Desarrollo de la propuesta 1. Situación Actual

Each Bluetooth device has a unique 48-bit Bluetooth device address (BD_ADDR). Bluetooth devices are required to form a piconet before exchanging data. Each piconet has a master unit that controls the channel access and frequency hopping sequence of all other nodes within the piconet, which are referred to as the slave units. In a Bluetooth piconet, the master node can simultaneously control up to seven slaves that are actively communicating with the master. The master will assign a 3-bit Logical Transport Address (LT_ADDR) to each of the active slave.

Several piconets can be interconnected via bridge nodes to create a scatternet. Bridge nodes are capable of time-sharing between multiple piconets, receiving packets from one piconet and forwarding them to another. As a bridge node is connected to multiple piconets, it can be a master in one piconet and act as slaves in other piconets. This is called a master/slave bridge (or M/S bridge). Alternatively, a bridge node can act as a slave in each of the piconets which it is connected to. This is called a slave/slave bridge (or S/S bridge).

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Bluetooth Networking 9-3

If a set of Bluetooth devices are within the transmission range of each other and need to exchange data, then a piconet formation algorithm is necessary to create a connected topology for the devices and to assign the role of each device (i.e., master or slave). The current Bluetooth specification [1] has standardized the procedures for piconet formation. This involves a sequence of steps including the inquiry, inquiry scan, page, and page scan phases.

Similarly, given a set of communicating Bluetooth devices that are either distributed over an area exceeding the coverage of a piconet, or whose number exceeds the eight active devices that can commu-nicate with each other in a piconet, a scatternet formation algorithm (SFA) is necessary to create a connected topology for the devices, and to assign the role of each device (i.e., master, slave, or bridge).

Although the current Bluetooth specification [1] defines what a scatternet is, a specific SFA is not specified.

How to efficiently self-organize the nodes into a high-performance ad hoc network has been the subject of intensive research [8–13].

In the current Bluetooth specification, all packet transmissions within a scatternet need to be routed via the master or bridge nodes. When the traffic load is high, these master or bridge nodes may become the network bottleneck. Furthermore, in order to support dynamic joining or leaving of mobile devices within a scatternet, a number of time slots in the master nodes may need to be allocated for the dynamic topology configuration updates. This further reduces the time slots available at the master nodes for data transmissions.

In [14] a two-phase scatternet formation (TPSF) algorithm is proposed with the aim of supporting dynamic topology changes while maintaining a high aggregate throughput. In the first phase, a control scatternet is constructed for control purposes (i.e., to support dynamic join/leave, route discovery, and so on). The second phase is invoked whenever a node needs to initiate data communications with another node. A dedicated piconet/scatternet is constructed on demand, between the communicating nodes. As the on-demand scatternet can dedicate all the time slots to a single communication session, it has the capability to provide a high throughput and a small end-to-end data transfer delay. The on-demand scatternet is torn down when the data transmissions are completed.

The control scatternet formation consists of three steps. In the first step, each node performs device discovery by exchanging control information with its neighbors. The second step consists of role deter-mination. The node with the highest number of neighbors among all its neighbors is selected as the master node. The last step is the creation of the control scatternet. The topology of the control scatternet has the following features. Each bridge node belongs to at most two piconets and acts as an S/S bridge.

Once the control scatternet has been formed, the master and bridge nodes stay in active mode, while all the pure slave nodes are put in the park mode.

The on-demand scatternet formation procedure is initiated by a source node that wants to transmit data to a destination node, and consists of two steps. First, a dynamic source routing based protocol is applied to enable piconet route discovery in the control scatternet. The piconet route discovery is achieved via the exchange of Route Request (RREQ) and Route Reply (RREP) messages. Then, all the master nodes along the piconet route select the participating nodes for the on-demand scatternet. This part is achieved via the exchange of Path Request (PREQ) and Path Reply (PREP) messages. Finally, the destination node initiates the connection setup via the paging procedure.

In [15] the mobility of Bluetooth devices is considered within a limited range and TPSF+ is proposed, which enhances the second phase (i.e., the on-demand scatternet formation phase) of the original TPSF algorithm. TPSF+ has the advantage of involving a small number of nodes participating in the demand scatternet route discovery procedure so as to avoid unnecessary route discoveries. The on-demand scatternet route discovery is limited to several piconets instead of all the piconets within the control scatternet. Results show that the route discovery procedure for TPSF+ is more efficient than the original TPSF.

The performance gain is illustrated here by comparing the proposed SFA, TPSF+ [15], with Bluenet [13].

In the simulation model, there are 40 nodes in total, and these nodes are placed in an area of 20 × 20 m². For each data point, the simulation was run 100 times and each run time was 120 seconds. The nonpersistent TCP (Transmission Control Protocol) on/off traffic is used. During the “on” periods,

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9-4 Mechatronic Systems: Devices, Design, Control, Operation and Monitoring

packets are generated at a constant burst data rate of 1440 Kbps. During the “off ” periods, no traffic is generated. Burst times and idle times follow the exponential distributions with an average “on” time of 0.5 seconds and an average “off ” time of 0.5 seconds. The packet size is 1000 bytes. The performance metrics include the aggregate throughput and the average end-to-end delay. The aggregate throughput is defined as the total throughput obtained by all the communication sessions. The end-to-end delay is determined from the time when the packet is created at the source node to the time when the packet is received at the destination node.

Figures 9.1 and 9.2 show the aggregate throughput and end-to-end delay of TPSF+ in comparisons with that of Bluenet, respectively. It is apparent that TPSF+ achieves a much higher aggregate throughput and FIGURE 9.1 Aggregate throughput versus number of sessions.

FIGURE 9.2 End-to-end delay versus number of sessions.

Number of communication sessions

1 2 3 4 5 6 7 8 9 10

Aggregate throughput (Mbps)

3.5

3

2.5

2

1.5

1

0.5

0

TCP Performance TPSF+

Bluenet

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

1 2 3 4 5 6 7 8 9 10

Number of communication sessions

Average delay (s)

TCP Performance Bluenet

TPSF+

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Bluetooth Networking 9-5

much lower end-to-end delay than Bluenet in a multi-hop scenario, with the performance differences increasing with the number of sessions. The performance bottleneck in Bluenet is due to the traffic load at the master and bridge nodes. TPSF+ avoids this bottleneck problem by setting up dedicated on-demand scatternet for each communication session.