1. An Introduction to Wireless LANs
Universität Karlsruhe
Institut für Telematik
An Introduction to Wireless LANs
Lecturer: Lei Guo, Yahoo!
Outline
Wireless LAN architecture
Medium access principle
MAC control
MAC management
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

2. Wireless LAN Architecture
Universität Karlsruhe
Institut für Telematik
Wireless LAN Architecture
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

3. Comparison: infrastructure vs. ad-hoc networks
Universität Karlsruhe
Institut für Telematik
Comparison: infrastructure vs. ad-hoc networks
infrastructure network
AP: Access Point AP AP
wired network AP
ad-hoc network
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

4. IEEE 802.11 - Architecture of an infrastructure network
Universität Karlsruhe
Institut für Telematik
IEEE Architecture of an infrastructure network
Basic Service Set (BSS)
group of stations using the same radio channel
Station (STA)
terminal with access mechanisms to the wireless medium and radio contact to the access point
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 (EES: Extended Service Set) based on several BSS
LAN
802.x LAN
STA1
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
BSS2
STA2
STA3
LAN
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 9

5. Wireless router in your home
Universität Karlsruhe
Institut für Telematik
Wireless router in your home
Cable modem
Wireless router
Internet
portal AP
NAT
Router
ESS2/BSS2
Your neighbor
channel 1
ESS1/BSS1
channel 6
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

6. 802.11 - Architecture of an ad-hoc network
Universität Karlsruhe
Institut für Telematik
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
ad-hoc routing:
possible but not specified in IEEE
LAN
STA1
IBSS1
STA3
STA2
IBSS2
STA5
STA4
LAN
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

7. IEEE standard 802.11: protocol stack
Universität Karlsruhe
Institut für Telematik
IEEE standard : protocol stack
fixed
terminal
mobile terminal
infrastructure
network
access point
application
application
TCP
TCP IP
LLC is same for all 802.x IP
LLC
LLC
LLC
MAC
MAC
802.3 MAC
802.3 MAC
PHY
PHY
802.3 PHY
802.3 PHY
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

8. Universität Karlsruhe Institut für Telematik
Layers and functions
MAC control
access mechanisms, fragmentation, encryption
MAC Management
synchronization, roaming, MIB, power management
PLCP Physical Layer Convergence Protocol
clear channel assessment signal (carrier sense)
PMD Physical Medium Dependent
modulation, coding
PHY Management
channel selection, MIB
Station Management
coordination of all management functions
Station Management
LLC
DLC
MAC
MAC Management
PLCP
PHY Management
See video (in reference 4) for physical layer technologies
PHY
PMD
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

9. Medium Access Principle
Universität Karlsruhe
Institut für Telematik
Medium Access Principle
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

10. Universität Karlsruhe Institut für Telematik
Motivation
Can we apply media access methods from fixed networks?
Ethernet: CSMA/CD
Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)
stop when collision is detected
Problems in wireless networks
signal strength decreases proportional to the square of the distance
the sender would apply CS and CD, but the collisions happen at the receiver
it might be the case that a sender cannot “hear” the collision, i.e., CD does not work
furthermore, CS might not work if, e.g., a terminal is “hidden”
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

11. Hidden and exposed terminals
Universität Karlsruhe
Institut für Telematik
Hidden and exposed terminals
Hidden terminals
A sends to B, C cannot receive A
C wants to send to B, C senses a “free” medium (CS fails)
collision at B, A cannot receive the collision (CD fails), neither can C
A is “hidden” for C, and vice versa
Exposed terminals
B sends to A, C wants to send to another terminal D
C has to wait, CS signals a medium in use
but A is outside the radio range of C, therefore waiting is not necessary
C is “exposed” to B, and vice versa A B C D
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

12. MACA - collision avoidance
Universität Karlsruhe
Institut für Telematik
MACA - collision avoidance
MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance
RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
after RTS/CTS exchange, medium is reserved
Signaling packets contain
sender address
receiver address
packet size
RTS/CTS collision
probability is small, since RTS/CTS packets are very short
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

13. Universität Karlsruhe Institut für Telematik
MACA examples
MACA avoids the problem of hidden terminals
A and C want to send to B
A sends RTS first
C waits after receiving CTS from B
MACA avoids the problem of exposed terminals
B wants to send to A, C to another terminal D
B sends RTS, and A
replies CTS
now C does not have to wait,
for it cannot receive CTS from A A C
RTS
CTS
CTS B A C
RTS
RTS
CTS B D
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

14. Universität Karlsruhe Institut für Telematik
Polling mechanisms
If one terminal can be heard by all others, this “central” terminal (e.g., AP) can poll all other terminals according to a certain scheme, e.g., round-robin polling
Polling is contention free, no collision
Infrastructure WLAN
PCF (Point Coordination Function)
polling to avoid collision
DCF (Distributed Coordination Function)
CSMA/CA with RTS/CTS to resolve hidden terminal problem
no exposed terminal problem in infrastructure WLAN (why?)
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

15. 802.11 Wireless LANs: MAC Control
Universität Karlsruhe
Institut für Telematik
Wireless LANs: MAC Control
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

16. Universität Karlsruhe Institut für Telematik
MAC layer I - DFWMAC
Traffic services
Asynchronous Data Service (infrastructure/ad-hoc, mandatory)
station sends packets with “best-effort”, no upper bound for delay
support of broadcast and multicast
implemented using DCF (Distributed Coordination Function)
Time-Bounded Service (infrastructure only, optional)
implemented using PCF (Point Coordination Function) with polling
DFWMAC (Distributed Foundation Wireless MAC)
DFWMAC-DCF CSMA/CA (mandatory)
collision avoidance via randomized “back-off” mechanism
reliable delivery with packet ACK (for unicast only)
DFWMAC-DCF w/ RTS/CTS (optional)
based on CSMA/CA, avoids hidden terminal problem
optional, but supported by most commodity products
DFWMAC-PCF (optional)
AP polls terminals according to a list, contention free
optional, not supported by most commodity products
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

17. Universität Karlsruhe Institut für Telematik
MAC layer II
Frame types
data frame: normal data packets
control frame: ACK, RTS, CTS
management frame: beacon, probe, association, authentication, …
Priorities for different frames
defined through different IFS (Inter Frame Spacing) duration
SIFS (Short IFS): highest priority, for ACK, CTS, polling response, …
PIFS (PCF IFS): medium priority, for time-bounded service (PCF)
DIFS (DCF IFS): lowest priority, for asynchronous data service (DCF)
no guaranteed, hard priorities
DIFS
DIFS
PIFS
SIFS
medium busy
contention
next frame t
direct access if medium is free  DIFS
random
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

18. 802.11 - CSMA/CA: sending unicast data frame
Universität Karlsruhe
Institut für Telematik
CSMA/CA: sending unicast data frame
contention window
(randomized back-off mechanism)
DIFS
DIFS
medium busy
next frame
direct access if medium is free  DIFS t
total waiting time
when has data to send: starts sensing the medium (Carrier Sense)
if the medium is free for the duration of DIFS, starts sending
if the medium is busy, wait for a free DIFS, then do back-off:
wait a random back-off time (contention window) to avoid collision
if the medium is busy again during the back-off time: stop back-off, wait for a free DIFS, then continue back-off (the contention window is set to the residual back-off time)
if collision: stop back-off, wait for a free DIFS, then redo back-off by doubling contention window size (until maximum), re-send data
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 12

19. 802.11 - competing stations - simple version
Universität Karlsruhe
Institut für Telematik
competing stations - simple version
DIFS
DIFS
DIFS
DIFS
boe
bor
boe
bor
boe
busy
station1
boe
busy
station2
busy
station3
boe
busy
boe
bor
station4
boe
bor
boe
busy
boe
bor
station5 t
busy
medium not idle (frame, ack etc.)
boe
elapsed backoff time
packet arrival at MAC
bor
residual backoff time
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

20. 802.11 - CSMA/CA: data frame acknowledgment
Universität Karlsruhe
Institut für Telematik
CSMA/CA: data frame acknowledgment
Sending a unicast packet: complete procedure
station has to wait for DIFS before sending data
receiver send ACK at once if the packet was received correctly
ACK cannot be delayed since it just wait a SIFS
automatic retransmission of data packets in case of transmission errors
Compared with CSMA/CD (Ethernet)
no collision detection (Ethernet has CD)
use ACK to confirm delivery except multicast/broadcast (Ethernet no ACK)
atomic operation
DIFS
data
sender
SIFS
ACK
receiver
DIFS
data
other
stations t
waiting time
contention
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

21. Universität Karlsruhe Institut für Telematik
– DFWMAC with RTS/CTS
Sending unicast packets
A sends RTS with net allocation vector (NAV, amount of time to use the medium) after waiting for DIFS, to reserve medium
B replies CTS after SIFS (if ready to receive)
A sends data to B, B replies ACK
other stations know medium reservation through NAV in RTS and CTS
atomic operation
DIFS
RTS
data A
sender
SIFS
SIFS
SIFS
CTS
ACK B
receiver
DIFS
NAV (RTS)
data
other
stations
NAV (CTS) C t
defer access
contention
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

22. 802.11 Wireless LANs: MAC Management
Universität Karlsruhe
Institut für Telematik
Wireless LANs: MAC Management
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

23. Universität Karlsruhe Institut für Telematik
MAC management
Synchronization
synchronize internal clocks
functions to support finding a WLAN
Power management
sleep-mode without missing a message
periodic sleep, frame buffering, traffic measurements
Roaming
scanning
changing access points
joining a network
MIB - Management Information Base
managing, read, write
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

24. Synchronization using a Beacon (infrastructure)
Universität Karlsruhe
Institut für Telematik
Synchronization using a Beacon (infrastructure)
each station maintains an internal clock
synchronized clocks are needed for
power management
PCF coordination
hopping sequence of radio frequency in physical layer (see video)
how to synchronize: AP broadcasts quasi periodic beacon frames (100 ms)
beacon interval B B B B
access
point
busy
busy
busy
busy
medium t B
value of the timestamp
beacon frame
synchronization in ad-hoc WLAN: more complex
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

25. Power management: motivation
Universität Karlsruhe
Institut für Telematik
Power management: motivation
Mobile devices are usually driven by battery powers
Wireless card consumes a great amount of energy in mobile devices
over 50% total energy for PDA
up to 10% total energy for laptop
Power modes of wireless card
Transmit mode
data transmission (sending)
power consumption: high
Receive mode
data receiving and listening
power consumption: medium
Sleep mode
power consumption: low
power consumption of a wireless card
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

26. 802.11 power management mechanism
Universität Karlsruhe
Institut für Telematik
power management mechanism
Idea: switch the transceiver off if not needed
power saving mode: a station sleep and awake
sender knows when to wake up for transmission
receiver needs to know when to wake up for receiving
unicast in infrastructure WLAN
AP buffers data for a station working in power saving mode
AP periodically broadcasts with beacon a Traffic Indication Map (TIM), indicating which station has data to receive
station periodically wakes up to receive TIM
station polls AP to receive data
broadcast/multicast in infrastructure WLAN
similar but no buffer and no poll (broadcast/multicast has no ACK)
needs another Traffic Indication Map (called Delivery TIM, DTIM)
period of DTIM is multiple of TIM
ad-hoc WLAN
more complex
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

27. Power saving with wake-up patterns (infrastructure)
Universität Karlsruhe
Institut für Telematik
Power saving with wake-up patterns (infrastructure)
before sleep: station notifies AP to have data buffered
TIM interval
DTIM interval D B T T d D B
access
point
busy
busy
busy
busy
medium p d
station t t0 t1 t2 t3 T
TIM D
DTIM
awake d
data transmission
to/from the station B
broadcast/multicast p
PS poll
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

28. Universität Karlsruhe Institut für Telematik
– Roaming
Scan
passive scan: listen into the medium for beacon signals
active scan: send probes into the medium and wait for an answer
Reassociation
station sends a Reassociation Request to new AP(s)
new AP replies Reassociation Response
success: join the new AP
failure: continue scanning
Handover
new AP: signals the new station to the distribution system (DS)
DS: updates data base (i.e., location information)
old AP: sends buffered data to new AP through DS, release resources
Advantage
seamless to end users, IP layer not affected
TCP/IP socket not broken (roaming in an ESS)
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

29. Universität Karlsruhe Institut für Telematik
IEEE family
802.11: legacy (1997)
2.4 GHz
1, 2 Mbps
802.11a: not widely used (2001)
5-6 GHz range
6, 9, 12, 18, 24, 36, 48, and 54 Mbps
802.11b: widely used before (1999)
1, 2, 5.5, 11 Mbps
802.11g: most popular now (2003)
backwards compatible with b
802.11n: MIMO antenna (2010)
up to 300 Mbps, 600 Mbps in the future
backwards compatible with a, b, g
product for n draft already on market
And others: d, e, f, h, i, …
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

30. Universität Karlsruhe Institut für Telematik
Summary: Key points
Architecture
protocol stack
Access principle of radio medium
hidden terminal problem
RTS/CTS, polling
MAC control
DCF CSMA/CA
DCF with RTS/CTS
MAC management
synchronization (infrastructure)
power saving mechanism (infrastructure)
References
Basic: J. F. Kurose, K. W. Ross, Computer Networking: A Top-Down Approach (4 ed.), Chapter 6
Advanced: J. Schiller, Mobile Communications (2 ed.), Chapter 7
Lab book: M. Gast, ® Wireless Networks: The Definitive Guide, O'Reilly
Physical layer video: click radio frequency technology to watch (optional)
Modern Networking Concepts: An Introduction to Wireless LANs
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller