802.11 Wireless LANs

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IEEE 802.11 Wireless LANs• 802.11b

– 2.4 GHz unlicensed band– Bit-rates 1-11 Mbps– direct sequence spread spectrum

(DSSS) in physical layer

• 802.11g– 2.4 GHz unlicensed band– Bit-rates 1-54 Mbps– Extended-Rate PHY (ERP) in

physical layer• 802.11n (MIMO)

– 2.4 and 5 GHz unlicensed bands– up to 600 Mbps– Uses advanced signal processing

and modulation techniques at physical layer to exploit multiple antennas and wider channels

• All use CSMA/CA for multiple access• All have multi-rate support for link adaptation• All allow both infrastructure-mode and ad-hoc mode

operation• All have same frame structure

• 802.11a– 5 GHz unlicensed band– Bit-rates 6-54 Mbps– Orthogonal Frequency Division

Multiplexing (OFDM) in physical layer

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802.11x StandardsIEEE standard Notes

802.11 First standard (1997). Specified the MAC and the original slower frequency-hopping and direct-sequence modulation techniques.

802.11a Second physical layer standard (1999), but products not released until late 2000.

802.11b Third physical layer standard (1999), but second wave of products.

TGc Task group that produced a correction to the example encoding in 802.11a. Since the only product was a correction, there is no 802.11c.

802.11d International roaming extensions (2001)

802.11e Quality-of-service (QoS) extensions, including packet bursting (2005)

802.11F Inter-access point protocol to improve roaming between directly attached access points (2003)

802.11g 54Mbps, 2.4GHz standard, backwards compatible with b (2003)

802.11h Spectrum managed 802.11a (5 GHz) for European compatibility (2004)

802.11i Improvements to security at the link layer (2004)

802.11j Enhancements to 802.11a to conform to Japanese radio emission regulations (2004)

802.11-2007 A new release of the standard that includes amendments a, b, d, e, g, h, i & j. (July 2007)

802.11k Radio resource measurement enhancements (2008)

802.11r Fast roaming (2008)

802.11y 3650-3700 MHz operation in the US (2008)

802.11n High throughput improvements using MIMO (multiple input, multiple output antennas) (Sep 2009)

802.11w Protected management frames (Sep 2009)

802.11x Standards in the Making

• IEEE 802.11p: WAVE—Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (July 2010)

• IEEE 802.11z: Extensions to Direct Link Setup (DLS) (September 2010)

• IEEE 802.11u: Interworking with non-802 networks (for example, cellular) (~ Dec 2010)

• IEEE 802.11v: Wireless network management (~ Dec 2010)

• IEEE 802.11s: Mesh Networking, Extended Service Set (ESS) (~ June 2011)

• IEEE 802.11mb: Maintenance of the standard. Will become 802.11-2011. (~ Dec 2011)

• IEEE 802.11ae: QoS Management (~ Dec 2011) • IEEE 802.11aa: Robust streaming of Audio Video Transport Streams (~ Mar 2012)

• IEEE 802.11af: TV Whitespace (~ Mar 2012) • IEEE 802.11ac: Very High Throughput <6 GHz; potential improvements over 802.11n:

better modulation scheme (expected ~10% throughput increase); wider channels (80 or even 160 MHz), multi user MIMO; (~ Dec 2012)

• IEEE 802.11ad: Very High Throughput 60 GHz (~ Dec 2012)

• IEEE 802.11ah: Sub 1Ghz (~ July 2013)

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Source: http://en.wikipedia.org/wiki/IEEE_802.11

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802.11 in the Big Picture

802.11

MAC: Medium Access ControlPLCP: Physical Layer Convergence ProcedurePMD: Physical Medium DependentFHSS: Frequency Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumHR/DSSS: High-Rate DSSSOFDM: Orthogonal Frequency Division MultiplexingERP: Extended-Rate PHY

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802.11 Architecture• Access Points (APs)

– Similar to base stations in cellular networks

– Perform wireless-to-wired bridging• Distribution System, or backbone

network (typically, Ethernet)• Basic Service Set (BSS)

– Group of communicating stations in a basic service area, similar to a cell in cellular networks

• Extended Service Set (ESS)– All APs in ESS share the same

network name, Service Set IDentifier (SSID)

ESS

Ad-hoc mode Infrastructure mode

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802.11 Network Services

Service Station or distribution service? Description

Distribution Distribution Service used in frame delivery to determine destination address in infrastructure networks

Integration Distribution Frame delivery to an IEEE 802 LAN outside the wireless network

Association Distribution Used to establish the AP which serves as the gateway to a particular mobile station

Reassociation Distribution Used to change the AP which serves as the gateway to a particular mobile station

Disassociation Distribution Removes the wireless station from the network

Authentication Station Establishes station identity (MAC address) prior to establishing association

Deauthentication Station Used to terminate authentication, and by extension, association

Confidentiality Station Provides protection against eavesdropping

MSDU delivery Station Delivers data to the recipient

Transmit Power Control (TPC)

Station/spectrum management

Reduces interference by minimizing station transmit power

Dynamic Frequency Selection (DFS)

Station/spectrum management

Avoids interfering with radar operation in the 5 GHz band

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802.11 Channels (UK)

200mW/23dBm EIRP limit

1W/30dBm EIRP limit

10mW/10dBm EIRP limit

Transmit Spectrum Mask

• To limit power leakage into adjacent channels

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Transmit spectrum mask for 802.11a

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802.11 Association

• AP channel determined automatically or assigned by AP admin

• Interference possible: channel can be same as that chosen by neighboring AP!

• Host: must associate with an AP– Scans channels, listening for beacon frames containing AP’s SSID

and MAC address– Selects an AP to associate with (left unspecified in the standard)– May perform authentication– Associate with the selected AP– Will typically run DHCP to get IP address in AP’s subnet

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802.11: Passive/Active Scanning

AP 2AP 1

H1

BSS 2BSS 1

122

3 4

Active Scanning: (1) Probe Request frame broadcast

from H1(2) Probe response frames sent from

APs(3) Association Request frame sent:

H1 to selected AP (4) Association Response frame

sent: Selected AP to H1

AP 2AP 1

H1

BSS 2BSS 1

12 3

1

Passive Scanning:(1) Beacon frames sent from APs(2) Association Request frame sent:

H1 to selected AP (3) Association Response frame sent:

Selected AP to H1

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IEEE 802.11: Multiple Access• Avoid collisions: 2+ nodes transmitting at same time• 802.11: CSMA - sense before transmitting

– Don’t collide with ongoing transmission by other node

• 802.11: no collision detection!– Difficult to receive (sense collisions) when transmitting due to weak

received signals (fading)– Can’t sense all collisions in any case: hidden terminal, fading– Goal: avoid collisions: CSMA/C(ollision)A(voidance)

• Uses a link-layer ACK/Retry (ARQ) scheme

AB

CA B C

A’s signalstrength

space

C’s signalstrength

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IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender1 If sense channel idle for DIFS then

Transmit entire frame (no CD)If no ACK, increase random backoff interval,

repeat 22 If sense channel busy then

Start random backoff timeTimer counts down while channel idleTransmit when timer expires

802.11 receiverIf frame received OK

Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

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Avoiding Collisions (more)

Idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

• Sender first transmits small request-to-send (RTS) packets to receiver using CSMA– RTSs may still collide with each other (but they are short)

• Receiver broadcasts clear-to-send CTS in response to RTS• CTS heard by all nodes

– Sender transmits data frame– Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets!

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Collision Avoidance: RTS-CTS Exchange

APA B

time

DATA (A)

reservation collision

defer

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framecontrol durationaddress

1address

2address

4address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 Frame: Addressing

Address 2: MAC addressof wireless host or AP transmitting this frame

Address 1: MAC addressof wireless host or AP to receive this frame

Address 3: MAC addressof router interface to which AP is attached

Address 4: used only in ad hoc mode

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Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3

802.11 frame

R1 MAC addr H1 MAC addr dest. address source address

802.3 frame

802.11 Frame: Addressing

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framecontrol durationaddress

1address

2address

4address

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

Type FromAPSubtype To

APMore frag WEPMore

dataPower

mgtRetry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

802.11 Frame: Moreduration of reserved transmission

time (RTS/CTS/DATA)frame seq #

(for duplicate filtering)

frame type(RTS, CTS, ACK, data)

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802.11 Mobility Support

BSS Transition

ESS Transition

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hub or switch

AP 2

AP 1

H1 BSS 2

BSS 1

802.11: Mobility Within Same Subnet

router• H1 remains in same IP subnet: IP address can remain same

• Switch: H1 associated with which AP?– self-learning: switch will

see frame from H1 and “remember” which switch port can be used to reach H1

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802.11 Handoff Procedure and Latency

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802.11 Rate Adaptation• Wireless channel time-

varying, so is received SNR

• AP and stations dynamically adapt transmission rate (modulation and coding scheme used) to track SNR variations

• Issues:– Responsiveness in

dynamic environments– Separating channel

related losses from interference losses

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1. SNR decreases (e.g., as node moves away from AP), BER increases2. When BER becomes too high, switch to lower transmission rate but with lower BER

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802.11 Rate Adaptation (Contd.)Office Environment

Home Environment

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802.11 Power Management

• Station-to-AP: “I am going to sleep until next beacon frame” (by setting power management bit in frame header)– AP knows not to transmit frames to that station

during that period, buffers them instead– Station wakes up before next beacon frame

• Beacon frame from AP: contains list of stations with AP-to-station frames buffered– Station will stay awake as long as there are AP-to-

station frames to be received; otherwise can sleep again until next beacon frame

802.11 Network Interface ImplementationA Schematic

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AmplifierFrequency conversionShielding

(De)modulationPhysical carrier sensing

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References

• J. F. Kurose and K. W. Ross, “Computer Networking: A Top-Down Approach,” 5th edition, Pearson Education, 2010.

• M. S. Gast, “802.11 Wireless Networks,” O’Reilly, 2005.

• S. Kawade and T. Hodgkinson, “License-Exempt Wireless Communication Systems,” BT Technology Journal, April 2007.

• A. Mishra, M. Shin and W. Arbaugh, “An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process,” ACM SIGCOMM Computer Communications Review (CCR), April 2003.