2. Wireless LANs: Characteristics
ā¢ Types
ā Infrastructure based
ā Ad-hoc
ā¢ Advantages
ā Flexible deployment
ā Minimal wiring difficulties
ā More robust against disasters (earthquake etc)
ā Historic buildings, conferences, trade shows,ā¦
ā¢ Disadvantages
ā Low bandwidth compared to wired networks (1-10 Mbit/s)
ā Proprietary solutions
ā Need to follow wireless spectrum regulations
3. Components/Architecture
ā¢ Station (STA) - Mobile node
ā¢ Access Point (AP) - Stations are connected to
access points.
ā¢ Basic Service Set (BSS) - Stations and the AP
with in the same radio coverage form a BSS.
ā¢ Extended Service Set (ESS) - Several BSSs
connected through APs form an ESS.
6. Wireless LAN: Motivation
ā¢ Can we apply media access methods from
fixed networks?
ā¢ Example CSMA/CD
ā Carrier Sense Multiple Access with Collision
Detection
ā send as soon as the medium is free, listen into
the medium if a collision occurs (original
method in IEEE 802.3)
7. ā¢ Medium access problems in wireless
networks
ā signal strength decreases proportional to the
square of the distance
ā sender would apply CS and CD, but the
collisions happen at the receiver
ā sender may not āhearā the collision, i.e., CD
does not work
ā CS might not work, e.g. if a terminal is āhiddenā
8. Difference Between Wired and
Wireless
ā¢ If both A and C sense the channel to be idle at the
same time, they send at the same time.
ā¢ Collision can be detected at sender in Ethernet.
ā¢ Half-duplex radios in wireless cannot detect
collision at sender.
A B C
A
B
C
Ethernet LAN Wireless LAN
9. ā¢ Hidden terminals
ā A and C cannot hear each other.
ā A sends to B, C cannot receive A.
ā C wants to send to B, C senses a āfreeā medium (CS fails)
ā Collision occurs at B.
ā A cannot receive the collision (CD fails).
ā A is āhiddenā for C.
ā¢ Solution?
ā Hidden terminal is peculiar to wireless (not found in wired)
ā Need to sense carrier at receiver, not sender!
ā āvirtual carrier sensingā: Sender āasksā receiver whether it
can hear something. If so, behave as if channel busy.
Hidden Terminal Problem
BA C
10. Medium access control layer
ā¢ Asynchronous data service (DCF)
- CSMA/CA
- RTS/CTS
ā¢ Time bounded service (PCF)
- Polling
ā¢ Inter-frame spacing (IFS)
- DIFS
- PIFS
- SIFS
12. ā¢ 802.11 avoids the problem of hidden terminals
ā A and C want to send to B
ā A sends RTS to B
ā B sends CTS to A
ā C āoverhearsā CTS from B
ā C waits for duration of Aās transmission
RTS/CTS
A B C
RTS
CTSCTS
14. Fragmentation
ā¢ Wireless LANs have high bit error rates.
ā¢ The probability of erroneous frame is much
higher for wireless links
ā¢ 802.11 uses fragmentation to reduce the
frame error rate.
16. PCF with Polling
ā¢ To provide time bounded service.
ā¢ Requires an access point.
ā¢ Access point polls each station during
contention free period.
ā¢ Becomes an overhead during light load
18. MAC management
ā¢ Synchronization
- finding and staying with a WLAN
- synchronization functions
ā¢ Power Management
- sleeping without missing any messages
- power management functions
ā¢ Roaming
- functions for joining a network
- changing access points
- scanning for access points
ā¢ Management information base
19. Synchronization
ā¢ Timing synchronization function (TSF)
ā¢ Used for power management
- beacons sent at well known intervals
- all station timers in BSS are
synchronized
21. Power Management
ā¢ Mobile devices are battery powered
- power management is important for
mobility
ā¢ 802.11 power management protocol
- allows transceiver to be off as much as
possible
- is transparent to existing protocols
22. Power management approach
ā¢ Allow idle stations to go to sleep
- stationās power save mode stored in
AP
ā¢ APs buffer packets for sleeping stations
- AP announces which stations have
frames buffered
- traffic indication map (TIM) sent with
every beacon
ā¢ Power saving stations wake up periodically
23. Power management
busybusy busybusy
p PS poll
Data transmission
to/from the station
Access
point
station
TIM interval
Broadcast/
multicast
D
B
B T T
medium
t
p
d
d
DTIM interval
d
24. Roaming
ā¢ Mobile stations may move
- beyond the coverage area of their AP
- but within range of another AP
ā¢ Re association allows station to continue
operation.
25. Roaming approach
ā¢ Station decides that link to its current AP is poor.
ā¢ Station uses scanning function to find another AP
ā¢ Station sends Re-association Request to new AP
ā¢ If Re-association Response is successful
- then station has roamed to the new AP
- else station scans for another AP
ā¢ If AP accepts Re-association Request
- AP indicates Re-association to the
distributed system
- Distributed system information is
updated
26. Scanning
ā¢ Scanning is required for many functions
- finding and joining a network
- finding a new access point during
roaming
ā¢ Passive scanning
- find networks simply by listening for
beacons
ā¢ Active scanning
- on each channel send a probe and wait
for probe response
27. References
ā¢ Brian P Crow, Indra Widjaja, J G Kim,
Prescott T Sakai. IEEE 802.11 Wireless
Local Area Networks. IEEE
Communications Magazine
ā¢ www.breezecom.com
ā¢ Jochen H. Schiller, Mobile
Communications