1. Module B WLAN – Protocol Aspects
Mobile Networks
Module B WLAN – Protocol Aspects
Prof. JP Hubaux

2. Reminder on frequencies and wavelenghts
twisted pair
coax cable
optical transmission
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m
3 THz
1 m
300 THz
VLF LF MF HF
VHF
UHF
SHF
EHF
infrared UV
visible light
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Frequency and wave length:
 = c/f
wave length , speed of light c  3x108m/s, frequency f

3. Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
simple, small antenna for handset
deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communication
small antenna
large bandwidth available
Wireless LANs use frequencies in UHF to SHF spectrum
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules (resonance frequencies)
Weather-dependent fading, signal loss caused by heavy rainfall etc.

4. Frequency allocation
Note: in the coming years, frequencies will become technology-neutral, at least within frequencies allocated to mobile phones (first row of the above table)

5. Characteristics of Wireless LANs
Advantages
flexibility
(almost) no wiring difficulties (e.g., historic buildings)
more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug...
Disadvantages
lower bitrate compared to wired networks
More difficult to secure

6. Scope of Various WLAN and WPAN Standards
Power consumption
Complexity I
Bluetooth
802.11a
802.11g
802.11
WPAN
802.11b
WLAN
802.11ac
Data rate
WPAN: Wireless Personal Area Network

7. Design goals for wireless LANs
low power
no special permissions or licenses needed to use the LAN
robust transmission technology
easy to use for everyone, simple management
protection of investment in wired networks (internetworking)
security, privacy, safety (low radiation)
transparency concerning applications and higher layer protocols
location awareness if necessary

8. Infrastructure vs. ad hoc networks
infrastructure network
AP: Access Point AP AP
wired network AP
Ad hoc network

9. IEEE 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
LAN
802.x LAN
STA1
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
BSS2
STA2
STA3
LAN 9

10. 802.11 - Architecture of an ad-hoc network
Direct communication within a limited range
Station (STA): terminal with access mechanisms to the wireless medium
Basic Service Set (BSS): group of stations using the same radio frequency
LAN
STA3
STA1
BSS1
STA2
LAN
BSS2
STA5
STA4

11. Interconnection of IEEE 802.11 with Ethernet
fixed terminal
mobile station
server
infrastructure network
access point
application
application
TCP
TCP IP IP
MAC
MAC
802.3 MAC
802.3 MAC
PHY
PHY
802.3 PHY
802.3 PHY

12. 802.11 - Layers and functions MAC MAC Management
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 IP
MAC
MAC Management
PLCP
PHY Management
PHY
PMD

13. 802.11b - Physical layer
2 versions: DSSS and FHSS (both typically at 2.4 GHz)
data rates 1, 2, 5 or 11 Mbit/s
DSSS (Direct Sequence Spread Spectrum)
DBPSK modulation (Differential Binary Phase Shift Keying) or DQPSK (Differential Quadrature PSK)
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
FHSS (Frequency Hopping Spread Spectrum)
spreading, despreading, signal strength
min. 2.5 frequency hops/s, two-level GFSK modulation (Gaussian Frequency Shift Keying)

14. 802.11 - MAC layer principles (1/2)
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 (called DFWMAC: Distributed Foundation Wireless MAC)
DCF CSMA/CA (mandatory)
collision avoidance via randomized „back-off“ mechanism
minimum distance between consecutive packets
ACK packet for acknowledgements (not for broadcasts)
DCF with RTS/CTS (optional)
avoids hidden terminal problem
PCF (optional and rarely used in practice)
access point polls terminals according to a list
DCF: Distributed Coordination Function
PCF: Point Coordination Function

15. 802.11 - MAC layer principles (2/2)
Priorities
defined through different inter frame spaces
no guaranteed, hard priorities
SIFS (Short Inter Frame Spacing)
highest priority, for ACK, CTS, polling response
PIFS (PCF IFS)
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
lowest priority, for asynchronous data service
DIFS
DIFS
PIFS
SIFS
medium busy
contention
next frame t
direct access if medium is free  DIFS
time slot
Note : IFS durations are specific to each PHY

16. CSMA/CA principles
contention window
(randomized back-off mechanism)
DIFS
DIFS
medium busy
next frame
direct access if medium has been free for at least DIFS t
time slot
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)
if another station occupies the medium during the back-off time of the station, the back-off timer stops (to increase fairness) 12

17. 802.11 – CSMA/CA broadcast = DIFS DIFS DIFS DIFS boe bor boe bor boe
busy
station1
boe
busy
station2
busy
station3
boe
busy
(detection by upper layer)
station4
boe
bor
boe
busy
(detection by upper layer)
station5 t
Here St4 and St5 happen to have the same back-off time
busy
medium not idle (frame, ack etc.)
boe
elapsed backoff time
packet arrival at MAC
bor
residual backoff time
The size of the contention window can be adapted
(if more collisions, then increase the size)
Note: broadcast is not acknowledged

18. See file B1-802-11-Traces.pdf
CSMA/CA unicast
Sending unicast packets
station has to wait for DIFS before sending data
receiver acknowledges at once (after waiting for SIFS) if the packet was received correctly (CRC)
automatic retransmission of data packets in case of transmission errors
DIFS
data
sender
SIFS
ACK
receiver
DIFS
data
other
stations t
waiting time
Contention window
The ACK is sent right at the end of SIFS (no contention)
See file B Traces.pdf

19. 802.11 – DCF with RTS/CTS 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
DIFS
RTS
data
sender
SIFS
SIFS
SIFS
CTS
ACK
receiver
DIFS
NAV (RTS)
data
other
stations
NAV (CTS) t
defer access
Contention window
RTS/CTS can be present for some packets and not for other
NAV: Net Allocation Vector

20. Fragmentation mode
DIFS
RTS
frag1
frag2
sender
SIFS
SIFS
SIFS
SIFS
SIFS
CTS
ACK1
ACK2
receiver
NAV (RTS)
NAV (CTS)
DIFS
NAV (frag1)
data
other
stations
NAV (ACK1) t
contention
Fragmentation is used in case the size of the packets sent has to be reduced (e.g., to diminish the probability of erroneous frames)
Each fragi (except the last one) also contains a duration (as RTS does), which determines the duration of the NAV
By this mechanism, fragments are sent in a row
In this example, there are only 2 fragments

21. 802.11 - MAC frame format Types Sequence numbers Addresses
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
bytes 2 2 6 6 6 2 6
0-2312 4
Frame
Control
Duration ID
Address 1
Address 2
Address 3
Sequence
Control
Address 4
Data
CRC
version, type, fragmentation, security, ...
detection of duplication

22. MAC address format DS: Distribution System AP: Access Point
DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
- infrastructure BSS : MAC address of the Access Point
- ad hoc BSS (IBSS): random number
RA: Receiver Address
TA: Transmitter Address

23. 802.11 - MAC management Synchronization Power management
Purpose
for the physical layer (e.g., maintaining in sync the frequency hop sequence in the case of FHSS)
for power management
Principle: beacons with time stamps
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
managing, read, write

24. Synchronization (infrastructure case)
beacon interval B B B B
access
point
busy
busy
busy
busy
medium t B
value of the timestamp
beacon frame
The access point transmits the (quasi) periodic beacon signal
The beacon contains a timestamp and other management information used for power management and roaming
All other wireless nodes adjust their local timers to the timestamp

25. Synchronization (ad-hoc case)
beacon interval B1 B1
station1 B2 B2
station2
busy
busy
busy
busy
medium t B
value of the timestamp
beacon frame
random delay (back-off)
Each node maintains its own synchronization timer and starts the transmission of a beacon frame after the beacon interval
Contention  back-off mechanism  only 1 beacon wins
All other stations adjust their internal clock according to the received beacon and suppress their beacon for the current cycle

26. 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 case
Traffic Indication Map (TIM)
list of unicast receivers transmitted by AP
Delivery Traffic Indication Map (DTIM)
list of broadcast/multicast receivers transmitted by AP
Ad-hoc case
Ad-hoc Traffic Indication Map (ATIM)
announcement of receivers by stations buffering frames
more complicated - no central AP
collision of ATIMs possible (scalability?)

27. Power saving (infrastructure case)
Here the access point announces data addressed to the station
TIM interval
DTIM interval D B T T d D B
access
point
busy
busy
busy
busy
medium p d
station t T
TIM D
DTIM
awake d
data transmission
to/from the station B
broadcast/multicast p
Power Saving poll: I am awake, please send the data

28. Power saving (ad-hoc case)
ATIM
window
beacon interval B1 A D B1
station1 B2 B2 a d
station2 t B A
transmit ATIM D
beacon frame
random delay
transmit data a d
awake
acknowledge ATIM
acknowledge data
ATIM: Ad hoc Traffic Indication Map (a station announces the list of buffered frames)
Potential problem: scalability (high number of collisions)

29. 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

30. MIMO – Multiple Input Multiple Output
Both the transmitter and the receiver use multiple antennas SU-MIMO: Single-user MIMO: exploits the presence of multiple transmit and receive antennas to improve both the capacity and the reliability of a transmission MU-MIMO: Multi-user MIMO: stations having multiple antennas can simultaneously transmit or receive multiple information flows
SU-MIMO:
MU-MIMO:
User 1 AP 1 3 2 4 AP 1 3 2 4
User 2
User
User 3

31. Multi-User Beamforming (MUBF)
Transmitter sends multiple streams concurrently to different users Improves theoretical system capacity compared to SUBF Now standardized in IEEE ac Channel sounding for pre-coding and zero- forcing (to null multi-user interference signal) High spectral efficiency R1
SUBF TX R2 R3 R4
MUBF
Courtesy Ed Knightly

32. Channel Sounding Timeline for 802.11acTx
Pilots
Data
CSI
ack
Rx A
CSI
ack
Rx B
CSI
ack
Rx C
Transmission Procedure
Select group and send channel sounding training sequence (Pilot Tones)
Receive channel state feedback (CSI) from each receiver serially
Construct steering weights and transmit data
Acknowledge transmission
Courtesy Ed Knightly

33. IEEE 802.11 – Standardization efforts
IEEE b
2.4 GHz band
DSSS (Direct-sequence spread spectrum)
Bitrates 1 – 11 Mbit/s
IEEE a
5 GHz band
Based on OFDM (orthogonal frequency-division multiplexing)
transmission rates up to 54 Mbit/s
Coverage is not as good as in b
IEEE g
2.4 GHz band (same as b)
Based on OFDM
Bitrates up to 54Mb/s
IEEE n
MIMO (multiple-input multiple-output)
40MHz channel (instead of 20MHz)
Can operate in the 5GHz or 2.4Ghz (risk of interference with other systems, however)
Bitrates up to 600Mb/s
IEEE ac
Extension of IEEE n; works 5GHz band; see recommended reading
IEEE e
Enhanced DCF: to support differentiated service
IEEE i
Security, makes use of IEEE 802.1x
IEEE p
For vehicular communications
IEEE s
For mesh networks

34. Conclusion on Wireless LANs
IEEE is the technology for wireless LANs
Developed over the last 20 years
Extremely widespread and successful
Excellent complement of cellular networks, especially with the emergence of smart phones
Found in most households and at almost all business buildings (with one major exception)
Envisioned also for mobile ad hoc networks (see next slides) and vehicular ad hoc networks
Interesting phenomenon: Fon

35. References
J. Schiller: Mobile Communications, Addison-Wesley, Second Edition, 2004
Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000
IEEE standards, available at

36. Ad Hoc On-Demand Distance Vector Routing (AODV)
Note: this and the following slides are provided here because AODV is used in the hands-on exercises. We will come back to this topic in a later module of the course.

37. AODV : Route discovery (1)K F H Q A E P S G D J B M R I L C N

38. AODV : Route discovery (2)K F H Q A E P S G D J B M R I L C N
Note: if one of the intermediate nodes (e.g., A) knows a route to D, it responds immediately to S
: Route Request (RREQ)

39. AODV : Route discovery (3)K F H Q A E P S G D J B M R I L C N
: represents a link on the reverse path

40. AODV : Route discovery (4)K F H Q A E P S G D J B M R I L C N

41. AODV : Route discovery (5)K F H Q A E P S G D J B M R I L C N

42. AODV : Route discovery (6)K F H Q A E P S G D J B M R I L C N

43. AODV : Route discovery (7)K F H Q A E P S G D J B M R I L C N

44. AODV : Route reply and setup of the forward pathK F H Q A E P S G D J B M R I L C N
: Link over which the RREP is transmitted
: Forward path

45. Route reply in AODV
In case it knows a path more recent than the one previously known to sender S, an intermediate node may also send a route reply (RREP)
The freshness of a path is assessed by means of destination sequence numbers
Both reverse and forward paths are purged at the expiration of appropriately chosen timeout intervals

46. AODV : Data delivery K F H Q A Data E P S G D J B M R I L C N
The route is not included in the packet header

47. AODV : Route maintenance (1)K F H Q A
Data E P S G X D J B M R I L C N

48. AODV : Route maintenance (2)K F H Q A
RERR(G-J) E P S G D X J B M R I L C N
When receiving the Route Error message (RERR), S removes the broken link from its cache.
It then initializes a new route discovery.

49. AODV (unicast) : Conclusion
Nodes maintain routing information only for routes that are in active use
Unused routes expire even when the topology does not change
Each node maintains at most one next-hop per destination

50. Hands-on exercises in room INF019
Next Week
Hands-on exercises in room INF019
Please read and bring with you the description of the hands-on exercises available at: