Kommunikationsnetz Franken e.V.
IEEE802.11
Wireless LAN
The broadband wireless Internet
Maximilian Riegel
wlan-tutorial.ppt-1 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
WLAN Dream Finally Seems to Happen...

Recently lots of serious WLAN activities have been announced
Big players have invested in WLAN (Cisco, Intel, Nokia)
 Integrated WLAN solutions appearing (Apple, IBM, Dell, ...)


Wireless IP solutions have lots of momentum!


People desire wireless IP terminals and access devices
WLAN offers a good mobile solution for indoor IP access
Added value for the user - Flexibility, user mobility
 Added value for ISP - solution for public
high speed IP access


WLAN standards are converging IEEE 802.11b rules

Interoperability has been the main obstacle
Wireless LAN
wlan-tutorial.ppt-2 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
The Wireless LAN market has taken off...

In the past:

Now:
Deployment of WLAN for vertical markets - moderate growth
Ubiquitous broadband wireless Internet access - the killer app
 IEEE802.11b 11 Mbps Wireless LAN everywhere
Office
Railway
Station
Airport
Public
W-LAN
Hospital
Congress hall,
Hotel
Semi-public
W-LAN
Office
Corporate
W-LAN
Plant
Remote
Access
Home
W-LAN
Campus
Wireless LAN
wlan-tutorial.ppt-3 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Wireless LAN IEEE802.11 Basic Architecture
local distribution network
CPE + NIC
Netscape
http
tcp
ip
802.2
ppp
Bluetooth
802.11
802.2
802.11 802.3
‚access router‘
ip
802.2 802.2
802.3 802.3
internet
apache
http
tcp
ip
802.2
ppp
Bluetooth
802.3
IEEE802.11
Wireless LAN
wlan-tutorial.ppt-4 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
What is unique about wireless?

Difficult media





Full connectivity cannot be assumed


interference and noise
quality varies over space and time
shared with “unwanted” 802.11 devices
shared with non-802 devices (unlicensed spectrum, microwave
ovens)
“hidden node” problem
Mobility
variation in link reliability
 battery usage: requires power management
 want “seamless” connections


Security
no physical boundaries
 overlapping LANs


Multiple international regulatory requirements
Wireless LAN
wlan-tutorial.ppt-5 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Industrial, Scientific and Medical (ISM) Bands
902 to 928MHz
2.400 to 2.4835GHz
26MHz
83.5MHz
5.725 to 5.850GHz
125MHz
FREQUENCY (GHz)
1
Low bandwidth
 Polluted by cellular
and cordless

Wireless LAN
2
3
4
Relatively clean spectrum
 DS radios good at rejecting
microwave interference
 Can fit several (11)
WLAN Channels

5
6
A band for the future
 No cost effective
technology yet

wlan-tutorial.ppt-6 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Wireless IEEE802.11 Standard
802.11 Standard supports 3 Physical Layers

Frequency hopping
Limited to 2Mbps data rate
 Requires more network overhead
 Has higher power density that can
generate interference


Direct sequence
Only PHY to support the 11Mbps data rate
 Low power density to minimize interference


Infrared

Range limited
Approved June 1997
802.11b approved September 1999
Wireless LAN
wlan-tutorial.ppt-7 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
IEEE802.11 Ad Hoc Mode
Peer-to-Peer Network

Independent networking




Use Distributed Coordination Function (DCF)
Forms a Basic Service Set (BSS)
Direct communication between stations
Coverage area limited by the range of individual stations
Wireless LAN
wlan-tutorial.ppt-11 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
IEEE802.11 Infrastructure Mode
Wired Network
BSS-A




Server
BSS-B
Access Points (AP) and stations (STA)
BSS (Basic Service Set): a set of stations controlled by a single
coordination function
Distribution system interconnects multiple cells via access points
to form a single network
Extends wireless coverage area and enables roaming
Wireless LAN
wlan-tutorial.ppt-12 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
IEEE 802.11 Network elements

Distribution system

Used to interconnect wireless cells
multiple BSS connected together form an ESS, Extended Service Set
 Allows mobile stations to access fixed resources


Not part of 802.11 standard
could be bridged IEEE LANs, wireless, other networks …
 Distribution System Services are defined


Access Points
Stations select an AP and “associate” with it
 Support roaming
 Provide other functions

time synchronization (beaconing)
 power management support
 point coordination function


Traffic typically (but not always) flows through AP

direct communication possible
Wireless LAN
wlan-tutorial.ppt-13 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
MAC Functionality

Independent and Infrastructure configuration support

Each BSS has a unique 48 bit address
Each ESS has a variable length address




MAC-level acknowledgment
allows for RTS/CTS exchanges (hidden node protection)
MSDU fragmentation
“Point Coordination” option (AP polling)


station scans for APs, association handshakes
Roaming support within an ESS


stations may power themselves down
AP buffering, distributed approach for IBSS


Optional support of “Wired Equivalent Privacy” (WEP)
Authentication handshakes defined





CSMA with collision avoidance
Association and Reassociation
Power management support
Authentication and privacy
Wireless LAN
wlan-tutorial.ppt-14 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
CSMA/CA Explained
Free access when medium
is free longer than DIFS
IFS: Inter Frame Space
DIFS
Contention Window
PIFS
DIFS
Busy Medium
SIFS
Backoff-Window
Next Frame
Slot time
Defer Access

Select Slot and Decrement Backoff as long as medium is idle.
Reduce collision probability where mostly needed.
Stations are waiting for medium to become free.
 Select Random Backoff after a Defer, resolving contention to avoid
collisions.


Efficient Backoff algorithm stable at high loads.
Exponential Backoff window increases for retransmissions.
 Backoff timer elapses only when medium is idle.


Implement different fixed priority levels
Wireless LAN
wlan-tutorial.ppt-17 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Carrier Sense Multiple Access
Collision Avoidance (CSMA/CA)
Station 1
Tx Data to STA 2
Short interval ensures ACK is sent
while other stations wait longer
ACK to STA1
Short deferral
Station 2
Rx data from STA 1
Distributed inter-frame deferral
Station 3 Detects channel busy
Detects channel busy
STA 3’s back-off is shorter than
STA 4’s therefore it begins
transmission first
Distributed interframe deferral
Random back-off
Tx Data
Distributed inter-frame deferral
Station 4 Detects channel busy
Wireless LAN
Detects channel busy
Distributed interframe deferral
Random back-off
Detects channel busy
wlan-tutorial.ppt-18 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
CSMA/CA + ACK protocol
DIFS
Data
Src
SIFS
Dest
Ack
Contention Window
DIFS
Other
Next MPDU
Defer Access

Backoff after Defer
Defer access based on Carrier Sense.

CCA from PHY and Virtual Carrier Sense state.

Direct access when medium is sensed free longer then DIFS,
otherwise defer and backoff.

Receiver of directed frames to return an ACK immediately when
CRC correct.

When no ACK received then retransmit frame after a random
backoff (up to maximum limit).
Wireless LAN
wlan-tutorial.ppt-19 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
IEEE802.11 Point Coordination Function (PCF)
CFP repetition interval
Contention Period
Contention Free Period
Access
Point
Beacon
Stations
D1+Poll
D2+Poll
U1+ACK
CF end
U2+ACK

Optional PCF mode provides alternating contention free and
contention operation under the control of the access point

The access point polls stations for data during contention
free period

Network Allocation Vector (NAV) defers the contention traffic
until reset by the last PCF transfer

PCF and DCF networks will defer to each other

PCF improves the quality of service for time bounded data
Wireless LAN
wlan-tutorial.ppt-20 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
“Hidden Node” Provisions
Problem – Stations contending for the medium do not Hear each other
Solution – Optional use of the Duration field in RTS and CTS frames with AP
CTS-Range
STA “B” cannot
receive data
from STA “A”
RTS-Range
STA “B”
Access Point
STA“A”
DIFS
STA A
Data
RTS
CTS
AP
Ack
STA “B” cannot detect carrier from STA “A”
STA B
Wireless LAN
Time period to defer access
is based on duration in CTS
Next MPDU
Back off after defer
wlan-tutorial.ppt-21 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Frame Formats
802.11 MAC Header
Bytes:
2
2
6
6
Frame Duration Addr 1
Control
ID
Bits: 2
Protocol
Version

2
4
Type
SubType
Addr 2
1
To
DS
6
2
6
Sequence Addr 4
Control
Addr 3
0-2312
4
Frame
Body
CRC
1
1
1
1
1
1
1
From
DS
More
Frag
Retry
Pwr
Mgt
More
Data
WEP
Rsvd
MAC Header format differs per Type:
Control Frames (several fields are omitted)
 Management Frames
 Data Frames


Includes Sequence Control Field for filtering of duplicate caused
by ACK mechanism.
Wireless LAN
wlan-tutorial.ppt-22 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Physical Layer Convergence Protocol (PLCP)
PLCP Protocol Data Unit

SYNC

SFD
SIGNAL
SERVICE
LENGTH
CRC




(gain setting, energy detection, antenna selection,
frequency offset compensation)
(Start Frame Delimiter; bit synchronization)
(rate indication; 1, 2, 5.5, 11 Mbit/s)
(reserved for future use)
(number of octets in PSDU)
(CCITT CRC-16, protects signal, service, length field)
Wireless LAN
wlan-tutorial.ppt-24 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Three PHYs

Frequency Hop Spread Spectrum

2.4GHz band, 1 and optional 2Mbps



2GFSK, 4GFSK (Gaussian Frequency Shift Keying)
2.5 hops/sec over 79 1MHz BW channels (North America)
Direct Sequence Spread Spectrum

2.4GHz band, 1 and 2Mbps
DBPSK, DQPSK (Differential Binary/Quadrature Phase Shift Keying)
 11 chip Barker sequence


2.4GHZ band, 5.5 and 11Mbps
CCK
 Complex spread functions


Baseband IR

Diffused infrared, 1 and 2Mbps, 16-PPM and 4-PPM
(Pulse Position Modulation)
Wireless LAN
wlan-tutorial.ppt-25 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Direct Sequence Spread Spectrum
RF Energy is Spread by XOR of Data with PRN Sequence
1
0
Data
1 bit period
Out
11 Bit Barker Code (PRN*)
1011011100010110111000
0100100011110110111000
11 chips
11 chips
1 bit
period
PRN
* PRN: Pseudorandom Number
Signal
Spectrum
Transmitter baseband signal before spreading
Transmitter baseband signal after spreading
Wireless LAN
Receiver baseband signal before matched filter (Correlator)
Receiver baseband signal after matched filter (De-spread)
wlan-tutorial.ppt-26 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
DSSS Transmit Spectrum and Channels
Transmit
Spectrum
Mask
0 dBr
Unfiltered
Sinx/x
-30 dBr
-50 dBr
fc -22 MHz
fc -11 MHz
fc
fc +11 MHz
fc +22 Mhz
Canne l
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Wireless LAN
USA
2412 MHz
2417 MHz
2422 MHz
2427 MHz
2432 MHz
2437 MHz
2442 MHz
2447 MHz
2452 MHz
2457 MHz
2462 MHz
N/A
N/A
N/A
ETSI
2412 MHz
2417 MHz
2422 MHz
2427 MHz
2432 MHz
2437 MHz
2442 MHz
2447 MHz
2452 MHz
2457 MHz
2462 MHz
2467 MHz
2472 MHz
N/A
Japan
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
2484 MHz
wlan-tutorial.ppt-27 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Power Management

Mobile devices are battery powered.


Power Management is important for mobility.
Current LAN protocols assume stations are always ready to
receive.

Idle receive state dominates LAN adapter power consumption over
time.

How can we power off during idle periods, yet maintain an
active session?

802.11 Power Management Protocol:
allows transceiver to be off as much as possible
 is transparent to existing protocols
 is flexible to support different applications


possible to trade off throughput for battery life
Wireless LAN
wlan-tutorial.ppt-31 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Power Management Approach

Allow idle stations to go to sleep


station’s power save mode stored in AP
APs buffer packets for sleeping stations.
AP announces which stations have frames buffered
 Traffic Indication Map (TIM) sent with every Beacon


Power Saving stations wake up periodically


listen for Beacons
TSF assures AP and Power Save stations are synchronized
stations will wake up to hear a Beacon
 TSF timer keeps running when stations are sleeping
 synchronization allows extreme low power operation


Independent BSS also have Power Management

similar in concept, distributed approach
Wireless LAN
wlan-tutorial.ppt-32 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Infrastructure Power Management
TIM-Interval
DTIM interval
Time-axis
TIM Busy Medium DTIM
TIM
AP activity
TIM
TIM
DTIM
Broadcast
Broadcast
PS Station
PS-Poll Tx operation

Broadcast frames are also buffered in AP.
all broadcasts/multicasts are buffered
 broadcasts/multicasts are only sent after Delivery Traffic Indication
Message (DTIM)
 DTIM interval is a multiple of TIM interval


Stations wake up prior to an expected DTIM.

If TIM indicates frame buffered
station sends PS-Poll and stays awake to receive data
 else station sleeps again

Wireless LAN
wlan-tutorial.ppt-33 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Scanning

Scanning required for many functions.




finding and joining a network
finding a new AP while roaming
initializing an Independent BSS (ad hoc) network
802.11 MAC uses a common mechanism for all PHY.
single or multi channel
 passive or active scanning


Passive Scanning


Find networks simply by listening for Beacons
Active Scanning

On each channel


Send a Probe, Wait for a Probe Response
Beacon or Probe Response contains information necessary to
join new network.
Wireless LAN
wlan-tutorial.ppt-34 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Active Scanning Example
Steps to Association:
Station sends Probe.
Access Point A
Access Point C
APs send Probe Response.
Station selects best AP.
Station sends Association
Request to selected AP.
AP sends Association
Response.
Initial connection to an Access Point
- ReAssociation follows a similar process
Wireless LAN
wlan-tutorial.ppt-35 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Roaming
Access Point B
Station 2
Access Point A
Station 5
Station 6
Access Point C
Station 4
Station 7
Station 3
Station 1

Mobile stations may move…
beyond the coverage area of their Access Point
 but within range of another Access Point

Reassociation allows station to continue operation
wlan-tutorial.ppt-36 (2000-11-26)
Wireless LAN
2000 © Maximilian Riegel

Kommunikationsnetz Franken e.V.
Roaming Approach

Station decides that link to its current AP is poor

Station uses scanning function to find another AP

or uses information from previous scans

Station sends Reassociation Request to new AP

If Reassociation Response is successful
then station has roamed to the new AP
 else station scans for another AP


If AP accepts Reassociation Request
AP indicates Reassociation to the Distribution System
 Distribution System information is updated
 normally old AP is notified through Distribution System

Wireless LAN
wlan-tutorial.ppt-37 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Privacy and Access Control

Goal of 802.11 is to provide “Wired Equivalent Privacy” (WEP)


Usable worldwide
802.11 provides for an Authentication mechanism
To aid in access control.
 Has provisions for “OPEN”
, “Shared Key” or proprietary
authentication extensions.


Optional (WEP) Privacy mechanism defined by 802.11.
Limited for Station-to-Station traffic, so not “end to end”
.
 Only implements “Confidentiality” function.
 Uses RC4 algorithm based on:

a 40 bit secret key (No Key distribution standardized)
 and a 24 bit IV that is send with the data.
 includes an ICV to allow integrity check.


Only payload of Data frames are encrypted.

Encryption on per MPDU basis.
Wireless LAN
wlan-tutorial.ppt-38 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
IEEE802.11 Architecture Overview

One MAC supporting multiple PHYs


Two configurations


currently Frequency Hopping, Direct Sequence and Infrared PHYs
“Independent” (ad hoc) and “Infrastructure”
CSMA/CA (collision avoidance) with optional “point coordination”
Connectionless Service




Transfer data on a shared medium without reservation
data comes in bursts
user waits for response, so transmit at highest speed possible
is the same service as used by Internet
Isochronous Service







reserve the medium for a single connection and provide a continues stream of bits,
even when not used
works only when cells (using the same frequencies) are not overlapping.
Robust against noise and interference (ACK)
Hidden Node Problem (RTS/CTS)
Mobility (Hand-over mechanism)
Security (WEP)
Power savings (Sleep intervals)
Wireless LAN
wlan-tutorial.ppt-44 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
IEEE802.11 - Current and future work
TGd
Regulatory updates
TGe
Enhancements of MAC
TGf
Inter Access Point Protocol
MAC
TGg
802.11b >20 Mbit/s Data Rate
2.4 GHz radio
2.4 GHz radio
Direct Sequence
Freq. Hopping
Spread Spectrum Spread Spectrum
1 Mbit/s
2 Mbit/s
2 Mbit/s
1 Mbit/s
InfraRed
1 Mbit/s
2 Mbit/s
2.4 GHz
Higher
data rate
extension
5 GHz High
data rate
extension
802.11b
802.11a
5.5 Mbit/s
11 Mbit/s
6,12,24 Mbit/s
9-54 Mbit/s
Legend: italic (and red) = optional
Wireless LAN
wlan-tutorial.ppt-45 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
WECA

Mission Statement
WECA’s mission is to certify interoperability of Wi-Fi (IEEE 802.11b
High Rate) products and to promote Wi-Fi as the global wireless
LAN standard across all market segments.

Current Activities:
Promote IEEE 802.11b HR technology in enterprise, home, and
education spaces
 One standard ---- everywhere


Consortium of Over 40 companies

Leading vendors




WLAN equipment, PC companies, chip companies, service
Published compliance matrix
Independent test lab (SVNL)
Wi-Fi seal of certified interoperability
Wireless LAN
wlan-tutorial.ppt-48 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
2.4 GHz Wireless LAN Standards Efforts
Data Rate
(Mbps)
10M
By Data Rate and Range
For 2.4GHz WLAN Applications
802.118M
b
6M
4M
2M
HomeRF
802.11
Bluetooth
0
10m
Wireless LAN
Range
30m
60m
100m
(meters)
wlan-tutorial.ppt-49 (2000-11-26)
2000 © Maximilian Riegel
Kommunikationsnetz Franken e.V.
Bluetooth

Backed by cellular industry


Not a network solution






Ericsson, Nokia, Intel, IBM,Toshiba
Simple point-to-point link
Low data rate (sub 1Mbps)
10cm to 10m range
Low power and low cost
Under 802.15 standard
Applications
Wireless desktop (replaces infrared)
 Cell phone, cordless phone, pager
 Internet bridge


For more data: http:// www.bluetooth.com
Wireless LAN
wlan-tutorial.ppt-50 (2000-11-26)
2000 © Maximilian Riegel