IEEE802.11. What is IEEE 802.11? Standard for wireless local area networks (wireless LANs) developed in 1990 by IEEE Intended for home or office use (primarily.

IEEE802.11

What is IEEE 802.11?•Standard for wireless local area networks (wireless LANs) developed in 1990 by IEEE

•Intended for home or office use (primarily indoor)

•802.11 standard describes the MAC layer, while other substandards (802.11a, 802.11b) describe the physical layer

•Wireless version of the Ethernet (802.3) standard

Wi-Fi Standard (802.11x) Mission: promote 802.11

interoperability as the global wireless LAN standard

Wi-Fi Board members include AMD, Apple, Cisco, Compaq, Dell, Epson, Ericsson, Fujistu, Gateway, HP, IBM, Intel, Microsoft, NEC, Nokia, Nortel, Philips, Samsung, Sharp, Sony, TDK, Toshiba,

802.11b LAN Configuration

Access PointDevice

Example 802.11b Wireless LAN Equipment

Intended Use Wireless Internet access inside hotel lobbies, conference

rooms, etc.

Wireless with your Latte? Wireless home networking

Wireless at the Airport

Wi-Fi Market in the News Wireless LAN equipment market

$969 Million in 2000 to estimated $4.5 Billion in 2006

In 2001: Microsoft adds 802.11 in Windows XP Major hotel chains install Wi-Fi Internet access 462 Starbucks stores offer wireless Internet Microsoft joins WECA board (the 802.11 alliance) Intel Joins WECA board

Most PC/Laptop manufacturers offer Wi-Fi

802.11b Interoperability

The Wireless Ethernet Compatibility Alliance (WECA) certification provides a standard for wireless technology, ensuring interoperability of products from different vendors.

WECA Certification Mark

Competing Short-Range Wireless Technologies

Short-range wireless solutions: Bluetooth 802.11 (Wi-Fi) HomeRF (not as popular)

Who will prevail? 802.11 more suitable for wireless LANs

(office, hotel, airport,…) Bluetooth is designed for personal area

networks – smart appliances, printers, scanners, etc.

Commercial WirelessData Communications

Present

Near Future

Cellular

19K

ProprietarySystems

128K

Digital Cellular

384K

Data Rate Bits per second

BluetoothTechnology

721K 11M 54M

802.11aTechnology

802.11bWireless LANs

Home RF

IEEE 802.11b Technology

The standard is believed to be very good, will be widely accepted and will allow hardware prices to decrease.

Developed by consortium of major companies with focus on interoperability.

Optimized for wireless LANs. Uses radio frequency signals in unlicensed

2.4GHz band to send and receive data. Uses Direct Sequence Spread Spectrum (DSSS)

RF method. Equipment dynamically selects lower data rates

as RF signal quality decreases: 11, 5.5, 2, 1 Mbits.

Allows roaming among radio access points.

• Standard adopted by Institute of Electrical and Electronics Engineers (IEEE) in September 1999.

IEEE 802.11b Technology

More Specifically

This in itself can form a standalone network as if wired devices were connected to a hub.

The Access Point sits on both the wireless network segment (space) and the wired segment, acting as a bridge from the wireless to the wired segments.

A bridge forwards data packets from one side to the other at the MAC layer.

• 802.11b defines how the RF channel is used, allowing multiple devices to communicate on the channel as if it were a wire.

Flavors of 802.11x

802.11 (1 Mbps) Older standard

802.11b (11 Mbps) Current technology

802.11g (22+ Mbps) Expected future

standard

802.11 Under the Hood(Wireless Ethernet)

Why can’t we use regular Ethernet for wireless? Ethernet: A sees B, B sees C, A sees C Wireless: Hidden node problem A sees B, B sees C, yet A does not see C

AB

C

802.11 Under the Hood(Wireless Ethernet)

Why can’t we use regular Ethernet for wireless? Ethernet: B sees C, C sees D B & C can’t

send together Wireless: B can send to A while C sends to D

A

B C

D

802.11 Under the HoodThe Protocol

Sender A sends Request-to-Send (RTS) Receiver B sends Clear-to-Send (CTS)

Nodes who hear CTS cannot transmit concurrently with A (red region)

Nodes who hear RTS but not CTS can transmit (green region)

Sender A sends data frame Receiver B sends ACK Nodes who hear the ACK can now transmit

RTS

CTS

AB

802.11 Collision Resolution Two senders might send RTS at the

same time Collision will occur corrupting the

data No CTS will follow Senders will time-out waiting for

CTS and retry with exponential backoff

More Wireless Challenges Problem: mobility

Nodes can move and get “out of touch” How to deal with intermittent

connectivity? Solution: hierarchical structure

A set of access points hooks into a wired distribution system

Access points cover the target area Roaming nodes connect to nearest

access point at any given time

Hierarchical Structure: Communication between roaming

nodes is via the distribution system

Distribution System

AP1AP2

AP3

A

BC D

E

F

Choosing the Access PointActive Scanning

Roaming node periodically sends probe frame

All APs within range reply with a probe response frame

Roaming node chooses best AP and replies to it with association request

AP acknowledges request with association response. AP notifies previous AP (if any) of the established association (handover).

Choosing the Access PointPassive Scanning APs advertise themselves periodically

by sending beacon frames Roaming nodes can associate

themselves with an AP by sending the association request

AP acknowledges request as before with an association response

Frame Format

ControlDurationAddr1Addr2Addr3 Addr4Control Data CRC

Distribution System

AP1AP2

AP3

A

BC D

E

F

Frame Type (RTS,CTS,…)ToDSFromDS

UltimateDestination (E)

ImmediateSender (AP3)

IntermediateDestination(AP1)

Source(A)

Physical Properties 802.11

Frequency hopping Direct sequence Diffused infrared

Range from Access Point to Computer

100 Feet

Access Point

Computers

Range = 0 to 700 feet

11 Mbits

5.5Mb2 Mb1 Mb

0 Mb

5.5Mb 11 Mb

Basement

1st

3rd

2nd

Electrical Engineering Building

Network Setup

Ethernet

MT

AP AP AP

MT MT

•Basic Network Setup is Cellular

•Mobile Terminals (MT) connect with Access Points (AP)

•Standard also supports ad-hoc networking where MT’s talk directly to MT’s

IEEE 802.11 Physical Layers

802.11b 802.11aModulation Scheme

DSSS OFDM

Spectrum (GHz) 2.4 – 2.485 5.15-5.35, 5.725-5.825

Data Rate (Mbps) 1 – 11 6 - 54Subchannels 11 overlapping 8 independent

Interference Microwave, Cordless Phones,Bluetooth, HomeRF, Light Bulbs!

HyperLAN II

Availability Today Late August?

Cost $250 AP, $100 PC Card

??? (same)

Media Access Control- Ethernet

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

If media is sensed idle, transmit If media is sensed busy, wait until idle and then

transmit immediately Collisions can occur if more than one user

transmits at the same time If a collision is detected, stop transmitting. Reschedule transmission according to exponential

backoff

Desktop System Desktop System Desktop System

Ethernet

Media Access Control (802.11) Would like to use CSMA

Nice for bursty traffic Make for seamless replacement of wired LANs with

wireless LANS Use CSMA, but can’t use CD

PT/PR ratio is too high Don’t want to waste energy on mobiles

Use Collision Avoidance instead Don’t always start transmitting immediately after

someone else

CSMA/CA Details SIFS (Short Interframe Space) DIFS (Distributed Interframe Space)

Packet A ACK

B CSIFS DIFS

Packet C ACK

SIFS DIFS

Packet B

Scenario:

•B and C want to transmit, but A currently has control of medium

•B randomly selects 7 slots of backoff, C selects 4 slots

•C transmits first, then B

What is HIPERLAN/2? European standard developed by ETSI/BRAN

(European Telecommunications Standards Institute/Broadband Radio Access Networks)

Physical Layer is very similar to 802.11a (OFDM operating in the 5 GHz spectrum)

Standard based on wireless ATM (Asynchronous Transfer Mode)

HIPERLAN/2 MAC

BCH – Miscellaneous header

FCH – Details how the DL and UL phases will be allocated

ACH – Feedback on which resource requests were received

RCH – Random access resource request

Performance Comparison

Conclusions IEEE 802.11 is a widely accepted standard in

the United States for wireless LANs Primarily a “cellular” random access scheme with

provisions for ad hoc networking and contention free access

802.11b products are available now, but better to wait for 802.11a products later this year

HIPERLAN/2 is being pushed in Europe Wireless ATM solution for real-time traffic Standard reflects the network topology

There is an effort to agree on one world-wide standard, keep your fingers crossed