1. Universität Karlsruhe Institut für Telematik ECE 591
Mobilkommunikation
SS 1998
Lecture 5: IEEE
ECE 591
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

2. IEEE Requirements
Wi-Fi often used by the public as a synonym for IEEE wireless LAN (WLAN).
Design for small coverage (e.g. office, home)
Low/no mobility
High data-rate applications
Ability to integrate real time applications and non-real-time applications
Use un-licensed spectrum

3. 802.11: Infrastructure Mode Architecture similar to cellular
networks station (STA)
terminal with access mechanisms to the wireless medium and radio contact to the access point
access point (AP)
station integrated into the wireless LAN and the distribution system
basic service set (BSS)
group of stations using the same AP
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 9

4. 802.11 - Architecture of an ad-hoc network
Freie Universität Berlin
Institut of Computer Science
Mobile Communications
2002
Architecture of an ad-hoc network
LAN
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
STA1
IBSS1
STA3
STA2
IBSS2
STA5
STA4
LAN
Prof. Dr.-Ing. Jochen Schiller

5. Freie Universität Berlin Institut of Computer Science
Mobile Communications
2002
IEEE standard
fixed
terminal
mobile terminal
infrastructure
network
access point
application
application
TCP
TCP IP IP
LLC
LLC
LLC
MAC
MAC
802.3 MAC
802.3 MAC
PHY
PHY
802.3 PHY
802.3 PHY
Prof. Dr.-Ing. Jochen Schiller

6. IEEE 802.11 Physical Layer Family of IEEE 802.11 standards:
unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz
300 MHz
GHz
GHz
and b/g
802.11a

7. The IEEE 802.11 Family Protocol Release Data Freq. Rate (typical)
Rate (max)
Range (indoor)
Legacy
1997
2.4 GHz
1 Mbps
2Mbps ?
802.11a
1999
5 GHz
25 Mbps
54 Mbps
~30 m
802.11b
6.5 Mbps
11 Mbps
802.11g
2003
802.11n
2008
2.4/5 GHz
200 Mbps
540 Mbps
~50 m

8. 802.11a Modulation
Use OFDM to divide each physical channel (20 MHz) into 52 subcarriers (312.5 KHz each)
48 data, 4 pilot
Adaptive modulation
BPSK: 6, 9 Mbps
QPSK: 12, 18 Mbps
16-QAM: 24, 36 Mbps
64-QAM: 48, 54 Mbps

9. 802.11 MAC Layer: Access Methods
DFWMAC-DCF CSMA/CA (mandatory)
collision avoidance via randomized “back-off“
ACK packet for acknowledgements
DFWMAC-DCF w/ RTS/CTS (optional)
additional virtual “carrier sensing: to avoid hidden terminal problem
DFWMAC- PCF (optional)
access point polls terminals according to a list

10. Hidden and Exposed Terminals
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
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
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 A B C
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

11. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Near and Far Terminals
Terminals A and B send, C receives
signal strength decreases proportional to the square of the distance
the signal of terminal B therefore drowns out A’s signal
C cannot receive A
If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer
Also severe problem for CDMA-networks - precise power control needed! A B C
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

12. MACA - collision avoidance
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
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
Signaling packets contain
sender address
receiver address
packet size
Variants of this method can be found in IEEE as DFWMAC (Distributed Foundation Wireless MAC)
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

13. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
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
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
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

14. MACA variant: DFWMAC in IEEE802.11
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
MACA variant: DFWMAC in IEEE802.11
sender
receiver
idle
idle
packet ready to send; RTS
data;
ACK
RxBusy
time-out;
RTS
wait for the
right to send
RTS;
CTS
time-out 
data;
NAK
ACK
time-out 
NAK;
RTS
CTS; data
wait for
data
wait for ACK
RTS; RxBusy
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

15. 802.11 CSMA/CA CSMA: Listen before transmit Collision avoidance
when transmitting a packet, choose a backoff interval in the range [0, CW]
CW is contention window
Count down the backoff interval when medium is idle
count-down is suspended if medium becomes busy
Transmit when backoff interval reaches 0

16. Congestion Avoidance: Example
busy
B1 = 25
B2 = 20
B1 = 5
data
wait
data
wait
B2 = 10
B2 = 15
busy
B1 and B2 are backoff intervals
at nodes 1 and 2

17. – RTS/CTS + ACK
Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium)
Receiver acknowledges via CTS (if ready to receive)
CTS reserves channel for sender, notifying possibly hidden stations
Sender can now send data at once, acknowledgement via ACK
Other stations store NAV distributed via RTS and CTS
DIFS
RTS
data
sender
SIFS
SIFS
CTS
SIFS
ACK
receiver
NAV (RTS)
DIFS
data
other
stations
NAV (CTS) t
defer access
new contention

18. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Not END
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller