiver would just "disappear" among other circuits.

Functionally, one talks about the three core protocols:

The logical link control and adaptation protocol (L2CAP), the service discovery protocol (SDP) and the RFCOMM protocol.

L2CAP, which adapts upper layer protocols over the Baseband, provides data services to the high layer protocols with protocol multiplexing capability, segmentation and reassembly operations, and group abstractions. Device information, services and the characteristics of the services can be queried using the SDP.

Like SDP, RFCOMM is layered on top of the L2CAP. As a ‘cable replacement’ protocol, RFCOMM provides transport capabilities for high-level services (e.g. OBEX protocol) that use serial line as the transport mechanism.

The Bluetooth air interface is based on a nominal antenna power of 0 dBm. Spectrum spreading has been added to facilitate optional operation at power levels up to 100 mW worldwide. This is accomplished by frequency hopping; 79 hops displaced by 1 MHz, starting at 2.402 GHz and stopping at 2.480 GHz.

Due to local regulations, the bandwidth is reduced in Japan, France and Spain. This is handled by an internal software switch. The maximum frequency hopping rate is 1600 hops/s. The nominal link range is 10 centimeters to 10 meters, but can be extended to more than 100 meters by increasing the transmit power.

The Bluetooth "Channels"

"Channels" can mean 3 different things in this context:

1. It could refer to the 79 (or 23) RF-channels, on individual frequencies 1 MHz apart
2. It could also refer to the communications channels, consisting of a pseudo-random hopping sequence through these 79 (or 23) RF-channels. Such a channel could more be likened with what is called a "session" in the OSI-model.
3. There are also 5 ”Logical Channels”, which are used for control purposes.

2 types of physical links are defined:

• SCO (Synchronous Connection-Oriented)
• ACL (Asynchronous Connection-Less)

The SCO link is point-to-point between master and slave. The master maintains the link by using reserved timeslots at regular intervals. Packet retransmissions are not allowed.

ACL provides packet-switched connections between the master and all active slaves. Packet retransmissions are usually applied to assure data integrity.
For each of these link types, 12 different packet types have been defined. 4 control packets are common to both links.

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