11.1.1 Historical Overview
The concept of a Personal Area Network (PAN) differs from that of other types of data networks (e.g. LAN, MAN, WAN) in terms of size, performance and cost (Figure 11.1).
PANs are the next step down from LANs and target applications that demand short-range communications inside the Personal Operating Space (POS) of a person or device. The term POS is used to define the space in the near vicinity of a person or device and can be thought of as a bubble that surrounds him. As the person goes through his regular daily activities, his POS changes to include a number of different devices (such as cellular phones, pagers, headphones, PC interfaces, etc.) with whom the ability for easy and transparent information exchange would be useful. PANs aim to provide such ability in an efficient manner.
There exist a number of different communication mediums to choose for implementing a PAN, such as electric and magnetic fields, radio and optical signal transmission. One of the first research concepts for PANs dates back to an IBM research project in 1996 and is known
Figure 11.1 The various kinds of wireless networks
as ‘Near-field Intra-body Communication PAN (NIC-PAN)’ [1]. This approach uses the human body as the communication medium, which can conduct electricity due to its natural content of salt. According to this approach, NIC-PAN-compliant devices worn by a user can communicate with each other through the user’s body, thus no wires are needed. Furthermore, a user wearing such a device can initiate communication with another device located on another user by means of a simple handshake. In order to transmit data between devices attached to the users’ bodies or clothing, the NIC-PAN device transmitter charges and discharges the human body, thus resulting in an oscillating potential appearing between the body and the environment. These changes of potential are picked up by the receiving NIC-PAN device and thus a communication channel is established. The electrical current used in this approach is approximately 1 nA, which is much lower than the natural electrical current of the human body. In the context of this research, a small prototype was built, which achieved data rates around 2.4 kbps.
However, the NIC-PAN approach did not evolve into something more than a research project, whereas the true revolution in the area of PANs was brought about by the use of wireless transmission-based PANs (WPANs1). The first attempt to define a standard for PANs dates back to an Ericsson project in 1994, which aimed to find a solution for wireless communication between mobile phones and related accessories (e.g. hands-free kits). This project was named Bluetooth, after the king that united the Viking tribes. Bluetooth was a promising approach and as a result, Nokia, Intel, Toshiba and IBM joined Ericsson to form the Bluetooth Special Interest Group (SIG) in May 1998. The purpose of the Bluetooth SIG is to develop a de facto standard for PANs that meets the communication needs of all mobile computing and communication devices located in a reduced geographical space regardless of their size or power budget. The size of the SIG has grown from the initial five members to nearly 2000 others at the present day. Any interested company is allowed to join the SIG provided that it lets all other members of the SIG use its patents royalty-free, in an effort to keep the standard open. Version 1.0 of the Bluetooth specification was released by the SIG in 1999, followed by version 1.1 in 2001. Both these versions support 64 kbps voice channels and asynchronous data channels either asymmetric, with a maximum data rate of 721 kbps in one direction and 57.6 in the other or symmetric with a 432 kbps maximum rate in both directions. Bluetooth uses Frequency Hopping Spread Spectrum (FHSS) modulation in the 2.4 GHz ISM band. The supported range is 10 m with the possibility of extending this to 100 m [2].
Another initiative from industry members to develop a PAN standard was made in 1997 by the formation of the HomeRF Working Group. The primary goal of this group is to enable interoperable wireless voice and data networking within the home. Version 1.0 of HomeRF was published in 1999. It supported four 32 kbps voice connections and data rates up to 1.6 Mbps at ranges up to 50 m. Version 2.0 of HomeRF was released in 2001 and increased these numbers to eight channels and 10 Mbps, respectively, making HomeRF more suitable than Bluetooth for transmitting music, audio, video and other high data applications. However, Bluetooth seems to have more industry backing. Like Bluetooth, HomeRF also supports voice and asynchronous data channels using FHSS modulation in the 2.4 GHz ISM band.
After the appearance of the Bluetooth and HomeRF initiatives, IEEE also decided to join the area of developing specifications for PANs. Thus the 802.15 Working Group [2–5] was
1Since all PAN technology alternatives today employ wireless transmission, the terms PAN and WPAN are used in this book synonymously.
formed in March 1999, with the responsibility of defining physical and MAC layer specifica- tions for PANs having low implementation complexity and low power consumption.
Although Working Group 802.11, which deals with Wireless LANs, already existed, it was decided that a new Working Group was needed for PAN standardization. This is attrib- uted to the fact that there is a much greater concern over power consumption, size, and product cost in PANs stemming from the demands for PAN devices compared to WLAN devices:
† Less size and weight, in order to be easily carried or worn for long periods of time. On the other hand, the size and weight of WLAN cards is a matter of secondary importance. This is because WLAN devices are typically either attached to portable computers, which of course are not carried by users all of the time, or to fixed desktop PCs.
† Lower cost, in order not to burden the total cost of the device. PAN devices aim to provide wireless connectivity to commercial electronic appliances. In order to enable small device size, PAN functionality will be integrated within the devices. To earn market acceptance, the cost burden of PAN functionality on the total cost of the device should be small. Users, on the other hand, can buy WLAN cards separately, thus the impact of their cost is less important.
There are four Task Groups (TGs) inside Working Group 802.15:
† TG1. This group is working on a PAN standard based on Bluetooth.
† TG2. This group aims to facilitate coexistence of PAN and WLAN networks.
† TG3. This group aims to produce a PAN standard with data rates exceeding 20 Mbps, while maintaining low cost, power consumption and interoperability with industry stan- dards.
† TG4. This group aims to produce a PAN standard that will enable low-rate operation while achieving levels of power consumption so low, that a battery life of months or years will be possible.
Due to the fact that industry consortia initiatives for PAN standards development preceded the initiative of IEEE, a key mission of the 802.15 Working Group will be to work closely with such consortia, such as Bluetooth and HomeRF, in order to achieve interoperability for PANs coexisting in a shared wireless medium.
11.1.2 PAN Concerns
There are certain issues that need to be taken into account when designing a PAN. The most obvious one affects all types of wireless networks and concerns the increased Bit Error Rate (BER) of the wireless medium. As has been mentioned, the primary reason for the increased BER is atmospheric noise, physical obstructions found in the signal’s path, multipath propa- gation and interference from other systems. The primary source of interference in the 2.4 GHz ISM band in which PANs operate, comes both from narrowband and wideband sources, such as microwave ovens. Apart from the good interference avoidance properties of SS modula- tion, which is employed for unlicensed transmission in the 2.4 GHz ISM band, Automatic Repeat Requests (ARQ) and Forward Error Control (FEC) techniques can also be used in this direction. Furthermore, PANs have to deal with interference from collocated PANs and WLANs, although this should not be a big problem due to the use of FHSS modulation.
Personal Area Networks (PANs) 301
PANs should provide full communication capability between all devices in the POS of a person. However, two PAN devices that initiate communication inside the POS should not interfere with other devices when this is not wanted. Consider for example the case of a person entering a conference room. After sitting in a chair, a PAN interface nearby could communicate with a handheld device of the person and deliver to him the information he wants. However, automatic initiation of communication with the devices of nearby confer- ence attendants may not be desirable for privacy reasons.
A person carrying a PAN-enabled device could find him-/herself in a diverse range of situations, whether personal or professional, that demand information delivery through the PAN device. Therefore, PAN devices should be compatible to enable information exchange in all cases. Returning to the conference example, imagine two attendants wanting to exchange information, discovering that their systems are not compatible and thus they need to exchange the information on paper. In such case, the market community would see PANs as a waste of time and money. Compatibility not only involves following a specific PAN standard but also software compatibility. For example, the file formats on each of the above devices should be able to be read by both devices.
PAN-enabled devices will be typically be carried by people for long periods of time. Thus, they need to be small enough in order to be carried around without burdening. PAN devices should therefore be as small and light as possible. Their energy efficiency should be enough in order not to trouble the user with frequent recharging of batteries, while maintaining a low device weight. Therefore, both low power consumption devices and high capacity batteries are desirable. Furthermore, the efficiency in terms of size and weight should not come at an expense over the price of PAN devices, in order to enable market acceptance.
Security issues are also crucial in PANs. Communications should be secure and difficult to eavesdrop. Consider the case of a malicious user approaching pedestrians carrying PAN devices. The unpleasant ‘Big Brother’ scenario of Orwell’s 1984 is obvious here and should be as difficult to achieve as possible. It should be very hard for the malicious user to obtain information regarding the unsuspected pedestrians, such as their names, home addresses, etc.
Therefore, robust authentication and encryption schemes should be developed in an effort to prevent unauthorized initiation of communication and eavesdropping. These schemes should be developed while keeping in mind the relatively low processing and power capabilities of PAN devices which stem from the requirements for reduced cost, size and weight.
Finally, as in the case of all wireless networks human safety issues are of great concern. A PAN device will typically be very close to the user for long periods of time and therefore even small dangers could potentially have some impact on the user over time. The good thing here is that PAN (like WLAN) devices typically transmit at power levels up to 0.5 W. Despite the fact that a final answer to the question of radiation threats to human health has yet to be given, it is reassuring for the consumers to know that the operating power levels of a PAN device are substantially lower than the 600-mW to 3-W range of common cellular phones.
11.1.3 PAN Applications
The main goal of PANs is freedom from cables and easy sharing of information between all kinds of wireless devices. The number of different possible applications can be very large. In the following we outline a representative set:
† Personal device synchronization. Automatic data synchronization between mobile wire- less equipment such as a mobile phone, notebook PC, etc. that execute similar applica- tions.
† Ad hoc connectivity. Transferring files, and other information to another user’s PAN- enabled device.
† Cordless computer. Wireless interfacing of devices like mice, keyboards, game pads to the computer.
† Cordless peripherals. Access to a variety of wireless peripherals including printers, scan- ners, fax, copier, storage systems, etc.
† Localized wireless LAN access. PAN-enabled devices can gain access to services offered by wired LANs through PAN-compatible Access Points (APs).
† Internet access. Downloads of email or browsing a web page using a PAN-enabled device, such as a mobile phone.
† Wireless synchronization. Synchronization of portable devices with the stationary servers via PAN APs.
† Cordless telephony/headset. A user selects a contact name from a handheld, the handheld wirelessly prompts the mobile phone in its proximity to dial the number and the audio from the call is wirelessly forwarded to the user’s headset.
† Home automation. Seamless transfer of commands to PAN-enabled home devices. For example, automatic unlocking of the door upon the arrival of the user at his home, or automatic tuning of the television to the user’s favorite channel upon his entrance to the room.
† Electronic purchases/reservations. PAN devices can be used to electronically book tick- ets. For example, the PAN device of a user can be programmed to instantly initiate a request for booking a ticket for a specific flight when the user enters the airport, thus avoiding the long queues and waiting times of the traditional booking procedures.
† Emergency situations. Medical devices with PAN interfaces can be used in order to increase the safety of patients. For example, pacemakers could be monitored and controlled remotely through PAN interfaces, or can be programmed to immediately call an ambulance while also transmitting the patient’s medical condition in the case of a heart attack or other serious health problem.
11.1.4 Scope of the Chapter
The remainder of this chapter provides a detailed presentation of technological alternatives in the PAN area. Section 11.2 presents the Bluetooth specification and discusses, among others, the way Bluetooth devices establish connections and exchange data. Furthermore, Blue- tooth’s provisions on security and power management are discussed. Section 11.3 is a similar discussion on the HomeRF standard. The chapter ends with a brief summary Section 11.4.