Master and slave operation Essay

Bluetooth devices exist in small ad-hoc network configuration with the ability to operate as either master or the slave; the specification also allows a mechanism for master and slave to switch their roles. The configurations can be single point, which is the simplest configuration with one master and one slave. Multipoint, called a Piconet, based on up to 7 slaves clustered around a single Master. And a third type called a Scatternet, this is a group of Piconets effectively hubbed via a single Bluetooth device acting as a master in one Piconet and a slave in the other Piconet. The Scatternet permits either larger coverage areas or number of devices than a single Piconet can offer. Figure 5 outlines the different master and slave topologies permitted for networks in the standard (see â€Å"Bluetooth: Goodbye Infrared†). The role of the master is to control the available bandwidth between the slaves, it calculates and allocates how often to communicate with each slave and locks them into the appropriate frequency hopping sequence. The specification describes an algorithm that calculates the hop sequence, the seed being based on the master’s device address and clock. In addition to hop sequence control, the master is responsible for transmit control by dividing the network into a series of time slots amongst the net members, as part of a Time Division Multiplexing (TDM) scheme. These time slots can consist of data and potentially additional voice traffic i. e. you will always need a data channel before you can add a voice channel. The time slot is defined as 625  µs and all packet traffic is allocated 1, 3 or 5 slots, grouped together in transmit and receive pairs. Prior to connection some operations such as inquiry, paging and scanning operations may sometimes occur on half slots (see â€Å"Bluetooth: Goodbye Infrared†). Figure 5: Point to point, Piconet and Scatternet. A. 2. 3 Voice and Data Links. Bluetooth carries communication traffic over two types of air interface links defined as Asynchronous ConnectionLess (ACL) or Synchronous Connection Oriented (SCO). During a connection the links carry voice and data traffic in the time slots and are categorised as either time critical, as used for voice and audio, or high speed non-time critical data with a mechanism for acknowledgement and re-transmission. The first link established between master and slave is the ACL link and carries high speed data that is insensitive to time. It is packet switched, as the data is sporadic in nature, asynchronous, contains asymmetric and symmetric services and uses a polling access scheme. A master may be permitted to have a number of ACL links up to the maximum number of slaves permitted by the specification but only one link is allowed between any two devices (see â€Å"Bluetooth: Goodbye Infrared†). Once an ACL has been established a SCO link can be created on top of the ACL link. The SCO link is circuit switched; it has symmetric synchronous services and has slot reservation at fixed intervals, making it suitable for time critical data such as voice. The specification restricts the number of SCO links that a master can support to three. Summarising the two types of links: ACL ? Packet constructed of a 72 bit access code, a 54 bit packet header, a 16 bit CRC and Payload data ? Largest data packet is DH5 giving 723. 2 Kb/s as highest data rate in one direction. ? Non time critical data ? Asynchronous ? Packet switched ? Polling access SCO ? Same access code and header as ACL packets ? ARQ (Automatic Repeat reQuest) and SEQ (SEQuence) flags redundant since flow control and re-transmissions do not apply ? Cyclic Redundancy Code (CRC) field is absent? Payload fixed at 30 bytes, with source data of 10, 20 or 30 bytes ? Circuit switched ? Symmetric synchronous services ? Slot reservation at fixed intervals A special case exists that mixes SCO and ACL packets. Known as the Data Voice (DV) packet it carries data and voice on regular intervals like the SCO. The voice data has no flow control or CRC as per SCO packets, whereas the data part of the DV packet has flow control, re-transmission of the data part is permitted and the data part is CRC protected (see â€Å"Bluetooth: Goodbye Infrared†).