802 Wireless Lan

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IEEE 802.11 Wireless LANStandard

Updated: 5/10/2011



IEEE 802.11 History andEnhancements

802.11 is dedicated to WLAN

The group started in 1990

Focusing on 2.4 GHz unlicensed band

2 Mbit BW – relatively slow

First standard that received industry support was 802.11b Accepted in 1999

Focusing on 2.4 GHz unlicensed band

2 Mbit BW – relatively slow



802.11 Standards

WiFi Alliance http://www.wi-fi.org/Read the handout!

http://www.wi-fi.org/






802.11 Standards

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



OSI Model



IEEE 802 Protocol Layers



IEEE 802 Protocol Layers•Encoding/decoding of signals•Preamble generation/removal(for synchronization)•Bit transmission/reception•Includes specification of thetransmission medium

•

On transmission, assemble datainto a frame with address anderror detection fields•On reception, disassemble frameand perform address recognitionand error detection•Govern access to the LANtransmission medium

Provide an interface tohigher layers and performflow and error controltransmission medium

LLC and MAC are separated:- The logic required to manage access to a shared-access medium not found in traditional layer 2 datalink control- For the same LLC, several MAC options may beprovided



Protocol Architecture

Functions of physical layer:

Encoding/decoding of signals

Preamble generation/removal (forsynchronization)

Bit transmission/reception

Includes specification of the transmission

medium



Protocol Architecture

Functions of medium access control (MAC)layer: On transmission, assemble data into a frame with

address and error detection fields

On reception, disassemble frame and performaddress recognition and error detection

Govern access to the LAN transmission medium

Functions of logical link control (LLC) Layer: Provide an interface to higher layers and perform

flow and error control



Separation of LLC and MAC

The logic required to manage accessto a shared-access medium not foundin traditional layer 2 data link control

For the same LLC, several MACoptions may be provided



MAC Frame Format

MAC control Contains Mac protocol

information - PRIORITY

Destination MAC address

Destination physicalattachment point

Source MAC address Source physical

attachment point

CRC

Cyclic redundancy check Error-detecting code

MAC is responsible for detecting errors and discarding frames with errors –Frames with no errors are sent to the LLC layer



Logical Link Control

LLC is in charge of ensuringtransmission of a link-levelPDU (Protocol Data Unit)

between end-to-end stations No intermediate node in between

Characteristics of LLC notshared by other link controlprotocols:

Must support multiaccess, shared-medium nature of the link

Relieved of some details of linkaccess by MAC layer

Note: SAP is the user address (service access point) / OSI terminology



LLC Services

Basic services: Provide mechanisms for addressing stations across the

medium

Services types: Unacknowledged connectionless service

Datagram-style service No flow- and error-control mechanisms

Data delivery not guaranteed

Connection-mode service

Logical connection set up between two users

Flow- and error-control provided

Acknowledged connectionless service Cross between previous two

Datagrams acknowledged

No prior logical setup

Skip LLC Protocol Section!



802.11 Architecture



Related Sub-layers

802.3 CSMA/CS related to Ethernet(star specification)

802.5 Token Ring specifications LAN 802.1 Management (Virtual LAN –

802.1q / Bridging 802.1d)



802.11 Sub-layers

The physical layer is divided into two sub layers:

PLCP: Physical Layer Convergence Protocol – gluesbetween MAC and Radio transmission; maps the MACframe and prepares it for transmission by adding

appropriate and header PMD: Physical Medium Dependent – transmits the

mapped framed in the air through the antenna

Service Access Point



Basic Terminologies –

Four Physical Components Distribution system: backbone

system used to relay framesbetween AP or between the APand the backbone

Access Point: Performs bridgingfunction (wireless-to-wired)

Medium: IR or RF physicalmedium

Stations: End users

Figure



IEEE 802.11 Architecture

Distribution system (DS)

Access point (AP)

Basic service set (BSS)

Stations competing for

access to shared wirelessmedium

Isolated or connected tobackbone DS through AP

Smallest building block

Extended service set - Two or more basic service setsinterconnected by DS- Requires a backbone (Ethernetor VLAN)



IEEE 802.11 Architecture

Independent Basic Service Set (IBSS)

Multiple independent stations (STA) can communicatewithin the boundary of a cell

We refer to the cell as Basic Service Set

Often P2P Used in single meetings with short duration

In a geographical area we can have multiple IBSS

Within IBSS the relation between STA and BSS isdynamic

STA moves / dies (temporary associating) We refer to IBSS structure as a peer-to-peer or ad-

hoc wireless network



IEEE 802.11 Architecture -Networks

Infrastructure Networks

All connections go through the AP

Nodes have to associate themselves tothe AP

Independent Networks

No access points (P2P)



•Each BSS has an address (SSID = Service Set ID)•SSID can be different

•BSS MAC-level addressof the AP•IBSS Random number Portal logic is

implementedin a Routeror bridgeAP can be part

of the station

/ providesaccess to DS



DS Issues

How AP communicate with one another totell about their associated stations Passing association information (using Inter-

access Point Protocol – IAPP)

No real standard

How to manage overlapping BSS in an ESS Multiple 802.11 networks can coexist

How to manage moving user from one BSSto another

How to distinguish between overlappingBSS and an IBSS



802.11 Network Operation

The network operation can bedefined by the services it provides

Nine different services

Services can be defined from STAor DS point of view

STA: Station and AP connections DS Services: Connecting AP to DS



802.11 Services

Typically divided into two types (in terms of distribution of messages within a DS)

Distribution service

Used to exchange MAC frames from station in one BSSto station in another BSS

E.g., Keeping track of mobile nodes and delivering toright node

Integration service

Transfer of data between station on IEEE 802.11 LANand station on integrated IEEE 802.x LAN

Muller page 394



What If Stations Are Moving?

Transition Types Based On Mobility No transition

Stationary or moves only within BSS

BSS transition Station moving from one BSS to another BSS in same ESS

Through association and re-association

ESS transition Station moving from BSS in one ESS to BSS within another ESS

This seamless transition is often provided by Mobile IP



Service Types

Distribution service(exchange betweenBSS)

Association

Re-association

Disassociation

Authentication

De-authentication

Privacy

Integration service(Between Gateways)

Distribution

Integration MSDU (MAC Service

Data Units)



Association-Related Services

Association

Establishes initial association between station and AP

Reassociation

Enables transfer of association from one AP toanother, allowing station to move from one BSS toanother / updates location

Of due to temporary disconnection

Disassociation

Association termination notice from station or AP



Access and Privacy Services

Authentication

Establishes identity of stations to each other

Use name or public key

Not mandatory by standards

Deathentication Invoked when existing authentication is terminated

Privacy

Prevents message contents from being read by unintendedrecipient

MSDU Delivery

MAC Service Data Unit Delivery

Responsible to ensure delivery of data to the STA



IEEE 802.11 Medium AccessControl

The key to 802.11 is understanding the MAC

Ethernet style

CSMA/CA – not so much /CD which takes too much overhead)

Supporting different media

Typically half-duplex connection

MAC challenges

Link quality (unpredictable in wireless)

Hidden node problem (not seeing unreachable nodes)

Simultaneous transmission of hidden nodes collision

MAC layer covers three functional areas:

Reliable data delivery

Access control

Security



Reliable Data Delivery

More efficient to deal with errors at the MAClevel than higher layer (such as TCP – but it istoo slow)

(Two) Frame exchange protocol Source station transmits data

Destination responds with acknowledgment (ACK)

If source doesn’t receive ACK, it retransmits frame

Four frame exchange

Source issues request to send (RTS) Destination responds with clear to send (CTS)

Source transmits data

Destination responds with ACK



Four FrameExchange (RTS/CTS)

Four frame exchange can resolve the hiddennode problem by silencing them via CTS

Often is required for high capacity networks

with high collision



Medium Access

Access to the wireless medium controlled by coordinationfunctions DSF (distributed coordination function)

Check the link before transmission Ethernet-like - Based on CSMA with backoff Used in IBSS

PCF (central coordination function) Not widely used A point of coordination is assumed – central station is assigned

to coordinate access) Ensures contention-free transmission

802.11 provides two ways to avoid collision

Physical detection of the medium – hardware-based sensing) Virtual – using network allocation network (NAV) / basically

use of various timers



Network allocation network(virtual) (NAV)

RTS FRM

CTS

SIFS=Short interframe spacePIFS=Point Coordination Function IFSDIFS=Distributed Coordinated Function IFS

At each station



Access Control

CentralizedPCF

NormalDelivery



Carrier Sense Multiple Access/ Collision Avoidance or Detection

CSMA/CA belongs to a class of protocols called multiple accessmethods

In CSMA, a station wishing to transmit has to first listen to the channelfor a predetermined amount of time so as to check for any activity on

the channel If the channel is sensed "idle" then the station is permitted to transmit

If the channel is sensed as "busy" the station has to defer itstransmission

This is the essence of both CSMA/CA and CSMA/CD

In CSMA/CA once the channel is clear, a station sends a signal telling

all other stations not to transmit

Figure p29/ 30 / 34

http://sss-mag.com/pdf/802_11tut.pdf



Contention Window

Channel busy defer the transmission; keep monitoring

After the IFS, the station backs off If the medium still idle transmit



Medium Access ControlLogic

IFS=interframe

space

busy

Random delay



Binary Backoff for DSSS-A station attempts to transmit repeatedly-When collision occurs (absence of ACK)

-the mean average delay is doubled



Management Operation

Mainly to solve power, reliability, andsecurity

Composed of three components

MLME : MAC layer management entity PLME : Physical layer management entity

SME : system management entity

Basic Functionalities Scanning

Authentication

Power conservation

Timing shynchronization

Figure



Scanning

The process of finding the network

Requires multiple parameters

BSSType – independent / infrastructure / both

BSSID - - individual / broadcast (anynetwork) SSID – specific network name or broadcast

ScanType – active (transmit a Probe Request)or passive (listen)

ChanList – list of channel to perform scanning



Passive / Active Scanning

Passive Scanning

Save power (not transmittinganything)

Typically has a ChanList to

listen for transmittedBEACONs

1- Wait for beacon frame2- check BSS information3-Sweep from chan to chan

Active Scanning

On Each designated channel a Probe Request is sent soliciting service (like Calling FIRE!) – everyoneresponses

Search for a specific SSID by broadcasting its name



Active Scanning

Note how medium access control is implemented!

Gainingaccess to the

channel isperformed

via DCFaccessprocess



Scan Report

Following each scan a report is generated

The following information is acquired

Beacon interval (how often the BSS is sending

its beacon) DTIM – Delivery traffic indication map (frames

indicating power management mechanism)

Timing Parameters - used for synchronization

PHY Parameter –

BSS Basic Rate



Example

HOMEWORK : Read Chapter 4Use OmniPeeK DemoSoftware and examineThe following frames:-Management frame in a beacon-Probe Request Frame-Beacon Frame

-Probe Response-ACK frame

http://books.google.com/books?id=TLUVG9yoGx4C&dq=802.11+Wireless+Gast&printsec=frontcover&source=bn&hl=en&ei=_a7ZS5HFDZD-sgPJm7yVAQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBsQ6AEwAw




Joining

STA decide on which BSS to join based onthe scan report

Joining is required to gain access to themedium

Before access association and authenticating isrequired

Joining a BSS will be based on severalparameters Matching PHY

WEP Negotiating timers, beacon interval, and power

saving mechanism



Authentication

Authentication is initiated by the STA 802.11 Authentication Open System No identify verification (Going to bank with no

id!)

Shared-Key WEP between both stations

One way authentication The AP is not authenticated by the station a

rogue AP can attack the station

Public Key authentication - 802.1x



Framing

MAC Frame Types Date Frame Control Frame Management Frame

Examples IBSS data frame Data Frame from AP Data Frame to AP RTS Frame CTS Frame

ACK Frame Management Frame Etc.



Frame



802.11 Standards

WiFi Alliance http://www.wi-fi.org/







References Good tutorials

http://grouper.ieee.org/groups/802/11/Tutorial/index.html

Google Book:

http://books.google.com/books?id=TLUVG9yoGx4C&dq=802.11+Wireless+Gast&printsec=frontcover&source=bn&hl=en&ei=_a7ZS5HFDZD-sgPJm7yVAQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBsQ6AEwAw#v=onepage&q&f=false














References

Click on the following links:

IEEE 802® GROUPS - PUBLIC WEB SITES

802 802.1 802.2 802.3

802.4 802.5 802.6

802.7 802.8 802.9

802.10 802.1

1

802.

12

802.1

4

802.1

5

802

.16

802.17 802.18

802.19

802.20

802.21

802.22

http://grouper.ieee.org/groups/802/index.html

http://grouper.ieee.org/groups/802/1/index.html

















http://home.knology.net/ieee80214






http://grouper.ieee.org/groups/802/17/







http://www.ieee802.org/21/






































http://grouper.ieee.org/groups/802/index.html



Homework –

802 Wireless Lan

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