Mobile Computing

Wireless Networks 33

it needs PCF services (polling) and what its power requirements are. The base station may

accept or reject the mobile station. However, if it is accepted, the mobile station has to authen-

ticate itself.

2. Disassociation: This service is used when either the mobile station or the base station

wants to break the connection. The base station may do so for maintenance purposes or if

it wants to go down. The mobile station may disassociate when it is leaving or shutting

down.

3. Re-association: This service is used when a mobile station wants to change the base station,

as when it moves from one cell to another.

4. Distribution: This service deals with routing of frames that are sent to the base station. If the

destination of the frame is local, it is sent directly over the air; else, it is forwarded over the

wired network.

5. Integration: This service is used if the frame has to be sent to a non-802.11 network. The

frame must be translated from the 802.11 format to the format of the destination network.

The station services are concerned with working within the same cell. They are used after a

mobile station has associated with a base station and are as follows:

1. Authentication: This service is used by the base station to check the identity of the mobile

station. Initially, the standard did not require the base station to prove its own identity to

the mobile station, but this defect in the standard is being corrected.

2. De-authentication: When a mobile station that has been authenticated wants to leave the

network, it is de-authenticated by the base station. After de-authentication, it cannot use the

network anymore.

3. Privacy: This service is used to ensure that the data in the wireless network is confidential.

Encryption is used for this purpose.

4. Data delivery: This service is the one used by mobile stations to send and receive data. It is

a reliable service requiring the higher layers to provide for error detection and correction.

3.3 Bluetooth

Bluetooth was developed in 1994 by a study interest group (SIG) consisting of IBM, Intel, Nokia

and Toshiba for connecting mobile phones or computing and communication devices without

the use of cables. In 2002 it was taken up by the IEEE wireless personal area network (WPAN)

Committee as the IEEE 802.15 standard, for the physical and data link layers.

It is a short-range, low-cost and power-efficient radio-frequency-based wireless technology

that supports both point-to-point and point-to-multipoint connections. It connects one hand-

held device to another Bluetooth-enabled device(s) within a 30-foot or 10-meter radius, such as

mobile phones, laptops, printers and other accessories. It is like having a universal remote for the

kinds of devices one uses every day and is oriented towards the mobile consumer wanting to do

digital imaging and multimedia applications.

Bluetooth operates in the unlicensed ISM band, with slight locational variations. Its essen-

tial characteristics are summarized in Table 3.1. Bluetooth-enabled devices can automatically

locate each other, but user action is necessary to make connections with other devices and to

form networks.

Eight devices can be connected in a Bluetooth network, known as a piconet. One of them

acts as the master and the others are called slaves. A scatternet is formed when two or more

piconets connect via a bridge node. This is shown in Figure 3.11.

34 Mobile Computing

Laptop C

(Master of piconet 3)

Laptop

Laptop Laptop A

(Master of piconet 1)

Piconet 1

Piconet 3

Laptop B

(Master of piconet 2)

Laptop D

User C’s PDA

Piconet 2

User B’s PDA

User B’s

mobile phone

Figure 3.11 Bluetooth Scatternet

In addition to the seven active slaves, there can be up to 255 parked nodes (in low power

state) in the net that can only respond to a beacon signal from the master. Slaves are dumb

devices, doing what the master tells them to do. The piconet is a TDM system, with the master

controlling the clock and determining which slave gets to communicate in which time slot. All

communication is between master and slave, not between slave and slave.

Characteristics Description

Physical Layer FHSS

Frequency Band 2.4–2.4835 GHz

Hop Frequency 1,600 hops/s

Data Rate 1 Mbps

Data and Network Security Provides three levels of security, two levels of device trust

and three levels of service security.

Operating Range 10 m

Throughput Around 720 Kbps

Advantages No wires and cables for many interfaces, can penetrate walls

and other obstacles, uses low power and minimal hardware.

Disadvantages May interfere with other ISM band technologies, has low

data rates.

Table 3.1 Bluetooth Features