Wireless Local Area Networks: Basics, MAC, Headershome.iitj.ac.in/~ramana/802-11.pdfWireless Local Area Networks: Basics, MAC, Headers Dr. Ramana I.I.T Rajasthan ... 2 Wireless LAN

Wireless Local Area Networks: Basics, MAC,Headers

Dr. Ramana

I.I.T Rajasthan

Dr. Ramana ( I.I.T Rajasthan ) Wireless Local Area Networks: Basics, MAC, Headers 1 / 34

Outline of the Lectures

1 Overview

2 Wireless LAN Technologies

3 IEEE 802.11 Services

4 IEEE 802.11 Medium Access Control


Overview

Introduction

Wireless LANs (WLANs or WiLANs or WiFi-LANs) have samecharacteristics (like limitted distance, shared broadcast medium)of Wired-LANs.But medium is unguided instead of guided.Wireless link characteristics

Decreased signal strength: radio signal attenuates as it propagatesthrough matter (path loss). (Ex. free-space path loss in dB L =20× Log10

4πdλ , i.e., free-space path loss = ( 4πd

λ )2 )Interference from other sources: standardized wireless networkfrequencies (e.g., 2.4 GHz) shared by other devices (e.g.,microwave devices) interfere as well.Multipath propagation: A phenomenon that results in radio signalsreaching the receiving antenna by two or more paths at slightlydifferent times.


Overview

WLAN Requirements

Throughput - MAC should be efficient to maximize the capacitySupport for large number of nodesConnection to backbone LANService Area - cover about 100 to 300mBattery power consumption - reduce the power consumptionTransmission robustness and security - vulnerable to interferenceand eavesdroppingCollocated network operations - more than one WLAN in thesame areaLicense-free operations - to cost cuttingHandoff - to enable roamingDynamic configuration - association and disassociation of a nodeshould take place without disruption to other nodes


Wireless LAN Technologies

Classification of Wireless Networks

Basic Service Set: A group of stations (fixed or mobile) contolledby a single coordinating entity, Access Point (AP).Distributed System: A system used to interconnect a set of BSSsand integrated LANs to create an Extended Service Set (ESS).Types of wireless configurations

Infrastructure mode - stations can communicate with each otherthrough an AP

WLANs: singlehop transmissions.Mesh Networks: multihop transmissions, have a relatively stabletopology.

Ad-hoc mode - stations can directly communicate with each otherto form an Independent BSS (IBSS) without connectivity to anywired backbone.

Bluetooth ad hoc networks: singlehop transmissionsMobile ad hoc networks: multihop transmissions, topology changesas nodes are mobile, no infrastructure support.



High-speed Backbone Wired LAN

(b) Ad hoc LAN

Figure 17.3 Wireless LAN Configurations

(a) Infrastructure Wireless LAN

Cell

Nomadicstation



BasicService Set

STA2

STA3

STA = stationAP = access point

STA4 BasicService Set

ExtendedService Set

IEEE 802.11 Architecture

STA6 STA7

Distribution System

IEEE 802.x LAN

STA1

AP

STA5

AP

portal



Standards at glance

IEEE 802.3 - Ethernet LANsIEEE 802.5 - Token LANsIEEE 802.11 - Wireless LANsIEEE 802.15 - Personal Area Networks (Bluetooth and Zigbee)IEEE 802.16 - Broadband wireless network (WiMAX)



(Cont.)

802.11a - 54 Mbps standard, 5 GHz signaling (ratified 1999)802.11b - 11 Mbps standard, 2.4 GHz signaling (1999)802.11e - Quality of Service (QoS) support (not yet ratified)802.11g - 54 Mbps standard, 2.4 GHz signaling (2003)802.11i- security improvements for the 802.11 family (2004)802.11n - OFDM version at 248 Mbps; MIMO version up to 600Mbps (formally voted into the standard in September 2009!)802.11u - internetworking with 3G / cellular and other forms ofexternal networks


IEEE 802.11 Services

Services

Station services - implemented in every 802.11 station includingAPs.

Delivery of dataAuthentication and deauthenticationPrivacy

Distribution Services - provided between BSSs; Implemented ineither AP or any other special purpose device.

Association - Association between AP and a stationDisassociation - association terminationReassociation - association with AP in another BSSDistribution - data transfer between BSSsIntegration - data transfer between 802.11 and 802.x


IEEE 802.11 Medium Access Control

Protocol Architecture

PointCoordination

Function (PCF)

Contention-freeservice

Contentionservice

IEEE 802.11 Protocol Architecture

MACLayer

Distributed Coordination Function

Logical Link Control

(DCF)

2.4-Ghzfrequency-

hoppingspread

spectrum1 Mbps2 Mbps

2.4-Ghzdirect-

sequencespread

spectrum1 Mbps2 Mbps

2.4-Ghzdirect

sequencespread

spectrum5.5 Mbps11 Mbps

2.4-GhzDS-SS

6, 9, 12,18, 24, 36,

48, 54 Mbps

Infrared1 Mbps2 Mbps

IEEE 802.11 IEEE 802.11a IEEE 802.11b

5-Ghzorthogonal

FDM6, 9, 12,

18, 24, 36,48, 54 Mbps

IEEE 802.11g



(Cont.)



MAC

FunctionalityReliable data deliveryFairly control accessProtection of data

DealsNoisy and unreliable mediumFrame exchange protocol - ACK (unlike in IEEE 802.3)Hidden Node Problem - RTS/CTS



Hidden Node Problem

Hidden node problem

Wireless stations havetransmission ranges and notall stations are within radiorange of each other.Simple CSMA will not work.While C is transmiting to B, Afalsely conclude that thechannel is idle, as it will nothear C’s transmission.



Exposed Node Problem

Exponsed node problem

This is the inverse problem.B wants to send to A andlistens to the channel.B hears C’s transmission (toD), B falsely assumes that itstransmission interfer with C’stranmission.



CSMA/CA (Collision Avoidance)

Collision detection is not possibleDifficult to receive (sense collisions) when transmitting due to weakreceived signals (fading)Can’t sense all collisions in any case: hidden terminal, fading

CSMA/CA operation:1-Persistent physical carrier sensing - Mandatory (like in Ethernet)Virtual carrier sensing - Optional

Mode of operationOnly physical CS: DATA - ACKBoth physical and virtual CS - RTS-CTS-DATA-ACK



MAC LogicWait for frame

to transmit

Wait IFS

IEEE 802.1 1 Medium Access Control Logic

No

Yes

Yes

Yes

No

No

Wait IFS

Mediumidle?

Stillidle?

Wait until currenttransmission ends

Exponential backoffwhile medium idle

Transmit frame

Transmit frame

Stillidle?



(Cont.)

1 Station with frame senses medium2 If idle, wait to see if remains idle for one DIFS.

If so, may transmit immediately3 If busy (either initially or becomes busy during DIFS)

Station defers transmission and continue to monitor until currenttransmission is overOnce current transmission is over, delay another DIFSIf remains idle, back off random time and again senseIf medium still idle, station may transmitDuring backoff time, if becomes busy, backoff timer is halted andresumes when medium becomes idleTo ensure stability, binary exponential backoff used



Binary Exponential Backoff Algorithm

Contention window (cw) of size 0 < cwminx ≤ cwmax

Generate a random number, r , from [0 . . . 2cwmin − 1]r represents number of slots to be deferred medium accessfor every successive ack loss - cwmin → cwmin + 1Default values for cwmin = 5 and cwmax = 10.Defines limits for the number of MAC retries for different types ofpackets.Short retry limit (7) for a control frame or a short frame smallerthan the RTSThreshold is retransmitted.Long retry limit (4) when a long packet frame is retransmitted.If retry count reaches their respective limits the frame isautomatically discarded.



MAC Timing

Defer access

DIFS

Immediate accesswhen medium is free

longer than DIFS

SIFS

PIFS

DIFS

Busy Medium Next frameBackoff window

Contention window

Slot time

Select slot using binary exponential backoff

(a) Basic Access Method

time

PCF (optional)

Contention-freeperiod

Variable length(per superframe)

Busy medium PCF (optional)DCF

Contention period

Superframe (fixed nominal length)

Superframe (fixed nominal length)

Foreshortened actualsuperframe period

PCFdefers

CF-Burst;asynchronoustraffic defers

(b) PCF Superframe Construction

IEEE 802.11 MAC TimingDr. Ramana ( I.I.T Rajasthan ) Wireless Local Area Networks: Basics, MAC, Headers 20 / 34


DCF in detail



Timings in IEEE 802.11

Interframe spacing (IFS) - to introduce priority among the transmissions andstations.SIFS - Short Interframce Space - The nominal time (in µsec) that the MAC andPHY will require to receive the last symbol of a frame at the air interface, processthe frame, and respond with the first symbol on the air interface of the earliestpossible response frame.SIFS time in µsec, aSIFS = aRxRFDelay + aRxPLCPDelay +aMACProcessingDelay + aRxTxTurnaroundTime.aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime +aMACProcessingDelay. (Slottime is the time that is good enough to determinethe transmission, if any, made by other station that accessed medium at thebegining of the previous slot).aCCATime - The minimum time (in microseconds) the CCA mechanism hasavailable to assess the medium within every time slot to determine whether themedium is busy or idle.aAirPropagationTime = Twice the propagation time (in microseconds) for asignal to cross the maximum distance between the most distant allowable STAsthat are slot synchronized.



(Cont.)

PCF-IFS time in µsec, aPIFS = aSIFS + SlotTimeDCF-IFS time in µsec, aDIFS = aSIFS + 2×SlotTimeEIFS - Extended IFS - Used when a node senses an idle mediumfollowing reception of an erronesous frame (incompleteframe/incorrect FCS), however to defer access for a longer periodin order to avoid collision with future packets that belong to thecurrent dialog.aEIFS = aSIFSTime + (8 x ACKSize) + aPreambleLength +aPLCPHeaderLngth + aDIFS.Relation: SIFS < PIFS < DIFS < EIFSExample -

802.11a 802.11bSlotTime 9 µsec 20 µsec

SIFS 16 µsec 10 µsecPIFS 25 µsec 30 µsecDIFS 34 µsec 50 µsec



How nodes selects their Access points? - Association

Before a node is allowed to send a data message via an AP, itshall first become associated with the AP.The technique for selecting an AP is called active or passivescanning.Active scanning

1 The node sends a Probe frame.2 All APs within reach reply with a Probe Response frame.3 The node selects one of the access points and sends that AP an

Association Request frame.4 The AP replies with an Association Response frame.

Passive scanningAPs periodically send a Beacon frame that advertises thecapabilities (ex. transmission rates supported) of the access point.A node can change to this AP based on the Beacon frame simply bysending it an Association Request frame back to the access point.

At any given instant, a node may be associated with no more thanone AP.



Disassociation/Reassociation

Disassociation - can be iniated by either AP or nodeAPs may need to disassociate nodes to enable the AP to beremoved from a network for service or for other reasons.Nodes shall attempt to disassociate when they leave a network.Disassociation is a notification, not a request. Disassociationcannot be refused by either party to the association.

ReassociationThe reassociation service is invoked to “move” a currentassociation from one AP to another.This keeps the DS informed of the current mapping between APand STA as the STA moves from BSS to BSS within an ESS.Reassociation also enables changing association attributes of anestablished association while the STA remains associated with thesame AP.Reassociation is always initiated by the mobile STA.



Framing in 802.11

Physical Layer - Responsible to the transmission of data over medium (RF)under the control of PMD as directed by the PLCP.

Physical Layer Convergence Procedure SublayerPhysical Medium Dependent Sublayer - takes the binary bits ofinformation from PLCP-PDU (PPDU) and transform them into RFsignals using carrier modulation and transmission techniques.Example, in 802.11b, Frequency band - 2.4 GHz, modulations -DBPSK, DQPSK, Complementary code keying (CCK) etc.,transmission technique - DSSS

MAC - which is responsible to arbiterate accesses to the medium - alsoresponsible for preparing MPDUs (MAC-PDU) or frames, which consist thefollowing basic components.

A MAC header, which comprises frame control, duration, address,and sequence control information, and, for QoS data frames, QoScontrol information;A variable length frame body, which contains information specific tothe frame type and subtype;A FCS, which contains an IEEE 32-bit CRC.Dr. Ramana ( I.I.T Rajasthan ) Wireless Local Area Networks: Basics, MAC, Headers 26 / 34


PPDU in 802.11b

Signal

8−bits

Length CRC

8−bits16−bits

SFD

128−bits

Sync

PSDU

Reserved

Preamble PLCP Header

16−bits8/16−bits

PPDU

1 Mbps DBPSK

SYNC - bitsteam that help receive to synchronize the clock prior toreceive the frameSFD - Indiacate the begining of frame F3A0Signal - Type of modulation to be used to receive the rest of PPDULength - Total frame lengthCRC - Protection to the PLCP headerLong and short preambles



Protocol Version Frame Type and

Sub Type To DS and From

DS More Fragments Retry Power

Management More Data WEP Order

FCDuration

/IDAddress

1Address

2Address

3Sequence

ControlAddress

4DATA FCS

2 2 6 6 6 2 6 0-2312 4 bytes

NAV information OR Short Id for PS-Poll

BSSID –BSS Identifier

TA - Transmitter RA - Receiver SA - Source DA - Destination

IEEE 48 bit address

Individual/Group Universal/Local 46 bit address

Sequence Number – 12 bit

Fragment Number – 4 bit

CRC-32 Polynomial

Upper layer data

802.11 MAC Frame



Address Fields

IEEE 48bit address comprises three fields:single-bit Individual/Group field: When set to 1, the address is thatof a group. if all bit are 1, that means broadcast.single-bit Universal/Local bit; when zero, the address is global andunique, otherwise it may no be unique and locally administered.

46bit address fieldsBSS Identifier (BSSID): unique identifier for a particular BSS. In aninfrastructure BSSID it is the MAC address of the AP. In IBSS, it israndom and locally administered by the starting station. This alsogive uniqueness. In the probe request frame and group addresscan be used.Transmitter Address (TA): MAC address of the station that transmitthe frame to the wireless medium. Always an individual address.Receiver Address (RA)Source Address (SA)Destination Address (DA)



(Cont.)

Function To DS From DS Address 1 Address 2 Address 3 Address 4IBSS 0 0 RA=DA SA BSSID N/A

From the AP 0 1 RA=DA BSSID SA N/ATo the AP 1 0 RA=BSSID SA DA N/A

Wireless DS 1 1 RA TA DA SA



Frame Subtypes

RTS CTS ACK PS-Poll CF-End & CF-End

ACK

Data Data+CF-ACK Data+CF-Poll Data+CF-ACK+CF-

Poll Null Function CF-ACK (nodata) CF-Poll (nodata) CF-ACK+CF+Poll

Beacon Probe Request & Response Authentication Deauthentication Association Request &

Response Reassociation Request &

Response Disassociation Announcement Traffic

Indication Message (ATIM)

CONTROL DATA MANAGEMENT



Control Frames

Assist in reliable data deliveryPower Save-Poll (PS-Poll)

Sent by any station to station that includes APRequest AP transmit frame buffered for this station while station inpower-saving mode

Request to Send (RTS)First frame in four-way frame exchange

Clear to Send (CTS)Second frame in four-way exchange

Acknowledgment (ACK)Contention-Free (CF)-end

Announces end of contention-free period part of PCFCF-End + CF-Ack:

Acknowledges CF-endEnds contention-free period and releases stations from associatedrestrictions



Data Frames-Data Carrying

Eight data frame subtypes, in two groupsFirst four carry upper-level data from source station to destinationstationData

Simplest data frameMay be used in contention or contention-free period

Data + CF-AckOnly sent during contention-free periodCarries data and acknowledges previously received data

Data + CF-PollUsed by point coordinator to deliver dataAlso to request station send data frame it may have buffered

Data + CF-Ack + CF-PollCombines Data + CF-Ack and Data + CF-Poll



Data Frames NO Data Carrying

Remaining four data frames do not carry user dataNull Function

Carries no data, polls, or acknowledgmentsCarries power management bit in frame control field to APIndicates station is changing to low-power state

Other three frames (CF-Ack, CF-Poll, CF-Ack + CF-Poll) same ascorresponding frame in preceding list (Data + CF-Ack, Data +CF-Poll, Data + CF-Ack + CF-Poll) but without data


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Wireless Local Area Networks: Basics, MAC, Headershome.iitj.ac.in/~ramana/802-11.pdfWireless Local Area Networks: Basics, MAC, Headers Dr. Ramana I.I.T Rajasthan ... 2 Wireless LAN