Simulation of Bluetooth Environment
Introduction to Wireless and Mobile Systems
Sudhindra Rao
03/11/04
Abstract
The Bluetoooth protocol is a short distance(10m to 100m) wireless communication protocol designed to work on freely available radio frequency of 2.4 GHz. This frequency band is under the ISM band which are free to use without regulations. This technology promises to replace wired networks with wireless ones and provide freedom and improvements which are not possible with wireless communications. With this technology devices can communicate to each other wirelessly without being hampered by any interferences from other devices. I try to simulate the behaviour of devices and the state transitions involved in setting up a Bluetooth piconet. Interesting insight gained by the simulations using a discrete event simulator called simjava has been presented.
Introduction to Bluetooth
The Bluetooth wireless technology was created to solve a simple problem ie to replace the cables used on mobile devices with radio frequency waves. The technology is a simple low-cost, low power technique easily available to all users. The radio frequency used is part of the freely available ISM band (2.4GHz).
A few interesting features of the Bluetooth technology can be quoted from [2] :
- Fast frequency hopping to minimize interference – this means that Bluetooth can provide robust data transfers/communication even in the presence of hostile radio frequency interference.
- Adaptive power output to minimize interference – Interference caused due to Bluetooth devices would be minimal. Also power control ensures that signals reach the desired destination.
- Short data packets – Since Bluetooth is for short distance message communication shorter data packets ensure faster delivery(even with retransmissions) – which is very critical with devices like mouse/keyboard.
- Continuous variable slope data(CVSD) for voice communication – can help withstand high error rates.
- Flexible packet types with 1,3,5 slots allocated can support short bursts and longer voice data.
- Used with mobile devices hence power control is integral part of Bluetooth(1mW transmission power).
Bluetooth devices form quick ad-hoc "piconet"’’s and communicate among the connected devices. Use of radio technology ensures there are no line-of-sight restrictions. Several piconets can be joined together using a common node creating a scatternet.
A piconet consists of a master(self-proclaimed) which can communicate with 8 other active nodes(slaves). If there are more than 8 nodes the rest of the nodes are marked as passive by the master. If one of the active node disconnects any of these nodes can change over to active.
Bluetooth Piconet formation
We discuss the formation of a piconet as explained in [1] for completeness. [1] should be referred to, for further details.
To form a piconet, the Bluetooth radio needs two parameters: the hopping pattern of the radio it wishes to connect to and the phase within that pattern. Bluetooth radios each have a unique "Global ID" that is used to create a hopping pattern. In forming a piconet, the master radio(one who wants to initiate communication) shares its Global ID with the other radios, which then become slaves and provide all the radios with the correct hopping pattern. The master also shares its clock offset (represented by the clock dial) with the slaves in the piconet, providing the offset into the hopping pattern.
Normally, radios not connected to the piconet are in "Standby" mode, where radios are listening for other radios to find them ("Inquire") and/or are listening for a request to form a piconet ("Page"). When a radio issues an Inquire command, listening radios will respond with their FHS packet (Global ID and clock offset), providing the inquiring radio with a list of Bluetooth radios in the area.
To form a piconet, a Bluetooth radio will page another radio with its Global ID (obtained by a previous inquiry). The paged radio will respond with its Global ID, and the master radio passes the paged radio an FHS packet. The paged radio then loads the paging radio's Global ID and clock offset, thus joining the master's piconet.
On joining the piconet, it is assigned a 3-bit Active Member Address (AMA) allowing other radios on the piconet to address it. Once the piconet has eight radios active, any more radios are assigned an 8-bit Passive Member Address (PMA) and put in “Park” mode. Any free AMA can be assigned to another radio wishing to join the piconet. This combination of AMA and PMA allows over 256 radios to actively reside on a piconet although only the eight radios with the AMAs can actively transfer data.
Figure 1: Bluetooth state diagram : Setup of Piconet. [1]
Figure 1 shows the state transition diagram for nodes in a piconet as depicted in [1]. Every node as explained starts with a listen in Standby mode. A master switches to inquiry mode when it has to find radios nearby. I hops around 32 freqeuncy bands and searches for a slave. Slaves respond with their hopping pattern after a delay of 2 slots. Slaves listen for about 10ms on a channel every 1.25sec. The master hops on 16 channels(based on its estimate of clock offset) and hopes to receive response from the slave. On successful response a slave is added to the network. Once the ID and the FHS pattern is known for a slave the communication can begin. In page mode the master checks if the slave is alive. If alive it connects to the slave and can transfer data.
The parked/passive nodes can be in Park, Sniff, Hold mode. In park mode they communicate with the master using beacon signals. In sniff mode the slaves can send data if a channel is free on an interval(used with devices like keyboards). In Hold mode the radio wakes up at given interval but no data is transferred. A radio in parked mode can get an active address if another communicating node disconnects.
With this information about piconet we simulate the Bluetooth network using simjava[3]. Simjava is chosen over others as it is built on Java and can be easily integrated with other java tools. Also creating applets and sharing the results/simulation is inbuilt in simjava.
We further present the state diagram of the simulation and attach a copy of the results and simulation source code. The state diagram shown here is a little modified version of the state diagram seen in literature and would help understand the simulation constraints and assumptions.
Figure 2: Master state diagram detail
Figure 3: Slave state diagram detail.
Some assumptions and simplifications for the simulation are :
- We assume a maximum of 16 nodes only – This helps us complete the simulation in limited time.
- The node ID and the frequency that they respond to is the same.
- We start the simulation from the Inquiry/inquiry scan state rather than standby and return to those states instead of standby.
References:
[1] James Kardach, “Bluetooth Architecture Overview”, Intel Technology Journal Q2, 2000 (
[2] Dharma Agrawal and Qing-An Zeng, “Introduction to Wireless and Mobile Systems”, Brooks/Cole (Thomson Learning) 2003, (Chapter 14)
[3]Simjava
Additional references:
[4]
[5] Bluetooth Primer
[6]Palowireless.com
- 1 -