WIRELESS NETWORKS. Erina Ferro

Transcription

1 WIRELESS NETWORKS Erina Ferro National Research Council (C.N.R.) ISTI, Institute of C.N.R. C.N.R. Research Area, Via Moruzzi 1, Pisa, Italy Phone: Fax: ISTI/C.N.R. 1

2 ISTI/C.N.R. 2

3 PRO and CONTRA of WIRELESS Approach with respect to wired LANs Mobility PRO movements freedom, with respect to the cabled networks. Dynamic joining ad leaving new wireless users can dynamically join or leave the network, move among different environments, create ad hoc networks for a limited time and then leave. Simple to deploy Wireless networks are simple to deploy and, in some cases, they have lower costs compared to wired LANs. CONTRA Lower reliability due to interference Higher power consumption Data security threats due to the inherent broadcast properties of the radio medium worries about user safety due to continued exposition to radio frequency, lower data rates ISTI/C.N.R. 3

4 Interference Due to collisions in the same frequency band among wireless terminals or due to common-use hardware (as a microwave oven). Reliability See interference Data Security Contrarily to the cabled networks, in the wireless networks the transmission link is open to all the wireless terminals in the coverage area of a transmitter. Various levels of criptography are necessary, thus reducing the performance and increasing the cost of the hardware. Power Consuming Batteries are used, due to mobility. Attention must be put to an efficient use of them. Users Security RF transmissions are dangerous for users? Transmission power must be reduced as much as possible. IF (Infra Red) optical transmitters must not damage the eyes. Throughput Physical and bandwidth limitations ISTI/C.N.R. 4

5 Other problems with wireless LANs Cell handoff Mobile terminals must be able to pass from a cell to another one in a transparent way. Frequency bands devoted to particular applications must be approved and licensed. Problem solved thanks to wireless standards which today use frequency bands that do not need to be licensed. ISM GHz U-NII GHz N/A around 850 nm ISTI/C.N.R. 5

6 TYPICAL WIRELESS LAN STRUCTURES Ad-hoc Networks No infrastructure, no coordinating terminal Coverage area limited by the single terminal range Networks with infrastructure There is a coordinating terminal which handles services such as the connection with a cabled network or with WAN networks such as the Internet ISTI/C.N.R. 6

7 AT PHYSICAL LAYER The standards Bluetooth and IEEE cover different techniques at physical layer: infrared ray communications, which are practically unused in commercial products different radio frequency modulation techniques: frequency hopping spread spectrum (FHSS), used by Bluetooth devices, direct sequence spread spectrum (DSSS), complementary code keying (CKK), and orthogonal frequency division multiplexing (OFDM), used in commercial devices. ISTI/C.N.R. 7

8 INFRARED WLAN They use the infrared portion of the spectrum. Used for many remote control devices ADVANTAGES: IR spectrum is virtually unlimited, thus high transmission capacity can be reached. No license problems. Radiation cannot pass through walls or other opaque surfaces, but it can be spread by colored objects. Thus a WAN in a room can be realized by using the roof or walls reflection. No interferences between two WLANs in two adjacent rooms Cost and complexity of IR devices quite low. DISADVANTAGES Being a luminous radiation, other luminous sources (sun or artificial light) can interfere. Power can be increased but consuming problems must be taken into account. ISTI/C.N.R. 8

9 Spread Spectrum WLAN The transmission medium is the radio frequency Spread spectrum technique was initially used in military environment because the spreading of the information content of a signal made the interception more difficult. Spread spectrum transmitters use same power levels than narrowband transmitters but the power spectrum density is lower as the signal spectrum is wider. THUS: spread spectrum and narrowband spectrum signal can occupy the same bandwidth with very low interference. Multi-path fading: signal reflection due fixed or mobile objects. As the path length is generally different, signals arrive to destination with different delays, which provoke differences of phase on the signals. At the receiver, the signal is the sum of the component signals, and it results to have lower amplitude than the original signal (fading). ISTI/C.N.R. 9

10 Techniques to implement the spread spectrum: Frequency hopping spread spectrum (FHSS) Direct sequence spread spectrum (DSSS) Frequency hopping spread spectrum (FHSP) Transmission by using a pseudo-random radio frequencies, hopping frequency by frequency at fixed time interval (hop rate). Only receivers with the correct hopping sequence can correctly receive data. ISTI/C.N.R. 10

11 Direct sequence spread spectrum (DSSS) Each single bit of the original signal is coded with a bit sequence ( chipping code ) which spreads the signal on a bandwidth wider than that one requested for transmission. The expansion is directly proportional to the number of bit constituting the chipping code. The spreading sequence is generated at a higher speed than the original signal data rate. In the transmitter the information signal is X-ORed with the chipping code. The derived symbol is modulated and transmitted. In the receiver the original signal is obtained by means of a despreading operation. TX signal before spreading RX signal before correlator TX signal after spreading RX signal after correlator (de-spreading) ISTI/C.N.R. 11

12 FHSS and DSSS COMPARISON FHSS Sensitive to noise during a single slot but on long scale they can have an almost error-free communication as it moves along the whole bandwidth. Modulation schemes simpler than in DSSS Maximum data rate is limited by the channel bandwidth, and they may cause higher interference on other systems due to the use of the whole band. DSSS Higher data rates reachable Modulation schemes more complex than in FHSS Predominance of the DS systems on the market. ISTI/C.N.R. 12

13 BLUETOOTH (BT) ISTI/C.N.R. 13

14 Standard for wireless communications based on low-cost, short-distance radio system. Range of applications (known as Wireless Personal Area Nework) Substitute the cables (computer, fax, joystick, mouse, printers.) Act as a bridge (towards already existing networks) Realize small ad-hoc networks. The original idea was born in 1994 at the Ericsson Mobile Communications. In 1998: Ericsson, Nokia, IBM, Toshiba, and Intel formed the Bluetooth SIG (Special Interest Group) In 1999: first release of the protocol In 2000: 3COM, Agere, Microsoft, and Motorola joined the SIG, and the first device (an headset for mobile phone) was put on the market. Bluetooth is currently at version 1.2. Since March 2002, version 1.1 has been compatible with the IEEE standard. ISTI/C.N.R. 14

15 THE PROTOCOL TCS=Telephony Control Specifications SDP=Service Discovery Protocol It does not define a radio interface only but a whole communication stack that allows the devices to find each other and advertise the services they offer. It is Level-organized. Some functions are distributed on more than one level. ISTI/C.N.R. 15

16 the Link Manager layer handles the type of link configuration, authentication, security, quality of service (QoS), power consumption, transmission scheduling. The Control supplies a command interface to the Link Manager and Baseband levels, thus providing a coherent interface to hardware developed by different manufactures. The L2CAP (Logical Link Control Adaptation Protocol) layer supplies connection-oriented and connectionless services to the upper levels. Its functions include: i) protocol multiplexing, ii) segmentation and reassembly of the protocol data units coming from the upper levels; iii) QoS support. It is possible to implement IP directly on L2CAP, but Bluetooth 1.1 does not define a profile which implements this facility. Thus IP is typically implemented by using PPP over RFCOMM, a profile emulating a serial port. RFCOMM is useful because many already existing applications are based on serial communications. Up to 60 connections can be simultaneously active between two Bluetooth devices. ISTI/C.N.R. 16

17 THE RADIO LEVEL Operates in the 2.4 GHz band (known as ISM bad in USA). As NO license is necessary, BT devices must tolerate possible presence of interferences. USA and most Europe: 83.5 MHz are available, where 79 RF channels are allocated, 1 MHz distant. Japan, France, and Spain: only 23 RF channels are available, 1 MHz distant. Channels are accessible via the FHSS technique, thus the physical channel is represented by a pseudo-random sequence of the 79 (or 23) available RF channels hops/s. Devices working with 79 channels do not work in countries where the available channels are 23, and viceversa Signal rate: 1 Msymbol/s, which corresponds to a theoretic transfer speed of 1 Mbps, as the adopted modulation scheme is the GFSK (Gaussian Shaped Frequency Shift Keying). Physical channel divided into 625 ms slots, and the devices change frequency once every packet (a packet may last 1, 3 or 5 slots). ISTI/C.N.R. 17

18 Timing Multislot packets ISTI/C.N.R. 18

19 GFSK modulation is a variation of the FSK (Frequency Shift Keying) modulation. FSH: a bit of information 1makes a positive shift in frequency with respect to the nominal carrier where the FH system is working a bit of information 0 makes a negative shift in frequency of the carrier GFSK uses a bandwidth-time (BWT) product equal to 0.5, and a modulation index in the range BWT is the product of the frequency distance between two adjacent signals (0.5 MHz, the bandwidth) and the lasting time of 1 symbol (1 µs, the time T) BWT=0.5 corresponds to the minimum separation distance between carriers such as to guarantee the orthogonality (no cross-correlation) between signals in adjacent channels. ISTI/C.N.R. 19

20 POWER CLASSES Bluetooth devices are divided in 3 power classes, according to the max output power of the transmitter Power class Max output power Range (with no obstacles) mw (20 dbm) 2.5 mw (4 dbm) 1 mw (0 dbm) 100 m 10 m 10 cm ISTI/C.N.R. 20

21 THE BT RECEIVER Actual sensitivity level of a RF receiver is the level of the input signal to which a BER of 0.1% corresponds. Minimum actual sensitivity level for a Bluetooth receiver is -70 dbm. Radio Bluetooth devices have a loop-back facility, useful to measure their own performance (in BER). ISTI/C.N.R. 21

22 THE BASEBAND LEVEL Link Manager (LM) requires a connection, and the relevant LCs, at both the ends, create the connection ad maintain it during time Link Controller (LC): devoted to the transmission of packets in response to link manager (LM) commands. Baseband is responsible for coding and decoding operations, timing control at low level, and handles the single packets of a connection. ISTI/C.N.R. 22

23 ADDRESSING BT address is 48 bit long, (Bluetooth device address, BD_ADDR), IEEE_MAC compatible. Divided in 3 parts: LAP: for the generation of the sync word and for the FH; UAP: to initialize the error correction mechanisms (HEC, header error correction, and CRC, cyclic redundancy check), and the FH; NAP: to initialize the motor used for the cryptography. ISTI/C.N.R. 23

24 MASTER, SLAVE, PICONET, & SCATTERNET BT devices need to know the same hopping frequency to communicate. BT devices can communicate in Master mode Slave mode Master decides the hopping sequence; slaves must synchronize in frequency and time with the hopping sequence of master. PICONET: is a set of maximum 8 devices (7 active slaves interconnected among them and synchronized with the same master). It is the simplest configuration of a Bluetooth network. ISTI/C.N.R. 24

25 PICONET ISTI/C.N.R. 25

26 SCATTERNET: several piconets are connected together. It is the topology over which a multihop wireless network can be built. ISTI/C.N.R. 26

27 A device present in more than one piconet must work for a certain number of slots in a piconet (using the relevant frequency hopping sequence), then it must change to the other piconet by changing the frequency hopping sequence. A device can work as a master in a piconet and as a slave in another one but not as a master in more than one piconet. When a mobile device looses contact with its piconet, a Supervisor Timeout ensures the closure of all the connections of those devices which exit the piconet coverage area. ISTI/C.N.R. 27

28 Park Mode: to save battery, some devices can stay in park mode, i.e. they are disconnected from the piconet activity but are periodically informed by the master, so that they can be again active in the piconet, if possible (i.e. if there are less than 7 active). In a piconet devices, the hopping sequence is unique and derives from the BD_ADDR and from the clock of the master. When a slave joins a piconet, it is informed by the master on its BD-ADDR and clock so that the master can compute the frequency hopping sequence. TRAFFIC on a piconet: voice data ISTI/C.N.R. 28

29 Master decides the transmission times and frequencies by allocating slots for voice traffic and slots for data traffic. DATA TRAFFIC: A slave can transmit only in response to a master transmission addressed to this slave; If the master has transmitted data to a slave in a slot x reserved to data traffic, slot x+1 is reserved to that slave for a possible data transmission (in response). VOICE TRAFFIC: A slave transmits in voice reserved slots (assigned by master) independently of a previous transmission by master towards that slave. MASTER-SLAVE COMMUNICATIONS only: data from slave A to slave B of the same piconet must pass through the master. This procedure is called Time Division Duplex (TDD) ISTI/C.N.R. 29

30 THE BLUETOOTH CLOCK BT uses a real-time clock, which is a 28-bit counter set to 0 at the device switch on, and then increased every 312,5 µ s (1/2 slot). The counter cycle lasts about one day (23.3 hours). Each BT device has its own Native clock (CLKN). Two more clocks are defined: Piconet clock (CLK), which coincides with the CLKN of the master. Each active device in the piconet must synchronize its CLKN with CLK (CLKN+ offset2 =CLK). Master-new slave clock (CLKE), used by the master during the creation of a connection towards a slave before the slave synchronizes with master (CLKN+offset1=CLK). ISTI/C.N.R. 30

31 Once a master-slave link has been set up, there are two possible services: Asynchronous connection-less (ACL) Synchronous Connection Oriented (SCO) ISTI/C.N.R. 31

32 ACL LINKS Master can have any number of ACL links with different slaves, but only one ACL link can exist between two devices (i.e. master cannot have two ACL links with the same slave). ACL link realizes a packet switch connection between master and slave Master decides which slave must receive or transmit Some ACL packets must be particularly protected against errors through a 16-bit CRC applied to the payload, optional 2/3 FEC convolutional code, and optional ARQ (automatic repeat request, i.e., packet retransmission on error). All slaves listen to the master, but if a slave is not able to decode its address in a received packet it is not allowed to transmit in the next slave->master slot. A packet transmitted by a slave with a special address may be broadcast by the master to all the active devices. ISTI/C.N.R. 32

33 The configuration of the ACL links, from the point of view of bandwidth and quality of service, is done through an interface offered by the Link Manager. The configurable parameters are: type of QoS (none, best effort, and guaranteed best effort, the latter being the default), the token rate (the data transfer rate guaranteed on that link; no default), the token rate bucket size (the buffer size for the received data, default is zero), the peak bandwidth (default is not specified), the latency (default is not specified), the delay variation (the maximum allowable difference between the delays of packets, default is not specified). ISTI/C.N.R. 33

34 SCO Links Furnishes: Symmetric link between master and slave Reserved bandwidth Exchange of data on a periodic basis by using slot devoted to this type of connection. A master may have up to 3 SCO links with 3 different slaves, at the same time. A slave may have up to 3 SCO links with the same master or with 2 different masters. SCO packets are never retransmitted. A slave can reply to a SCO packet, in the reply-sco slot, even if not able to decode its address. A periodic SCO transfer can be interrupted only by a broadcast LMP (Link Manager Protocol) message. ISTI/C.N.R. 34

35 ACL transfer SCO transfer ISTI/C.N.R. 35

36 BT PACKETS STRUCTURE ISTI/C.N.R. 36

37 Access Code: identifies packets arriving from a specific master. Channel Access Code (CAC). Derived from the BD_ADDR of the master, it is used to identify packets of a certain piconet. Device Access Code (DAC). Used in the page phase. General Inquiry Access Code (GIAC). Used by all the BT devices of a piconet during the inquiry phase. Fixed by BT specs. Dedicated Inquiry Access Code (DIAC). Same function as GIAC, but used to restrict the inquiry to specific BT devices. HEADER: packet s info. 18 bits long, 1/3 FEC coded. AM_ADDR : active member address. Up to 7 slaves can be addressed. Type: Packet type (SCO, ACL, NULL or POLL) +payload FEC used+number of used slots FLOW: if ON, overflow ARQN: automatic repeat request. (No). Only for ACL links. If a packet is correctly received, the slave sets ARQN to 1 in the response packet. Otherwise master retransmits until ARQN=1 or the flushing timeout forces the transmission of a new packet. SEQN: indicates if retransmitted packet or a new one. HEC: header error check. 8 bit CRC applied to the header. ISTI/C.N.R. 37

38 BT LINK CONTROLLER Statuses of a BT device: Standby Connection Sub-statuses. Temporary, used to create a new piconet or to add new slaves to an already existing piconet. Page (used for a new connection between 2 BT devices) Page scan (used for a new connection between 2 BT devices) Inquiry (search for a new BT device) Inquiry scan (status for being discovered) Master response (used for a new connection between 2 BT devices) Slave response (used for a new connection between 2 BT devices) Inquiry response (send info to the discoverer device) ISTI/C.N.R. 38

39 HOPPING SEQUENCES 10 different hopping sequences are defined, associated to the previous statuses and sub-statuses, 5 for countries where 79 channels are available, and 5 for countries where 23 channels are available. Page hopping sequence: 32 (16) frequencies among the 79 available. Periodic sequence with period = 32 (16). Page response sequence: 32 (16) frequencies in one-to-one correspondence to the page hopping sequence. Inquiry sequence: 32 (16) frequencies among the 79 available. Periodic sequence with period = 32 (16). Inquiry response sequence: 32 (16) frequencies in one-toone correspondence to the inquiry hopping sequence. Channel hopping sequence: used in the Connection status, constituted by 79 (23) frequencies distributed in the available bandwidth. ISTI/C.N.R. 39

40 INQUIRY: procedure used by the BT standard to allow applications where the source unit does not know the destination address, for ex. to discover new BT devices in the coverage area of the piconet. Those must be in inquiry scan status. The device in inquiry state collects the BD_ADDR and the clocks of all the devices which reply to it. Master sends inquiry messages and waits for responses. Slave replies sending its parameters after a back-off procedure to avoid collisions (not frequent event due to the frequencies hopping): A random number between 0 and 1023 is selected Slave returns in connection (scatternet) or standby (piconet) state for a number of slots equal to the selected number After this back-off time, slave returns in inquiry scan state If another inquiry message is received, slave moves to inquiry response state and sends a packet exactly 625 µs after the reception of the inquiry message. If not, slave returns in connection or standby state. ISTI/C.N.R. 40

41 PAGE: a connection is established. To establish a connection only the BD_ADDR of the destination is necessary, while the destination CLKN is useful to fasten the page procedure. In fact the master uses an estimation of the slave CLKE to make a prevision on the frequency which will be used by the slave to start the page scan phase. The unit which starts the page procedure automatically becomes the master. BT defines a mandatory page scheme. Page phase follows inquiry phase. In page scan status a BT device listens to the channel searching for page packets to it addressed. The scan period lasts for about 20 s. In this period a frequency for the scan is selected according to the page hopping sequence. A slave in page scan status starts a synchronization procedure between master and slave, after receiving a page message from the master. ISTI/C.N.R. 41

42 After receiving the page message, after 625 µs the slave replies with a packet with the same ID, by using a frequency belonging to the page response frequency. After sending this response msg, the slave activates its receiver and waits for the FHS (frequency hopping sequence) packet containing the master parameters. If correctly received, slave sends back an Ack message by using the page response frequency. After sending this message, the slave transceiver synchronizes on the channel hopping sequence and slave moves to the connection state. Master initializes the connection mode by sending a POLL packet, to which the slave may reply with a packet of any type (typically a NULL packet). If the POLL packet is not received or the reply to it is not received, both master and slave return in the page and page scan statuses, respectively. ISTI/C.N.R. 42

43 PAGE PROCEDURE ISTI/C.N.R. 43

44 CONNECTION-ACTIVE mode CONNECTION-HOLD mode: an ACL link toward a slave can be put in hold. Links SCO are supported. This mode is useful to solve problems of capacity in terms of available bandwidth. A BT device can use the hold mode to temporarily stop the data transfer on an ACL link to save capacity for inquiry, page, scan or for passing to work on another piconet. A low power state can be entered CONNECTION SNIFF mode: a slave can reduce it listening activity on the channel. Master can transmit to slave which is in sniff state only by using reserved slots (towards that slave), called sniff slots. CONNECTION-PARK mode: a slave does not want to be really active in a piconet but wants to remain synchronized. Low power state It is a way to allow more than 7 slaves in the piconet (only 7 active) Master creates a beacon channel to allow synchronization of slaves in connection park mode. Master uses it for: Transmitting special packets for slave re-synch. Transmitting special messages of changes in the beacon channel structure Broadcasting packets Unparking some slaves ISTI/C.N.R. 44

45 STRUCTURE OF A PICONET A BT device can participate in more than one piconet if working in time multiplexing mode. During the hold, park, or sniff mode in a piconet, a BT device can become active in another piconet Master-slave switch between piconets ISTI/C.N.R. 45

46 HIGHER LEVELS IN THE BT STACK Host Controlled Interface (HCI): interface to commands for the link manager and baseband layers. Logical Link Control and Adaptation Layer (L2CAP): furnishes connection-oriented and connection-less services to the higher levels. Protocol multiplexing,qos (it can be negotiated), segmentation and reassembling of the PDU coming from higher levels and with size larger than 64KB. It is possible the realization of TCP/IP transmissions directly on L2CAP, but this must be better defined. RFCOMM: protocol used to emulate the RS232 serial ports. Up to 60 connections at the same time between two BT devices. Most of the TCP/IP implementations are based on PPP realised on RFCOMM. SDP (Service Discovery Protocol): allows to discover which services (and their characteristics) are around a mobile device. ISTI/C.N.R. 46

47 SECURITY Bluetooth security is divided into three modes: Mode 1: non-secure Mode 2: Service Level enforced security (after channel establishment) Mode 3: Link Level enforced security (before channel establishment). Authentication and encryption at the link level are handled through the use of four basic entities: the Bluetooth device address, which is a 48-bit unique identifier assigned to each device; a private authentication key (random number); a private encryption key (random number); a 128-bit frequently changing random number, dynamically generated by each device. A PIN should be entered for each Bluetooth device at initialization; however, key distribution is not covered by the Bluetooth specifications. ISTI/C.N.R. 47

48 IEEE ISTI/C.N.R. 48

49 In 1997 IEEE Institute approved a standard for wireless LAN called This standard specified: the development of devices able to transfer data at 1 or 2 Mbps, the MAC and PHY levels for transmissions in the ISM 2.4 GHz band. In 1999 an improvement was approved, called b, which allowed a speed rate up to 11 Mbps (Wi-Fi name on the market). Modifications to the modulation techniques (PHY level) An optional coding technique (PBCC, packet binary convolutional code) allows to reach 22 Mbps. In 1999 also another standard was defined, a (Wi-Fi 5 on the market) Modifications to the MAC and PHY levels for using the OFDM (orthogonal frequency division multiplexing) modulation technique to reach up to 54 Mbps Utilized band is the U-NII (Unlicensed National Information Infrastructure) 5 GHz. This band is available in USA only. Currently not working in Europe. In 2003, the IEEE approved g as a further evolution of the standard g provides the same performance as a, while working in the 2.4 GHz band, which makes it deployable in Europe. Compatibility with b devices is guaranteed. ISTI/C.N.R. 49

50 The IEEE standards family Standard Description Status IEEE WPAN; Bluetooth Approved 2002 compatible; 2.4 GHz IEEE IEEE a (Wi-Fi5) IEEE b (Wi-Fi) IEEE g IEEE e IEEE f IEEE h WLAN; up to 2 Mb/s; 2.4 GHz WLAN; up to 54 Mb/s; 5 GHz WLAN; up to 11 Mb/s; 5 GHz WLAN; up to 54 Mb/s; 2.4 GHz New distribution functions for QoS IAPP Protocol) (Inter-AP Use of the 5 GHz band in Europe Approved 1997 Approved 1999 Approved 1999 Approved 2003 Task group development Task group development Task group development IEEE i New standards encryption Task group development ISTI/C.N.R. 50

51 WLAN ARCHITECTURE Cellular architecture (area where services must be present is called cell) Each cell is called BSS (Basic Service Set) Each BSS is a set of STA (stations), which are fixed or mobile devices IEEE compatible. STAs are controlled by means of a coordination function (CF). Two CF are possible in : Distributed Coordination Function (DCF), mandatory for each BSS. No master station. Point Coordination Function (PCF), optional. Master station. A Coordination Function is a set of rules for the access to the transmission medium and for the realization of the various services offered by the protocol (therefore by the STA). ISTI/C.N.R. 51

52 Simplest configuration is the independent BSS (IBSS), which is an adhoc network constituted by at least 2 STA. A BSS can be part of a wider network, the extended service set (ESS). An ESS is formed by: A series of BSS among them interconnected by means of a distribution system (DS), which could be a cabled network (as Ethernet). From a logical point of view BSS and DS utilize different transmission media: BSS uses the wireless medium (WM), while DS uses the distribution system medium (DSM). IEEE is independent of a specific transmission medium, thus WM and DSM can be (or not) the same IEEE specs refer to the wireless medium only. STA connected to DS are called Access Points (AP). Two categories: Station services (SS) Distribution system services (DSS) ISTI/C.N.R. 52

53 DSS services are realized througth the APs. DSS services allow the MAC to transport packets between STAs which cannot communicate because not in the same area. PORTAL is a device which allows the interconnection between a LAN and another network 802.x (more or less a bridge). ISTI/C.N.R. 53

54 The available bandwidth is divided into 14 partially overlapping channels, each 22 MHz wide. All the devices in the same BSS (either infra-structured or ad-hoc) use the same channel. One among three techniques is used for coding: the Direct Sequence Spread Spectrum (DSSS), which uses a Barker sequence, is adopted for the 1 and 2 Mb/s signal rates; the Complementary Code Keying (CKK), defined in b, is used for the 5.5 and 11 Mb/s signal rates; the Orthogonal Frequency Division Multiplexing (OFDM), defined in g, which is used for 6, 12, 18, 24, 36, 48 and 54 Mb/s. Many other optional modulation schemes are defined in the standard, but we will not mention them. ISTI/C.N.R. 54

55 DSSS uses an 11-bit Barker sequence, so each sequence of 11 chips codifies a single information bit. The modulation rate is 1 Msymbol/s using either BPSK (Binary Phase Shift Keying) or QPSK (Quadrature Phase Shift Keying), for transmission rates of 1 or 2 Mb/s, respectively. With CCK, a 16-bit sequence transmitted on the channel codifies either 4 or 8 information bits. The modulation is QPSK at Msymbol/s, for signal rates of either 5.5 or 11 Mb/s. Note that in both DSSS and CCK cases the chip rate is 11 Mchip/s, which means that the lowest layer of the radio section is the same, the difference laying in the coding. OFDM uses a comb of 52 sub-carriers (48 for data) with a spacing of MHz and a symbol duration of 4 µs, for a total of 12 Msymbols/s. Each symbol is protected with a convolutional code of either 3/4, 2/3 or 1/2 rate, using MQAM modulation (M-ary Quadrature Amplitude Modulation) with M being 2, 4, 16 or 64. The resulting combinations provide signal rates of 6, 12, 18, 24, 36, 48 and 54 Mb/s ISTI/C.N.R. 55

56 SERVICES OFFERED IN services in total: 6 devoted to the transport of packets among STAs, and 3 devoted to control the access and the security of the WLAN Services are realized by means several types of MAC frame: some of management type, other of data type. Authentication Deauthentication Privacy Distribution Integration Transition Association Disassociation Reassociation ISTI/C.N.R. 56

57 Authentication: between two STAs which want to communicate. At various levels: link level, user level (supported but not specified in the standard). De-authentication: to cancel an authentication. Privacy: for data security Distribution: main service used by a STA. Used by any message from or to a STA in an ESS. Integration: it allows the transportation of a data unit between a WLAN and the distribution dystem through a portal. Transition: 3 types of transitions which define the mobility concept: No transition Static Local movement (in the coverage area of the STA in communication) BSS-Transition: movements of a STA from a BSS to another one, in the same ESS ESS-Transition: movements of a STA from a BSS belonging to an ESS to another BSS in another ESS. ISTI/C.N.R. 57

58 Association: a STA communicates its presence to the AP of its BSS. Only the association with one AP is possible so that the distribution service will found one only AP to transport the DS messages. Disassociation: an association is cancelled. Re-association: in order to support the BSS-transition, this service must be invoked. An association is moved from an AP to another AP (when the STA moves from a BSS to another). ISTI/C.N.R. 58

59 ISTI/C.N.R. 59

60 MAC LEVEL OF IEEE The MAC protocol is comprehensive of a set of functions which realize all the possible operations of a WLAN MAC level handles and maintains all the communications among STAs, by coordinating the access to the shared transmission medium. ISTI/C.N.R. 60

61 TIME INTERVALS INTER FRAME SPACE (IFS): is the time interval specified by the CSMA/CA protocol between the transmissions of two subsequent frames. A STA which wants to transmit must be sure that the transmission medium be free during an entire IFS. 4 different IFS: Short IFS (SIFS). Used for the immediate ACK of a frame, for responses clear to send like, for request to send frames, for delimiting MPDU (MAC protocol data unit) transmissions, for polling reply, and in the replies to any frame sent by the AP during the contention free period (CFP). Point Coordination Function IFS (PIFS). Used by the STAs operating in PCF to get access to the transmission medium during the CFP. Distributed Coordination Function IFS (DIFS). Used by the STAs operating in DCF to get access to the transmission medium and to transmit data or control frames. Extended IFS (EIFS). Longest IFS. Used by a STA which received an incomprehensible packet. This time is necessary to protect the station (which do not understand the lasting info necessary to the virtual carrier sense) against collisions with future packets belonging to the same message. ISTI/C.N.R. 61

62 DISTRIBUTED COORDINATION FUNCTION (DCF) Basic access mechanism, based on the carrier sense multiple access with collision avoidance CSMA/CA mechanism. Must be implemented in all the STAs and it is implemented both in the adhoc networks and in the configurations with infrastructure (AP). A STA, before transmitting, must listen at the transmission medium to be sure that another STA is not transmitting. If, according to the rules of the carrier sensing, the STA decides the medium is free, it transmits a frame. Frame directed to a specified STA (unicast): the receiving STA must send a positive ACK. If ACK not received by the transmitting STA, this one retransmits the frame. A STA has info relevant the transmission duration of another STA by means of the duration/id field in the header of the frames (even if fragmented). ISTI/C.N.R. 62

63 BACKOFF PROCEDURE: after an IFS, and after any successful transmission, a STA must select a random delay > or = to DIFS or to EIFS (random backoff interval), and starts another transmission while decrementing a counter (random backoff interval counter) each time the medium is available. Random Backoff = Random() x Slot Time slot time is the granularity of time intervals such as IFS. Some standard values for the various PHY: SIFS 28 µs, Slot time 50 µs (FHSS); SIFS 10 µs, Slot time 20 µs (DSSS, Hi-rate DSSS, IR) Random () is a pseudo-causal integer number uniformely distributed in the range [0, CW]. CW is the contention window, in the range [CVmin, CVmax] ISTI/C.N.R. 63

64 Each STA has a retry counter, which is incremented each time a transmission is unsuccessful. An increment of the retry counter provokes an increment of CW. When CW=CWmax, further unsuccessful transm. will not modify this value. CW is reset to CWmin after any successful transmission. Some CW standard values for various PHY: CWmin 15, CWmax 1023 (FHSS); CWmin 31, CWmax 1023 CWmin 63, CWmax 1023 (IR) (DSSS, Hi-rate DSSS) ISTI/C.N.R. 64

65 ISTI/C.N.R. 65

66 DCF BASE ACCESS PROCEDURE ISTI/C.N.R. 66

67 FRAGMENTING/REASSEMBLING MAC Service Data Units (MSDU) and MAC Management Protocol Data Units (MMPDU) can be fragmented in MAC Protocol Data Units (MPDU in order to increase the transmission efficiency (in case of interferencers, high traffic, etc.) This facility is usable only in unicast mode, not for multicast or broadcast. Fragmentation Threshold: is a parameter which defines the dimension of the frames where an MSDU is fragmented. ISTI/C.N.R. 67

68 HIDDEN NODES STA A is hidden to STA B when they cannot transmit directly, even if in the same BSS. ISTI/C.N.R. 68

69 STA B will receive packets both from A and from C, thus increasing the collision probability. To solve this problem the RTS/CTS (Request To Send/Clear To Send) mechanism is used. STA A sends to B a frame RTS type. Stations which correctly receive this frame know how long it will last. Thus they adjust their NAV (Network Allocation Vector) in such a way to define their access to the medium after A. STA C does not receive anything from A. STA B, once received the RTS frame from A, replies with the CTS frame, after a SIFS interval, communicating to be ready to receive data. CTS frame is received from C also, which adjust its NAV according to the CTS frame header. Data frame will be transmitted a SIFS after the transmission of the CTS frame, with no medium control. ISTI/C.N.R. 69

70 This mechanism is optional and introduces the overhead due to the presence of the 2 RTS/CTS frames. It can be configured in such a way to be active only for transmissions of data whose length is > than a threshold value (TRSThreshold) If this threshold value is 0, RTS/CTS can be applied to all transmissions. If the threshold is > than max possible packet length, no frame will be sent with RTS/CTS mechanism. ISTI/C.N.R. 70

71 Frames not correctly received are retransmitted by the sender until: Correctly received Retry limit received Each STA has 2 counters: STA short retry count (SSRC) STA long retry count (SLRC) SSRC: incremented each time the transmission of a MAC MSDU or MMPDU frame fails, whose dimension is less than or equal to TRSThreshold reset when transmission successful SLRC: incremented each time the transmission of a MAC MSDU or MMPDU frame fails, whose dimension is greater than TRSThreshold reset when transmission successful Retransmissions retries end when SSRC or SLRC reach the limits values, respectively. When one of he limits is reached, the MSDU or MMPDu is discarted. ShortRetyLimit and LongRetryLimit fixed to value of 7. A STA must be able to detect, at MAC level, the reception of a duplicated frame by means of Sequence and fragment numbers. ISTI/C.N.R. 71

72 POINT COORDINATION FUNCTION Optional MAC access (only in infra-structured networks). A logical entity, called Point Coordinator (PC), in the AP of the BSS determines via a polling mechanism which STA is authorized to transmit. STAs which optionally implement the polling reply facility are called pollable STAs. PCF uses a virtual carrier sense mechanism together with an access mechanism based on priority. This method could be used to realise a contention-free (CF) access mechanism because the PC controls the access to the medium. IEEE standard states that DCF and PCF must co- exist: when a PC is present in a BSS, the two methods alternate, thus crating a Contention Free Period (CFP), to which a Contention Period (CF) follows. ISTI/C.N.R. 72

73 The point coordination function is used for transfer of frames during the contention free period. Standard imposes that a contention period lasting sufficiently for transmitting at least one complete frame must be always present, in order to allow the transmission of Management frames. Each contention free period begins with a special frame called beacon, which contains important info relevant the contention free period itself. ISTI/C.N.R. 73

74 The point coordinator may end a contention free period before its foreseen duration (parameter CFP-MaxDuration). This may be due to the lack of traffic, for ex. On the other end, the traffic sent in the contention period (which follows the CFP) may cause a delay in the transmission of next becon. In this situation, next CFP is shortened of this delay. Standard foresees that if the transmission medium is busy by a DCF frame, the beacon transmission must be delayed until the DCF transmission is finished Thus in alternating the two access methods, a minimum duration for the contention period is guaranteed, but no minimum for the CFP is guaranteed (a maximum is guaranteed). It means that CFP does not guarantee a minimum transmission bandwidth. ISTI/C.N.R. 74

75 BASIC ACCESS PROCEDURE The point coordinator (PC) logic entity in the AP of an infrastructured BSS obtains the transmission medium control at the beginning of each contention free period and tries to maintain this control for the whole duration of the CFP by using an interframe space (IFS) lower than that one used by the STAs (PIFS), which operate in DCF mode and use a DIFS time interval between the frame transmissions. Thus, at the beginning of a CFP, the PC controls the transmission medium and, if it is free for a time greater than or equal to a PIFS, the PC transmits the beacon. The STAs set their NAV on the CFP max duration, in order to avoid any possible transmission not caused by poll of the PC. STA CF-pollable and PC do not use RTS/CTS mechanism during CFP. ISTI/C.N.R. 75

76 A pollable STA polled by the PC only can transmit an MPDU to the PC or to another STA, and can piggyback the ACK relevant to a frame received from the PC. The reception of the frame sent by this STA must be confirmed by sending an ACK after a SIFS (as in DCF mode), thus confirming that PCF method depends on DCF. If this ACK is not received, the STA authorized by the PC to the transmission cannot retransmit the frame until a new polling by the PC, or it can decide to retransmit it during the contention period. The PC can retry the retransmission of a non-acked frame after a PIFS. ISTI/C.N.R. 76

77 PCF: transmission from PC towards a STA ISTI/C.N.R. 77

78 PCF: transmission from STA towards another STA ISTI/C.N.R. 78

79 SUMMARY OF THE PCF RULES PC can send unicast, multicast or broadcast frames to each active STA and to CF-pollable STAs (in power save mode) During the CFP period, each CF-pollable STA must operate after a SIFS as follows: If it received a frame Data+CF-Poll or Data+CF-ACK+CF-Poll, it has to reply with Data+CF-ACK or CF-ACK if no data to transmit If it received a frame CF-Poll it has to reply with Data or Null If it received any other frame (data or management), it has to reply with an ACK. Not CF-Pollable STAs follow the DCF rules: At the reception of any frame (data or management), they reply with an ACK after a SIFS, When a CF-pollable STA is polled by the PC, it can send a frame to any STA or to the PC. If to PC, or through PC because addressed to DS, PC itself must confirm the reception of the frame by using CF-ACK after a SIFS. If to a STA, the destination STA will confirm the reception with an ACK after a SIFS. ISTI/C.N.R. 79

80 SYNCHRONIZATION Synchronization is based in each STA of a BSS on the TSF (Timing Synchronization Function) an on the TSF timer. WLAN with infrastructure: The AP realizes the TSF function and checks the common clock. At the beacon period, the AP transmits beacon frames containing a copy of its TSF timer (timestamp), to allow the other STAs of the BSS to synchronize. A STA, after reception of the beacon of the AP of its BSS (others are rejected), must accept the info relevant to the timestamps of the AP, and must adjust its TSF timer to the received timestamp. ISTI/C.N.R. 80

81 Ad-hoc WLAN (or IBSS): TSF function is implemented with a distributed algorithm among all the STAs of a IBSS. Beacon generation is distributed. The beacon period parameter is fixed by the STA which initializes the IBSS. The beacon period states the times for sending the becon. At each becon generation time, each STA must: Suspend any backoff procedure Compute the random delay in the range [0; 2 x CWmin x SlotTime] and wait for it If in the meantime a beacon is received, generated by another STA, the delay is cancelled, the procedure for sending the beacon interrupted, and the backoff possibly suspended restart; If no other beacon received, this STA sends its own beacon with its timestamp. ISTI/C.N.R. 81

82 POWER MANAGEMENT and POWER SAVE POWER MANAGEMENT IN AN INFRA STRUCTURED WLAN As far as the power consuming is concerned, a STA can be in : Awake state. Full working Doze state. The STA cannot neither receive nor transmit, thus consuming little power. STA must communicate to the AP the change in working mode by means of the Power Management bit in the Frame Control Field of the packets. Any change is possible only at the end of the exchange of frames in course. No MSDU transmissions from the AP to a STA in POWER SAVE (PS) mode, but all traffic relevant to this STA must be collected and transmitted at fixed times. STA in power save listens at the channel periodically to receive the beacon where the Traffic Indication Map (TIM) info is present, which is a sort of virtual map of the traffic for all the stations in PS. ISTI/C.N.R. 82

83 If a STA in power save state has in the AP traffic addressed to itself: Transmits to the AP a power-save-poll frame to declare that it is ready to receive the data. AP can reply later or send immediately a buffered MSDU If TIM indicates that buffered frame will be sent during the CFP, the (pollable) STA will not send any Power save-poll frame but will wait for its turn in the CFP to receive data from the AP, remaining in Awake Mode (AM) for the whole CFP. After a certain timeout, too old traffic buffered in the AP is cancelled (aging function) ISTI/C.N.R. 83

84 POWER MANAGEMENT IN AN AD-HOC WLAN (IBSS) Same as for the infra-structured WLAN. ATIM (Ad-Hoc TIM) instead of TIM. As there is no AP, a STA can only make an estimation (how it is done is out of the standard) of the state of another STA in the IBSS. Indications: Based on the info relevant the power management transmitted by the other STAs Info such as the sequence of the successful and unsuccessful transmissions towards the other STAs. In this case the RTS/CTS mechanism can facilitate the power save mode estimation of the STAs: if an RTS frame is sent but no CTS is received, the sending STA can assume that the destination STA is in power save mode. ISTI/C.N.R. 84

85 SCAN and JOIN PROCEDURES SCAN function is used by a STA: To find a network and to connect to it To find a new AP (roaming) To initialize an IBSS (ad hoc netwok) Two types of SCAN: Passive. Listens to each channel specified in the channel list for the relevant time, waiting for beacon frames. At the end of the scan phase stores all the info collected for a possible future join action. Active. Waits for the stated delay (parameter ProbeDelay) Accesses the medium in DCF mode and Broadcast a Probe request file Restarts a Probe Timer and continues to send Probe files until detects the medium free and the probe timer is less than Min ChannelTime After that, Resets the NAV and passes to another channel ISTI/C.N.R. 85

86 Probe Response must be sent in DCF mode by the STA to which the probe request was addressed In an IBSS the STA which generated the last beacon must reply with a probe response. In this case it may be that more than one STA in the IBSS responds to the probe request because more than STA sends a beacon. ISTI/C.N.R. 86

87 ROAMING Transition of mobile stations from a BSS to another one. Only in networks with infrastructure. Standard defines the re-association function. Operations: Scan phase. At the end of this a STA has a list of AP which can be ordered, for ex., according to the signal level. AP selection. Send a Re-association Request frame to the selected AP. It contains also the address of the AP to which the STA is currently associated. The selected AP sends the STA a Re-association Response frame (accepted or refused). The two APs involved (old and new) must exchange info. The protocol used is not defined in the standard. An attempt of standardization is the IAPP protocol (Inter Access Point Protocol). ISTI/C.N.R. 87

88 IEEE FRAME FORMAT Duration/ID: to update the NAV of the STAs which access the medium in DCF mode. Frame Body: variable length ISTI/C.N.R. 88

89 IEEE PHYSICAL LAYER Infra Red Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) ISTI/C.N.R. 89

90 PHY IEEE FHSS operates in ISM 2.4 GHz band In Europe (with the exclusion of France and Spain): 79 ho channels, 1 MHz spaced. Max =100, min = 10 mw EIRP (Equivalent Isotropic Radiated Power). GFSK signal modulation Hop rate is 2.5 hop/s, i.e. the operative frequency changes every 400ms. ISTI/C.N.R. 90

91 PHY IEEE DSSS operates in ISM 2.4 GHz band Each channel is.33 MHz large; central frequencies must be distant 25MHz each other to limit the interference. 11 symbol Barker sequence to code the data flow. ISTI/C.N.R. 91

92 BLUETOOTH AND IEEE COMPARISON AT THE MAC LEVEL ISTI/C.N.R. 92

93 The two standards are compare at the MAC level, in particular as far as the following items are concerned: the ability to create an efficient network, particularly with regard to the maximum number of terminals which can be handled in a basic cell, the creation speed of the networks, and how the networks are created and maintained; the characteristics of the network topology, particularly with regard to the ability to extend the basic cells, to interconnect with other network types, and the routing problems; the characteristics of the links among the devices of a single basic cell, and the maximum attainable throughput; the ability to offer a given quality of service. ISTI/C.N.R. 93

94 SOME DEFINITIONS NETWORK SIZE: the maximum number of devices belonging to the network basic block, i.e. the Piconet for Bluetooth and the BSS for IEEE SPATIAL CAPACITY: the ratio between aggregated data transfer speed and used transmission area. NETWORK CREATION SPEED: how fast the setup of links among devices of the same basic network unit is. ISTI/C.N.R. 94

95 Max number of devices in the basic cell Extension of the basic cell Maximum signal rate Procedures used for the network setup Nominal range Spatial capacity Average speed in network setup without external interferences BLUETOOTH 8 active devices; 255 in park mode Scatternet 1 Mb/s Inquiry, Page 10 m About 100 [kb/s m 2 ] 5 s + n 1.28 s, where n is the number of slaves in the Piconet, ranging from 1 to 7. IEEE Unlimited in ad hoc networks (IBSS); up to 2007 devices in infrastructured networks. ESS 11 Mb/s in b, 54 Mb/s in g Scan and Authentication for ad hoc networks; Scan, Authentication and Association for infrastructured networks. 100 m About 15 [kb/s m 2 ] n c 1.35 ms for an unsaturated network, c probed channels (1 c 13), n stations (excluding the AP), active scan, infrastructured topology. ISTI/C.N.R. 95