Bluetooth in Mobile Devices

Bluetooth in Mobile Devices Vidar Rinne Mälardalen University School of Innovation, Design and Engineering Computer Science: Game Development vre03001@student.mdh.se Abstract The basic idea of Bluetooth was to replace cables with a wireless technology that was both cheap and energy efficient. Today, Bluetooth has become a standard and is used in many electronic devices, especially mobile devices like mobile phones, headsets, PDAs etc. more than anyone could ever imagine at the time of its early days. How could a basic idea of just cutting the cables become a worldwide standard? How did Bluetooth outperform its competitors? In this overview paper, we answers these questions and explain the basics of Bluetooth technology including features, security, protocol stack and layers, limitations and performance. We also cover how the technique has been developed until today, how it is expected to be in near future, how it weighs against WLAN, Zigbee and how Bluetooth outperformed IrDA. Keywords Bluetooth, wireless communication, security, profiles, protocol stack, application layers, Special Interest Group, SIG, WLAN, Zigbee. 1 Introduction Bluetooth has become important for communication between mobile devices because of its low cost, both in terms of money, power usage and that it has no need of cables [1; chapter 4.6, pp. 310 311]. The need of cables for connection between mobile devices would not be an option in the long run. An early alternative solution for wireless communication was Infrared Data Association (IrDA), but because of its very limited range and easily breakable connection such as a piece of paper between the devices would kill the link, IrDA was Mattias Andersson Mälardalen University School of Innovation, Design and Engineering Mecatronic man04008@student.mdh.se not an option [3; chapter 9, pp.179 183], [4; chapter 9, pp.196 200]. Today, Bluetooth is a standard for wireless communication and is widely used. Nowadays, it is difficult to find a mobile phone without Bluetooth. The Bluetooth saga began in 1994 when engineers at Telefonaktiobolaget LM Ericsson, later on Ericsson and now Sony Ericsson, realized that within a short period, it would be necessary with a standard for low cost and low energy wireless communication between mobile devices, especially between mobile phone and its headset, which will be used as a cable replacement. The plan was to use the unlicensed frequency band (2.4 to 2.48 GHz in this case). In the year of 1997, Ericsson decided it was better if more companies used the technology, otherwise, the technology would probably not survive, and therefore, Ericsson began conversations with other companies. One year later, the Bluetooth Special Interest Group (SIG) was founded. SIG was in cooperation with several large companies such as Ericsson, Intel Corporation, IBM, Nokia Corporation and Toshiba Corporation. The Bluetooth SIG released the first version of Bluetooth, Bluetooth v1.0, in July 1999. After the release of the specification, the interest among other companies raised and in December, four more promoters (3Com, Lucent Technologies, Microsoft and Motorola) joined the group. In the first year of the new millennium, the first Bluetooth products came out to the market. In the same year, the Bluetooth technology was wide spread and thousands of other companies joined the Bluetooth SIG [1; chapter 4.6, pp. 310 311], [3; chapter 1, pp.5 8], [4; chapter 2, pp.18 25]. Bluetooth v1.0 had some limitations, a few of the most important changes made until today were in general improvements of data rate and security but 1

also the software layers to improve usability [4; chapter 10, pp.218], [5a]. In this paper, we will cover the development, improvements and problems in various Bluetooth versions including features, limitations, technical issues, security, how Bluetooth stands against WLAN, Zigbee and IrDA and what is expected in near future. 2 The Development of Bluetooth When developing Bluetooth, the idea was mainly to cut the cable [6a]. It can easy become a snake nest when a cable from every device that needs to be connected lies among each other. Since very few cables between mobile devices are very long, it was not supposed that the width of the wireless range should be very long, it was more important with a low power consuming [4; chapter 2, pp.18 25]. Most of the layers in the Bluetooth stack are based on already existing layers and protocols [3; chapter 5, pp.60]. In the earliest versions, the protocols were made just for a specific purpose [1; chapter 4.6.2, pp 312 13]. A group, whose task was to find bugs and security holes, was selected and eventually mistakes would be corrected in future versions [4; chapter 10, pp.218]. 2.1 The Name Bluetooth The name Bluetooth comes from a Danish old Viking king, Harald Blåtand, Blåtand is Danish for Bluetooth. His last name comes from that he had a blue tooth. The king also had a great ability to unite folks and make them speak to each other, the same as Bluetooth is meant to do [4; chapter 2, pp.25 28]. The Bluetooth logo is a conjunction of the runic letters for H and B [4; chapter 2, pp.25 28]. Figure 2.1 The Bluetooth logo 3 Bluetooth and Networking TV broadcasting is an early example of one waycommunication, a transmitter sends out radio waves of a specific frequency and a receiver receives it. The transmitter just sends out and takes no notice of however if it reaches the receiver. It is up to the receiver to catch the data, fully, corrupted, or not at all. Bluetooth on the other hand is an example of twoway communication. This means that both sides can transmit at the same time and that acknowledgement packet can confirm the connection status and it s possible to resend corrupted data if needed. If an audio connection is set, it is better with a few packet losses because it would be choppy and might have long time delays, but if a document is to be sent, it is probably better to wait a few moments extra to get the whole file. 3.1 Radio Wave Communication Bluetooth uses radio waves to communicate. Radio waves instead of wires has been used for a long time, mostly known are radio, TV and satellites. The frequency of the waves in Bluetooth is between 2.4 and 2.48 GHz. This range is often called the 2.4 GHz or ISM (industrial, scientific, medical), it is unlicensed [4; chapter 1, pp.8 9], and the radio waves in this frequency fade away rather fast, normally 10 100 meters for Bluetooth and Zigbee [9], 100 meters for WLAN (Wireless Local Area Network) [4; chapter 9, pp.203 209], depending on the power usage of the radios. Short range means that it is possible to have many different nets in a relative close distance without interfering each other. That is why the frequency is widely used in WLANs all over the world. This frequency also makes water molecules vibrate and is therefore used in microwave ovens [4; chapter 6, pp.118 122], [7]. Bluetooth uses frequency hopping to avoid interferences from other devices using the same frequency. There is a small risk that Bluetooth s frequency hopping will corrupt for example a WLAN transmission [7]. 3.2 Interconnection A piconet is formed when a slave is connected to a master. The piconet topology in Bluetooth uses 3 bit addresses, which means 2 3 = 8 devices in a topology, 2

whence at least one must be a master [2; chapter 2. pp 28 30]. Scatternet is several different piconets that are interconnected into a greater network. In a scatternet, a device could belong to different piconets and got different tasks for different piconets [1; chapter 4.6.1, pp. 311], [2; chapter 2. pp 28 30]. Figure 3.1 (the original picture is located in [2; chapter 2. pp 28 30]) describes three piconets (the large circles) making a scatternet. The unit inside the intersection between piconet A and B acts as a master in B and as a slave in A [2; chapter 2. pp 28 30]. A slave can act as a bridge slave like the one between Piconet A and C [1; chapter 4.6.1, pp. 311]. 4.1 The Protocol Stack The Bluetooth protocol stack is not similar to OSI (Open System Interconnection) reference model, TCP/IP, 802 or any other networking protocol [1; Many protocols are packed into layers [1; chapter 4.6.3. pp 313 316], we will describ e the layer s, from bottom to top. Application/Profiles Audio Other LLC RFcomm Telephony SDP Logical link control adaption protocol Control Link manager Baseband Physical radio Figure 4.1 The Bluetooth v1 stack. The original picture can be found in [1; chapter 4.6.3. pp 313 316] Figure 3.1 Piconets making a scatternet 4 Version 1 specifications Version 1.0 has a theoretical limit of 1 Mbps with an estimated practical speed of approximately 720 kbps, the lost bandwidth is used for header information [2; chapter 2.3.4, pp.31], [5b]. The basic developments in version 1.1 were merely fixing some small issues, bugs, implementation of Received Signal Strength Indicator (RSSI), which is used to measure and control power levels. Version 1.1 also added support for non secure channels. For version 1.2, the main extends were implementation of adaptive frequency hopping spread spectrum, which is used to avoid overloaded channels. Faster discovery and connection between devices, faster effective transmission speed and retransmission of corrupted packets were also added [5a], [8]. Radio Layer: The task of the radio layer is to make the data bits travel from the sender to the receiver. The frequency bands are divided into 79 pieces of 1 MHz channels and uses frequency hopping. When transferring data, its transfer in 625 µs per channel, then it hopes to another channel. This gives about 1600 hops per second. Every unit connected to the server shift frequency at the same time [1; chapter 4.6.3. pp 313 316]. Baseband Layer: The base band layer s task is to divide data into frames and defines which devices that will be masters and what role the masters will have. When all masters are defined, all remaining devices are set to slaves. The layer also synchronizes the frequency hopping and allocates several timeslots, which are 625 µs each. This is called Time Division Multiplexing (TDM). Each frame can be either 1, 3 or 5 slots. Masters always starts at even slots and slaves on uneven [1; Logical Link Control and Adoption Protocol Layer: Logical Link Control and Adoption Protocol (L2CAP) conceal the lower layers from the higher layers. It also divides packets of an upper limit of 64 KB to frames. Furthermore, it takes care of multiplexing and demultiplexing. When the L2CAP receives a packet from a lower layer, it decides what upper laye r to forward it to [1; 3

There has been a main extension to version 2.0 and then some minor improvements. The most important thing added in 2.0 compared to the earlier version was Enhanced Data Rate (EDR). With EDR enabled the theoretical speed reaches 3 Mbps or 2.1 Mbps in real life, which is about three times faster than previous versions. The technology that made this speed avail able is a combination of Gaussian Frequency Shift Keying (GFSK) and Phase Shift Keying (PSK) modulation while earlier versions only used GFSK. It also has better power efficiency since it has a shorter operating time (duty cycle). This version has higher power consumption but since the transfer speed is faster, it will use less time and therefore less power [5b], [8a]. This version also uses a maximum of 128 bits (1 8 bytes) for the PIN code compared to 48 bits in the first versions [6b]. The Link Manager Protocol (LMP) works parallel with the L2CAP but has no connection to upper layers. The LMP takes care of the establishment for logical link setups between devices like power management, authentication and quality of service [1; chapter 4.6.3. pp 313 316]. Radio Frequency Communication: The radio frequency communication protocol (RFcomm) emulates a serial port connection. Serial port devices can be mice, keyboards, modems etc [1; Telephony: This is a real time protocol that is used for voice and data calls, including setup and termination of calls [1; Service Discovery Protocol: The Service Discovery Protocol (SDP) discovers other Bluetooth devices, establishes connection, and examines what features are available for each connection [1; chapter 4.6.3. pp 313 316]. Audio Protocol: The audio Protocol is, as it sounds used for audio. It does not need any lower layers other than Baseband and Physical Radio. This is directly available from the applications and do not need to go through L2CAP [3; chapter 5, pp.70 72]. 4.2 Profiles There are several different profiles in Bluetooth where each profile has a specific purpose depending of what the task is, for example voice or file transferring. The different profiles are described below. Generic Access: Its main task is to set up and keep a reliable communication between master and slave. Applications are built upon it. This is a necessary profile and should be implemented in all Bluetooth devices [1; Service Discovery: This protocol finds out which services are available of the other device. This protocol should be implemented for Bluetooth to work [1; Serial Port: This profile emulates a serial link for application that is programmed for serial ports so that the data can be sent over a wireless connection via Bluetooth [1; Generic Object Exchange: It is a relationship of clientserver. The special is that a slave could be either a client or a server [1; LAN Access: This profile is used for when a Bluetooth device connects to a network [1; chapter 4.6.3. pp 313 316]. Dial up Networking: Allows a computer to use mobile phones as modem and make calls [1; chapter 4.6.3. pp 313 316]. Fax: This is similar as dial up networking but instead it sends and receives fax [1; Cordless Telephony: This is used when using a mobile phone as a telephone receiver (horn) connected to some kind of base station [1; chapter 4.6.3. pp 313 316]. Intercom: Allows two mobile phones to act as walkie between mobile phone and hands free device [1; talkies [1; Headset: Is the name when using wireless channels Object Push: This profile is used to exchange simple objects as business cards, pictures or data files [1; File Transfer: A profile for more general file transfer than just small files and pictures using object push [1; Synchronization: Allows synchronization of files etc. between a computer and a mobile phone or PDA [1; 5 Version 2 Specification 4

Version 2.1 uses an extended inquiry response, which means that when discovering other devices it gets more information like the services it supports, name and some other pairing information. This way it is easier to filter devices before a connection. Another thing added is the Sniff Subrating, which means that a device like a mouse or a keyboard can make a decision of how often it will send ping or keep alive packets. In this way, it can choose a longer ping time up to about 10 seconds, which will give a longer battery time. When in this low power sniff mode it can save up to about a third more power. Compared to earlier versions these keep alive packets were sent several times each second. An Encryption Pause Resume function has also been added to this version; it allows the devices to refresh their encryption keys, which then offers much better security for long time connections. A fourth thing is Secure Simple Pairing, which improves the security. Near Field Communication makes it possible to automatically connect to for example a headset that is in very close range, which has a NFC radio implemented. This function is a part of the Secure Simple Pairing. Another example is when bringing two devices near each other they will synchronize information. The Bit Error Rate (BER) performance has also improved in this version [5b], [8a]. 6 Security Security is important for all networks and especially for wireless commu nication where it is easy to pick up and listen to the radio waves, which are broadcasted out in the open air. Therefore, it is necessary to encrypt the data and use device authentication to avoid un authorized users. 6.1 Bluetooth Security Modes Bluetooth uses three security modes: Non Secure: There is no security at all [3; chapter 12, pp.215 216], [4; chapter 7, pp.152 157]. Service Level Enforced Security: After a connection is established, the security mode is initiated. This security mode is initiated in the L2CAP layer [3; chapter 12, pp.215 216], [4; chapter 7, pp.152 157]. Link Level Enforced Security: The security mode is initiated before the connection is established. This is the most secure mode of them all. This security mode is initiated in the LMP layer [3; chapter 12, pp.215 216], [4; chapter 7, pp.152 157]. 6.2 Key Management The Key Management uses three types of keys, which are PIN code, private link key and private encryption key. Personal Identification Number (PIN) Code is a userdefined code. All users that connect to the network must enter the same code before a connection is established [4; chapter 7, pp. 152 157]. Private Link Key (also called authentication key) is a set of four different types that are created at every transmission. The first called unit key and is, decided by one of the sides, the device address can be used to calculate one. The second is a combination key that is determined from both sides. There also is a type called master key, which is the centre of a network (piconet). This is used when a master is going to send the same information to all slaves. The last kind is an initialization key that is used to protect initialization parameters [4; chapter 7, pp.152 157]. Private Encryption Key is made of the link key. The key is changed at every time the encryption is needed. The key can also be of different lengths [4; chapter 7, pp.152 157]. What happens in the security mode, see picture 6.1 below redrawn from the original image in [4; chapter 7, pp.152 157]. PIN Link Key Encryption Key Authentication Encryption Figure 6.1 Security steps. PIN Link Key Encryption Key The first step is that the users enter a PIN code. The second step is that a private link key is generated on a side and exchanges it with the other side. The third step is that a private encryption key is generated on a side and exchanges it with the other side. If any of th ese keys do not match, the connection will be aborted [4; chapter 7, pp.152 157]. 5

7 Problems Bluetooth v1 was highly criticized due to security. An aspect was that the security keys, that used 48 bits, which was claimed to be of too short length. It was said that it easily could be hacked [4; chapter 10, pp.218]. Before Bluetooth implemented adaptive frequency hopping, it could easier be interfered by for example WLAN and microwave ovens. Even after this implementation, it still can be interfered if a microwave oven is within a very short range [4; chapter 6, pp.118 122], [7]. Bluetooth version 1 also has a low practical speed compared to the theoretical speed. The speed of B luetooth v2 was increased b y three to a theoretical speed of 3 Mbps but even in this version suffers from packet headers which limits the pay load size (the actual data speed) to just a bit over 2 Mbps [5d], [8a]. Even if this version of Bluetooth is faster than previous versions, it still is very limited compared to WLAN, which rarely is below 54 Mbps. The first hardware of the Bluetooth radio used two chips, one for radio and another for controller. Because of this, the radio used much more power than it was supposed to. Later versions integrated the two chips into a single one, which resulted in lower power usage [4; chapter 10, pp.219 220]. 8 Performance and Results WLAN can draw more power and is therefore stronger in speed and range; it is also more expensive to build than Bluetooth [4; chapter 9, pp.203 209]. WLAN is made more for replacing cables in networks compared to Bluetooth, which was first built to replace cables between two or a few mobile devices. WLAN has a more further complex setup while Bluetooth was made to make its connection simple and fast [4; chapter 9, pp.203 209]. On the other hand, WLAN has more improved security [8b]. If interference between Bluetooth and WLAN occurs, Bluetooth will try to send the packet on a different channel and WLAN will see this as a collision, slow down and try to resend later in the same way as Ethernet does [7]. IrDA was already on the market when the first Bluetooth products arrived. At that time, Bluetooth was expensive, drew more power and its data transfer was significantly slower. However, IrDA had a few major drawbacks. As a comparison to, between IrDA had a very short range, about 1 m, the devices had to point to each other within a 30 degree angle and a thin piece of paper between the devices would interrupt the transmission. Since Bluetooth uses radio waves and IrDA uses light, Bluetooth is capable of handling the largest drawbacks of IrDA [4; chapter 9, pp.196 200], [4; chapter 9, pp.203 209]. Since IrDA cannot operate outside a 30 degree angle, it is not suitable for networks as technologies based on radio waves [4; chapter 9, pp.196 200]. When Bluetooth s data rate was increased with its newer versions, it speeded up the downfall of IrDA as a cable replacement compared to Bluetooth [4; chapter 9, pp.196 200]. The technique in Zigbee is very similar to Bluetooth s but is rather used for Industrial control and monitoring. Zigbee was designed to have very low power consumption and the data rate is much slower than in Bluetooth. When developing new versions of Bluetooth, it has always supported backward capability. It means that different versions can interact with each other [5a]. The data transfer speed is always at its maximum limitation and depending on how far the transmission range is, the power consumption will increase acc ording to the power classes seen below. There are three power classes, which describe how much power is needed for a specific range: Class 1: has a range of 100 meters and requires 100 mw. Class 2: has a range of 10 meters and requires 2.5 mw. Class 3: has a range of 1 meter and requires 1 mw. As the class scheme shows, the power demand grows exponentially in proportion to the range [4; Appendix B, pp.252]. 6

9 What to Expect in the Future The next version, which is expected to be called WiMedia Alliance, is going to be quite similar to current versions but according to an announcement from the Bluetooth SIG, it will be using Multi Band Orthogonal Frequency Division Multiplexing on the Ultra Wideband (UWB). This technology will raise the speed from 3 Mbps to about 480 Mbps (theoretical speed) and in this way it will meet the high speed demands of today [5c], [8c]. 10 Summary and Conclusion Almost every new mobile phone supports Bluetooth. From the early days when some engineers at Ericsson decided to invent Bluetooth as a cable replacement, it has grown to a worldwide industry. Already opponents, such as IrDA, had no chance in the long run compared to Bluetooth s more advanced technique. When Zigbee hit the market later on, Bluetooth was already widely used and even if Zigbee required much less power consumption, the data rate was much slower than Bluetooth s. The main purpose of the Zigbee development was not to cover the same areas as Bluetooth. Bluetooth s usability is constantly developing for new areas and is today intersecting with WLAN. Today s versions of Bluetooth is three time faster than the earliest but according to the present days, the speed isn t very impressive, although next version, that is expected to hit the market in a short period of time, the speed will be fast enough for today s demands and the speed will even be faster than current WLANs. Security is still not as strong as in recent WLAN but has been improved compared to earlier versions when it didn t was expected to be as WLAN alike as it has become. The area of Bluetooth usability has spread and is now used in printers for example. Printers are not very mobile so the original idea of cable replacement in just mobile devices has broadened too many electronic devices. When the approach Bluetooth in non mobile devices, for examples printers, it came up ideas, that Bluetooth could replace almost every wire between electronic devices. It is however not very common in nowadays. 11 References 1. Andrew S. Tanenbaum. Computer Networks, 4 th Edition. Pearson Education, Inc. ISBN: 0 13 038488 7. 2003. 2. Javier García Castaño. Algorithms and Protocols Enhancing Mobility Support for Wireless Sensor Networks Based on Bluetooth and Zigbee. Mälardalen University Press. ISBN: 91 85485 21 7. 2006. 3. Brent A. Miller, Chatschik Bisdikian. Bluetooth Revealed. Prentice Hall, Inc. ISBN: 0 13 090294 2. 2001. 4. Michael Miller. Discovering Bluetooth. Sybex, Inc. ISBN: 0 7821 2972 2. 2001. 5. BlueTomorrow: http://www.bluetomorrow.com a. Bluetooth Core Specification Versions: http://www.bluetomorrow.com/content/section/100/ 275/ b. Bluetooth Specifications: http://www.bluetomorrow.com/content/section/13/4 1/ c. High Speed Bluetooth: http://www.bluetomorrow.com/content/section/344/ 466/ 6. Bluetooth: www.bluetooth.com a. How Bluetooth Technology Works http://www.bluetooth.com/bluetooth/technology/wo rks/ b. Specification of the Bluetooth System, Volume 0, 2007 07 26. Core Specification v2.1 + EDR, Key Types, chapter 3.1. http://www.bluetooth.com/bluetooth/technology/buil ding/specifications/ 7. HP Invent: Wi Fi and Bluetooth Interference Issues, 2002. http://www.hp.com/rnd/library/pdf/wifi_bluetooth_co existance.pdf 8. Bluetooth: http://en.wikipedia.org/wiki/bluetooth a. Specifications and Features: http://en.wikipedia.org/wiki/bluetooth#specifications_ and_features b. WiFi: http://en.wikipedia.org/wiki/bluetooth#wi Fi c. Future of Bluetooth: http://en.wikipedia.org/wiki/bluetooth#future_of_blu etooth 9. Software Technologies Group: How does ZigBee compare with other wireless standards? http://www.stg.com/wireless/zigbee_comp.html 7