Wireless LAN - Mobile Communications - Lecture Slides, Slides of Mobile Communication Systems

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IEEE 802.

Wireless LAN

Wireless LANs: Characteristics

• Types
  • Infrastructure based
  • Ad-hoc
• Advantages
  • Flexible deployment
  • Minimal wiring difficulties
  • More robust against disasters (earthquake etc)
  • Historic buildings, conferences, trade shows,…
• Disadvantages
  • Low bandwidth compared to wired networks (1-10 Mbit/s)
  • Proprietary solutions
  • Need to follow wireless spectrum regulations

infrastructure network

ad-hoc network

AP AP

AP

wired network

AP: Access Point

Source: SchillerDocsity.com

Protocol Architecture

MAC sublayer

MAC layer

management

PLCP sublayer

PMD sublayer

Physical layer

management

• Medium access problems in wireless

networks

• signal strength decreases proportional to the square of the distance
• sender would apply CS and CD, but the collisions happen at the receiver
• sender may not “hear” the collision, i.e., CD does not work
• CS might not work, e.g. if a terminal is “hidden”

Difference Between Wired and

Wireless

• If both A and C sense the channel to be idle at the same

time, they send at the same time.

• Collision can be detected at sender in Ethernet.
• Half-duplex radios in wireless cannot detect collision at

sender.

A B C

A

B

C

Ethernet LAN (^) Wireless LAN

Medium access control layer

• Asynchronous data service (DCF)

- CSMA/CA

- RTS/CTS

• Time bounded service (PCF)

- Polling

• Inter-frame spacing (IFS)

- DIFS

- PIFS

- SIFS

CSMA/CA

station

station

station

station

station

t

DIFS DIFS DIFS DIFS

busy backoff Residual backoff

RTS/CTS

RTS

sender

receiver

Other stations

t

DIFS SIFS

DIFS

CTS

SIFS Data

ACK

SIFS

NAV (RTS)

NAV (CTS) bo^ Data

Fragmentation

• Wireless LANs have high bit error rates.

• The probability of erroneous frame is much

higher for wireless links

• 802.11 uses fragmentation to reduce the

frame error rate.

PCF with Polling

• To provide time bounded service.

• Requires an access point.

• Access point polls each station during

contention free period.

• Becomes an overhead during light load

PCF with polling

D

AP

station

station

t

PIFS SIFS (^) SIFS

U

SIFS D

U

SIFS

NAV (RTS)

Synchronization

• Timing synchronization function (TSF)

• Used for power management

- beacons sent at well known intervals

- all station timers in BSS are

synchronized

Synchronization

B

busy busy B B busy busy B B beacon Value of time stamp Access point medium Beacon interval B

t