Presentation is loading. Please wait.

Presentation is loading. Please wait.

Wireless Local Area Networks (WLANs)

Similar presentations


Presentation on theme: "Wireless Local Area Networks (WLANs)"— Presentation transcript:

1 Wireless Local Area Networks (WLANs)
RETI RADIOMOBILI

2 Copyright Quest’opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare: oppure inviare una lettera a: Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. This work is licensed under the Creative Commons NoDerivs-NonCommercial License. To view a copy of this license, visit: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. RETI RADIOMOBILI

3 WLAN Technologies IEEE Hiperlan RETI RADIOMOBILI

4 IEEE RETI RADIOMOBILI

5 Bibliography Tutorial on WLAN ,by Crow, in the IEEE Communications Magazine, 1997 (in English) A.Tanenbaum, Computer networks, 4th ed., Prentice-Hall, 2002 (in English) M. S. Gast, Wireless Networks - The definitive Guide, O’Reilly 2002 (in English) Supporting Material (in English) RETI RADIOMOBILI

6 IEEE Wireless LAN standard specifying a wireless interface between a client and a base station (or access point), as well as between wireless clients Defines the PHY and MAC layer (LLC layer defined in 802.2) Physical Media: radio or diffused infrared Standardization process begun in 1990 and is still going on (1st release ’97, 2nd release ’99, ‘03) RETI RADIOMOBILI

7 Protocol Stack RETI RADIOMOBILI

8 Standards Evolution IEEE The original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard (1999) IEEE a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001) IEEE b - Enhancements to to support 5.5 and 11 Mbit/s (1999) IEEE c - Bridge operation procedures; included in the IEEE 802.1D standard (2001) IEEE d - International (country-to-country) roaming extensions (2001) IEEE e - Enhancements: QoS, including packet bursting (2005) IEEE f - Inter-Access Point Protocol (2003) Withdrawn February 2006 IEEE g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003) IEEE h - Spectrum Managed a (5 GHz) for European compatibility (2004) IEEE i - Enhanced security (2004) IEEE j – Spectrum extensions for Japan (2004) IEEE n – High-speed (up to 540 Mb/s) WLAN IEEE p - WAVE - Wireless Access for the Vehicular Environment IEEE s - ESS Mesh Networking RETI RADIOMOBILI

9 IEEE 802.11 PHY Layer Activities
IR Mbps 2.4GHz FHSS Mbps 802.11g 2.4GHz OFDM 54Mbps (approved in June’03) 2.4GHz DSSS Mbps 5GHz OFDM 802.11b 5-11Mbps 802.11d / TG d Regulatory Domain Update 802.11a 6-54Mbps 802.11h 5GHz Spectrum Managment RETI RADIOMOBILI

10 IEEE 802.11 MAC Layer Activities
802.11e / TG e MAC Enhanced QoS 802.11f / TG f Inter-AP Protocol High Throughput Radio Resource Managment 802.11i / TG i Security Mechanisms RETI RADIOMOBILI

11 IEEE 802.11 (Radio) Evolution
Standard 802.11 802.11b (Wi-Fi) 802.11a 802.11g Approval July 1997 Sep. 1999 June ‘03 Bandwidth 83.5 MHz 300 MHz Operation frequency GHz GHz GHz No. of non-overlapping channels 3 Indoor / Outdoor 4 Indoor 4 Indoor/Outdoor Data rate / channel 1,2 Mbps 1,2,5.5,11 Mbps 6,9,12,18,24,36, 48,54 Mbps 1,2,5.5,6,9, 11,12,18,24,36,48,54Mbps PHY layer FHSS, DSSS DSSS OFDM DSSS / OFDM RETI RADIOMOBILI

12 Architecture BSS (Basic Service Set): set of nodes using the same coordination function to access the channel BSA (Basic Service Area): spatial area covered by a BSS (WLAN cell) BSS configuration mode with infrastructure: the BSS is connected to a fixed infrastructure through a centralized controller, the so-called Access Point (AP) ad hoc mode RETI RADIOMOBILI

13 WLAN with Infrastructure
BSS contains: wireless hosts access point (AP): base station BSS’s interconnected by distribution system (DS) RETI RADIOMOBILI

14 Ad Hoc WLANs Ad hoc network: IEEE stations can dynamically form a network without AP and communicate directly with each other Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield IETF MANET (Mobile Ad hoc Networks) working group RETI RADIOMOBILI

15 Extended Service Set (ESS)
Several BSSs interconnected with each other at the MAC layer The backbone interconnecting the BSS APs (Distribution System) can be a: LAN (802.3 Ethernet/802.4 token bus/802.5 token ring) wired MAN IEEE WLAN An ESS can give access to the fixed Internet network through a gateway node If fixed network is a IEEE 802.X, the gateway works as a bridge thus performing the frame format conversion RETI RADIOMOBILI

16 Ad hoc networking Independent BSS (IBSS)
Possible Scenarios (1) Ad hoc networking Independent BSS (IBSS) STA STA AP STA Distribution system IEEE 802.X STA STA AP Network with infrastructure STA STA RETI RADIOMOBILI

17 Possible Scenarios (2) Ad hoc WLAN Distribution System STA AP AP STA
WLANs with infrastructure RETI RADIOMOBILI

18 Joining a BSS Scanning Authentication Association
BSS with AP: Both authentication and association are necessary for joining a BSS Independent BSS: No authentication neither association procedures are required for joining an IBSS RETI RADIOMOBILI

19 Joining BSS with AP: Scanning
A station willing to join a BSS must get in contact with the AP. This can happen through: Passive scanning The station scans the channels for a Beacon frame (with sync. info) that is periodically sent by the AP Active scanning (the station tries to find an AP) The station sends a ProbeRequest frame All APs within reach reply with a ProbeResponse frame RETI RADIOMOBILI

20 Joining BSS with AP: Authentication
Once an AP is found/selected, a station goes through authentication Open system authentication (default, 2-step process) Station sends authentication frame with its identity AP sends frame as an ack / nack Shared key authentication Stations and AP own shared secret key previously exchanged through secure channel independent of (e.g. set in AP and typed by station user) Stations authenticate through secret key (requires encryption via WEP): challenge & response RETI RADIOMOBILI

21 Joining BSS with AP: Association
Once a station is authenticated, it starts the association process, i.e., information exchange about the AP/station capabilities and roaming STA -> AP: AssociateRequest frame AP -> STA: AssociationResponse frame New AP informs old AP via DS in case of roaming Only after the association is completed, a station can transmit and receive data frames RETI RADIOMOBILI

22 IEEE / b RETI RADIOMOBILI

23 Physical Layer Three different access techniques: Infrared (IR)
Frequency hopping spread spectrum (FHSS) Direct sequence spread spectrum (DSSS) RETI RADIOMOBILI

24 Infrared Works in the regular IR LED range, i.e., 850-950 nm
Used indoor only Employes diffusive transmissions, nodes can receive both scattered and line-of-sight signals 2 Mbps obtained through 4-pulse position modulation (4-PPM), i.e., 2 information bits encoded with 4 bits Max output power: 2W Not really used – IrDA is more common and cheaper RETI RADIOMOBILI

25 Spread Spectrum Idea: spread signal over wider frequency band than required Frequency Hopping : transmit over random sequence of frequencies Direct Sequence random sequence (known to both sender and receiver), called chipping code RETI RADIOMOBILI

26 FHSS Not really used anymore Frequency band: ISM @ 2.4 GHz
In the U.S., the FCC has specified 79 ISM frequency channels with width equal to 1 MHz. Central frequency GHz 3 channels each corresponding to 1Mbps with GFSK modulation 20 ms dwell time  50 hops/s RETI RADIOMOBILI

27 FHSS (2) Three sets of hopping frequency sequences are used to reduce interference between adjacent BSSs. Each set includes 26 sequences RETI RADIOMOBILI

28 DSSS (1) Radiated power is limited Typical values: 85 mW
Frequency band: ISM 2.4 GHz Band divided into 14 channels, each 22 MHz wide To avoid interference, only channels 1,6,11 are used (which are spaced by  25MHz) No more than 3 adjacent BSSs should be allowed Adjacent BSSs coexist without interfering with each other if the separation between their f0 is at least equal to 25MHz RETI RADIOMOBILI

29 Overlapping Frequency Channels
RETI RADIOMOBILI

30 RETI RADIOMOBILI

31 Selected Channels RETI RADIOMOBILI

32 Channel 1 Channel 6 Channel 11 RETI RADIOMOBILI

33 RETI RADIOMOBILI

34 DSSS (2) In the case of 1 and 2 Mb/s data rates:
Spreading made by using the Barker sequence with length of 11 chips. Transmission rate=11Mchip/s Barker sequence is fixed for all stations within a BSS Modulation scheme: DBPSK (Differential Binary Phase Shift 1 Mbps: occupied bandwidth = 11 MHz (base band, 22 MHz double sided) -> 11MHz/(11chip/symbol) = 1Mbps; SF=11 DQPSK (Differential Quadrature Phase Shift 2 Mbps, occupied bandwidth = 11 MHz (base band, 22 MHz double sided), 11 chip/symbol -> SF = 5.5 RETI RADIOMOBILI

35 DSSS (3) In the case of 5.5 and 11 Mb/s data rates:
CCK (Complementary Code Keying): a code book and a DQPSK modulation are used. 2 (6 in the case of 11 Mb/s) information bits determine the 8-bit code word, 2 bits determine the phase Range Indoor: 91 1 Mbps; Mbps Outdoor: 460 1Mbps; Mbps RETI RADIOMOBILI

36 Advantage of Multi-rate
Direct relationship between communication rate and the channel quality required for that rate As distance increases, channel quality decreases Thus tradeoff between communication range and link speed Multi-rate provides flexibility to meet both consumer demands and coverage requirements 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Add that MAC adaptively sets rate Add that multi-rate allows both longer range and higher speed (but not at same time) Multi-rate used by most existing standards (802.11abg HiperLANII, etc.) and likely to continue into the future. Very little research on the effects of multi-rate on ad hoc networks. Lucent Orinoco b card ranges using NS2 two-ray ground propagation model RETI RADIOMOBILI

37 Rate Adaptation Stations constantly perform operations to detect and automatically set the best data rate Control information always basic rate Standard does not specify how to adapt transmission speed Automatic Rate Adaptation: based on SIR measurements over moving window RETI RADIOMOBILI

38 Auto Rate Selection Auto Rate Fallback (ARF) [Monteban97]
Adaptive, based on success/failure of previous packets Simple to implement Doesn’t require the use of RTS/CTS or changes to specs Receiver Based Auto Rate (RBAR) [Holland01] Receiver uses SNR measurement of RTS to select rate and notifies it to the sender through CTS Faster & more accurate in changing channel Requires some tweaks to the header fields The first commercial implementation that exploits this multi-rate capability is termed Auto Rate Fallback (ARF) [6], used in Lucent's WaveLAN II devices. With ARF, senders use the history of previous transmission error rates to adaptively select future (attempted) transmission rates. That is, after a number of consecutive successful transmissions, the sender changes its modulation scheme to attempt transmission at a higher rate, and vice versa after consecutive losses. Consequently, if a mobile user has (for example) a perpetually high-quality channel, the user will eventually transmit at higher data rates. ARF may works not so well when low SNR is due to fading instead of distance, i.e., in presence of quick propagation conditions changes. An enhanced protocol to exploit the multi-rate capabilities of IEEE a termed Receiver Based Auto Rate (RBAR). The key idea of RBAR is for receivers to control the sender's transmission rate. In IEEE a, all RTS/CTS messages must be sent at the base rate to ensure that all stations are able to receive these messages error free.In RBAR RTS and CTS carries rate and length (from which the time duration of the frame exchange can be derived) instead of the time duration. RBAR uses physical-layer analysis of the received RTS message to determine the maximum possible transmission rate for a particular bit error rate. The receiver inserts this rate into a special field of the CTS message to inform the sender and other overhearing nodes of the potentially modified rate. The sender specified the updated rate to be used to send data in a new header field of the MAC data frame. This message is termed reservation-sub-header (RSH) and is inserted preceding data transfer. With the RSH message, overhearing nodes can modify their NAV values to the new potentially decreased transmission time. In this way, RBAR quickly adapts to changing channel conditions. RBAR requires the following changes: RTS and CTS carry rate & length, data frame carries RSH, and the Signal field of the PLCP Header carries the rate at which RSH and the data frame are tx. Prism 2.5: it mainatins a bit rate for each STA the card talks to, starting from 11Mbps. If we have 8 packet failures in a row, the rate is decreased to the next lower rate. If a packet is acked and no failure occurs in the next 10 s, then the rate is increased to 11Mbps. RETI RADIOMOBILI

39 IEEE 802.11 MAC Protocol Performs the following functions:
Resource allocation Data segmentation and reassemby MAC Protocol Data Unit (MPDU) address MPDU (frame) format Error control RETI RADIOMOBILI

40 Time Units (Slots) Time is divided into intervals, called slots
A slot is the system unit time and its duration depends on the implementation of the physical layer (it accounts for TX/RX turnaround time and Power detection time) 802.11b: 5 μs turnaround + 15 μs power detection = 20 μs Stations are synchronized with the AP in the infrastructure mode and among each other in the ad hoc mode  the system is synchronous Synchronization maintained through Beacon frames RETI RADIOMOBILI

41 IFS – InterFrame Space InterFrame Space (IFS)
time interval between frame transmissions used to establish priority in accessing the channel 4 types of IFS: Short IFS (SIFS) Point coordination IFS (PIFS) >SIFS Distributed IFS (DIFS) >PIFS Extended IFS (EIFS) > DIFS Duration depends on physical level implementation RETI RADIOMOBILI

42 Short IFS (SIFS) To separate transmissions belonging to the same dialogue Shortest IFS  Associated to the highest priority Its duration depends on: Propagation time over the channel Time to convey the information from the PHY to the MAC layer Radio switch time from TX to RX mode 802.11b: 10μs RETI RADIOMOBILI

43 Point Coordination IFS (PIFS)
Used to give priority access to Point Coordinator (PC) Only a PC can access the channel between SIFS and DIFS PIFS=SIFS + 1 time slot SIFS < PIFS RETI RADIOMOBILI

44 Distributed IFS (DIFS)
Used by stations waiting for a free channel to contend Set to: PIFS + 1 time slot SIFS < PIFS < DIFS RETI RADIOMOBILI

45 Extended IFS (EIFS) Used by a station when the PHY layer notifies the MAC layer that a transmission has not been correctly received Waits more before trying to access the channel, as a different station may correctly receive the frame and reply with an ACK, and we do not want to disrupt the ACK with a new transmission SIFS < PIFS < DIFS < EIFS RETI RADIOMOBILI

46 MAC Frames Three frame types are defined
Control: positive ACK, handshaking for accessing the channel (RTS, CTS) Data Transfer: information to be transmitted over the channel Management: connection establishment/release, synchronization, authentication. Exchanged as data frames but are not reported to the higher layer RETI RADIOMOBILI

47 Data Transfer Distributed, asynchronous data transfer for delay-tolerant traffic (like file transfer) DCF (Distributed Coordination Function) Centralized, synchronous data transfer for real-time traffic (like audio and video) PCF (Point Coordination Function): based on the polling of the stations and controlled by the AP (PC) Its implementation is optional (not really implemented) RETI RADIOMOBILI

48 DCF Access Scheme RETI RADIOMOBILI

49 DCF basic features DCF implementation is mandatory
Broadcast wireless medium: multiple access Distributed scheme: lack of central coordination Stations have a single network interface, and can perform only one action at a time: trasmit or receive (no Collision Detection) Random Multiple Access CSMA/CA RETI RADIOMOBILI

50 CSMA Carrier Sense Multiple Access If a node needs to transmit data
senses the channel (Carrier Sensing) for a DIFS period if the channel is idle after DIFS, the station transmits if the channel becomes busy during the DIFS period, the station waits until the transmission is ended before trying to transmit again If a node receives data correctly replies with an ACK after SIFS from end of data reception DIFS SIFS DIFS DATA source ACK destination RETI RADIOMOBILI

51 CSMA Carrier Sensing is performed in two ways in DCF
Physical Carrier Sensing: the station senses the channel by means of its network interface Virtual Carrier Sensing: the station uses information about ongoing data transmissions to avoid transmission when DATA is received, other stations set a Network Allocation Vector (NAV) to the end of data exchange (ACK included), and stay silent until the NAV expires DIFS SIFS DIFS DATA source ACK destination NAV other station RETI RADIOMOBILI

52 CSMA Random multiple access: stations contend for the channel
Each transmission requires a contention one single data frame sent every time Collisions can occur Wireless channel can cause errors on bits Automatic Retransmission reQuest (ARQ) stop&wait used to retransmit non-ACK’d frames up to retryLimit times collision DIFS DATA source A DATA source B RETI RADIOMOBILI

53 CSMA/CA CSMA with Collision Avoidance
When a station senses the channel idle after DIFS it starts a Random BackOff (BO) before transmitting data DIFS without BO DATA source A destination DATA source B BO 4 DIFS SIFS with BO 4 3 2 1 DATA source A BO 8 ACK destination 8 7 6 5 NAV source B RETI RADIOMOBILI

54 CSMA/CA After DIFS expires (and channel is still idle!)
contending stations each extract a BO BOs are decremented by each station the first station whose BO goes to zero transmit other stations sense a transmission has started freeze their BO to the current value set their NAV to the end of the transmission When transmission ends (after the ACK) contending stations all wait DIFS contending stations resume their BOs decrement RETI RADIOMOBILI

55 CSMA/CA A station successfully completing a transmission, always extracts a new BO (Post-BackOff), even if it has no data waiting to be sent After DIFS from ACK reception, it starts decrementing the Post-BackOff, which behaves like a standard one This means that a station can wait just DIFS before sending data in two cases only (assuming an idle channel) the station has just joined the BSS the station receives a packet to send after it has already decremented its Post-BackOff to zero RETI RADIOMOBILI

56 new BackOff extraction
CSMA/CA Example: 3 contending stations BO 3 DIFS SIFS DIFS DIFS 4 3 2 1 3 2 1 A BO 6 SIFS 6 5 4 3 2 1 6 5 B SIFS 8 7 6 5 4 3 2 2 1 C received packet to send DATA SIFS BO 4 D NAV new BackOff extraction ACK 1 backoff RETI RADIOMOBILI

57 CSMA/CA Collisions are still possible
two stations can extract the same BO value The probability of a collision depends on the number of contending stations more contending stations higher probability that two stations pick the same BO value To reduce the probability of collision in presence of many stations, the range of the BO is increased more BO values to pick from lower probability that two stations choose the same BO value disadvantage: delay is increased RETI RADIOMOBILI

58 CSMA/CA Random Backoff is computed as CW is the Contention Window:
BO = slotTime * uniform[0,CW] CW is the Contention Window: CW is an integer always in the interval [CWmin,CWmax] CW is initially set to CWmin CW is doubled after every failed tranmsission, up to a value CWmax CW is reset to CWmin after a successful transmission Standard values for DCF: CW = 2 ( CW + 1 ) - 1 CWmin 31, 63, 127, 255, 511, 1023 CWmax RETI RADIOMOBILI

59 CSMA/CA The PHY layer can inform the MAC layer that an erroneous transmission has been sensed The error might be related to the position of the station, and other stations might receive the frame correctly As a consequence, the station that sensed the erroneous frame (A) must stay silent for the time needed for a possible reply (ACK) from the destination of the transmission Station A waits EIFS after the end of reception of the erroneous frame (when channel becomes idle) RETI RADIOMOBILI

60 CSMA/CA BO 4 without EIFS source + destination DATA ACK DATA
DIFS SIFS without EIFS source + destination 4 3 2 1 DATA ACK 1 DATA other station DIFS BO 4 DIFS SIFS DIFS source + destination with EIFS 4 3 2 1 DATA ACK other station EIFS BO 4 DIFS SIFS source + destination 4 3 2 1 DATA ACK ACK other station EIFS RETI RADIOMOBILI

61 CSMA/CA: problems Long time to detect a collision Hidden Terminal
must wait for missing ACK, the whole frame must be transmitted Hidden Terminal stations may not be all within transmission range OR DIFS 4 3 2 1 DATA SIFS 2 1 DATA RETI RADIOMOBILI

62 RTS/CTS Solution: handshaking phase before data transmission
the sender asks permission to transmit with a Ready To Send (RTS) control frame The receiver grants transmission with a Clear To Send (CTS) control frame All the handshaking control frames are sent at basic transmission rate (usually 1Mbps) to ensure maximum resilience to channel errors DIFS SIFS SIFS 2 1 RTS DATA SIFS SIFS CTS ACK NAV RETI RADIOMOBILI

63 RTS/CTS Neighboring stations all set/update their NAV upon every RTS/CTS/DATA/ACK frames reception RTS (20 bytes) and CTS (14 bytes) small frames still add overhead to transmission RTS/CTS handshaking only used for large frames only packets larger than a RTS/CTS threshold are preceded by a RTS/CTS handshaking The RTS/CTS threshold also determines the maximum number of retransmissions of a packet shortRetryLimit (7) if packet size ≤ RTS/CTS threshold longRetryLimit (4) if packet size > RTS/CTS threshold RETI RADIOMOBILI

64 RTS/CTS Long collision detection times are avoided
Collision detected SIFS after RTS transmission ACK timeout DIFS 2 1 DATA source A SIFS destination 2 1 DATA source B BO 2 DIFS DIFS SIFS SIFS 2 1 RTS 2 1 RTS DATA SIFS SIFS ACK timeout BO 4 CTS ACK 2 1 RTS 4 3 NAV RETI RADIOMOBILI

65 RTS/CTS Long collision detection times are avoided
Collision detected SIFS after RTS transmission DIFS 2 1 DATA ACK timeout source A SIFS destination 2 1 DATA source B BO 2 DIFS DIFS SIFS SIFS 2 1 RTS ACK timeout 2 1 RTS DATA SIFS SIFS BO 4 CTS ACK 2 1 RTS 8 7 6 5 4 3 NAV RETI RADIOMOBILI

66 RTS/CTS Hidden terminal problem is mitigated
intermediate station informs out-of-range nodes of the ongoing transmission DIFS 4 3 2 1 DATA SIFS 8 7 6 5 4 3 2 1 DATA DIFS SIFS SIFS 4 3 2 1 RTS DATA SIFS SIFS CTS ACK 8 7 6 5 4 3 2 NAV RETI RADIOMOBILI

67 RTS/CTS RTS/CTS does not solve all the problems
Complex Hidden Terminal sends DATA to sends RTS to sends DATA to sends CTS to does not receive CTS from (collision with  ) and can disrupt  RETI RADIOMOBILI

68 RTS/CTS RTS/CTS does not solve all the problems Exposed Terminal
sends RTS to sends CTS to receives RTS from and avoids transmission to but a transmission from to would be ok! RETI RADIOMOBILI

69 MACA Mulitple Access with Collision Avoidance
Scheme which first intrdouced the RTS/CTS mechanism [Karn’90] It also proposed a solution to the Exposed Terminal problem if a station A hears a RTS, but does not hear the CTS afterwards, it means it can sense the transmitter but not the receiver assuming symmetric channels, the receiver cannot sense station A station A resets its NAV (was set by RTS) and can transmit without colliding with the receiver of the other transmission RETI RADIOMOBILI

70 Data Fragmentation (1) An MSDU is fragmented into more than one frame (MPDU) when its size is larger than a certain fragmentation threshold In the case of failure, less bandwidth is wasted All MPDUs have same size except for the last MPDU that may be smaller than the fragmentation threshold PHY and MAC headers are inserted in every fragment -> convenient if the fragmentation threshold is not too little RETI RADIOMOBILI

71 Data Fragmentation (2) MPDUs originated from the same MSDU are transmitted at distance of SIFS + ACK + SIFS The transmitter releases the channel when the transmission of all MPDUs belonging to an MSDU is completed the ACK associated to an MPDU is lost RETI RADIOMOBILI

72 Data Fragmentation (3) Backoff counter is increased for each fragment retransmission belonging to the same frame The receiver reassembles the MPDUs into the original MSDU that is then passed to the higher layers Broadcast and multicast data units are never fragmented RETI RADIOMOBILI

73 DCF Access Scheme RETI RADIOMOBILI

74 Basic Characteristics
Its implementation is mandatory DCF is based on the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme: stations that have data to transmit contend for accessing the channel a station has to repeat the contention procedure every time it has a new data frame to transmit RETI RADIOMOBILI

75 IEEE 802.11 MAC Protocol Overview: CSMA/CA
CSMA: sender - if sense channel idle for DIFS seconds then transmit entire frame (no collision detection) -if sense channel busy then binary backoff (CA) CSMA receiver: if received OK return ACK after SIFS RETI RADIOMOBILI

76 IEEE 802.11 MAC Protocol Overview
CSMA Protocol: others NAV: Network Allocation Vector frame has transmission time field others (hearing data) defer access for NAV time units RETI RADIOMOBILI

77 Hidden Terminal Effect
hidden terminals: A, C cannot hear each other obstacles, signal attenuation collisions at B goal: avoid collisions at B CSMA/CA with handshaking RETI RADIOMOBILI

78 IEEE 802.11 MAC Protocol Overview: Handshaking
CSMA/CA: explicit channel reservation sender: send short RTS (Request To Send) receiver: reply with short CTS (Clear To Send) CTS reserves channel for sender, notifying (possibly hidden) stations avoid hidden station collisions RETI RADIOMOBILI

79 IEEE 802.11 MAC Protocol Overview: Handshaking
RTS and CTS are short: collisions less likely, of shorter duration end result similar to collision detection DCF allows: CSMA/CA CSMA/CA with reservations RETI RADIOMOBILI

80 The DCF Access Scheme Basic the simplest scheme
used when the data frames to be transmitted have a fairly short duration With handshaking uses additional control frames for channel access designed to solve the hidden terminals problem provides higher reliability in data transmission RETI RADIOMOBILI

81 DCF The Basic Access Mode
RETI RADIOMOBILI

82 Carrier Sensing Used to determine whether the channel is busy or idle
Performed at the physical layer (physical carrier sensing) and at the MAC layer (virtual carrier sensing) Physical carrier sensing: detection of nearby energy sources Virtual carrier sensing: the frame header indicates the duration of the MAC PDU (MPDU) included in the frame RETI RADIOMOBILI

83 Network Allocation Vector (NAV)
Used by the stations nearby the transmitter to store the duration of the frame that is occupying the channel The channel will become idle when the NAV expires Upon the NAV expiration, stations that have data to transmit listen to the channel again RETI RADIOMOBILI

84 Using DIFS and SIFS Transmitter: it senses the channel
if the channel is idle, it waits a time equal to DIFS if the channel remains idle for DIFS, it transmits its MPDU RETI RADIOMOBILI

85 Using DIFS and SIFS Receiver:
computes the checksum thus verifying whether the transmission is correct if so, it sends an ACK after a time equal to SIFS it should always transmit an ACK with a rate less than or equal to the one used by the transmitter RETI RADIOMOBILI

86 Using DIFS and SIFS Neighbors:
set their NAV to the value indicated in the transmitted MPDU NAV set to: the MPDU tx time + 1 SIFS + ACK RETI RADIOMOBILI

87 MPDU Transmission DATA SIFS source DIFS ACK destination others NAV
RETI RADIOMOBILI

88 Frame Retransmissions
A frame transmission may fail because of collision or error on the radio channel A failed transmission is re-attempted till a max no. of retransmissions is reached ARQ scheme (Stop&Wait) RETI RADIOMOBILI

89 Collision Avoidance (CA)
Backoff procedure If a station senses the channel as busy, it waits for the channel to become idle As soon as the channel is idle for DIFS, the station computes the backoff time interval sets the backoff counter to this value and then decreases it with time The station will be able to transmit when its backoff counter reaches 0 RETI RADIOMOBILI

90 MPDU Transmission DATA SIFS source DIFS ACK destination others DIFS CW
backoff DIFS ACK DIFS CW NAV CW=Contention Window RETI RADIOMOBILI

91 Backoff Value Integer value corresponding to a number of time slots
The number of slots is a r.v. uniformly distributed in [0,CW] CW is the Contention Window and at each transmission attempt is updated as follows: For i=1, CW1=CWmin For i>1, CWi=2(CWi-1+1)-1 with i>1 being the no. of consecutive attempts for transmitting the MPDU For any i, CWi CWmax RETI RADIOMOBILI

92 Backoff Decrease While the channel is busy, the backoff counter is frozen While the channel is idle, the station decrements the backoff value until the channel becomes busy or the backoff counter reaches 0 RETI RADIOMOBILI

93 Accessing the Channel If more than one station decreases its counter to 0 at the same time → collision Colliding stations have to recompute a new backoff value RETI RADIOMOBILI

94 Post-Backoff After completing a transmission, a station waits for the ACK and then performs the backoff procedure, no matter whether it has a new MPDU to transmit or not Only two cases where an STA sends a frame after having sensed the channel as idle for DIFS: the STA has just entered the BSS, or its queue is idle and its post-backoff time has already passed RETI RADIOMOBILI

95 Basic DCF: An Example Stations A/C Station B Data ACK SIFS Backoff
wait DIFS RETI RADIOMOBILI

96 Backoff Procedure Station A Station B Station C Station D Station E
Decremented Backoff Remaining Backoff DIFS DIFS DIFS DIFS Station A Station B Station C Station D Station E CW Frame Backoff wait Frame CW wait Frame wait Frame wait Frame RETI RADIOMOBILI

97 Data Fragmentation (1) An MSDU is fragmented into more than one frame (MPDU) when its size is larger than a certain fragmentation threshold In the case of failure, less bandwidth is wasted All MPDUs have same size except for the last MPDU that may be smaller than the fragmentation threshold PHY and MAC headers are inserted in every fragment -> convenient if the fragmentation threshold is not too little RETI RADIOMOBILI

98 Data Fragmentation (2) MPDUs originated from the same MSDU are transmitted at distance of SIFS + ACK + SIFS The transmitter releases the channel when the transmission of all MPDUs belonging to an MSDU is completed the ACK associated to an MPDU is lost RETI RADIOMOBILI

99 Data Fragmentation (3) Backoff counter is increased for each fragment retransmission belonging to the same frame The receiver reassembles the MPDUs into the original MSDU that is then passed to the higher layers Broadcast and multicast data units are never fragmented RETI RADIOMOBILI

100 Recontending for the Channel
A station recontends for the channel when it has completed the transmission of an MPDU but still has data to transmit an MPDU transmission fails and the MPDU must be retransmitted Recall that before recontending for the channel, a station must perform the backoff procedure RETI RADIOMOBILI

101 EIFS Used by a station, let’s say A, when the PHY layer notifies the MAC layer that a transmission is in error Starts from the instant when the PHY layer detects an idle channel after the erroneous frame, without regard to the virtual carrier-sense mechanism RETI RADIOMOBILI

102 EIFS Reception of an error-free frame during the EIFS resynchronizes A to the actual busy/idle state of the channel In this case, the EIFS is terminated and a normal medium access (using DIFS and, if necessary, backoff) is resumed EIFS must be long enough so that another station can acknowledge what was, for A, an erroneous frame before A accesses the channel RETI RADIOMOBILI

103 Example B->C; due to channel error A doesn’t receive correctly.
As B’s tx ends, A must wait EIFS before accessing the channel (in the meanwhile B can rx the ACK from C) RETI RADIOMOBILI

104 DCF Basic Access Sender transmits a frame
Summary DCF Basic Access Sender transmits a frame Receiver sends ACK when it gets the frame Neighbors: silent until the ACK if they heard the NAV otherwise, they detect the channel busy as they start sensing it Collisions no collision detection known when ACK is not received exponential backoff (collision avoidance) RETI RADIOMOBILI

105 DCF Access with handshaking
RETI RADIOMOBILI

106 Access with Handshake Used to reserve the channel Why? Hidden stations
Need to avoid collisions, especially when frame is large: Colliding stations keep transmitting their MPDU; the larger the MPDU involved in the collision, the more bandwidth is wasted Particularly useful when a large no. of STAs contend for the channel RETI RADIOMOBILI

107 RTS/CTS Handshaking procedure uses the Request to Send (RTS) and Clear to Send (CTS) control frames RTS / CTS should always be basic rate Access with handshaking is used for frames larger than an RTS_Threshold RETI RADIOMOBILI

108 DCF with Handshaking Transmitter:
send a RTS (20 bytes long) to the destination Neighbors: read the duration field in RTS and set their NAV Receiver: acknowledge the RTS reception after SIFS by sending a CTS (14 bytes long) read the duration field in CTS and update their NAV start transmitting upon CTS reception RETI RADIOMOBILI

109 MPDU Transmission & NAV
DIFS SIFS RTS source DATA CTS ACK destination SIFS SIFS others NAV (RTS) CW NAV (CTS) backoff DIFS NAV (data) RETI RADIOMOBILI

110 Solving the Problem of Hidden Terminals
C D B C: RTS ; A knows that B is busy comunicating with C C B: CTS ; D knows that C is busy receiving B’s transmission RETI RADIOMOBILI

111 Notice that… If a station detects the RTS message only, it will set its NAV accordingly If the station originating the RTS does not receive the CTS within a certain timeout, it thinks that the destination is unreachable it starts the backoff procedure to re-attempt a channel access RETI RADIOMOBILI

112 Notice that… A station, which receives the CTS only, will set its NAV accordingly even if it did not receive the corresponding RTS In this way, the hidden terminals problem is solved However, if an STA close to the receiver does not hear the CTS because of collision, it may attempt at accessing the channel and cause collision at the receiver RETI RADIOMOBILI

113 The Problem of Hidden Terminals
C D E A<->E; B listens to A-E cannot hear C sending a CTS as reply to D’s RTS B then can transmit an RTS toward C causing C while also D is transmitting toward C RETI RADIOMOBILI

114 MACA (Multiple Access with Collision Avoidance Karn ’90)
Algorithm that originated the RTS/CTS mechanism used in MACA avoids the hidden terminals problem as done in MACA avoids the exposed terminals problem too RETI RADIOMOBILI

115 The Problem of Exposed Terminals
C D C D: RTS D C: CTS ; B does not hear D sending the CTS, thus it could transmit to A without causing collision at D RETI RADIOMOBILI

116 MACA MACA avoids the exposed terminals problem as follows:
If a station detects an RTS message, it will set its NAV accordingly However, if it does not hear the corresponding CTS within a certain timeout, it thinks that it can attempt at accessing the channel and resets its NAV RETI RADIOMOBILI

117 The Problem of Exposed Terminals
C D C D: RTS D C: CTS ; B does not hear D sending the CTS, thus it transmits to A RETI RADIOMOBILI

118 MACAW (MACA Wireless) It uses the following sequence of messages: RTS-CTS-DS (Data Sending)-Data-ACK DS is introduced because: If B does not hear the CTS from D, B can access the channel If B sends a RTS to A while C has already started transmitting to D, B may be unable to receive the CTS from A (C’s transmission and A’s transmission would collide at B) RETI RADIOMOBILI

119 MACAW To solve the problem, C sends a DS message to inform its neighbors that data transmission will take place No other transmission with RTS/CTS by C’s neighbors should be attempted during this time RETI RADIOMOBILI

120 Retransmission Counters
Each station uses two counters: Long_Retry_Counter (LRC) if the MPDU to transmit is larger than the RTS_Threshold Short _Retry_Counter (SRC) otherwise Long_Retry_Limit (LRL): max value of LRC Short _Retry_Limit (SRL): max value of SRC RETI RADIOMOBILI

121 Retransmissions Counters
If the transmitter does not receive the CTS as a reply to the RTS frame, after a certain timeout it increases its SRC If the transmitter does not receive the ACK for its MPDU frame, after a certain timeout: it increases its LRC if MPDU size  RTS_Threshold it increases its SRC if MPDU size < RTS_Threshold As soon as either one of the 2 counters reaches its maximum value (SRL and LRL, respectively), the frame is discarded RETI RADIOMOBILI

122 DCF with Handshaking Summary Sender transmits RTS frame
Receiver replies with CTS frame Neighbors: see either RTS or CTS: keep quiet Receiver sends ACK when has the frame neighbors silent until ACK transmission ends Collisions no collision detection known when CTS is not received exponential backoff (collision avoidance) RETI RADIOMOBILI

123 Remarks on DCF In the long term, it gives all nodes same chance to access the channel It is the only possible coordination function in the case of ad hoc network configuration when an infrastructure exists, the PCF can be implemented in conjuction with the DCF RETI RADIOMOBILI

124 PCF Centralized access scheme
RETI RADIOMOBILI

125 Basic Characteristics
Used for services with QoS requirements, it provides a contention-free access to the channel Needs a Point Coordination (PC) that polls the stations → it can be implemented in networks with infrastructure only (AP=PC) Stations enabled to operate under the PCF mode are said to be CF-aware (CF=Contention Free) RETI RADIOMOBILI

126 PCF Stations declare their participation in the CF phase in the Association Request PC builds the polling list based on the received requests Polling list is static Implementation of the polling list and tables are left to the system operator RETI RADIOMOBILI

127 PCF Duration Designed to coexist with the DCF
The Collision Free Period (CFP) Repetiton Interval (or Superframe) determines the repetition frequency of the PCF with respect to the Collision Period (CP), during which the DCF is performed CFP starts with a beacon signal periodically broadcast by the AP used to synchronize stations The CFP terminates with a frame of CF_end RETI RADIOMOBILI

128 CFP Repetition Interval or Superframe
Coexistence between DCF and PCF CFP Repetition Interval or Superframe B B PCF DCF PCF DCF NAV NAV RETI RADIOMOBILI

129 PCF Duration Max CFP duration determined by parameter CFP_Max_Duration (included in the beacon) Min CFP_Max_Duration: 2 MPDUs with max length + 1 beacon frame + 1 CFP_end frame Max CFP_Max_Duration: CFP repetition interval – (RTS+CTS+1 MPDU with max length + ACK) CFP duration determined by PC based on traffic load When a CFP starts, all stations set their NAV to CFP_Max_Duration RETI RADIOMOBILI

130 Superframe and PCF Protocol
TBTT Superframe Max Contention Free Period P Busy Medium S S S S Contention Period CF-End D1+ Poll D2+ACK+Poll D3+Poll B Ack U1+ ACK Null+ACK U3+ ACK S S S S NAV Reset NAV TBTT: Target Beacon Transmission Time D1, D2, D3: frames sent by PC U1, U2, U3: frames sent by polled station B: beacon frame (sent by AP) D=CF-Downlink U=CF-UPlink S=SIFS P=PIFS RETI RADIOMOBILI

131 CFP Access When CFP has to start, the PC senses the channel. If idle and still so for a PIFS, the PC broadcasts the beacon frame In CFP, stations can transmit only in response to a PC’s poll, or to acknowledge an MPDU After SIFS from the beacon, the PC transmits a CF-Poll frame or a data frame or a data frame + a CF-Poll frame RETI RADIOMOBILI

132 CFP Access The PC MAY end the CFP by sending a CFP_end frame even right after its first transmission (a CF-ACK or a data frame or a data+CF-ACK) In the case the CFP goes on, the polled station can reply after a SIFS interval by sending a data frame a data frame + CF-ACK (if it received data) a NULL frame (+ ACK) if it does not have any data RETI RADIOMOBILI

133 CFP Access As the PC receives a data frame+CF-ACK it waits SIFS
then it transmits a data frame+CF-ACK+CF- Poll to a different station If the PC does not receive the CF-ACK as expected, it waits a PIFS time and then transmits to the next station in the polling list RETI RADIOMOBILI

134 What’s the Problem in WLAN QoS
PCF designed to provide QoS to real-time traffic What makes QoS in difficult? Unpredictable beacon delay An STA does not initiate a transmission after TBTT, but continues its on-going transmission thus beacon frames may be delayed The larger the frame size, the longer the beacon delay (up to 4.9 ms) Unknown transmission duration Static polling list -> polling overhead RETI RADIOMOBILI

135 More details on RETI RADIOMOBILI

136 Power Saving Typically, 802.11 cards have high power consumption:
Ptx=1.6 W, Prx=1.45 W, Pidle=1.15 W, Pdoze=0.085 W To reduce energy expenditure, stations can go into Power Saving Mode (PSM) RETI RADIOMOBILI

137 Power Saving Mode (with AP)
AP periodically transmits Beacon (for sync.) Stations which want to move into PSM select their “waking up period” (as a multiple of the Beacon period) and inform the AP The AP maintains a record of the stations in PSM and buffers packets until stations wake up Upon sending a beacon, the AP includes in the Traffic Indication Map (TIM) field which stations in PSM have waiting data RETI RADIOMOBILI

138 Power Saving Mode (with AP)
Stations in PSM monitor beacon transmissions every waking up period: if there are data for them they remain awake and poll the AP for it otherwise they go back to sleep Multicast messages are transmitted at an a-priori known time All stations who wish to receive this information should wake up RETI RADIOMOBILI

139 Power Saving Mode (with AP)
Stations with waiting data backoff before sending a PS-Poll message If PS-Poll is successful, AP sends data frame after SIFS If there are more frames at the AP for that station, AP sets the MoreData bit to 1 and the station will send another Poll RETI RADIOMOBILI

140 Generic Frame Format (for all frames)
PLCP Header MAC Header Frame Body (payload) Preamble CRC RETI RADIOMOBILI

141 Preamble and PLCP Header
Preamble (PHY basic rate) Sync - An 80 bit sequence of alternating 0s and 1s Start Frame Delimiter (SFD) - 16-bit pattern: (for frame timing) PLCP Header basic rate) Length Word - No. of bytes in the frame (used by the PHY layer) Signaling Field – for data speed HEC – 16-bit CRC for the header RETI RADIOMOBILI

142 MAC Header+Frame Body+CRC
0-2312 2 2 6 6 6 2 6 4 Frame Control Duration or ID Addr. 1 Addr. 2 Addr. 3 Sequ. Control Frame Body Addr. 4 CRC 2 2 4 1 1 1 1 1 1 1 1 Protocol Version Power Mngmt From DS More Frag. More Data Type SubT To DS Retry WEP Order Length of the MAC Data and CRC fields in octects Length of the Frame Controld fields in bits RETI RADIOMOBILI

143 Frame Control Field Protocol Version
To differentiate among e.g , a, b, g Type and Subtype Frame type: management (e.g., Beacon, Probe, Association), control (e.g., RTS, CTS, ACK, Poll), or data There are more than 30 different subtypes of frame RETI RADIOMOBILI

144 Frame Control Field ToDS / FromDS
Whether a frame destined to the DS or not FromDS=0,ToDS=0: Mng&Control frames, Data frames within an IBSS FromDS=1,ToDS=0: data frame to a station in an infrastructure network FromDS=0,ToDS=1: data frame from a station in an infrastructure network FromDS=1,ToDS=1: data frame on a wireless bridge RETI RADIOMOBILI

145 Frame Control Field More Fragments To signal more incoming fragments
Retry 1 if it is a retransmission Power Managment To signal that the station is changing from Active to Power Save mode (or vice-versa) More Data There are more frames buffered for this station RETI RADIOMOBILI

146 Frame Control Field WEP
Indicates whether the frame body is encrypted or not Order The frame is in a stream that is strictly ordered RETI RADIOMOBILI

147 Other MAC Header Fields
Duration / ID Duration: used for NAV calculation ID: Station ID for polling in PSM Sequence Control Frame numbering and fragment numbering RETI RADIOMOBILI

148 Other MAC Header Fields
Standard 48-bit long IEEE address Address 1 Recipient address if ToDS=0, then end station’s address if ToDS=1, BSSID (if FromDS=0) or bridge (if FromDS=1) Address 2 Transmitter address if FromDS=0, then source station’s address If FromDS=1, BSSID (if ToDS=0) or bridge (if ToDS=1) RETI RADIOMOBILI

149 Other MAC Header Fields
Address 3 If FromDS=ToDS=0, BSSID If FromDS=0, ToDS=1, final destination address If FromDS=1, ToDS=1, final destination address Address 4 Original source address Set only when a frame is transmitted from one AP to another, i.e., if FromDS=ToDS=1 RETI RADIOMOBILI

150 Example: RTS Frame Frame Control Duration RA TA CRC MAC Header Duration (in s): Time required to transmit next (data) frame + CTS + ACK + 3 SIFS RA: Address of the intended immediate recipient TA: Address of the station transmitting this frame RETI RADIOMOBILI

151 Example: CTS Frame Frame Control Duration RA CRC MAC Header Duration (in s): Duration value of previous RTS frame  1 CTS time  1 SIFS RA: The TA field in the RTS frame RETI RADIOMOBILI

152 Example: ACK Frame Frame Control Duration RA CRC MAC Header Duration: set to 0 if More Fragments bit was 0, otherwise equal to the duration in previous frame  1 ACK  1 SIFS RA: copied from the Address 2 field of previous frame RETI RADIOMOBILI

153 Some Numerical Values…
PHY preamble: 18 bytes (long) or 9 bytes (short), 1 Mbps PHYHDR: 6 bytes, 1 Mbps MACHDR: 34 bytes, same rate as the one used to send the frame ACK=Preamble + PHYHDR+14 bytes RETI RADIOMOBILI

154 Bluetooth vs. IEEE 802.11b Property BT IEEE 802.11b Application
Cable replacement technology Wireless version of Ethernet LAN Type of devices Ideal for Cellular Phones Limited applicability for handheld devices Bandwidth ISM Range <10 meters >100 meters Data rate 1 Mbps 11 Mbps Power Consumption Limited Expensive Physical layer FHSS DSSS Type of use Very simple Complex RETI RADIOMOBILI

155 IEEE Evolution RETI RADIOMOBILI

156 IEEE 802.11 (Radio) Evolution
Standard 802.11 802.11b (Wi-Fi) 802.11a 802.11g Approval July 1997 Sep. 1999 June ‘03 Bandwidth 83.5 MHz 300 MHz Operation frequency GHz GHz GHz No. of non-overlapping channels 3 Indoor / Outdoor 4 Indoor 4 Indoor/Outdoor Data rate / channel 1,2 Mbps 1,2,5.5,11 Mbps 6,9,12,18,24,36, 48,54 Mbps 1,2,5.5,6,9, 11,12,18,24,36,48,54Mbps PHY layer FHSS, DSSS DSSS OFDM DSSS / OFDM RETI RADIOMOBILI

157 IEEE a RETI RADIOMOBILI

158 Physical Layer Standard approved years ago, but difficulties due to higher frequency (5GHz) and costs UNII 5 GHz bands In U.S.: UNII-1: 4 channels for indoor use UNII-2: 4 channels for indoor/outdoor use UNII-3: 4 channels for outdoor bridging In Europe difficulties due to Hiperlan II, but now it is approved RETI RADIOMOBILI

159 Physical Layer OFDM (Orthogonal Frequency Division Modulation) as transmission technology Very good performance against multipath Modulation: BPSK, QPSK, 16-QAM, 64-QAM Data rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps Reduced range slot=9μs, SIFS=16μs, PIFS=25μs, DIFS=34μs, CWmin=15, CWmax=1023 RETI RADIOMOBILI

160 OFDM Orthogonal Frequency Division Multiplexing (OFDM) distributes data over multiple, adjacent, frequency channels Channels are narrow-band with carriers very close to each other Each channel is orthogonal w.r.t. the others (spectra have zeros in correspondence of the other carriers) -> no co-channel interference RETI RADIOMOBILI

161 OFDM In practice, each user transmits over multiple narrow-band channels in parallel, hence at low bit rate Low bit rate transmissions imply increased robustness against delay spread on the multipath channel Continuous transmissions at low bit rate require low power consumption RETI RADIOMOBILI

162 OFDM in a RETI RADIOMOBILI

163 802.11a Transmission speed up to 54 Mbps
Products on the market are capable of 108 Mbps (Atheros turbo mode) Will IEEE adopt this? Does the higher frequency have an essential impact on the communication range? Corresponding to the ETSI Hiperlan II RETI RADIOMOBILI

164 802.11a vs. 802.11b 8 independent channels with 802.11a (3 in 802.11b)
Max data speed is 5 to 10 times higher Power consumption is similar, although with a it takes 4 to 9 times less energy to transmit a given length packet (due to the higher speed) RETI RADIOMOBILI

165 802.11a vs b No other existing equipment interfere (yet) including microwaves, b or Bluetooth Atheros claims that during real throughput measurements b never superseded a (in a typical office environment despite the higher frequency band usage – see diagrams in the next slides) RETI RADIOMOBILI

166 802.11a vs b RETI RADIOMOBILI

167 802.11a vs b RETI RADIOMOBILI

168 802.11a vs b RETI RADIOMOBILI

169 802.11a vs b RETI RADIOMOBILI

170 802.11a vs b RETI RADIOMOBILI

171 IEEE g RETI RADIOMOBILI

172 IEEE 802.11g Standard 802.11g approved in June 2003
Operates in the ISM 2.4 GHz bands Backward compatible with b Uses OFDM as transmission technology Modulation: BPSK, QPSK, 16-QAM, 64-QAM Data rates: 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36,48, 54 Mbps Power consumption similar to b RETI RADIOMOBILI

173 “All g” Operational Mode
Slot time=20 μs / Short slot time=9 μs SIFS=10 μs, CWmin=15, CWmax=1023 Basic rates determined by the AP (may be greater than 1Mbps), for management and control frames, as well as multicast and broadcast data frames Actual throughput: ≈20 Mbps RETI RADIOMOBILI

174 Backward Compatible Slot time=20μs SIFS=10 μs, CWmin=31, CWmax=1023
NAV distribution Protection mechanisms CTS-to-itself @basic rate, to notify duration to all RTS / CTS same scope, better for hidden terminals DSSS-OFDM: frame with DSSS preamble and header, and OFDM payload (no need for protection) Actual throughput: ≈10 Mbps RETI RADIOMOBILI

175 Available Products 802.11 a/b/g combo-card Ad hoc mode support
Typically power control Improved security functions RETI RADIOMOBILI

176 HIPERLAN RETI RADIOMOBILI

177 General Characteristics
Standard ETSI (European Telecommunications Standards Institute) HIPERLAN/1 (H/1) and HIPERLAN/2 (H/2 (1999) Frequency bands: GHz & GHz H/1 bit rates up to: 23.5 Mbps for data traffic (asynchronous access) 2 Mbps for real-time traffic GHz provides bit rates up to 54 Mb/s (as IEEE a) RETI RADIOMOBILI

178 General Characteristics
Stationary or slowly moving nodes (speed up to 36 Kmph) Nodes transmission range up to: 50 high bit rate 800 low bit rate Modulation scheme: GMSK for H/1 OFDM for H/2 Configuration mode: ad hoc or with AP Our focus on configuration with AP RETI RADIOMOBILI

179 H/2 Protocol Stack Link Layer Higher Layers Convergence Layer RLC DLC
PHY MAC Convergence Layer RLC (control plane) DLC (user plane) Link Layer Higher Layers RETI RADIOMOBILI

180 H/2 MAC More than one frequency channel available
Over each channel, TDD/TDMA access scheme Time is slotted - Frame duration=2 ms Dynamic capacity assignment in uplink and downlink BCH FCH ACH DL phase UL phase RCHs MAC-Frame Broadcast CH - Frame CH- Access feedback CH - Random CH RETI RADIOMOBILI

181 H/2 MAC: Transport Channels
Broadcast CHannel (BCH): In DL to convey information concerning the whole radio cell, e.g., AP ID, network ID, etc. Frame CHannel (FCH): In DL to convey information on the MAC frame structure (e.g., resource grant announcement) Access feedback CHannel (ACH): In DL to transport ack or nack to transmission requests sent by the terminals in previous frame Random CHannel (RCH): In UL to send signaling data (e.g., resource request, association request) RETI RADIOMOBILI

182 H/2 MAC A resource request to the AP contains the number of PDUs that are waiting to be transmitted Requests sent using ALOHA scheme, in the correspondance of the time slots allocated by AP Number of contention slots determined by AP depending on required max/mean delay access In case there is not a collision, a node is notified by AP through ACH in the next frame In case of collision, the node computes a backoff time as a random number of time slots RETI RADIOMOBILI

183 H/2 MAC If resource request is successfully, the node passes to contention-free mode In contention-free mode, AP schedules uplink/downlink transmissions Periodically, AP can ask nodes about their buffer occupancy level RETI RADIOMOBILI

184 H/2: RLC Authentication and other security functions
RRC, handover management, power saving and power control Establishment and release of user connections RETI RADIOMOBILI

185 H/2 DLC - Error Control Acknowledged mode: ARQ scheme (SR-like)
Repetition mode: repetion of the transfered data without using any feedback channel Transmission of some PDUs is repeated (retransmitted PDUs arbitrary chosen by the sender) Receiver accepts all PDUs having a sequence number within the receiver window Unacknowledged mode: use PDU sequence numbers. PDUs in error are discarded while correct PDU are passed to higher layers RETI RADIOMOBILI

186 H/2 Convergence Layer Mapping between higher layer connections / priorities and DLC connections / priorities Flexible amount of QoS classes Segmentation and reassembly to / from 48-byte packets Multicast & broadcast handling RETI RADIOMOBILI

187 Hiperlan vs. 802.11 Similarities:
Support ad hoc and with AP configuration Use OFDM Contention-based channel access Bit rate comparable to wired LAN LLC same as in wired LAN Differences: TDD/TDMA in Hiperlan, CSMA/CA in In Hiperlan more attention to real-time traffic RETI RADIOMOBILI


Download ppt "Wireless Local Area Networks (WLANs)"

Similar presentations


Ads by Google