Download as PDF, PPTX
![Exponential Backoff and Contention Window
• STA wants to transmit a frame
• When the medium is idle, STA will wait = DIFS (or EIFS if error in last frame
received) + Contention Window
• Backoff Time = Random() × aSlotTime (= 20 µs for DSSS)
• Random() = Pseudo-random int drawn from a uniform distribution over [0, CW]
• CW = 2p
- 1
• aCWmin = 31 (for DSSS, p = 5) ≤ CW ≤ aCWmax = 1023 (p = 10)
• Collisions only occur if 2+ STAs select the same random value
• If collision occur, p is increased by 1 after every consecutive collision
• Reduces the probability of the nodes selecting the same random value again
• Once a frame is successfully transmitted or the retry limit is reached, CW is reset to CWmin
51](https://image.slidesharecdn.com/wifi-v3-200129224729/75/WiFi-IEEE-802-11-51-2048.jpg)
![Collision Avoidance (CA)
• STA detects that the medium has been idle for DIFS
• It sets its BackOff (BO) counter to the randomly selected value drawn
from [0, CW]
• It decrements the BO for each aSlotTime that the medium is idle
• If the BO counter reaches 0, it transmits its data frame
• If another STA begins transmitting before the BO reaches 0, it pauses
decrementing the value until it detects that the medium is idle again
52](https://image.slidesharecdn.com/wifi-v3-200129224729/75/WiFi-IEEE-802-11-52-2048.jpg)
![1. UDP with RTS/CTS
Starts sensing the
medium (idle)
DataAP
STA-A
STA-B
MAC
ACK
Defers access
timeRTS
CTS
RTS
DIFS
SIFS
BO: RandInt[0,CWmin]
102](https://image.slidesharecdn.com/wifi-v3-200129224729/75/WiFi-IEEE-802-11-102-2048.jpg)
![2. UDP without RTS/CTS
Starts sensing the
medium (idle)
DataAP
STA-A
STA-B
MAC
ACK
Defers access
time
Data
What happens if the
MAC-ACK is not received?
Could be the Data packet
was lost or the MAC-ACK
was lost.
Retransmission: Receiver of the frame should
transmit an ACK immediately. If no ACK is
received then sender retransmits the frame
after a random backoff.
But how useful is retx for real-time VoIP or
video?
DIFS
SIFS
BO: RandInt[0,CWmin]
103](https://image.slidesharecdn.com/wifi-v3-200129224729/75/WiFi-IEEE-802-11-103-2048.jpg)
![3. TCP … it gets worse...
Starts sensing the
medium (idle)
TCP Data
AP
STA-A
STA-B
MAC
ACK
Defers access
time
TCP ACK
MAC
ACK
Data
What happens if the TCP-Data, TCP-MAC-ACK, TCP-Ack, TCP-MAC-ACK_ACK gets
lost? Not even lost – but maybe just delayed…. Not only are there
retransmissions at the MAC layer, but then the TCP congestion control kicks in
invoking the TCP-retransmission and the Congestion control.
Very lucky here.. But what if STA-B won the right to
transmit instead.
DIFS
SIFS
BO: RandInt[0,CWmin]
104](https://image.slidesharecdn.com/wifi-v3-200129224729/75/WiFi-IEEE-802-11-104-2048.jpg)
![Arbitration Inter-Frame Space (AIFS)
• AIFS are used by QSTAs to transmit all data and management frames and
the following control frames: PS-Poll, RTS, BlockAckReq
• A QSTA using EDCA shall obtain a TXOP for an AC if the CS mechanism
determines that the medium is idle at the AIFS[AC] slot boundary, after a
correctly received frame, and the backoff time for that AC has expired
• AIFS Number (AIFSN)
• nb of slots after a SIFS a non-AP QSTA should defer before either invoking a backoff
or starting a transmission (minimum value = 2 for non-AP, 1 for QAP)
• AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime
• A QSTA using EDCA shall not transmit within an EIFS-DIFS+AIFS[AC] plus any
backoff time after that QSTA determines that the medium is idle following
reception of an incorrect frame
115](https://image.slidesharecdn.com/wifi-v3-200129224729/75/WiFi-IEEE-802-11-115-2048.jpg)
More Related Content
WiFi - IEEE 802.11
- 1. WiFi – IEEE802.11 Damien Magoni – University of Bordeaux & Liam Murphy et al. – University College Dublin 2015/09/01 Version 3
- 2. Attribution • The materialcontained inside is intended for teaching. • This document is licensed under the CC BY-NC-SA license. • All figures and text borrowed from external sources retain the rights of their respective owners. 2
- 3. Acknowledgments • These slidesare originating from Prof. Liam Murphy’s lectures on WiFi at the school of CS at UCD. • They were subsequently extended by J. Fitzpatrick, S. Djahel, and N. Cranley at UCD. • I have completed the existing parts and have added part 2 on the physical layer and part 6 on 802.11e. • Part 2 material comes from the presentation of Imad Aad at IN’Tech 2002 on 802.11 and from the lecture notes of Herbert Haas on modulation techniques. • Part 6 material comes from the IEEE standard. 3
- 4. Table of Contents 1.Overview 2. PHY layer 3. MAC layer 4. Improvements 5. Applications 6. QoS and 802.11e 4
- 5.
- 6. Advantages • It’s wireless…mobility • Plug and play • Requires less infrastructure • Quick, easy to install and maintain, simplified wiring • Cost-efficient • Flexible topologies – Infrastructure, Mesh, Ad-hoc networking • Easily expandable, scalable • Handover capabilities and “roaming” • Backup for cellular network 6
- 7. Disadvantages • Bandwidth andspeed – Slower than wired networks • Security – Much harder to secure than wired • Reliability – Radio suffers from interference • Range – Typical ranges under normal indoor conditions are 20m-50m • Struggles with large number of users .. CSMA/CA • Non-deterministic • Lots of WLAN standards and constantly evolving 7
- 8. WiFi • Most successfullocal wireless networking technology • Using Unlicensed ISM Band • Wi-Fi Alliance • Industry organisation • Promotes interoperability • Compatibility • Standard compliance • Certification 8
- 9. IEEE 802.11 • IEEE:Not-for-profit Professional association • 802 is the LAN/MAN standards committee • WLAN technology is standardised by the IEEE • Part .11 is for WLAN • Also have ISO/IEC equivalents • Following standards ensures compatibility and interoperability 9
- 10.
- 11.
- 12. 802.11 in theOSI Protocol Stack Application Presentation Session Transport Network Data Link Physical • Open System Interconnect (“OSI model”) • 7 Layers of abstract functional blocks for data communication • Each provides service to block above/below • Allows changes to each without modifying entire system HTTP, FTP, SSH TCP, UDP, SCTP IPv4, IPv6 Ethernet, 802.11, ISDN, ATM Wired, Wireless, Optical802.11 12
- 13. WLAN Stack 13 Application Presentation Session Transport Network Data Link Physical MAC– CSMA/CA Physical LLC - IEEE 802.2 802.11 2Mbps S-Band FHSS 802.11G 54Mbps S-Band OFDM 802.11A 54Mbps C-Band OFDM 802.11B 11Mbps S-Band DSSS
- 14. WLAN Network Types IndependentBSS / Ad-hoc Infrastructure BSS • BSS (Basic Service Set) – A group of stations that can communicate • Defined by propagation characteristics of the wireless medium • SSID (Service Set Identifier) – Advertised Network Name • STA (Station) – User terminal connected to other STA or AP • AP (Access Point) – Station with access to external network 14
- 15. Extended Service Sets(ESS) • BSS only covers limited area • ESS allows creation of larger network • Multiple APs connected to the same DS (Distribution System) • Backbone network used to forward packets between APs • APs have the same SSID • STA’s can roam from BSS1 to BSS2 15
- 16. Access Point (AP) •Basic Parameters • Service Set Identifier (SSID) to identify your network • Protocol 802.11a/b/g/n … • Channel assignment • Power levels • The AP sends out beacons announcing the SSID, data rate, … • Security • Protocols: WEP (don’t use it), WPA, WPA2 (IEEE 802.11i), WPS (don’t use it) • Encryption protocols: TKIP (RC4 based), CCMP (AES based) • Authentication protocols (EAP): EAP-TLS, EAP-TTLS, PEAP-TLS, EAP-FAST… 16
- 17. Client (STA) Association •Client scans all channels • Client listens to beacons • Client associates with the AP • Obtains the MAC and security settings • AP/Radius server authenticates the client • Client keeps scanning – just in case the network is poor 17
- 18. Beacons and Scanning •Beacon Frames are used to announce the existence of the network • Periodically transmitted by APs (e.g. every 100ms) • Lowest data rate, maximum power • Provides synchronisation, BSS and address information • The user station’s behaviour • Passive Scanning: Scans each available channel looking for beacon frames • Active Scanning: Does not scan; rather, transmits a probe request – AP will respond with a probe response containing beacon information • Active scanning is faster but more intrusive. Active scanning is now more common (allows faster re-connection) • Association/Authentication: having found an AP to attach to, the user station must register with the AP 18
- 19. WLAN Reliability • Wirelessis challenging • Interference, obstacles etc… • Wireless networks have unclear boundaries • Received signal = transmitted signal + interference + noise • Signal strength decays with distance • STA hears signal only if • Received Power > Carrier Sense Threshold (CST) • WLAN has built-in reliability 19
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- 32. Complementary Code Keying(CCK) • Modulation scheme adopted to supplement the Barker code • Due to the shorter chipping sequence in CCK (8 bits versus 11 bits in Barker code) • Less spreading higher data rate more susceptible to narrowband interference shorter radio transmission range • CCK also has more chipping sequences to encode more bits • 8 chipping sequences at 5.5 Mbit/s and 64 chipping sequences at 11 Mbit/s • Barker code only has a single chipping sequence • CCK uses approximately the same bandwidth and utilise the same preamble and header as pre-existing 1 and 2 Mbit/s 802.11 • 802.11g networks employ CCK when operating at 802.11b speeds 32
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- 35. OFDM Transmitter IdealModel 35
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- 39.
- 40. MAC Functions • Framedelimiting and recognition • Addressing of destination stations • both as individual stations and as groups of stations • Access control to the physical transmission medium • DCF, PCF • Protection against errors • Frame Check Sequences • ACKs • RTS/CTS 40
- 41.
- 42. MAC Access Modesdetailed • DCF (Distributed Coordination Function) • Contention based medium access • Uses CSMA/CA: listen-before-transmit • Random backoff • Most prevalent MAC access mechanism • PCF (Point Coordination Function) • Contention Free (CF) • Point coordinator (PC) (in the AP) controls medium access • Not widely implemented • Both can operate simultaneously 42
- 43. Distributed Coordination Function •Enables automatic medium sharing • DCF = CSMA/CA + ACKs for unicast traffic • CSMA/CA • CS = Carrier Sensing, i.e. listening to signal = wait • MA = Multiple Access, i.e. fair shared medium, best effort • CA = Collision Avoidance, i.e. more waiting if there is congestion, only transmit if the medium is idle • Even if there is only one STA in the BSS • STA is contending with the AP to transmit data • Try uploading and downloading high bandwidth video simultaneously 43
- 44. Carrier Sense (CS) •Physical and virtual CS functions are used to determine the state of the medium (busy/idle) • Physical CS mechanism provided by the PHY layer • Virtual CS mechanism provided by the MAC sublayer • Using a Network Allocation Vector (NAV) • Enables to predict future traffic • CS combines the NAV state, the STA transmitter status and the physical CS to determine the state of the medium (busy/idle) 44
- 45.
- 46. Inter-Frame Spaces (IFS) •Access and transmission on the medium is controlled by a number of precise time intervals or IFS, listed from shortest to longest: • SIFS: short interframe space • PIFS: PCF interframe space • DIFS: DCF interframe space • EIFS: extended interframe space • SIFS < PIFS < DIFS < EIFS • These are MANDATORY gaps preceding Control and Data frames 46
- 47. Short IFS (SIFS) •Minimum Inter Frame Space, determined per PHY • 28 µs for FH, 10 µs for DS, 16 µs for OFDM • Used to separate control transmissions belonging to a single dialog • Data ACK and CTS frames • the second or subsequent MPDU of a fragment burst • by a STA responding to any polling by the PCF • by a PC for any types of frames during the CFP 47 Sender Node A Receiver Node B RTS CTS S I F S Sender Node A Receiver Node B DATA ACK S I F S
- 48. Point Coordination FunctionIFS (PIFS) • Only used in PCF mode • Used by STAs to gain priority access to the medium • A STA using the PCF shall be allowed to transmit CF traffic after its CS mechanism determines that the medium is idle at the TxPIFS slot boundary • PIFS = aSIFSTime +aSlotTime (determined by PHY) • At the beginning of the CFP, all STAs except the AP are required to wait for DIFS before they can transmit, the AP can access the medium before the other STAs as PIFS < DIFS 48
- 49. Distributed Coordination FunctionIFS (DIFS) • Used in DCF mode • A DIFS must follow after a busy period • Then a STA can start a new transmission if • Backoff time has expired • Medium is idle • DIFS = aSIFSTime + 2 x aSlotTime 49 Sender Node A Receiver Node B DATA ACK S I F S D I F S T S DATA ACKS I F S T S T S D I F S T S T S DATA BO=3 BO=2
- 50. Extended IFS (EIFS) •Used by the DCF when a frame transmission was begun that did not result in the correct reception of a complete MAC frame with a correct FCS value • EIFS shall begin following indication by the PHY that the medium is idle after detection of the erroneous frame, without regard to the virtual CS mechanism • EIFS is defined to provide enough time for another STA to acknowledge what was, to this STA, an incorrectly received frame before this STA commences transmission • EIFS = aSIFSTime + DIFS + ACKTxTime • Reception of an error-free frame during the EIFS resynchronizes the STA to the actual busy/idle state of the medium, so the EIFS is terminated and normal medium access (using DIFS and, if necessary, backoff) continues following reception of that frame 50
- 51. Exponential Backoff andContention Window • STA wants to transmit a frame • When the medium is idle, STA will wait = DIFS (or EIFS if error in last frame received) + Contention Window • Backoff Time = Random() × aSlotTime (= 20 µs for DSSS) • Random() = Pseudo-random int drawn from a uniform distribution over [0, CW] • CW = 2p - 1 • aCWmin = 31 (for DSSS, p = 5) ≤ CW ≤ aCWmax = 1023 (p = 10) • Collisions only occur if 2+ STAs select the same random value • If collision occur, p is increased by 1 after every consecutive collision • Reduces the probability of the nodes selecting the same random value again • Once a frame is successfully transmitted or the retry limit is reached, CW is reset to CWmin 51
- 52. Collision Avoidance (CA) •STA detects that the medium has been idle for DIFS • It sets its BackOff (BO) counter to the randomly selected value drawn from [0, CW] • It decrements the BO for each aSlotTime that the medium is idle • If the BO counter reaches 0, it transmits its data frame • If another STA begins transmitting before the BO reaches 0, it pauses decrementing the value until it detects that the medium is idle again 52
- 53. 53 D I F S T S DATA ACK S I F S T S T S T S T S BO=3 DATA ACK S I F S D I F S D I F S T S DATA A B C Medium Idle Backoff Node B wins PAUSEBO PAUSE BO PAUSE BO BO=2 BO=3 Medium Idle Resume Backoff Node A wins BO=1BO=6 BO=5 Resume Backoff Node C wins A and C set their NAV
- 54. Reliability and Retransmissions •For each data frame transmitted, an ACK is transmitted by the receiver • If the ACK is not received, the sender will try to retransmit the frame again • Retry Counters: the number of times the sender will try to retransmit the frame • Short Retry Counter: number of times to retransmit a short frame • Long Retry Counter • When the Retry Counter reaches 0, the frame is discarded • Reliability mechanism is designed to overcome 2 issues which can occur: • Hidden Nodes • Exposed Nodes 54
- 55. Hidden Nodes • Bcan “hear” A and C but A cannot “hear” C.. They are hidden from each other • A is transmitting to B • But C can’t “hear” this transmission • C transmits to B • … COLLISION … both transmissions may be lost A B C Nodes A and C Are hidden from each other 55
- 56. Solution to HiddenNodes • 802.11 MAC uses 2 control frames • Request to Send (RTS) • Clear to Send (CTS) • These are very short frames • Aim to reduce the collision period • Better to collide on a shorter frame than a longer data frame 56
- 57. RTS/CTS Operation • NodeA wants to transmit to Node B • Case 1: • A sends an RTS and waits for a CTS from B • B sends a CTS to A, BUT C also “hears” the CTS • Node A receives the CTS .. Its ok to transmit • Node A transmits the DATA • Node B transmits the ACK • Node C “hears” the ACK … and knows that it can try again to transmsit • Case 2: • A and C BOTH send an RTS to B • Collision occurs • But since RTS are very short .. Its only a short collision 57
- 58. 58 A B C RTS CTS DATA NodeC can “hear” the CTS transmitted by Node B RTS RTS ACK Node C can “hear” the ACK transmitted by Node B Case 1 Case 2 Short collision
- 59. Impact of RTS/CTS •Node A sends an RTS • All nodes in range of A must WAIT until transmission is completed • Node B sends the CTS • All nodes in range of B must WAIT until they “hear” the ACK from Node B 59 A B C Inefficient!! Not needed if ALL STA are in range WAITING!! WAITING!!
- 60. Exposed Nodes ReducedThroughput 60 • Node A is transmitting to Node D • Node B wants to transmit to Node C • But Node B “thinks” that if it transmits, it will interfere with Node A • … So Node B WAITS …. Inefficient and reduced throughput A B C D Exposed Node
- 61. dot11RTSThreshold Parameter • TheMAC layer parameter dot11RTSThreshold indicates the minimum required length of a frame for the frame to be preceded by RTS and CTS frames • Short frames are not preceded by RTS & CTS frames • If Length > dot11RTSThreshold use RTS/CTS • Can be set on a per-STA basis • The default value of dot11RTSThreshold is 2347 Bytes (i.e., no RTS/CTS) 61
- 62. Network Allocation Vector(NAV) • How does a STA know how long to wait while a transmission is taking place? • 802.11 frames carry a Duration field • Specifies the transmission time required for the frame, during which the medium will be busy • Duration information announced in RTS/CTS frames • Duration information available in the MAC headers of all frames sent during the CP • Each STA sets and updates its NAV using these Duration fields • NAV prevents other stations from accessing the medium until the transmission is complete • NAV counting down to zero at a uniform rate (timer) • NAV > 0 -> medium is busy, NAV = 0 -> medium is idle • Medium considered busy when the STA is transmitting 62
- 63. NAV Timeline 63 Sender Node A Receiver NodeB NAV Node C RTS CTS Frame ACK NAV RTS NAV CTS SIFS SIFS SIFS DIFS t t t RTS Duration Field = CTS + Data/Mgmt + ACK + 3xSIFS CTS Duration Field = Data + ACK + 2xSIFS
- 64.
- 65. NAV for PowerSave • Wireless stations are often battery powered, so in order to conserve power the stations may enter a power-saving mode. • Constantly sensing the medium consumes power • When the NAV = 0, an STA will wakeup to sense the medium again. 65
- 66. Point Coordination Function •Optional, not widely implemented • Wireless channel is divided into superframes • Superframe = Contention Free Period (CFP) + Contention Period (CP) • At the beginning of CFP, the Point Coordinator (PC) (in the AP) contends for access of the wireless channel • If an AP acquires the channel, a polling/granting policy is applied for transmission • Used in 802.15.4 Zigbee for WSN 66
- 67. B B CFP repetitioninterval Superframe CFP Busy medium B CP (DCF)CP (DCF) Superframe • Superframe = CFP + CP • Designed for delay-sensitive/real-time applications • B = Beacon frame (sent by AP to indicate start of CFP) • CPF = Contention-Free Period (reserved for real-time traffic) • CP = Contention Period (normal DCF operation) CFP Reduced CFP due to the busy medium (it is not possible to cut off active DCF transmissions) 67
- 68. Protecting CFP fromCP CFP operation is guaranteed in two ways 1. The NAV value in the beacon signal = length of CFP 2. Usage of PIFS (instead of DIFS) within the CFP • PIFS < DIFS B B CFP Busy medium B CP (DCF)CP (DCF) CFP NAV NAV 68
- 69. Polling during thePCF of the CFP B SIFS Set by beacon frame S1 DATA SIFS SIFS PC (AP) STAs NAV SIFS PIFS SIFS S3 CPCFP Poll S1 Poll S2 Poll S3 + data CFP end Begin of CFP AP sends a Beacon containing the NAV so the duration of the CFP is known by all AP polls STA1, STA1 sends its data AP polls STA2, STA2 does NOT have any data to send, wait for PIFS AP polls STA3 and also sends some data to STA3, STA3 has some data to send End of CFP 69
- 70. Shortcomings of thePCF • PCF designed for RT applications but does not offer extensive QoS • No differentiation possible between traffic types/classes • CPF length cannot be dynamically changed according to traffic needs • No mechanism for STAs to communicate QoS requirements to the AP • Different maximum packet lengths cannot be enforced • Increased delays – waiting to be polled in a round robin • Wasteful – polling STAs that have nothing to send • AP needs to contend for the channel at the beginning using DCF • Effective period of CFP may vary 70
- 71. General Frame Format •Generic MAC frame • The 802.11 MAC uses a different frame structure than Ethernet • 4 Address fields • Not used all the time • Overhead of 34 bytes • Max payload 2312 bytes 71 Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS 2 2 6 6 6 2 6 0-2312 4Bytes MAC Header
- 72. Frame Control Field 72 22 6 6 6 2 6 0-2312 4Bytes Protocol Type= data Sub type To DS 2 2 4 1 From DS 1 More Frag 1 Retry 1 Pwr Mgmt 1 More Data 1 Prot Frame 1 Order bit 1Bits • Protocol Version: version of 802.11 MAC (currently 0) • Type: Frame Type (e.g. Management, Control or Data) • Sub Type: Specific Frame Subtype – e.g. 1000 = beacon, 1011 = RTS • To DS / From DS: Specify if frame is destined for the DS or is coming from the DS Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS
- 73. Frame Control Fieldcont’ • More Frag: Specifies if frame has been fragmented • Retry: Is a retransmission • Power Management: Indicates that sending station will power down after transmission • More Data: Informs a sleeping station that data is available • Protected Frame: Security is being used on Frame • Order Bit: Strict ordering must be used Protocol Type= data Sub type To DS 2 2 4 1 From DS 1 More Frag 1 Retry 1 Pwr Mgmt 1 More Data 1 Prot Frame 1 Order Bit 1Bits Bytes Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS 2 2 6 6 6 2 6 0-2312 4 73
- 74. Duration/ID Field • Duration:Three Purposes • Setting the NAV: Defines the amount of time in msec the medium will be busy for • CFP Frames: Essentially sets the NAV to the maximum value during contention free periods • PS-Poll Frames: Used by stations to inform the AP that they have woken from power saving state Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS 2 2 6 6 6 2 6 0-2312 4Bytes 74
- 75. Address Fields • Alladdresses are 48 bit IEEE MACs (01:23:45:67:89:ab) • Destination: Identifies the final recipient • Source: Identifies the original source • Receiver: Identifies the receiver which should process the frame (Intermediate node) • Transmitter: Identifies the transmitter of the frame (Intermediate node) Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS 2 2 6 6 6 2 6 0-2312 4Bytes 75
- 76. Frame Body • Alsocalled the “Data Field” • Contains the higher layer payload • 802.11 supports up to 2312 bytes of data Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS 2 2 6 6 6 2 6 0-2312 4Bytes 76
- 77. Seq-Ctl & FCS •Sequence Control – Used for detecting and discarding of duplicate frames • Frame Check Sequence – Often called the CRC, used to check frame integrity • G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 Frame Control Duration /ID Address 1 Address 2 Address 3 Seq-ctl Address 4 Frame Body FCS 2 2 6 6 6 2 6 0-2312 4Bytes 77
- 78. Control Frames Format:RTS • RA: identifies the immediate receiver • TA: identifies the transmitter of the RTS • Duration = CTS + Data/Mgmt + ACK + 3xSIFS • measured in micro-seconds • i.e., CTS->SIFS->DATA->SIFS->ACK->SIFS 78 Frame Control Duration RA TA FCS 2 2 6 6Bytes 4 MAC Header
- 79. Control Frames Format:CTS 79 • RA: identifies the immediate receiver, i.e. the sender of the previous RTS • Duration = Data + ACK + 2xSIFS Frame Control Duration RA FCS 2 2 6Bytes 4 MAC Header
- 80. Control Frames Format:ACK 80 • RA: identifies the immediate receiver, i.e. the sender of the previous DATA frame • Duration = 0 if More Frag = 0 …otherwise: • Duration = DATA + ACK + 2xSIFS Frame Control Duration RA FCS 2 2 6Bytes 4 MAC Header
- 81.
- 82. The 802.11 AlphabetSoup • Only one standard… but a large number of 802.11 Amendments • Naming scheme is done per letter, as the working group is established • Not when it was published! • Some define both MAC/PHY, others just apply to the MAC • Some operate in conjunction with others, e.g. 802.11g/802.11e 82
- 83. Common 802.11 Amendments •802.11e – QoS enhancements to MAC layer • 802.11f – Inter access point protocol • 802.11s – Mesh networking • 802.11k – Radio resource management • 802.11n – MIMO • 802.11ac – MU-MIMO 83
- 84. Lastest IEEE 802.11standards/amendments • 802.11af-2013: Television White Spaces (TVWS) Operation • 802.11ac-2013: Enhancements for Very High Throughput for Operation in Bands below 6 GHz • 802.11ad-2012: Enhancements for Very High Throughput in the 60 GHz Band • 802.11ae-2012: Prioritization of Management Frames • 802.11aa-2012: MAC Enhancements for Robust Audio Video Streaming • 802.11-2012: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications (latest global version, 2793 pages!) • 802.11u-2010: Interworking with External Networks 84
- 85. Still Evolving… 85 2nd Generation 1999-2002 3rdGeneration 2003-2008 4th Generation 2009-2011 802.11 1, 2Mbps FHSS 802.11A 54Mbps, OFDM, 5GHz 802.11B 11Mbps, DSSS, 2.4GHz 802.11G 54Mbps, OFDM, 2.4GHz 802.11N 600Mbps, OFDM, 2.4/5GHz, 20/40MHz/ch MIMO/SISO 802.11AC 5GHz 802.11AD 60GHz 5th Generation 2012- 2013 2012 1st Generation 1997-1998 802.11E 11Mbps, QoS, 2.4GHz
- 86. Comparison Chart 86 Freq Band (GHz) Bandwidth (MHz/ch) ModulationMax Phy Rate (Mbps) Antennas 802.11 2.4 20 DSSS, FHSS 2 SISO 802.11b 2.4 20 DSSS 11 SISO 802.11a 5 20 OFDM 54 SISO 802.11g 2.4 20 DSSS, OFDM 54 SISO 802.11n 2.4/5 20/40 OFDM 600 MIMO 802.11ac 5 20/40/80/ 80+80,160 OFDM 6.9 Gbps MIMO/ MU-MIMO 802.11ad 60 2.16 GHz OFDM / SC-OFDM (Single carrier) 6.7 Gbps MIMO/ MU-MIMO
- 87. 802.11n • Features • MultipleInput (tx antennas) Multiple Output (rx antennas) (MIMO) exploits multipath, each rx antenna gets a slightly different version of the signal allowing for successful decoding • 40 MHz channel bandwidth • Frame aggregation using Block ACKs from .11e • Optional features • 2 to 4 spatial streams • Transmit Beamforming (TxBF) • 400 ns short guard interval (SGI) • 64-QAM, 40 MHz channel, 400 ns GI, 4 spatial streams -> 600 Mbps 87
- 88. Multipath 88 Multipath with 11a/b/g: Reflections can interfere and distort the signal. When using MIMO and its multiple receiving antennas, however, the effects of multipath become additive - multiple messages can be received by multiple antennas, and combined. http://www.radio-electronics.com/info/antennas/mimo/formats-siso-simo-miso-mimo.php
- 89. 802.11ac • Operates inthe 5GHz frequency band • Multiple User-MIMO (MU-MIMO) enables multiple users to simultaneously access the same channel by leveraging users’ interferences (spatial signature) • New mandatory features • 80 MHz channel bandwidth • New optional features • 5 to 8 spatial streams • 160 MHz channel bandwidth (contiguous 80 + 80) • 80 + 80 MHz channel bonding (discontiguous 80 + 80) • MCS 8/9 (256-QAM), 10/11 (1024-QAM, non standard) • 256-QAM, 160 MHz channel, 400 ns GI, 8 spatial streams -> 8 x 866.7 Mbps = ~6934 Mbps 89
- 90. 802.11ad - WiGig •4 bands around 60GHz • 2.16GHz channel bandwidth • Can not penetrate walls but can propagate by reflection • Beamforming for ranges up to 10 m • Backward compatibility for tri-band (2.4, 5 and 60GHz) devices • Low power single carrier data rate up to 2.5Gbps • 64-QAM, -47dBm sensitivity, -26dB Error Vector Magnitude (EVM) in constellation -> ~6757 Mbps 90
- 91. 802.11af - WhiteFi,Super WiFi • WLAN operation in TV white space spectrum in the VHF and UHF bands between 54 and 790 MHz • Frequency channels 6 to 8 MHz wide, depending on the regulatory domain • OFDM as in 802.11ac • Range up to 1 km, increased compared to 802.11 a/b/g/n/ac • Up to four channels may be bonded in either one or two contiguous blocks • MIMO operation possible with up to 4 streams used for either space–time block code (STBC) or multi-user (MU-MIMO) operation • 256-QAM, 8MHz channel, 2.25µs GI, 4 spatial streams, 4 bonded channels -> ~569 Mbps 91
- 92.
- 93. WiFi is slow •Application types and hungry apps • Lack of bandwidth • Poor configuration • Real throughput 93
- 94. Application Types • Voice •Video • Data • E-mail • Browsing/Surfing • Business applications • File transfer/sharing • Gaming 94
- 95. Voice • Not bursty,not bandwidth demanding • Small packets sent frequently • Eg 10ms voice per pkt = 100pkts per sec • Very strict latency and loss rate requirements • People are particularly sensitive to latency • VoIP quality is measured using the E-model • Ro = SNR • Is = combination of all impairments which occur more or less simultaneously with the voice signal • Id = impairments caused by delay • Ie-eff = effective equipment impairment factor represents impairments caused by low bit-rate codecs and impairments due to randomly distributed packet losses • A = compensation of impairment factors • WiFi can not guarantee or even bound latency or loss 95
- 96. Video • Bandwidth demanding •Somewhat loss resilient but error propagation issues • Encoded as a series of frames • Example: 20fps = 1 frame every 50 ms • Different frame types: I, P, B • Group of Pictures (GoP) = periodic sequence of P+B frames between I-frames • Example: 20fps might have GoP sequence (I-PbbPbbPbb)x2 • I-frames are large and generate a burst of packets • Encode video at a CBR to avoid bursts • No as CBR is the average smoothed bit rate, cannot avoid packet bursts generated by I-frames Frame size 96
- 97. Gaming • Fast interactivity-> Low latency, low loss rate • Asynchronous and bursty • Often adds VoIP on top for team cooperation • May be high bandwidth for Web gaming 97
- 98. Upgrade WiFi toWiGig • Throw bandwidth at the problem, that’ll make it all go away … • Upgrade to a higher rate PHY (theoretical limits) • 802.11n: 600Mbps • 802.11ac/ad: 6Gbps • Temporary quick fix • Contention issues remain • Hungrier apps, e.g. HDTV • Backward compatibility and device chipsets 98
- 99. Typical WiFi ConfigParameters Parameter If Increased… If Decreased… Beacon interval Better throughput and longer battery life Faster mobility and handoff RTS/CTS On/Off Off: Better throughput On: Hidden node access RTS Threshold Frames > Threshold use RTS/CTS Increase throughput if no hidden terminals Higher throughput with many hidden terminals Fragmentation Threshold Frames > Threshold are fragmented Increased throughput in error free channel Increased throughput in error prone channel + increased latency Long/short retry limits Max number of retransmission attempts Lower pkt loss rate, increased backoff and reduced throughput Higher pkt loss rate, smaller buffer required Auto-rate adaptation AP can be in auto mode, dynamically switch PHY rates to limit interference Power Increase antenna power to increase coverage and range. Typ. max power value of 20 dBm or 100 mW which covers an area of 100 meters But in reality these are not always available …. 99
- 100. WiFi Auto-Rate Adaptation •Clients can shift data rates on a transmission-by-transmission basis • AP can support multiple clients at multiple speeds depending upon the location of each client • Higher rates require stronger signals at the receiver • Lower rates have a greater range • STAs always try to communicate at highest rate • STAs reduce the rate only if transmission errors and transmission retries occur 100
- 101. Interaction with HigherLayers STA-A STA-B AP Wired Ethernet 1. UDP with RTS/CTS 2. UDP without RTS/CTS 3. TCP 101
- 102. 1. UDP withRTS/CTS Starts sensing the medium (idle) DataAP STA-A STA-B MAC ACK Defers access timeRTS CTS RTS DIFS SIFS BO: RandInt[0,CWmin] 102
- 103. 2. UDP withoutRTS/CTS Starts sensing the medium (idle) DataAP STA-A STA-B MAC ACK Defers access time Data What happens if the MAC-ACK is not received? Could be the Data packet was lost or the MAC-ACK was lost. Retransmission: Receiver of the frame should transmit an ACK immediately. If no ACK is received then sender retransmits the frame after a random backoff. But how useful is retx for real-time VoIP or video? DIFS SIFS BO: RandInt[0,CWmin] 103
- 104. 3. TCP …it gets worse... Starts sensing the medium (idle) TCP Data AP STA-A STA-B MAC ACK Defers access time TCP ACK MAC ACK Data What happens if the TCP-Data, TCP-MAC-ACK, TCP-Ack, TCP-MAC-ACK_ACK gets lost? Not even lost – but maybe just delayed…. Not only are there retransmissions at the MAC layer, but then the TCP congestion control kicks in invoking the TCP-retransmission and the Congestion control. Very lucky here.. But what if STA-B won the right to transmit instead. DIFS SIFS BO: RandInt[0,CWmin] 104
- 105. Realistic Throughput Different modulations Exponentialdecay of throughput with increased contention E.g. 802.11b: PHY 11Mbps, best case 7Mbps, 10 users 720kbps, 20 users 360kbps 105
- 106.
- 107.
- 108. IEEE 802.11e-2005 • Amendmentincorporated into the published 802.11-2007 standard • Hybrid Coordination Function (HCF) with 2 modes • EDCA • HCCA • Can cooperate with non-QoS devices • nQSTA are regular stations: only DCF (optional PCF) • QSTA are QoS enabled stations: DCF and HCF are present 108
- 109. Enhancements of 802.11e •Contention Free Bursts (CFB) allow stations to send several frames in a row without contention, if the allocated Transmission Opportunity (TXOP) permits • Block Ack (optional) • No Ack needed for applications where retransmission cannot be used due to strict delay requirements • Direct Link Setup (DLS) enables direct communication between STAs without involving the AP 109
- 110.
- 111. Hybrid Coordination Function(HCF) • Hybrid Coordinator (HC) in the QoS enabled AP (QAP) • HCF has 2 access modes • Enhanced Distributed Channel Access (EDCA) similar to DCF but with different priority levels for different services (such as DiffServ) • HCF Controlled Channel Access (HCCA) is a CSMA/CA-compatible polling- based access method similar to PCF (optional, not widely implemented) 111
- 112. Enhanced Distributed ChannelAccess (EDCA) • EDCA divides the traffic into 8 different User Priorities (UP, same as in 802.1D) which are mapped onto 4 Access Categories (AC) • All MSDUs having the same UP belong to a Traffic Category (TC) • Each AC’s access is controlled by using 4 differentiating parameters • Transmission Opportunity (TXOP) specifies the starting time and maximum duration during which a station can transmit 1+ frames • Minimum idle duration time is not the DIFS but the Arbitration Interframe Space (AIFS) which is a variable DIFS • Minimum contention window size (aCWmin) • Maximum contention window size (aCWmax) 112
- 113.
- 114. Transmission Opportunity (TXOP) •Under HCF, the basic unit of allocation of the right to transmit onto the medium is the TXOP • Each TXOP is defined by a starting time and a defined maximum length/duration • TXOP may be obtained in 2 ways • by a QSTA winning an instance of EDCA contention (EDCA TXOP, length defined in beacon frame) during the CP • by a QSTA receiving a QoS(+)CF-Poll frame (HCCA TXOP or polled TXOP, includes its length) from HC during the CFP or the CP 114
- 115. Arbitration Inter-Frame Space(AIFS) • AIFS are used by QSTAs to transmit all data and management frames and the following control frames: PS-Poll, RTS, BlockAckReq • A QSTA using EDCA shall obtain a TXOP for an AC if the CS mechanism determines that the medium is idle at the AIFS[AC] slot boundary, after a correctly received frame, and the backoff time for that AC has expired • AIFS Number (AIFSN) • nb of slots after a SIFS a non-AP QSTA should defer before either invoking a backoff or starting a transmission (minimum value = 2 for non-AP, 1 for QAP) • AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime • A QSTA using EDCA shall not transmit within an EIFS-DIFS+AIFS[AC] plus any backoff time after that QSTA determines that the medium is idle following reception of an incorrect frame 115
- 116.
- 117. AC Parameter Sets DSSSOFDM 117
- 118. Scheduling the ACqueues • Consider an AP has some VoIP, video, and background data to send to STA • Internal collision resolution: High priority AC wins the right to transmit, but low priority AC back off as if it experiences a collision • Highest priority traffic “statistically” wins 118
- 119.
- 120. Which AC Parameters? •VoIP/Gaming – short AIFS, low CWmin, for quick access • Packet every 10 or 20 ms • Video – large TXOP to allow for bursts generated by an I-frame • Packet bursts every 40 - 50 ms • TCP – low AIFS and CW to minimise TCP-ACK RTT and hopefully avoid TCP congestion control • Best-effort and Background – values larger than the above 120
- 121. HCF Controlled Access(HCCA) • Based on the HC using polling for controlling the traffic • HC can poll stations during CFP and also during CP • By using Controlled Access Phases (CAP) • During the CP, all stations function in EDCA • Traffic can be classified • Use of Traffic Streams (TS) • Traffic Specifications (TSPEC) in frames • Traffic Classification (TCLAS) elements in devices 121
- 122. Traffic Streams (TS) •QSTAs can send TSPECs describing the traffic characteristics and the QoS requirements (rate, delay, …) of a TS • Reserve resources within the HC and modify the HC’s scheduling behavior • Allow other parameters to be specified that are associated with the TS (e.g., traffic classifier and acknowledgment policy) • A TS may have one or more TCLAS element associated with it • The AP uses the parameters in the TCLAS elements to filter the MSDUs belonging to this TS for delivery as part of the TS • HC is thus not limited to per-station queuing and can provide per-session service • Stations provide lengths of their queues for each TS and the HC can use this info 122
- 123. Controlled Access Phase(CAP) • HCCA uses the HC’s higher priority to initiate frame exchanges and to allocate TXOPs to itself and other STAs in order to provide limited- duration Controlled Access Phase (CAP) for CF transfer of QoS data • CAP is a time period when the HC maintains control of the medium, after gaining access by sensing the channel to be idle for a PIFS • CAP might span multiple consecutive TXOPs and can contain polled TXOPs • Delivery Traffic Indication Message (DTIM): interval between the consecutive TxTimes of beacons containing a DTIM 123
- 124. CAPs in CFPsand CPs 124
- 125. TXOP Structure andTiming • Any QoS data frame with CF-Poll contains a TXOP limit in its QoS Control field • The ensuing polled TXOP is protected by the NAV set by the Duration field of the frame that contained the QoS (+)CF-Poll function • Within a polled TXOP, a STA may initiate the transmission of 1+ frame exchanges each separated by a SIFS • The STA shall not initiate transmission of a frame unless the transmission and any acknowledgment or other immediate response are able to complete prior to the end of the remaining TXOP duration • A TXOP or transmission within a TXOP shall not extend across • TargetBeaconTxTime, dot11CFPMaxDuration (if during CFP), dot11MaxDwellTime (if using an FH PHY), or dot11CAPLimit • The HC shall verify that the full duration of any granted TXOP meets these requirements so that STAs may use the time prior to the TXOP limit of a polled TXOP without checking for these constraints 125
- 126. Polled TXOP GrantedTiming 126
- 127. Block Ack • Improvesthe channel efficiency • Transmit a block of data frames • each separated by a SIFS period • Aggregating several acknowledgements into ONE frame • Two types of Block Acks • immediate for high-bandwidth, low latency traffic • delayed for applications that can tolerate moderate latency 127
- 128. Block Ack MessageSequence 128
- 129. Immediate Block AckSequence 129
- 130. Delayed Block AckSequence 130
- 131. No Ack Policy •Usage of No Ack is determined by the policy at the QSTA • When No Ack policy is used, there is no MAC-level recovery • Reduced reliability due to the increased probability of lost frames from interference, collisions, or timevarying channel parameters • A protective mechanism (e.g., HCCA, RTS/CTS) should be used to reduce the probability of other STAs transmitting during the TXOP 131
- 132. Direct Link Setup(DLS) • DLS is a protocol that enables a STA in a BSS to transmit frames directly to another STA in the same BSS • DLS enables to exchange rate set and data between the sender and the receiver • A recipient may be in PS mode, in which case it can be awakened only by the AP • DLS prohibits the STAs going into PS mode for the duration of the direct stream as long as there is an active DLS between the two STAs • Setup and teardown procedures go through the AP • DLS does not apply in an IBSS, where frames are always sent directly from one STA to another 132
- 133. Considerations • QoS canbe finely configured with HCCA • QSTA can request specific transmission parameters (data rate, jitter, …) • Requirements for implementing HCCA • Stations only need to be able to respond to poll messages • On the AP side, a scheduler and queuing mechanism is needed • .11e not widely supported especially HCCA • Difficulty to set optimal settings for the different traffic types and ACs • PCF is not widely implemented in WLAN but is in WSN • With the takeoff of WSNs due to IoT and M2M, PCF might make a comeback 133