4. Wi-Fi Access Technologies

Transcription

1 4. Wi-Fi Access Technologies Prof.dr.ing. Ion Marghescu 1

2 4.1 Preliminary Issues Today, when wireless local area networks (WLAN) based on the standard IEEE b / g (Wi-Fi) are so widespread, few still think about 20 years ago such networks were reserved for niche applications. This chapter will describe Wi-Fi networks and how Wi-Fi IEEE standard evolved and continues to evolve in order to provide higher rates of data transmission and an increasingly high quality of service. 2

3 Elements underlying the current standard IEEE used in wireless networks will be analysed: presentations being made for the two lower layers of the OSI reference model: - physical (PHY) - Media Access Control (MAC). It will be highlighted that the latest and most performant versions of the standard are using OFDM modulation technique and some specific aspects will analyzed. 3

4 The presentation of the link layer will highlight: How devices access the wireless network; How data packets are transmitted by the MAC layer protocols. The data link layer has two sublayers: logical link control (LLC) and media access control (MAC). 4

5 4.2 Getting Started with IEEE Network Topology WiFi equipment are either stations or access points. Stations (STA) are client equipment. These can be: included into a LAN card installed on a desktop a USB adapter a PC or PCMCIA card can be integrated into portable computers (laptop) or other devices. 5

6 Access points (AP) form a bridge between wireless networks and wired ones; Each device in a wireless network includes a radio transceiver. Two or more stations form a Basic Service Set (BSS) when two or more stations have recognized each other and formed a network. The network can be configured in two ways: 1. Peer-to-peer Network (ad hoc mode); 2. Client/Server Network (structured network). 6

7 Peer-to-peer networks are similar to wired networks, just the cables are missing; Two or more stations can communicate with each other without an access point; When two or more stations create an ad hoc network, this is called Independent Basic Service Set (IBSS). Client/Server networks (structured networks) consist of multiple stations connected to an access point. The access point works like a bridge to a wired network 7

8 A Basic Service Set (BSS) in this configuration is said to work in infrastructure mode. An Extended Service Set (ESS) is established when multiple sets of Basic Service Sets, overlapping or not (each containing an AP), are connected together through a distribution system (usually - a wired Ethernet LAN). Sets with coverage areas that overlap must use different channels to avoid interference. 8

9 Typical Extended Service Set The distance by which the access point interacts with the stations is up to 100 m (depending on the data transmission rate); The overall range of the ESS is limited only by the range of the wired distribution system. Extensive sets can also cover wider areas, up to several kilometers in radius, using directive antennas for wireless connections. 9

10 4.2.2 Evolution of IEEE standard The IEEE (Institute of Electrical and Electronics Engineers) 802 Standards Committee publishes the series of standards called IEEE 802.xx; These initially included the local networks (LAN) and metropolitan networks (MAN), but now also include personal networks (PAN); IEEE 802 committee documented the standardization of processes and procedures that deal with the first two layers of OSI Reference model: Medium access control (MAC) layer and physical layer (PHY). 10

11 IEEE 802 Committee is divided in Working Groups numbered from to ; Each group studies different subjects about data transmission networks and develops standards which are than named with the code of the author group; The first two groups (security and management of network) and (Logical Link Control - LLC) deal with standards applying both to wired and wireless networks. 11

12 The working group develops standards for wireless local area networks (LAN); The working groups are further divided into Task Groups, marked from a to z which study various additions and improvements that can be brought to standards. 12

13 4.2.3 The Initial Standard The first standard for wireless Ethernet networks, IEEE , was adopted in 1997 and improved in 1999; It specified three different technologies for the physical layer (PHY): Diffused infrared at1 Mbps speed; Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS). The last two technologies offered speeds of up to 2 Mbps working in 2.4 GHz band (ISM); 13

14 As the wired networks of that time allowed speeds of up to 10 Mbps with much lower costs, the initial standard had limited success. 14

15 4.2.4 Current Standards After two years (since 1999), the initial standard has evolved in two directions: The b Task Group developed the specification with the same name that allows for data rates of up to 11 Mbps (therefore comparable with traditional networks) and that maintains compatibility with the initial standard; It operates in the same frequency range 2.4 GHz, and is a direct extension of the DSSS modulation technique defined in the initial standard. 15

16 802.11b incorporates a more efficient coding scheme called Complementary Code Keying (CCK) in order to reach the maximum rate of 11 Mbps. A second coding scheme, Packet Binary Convolutional Coding (PBCC), was included as an option for higher performance (5,5 and 11 Mbps), because it insures a 3 db gain by coding. 16

17 The a Task Group was formed during the development of b and published its results at the same time, referring to different frequency band, of 5,2 GHz ( Unlicensed National Information Infrastructure U-NII); Transfer rates of up to 54 Mbps were obtained; While standard b uses a modulation with a single carrier frequency, a uses the modulation technique OFDM; It is necessary to use an error correction code, resulting in a useful transmission rate of around 20Mb / s. 17

18 Because it uses the radio spectrum around the frequency 5 GHz, standard a is not compatible with b or with the initial standard ; The first products based on standard b appeared on the market at the beginning of 2000 and were accepted in short time as an industrial standard, with a price substantially reduced compared to previous products. 18

19 It must be mentioned that the unlicensed frequency band of 2,4 GHz is used by many other devices (Bluetooth equipment, cordless telephones, microwave ovens etc); This leads to interferences, but the low cost, good coverage range and worldwide availability of the frequency band contributed to its fast spreading. 19

20 802.11a products appeared on the market two years later; These had clear advantages, such as: much higher transmission speed, less problems with interferences; However they managed to impose only partially on the wireless network market for business users. 20

21 The decisive factor : the high cost of implementing a wireless network in the 5 GHz frequency band; Also, as the carrier frequency is increasing, the coverage area decreases and the attenuation when passing through walls or solid objects also increases; Therefore, sometimes there was need for more access points to insure a coverage equivalent to the one corresponding to specifications of b. 21

22 Usualy, at lower speeds, b provides a better coverage, while, at higher speeds, a provides a coverage radius equivalent to or slightly higher ( the regulated band 5 GHz is much less affected by interference); These two lines of development supported in turn additions and improvements; in November 2003 two new wireless transmission standards were ratified : g and h; The first is an extension of specifications b, and the second is an extension of specification a. 22

23 The task of g group to find solutions for increasing the speed of transmission in the 2.4 GHz band was not easy, being almost abandoned in late 2001, due to lack of consensus on the final solution; Then things took a favorable turn in 2002, when the first products based on this new specification appeared on the market. 23

24 Looking to the present situation, one notes that this specification became the basis of Wi-Fi communications, being included since 2005 in most laptops and portable devices; The cost of devices is lower than for a devices and, thanks to new technologies, was only slightly higher than equipment that supports b. 24

25 Dual-band products that support both a and b quickly became dual-band/tri-mode, supporting, in a single adapter card or a single access point, specifications a, b and g. The price of such implementations is comparable to the implementation of a network based only on a 25

26 Using the table below, transmission rates for specifications a, b and g can be compared, according to modulation and coding techniques used. 26

27 Activity of h group marked the beginning of the implementation of cognitive radio technologies in Wi- Fi equipment; The starting point for the new specifications aimed at making Wi-Fi networks compatible with some European and Japanese regulations on the 5 GHz band; Here are a series of problems in terms of interference with satellites or military radars that use the same band. 30

28 To solve such problems h standard introduced two innovative mechanisms: Dynamic Frequency Selection ( DFS ) management of available spectrum - access points avoid channels containing radar signals and the energy is spread over the entire bandwidth available ( to reduce interference with satellites); Transmitted Power Control ( TPC ) - adjusts the average emission power so as not to exceed the maximum regulated value ( reduces interference with satellites). 31

29 The problem of ensuring service quality and enhanced security for Wi-Fi transmissions was put forward before the ratification of g and h; Initially, both tasks were assigned to e Task Group; Later on, security issues were completely delegated to a new Task Group i; This group, along with Wi-Fi Alliance, have developed a robust and interoperable security standard that was called Wi-Fi Protected Access (WPA). 32

30 WPA 2003 represented a significant jump in Wi-Fi security; WPA solved the known weaknesses of WEP - the native security mechanism in the standard by adding more extensions to the MAC. Thus, WPA ofers stronger data encryption, and also involves a secure authentication procedure which was almost absent in the initial security protocol. 33

31 Later a new version was developed- WPA2 - using a new encryption algorithm Advanced Encryption Standard (AES); The resulted standard i approved in 2004 adds to mobility and flexibility offered by Wi-Fi technology the guarantee of a high level security of transmitted data. 34

32 The goal of the e Task Group standard to support both business applications as well as residential users, especially in terms of multimedia applications; This required changes in MAC - to expand support for applications involving requirements on quality of service; The proposed solutions guarantee collision avoidance and provide mechanisms for scheduling transmissions and improve the robustness of communication channel; It gives priority to ensure the necessary bandwidth to real-time traffic (interactive), while ensuring that traffic on other channels is not interrupted; The transmission of sound and images in real time via wireless networks became reality in 2005 with the ratification of e extension. 35

33 In 2003, the ma Task Group was authorized to handle the compilation of most amendments brought over time to the 1999 version of the standard; The group created a single document that unified the 8 amendments (a, b, d, e, g, h, i, j) with the base standard; After ratification of 8 March 2007, the standard REVma was renamed

34 The latest standard added is n that improves the previous standards in higher transmission rate reached - more than 100 Mbps, by using Multiple-input Multipleoutput (MIMO) technology and other mechanisms; The standard was approved in 2009, but many equipment manufacturers have started earlier migration to this new technology, based on the outline number 2 of the proposal n; In 2012 Version IEEE mb was approved, which was a Standard maintenance. A single document that merged ten amendments (802.11k, r, y, n, w, p, z, v, u, s) with the 2007 base standard. The new standard is referred to as IEEE ; and is the current complete version of standard

35 Revision 2012 gives users, in one document, the IEEE standard for wireless local area networks (WLANS) with all the amendments that have been published till March

36 IEEE Evolution Timeline 39

37 4.3 Physical layer of Wireless Networks (PHY) The functions of the physical layer are related to the creation of radio channels enabling data transmission between WLAN nodes. The physical layer defines the means of transmitting the raw bits rather than logical data packets over the radio link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to the medium. signal that is transmitted over the radio transmission 48

38 Evolution of IEEE standards with respect to the PHY layer 49

39 4.3.1 Radio Channels in Standard According to standard, the available bandwidth is divided in channels 22 MHz each, with a quite large overlap between channels; For example, band 2,4000 2,4835 GHz is divided in 13 channels, spaced at only 5 MHz; Japan added a 14 th channel. 50

40 Channel availability varies from country to country, being limited by how radio resources are allocated to various services; Most European countries allow currently using the 13 channels; US and some countries in Central and South America limits the number of available channels 11. Of these only three are non-overlapping, as can be seen from the figure on the next slide. 51

41 Non overlapping channels in b/g 52

42 U-NII (Unlicensed National Information Infrastructure) band of 5,2 GHz used by a allows 23 non-overlapping channels (originally there were only 12), as can be seen from the figures given below. 53

43 Non-overlapping channels in a 54

44 In addition to the center frequency, standard specifies the spectral mask that defines the power spectral density variation for each channel; It requires power to decrease by at least 20 db of peak power at a distance of ± 11 MHz off the center frequency; One result is that only four channels may be identified in Europe (1,5,9 and 13) and three in America (1,6 and 11) which do not overlap; 55

45 The spectral mask also specifies that at ± 22 MHz off center frequency the transmitted power density must decrease by at least 50 db, so it is accepted that channel energy does not extend beyond these limits; It is more correct to say that, given the separation between channels, the overlapping signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other channel; The near-far problem must also be considered, mostly specific to CDMA transmissions; a transmitter can impact a receiver even on a "non-overlapping" channel, but only if it is close to the victim receiver (within a meter) or operating above allowed power levels. 56

46 Should transmitters work on other channels than 1,6 and 11, the overlap between channels can cause unacceptable degradation of signal quality and bit rate of transmission; In some special situations overlapping channels are used in order to increase the number of available channels; As a conclusion, the following diagrams illustrate variation of transfer capacity provided by each standard, depending on the number of channels available. 57

47 58

48 4.3.2 OFDM Signal Parameters It has been proven that the most performing Wi-Fi network versions use the OFDM modulation technique: a, g n, ac, ad; The main parameters of OFDM modulation used in IEEE a/g are listed in the table: 59

49 Analysis starts with guard interval duration Tg = 800ns; In order to limit the loss SNR to just over 1 db, total symbol duration is 4 s. So the actual symbol duration is 3.2 s; As a result, interval between subcarriers is khz; OFDM signal consists of 52 subcarriers of which 48 for data and 4 for synchronization (pilot). The four synchronization subcarriers are used to track the residual offset of subcarriers. In order to achieve this goal, modulation techniques are used starting with BPSK and ending with 64-QAM. 60

50 To correct the data carried by subcarriers affected during transmission a predictive coding (FEC) is used, that makes the encoded data rate to reach 6 to 54Mbps; For example, a convolutional code with ½ rate may be used. This solution, combined with modulation type BPSK, QPSK, respectively 16-QAM, allows for net data throughputs of 6, 12 or 24 Mbps, respectively; Higher coding rates (2/3 şi ¾) are obtained by puncturing the codes of rate ½; Using code rate 2/3 and modulation 64 QAM results in a data rate of 48Mbps; Using code rate ¾ and modulation BPSK, QPSK, 16- QAM şi 64-QAM results in data rates of: 9, 18, 36 and 54 Mbps. 61

51 4.3.3 OFDM Signal Processing RF TX DAC Binary input data Coding Interleaving QAM mapping Pilot Insertion Serial to Parallel Parallel to Serial Add cyclic extension and windowing I/Q output signals IFFT (TX) Binary output data Decoding Deinterleaving QAM Demapping Channel Correction Parallel to Serial FFT (RX) Serial to Parallel Remove cyclic extension Symbol timing Frequency corrected input signal Block diagram of OFDM processing is similar to the one used in OFDM signal analysis RF TX ADC Timing and Frequency Synchronization 62

52 Data to be transmitted is encoded with a standard convolutional encoder of code rate ½; The other codes are obtained by reduction (puncturing) of bits of coded data stream ½; After interleaving, the binary signal is converted to binary QAM symbols; To facilitate coherent reception, four pilot symbols corresponding to pilot subcarriers are added to each 48 data values block; In this way will result blocks of 52 QAM values per OFDM symbol, which are modulated onto 52 subcarriers by Inverse Fast Fourier Transform (IFFT - 64 points); 63

53 To make the system robust to multipath propagation, a cyclic prefix is added. Further, windowing is applied to get a narrower output spectrum; After this step, the digital output signals can be converted to analog signals, which are then upconverted to the 5 GHz band, amplified and transmitted through an antenna; The OFDM receiver performs the reverse operations of the transmitter, together with additional training stages to prepare the equipment for receiving the data; First, the receiver has to estimate frequency offset and symbol timing, using special training symbols; 64

54 Then, it can do a Fast Fourier Transform for every symbol to recover the 52 QAM values of all subcarriers. The training symbols and pilot subcarriers are used to correct for the channel response as well as remaining phase drift; The QAM values are then demapped into binary values, after which a Viterbi decoder can decode the information bits. 65

55 4.3.4 Training the receiver for data processing The format of a frame structured according to standard IEEE a is: The Physical Layer Convergence Procedure (PLCP) preamble is used to acquire the incoming signal and train and synchronize the receiver. The PLCP preamble consists of 12 symbols, ten of which are short symbols, and two long symbols. 66

56 The header includes a series of fields such as: rate, reserved, length, parity, tail, service; These are carried by a single OFDM symbol, coded BPSK, and known as the signal symbol; Fields: data (PSDU -Physical Service Data Layer), tail and pad complete the remaining structure of the frame; The data sequence is transmitted using a variable number of OFDM symbols. 67

57 The preamble serves for: detecting the beginning of frame automatic gain control (AGC) symbol synchronization coarse estimation of carrier frequency and estimation of channel; These tasks are executed before decoding the actual data. 68

58 The structure of OFDM preamble as specified by IEEE a is described below: The first part of the preamble consists of 10 repetitions of a training symbol with a duration of 800 ns, which is only a quarter of the FFT interval of a normal data symbol. 69

59 These short symbols are produced by using only nonzero subcarrier values for subcarrier numbers which are a multiple of 4. Hence, of all 52 possible subcarrier numbers from -26 to +26, only the subset {-24, -20, - 16, -12, -8, -4, 4, 8, 12, 16, 20, 24} is used. 70

60 The short training symbols are followed by a long training symbol which contains 52 QPSK modulated subcarriers like a normal data symbol. The length of this training symbol is twice that of a data symbol, which is done for two reasons: First, it makes it possible to do a precise frequency estimation on the long symbol. The second reason for the long symbol is to obtain reference amplitudes and phases for doing coherent demodulation. By averaging the two identical parts of the long training symbol, coherent references can be obtained with a noise level that is 3 db lower than the noise level of data symbols 73

61 Both the long and short symbols are designed in such a way that the peak-to-average power (PAP) ratio is approximately 3 db, which is significantly lower than the PAP ratio of random OFDM data symbols; This guarantees that in the training phase, the degradation caused by non-linear amplifier distortion will be smaller than the distortion of the data symbols; At the same time, a correlator with a simpler can be implemented. structure 74

62 After processing the preamble, there is still one training task left, which is tracking the reference phase; This is necessary because, even with precise adjustments, there will always be some remaining frequency offset which causes a common phase drift on all subcarriers; In order to track this phase drift, 4 of the 52 subcarriers contain pilot values known to receiver. The data carried by pilots are scrambled by a length 127 pseudo-noise sequence to avoid spectral lines exceeding the average power density of the OFDM spectrum. 75

63 4.4 The MAC layer (link layer) Generalities As already explained, the services and protocols specified in all IEEE 802.x standards correspond to the lower two layers (Data Link and Physical) of the sevenlayer OSI networking reference model; IEEE 802 committee splits the OSI Data Link Layer into two sub-layers named Logical Link Control (LLC) and Media Access Control (MAC). 77

64 The standard describing the Logical Link Control (LLC) sublayer is 802.2, used by all other standards issued by 802 Commitee; MAC sublayer has as main characteristic parameters: packet format (size, headers) mechanisms for access to the communication channel. facilities related to network management. As functions, it includes: access to the wireless channel; association and disassociation from a network; security services. 78

65 Services supported by MAC sublayer Station Services (SS) Authentication De-authentication Privacy Data delivery (MAC Service Data Units -MSDU) Distribution Services System (DSS) Association Disassociation Reassociation, 79

66 4.4.2 Description of MAC sublayer The main task of MAC protocols is to regulate the use of the medium; This is achieved through a channel access mechanism that implements a procedure to divide radio resources between nodes; Following this protocol each node can determine the times at which can transmit data or when it should. receive data. The channel access mechanism is the core of the MAC protocol. 80

67 For the radio channel as medium, there are three main classes of channel access mechanisms: TDMA; CSMA/CA); Polling ; Some aspects about each shall be presented, as well as the extent of involvement in

68 a. TDMA TDMA (Time Division Multiplex Access) is a simple channel access mechanism based on a specific node, the base station, which has the responsibility to coordinate the nodes of the network and mediates the data flow; The time on the channel is divided into time slots, which are generally of fixed size. Each node of the network is allocated a certain number of slots where it can transmit. Slots are usually organized in a frame, which is repeated on a regular basis. 83

69 TDMA is well suited for phone applications (cellular networks, DECT) because those applications have predictable needs (fixed and identical bit rate); TDMA is not well suited for data networking applications, because its lack of flexibility. TDMA is connection oriented, while IP traffic is connectionless. TDMA use fixed size packets and usually symmetrical link, while IP traffic has variable size packets. Data transmission protocols for data networks should be oriented for packet switching and variable size packets and/or bursts of data; TDMA is very much dependant of the quality of the frequency band and suffers and cannot adapt to bursts of interference often found on unlicensed radio bands. 84

70 b. CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) This is a network arbitration protocol that involves sensing the radio communication medium to avoid collisions; It is implemented in any network as part of the Distributed Coordination Function DCF; CSMA/CA implements an asynchronous message passing mechanism, without a dedicated circuit (connectionless); This offers a best effort service, without guaranteeing a transmission rate (bandwidth) or latency. 85

71 It is suited for network protocols such as TCP/IP, adapts quite well with the variable condition of traffic and is quite robust against interferences; CSMA/CA is derived from CSMA/CD (Collision Detection), which is the base of Ethernet data networks (wired).the main difference is the collision avoidance mechanism : on a wire, the transceiver has the ability to listen while transmitting and so to detect collisions; If a radio node could listen on the channel while transmitting, the signal of its own transmissions would mask all other signals on the air. So, the protocol can't directly detect collisions and only tries to avoid them. 86

72 CSMA/CA Channel Access Mechanisms The node that wants to send a packet, listens to the radio channel (this is carrier sensing) checking whether or not there is activity. The device can begin transmission of a packet if the channel is free (idle).if channel is busy, it waits for completion of the current transmission; 87

73 At the beginning of contention (disputing) period, it initiates a timer of randomly chosen duration; When its contention timer expires, the node listens again on the channel; The node having chosen the shortest contention delay wins and transmits its packet. The other nodes just wait for the next contention (at the end of this packet); Because the contention is a random number and done for every packets, each node is given an equal chance to access the channel (on average - it is statistic); Collisions cannot be detected on the radio medium and devices require some time to switch from receive to transmit; 88

74 c. Polling Polling is a solution in between TDMA and CSMA/CA; The base station retains total control over the channel, but the frame content is no more fixed, allowing variable size packets to be sent.; The base station sends a specific packet (a poll packet) to trigger the transmission by a certain node; The node just waits to receive a poll packet, and upon reception starts transmission. 90

75 Polling channel Access Mechanisms Polling can be implemented as a connection oriented service (similar to TDMA, but with higher flexibility in packet size) or a packet oriented connectionless service (asynchronous). 91

76 The base station can either poll permanently all the nodes of the network just to check if they have something to send (this usually applies in networks with limited number of nodes), or the protocol may use reservation slots when each node can request a connection or to transmit a packet (depending if the MAC protocol is connection oriented or packet oriented). Reservation slots shall be explained later. 92

77 Polling is used in networks that operate in infrastructure mode (where stations are connected to the network through an Access Point -AP. This mode is optional, and only very few APs or Wi-Fi adapters actually implement it); For such networks, the access mechanism is provided as an optional access method - Point Coordination Function (PCF); It resides in a point coordinator also known as Access Point (AP), to coordinate the communication within the network. The AP waits for duration of PCF Interframe Space (PIFS) rather than duration of DCF Interframe Space (DIFS) to take the channel. PIFS is less than DIFS duration and hence the point coordinator always has the priority to access the channel. 93

78 For and wireless LAN, all the polling packets have to be transmitted over the same bandwidth of the radio channel, generating much more overhead; More recent system use reservation slots, which is a more flexible system, but still require significant overhead; As CSMA/CA offers ad-hoc networking (no need of a base station) and similar performance, it is usually preferred in most wireless LANs. For example, most vendors prefer to use the distributed mode DCF and CSMA/CA over the PCF coordinated mode (polling). As such, there are stations that are not able to work in the PCF mode. A network that works only in PCF mode should allow these stations to access the medium, leaving enough time between periods of interrogation for mode DCF. 94

79 4.4.4 MAC specific techniques MAC protocols use additional techniques to improve the performance of CSMA/CA: a. MAC Retransmissions b. Fragmentation (and reassembly) c. RTS/CTS d. Reservation and service slots 95

80 a. MAC Retransmissions The main problem of the CSMA/CA protocol is that the transmitter can't detect collisions on the radio channel; There is also a higher error rate on the air than on a cable channels, so there could be a higher rate of corrupted packets; TCP protocol cannot cope well with packet losses at the MAC layer, sensing this as congestion and reacting by reducing the transmission rate without real reason; 96

81 In order to avoid losing packets on the radio channel, MAC protocols also implement MAC level retransmissions and positive acknowledgement: each time a node receives a packet, it sends back immediately a short message (ACK) to the transmitter to indicate that it has successfully received the packet without errors. If after sending a packet the transmitter doesn't receive an ACK, it decides that the packet was lost and it will retransmit the packet and start a counter; MAC retransmissions in CSMA/CA 97

82 When the counters expires or the number of confirmation of successful reception reaches 10, the transmission rate is raised to the next level and the counter is reset; If, instead, for the retransmitted data is not received confirmation, the transmission rate is decreased again and the counter is restarted; This algorithm is purely heuristic and conservative as possible, being unable to react quickly to changing conditions of the communication channel; In other words, the transmitter can try to increase or decrease the transmission rate to test the channel condition, without considering the actual cause of these losses - errors due to interference or collision 98

83 Most MAC protocols use a stop and go mechanism, they transmit the next packet of the queue only if the current packet has been properly acknowledged; The purpose is to make the protocol simpler, minimize latency and avoid desenquencing packets; The acknowledgements are included in the MAC protocol, so they are guaranteed not to collide (the contention period starts after the ACK) - see previous figure; These ACKs are very different from the TCP ACKs, which work at a different level (and on a different time frame). Of course, broadcast and multicast packets (with more destinations) are not acknowledged, so they are more likely to be lost. 99

84 c. Fragmentation (and reassembly) The radio medium has a higher error rate than a wire. For this reason, as explained before, most devices products include MAC level retransmissions to avoid losing packets. MAC level retransmissions solve this problem, but the method is not really performant; If the packet to transmit is long and contains only one error, the node needs to retransmit it entirely; If the error rate is quite high, it can happen that the probability of error in a large packet is dangerously close to 1 (a packet can't fit between the bursts of errors due to fading or interference), so the packet cannot be transmitted correctly. 100

85 This is why in some situations fragmentation is used. Fragmentation is sending the big packets in small pieces over the medium. This technique certainly adds some overhead, because it duplicates packet headers in every fragments and each fragment is individually checked and retransmitted if necessary. MSDU = MAC Service Data Unit Fragmentation of long packets 101

86 Transmission of fragments is by a simple send and wait mechanism, the station transmitting is not allowed to transmit another fragment either until receiving confirmation, or after deciding that the packet was retransmitted too many times and giving up on the packet; The first advantage is that in case of error, the node needs only to retransmit one small fragment, so it is faster; The second advantage is that if the medium is very noisy, a small packet has a higher probability to get through without errors, so the node increases its chance of success in bad conditions. For defragmentation, each fragment contains information to allow the complete MSDU to be reassembled from its constituent fragments. The header of each fragment contains the information that is used by the destination STA to reassemble the MSDU. 102

87 c. RTS/CTS An interesting issue in transmissions of radio waves is the attenuation of the signal that makes difficult detection of activity by all members of a Basic Service Set (BSS); A first such problem is known as the "exposed terminal problem" and can be stated as follows: A and C stations are within range of station B but not in the coverage of station D. A transmits to B, and C wants to send to D, but it is impossible because it detects that activity; The second problem is called "hidden terminal problem" and refers to the following situation: A and C stations are within range of the station B, but not within range of each other, so they know nothing, directly, one about the activity of the other. 103

88 If A already transmits to B, C finds that the medium is free and can begin to transmit a packet to B, so there will be a collision; Since the transmissions are based on a carrier detection mechanism, the nodes A and C are ignoring each other and transmit at the same time. This is usually a good thing because allows reuse of frequencies; But if simultaneous transmissions are made to a node placed between the two with a comparable power, this leads to collisions. This node could be impossible to reach because of these collisions or the latency will increase; The main reason is that the transmitter tries to estimate if the channel is free at the receiver with only local information. The situation might be quite different between those two locations. 104

89 An simple solution to this problem is to use RTS/CTS (Request To Send/Clear To Send) handshaking: before sending a packet, the transmitter node sends a RTS packet and waits for a CTS packet from the receiver. The reception of a CTS indicates that the receiver node is able to receive the RTS, (the channel is clear in its area) and the data packet is sent; At the same time, every node in the range of the receiver (that could cause collisions) hears the CTS or the RTS so understands that a transmission is going on. 105

90 The RTS and CTS messages contain the size of the expected transmission, so the nodes will know how long the transmission will last. All nodes will wait long enough before accessing the channel after hearing the CTS, even if their carrier sense indicate that the medium is free. Solving the hidden node problem in CSMA/CA by RTS/CTS mechanism 106

91 IEEE MAC collision avoidance mechanism implements a Network Allocation Vector (NAV), which is a counter value that indicates to a station the amount of time that remains before the medium will become available and is reset each time the station receives a control packet. Even if the medium does not appear to be carrying a transmission by the physical carrier sense, the station may avoid transmitting; A station receiving a valid data frame shall update their NAV with the information received in the Duration/ID field of the frame, in certain conditions; This value will reset the NAV and the station will not attempt to transmit before expiration of the NAV timer. 107

92 NAV duration information is transmitted also in the RTS/CTS packets; Figure below indicates the NAV for STAs that may receive the RTS frame, while other STAs may only receive the CTS frame, resulting in the lower NAV bar as shown (with the exception of the STA to which the RTS was addresed. was addressed). Exchange of messages RTS/CTS/data/ACK and NAV setting 108

93 In conclusion, transmission of a packet in the RTS/CTS mechanism involves the transmission of 4 frames: RTS, CTS, the useful data and the confirmation of correct reception ACK; The supplementary handshaking packets will constitute information overhead that may disturb in certain situations; Therefore, the RTS / CTS mechanism is not recommended for the transmission of short frames or networks with few clients; In exchange, the mechanism can avoid collisions in cases of long data frames which will introduce a larger overhead due to retransmissions. 109

94 Using the RTS/CTS mechanism avoids collisions that would affect the long data frames, collisions can occur only for RTS and CTS frames which is much shorter; The decision to use the RTS/CTS mechanism is taken by the transmitting station when the size of the data frame on hold is greater than or equal to a threshold value (RTS Threshold).Typically, this threshold is set at the highest possible value (2347 bytes), which impedes using the mechanism using RTS/CTS; 110

95 Standard e specifies an exception case in which transmissions can be initiated regardless of RTS threshold value. For example, an RTS frame can be transmitted to book a time called Transmission Opportunity (TXOP). A TXOP is an interval of time when a station has the right to initiate transmissions, defined by a starting time and a maximum duration. This allows consecutive transmissions of multiple data frames. This significantly improves the quality of service (QoS), especially in crowded wireless networks. 111

96 4.4.3 Structure of MAC frame and addressing mode IEEE standard specifies three main types of frames: a. Data frames Data frames carry the useful information; b. Control frames Control frames provide mechanisms to control transmissions (RTS, CTS, ACK); c. Management frames IEEE management frames enable stations to establish and maintain communications. Used for Station association, dissociation, timing, synchronization and authentication etc. 112

97 Addressing mode Each device connected in a network has a 48 bits address called MAC address, which is used for unique identification of each equipment; There are individual addresses, multi-destination or broadcasting addresses, such as FF-FF-FF-FF-FF; Structure of a MAC frame is given in the next slide 113

98 Structure of a MAC frame MAC header Frame control Duration/ Identification Address 1 Address 2 Address 3 Control sequence Address 4 Data Frame Check Sequence(FCS) 2 bytes 2 bytes 6 bytes 6 bytes 6 bytes 2 bytes 6 bytes Max 2312 bytes 4 bytes 114

99 The Frame control field is used to transfer control information between devices; Its subfields contain identifying elements such as protocol version, frame type, information on location of fragment in data packet, the state of energy management, security level, etc. Structure of Frame Control Field 115

100 The Duration field indicates the time required for transmission of frame. If the value is less than 32768, the duration will be used to update a network allocation vector (NAV); The first three address fields are used to address the Access Point receiver, the Access Point transmitter and the destination device. If the frame is transferred through multiple Access Points (wireless distribution systems), the fourth field indicates the address of the device that sent the frame. The Control sequence is used to manage fragmentation MSDU (MAC Service Data Layer) and to detect duplicate frames. It mentions the order of the different fragments of the same frame. 116

101 The Data or Frame body field contains the useful information; The field Frame check sequence (FCS) contains a cyclic code error detection CRC (Cyclic Redundancy Check) calculated for all fields in the header and the data frame body. This allows to check the integrity of received frames. 117

102 Structure of RTS frame RA = address of receiving station; TA = address of transmitting station; Duration (in µs) = The duration value is the time required to transmit the pending data or management frame, plus one CTS frame, plus one ACK frame, plus three SIFS intervals (short interframe space); The purpose is to transmit the duration to stations in order for them to update their NAV to prevent transmissions from colliding with the data or management frame that is expected to follow. 118

103 Structure of CTS frame RA = address of receiving station (copied from address TA of frame RTS) Duration = The duration value is the value obtained from the Duration field of the immediately previous RTS frame, minus the time required to transmit the CTS frame and its SIFS interval. 119

104 Structure of ACK Frame RA = address of receiving station (copied from address field 2 of previous frame); Duration = is set to zero if the immediately previous directed data was not fragmented. If the immediately previous directed data or management frame, the duration value is the value obtained from the Duration field of the immediately previous data or management frame, minus the time, required to transmit the ACK frame and its SIFS interval. 120

105 4.4.4 Interframe spaces (IFS) The time interval between frames is called the IFS. A STA shall determine that the medium is idle through the use of the carrier-sense function for the interval specified. Four different IFSs are defined to provide priority levels for access to the wireless channel; they are listed in order, from the shortest to the longest. a) SIFS - short interframe space b) PIFS - PCF interframe space c) DIFS - DCF interframe space d) EIFS extended interframe space 121

106 a. SIFS (Short Interframe Space) - used to separate transmissions belonging to a single dialog (confirmation of fragment). SIFS is the shortest IFS; There is always at most one station to use this interval, giving it priority to take over the channel over all other stations; SIFS value is set at the physical level and is calculated so that the transmitting station can return to receive mode and be able to decode the packet to be received; For example in b this value is set to 28 µs (for spread spectrum modulation technique in frequency hopping mode). 122

107 b. PIFS (Point Interframe Space) The PIFS shall be used only by stations (Access Point of Point Coordinator) operating under the Point coordination function (PCF) to gain priority access to the medium to start a new transmission. A station using the PCF shall be allowed to transmit contention-free traffic after its carrier sense mechanism determines that the medium is idle. PIFS duration is equal to SIFS + slot duration, in total 78 µs. 123

108 c. DIFS (DCF Interframe Space) The DIFS shall be used by stations operating under the Distributed coordination function (DCF) medium access technique to start a transmission of data frames or management frames. A station using the DCF shall be allowed to transmit if its carrier-sense mechanism determines that the medium is idle. DIFS duration value is (total 128 µs). PIFS + duration of a slot 124

109 d. EIFS (Extended Interframe Space) is a longer IFS used by a station that received a packet it did not understand. The EIFS shall be used by the DCF whenever the PHY has indicated to the MAC that a frame transmission was begun that did not result in the correct reception of a complete MAC frame with a correct FCS value; EIFS is needed to prevent the station (which could not understand the duration information) from colliding with a future packet belonging to the current dialogue. 125

110 4.4.5 Algorithm for association of a station to an Access Point Steps taken by a station entering the coverage area of a Basic Service Set (BSS) or is powered on inside this area: 1. Station search for available Access Points; Access points may not transmit all data for security reasons and then the station must know the Service Set Identifier (SSID) accordingly. 2. Station identifies an Access Point, wants to join it and sends an association request to the Access Point; A "handshake" type exchange of information takes place for authentication of client that will join the network. 126

111 3. After authentication the station is associated with the Access Point and can transmit and receive packets; 4. After using network services, the station must be disassociated from the Access Point; 5. Access points also use "downtime" counters to disassociate inactive stations. 127

112 4.4.6 Synchronization and Power Management Within the MAC are also implemented synchronization and power management functions; Stations must maintain permanent synchronization, this being necessary for other functions including power saving function; These functions can be activated only in networks that operate in infrastructure mode, where all stations adjusts its own clock by the clock of the Access points to which are associated. 128

113 Synchronization is performed as follows: Access Points transmit periodic frames called signaling frames which contain the actual value of the clock at the moment of actual frame transmission (not the moment of adding to the transmission stack, because the delay caused by CSMA algorithm can be significant); Stations receiving the message then check their own clock value at the reception moment and correct it to maintain clock synchronization with the access point. This prevents synchronization losses that may occur after several hours of operation. 129

114 There are two ways in which a station can identify access points and receive timing information: Passive scanning - in this case station only expects to receive a signaling frame (or synchronization) periodically sent from the Access Point. Beacon frames are being sent out from the AP (typically every 100 millisecond) to announce the presence of a wireless LAN. This frame contains many information like SSID, Capability information, Supported rates etc; Active scanning - where the station is trying to find an Access Point transmitting probe request frames and waiting for the reply from AP. This saves time spent scanning. Both are valid, they are chosen according to the accepted compromise between power consumption and performance. 130

115 In wireless local area networks are used often mobile terminals for which energy is a precious resource and therefore the standard addresses the problem of defining energy saving mechanism that allows stations to fully enter standby for long periods of time without losing information addressed to them; Thus, in infrastructure type networks, stations may choose how to control the power and will then inform the AP through an appropriate frame about this. 131

116 The main idea behind this mechanism is that the AP has updated information about the stations that are in Power Save mode and keeps in buffers packets for those stations until they either require the packets by sending a probe request or change their mode of operation; Access Point transmits periodically (as part of signaling frames) information about which stations located in Power Save mode have frames in memory buffer, so those stations should wake up from standby to receive one of these signaling frames. 132

117 If there are indications that in the buffer of the AP there is a frame waiting to be sent, the station should remain "awake" and send a polling message to receive them; If the station is, for example, in "low power mode, it listens only to signals sent by the AP, and this one keeps the packets in buffer until the station will send a message "PS-Poll ; Access Point also stores in buffer all multicast (multidestination) or broadcast messages and forwards them at a preset time (specified in a Delivery Traffic Indication Message-DTIM) when all stations that are in powersaving state and want to receive this kind of messages will again become "active". 133