10 Wireless LAN.rar (3,116Kb)

  Wireless LANs Wireless LANs

  EPL 657 EPL 657

  Panayiotis Kolios

  Panayiotis Kolios

  Contains slides and ideas from Teknillinen Korkeakoulou, Finland: Wireless personal, local, metropolitan,

WIRELESS LAN (WLAN) WIRELESS LAN (WLAN)

  

  Selected topics

  Selected topics » Introduction Introduction » WLAN aims

  WLAN aims » WLAN characteristics

  WLAN characteristics » WLAN design goals

  WLAN design goals » Infrared vs radio transmission

  Infrared vs radio transmission »

  Infrastructure-based vs ad-hoc networks Infrastructure-based vs ad-hoc networks » WLAN Standards

  WLAN Standards »

  IEEE 802.11

  IEEE 802.11 » WLAN Roaming

  WLAN Roaming » WLAN Security

  WLAN Security » WLAN enhancements WLAN enhancements » WLAN design issues

  WLAN design issues »

  Other technologies (separate slides) Other technologies (separate slides)

  

Why Wireless LANs (WLANs)

Why Wireless LANs (WLANs)

   Mobility (portability)

  Mobility (portability) and Flexibility and Flexibility

  

Places where there is no cabling infrastructure /

  

Places where there is no cabling infrastructure /

Hard to wire areas

  Hard to wire areas 

  Reduced cost of wireless systems Reduced cost of wireless systems

   Improved flexibility of wireless systems

  Improved flexibility of wireless systems 

  Cost Cost

• – Relatively low cost of deployment

  Relatively low cost of deployment

• – Continual drop in price for WLAN equipment

  Continual drop in price for WLAN equipment

  

Wireless LAN Applications

Wireless LAN Applications

   LAN Extension

  LAN Extension 

  Cross building interconnection Cross building interconnection

   Nomadic access

  Nomadic access 

  Ad hoc networks Ad hoc networks

  Vertical Markets Vertical Markets 

  Airport Airport

  Center



  Convention Center

  



Convention

  College campus campus

  



College

  Starbuck Starbuck

  Hotel



  



Hotel

  Stock market



  Factory foor Factory foor

   Stock market

  Warehouse Warehouse

  Retail stores 

   Retail stores

  Ofice workers Ofice workers

  Hospital 

   Hospital

  Home networking networking

   Home

  Miscellaneous Miscellaneous

  

Example WLAN

Example WLAN

deployment - Hotel deployment - Hotel

   Competing Technologies

  Competing Technologies 

  Wired Ethernet (802.3) Wired Ethernet (802.3)

   Phone Line

  Phone Line  xDSL xDSL

   Power Line

  Power Line 

  Proposed: Wireless LAN (802.11) Proposed: Wireless LAN (802.11)

  

Why: Price/Performance and ease of

  

Why: Price/Performance and ease of

deployment deployment

   Current status: almost all major hotel

  Current status: almost all major hotel chains in major (and not so major) chains in major (and not so major) cities cities

  Wireless LANs Wireless LANs

  

Wireless LAN considerations

Wireless LAN considerations

   Throughput

  Throughput 

  Number of nodes Number of nodes

   Connection to backbone

  Connection to backbone 

  Service area Service area

   Battery power consumption

  Battery power consumption 

  Transmission robustness and security Transmission robustness and security

   Collocated network operation

  Collocated network operation 

  License free operation License free operation

   Handoff/roaming

  Handoff/roaming 

  Dynamic configuration Dynamic configuration

  WLANs goal WLANs goal 

  A mature market introducing the flexibility of wireless A mature market introducing the flexibility of wireless access into office, home, or production environments. access into office, home, or production environments.

   Typically restricted in their diameter to buildings, a

  Typically restricted in their diameter to buildings, a campus, single rooms etc. campus, single rooms etc.

   The global goal of WLANs is to

  The global goal of WLANs is to replace office cabling, replace office cabling,

increase flexibility of connection especially for portable

increase flexibility of connection especially for portable

devices and, additionally, to introduce a higher flexibility devices and, additionally, to introduce a higher flexibility for ad hoc communication in, e.g., group meetings for ad hoc communication in, e.g., group meetings .

  .

  WLAN characteristics

WLAN characteristics

   Advantages:

  Advantages:

• – very

  very flexible flexible

within radio coverage

within radio coverage

• – ad-hoc networks

  ad-hoc networks without without previous previous planning planning possible possible

• – wireless networks allow for the

  wireless networks allow for the design design of small, of small, independent devices independent devices

• – more

  more robust robust against disasters (e.g., earthquakes, fire) against disasters (e.g., earthquakes, fire)

  WLAN characteristics WLAN characteristics 

  Disadvantages: Disadvantages:

• – typically
• – 300 Mbit/s) due to limitations in radio transmission,
• – 300 Mbit/s) due to limitations in radio transmission,

  typically lower bandwidth lower bandwidth compared to wired networks (~11 compared to wired networks (~11

  higher higher error rates error rates due to interference, and due to interference, and higher delay/delay higher delay/delay variation variation due to extensive error correction and error detection due to extensive error correction and error detection mechanisms mechanisms

  »

  offer lower QoS offer lower QoS

• – a number of
• – products have to follow many

  a number of proprietary solutions proprietary solutions

  , especially for higher bit- , especially for higher bit- rates, standards take their time (e.g., IEEE 802.11n) rates, standards take their time (e.g., IEEE 802.11n)

  »

  standardized functionality plus many enhanced features

  standardized functionality plus many enhanced features »

  additional features only work in a homogeneous environment (i.e., additional features only work in a homogeneous environment (i.e., when adapters from same vendors used) when adapters from same vendors used)

  products have to follow many national national restrictions restrictions if working if working wireless, it takes a very long time to establish global solutions wireless, it takes a very long time to establish global solutions

  

WLAN design goals

WLAN design goals

   global global

  , , seamless operation seamless operation of WLAN products of WLAN products

   low power low power for battery use (special power saving modes and for battery use (special power saving modes and power management functions) power management functions)

   no special permissions or licenses needed ( no special permissions or licenses needed ( license-free license-free band) band)

   robust robust transmission technology transmission technology

   easy easy to use for everyone, simple management to use for everyone, simple management

   protection of investment protection of investment in wired networks (support the same in wired networks (support the same data types and services) data types and services)

   security security

• – no one should be able to read other’s data,

• – no one should be able to read other’s data,

  privacy privacy

• – –

  no one should be able to collect user profiles, no one should be able to collect user profiles, safety safety

• – low
• – low

  radiation radiation

  

Known problems with WLANs



  Wireless link characteristics: media is error prone and the bit error rate ( BER ) is very high compared to the BER of wired networks.

   Carrier Sensing/collision detection is difficult in wireless networks because a station is incapable of listening to its own transmissions in order to detect a

  (more later) collision .

   The Hidden Terminal problem also decreases the

  (more later) performance of a WLAN .

   Mobility (variation in link reliability, seamless

  (more later) connections required, battery limitations)

  

Wireless Link Characteristics

Wireless Link Characteristics

Differences from wired link ….

  Differences from wired link ….

• – decreased signal strength:
• – interference from other sources:

  decreased signal strength: radio signal attenuates as radio signal attenuates as it propagates through matter (path loss) it propagates through matter (path loss)

  interference from other sources: standardized standardized wireless network frequencies (e.g., 2.4 GHz) shared wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); also devices (e.g. by other devices (e.g., phone); also devices (e.g. motors) interfere as well ( motors) interfere as well ( noise noise

  ) )

• – multipath propagation:
• – shared
• – more difficult

  multipath propagation: radio signal reflects off radio signal reflects off objects, arriving at destination at slightly different objects, arriving at destination at slightly different times ( times ( channel quality varies over time channel quality varies over time

  ) )

  shared with other technologies and spectrum users with other technologies and spectrum users

  more difficult security security

  (shared medium) (shared medium)

  … …

  

. make communication across (even a point to point)

. make communication across (even a point to point)

wireless link much more “difficult” wireless link much more “difficult”

  

Wireless LAN Radio Technology

Wireless LAN Radio Technology

   Infrared (IR) LANs

  Infrared (IR) LANs 

  Spread spectrum LANs Spread spectrum LANs

   Narrow band microwave

  Narrow band microwave 

  Laser beam Laser beam

  ISM frequency bands

  ISM frequency bands

  ISM (Industrial, Scientific and Medical) frequency bands:

• 900 MHz band (902 … 928 MHz)
• 5.8 GHz band (5.725 … 5.850 GHz)

2.4 GHz band (2.4 … 2.4835 GHz)

  Anyone is allowed to use radio equipment for transmitting in these bands (provided specific transmission power limits are not exceeded) without obtaining a license.

   Several WLAN standards:

  Several WLAN standards:

• – IEEE 802.11b
• – The same radio spectrum is used by

  IEEE 802.11b offering 11 Mbit/s at 2.4 GHz offering 11 Mbit/s at 2.4 GHz

  The same radio spectrum is used by Bluetooth Bluetooth

  » A short-range technology to set-up wireless personal area

  A short-range technology to set-up wireless personal area networks with gross data rates less than 1 Mbit/s networks with gross data rates less than 1 Mbit/s

• – IEEE
• – IEEE
• – IEEE

  IEEE 802.11a 802.11a

  , operating at 5 GHz and offering gross , operating at 5 GHz and offering gross data rates of 54 Mbit/s data rates of 54 Mbit/s

  IEEE 802.11g 802.11g offering up to 54 Mbit/s at 2.4 GHz. offering up to 54 Mbit/s at 2.4 GHz.

  IEEE 802.11n 802.11n up and coming standard up to 300 Mbit/s up and coming standard up to 300 Mbit/s

  (two spatial streams; 600 Mbit/s with 4 spatial streams) (two spatial streams; 600 Mbit/s with 4 spatial streams)

  WLAN Standards WLAN Standards

  WLAN Standards WLAN Standards Wireless LAN

2.4 GHz

  5 GHz 802.11 _{(2 Mbps)} 802.11b _{(11 Mbps)} 802.11g _{(22-54 Mbps)} HiSWANa _{(54 Mbps)} 802.11a _{(54 Mbps)} HiperLAN2 _{(54 Mbps)} HomeRF 2.0 _{(10 Mbps)} Bluetooth _{(1 Mbps)} HomeRF 1.0 _{(2 Mbps)} 802.11e _(QoS) 802.11i _(Security) 802.11f _(IAPP) 802.11h _(TPC-DFS)

  802.11n (300Mb/sec) IEEE 802 standardisation framework 802.1 Manage- ment 802.3

  MAC 802.3 PHY 802.5

  MAC 802.5 PHY 802.11

  PHY 802.11a PHY 802.11b

  PHY 802.11g PHY 802.2 Logical Link Control (LLC)

  802.11 Medium Access Control (MAC) CSMA/CD (Ethernet)

CSMA/CA

  Token Ring CSMA/CA (Wireless LAN)

  802.11n (300Mb/sec) IEEE 802 wireless network technology options Network definition

  IEEE standard Known as Wireless personal area

  IEEE 802.15.1 Bluetooth network (WPAN) Low-rate WPAN (LR-

  IEEE 802.15.4 ZigBee WPAN) Wireless local area

  IEEE 802.11 WiFi network (WLAN) Wireless metroplitan

  IEEE 802.16 WiMAX area network (WMAN)

  IEEE 802.11 standard

  IEEE 802.11 standard 

  As the As the standards standards number indicates, this standard belongs number indicates, this standard belongs to the group of to the group of

  802.x 802.x LAN standards

  

LAN standards

.

  .

   This means that the standard specifies the

  This means that the standard specifies the physical and physical and medium access layer medium access layer adapted to the special requirements adapted to the special requirements of wireless LANs, but of wireless LANs, but offers offers the the same interface as the same interface as the others to higher layers others to higher layers to maintain interoperability. to maintain interoperability.

   The primary goal of the standard was the specification of a

  The primary goal of the standard was the specification of a simple and robust WLAN simple and robust WLAN which offers time-bounded and which offers time-bounded and asynchronous services. asynchronous services.

  up to 54 Mbps

   802.11g

• – 2.4-5 GHz range
• – 2.4-5 GHz unlicensed spectrum
• – up to 11 Mbps
• – up to 54 Mbps
• – Benefits from the better

  2.4-5 GHz range

  

IEEE 802.11 Wireless LAN

  

IEEE 802.11 Wireless LAN



  802.11b 802.11b

  2.4-5 GHz unlicensed spectrum

  up to 11 Mbps

  802.11g

• – direct sequence spread spectrum (DSSS) in direct sequence spread spectrum (DSSS) in physical layer
• – 5-6 GHz range

  802.11a

   802.11n:

   all use CSMA/CA for multiple access all use CSMA/CA for multiple access Characteristics of selected wireless

Characteristics of selected wireless

link standards link standards

  Some QoS guarantees can be given Some QoS guarantees can be given only via polling using PCF only via polling using PCF

  providing some QoS providing some QoS »

  MAC enhancements for

  IEEE 802.11e

  IEEE 802.11e

  

  typically 200 Mbps

  2.4-5 GHz range

  multiple antennae

  802.11n: multiple antennae

  » Backward compatible to 802.11b Backward compatible to 802.11b

  5-6 GHz range

  GHz compared to 5 GHz

  GHz compared to 5 GHz

  propagation characteristics at 2.4

  propagation characteristics at 2.4

  Benefits from the better

  physical layer » all hosts use same chipping code all hosts use same chipping code

   802.11a

• – up to 54 Mbps
• – 2.4-5 GHz range
• – typically 200 Mbps

  Depending on the SNR, propagation conditions and distance between sender conditions and distance between sender and receiver, data rates may drop fast and receiver, data rates may drop fast

  2.4 GHz » Depending on the SNR, propagation

  up to 54 Mbps » Shading is much more severe compared to Shading is much more severe compared to

2.4 GHz

• – MAC enhancements for

  802.11n

  200

  802.11a,g

  54

  802.11a,g point-to-point data s)

  802.11b

  5-11

  802.16 (WiMAX) bp M

  3G cellular (

  4 UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO

  te enhanced ra

  802.15

  1

  a at D

  .384

UMTS/WCDMA, CDMA2000

  3G

  2G

  .056

IS-95, CDMA, GSM

  Indoor Outdoor Mid-range Long-range

  10-30m 50-200m

  outdoor outdoor

  200m – 4 Km

  5Km – 20 Km

  Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks wireless networks  Infrastructure networks provide access to other networks.

  Infrastructure networks provide access to other networks.

  

Communication typically takes place only between the

  

Communication typically takes place only between the

wireless nodes and the access point, but not directly wireless nodes and the access point, but not directly between the wireless nodes. between the wireless nodes.

   The access point does not just control medium access, but

  The access point does not just control medium access, but

also acts as a bridge to other wireless or wired networks.

also acts as a bridge to other wireless or wired networks.

  AP AP AP wired network AP: Access Point

  Infrastructure- based wireless networks

  Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks wireless networks 

  

Several wireless networks may form one logical wireless network:

Several wireless networks may form one logical wireless network:

• – The access points together with the fixed network in between can connect

  The access points together with the fixed network in between can connect

  

several wireless networks to form a larger network beyond actual radio

  several wireless networks to form a larger network beyond actual radio coverage. coverage.

   Network functionality lies within the access point (controls network

  Network functionality lies within the access point (controls network flow), whereas the wireless clients can remain quite simple. flow), whereas the wireless clients can remain quite simple.

   Can use different access schemes with or without collision.

  Can use different access schemes with or without collision.

• – Collisions may occur if medium access of the wireless nodes and the access Collisions may occur if medium access of the wireless nodes and the access point is not coordinated.

  point is not coordinated.

  » If only the access point controls medium access, no collisions are possible.

  If only the access point controls medium access, no collisions are possible. _

  Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes) Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes) _ The access point may poll the single wireless nodes to ensure the data rate.

  The access point may poll the single wireless nodes to ensure the data rate.

   Infrastructure-based wireless networks lose some of the

  

Infrastructure-based vs ad-hoc

Infrastructure-based vs ad-hoc

wireless networks wireless networks

  Infrastructure-based wireless networks lose some of the flexibility wireless networks can offer in general: flexibility wireless networks can offer in general:

• – They cannot be used for disaster relief in cases where no

  

They cannot be used for disaster relief in cases where no

infrastructure is left. infrastructure is left.

  

Infrastructure-based vs ad-hoc

Infrastructure-based vs ad-hoc

wireless networks wireless networks Ad-hoc

wireless

networks

   No need of any infrastructure to work

  No need of any infrastructure to work

• – greatest possible flexibility

  greatest possible flexibility 

  Each node communicate with other nodes, so no access Each node communicate with other nodes, so no access point controlling medium access is necessary. point controlling medium access is necessary.

• – The complexity of each node is much higher

  The complexity of each node is much higher

  Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks wireless networks 

  Nodes within an ad-hoc network can only communicate if Nodes within an ad-hoc network can only communicate if they can reach each other physically they can reach each other physically

• – if they are within each other’s radio range

  if they are within each other’s radio range

• – if other nodes can/want to forward the message

  if other nodes can/want to forward the message 

  

IEEE 802.11 WLANs are typically infrastructure-based

  

IEEE 802.11 WLANs are typically infrastructure-based

networks, which additionally support ad-hoc networking

networks, which additionally support ad-hoc networking

   Bluetooth is a typical wireless ad-hoc network

  Bluetooth is a typical wireless ad-hoc network

  Elements of a wireless network Elements of a wireless network

  network infrastructure

  wireless hosts wireless hosts  laptop, PDA, IP phone

  laptop, PDA, IP phone

   run applications

  run applications

   may be stationary (non-

  may be stationary (non-

  mobile) or mobile

  mobile) or mobile

• – wireless does

  wireless does

  not not always

  always

  mean mobility

  mean mobility

  802.11 access points

  sending packets between

  802.11 access points

  e.g., cell towers,

  “area”

  “area”

  wireless host(s) in its

  wireless host(s) in its

  wired network and

  wired network and

  sending packets between

  Elements of a wireless network Elements of a wireless network

  relay - responsible for

   relay - responsible for

  wired network

  wired network

  typically connected to

   typically connected to

  base station base station

  network infrastructure

• – e.g., cell towers,

Elements of a wireless network Elements of a wireless network

  network infrastructure

  wireless link wireless link

   typically used to connect

  typically used to connect

  mobile(s) to base station

  mobile(s) to base station

   also can be used as

  also can be used as

  backbone links

  backbone links

   multiple access protocol

  multiple access protocol

  coordinates link access

  coordinates link access

   various data rates, various data rates, transmission distance

  transmission distance Elements of a wireless network Elements of a wireless network

  network infrastructure

  infrastructure mode infrastructure mode

   base station connects

  base station connects

  mobiles into wired

  mobiles into wired

  network

  network

   handoff: mobile changes

  handoff: mobile changes

  base station

  base station Elements of a wireless network Elements of a wireless network

  Ad hoc mode Ad hoc mode  no base stations

  no base stations

   nodes can only transmit

  nodes can only transmit

  to other nodes within link

  to other nodes within link

  coverage

  coverage

   nodes organize

  nodes organize

  themselves into a

  themselves into a

  network: route among network: route among themselves

  themselves

  Node disconnected from the rest of the ad-hoc network

  

WLAN components

WLAN components

  Figure 2.11

  Figure 2.11

   Photographs of popular 802.11b WLAN equipment. Access Photographs of popular 802.11b WLAN equipment. Access points and a client card are shown on top, and PCMCIA Client card is points and a client card are shown on top, and PCMCIA Client card is

  IEEE 802.11 terminology

  IEEE 802.11 terminology System Architecture of an infrastructure network

  System Architecture of an infrastructure network 

  Basic Service Set ( BSS )

  Basic Service Set ( BSS ) 802.11 LAN

  802.x LAN

• – group of stations using the same

  group of stations using the same

  radio frequency

  radio frequency STA ₁

  

  Access Point ( AP )

  Access Point ( AP ) BSS ¹

• – station integrated into the wireless

  station integrated into the wireless Portal Access

  LAN and the distribution system LAN and the distribution system Point

  

  Station ( STA )

  Station ( STA ) Distribution System

• – terminal with access mechanisms to terminal with access mechanisms to the wireless medium and radio

  the wireless medium and radio Access ESS

  Point

  contact to the access point

  contact to the access point 

  Portal Portal

  BSS ₂

• – bridge to other (wired) networks

  bridge to other (wired) networks 

  Distribution System ( DS )

  Distribution System ( DS ) STA ²

• – STA _{3 interconnection network to form one}

  interconnection network to form one 802.11 LAN logical network logical network

IEEE 802.11 BSS

  

  IEEE 802.11 allows the building of ad hoc

  IEEE 802.11 allows the building of ad hoc

networks between stations, thus forming one or

networks between stations, thus forming one or

more BSSs. more BSSs.

• – In this case, a BSS comprises a group of stations using

  In this case, a BSS comprises a group of stations using the same radio frequency. the same radio frequency.

• – Several BSSs can either be formed via the distance

  Several BSSs can either be formed via the distance between the BSSs or by using different carrier between the BSSs or by using different carrier frequencies. frequencies.

  

Distribution System (DS)

Distribution System (DS)

   Used to interconnect wireless cells

  Used to interconnect wireless cells (multiple BSS to form an ESS)

  (multiple BSS to form an ESS) 

  

Allows multiple mobile stations to access

Allows multiple mobile stations to access

fixed resources fixed resources

   Interconnects 802.11 technology

  Interconnects 802.11 technology

  

Access Points (AP)

Access Points (AP)

   Allows stations to associate with it

  Allows stations to associate with it 

  Supports Point Coordination Function (PCF) Supports Point Coordination Function (PCF)

   Provides management features

  Provides management features

• – Join/Associate with BSS

  Join/Associate with BSS

• – Time synchronisation (beaconing)

  Time synchronisation (beaconing)

• – Power management

  Power management  all traffic flows through APs all traffic flows through APs

   Supports roaming

  Supports roaming

  IEEE standard 802.11

  IEEE standard 802.11 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

  IEEE 802.11 protocol

  IEEE 802.11 protocol 

  Protocol architecture aims Protocol architecture aims

• – Applications should not notice any difference apart

  Applications should not notice any difference apart from the lower bandwidth and perhaps higher access from the lower bandwidth and perhaps higher access time from the wireless LAN. time from the wireless LAN.

  » WLAN behaves like, perhaps a ‘slower’, wired LAN.

  WLAN behaves like, perhaps a ‘slower’, wired LAN.

• – Consequently, the higher layers (application, TCP, IP)

  Consequently, the higher layers (application, TCP, IP) look the same for the wireless node as for the wired look the same for the wireless node as for the wired node. node.

• – The differences are in

  The differences are in physical and link layer physical and link layer

  » different media and access control different media and access control

  IEEE 802.11 protocol

  IEEE 802.11 protocol

• – The

  The physical layer physical layer provides a carrier sense signal, handles provides a carrier sense signal, handles modulation and encoding/decoding of signals. modulation and encoding/decoding of signals.

• – The basic tasks of the MAC-medium

  The basic tasks of the MAC-medium access control access control protocol protocol

comprise medium access, fragmentation of user data, and

comprise medium access, fragmentation of user data, and

encryption. encryption.

   The standard also specifies

  The standard also specifies management layers management layers .

  .

• – The MAC management supports the

  The MAC management supports the association association and re- and re- association of a station to an access point and association of a station to an access point and roaming roaming between between different APs. different APs.

• – Furthermore, it controls

  Furthermore, it controls authentication authentication mechanisms, encryption, mechanisms, encryption, synchronization synchronization of a station with regard to an AP, and of a station with regard to an AP, and power power management management to save battery power. to save battery power.

  IEEE 802.11

  IEEE 802.11 

  Physical layer Physical layer

• – Includes the provision of the

  Includes the provision of the Clear Channel Assessment-CCA

  Clear Channel Assessment-CCA signal (energy detection). signal (energy detection).

• – This signal is needed for the MAC mechanisms controlling

  This signal is needed for the MAC mechanisms controlling medium access and indicates if the medium is currently idle. medium access and indicates if the medium is currently idle.

• – A number of physical channels

  A number of physical channels Logical Link Control (LLC)

  802.11 802.11 802.11

802.11a

802.11b 802.11g Media Access Control (MAC)

  

Physical layer

Physical layer

  Wireless Transmission Infrared (IR)

  Radio Frequency (RF) Spread Spectrum

  Frequency Hopping Direct Sequence

  Orthogonal Frequency Division Multiplexing

  

Infrared vs radio transmission

Infrared vs radio transmission

_ Radio typically using the license free frequency

  Infrared light Infrared light band at 2.4 GHz

  

  uses IR diodes, diffuse light reflected

  uses IR diodes, diffuse light reflected 

  at walls, furniture etc, or directed light

  at walls, furniture etc, or directed light Advantages

  if a LOS exists btn sender and receiver

  if a LOS exists btn sender and receiver

• – experience from wireless WAN

   (microwave links) and mobile phones

  Advantages Advantages _ can be used simple, cheap, available in many mobile simple, cheap, available in many mobile devices (PDAs, laptops, mobile phones) devices (PDAs, laptops, mobile phones) _

• – coverage of larger areas possible (radio no licenses needed

  can penetrate (thinner) walls, furniture no licenses needed etc.)

• – higher transmission rates (~11 – 54

   Disadvantages Disadvantages _

  Mbit/s) interference by sunlight, heat sources interference by sunlight, heat sources etc. etc.

   _ Disadvantages many things shield or absorb IR light many things shield or absorb IR light _

• – very limited license free frequency cannot penetrate obstacles (e.g., walls)

  cannot penetrate obstacles (e.g., walls) _ bands low bandwidth (~115kbit/s, 4Mbit/s) low bandwidth (~115kbit/s, 4Mbit/s)

• – shielding more difficult, interference with

   other senders, or electrical devices

  Example Example _ IrDA (Infrared Data Association) IrDA (Infrared Data Association)

   Example interface available everywhere interface available everywhere

  

Example WLAN physical

Example WLAN physical

layer layer

  802.11g is the most popular physical layer, operating in

  the same band as 802.11b The signal format is

  802.11 Medium Access Control (MAC) OFDM (Orthogonal

CSMA/CA

  Frequency Division Multiplexing)

  802.11 802.11a 802.11b 802.11g

  Data rates supported:

  various bit rates from PHY PHY PHY PHY 6 to 54 Mbit/s (same

  as 802.11a)

  ISM band: 2.4 … 2.4835 GHz The ISM band at 2.4 GHz can be used by anyone as long as (in Europe...)

  Transmitters using FH (Frequency Hopping) technology:

• Total transmission power < 100 mW
• Power density < 100 mW / 100 kHz

  Transmitters using DSSS technology:

  ETSI EN 300 328-1 requirements

  ISM

  ISM frequency band at 2.4 GHz frequency band at 2.4 GHz

• Total transmission power < 100 mW
• Power density < 10 mW / 1 MHz

  802.11 spectrum at 2.4 GHz 802.11 spectrum at 2.4 GHz .

  Divided into overlapping channels

  For e.g. the 2.4000–2.4835 GHz band is divided into 13 channels each of width 22 MHz but spaced only 5 MHz apart, with channel 1 centred on 2.412 GHz and 13 on 2.472 GHz Availability of channels is regulated by country (e.g. Japan adds a 14th channel 12 MHz above channel 13).

  

Given the separation between channels 1, 6, and 11, the signal on any channel should

be sufficiently attenuated to minimally interfere with a transmitter on any other

  Recall: Free-space loss is Recall: Free-space loss is

dependent on frequency

dependent on frequency

  The free-space loss L of a radio signal is:

  2

  2 4 d

4 df

 

      L

        c

      

  where d is the distance between transmitter and receiver,  is the rf wavelength, f is the radio frequency, and c is the speed of light. The formula is valid for d >> , and does not take into account antenna gains (=> Friis formula) or obstucting elements causing additional loss.

  

Free-space loss examples

Free-space loss examples

  For example, when d is 10 or 100 m, the free-space loss values (in dB) for the different ISM bands are:

  d = 10 m d = 100 m f = 900 MHz f = 2.4 GHz f = 5.8 GHz L = 51.5 dB L = 71.5 dB

  L = 60.0 dB L = 80.0 dB L = 67.7 dB L = 87.7 dB Maximum channel data rates Maximum channel data rates

  Network Maximum data rate

  IEEE 802.15.1 WPAN

  1 Mbit/s (Bluetooth v. 1.2)

  (Bluetooth)

  3 Mbit/s (Bluetooth v. 2.0)

  IEEE 802.15.4 LR- 250 kbit/s WPAN (ZigBee)

  IEEE 802.11 WLAN

  11 Mbit/s (802.11b)

  (WiFi)

  54 Mbit/s (802.11g)

  IEEE 802.16 WMAN 134 Mbit/s (WiMAX)

  Modulation / Signal spreading Modulation / Signal spreading

  

Network Modulation / spreading method

  IEEE 802.15.1 WPAN Gaussian FSK / FHSS (Bluetooth)

  IEEE 802.15.4 LR- Offset-QPSK / DSSS WPAN (ZigBee)

  IEEE 802.11 WLAN DQPSK / DSSS (802.11b) (WiFi)

64-QAM / OFDM (802.11g)

  IEEE 802.16 WMAN 128-QAM / single carrier (WiMAX) 64-QAM / OFDM

  802.11: advanced capabilities 802.11: advanced capabilities _-1

  10 _{QAM256 (8 Mbps)} Rate Adaptation

  Rate Adaptation _{10 -2 BPSK (1 Mbps) QAM16 (4 Mbps)}  _{10 -3 operating point} base station and base station and

  R _-4 E ₁₀ B mobile dynamically mobile dynamically _{10 -5} change transmission change transmission _{10 10 -7 -6} rate (physical layer rate (physical layer _{10 20 30 40} SNR(dB) modulation technique) modulation technique)

  1. As node moves away from

  as mobile moves, as mobile moves,

  base station SNR

  SNR varies SNR varies

  decreases, BER increase

  2. When BER becomes too high, switch to lower transmission rate but with lower BER

IEEE 802.11: MAC

  overview overview

   Two basic access mechanisms have been defined for IEEE

  Two basic access mechanisms have been defined for IEEE 802.11

  802.11

• – CSMA/CA

CSMA/CA

  ( ( mandatory mandatory

  ) summarized as ) summarized as distributed distributed coordination function ( coordination function ( DCF DCF

  

)

)

  »

  Optional method (

  Optional method (

  RTS/CTS)

  avoiding the hidden terminal problem

RTS/CTS)

• – A
• – DCF
• – only asynchronous data service in ad-hoc network mode

  »

  access point polls terminals according to a list

  access point polls terminals according to a list