e-pg Pathshala Subject : Computer Science Module: Bluetooth Paper: Computer Networks Module No: CS/CN/37 Quadrant 1 e-text In our journey on networks, we are now exploring wireless networks. We looked at 802.11 protocol used for WLANs in the previous module. In this module we will focus on Bluetooth. Learning Objectives: In this module, we will examine the following with respect to Bluetooth : Components Connection setup Details 37.1Bluetooth Introduction Bluetooth is a technology that is used for very short range communication between mobile phones, PDAs, notebook computers and other personal or peripheral devices to replace cables connecting electronic devices. It operates in the license-exempt band at 2.45 GHz with FHSS. It has a range of only 10 m. The communication devices typically belong to one individual or group, hence it is sometimes categorized as Personal Area Network (PAN). Version 2.0 provides speeds up to 2.1 Mbps. Power consumption is low. Bluetooth is specified by an industry consortium called the Bluetooth Special Interest Group. It specifies an entire suite of protocols, going beyond the link layer to define application protocols, which it calls profiles, for a range of applications. There is a profile for synchronizing a PDA with personal computer. Another profile gives a mobile computer access to a wired LAN, and so on. 37.2 Bluetooth architecture Two or more devices sharing the same channel form a piconet. A piconet consists of a master device and up to seven slave devices. Any communication is between the master and a slave. The slaves do not communicate directly with each other. A slave can be parked: set to an inactive, low-power state. Two or more piconets form a scatternet (Fig. 37.1).
Figure 37.1 Bluetooth Scenario Master is the device in a Piconet whose clock and hopping sequence are used to synchronize all other devices (slaves) in the Piconet. It also carries out paging procedure and connection establishment. Slaves are units within the piconet that are synchronized to the master via its clock and hopping sequence. After connection establishment, slaves are assigned a temporary 3 bit member address to reduce the number of addressing bits required.
37.2.1 Piconets Point to Point Links are connected in a master - slave relationship, with one Bluetooth device functioning as master and the other as slave. Bluetooth devices in general are capable of functioning as masters or slaves. A Piconet is a network formed by a Master and one or more slaves (maximum 7). Each piconet is defined by a different hopping channel to which users synchronize to. Each piconet has max capacity (1 Mbps). The hopping pattern is determined by the master. The Piconet Structure is shown in Fig. 37.2. One device acts as the master, there are 4 active slaves. Other slaves are parked and some act as standby. Figure 37.2 Piconet example There are two different types of physical links that are supported - Synchronous Connection Oriented (SCO), and Asynchronous connectionless (ACL). SCO supports Point-to-Point full duplex between master and slave. It is established once by the master and kept alive till it is released by the Master. Master reserves slots used for SCO link on the channel to preserve time sensitive information. It is typically
intended for use with time-bounded information such as audio or video. It provides a circuit-switched connection where data is regularly exchanged. Retransmission is not necessary, since data is real-time. Up to 3 SCO links are supported per piconet. ACL on the other hand, is a momentary link between master and slave. No slots are reserved. It is a Point to Multipoint connection. Symmetric and asymmetric links are possible. It is primarily designed to cater to data traffic. This is a packet switched connection where data is exchanged sporadically as and when data is available from higher up the stack. Data integrity is checked through error checking and retransmission. We can have one ACL link between a master and a slave. 37.3 Protocol Stack in Bluetooth The protocol stack is shown in Fig.37.3. The layers below the red line are implemented on the Bluetooth hardware, while the protocols above the red line are software modules. Figure 37.3 Bluetooth protocol stack The bottom group is referred to as the Transport Protocol Group, and has the following functions. Radio Frequency (RF) is responsible for sending and receiving modulated bit streams. Baseband defines the timing, framing, and flow control on the link.
Link Manager is responsible for managing the connection states, enforcing fairness among slaves, and power management. Logical Link Control &Adaptation Protocol handles multiplexing of higher level protocols, Segmentation & reassembly of large packets, device discovery & QoS. 37.4 Packet Structure The packet structure is given in Fig. 37.4. It consists of an access code, header and payload. The payloads for voice (SCO) and data (ACL) differ. There is no CRC for voice, whereas CRC is present for data. Figure 37.4 Blu etooth packet structure The Access Code is used for Synchronization, Identification, and Signaling. Accordingly, there are different types of access codes. Channel Access Code (CAC) identifies a piconet. Device Access Code (DAC) is used for signaling procedures like paging and response paging. Inquiry Access Code (IAC) is used during device discovery. There are two types of IAC. General IAC (GIAC) is common to all devices, whereas, Dedicated IAC (DIAC) is for a dedicated group of Bluetooth devices. We will look at its use when we look at the connection setup. 37.5 Bluetooth State Machine The Bluetooth device could be in one of the following states: Standby, Inquiry, Page, Connected, and Transmit. In the connected state, it could be in one of 3 modes park, hold or sniff. The state machine given in Fig. 37.5 shows the state transitions. The device is initially in the standby state. When it wants to be discovered it enters the inquiry state. A process known as inquiry scan is carried out to discover devices in this state.
Inquiry Channel Figure 37.5 State machine Inquiry: A device that wants to be discovered will periodically enter the inquiry scan mode and listen for inquiry packets. Inquiry packets are sent by a device addressed to GIAC or DIAC. Transmission is repeated on the inquiry hop sequence of frequencies. When an inquiry message is received in the inquiry scan state, a response packet (Frequency Hopping Synchronization - FHS) containing the responding device address must be sent after a random number of slots. The inquiring device sends out an inquire on 16 different frequencies (16 channel train) Fig. 37.6. The receiver (device in standby mode), performs an inquire scan long enough for an inquiring device to send the inquire on 16 frequencies. It does the inquire scan frequent enough so that it is guaranteed to wake up during a 16 channel train. 16 Channel Train 18 16 14 12 10 8 Series1 6 4 2 0 0 2 4 6 8 10 12 14 16 18 Slot Figure 37.6 Inquiry Hop Train
When a device receives inquire, it will wait for a random time between 0 and.32 seconds before sending an FHS packet as a response. This is done to avoid collision with another device that also wants to send an FHS packet. The FHS packet contains, Device ID and Clock. After the inquiring device is done with inquiring procedure, it knows all of the radios (that are discoverable) within the range. Page: The next state that it moves to is the Page state. In this state, the master asks the slave, Will you connect to me?. The master uses the clock information obtained in the inquire state, about the slave to be paged, to determine where in the hop sequence, the slave might be listening in the page scan mode. The master sends a page message. The slave does a Page Scan. The page scan substate can be entered by the slave from the standby state or the connection state. In this state it listens to packets addressed to its DAC. On receiving the page message, the slave enters the slave page response substate. It sends back a page response consisting of its ID packet which contains its DAC, at the frequency for the next slot from the one in which the page message was received. We can see that this process is very similar to inquire. But this state is where it establishes the actual Piconet connection with a device that it knows about. The connection process is a 6 step communication between the master and the slave as shown below: Step Message Direction Hopping Pattern Pattern Source and Clock 1 Slave ID Master to Slave Page Slave 2 Slave ID Slave to Master Page Response Slave 3 FHS Master to Slave Page Slave 4 Slave ID Slave to Master Page Response Slave 5 1st Master Packet Master to Slave Channel Master 6 1st Slave Packet Slave to Master Channel Master It is a series of exchanges between the master and slave. At the end of this exchange, the master and slave are connected. In the Connected state, there are three Power Control Modes : Sniff, Hold and Park. Sniff mode is a low power mode in which the listening activity of the slave is reduced. In this mode, the slave listens for transmissions only at fixed intervals T sniff, at the offset slot D sniff for N sniff times. These parameters are given by the master when it issues the SNIFF command to the slave. In the Hold mode, the slave temporarily (for T hold sec) does not support ACL packets on the channel (possible SCO links will still be supported). By doing this, some channel capacity can be made free to do other things like scanning, paging, inquiring, or
attending to another piconet. The slave unit keeps its active member address (AM_ADDR). Park Mode is a very low power mode with very little activity. The slave however, stays synchronized to the channel. The parked slaves regularly listen for beacon signals at intervals decided by the beacon structure communicated to the slave during the start of parking. The parked slave has to be informed about a transmission in a beacon c hannel which is supported by the master to keep parked slaves in synchronization and send them any other information. Any message to be sent to a parked member are sent over the broadcast channel. It also helps the master to have more than seven slaves. 37.6 Frequency hopping Bluetooth channel is represented by a pseudo random hopping sequence through the entire 79 RF frequencies. Nominal hop rate is 1600 frequency hops per second. Channel Spacing is 1 MHz. The frequency range is 2400-2483.5 MHz. For a channel k, where k goes from 0 to78, the RF frequency for the channel will be 2402+k MHz. On the allotted channel, Bluetooth devices use Time-Division Duplex (TDD) scheme (Fig. 37.7). The channel is divided into slots, of 625 s duration. One packet can be transmitted per slot. Subsequent slots are alternatively used for transmitting and receiving. There is a strict alternation of slots between the master and the slaves. Master can send packets to a slave only in EVEN slots. Slave can send packets to the master only in the ODD slots. Figure 37.7 Frequency hopping per slot
It hops for every packet, and the packets are kept very short. A frame is of size 366 bits, and occupies a slot. The payload size after removing the header is : 366-72-54=240 bits = 30 bytes. Slots can be reserved for voice in a synchronous link. Frames can occupy up to 5 slots to improve channel efficiency. Slots may be reserved or allocated. Both these slots are interleaved to support synchronous (SCO) and asynchronous data (ACL) as shown in Fig, 37.8. Reserved slots are for time-bounded info, for e.g. voice. If 1 byte is transmitted every 0.125 ms, 30 bytes take up 3.75ms. 3.75ms/625μs = 6. Thus 1 in 6 slots will be used for transmitting an audio packet of 30 bytes. Allocated slots are Asynchronous and on-demand. They help in Collision-free polling, reservation, and allocation. Figure 37.8 Time Slots in the SCO Link and the ACL Link 37.6 IEEE 802.11 vs. Bluetooth
Having discussed both IEEE 802.11 and Bluetooth, it is time to look at a comparison of the two protocols. This is shown in Table 37.1 It can be seen that the two standards are complementary to each other and hence coexist by serving different purposes. Table 37.1 IEEE 802.11 vs Bluetooth IEEE 802.11 Bluetooth Frequency 2.4 GHz (802.11, 802.11b) 5 GHz (802.11a) Data rate 1, 2 Mb/s (802.11) 5.5, 11 Mb/s (802.11b) 54 Mb/s (802.11a) 2.4GHz 1 3 Mb/s (53-480 Mb/s in proposal) Range round 100 m within 1-100 m, depending on the class of power Power consumption PHY specification higher (with 1W, usually 30 100 mw) Infrared OFDM FHSS DSSS lower (1 mw 100 mw, usually about 1mW) (adaptive) FHSS MAC DCF PCF Slot allocation Price Higher Lower Major application Wireless LAN Short-range connection 37. 7 Summary We have had an overview of the Bluetooth technology, and have introduced the terminology used in Bluetooth. We have discussed about Piconets, Connection state machine, Protocol stack, and frequency hopping. Acknowledgements & References :
1. Computer Networking: A Top Down Approach Featuring the Internet, 6th edition. Jim Kurose, Keith Ross Addison-Wesley, 2012. 2. Computer Networks: A systems Approach, 4 th edition, David Peterson, Davie, Morgan Kauffman, 2012. 3. Computer Networks An Open Source Approach, Ying-Dar Lin, Ren-Hung Hwang, Fred Baker, McGraw Hill, 2012.