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Advanced Lab in Computer Communications: Meeting 3 - Wi-Fi 802.11

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The document discusses the IEEE 802.11 standards for wireless networks. It describes how wireless networks are standardized by IEEE under the 802 LAN/MAN committee. It provides an overview of 802.11, noting it was adopted in 1997 and defines the MAC sublayer, protocols, and physical layers. It then summarizes some of the main 802.11 standards including 802.11b, 802.11g, 802.11a, and 802.11n, outlining their data rates and technologies like OFDM and MIMO.

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Advanced Lab in Computer Communications: Meeting 3 - Wi-Fi 802.11

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ferronatto11
72 views29 pages
The document discusses the IEEE 802.11 standards for wireless networks. It describes how wireless networks are standardized by IEEE under the 802 LAN/MAN committee. It provides an overview of 802.11, noting it was adopted in 1997 and defines the MAC sublayer, protocols, and physical layers. It then summarizes some of the main 802.11 standards including 802.11b, 802.11g, 802.11a, and 802.11n, outlining their data rates and technologies like OFDM and MIMO.

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The document discusses the IEEE 802.11 standards for wireless networks. It describes how wireless networks are standardized by IEEE under the 802 LAN/MAN committee. It provides an overview of 802.11, noting it was adopted in 1997 and defines the MAC sublayer, protocols, and physical layers. It then summarizes some of the main 802.11 standards including 802.11b, 802.11g, 802.11a, and 802.11n, outlining their data rates and technologies like OFDM and MIMO.

Copyright:

© All Rights Reserved
Download as PDF, TXT or read online from Scribd
Download as pdf or txt
72 views29 pages

Advanced Lab in Computer Communications: Meeting 3 - Wi-Fi 802.11

Uploaded by

ferronatto11
The document discusses the IEEE 802.11 standards for wireless networks. It describes how wireless networks are standardized by IEEE under the 802 LAN/MAN committee. It provides an overview of 802.11, noting it was adopted in 1997 and defines the MAC sublayer, protocols, and physical layers. It then summarizes some of the main 802.11 standards including 802.11b, 802.11g, 802.11a, and 802.11n, outlining their data rates and technologies like OFDM and MIMO.

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Advanced Lab in Computer

Communications
Meeting 3 Wi-Fi 802.11

Instructor: Dima Laschov

Standardization of Wireless Networks





Wireless networks are standardized by IEEE.


Under 802 LAN/MAN standards committee.

ISO
OSI
7-layer
model

Application
Presentation
Session
Transport

IEEE 802
standards

Network

Logical Link Control

Data Link

Medium Access (MAC)

Physical

Physical (PHY)

IEEE 802.11 Overview


Adopted in 1997 and
clarified in 1999.
(802.11 legacy) data
rates of 1 and 2 (Mbit/s)
Defines;
 MAC sublayer
 MAC management
protocols and services
 Physical (PHY) layers


Wireless 802.11


802.11x is a set of standards derived from the


original IEEE 802.11 also known as 802.11legacy

The standard defines the PHY and the MAC layers.

802.11b (11 Mbit/s) and 802.11g (54 Mbit/s) use 2.4


GHz band. Same band used by Bluetooth, portable
phones, and microwaves!

802.11a uses 5GHz data rate of 54 Mbit/s.

802.11n is a MIMO extension of the previous


standards (54 Mbits/s to 600 Mbits/s)

Devices

802.11 Basics


2 Types of devices: STA and AP




Station (STA): A computer or device with a


wireless network interface.
Access Point (AP): Device used to bridge the
wireless-wired boundary, or to increase distance
as a wireless packet repeater.

Basic Service Set (BSS) provides the basic


building-block of an 802.11 wireless LAN.



Independent BSS (IBSS)


Infrastructure BSS

802.11 Basics


Basic service set identification (BSSID)




in infrastructure mode, the BSSID is the MAC


address of the wireless AP
for an IBSS, the BSSID is a locally administered
MAC address generated from a 46-bit random
number

Extended Service Set (ESS): A set BSSs


and wired LANs with Access Points that
appear as a single logical BSS.

802.11 Basics


2 Operation modes:
Ad Hoc No AP is used. Called also p2p
mode or IBSS.
Infrastructure Mode All stations are
connected to a AP.

802.11 Architecture
ESS
Existing
Wired LAN

AP
STA

BSS

STA

AP
STA

BSS

STA

Infrastructure
Network
STA
Ad Hoc
Network

STA

BSS

BSS
STA

STA

Ad Hoc
Network

MAC Protocols
2 Types of MAC protocols:
DCF

(Distributed Coordination Function)


Binary exponential backoff algorithm
CSMA/CA
CSMA- Carrier Sense Multiple Access
CA Collision Avoidance

In wireless we can not listen and transmit simultaneously, thus CD is


impossible.
PCF

(Point Coordination Function)A centralized protocol used in infrastructure mode only.

CSMA/CA

Before transmitting, the channel should be quiet for at least DIFS.


Afterwards the station can transmit.

If the channel is busy, wait until the channel is clear then wait DIFS + backoff time.

If transmission is sensed during the backoff time, the backoff timer freezes, and resumes after the
channel is free for another DIFS.

The backoff time is doubled at every collision detected, until it reaches a maximum.

MAC Level Ack Messages:


After a successful transmission an ACK should be transmitted after SIFS seconds.

After transmission wait for ACK, if no ACK is received, the stations will retransmit.

DIFS>SIFS
ACK is favored upon regular transmission

DIFS- Data Interframe Space


SIFS- Short Interframe Space

CSMA/CA Cont.









Can 2 data frames collide?


Yes!
If 2 stations hear a quiet channel for DIFS seconds (hidden node).
Can a ACK frame collide with a different frame?
No!
Cant collide with a data frame because DIFS>SIFS
Cant collide with another ACK, because that means that 2
successful data frames were transmitted simultaneously. The latter
data frame must have started while the channel was busy, causing a
collision (no ACK after collision).
Is there a need for an LLC layer?
No!
The MAC layer with ACKs provides in order transmission with no
errors.

2 Observations on CSMA/CD


Transmitter can send/listen concurrently




If (Transmitted - Sensed = null)? Then success


Some slides based
on:

The signal is identical at Tx and Rx




Non-dispersive

Wireless Medium
Access Control
Romit Roy
Choudhury
Wireless Networking
Lectures
Duke University

The TRANSMITTER can detect if and


when collision occurs
13

Unfortunately

Both observations do not hold for wireless

Because

14

Wireless Media Disperse Energy


A cannot send and listen in parallel
C
A

Signal
power
Signal not same at different locations
Transmitter can only hear itself
Cannot determine signal quality at receiver

Distance
15

Collision Detection Difficult


A
D

Signal not same at different locations


Transmitter can only hear itself
Cannot determine signal quality at receiver

16

The Hidden Terminal Problem

Both stations might transmit to the middle station simultaneously,


because they dont hear each other.

The stations will transmit the entire packet, because they cant do
CD.

IEEE 802.11
RTS = Request
To Send

CTS = Clear
To Send
M
Y
S

RTS

CTS
X
K

18

IEEE 802.11
silenced
M
Y
S

Data

silenced

ACK
X

silenced
K

silenced

19

Virtual Carrier Sense: RTS/CTS




An attempt to handle the hidden node problem (optional mechanism)

The message length is transmitted in the RTS and in the CTS messages, allowing the
NAV times to be derived.

The RTS will cause all the stations that hear the transmitter to be quiet for NAV(RTS)
The CTS will cause all the stations that hear the receiver to be quiet for NAV(CTS)
RTS/CTS Request/Clear To Send
NAV Network Allocation Vector

DCF Operation

RTS/CTS


Does it solve hidden terminals ?




Assuming carrier sensing zone = communication


zone
E

RTS
F

CTS
A

CTS

EEdoes
doesnot
notreceive
receiveCTS
CTSsuccessfully
successfully
Can
Canlater
laterinitiate
initiatetransmission
transmissionto
toD.
D.
Hidden
Hiddenterminal
terminalproblem
problemremains.
remains.
22

CTS-to-self


Defined in 802.11g as an alternative to RTS/CTS

The transmitter sends a CTS packet before


transmission, waits SIFS and then transmits the
data packet

With RTS/CTS the sender asks for permission, and


the AP tells everyone how long the sender will use
the channel. In CTS-to-self the transmitter
announces to everyone that it plans to transmit.

When is it used?

PCF- Point Coordination Function










PCF is available only in infrastructure mode (in the


presence of an AP)
AP sends beacon frames at regular intervals.
Between beacon frames are 2 periods:
Contention Free Period (CFP) and Contention
Period (CP)
In CP DCF is used.
In CFP the AP assigns Contention Free-Poll (CFPoll) packets to each station, one at a time, to give it
the right to send a packet.

802.11 Legacy


Implementation of CSMA/CA

ISM Band (2.4GHz) Industrial Scientific and Medical also used by


the Bluetooth standard

802.11a



OFDM modulation up to 54Mpbs (~20Mbps in real life).


52 Orthogonal sub-carriers modulated in regular single carrier modulation (BPSK,
QPSK, QAM16 or QAM64) at low symbol rate (4 usec per symbol).
The main advantage of multiple sub-carriers over a single carrier is the ability to cope
with severe channel conditions
5GHz high band is absorbed easily forcing the STAT to be almost in line of sight with
the AP.
A need to deploy more APs.
Because of a lack of 5GHz HW, and the distance problem 802.11a was not widely
adopted

802.11b



2.4 GHz (low band)


CCK modulation (a variant of CDMA used by cell phones). This
allowed fast development by upgrading CDMA chipsets.
Adaptive Rate Selection (according to signal quality) 11, 5, 2 or
1 Mbps
Actual throughput is 5.9Mbps under TCP and 7.1 Mbps under
UDP

802.11g



54 Mbps at low band (actual 24.7 Mbps)


OFDM modulation (like in 802.11a) for high data rates,
and reverts to CCK for low data rates.
Wide spread even before ratification by home
consumers. Corporate customers and big equipment
users held back.

802.11n

Very high (theoretical) data rates - 540 Mbps

MIMO is used to increase data throughput.

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