University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Mobile Communications
Chapter 7: Wireless LANs
Characteristics
 IEEE 802.11


PHY
MAC
 Roaming
 IEEE 802.11a, b, g, e
HIPERLAN
 Bluetooth
 Comparisons


Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.1
Comparison: infrastructure vs. ad-hoc networks
infrastructure
network
AP
AP
wired network
Infrastructure networks:
• provide access to other networks
• typically communications between the wireless node and the
access point, but not directly between the wireless nodes.
• the access points controls medium access, but also act as a
bridge to other wireless or wired networks
• in the figure three WLANs with different coverage
areas are connected
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
AP: Access Point
AP
• most of the network functionality lies within
the access points, whereas the wireless clients
can remain quite simple.
• the MAC can be centralised to the access point
or be distributed oven the wireless clients.
• these wireless networks do rely on the access
points.
MC SS02
7.2
7.1
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Comparison: infrastructure vs. ad-hoc networks
ad-hoc network
Ad-hoc networks:
• no need for any infrastructure to work.
• each node can communicate directly with other nodes
• no access point controlling media access is necessary
• nodes within an ad-hoc network can communicate
directly or via other nodes
• the complexity of each node is higher because each node has
- implement MAC-mechanisms
- handle hidden or exposed terminal problem
- perhaps priority mechanisms to provide a certain QoS
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.3
IEEE standard 802.11 – Protocol architecture
The basic IEEE 802.11 standard!!
fixed
terminal
mobile terminal
infrastructure
network
access point
application
application
TCP
TCP
IP
LLC
IP
LLC
802.11 MAC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 PHY
802.11 PHY
802.3 PHY
802.3 PHY
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
LLC
MC SS02
7.4
7.2
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
DSSS PHY packet format (More important!!!)
Synchronization

synch., gain setting, energy detection, frequency offset compensation
SFD (Start Frame Delimiter)

1111001110100000 – indicates the start of a frame
Signal

data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)
Service

Length
future use, 00: 802.11 compliant

length of the payload
HEC (Header Error Check)

protection of signal, service and length, x16+x12+x5+1
128
synchronization
16
SFD
PLCP preamble
8
8
16
16
signal service length HEC
bits
variable
payload
PLCP header
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.5
802.11 - MAC layer I – DFWMAC
Distributed Foundation Wireless Medium Access Control
Asynchronous Data Service (mandatory)
exchange of data packets based on “best-effort”
 support of broadcast and multicast


Time-Bounded Service (optional)

PHY

DLC
Traffic services
LLC
MAC
PLCP
PMD
implemented using PCF (Point Coordination Function)
Access methods

DFWMAC-DCF CSMA/CA (mandatory) (DCF: distributed coordination function)*



DFWMAC-DCF w/ RTS/CTS (optional)



collision avoidance via randomized „back-off“ mechanism
ACK packet for acknowledgements (not for broadcasts)
Distributed Foundation Wireless MAC
avoids hidden terminal problem
DFWMAC- PCF (optional) (PCF: point coordination function)

access point polls terminals according to a list
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
MC SS02
7.6
7.3
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
802.11 - MAC layer II – General!
Priorities

defined through different inter frame spaces (IFS)
 no guaranteed hard priorities
 SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response

PIFS (PCF, Point Coordination Function IFS)

DIFS (DCF, Distributed Coordination Function IFS)


medium priority, for time-bounded service using PCF
lowest priority, for asynchronous data service
DIFS
DIFS
PIFS
SIFS
medium busy
direct access if
medium is free
contention
next frame
t
DIFS
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.7
802.11 - competing stations - simple version
DIFS
DIFS
station1
station2
DIFS
bo e
bo r
bo e
busy
DIFS
bo e bo r
bo e
busy
bo e busy
bo e bo r
bo e
bo e bo r
busy
station3
station4
bo e bo r
station5
t
busy
medium not idle (frame, ack etc.)
bo e elapsed backoff time
packet arrival at MAC
bo r residual backoff time
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
busy
MC SS02
7.8
7.4
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Motivation - 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)
 A is “hidden” for C

Exposed terminals
A
B
C

B sends to A, C wants to send to another terminal (not A or B)
 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
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.9
Fragmentation
If a node senses the wireless channel to be very disturbed, the node may
choose to fragment its sending frame into small sub frames (frags).
DIFS
sender
RTS
frag1
SIFS
receiver
CTS SIFS
frag2
SIFS
ACK1 SIFS
SIFS
ACK2
NAV (RTS)
NAV (CTS)
DIFS
NAV (frag1)
NAV (ACK1)
other
stations
data
contention
t
In the frags, the control field gives information to other stations of how
to set their NAV.
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
MC SS02
7.10
7.5
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
802.11 - Frame format – MAC frames
Types

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
Frame Duration/ Address Address Address Sequence Address
Control
ID
1
2
3
Control
4
bits
2
2
4
1
1
1
1
1
1
1
0-2312
4
Data
CRC
1
Protocol
To From More
Power More
Type Subtype
Retry
WEP Order
version
DS DS Frag
Mgmt Data
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.11
Special Frames: ACK, RTS, CTS
Acknowledgement
bytes
ACK
2
Frame
Control
2
6
Receiver
Duration
Address
4
CRC
Request To Send
bytes
RTS
2
Frame
Control
2
6
6
Receiver Transmitter
Duration
Address Address
4
CRC
Clear To Send
bytes
CTS
2
Frame
Control
6
Receiver
Duration
Address
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
2
MC SS02
4
CRC
7.12
7.6
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Synchronization using a Beacon (infrastructure)
Each node of an 802.11 network maintains an internal clock!
To synchronize the clocks of all nodes, IEEE 802.11 specifies a
Timer synchronization function (TSF)
Within a BSS (basic server set), timing is conveyed by the (quasi)periodic transmission of a beacon.
A beacon contains a timestamp and other management information used for power management
and roaming (e.g. identification of the BSS).
Within infrastructure-base networks, the access point performs synch by transmitting beacons
beacon interval
access
point
B
B
busy
medium
busy
B
B
busy
busy
t
value of the timestamp
B
beacon frame
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.13
Power saving with wake-up patterns (infrastructure)
Example: one access point and one station
The AP transmits a beacon frame each beacon interval.
This interval is equal to the TIM interval.
The AP maintains a delivery traffic indication map (DTIM) interval.
TIM interval
access
point
DTIM interval
D B
T
busy
medium
busy
T
d
D B
busy
busy
p
station
d
t
T
TIM
D
B
broadcast/multicast
DTIM
awake
p PS poll
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
d data transmission
to/from the station
MC SS02
7.14
7.7
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Channel plan for IEEE 802.11b
Channel Frequency (MHz)
US/Canada
Europé
Japan
1
2
3
4
5
6
7
8
9
10
11
12
13
14
x
x
x
x
x
x
x
x
x
x
x
-
x
x
x
x
x
x
x
x
x
x
x
x
x
-
x
x
x
x
x
x
x
x
x
x
x
x
x
x
2412
2417
2422
2427
2432
2437
2442
2447
2452
2457
2462
2467
2472
2484
Altogether 14 channels have been defined as the table shows.
For each channel a center frequency is given.
Depending on national restrictions 11 (US/Canada), 13 (Europe) or
14 channels (Japan) can be used.
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.15
Channel selection (non-overlapping)
Non-overlapping usage of channels for an IEEE 802.11b installation with minimum interference.
The spacing between center frequencies should be at least 25 MHz.
The occupied bandwidth of the main lobe of the signal is 22 MHz.
Europe (ETSI)
Users can install overlapping cells for WLANs
using three non-overlapping chanells.
channel 1
2400
2412
channel 7
channel 13
2442
2472
2483.5 [MHz]
22 MHz
US (FCC)/Canada (IC)
channel 1
2400
2412
channel 6
channel 11
2437
2462
2483.5
[MHz]
22 MHz
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
MC SS02
7.16
7.8
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
WLAN: IEEE 802.11 – some examples
802.11-2007: A new release of the standard that includes amendments
a, b, d, e, g, h, i & j. (July 2007)
802.11e: MAC Enhancements – QoS –
Enhance the current 802.11 MAC to expand support for applications with
Quality of Service requirements, and in the capabilities and efficiency of the
protocol.
802.11g: Data Rates at 2.4 GHz; 54 Mbit/s, OFDM, (backwards
compatible with b) (2003)
802.11s: Mesh Networking, Extended Service Set (ESS) (June 2011)
802.11ad: Very High Throughput 60 GHz (~Dec 2012)
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.17
Ad Hoc Networks
• Power is an issue
• All stations are mobile
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
MC SS02
7.18
7.9
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Broadband Internet access via mesh networks
• Power is NOT an issue
• Stations are NOT mobile
FL, UniK 4290: Introduction, 28.01.08
19
Mesh networking with Ad hoc networks
FL, UniK 4290: Introduction, 28.01.08
Jochen H. Schiller
1999
20
7.10
University of Karlsruhe
Institute of Telematics
Mobile Communications
Chapter 7: Wireless LANs
Sensor networks
• Power is an issue
• Stations are NOT mobile
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
7.21
MC SS02
7.22
End
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Jochen H. Schiller
1999
7.11