1. IEEE WLAN

2. IEEE Protocol Stack
For centralized contention-free channel access
For distributed contention-based channel access

3. Possible Network Topologies
BSS mode
ESS mode

4. 802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for AP
interference possible: channel can be same as that chosen by neighboring AP!
host: must associate with an AP
scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address
selects AP to associate with
may perform authentication
will typically run DHCP to get IP address in AP’s subnet

5. Distributed Coordination Function (DCF)

6. Binary Exponential Backoff
Backoff Counter is randomly selected from [0,CW],where CW is contention window
For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax)
CW  2 (CW+1)-1
If successful transmission,
CW  CWmin
Reduces the collision probability

7. Virtual Carrier SensingB C D E
RTS
RTS
CTS
CTS
DATA
DATA
ACK
ACK
Any node hearing RTS or CTS sets up their NAV (network allocation vector) until end of ACK.
NAV set -> node silent (act as if carrier busy).

8. Timeline
If carrier busy (physical or virtual), schedule transmission after a random backoff when carrier is free.
Average backoff interval is doubled for each failed attempt.
RTS
DATA
Transmitter
SIFS
SIFS
DIFS
CTS
ACK
Receiver
SIFS
Nodes that hear
transmitter
DIFS
NAV (RTS)
NAV (CTS) t
Nodes that hear
receiver
Defer access
Random
backoff
Another
transfer

9. RTS/CTS Mechanism (Optional)
RTC/CTS solves HTP
But, non-negligible overhead
If frame size > RTSthreshhold,
RTS-CTS-DATA-ACK
Otherwise,
DATA-ACK
802.11b
tslot
20usec
SIFS
10usec
PIFS
SIFS + tslot
DIFS
SIFS + 2*tslot
EIFS
> DIFS

10. Collisions are not completely avoided in IEEE 802.11 !!
H does not sense any signal during D’s DATA tx
H may transmit
Collision in E’s reception

11. Energy Conservation: Power control
Power control has two potential benefit
Reduced interference & increased spatial reuse
Energy saving
If C reduces transmit power, it can still communicate with D
Reduces energy consumption at node C
Allows B to receive A’s transmission (spatial reuse)

12. frame: addressing
frame
control
duration
address 1 2 4 3
payload
CRC 6
seq
Address 4: used only in ad hoc mode
Address 1: MAC address
of wireless host or AP
to receive this frame
Address 3: MAC address
of router interface to which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame

13. 802.11 frame: addressing Internet router H1 R1 AP
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
frame H1 R1
R1 MAC addr AP MAC addr
dest. address
source address
802.3 frame

14. 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved
transmission time (RTS/CTS)
frame
control
duration
address 1 2 4 3
payload
CRC 6
seq
Type
From AP
Subtype To
More
frag
WEP
data
Power
mgt
Retry
Rsvd
Protocol
version 2 4 1
frame type
(RTS, CTS, ACK, data)

15. The Other IEEE 802.11 Efforts 802.11e 802.11h 802.11i 802.11n 802.11r
Provides QoS support by differentiating traffic streams
Applicable to PHY a, b, and g
802.11h
Supplementary to MAC layer so as to comply with European regulations for 5 GHz WLAN
802.11i
Security enhancement
802.11n
Enhancement for higher throughput (> 100 Mbps )
Decrease overhead within protocol
Packet preamble, CW, ACK, IFS parameters
802.11r
Speed up handoff between APs (Fast BSS-Transition)
Important for VoWLAN
802.11s
Support mesh networks