Mobile Communications Chapter 7: Wireless LANs · IEEE 802.11a, b, g, e HIPERLAN Bluetooth ... Comparison: infrastructure vs. ad-hoc networks ... •HiperLAN2 •Bluetooth

University of Karlsruhe

Institute of Telematics

Mobile Communications

Chapter 7: Wireless LANs

Jochen H. Schiller

1999 7.1

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.1



Characteristics

IEEE 802.11

PHY

MAC

Roaming

IEEE 802.11a, b, g, e

HIPERLAN

Bluetooth

Comparisons


Characteristics of Wireless LANs (WLANs)

Advantages

very flexible within the reception area

Ad-hoc networks without previous planning possible

(almost) no wiring difficulties (e.g. historic buildings, firewalls)

more robust against disasters like, e.g., earthquakes, fire

Disadvantages

typically low bandwidth compared to wired networks

(50-300 Mbit/s user data)

many proprietary solutions, especially for higher bit-rates, standards take

their time (e.g. IEEE 802.11). See IEEE802.11g, IEEE802.11n!

products have to follow many national restrictions if working wireless, it

takes a very long time to establish global solutions like, e.g., IMT-2000





Jochen H. Schiller

1999 7.2


Design goals for wireless LANs

global, seamless operation

low power for battery use

no special permissions or licenses needed to use the LAN

robust transmission technology – other electrical devices can interfere!

simplified spontaneous cooperation at meetings, i.e. simple set up

easy to use for everyone, simple management

security (no one should be able to read my data), privacy (no one should

be able to collect user profiles), safety (low radiation)


Comparison: infrared vs. radio transmission

Infrared

uses IR diodes, diffuse light,

multiple reflections (walls,

furniture etc.)

Advantages

simple, cheap, available in

many mobile devices

no licenses needed

simple shielding possible

Disadvantages

interference by sunlight, heat

sources etc.

many things shield or absorb IR

light

low bandwidth

Example

IrDA (Infrared Data Association)

interface available everywhere

Radio

typically using the license free

ISM band at 2.4 GHz

Advantages

experience from wireless WAN

and mobile phones can be used

coverage of larger areas

possible (radio can penetrate

walls, furniture etc.)

Disadvantages

very limited license free

frequency bands

shielding more difficult,

interference with other electrical

devices

Example

IEEE802.11, HIPERLAN,

Bluetooth





Jochen H. Schiller

1999 7.3


Comparison: infrastructure vs. ad-hoc networks

infrastructure

network

APAP

AP

wired network

AP: Access Point

Infrastructure networks:

• provide access to other networks

• typically communications between the wireless node and the

access point, but not directly between the wireless nodes.

• the access points controls medium access, but also act as a

bridge to other wireless or wired networks

• in the figure three WLANs with different coverage

areas are connected

• most of the network functionality lies within

the access points, whereas the wireless clients

can remain quite simple.

• the MAC can be centralised to the access point

or be distributed oven the wireless clients.

• these wireless networks do rely on the access

points.


Comparison: infrastructure vs. ad-hoc networks

ad-hoc network

Ad-hoc networks:

• no need for any infrastructure to work.

• each node can communicate directly with other nodes

• no access point controlling media access is necessary

• nodes within an ad-hoc network can communicate

directly or via other nodes

• the complexity of each node is higher because each node has

- implement MAC-mechanisms

- handle hidden or exposed terminal problem

- perhaps priority mechanisms to provide a certain QoS





Jochen H. Schiller

1999 7.4


Ad Hoc Networking

• Self-organizing and adaptive

• Can be de-formed on the fly

• No system administration

• Most often battery operated devices

• Multiple hops may be needed, i.e.

each node may act as a router


WLAN

The IEEE 802.11 family

IEEE 802.11b 2.4 GHz, 11 Mbps

IEEE 802.11a 5 GHz, 54 Mbps

IEEE 802.11g 2.4 GHz, 54 Mbps

IEEE 802.11n 2.4 GHz 200 Mbps

In this chapter we will examine three WLANs

• IEEE 802.11

• HiperLAN2

• Bluetooth

The first two are typically infrastructure-based networks, which additionally

support ad-hoc networking, whereas Bluetooth is a typical ad-hoc network.





Jochen H. Schiller

1999 7.5


802.11 - Architecture of an infrastructure network

Station (STA)

terminal with access mechanisms to the

wireless medium and radio contact to the

access point

Basic Service Set (BSS)

group of stations using the same radio

frequency

Access Point

station integrated into the wireless LAN and

the distribution system

Portal

bridge to other (wired) networks

Distribution System

interconnection network to form one logical

network (ESS: Extended Service Set) based

on several BSS.

The architecture of the distributing

system is not specified furher in the

IEEE 802.11.

Distribution System

Portal

802.x LAN

Access

Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access

Point

STA1

STA2 STA3

ESS


802.11 - Architecture of an infrastructure network

Stations can select an AP and

associate with it.

The APs support roaming, (i.e.

changing access points)

The distribution system handles data

transfer between the different

APs.

APs provide synchronization within a

BSS (Basic Service Set)

APs support power management

APs can control medium access to

support time-bounded services.

Distribution System

Portal

802.x LAN

Access

Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access

Point

STA1

STA2 STA3

ESS





Jochen H. Schiller

1999 7.6


802.11 - Architecture of an ad-hoc network

Direct communication within a limited

range

Station (STA):

terminal with access mechanisms to

the wireless medium

Independent Basic Service Set

(IBSS):

group of stations using the same

radio frequency

Here two IBSS are shown. They

could be separated by either

- distance

- using different frequencies or

codes

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3


IEEE standard 802.11 – Protocol architecture

mobile terminal

access point

fixed

terminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructure

network

LLC LLC

The basic IEEE 802.11 standard!!





Jochen H. Schiller

1999 7.7


802.11 - Layers and functions

The standard also specifies management

layers and the station management:

MAC Management

synchronization, roaming, MIB

(management information base), power

management

PHY Management

channel selection, MIB

Station Management

coordination of all management functions

MAC

access mechanisms, fragmentation, encryption

PLCP Physical Layer Convergence Protocol

Provides a carrier sense signal, called clear channel assessment signal

PMD Physical Medium Dependent

modulation, encoding /decoding

PMD

PLCP

MAC

LLC

MAC Management

PHY Management

PH

YD

LC

Sta

tio

n M

anagem

ent


802.11 - Physical layer Note: Original 802.11!

3 versions: 2 radio (typ. 2.4 GHz), 1 IR

data rates 1 or 2 Mbit/s

FHSS (Frequency Hopping Spread Spectrum)

spreading, despreading, signal strength, typ. 1 Mbit/s

min. 2.5 frequency hops/s (USA), two-level GFSK modulation

DSSS (Direct Sequence Spread Spectrum)

DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying),

DQPSK for 2 Mbit/s (Differential Quadrature PSK)

preamble and header of a frame is always transmitted with 1 Mbps, rest

of transmission 1 or 2 Mbit/s

chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)

max. radiated power 1 W (USA), 100 mW (EU), min. 1mW

Infrared

850-950 nm, diffuse light, typ. 10 m range

carrier detection, energy detection, synchonization





Jochen H. Schiller

1999 7.8


FHSS (Frequency Hop Spread Spectrum) PHY packet format

synchronization SFD PLW PSF HEC payload

PLCP preamble PLCP header

80 16 12 4 16 variable

Synchronization

010101... pattern – used for synchronization and signal detection

by the CCA (Clear Channel Assessment)

SFD (Start Frame Delimiter)

0000110010111101 start pattern – indicates the start of the frame

PLW (PLCP_PDU Length Word)

length of payload in bytes incl. 32 bit CRC of payload, PLW < 4096

PSF (PLCP Signaling Field)

data of payload (1 or 2 Mbit/s)

HEC (Header Error Check)

CRC with x16+x12+x5+1

PLCP: Physical Layer

Convergence Protocol

PMD

PLCP

MAC

LLC

PH

YD

LC


DSSS PHY packet format (More important!!!)

synchronization SFD signal service HEC payload


128 16 8 8 16 variable bits

length

16

Synchronization

synch., gain setting, energy detection, frequency offset compensation

SFD (Start Frame Delimiter)

1111001110100000 – indicates the start of a frame

Signal

data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)

Service Length

future use, 00: 802.11 compliant length of the payload

HEC (Header Error Check)

protection of signal, service and length, x16+x12+x5+1





Jochen H. Schiller

1999 7.9


802.11 - MAC layer I – DFWMAC

Distributed Foundation Wireless Medium Access Control

Traffic services

Asynchronous Data Service (mandatory)

exchange of data packets based on “best-effort”

support of broadcast and multicast

Time-Bounded Service (optional)

implemented using PCF (Point Coordination Function)

Access methods

DFWMAC-DCF CSMA/CA (mandatory) (DCF: distributed coordination function)*

collision avoidance via randomized „back-off“ mechanism

ACK packet for acknowledgements (not for broadcasts)

DFWMAC-DCF w/ RTS/CTS (optional)

Distributed Foundation Wireless MAC

avoids hidden terminal problem

DFWMAC- PCF (optional) (PCF: point coordination function)

access point polls terminals according to a list

PMD

PLCP

MAC

LLC

PH

YD

LC


802.11 - MAC layer II – General!

Priorities

defined through different inter frame spaces (IFS)

no guaranteed hard priorities

SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response

PIFS (PCF, Point Coordination Function IFS)

medium priority, for time-bounded service using PCF

DIFS (DCF, Distributed Coordination Function IFS)

lowest priority, for asynchronous data service

t

medium busySIFS

PIFS

DIFSDIFS

next framecontention

direct access if

medium is free DIFS





Jochen H. Schiller

1999 7.10


DFWMAC-DCF CSMA/CA (DCF: distributed coordination function)

station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)

To provide fainess an backoff timer is added!

if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

t

medium busy

DIFSDIFS

next frame

contention window

(randomized back-off

mechanism)

slot time

direct access if

medium is free DIFS

802.11 - CSMA/CA access method – the first


802.11 - CSMA/CA access method – the first

Sending unicast packets

station has to wait for DIFS before sending data

receivers acknowledge at once (after waiting for SIFS) if the packet was

received correctly (CRC)

automatic retransmission of data packets in case of transmission errors

t

SIFS

DIFS

data

ACK

waiting time

other

stations

receiver

senderdata

DIFS

contention





Jochen H. Schiller

1999 7.11


802.11 - competing stations - simple version

t

busy

boe

station1

station2

station3

station4

station5

packet arrival at MAC

DIFS

boe

boe

boe

busy

elapsed backoff time

borresidual backoff time

busy medium not idle (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFS

boe busy

boe

boe

bor

bor


802.11 – DFWMAC-DCF with RTS/CTS extension

To avoid the hidden terminal problem!!

Sending unicast packets

station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium)

acknowledgement via CTS after SIFS by receiver (if ready to receive)

sender can now send data at once, acknowledgement via ACK

other stations store medium reservations distributed via RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

other

stations

receiver

senderdata

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV: Net Allocation Vector

The second!





Jochen H. Schiller

1999 7.12


Fragmentation

t

SIFS

DIFS

data

ACK1

other

stations

receiver

senderfrag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

In the frags, the control field gives information to other stations of how

to set their NAV.

If a node senses the wireless channel to be very disturbed, the node may

choose to fragment its sending frame into small sub frames (frags).


DFWMAC-PCF I

PIFS

stations„

NAV

wireless

stations

point

coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

The third!

tstations„

NAV

wireless

stations

point

coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contention

period

contention free period

t2 t3 t4





Jochen H. Schiller

1999 7.13


802.11 - Frame format – MAC frames

Types

control frames, management frames, data frames

Sequence numbers

important against duplicated frames due to lost ACKs

Addresses

receiver, transmitter (physical), BSS identifier, sender (logical)

Miscellaneous

sending time, checksum, frame control, data

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

Control

Address

4Data CRC

2 2 6 6 6 62 40-2312bytes

Protocol

versionType Subtype

To

DS

More

FragRetry

Power

Mgmt

More

DataWEP

2 2 4 1

From

DS

1

Order

bits 1 1 1 1 1 1


802.11 - Frame format

Frame control: see next slide

Duration/ID: indicating the time the medium is occupied (in micro s)

Address 1 to 4: IEEE 802 MAC addresses (48 bits each), see coming slide

Sequence ctrl: sequence number to filter duplicates

Data: The MAC frame contains arbitrary data (max 2312 bytes),

which is transferred transparently from sender to receivers

CRC: 32 bit checksum is used to protect the frame as is common

practice in all 802.x networks

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

Control

Address

4Data CRC

2 2 6 6 6 62 40-2312bytes

Protocol

versionType Subtype

To

DS

More

FragRetry

Power

Mgmt

More

DataWEP

2 2 4 1

From

DS

1

Order

bits 1 1 1 1 1 1





Jochen H. Schiller

1999 7.14


802.11 - Frame format

Protocol version: the current protocol version, fixed to 0 now. If major future

revisions, this value will be increased

Type: type of frame: management, control, data

Subtype: several subtypes exist, like for different control frames

To DS/From DS: see coming slides

More Frag If more fragments to follow

Retry: if the current frame is a retransmission the bit is set to 1

Power man: the mode of the station after a successful transmission of a frame, set to 1 if

the station goes into power-save mode

More Data: if the station has more data to send

WEP: Wireless equivalent privacy. Security

Order: if this bit is set to 1 the received frames must be processed in order

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

Control

Address

4Data CRC

2 2 6 6 6 62 40-2312bytes

Protocol

versionType Subtype

To

DS

More

FragRetry

Power

Mgmt

More

DataWEP

2 2 4 1

From

DS

1

Order

bits 1 1 1 1 1 1


MAC address format

scenario to DS fromDS

address 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -

infrastructurenetwork, from AP

0 1 DA BSSID SA -

infrastructurenetwork, to AP

1 0 BSSID SA DA -

infrastructurenetwork, within DS

1 1 RA TA DA SA

DS: Distribution System

AP: Access Point

DA: Destination Address

SA: Source Address

BSSID: Basic Service Set Identifier

RA: Receiver Address (AP)

TA: Transmitter Address (AP)

MAC frames can be transmitted

between mobile stations

between mobile stations and an access point

between access points over a DS

2 bits within the Frame Control Field “to DS” and “from DS”,

differentiate these cases and control the meaning of the four

addresses used.





Jochen H. Schiller

1999 7.15


Special Frames: ACK, RTS, CTS

Acknowledgement

Request To Send

Clear To Send

Frame

ControlDuration

Receiver

Address

Transmitter

AddressCRC

2 2 6 6 4bytes

Frame

ControlDuration

Receiver

AddressCRC

2 2 6 4bytes

Frame

ControlDuration

Receiver

AddressCRC

2 2 6 4bytes

ACK

RTS

CTS


802.11 - MAC management

MAC management plays a central role in an IEEE 802.11 station.

It more or less controls all functions related to system integration!

Synchronization

try to find a LAN, try to stay within a LAN

timer etc.

Power management

sleep-mode without missing a message

periodic sleep, frame buffering, traffic measurements

Association/Reassociation

integration into a LAN

roaming, i.e. change networks by changing access points

scanning, i.e. active search for a network

MIB - Management Information Base

All parameters representing the current state of a wireless station and an

access point are stored within a MIB for internal and external access

Managing, read, write





Jochen H. Schiller

1999 7.16


Synchronization using a Beacon (infrastructure)

beacon interval

tmedium

access

pointbusy

B

busy busy busy

B B B

value of the timestamp B beacon frame

Each node of an 802.11 network maintains an internal clock!

To synchronize the clocks of all nodes, IEEE 802.11 specifies a

Timer synchronization function (TSF)

Within a BSS (basic server set), timing is conveyed by the (quasi)periodic transmission of a beacon.

A beacon contains a timestamp and other management information used for power management

and roaming (e.g. identification of the BSS).

Within infrastructure-base networks, the access point performs synch by transmitting beacons


Synchronization using a Beacon (ad-hoc)

tmedium

station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

station2

B2 B2

random delay

For ad-hoc networks, the situation is slightly more complicated as they

do not have an access point for beacon transmission.

In this case, each node maintains its own synchronization timer and starts transmission

of a beacon frame after the beacon interval.

However, the standard random backoff algorithm is also applied to the beacon frames so

only one beacon wins.

All other stations now adjust their internal clocks according to the received beacon and

suppress their beacons for this cycle.





Jochen H. Schiller

1999 7.17


Power management

Idea: switch the transceiver off if not needed

States of a station: sleep and awake

Timing Synchronization Function (TSF)

stations wake up at the same time

Infrastructure

Traffic Indication Map (TIM), sent with the beacons

list of unicast receivers transmitted by AP

Delivery Traffic Indication Map (DTIM)

list of broadcast/multicast receivers transmitted by AP

Ad-hoc

Ad-hoc Traffic Indication Map (ATIM)

announcement of receivers by stations buffering frames

more complicated - no central AP

collision of ATIMs possible (scalability?)


Power saving with wake-up patterns (infrastructure)

TIM interval

t

medium

access

pointbusy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

p PS poll

p

d

d

ddata transmission

to/from the station

Example: one access point and one station

The AP transmits a beacon frame each beacon interval.

This interval is equal to the TIM interval.

The AP maintains a delivery traffic indication map (DTIM) interval.





Jochen H. Schiller

1999 7.18


Power saving with wake-up patterns (ad-hoc)

awake

A transmit ATIM D transmit data

t

station1

B1 B1

B beacon frame

station2

B2 B2

random delay

A

a

D

d

ATIM

window beacon interval

a acknowledge ATIM d acknowledge data


802.11 - Roaming

No or bad connection? Then perform:

Scanning

scan the environment, i.e., listen into the medium for beacon signals or

send probes into the medium and wait for an answer

Reassociation Request

station sends a request to one or several AP(s)

Reassociation Response

success: AP has answered, station can now participate

failure: continue scanning

AP accepts Reassociation Request

signal the new station to the distribution system

the distribution system updates its data base (i.e., location information)

typically, the distribution system now informs the old AP so it can release

resources





Jochen H. Schiller

1999 7.19

IEEE 802.11b (IEEE 1999)

Supplement to the original standard (Higher speed physical layer

extension in the 2.4 GHz band)

A new physical layer (JUST A NEW PHYSICAL LAYER!)

All the MAC schemes, management procedures, etc are still used.

Depending on the current interface and the distance between sender

and reciever 802.11 systems offer 11, 5.5, 2 or 1 Mbps

Next slide shows the mandatory packet format for 802.11b which is

similar to the original. – There are others as well. However all control

packets follow the mandatory for all stations to understand.

Operates on certain frequencies at the 2.4 GHz ISM band.

These frequences depend on national regulations.



IEEE 802.11b – PHY frame formats




length

16

192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

Long PLCP PPDU format (PLCP: Physical Layer Convergence Protocol)

The format is similar to the original 802.11 format.

One difference is the rate encoded in the signal field in multiples of 100 kbps.

Signal+Service = 16 bits! =>

0x0A: 1 Mbps; 0x14: 2 Mbps; 0x37: 5.5 Mbps; 0x6E: 11 Mbps

Note that the preamble and the header are always transmitted at 1 Mbps!





Jochen H. Schiller

1999 7.20


IEEE 802.11b – PHY frame formats




length

16

192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

short synch. SFD signal service HEC payload

PLCP preamble

(1 Mbit/s, DBPSK)

PLCP header

(2 Mbit/s, DQPSK)


length

16

96 µs 2, 5.5 or 11 Mbit/s

Long PLCP PPDU format (PLCP: Physical Layer Convergence Protocol)

Short PLCP PPDU format (optional)

Channel plan for IEEE 802.11b


Channel Frequency (MHz) US/Canada Europé Japan

1 2412 x x x

2 2417 x x x

3 2422 x x x

4 2427 x x x

5 2432 x x x

6 2437 x x x

7 2442 x x x

8 2447 x x x

9 2452 x x x

10 2457 x x x

11 2462 x x x

12 2467 - x x

13 2472 - x x

14 2484 - - x

Altogether 14 channels have been defined as the table shows.

For each channel a center frequency is given.

Depending on national restrictions 11 (US/Canada), 13 (Europe) or

14 channels (Japan) can be used.





Jochen H. Schiller

1999 7.21


Channel selection (non-overlapping)

2400 [MHz]2412 2483.52442 2472

channel 1 channel 7 channel 13

Europe (ETSI)

US (FCC)/Canada (IC)

2400 [MHz]2412 2483.52437 2462

channel 1 channel 6 channel 11

22 MHz

22 MHz

Non-overlapping usage of channels for an IEEE 802.11b installation with minimum interference.

The spacing between center frequencies should be at least 25 MHz.

The occupied bandwidth of the main lobe of the signal is 22 MHz.

Users can install overlapping cells for WLANs

using three non-overlapping chanells.


WLAN: IEEE 802.11b

Data rate

1, 2, 5.5, 11 Mbit/s, depending on

SNR

User data rate max. approx. 6

Mbit/s

Transmission range

300m outdoor, 30m indoor

Max. data rate ~10m indoor

Frequency

Free 2.4 GHz ISM-band

Security

Limited, WEP insecure,

Availability

Many products, many vendors

Quality of Service

Typ. Best effort, no guarantees (unless polling is used, limited support in products)

Special Advantages/Disadvantages

Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system

Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only





Jochen H. Schiller

1999 7.22

IEEE 802.11a

Initially aimed at the US 5 GHz U-NII (Unlicenced National Information

Infrastucture) bands.

Offering up to 56 Mbps using OFDM (IEEE, 1999).

The first products were offered in 2001, but harmonization between

IEEE and ETSI took some time.

ETSI (in Europe) defines different frequecy bands: 5.15-5.35 and 5.47-

5.725 GHz.

ETSI also requires: Dynamic frequency selection (DFS) and transmitt

power control (TPC).

DFC and TPC are not necessary, if the transmission power stays

below 50 mW

The physical layer of 802.11a and the ETSI standard HiperLAN2 has

been jointly developed, so both physical layers are almost identical.

However, 802.11a products were available first .


IEEE 802.11a

IEEE 802.11a uses the same MAC layer as all IEEE 802.11 physical

layers do.

To be able to offer data rates up to 54 Mbps IEEE 802.11a uses many

different technologies.

The system uses OFDM (52 subcarriers) that are modulated using

BPSK, QPSK, 16-QAM, or 64-QAM.

To mitigate transmission errors, FEC is applied using coding rates of

1/2, 2/3 or 3/4

Similar to 802.11b several operating channels have been standardized

to minimize interference.

Due to the nature of OFDM, the PDU on the physical layer of IEEE

802.11a looks quite different from 802.11b or the original 802.11

physical layers.






Jochen H. Schiller

1999 7.23


Operating channels for 802.11a / US U-NII

5150 [MHz]5180 53505200

36 44

16.6 MHz

Center frequency =

5000 + 5*channel number [MHz]

Channel spacing is 20 MHz

The occupied bandwidth is 16.6 MHz

channel40 48 52 56 60 64

149 153 157 161

5220 5240 5260 5280 5300 5320

5725 [MHz]5745 58255765

16.6 MHz

channel

5785 5805


IEEE 802.11a – PHY frame format

rate service payload

variable bits

6 Mbit/s

PLCP preamble signal data

OFDM symbols12 1 variable

reserved length tailparity tail pad

616611214 variable

6, 9, 12, 18, 24, 36, 48, 54 Mbit/s

PLCP header

IEEE 802.11a uses OFDM (orthogonal frequency division multiplex) coding (2.6.6).

Thus the PDU (Packet data unit) on the physical layer looks quite different from

802.11b or the original 802.11 physical layers.





Jochen H. Schiller

1999 7.24


WLAN: IEEE 802.11a

Data rate

6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR

6, 12, 24 Mbit/s mandatory

Transmission range

100m outdoor, 10m indoor 54 Mbit/s up to 5 m,

48 up to 12 m,

36 up to 25 m,

24 up to 30m,

18 up to 40 m,

12 up to 60 m

Frequency

Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band

Security

Limited, WEP insecure,

Quality of Service

Typ. best effort, no guarantees (same as

all 802.11 products)

Special Advantages/Disadvantages

Advantage: fits into 802.x standards, free

ISM-band, available, simple system,

uses less crowded 5 GHz band

Disadvantage: stronger shading due to

higher frequency, no QoS

IEEE 802.11n


• OFDM

• MIMO

• 40 MHz channels

• 2.4 GHz (in fact also in the 5 GHz band)

• up to 600 Mbps





Jochen H. Schiller

1999 7.25


WLAN: IEEE 802.11 – some examples

802.11-2007: A new release of the standard that includes amendments

a, b, d, e, g, h, i & j. (July 2007)

802.11e: MAC Enhancements – QoS –

Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol.

802.11g: Data Rates at 2.4 GHz; 54 Mbit/s, OFDM, (backwards compatible with b) (2003)

802.11s: Mesh Networking, Extended Service Set (ESS) (June 2011)

802.11ad: Very High Throughput 60 GHz (~Dec 2012)

Ad Hoc Networks


• Power is an issue

• All stations are mobile





Jochen H. Schiller

1999 7.26

FL, UniK 4290: Introduction, 28.01.0851

Broadband Internet access via mesh networks

• Power is NOT an issue

• Stations are NOT mobile

FL, UniK 4290: Introduction, 28.01.0852

Mesh networking with Ad hoc networks





Jochen H. Schiller

1999 7.27

Sensor networks


• Power is an issue

• Stations are NOT mobile


End

Documents

Mobile Communications Chapter 7: Wireless LANs · IEEE 802.11a, b, g, e HIPERLAN Bluetooth ... Comparison: infrastructure vs. ad-hoc networks ... •HiperLAN2 •Bluetooth