1. 802.11 Specification overview
Pravin Bhagwat
Specification
overview
P. Bhagwat
Tutorial:

2. 802.11 Specifications MAC PHY LLC MAC sublayer MAC Layer Management
MAC Mgmt Service
Interface
MAC Service
Interface
LLC
MAC sublayer
MAC Layer
Management
WEP
MAC
Mgmt
MAC
MIB
PHY Service
Interface
PHY Mgmt Service
Interface
PLCP Sublayer
PHY
PHY layer Management
DSSS FH IR
OFDM
PMD Sublayer
P. Bhagwat

3. 802.11 Specifications LLC MAC sublayer MAC Management PHY Layer
MAC Service
Interface
(clause 6)
MAC Mgmt Service
Interface (clause 10)
MAC sublayer
MAC Management
MAC framing (clause 7)
MAC operation (clause 9)
WEP (clause 8)
State Machines (Annex C)
Protocols (clause 11)
State Machines (Annex C)
MIBs (Annex D)
PHY Service
Interface (clause 12)
PHY Mgmt Service
Interface (clause 13)
PHY Layer
PHY Management
FH (clause 14)
DSSS (clause 15)
Infrared (clause 16)
OFDM (clause 17)
High rate DSSS (clause 18)
MIBs (Annex D)
P. Bhagwat

4. System Architecture
Basic Service Set (BSS): a set of stations which communicate with one another
Independent Basic Service Set (IBSS)
Infrastructure Basic Service Set (BSS)
AP provides
connection to wired network
relay function
stations not allowed to communicate directly
only direct communication possible
no relay function
P. Bhagwat

5. ESS: a set of BSSs interconnected by a distribution system (DS)
Extended Service Set
ESS: a set of BSSs interconnected by a distribution system (DS)
ESS and all of its stations appear to be a single MAC layer
AP communicate among themselves to forward traffic
Station mobility within an ESS is invisible to the higher layers
P. Bhagwat

6. 802.11 Specifications Applications Control MAC PHY
LLC
WEP
MAC
Mgmt
MAC
MIB
PHY
DSSS FH IR
OFDM
Specification of layers below LLC
Associated management/control interfaces
P. Bhagwat

7. 802.11 PHY Applications Control MAC PHY LLC MAC Mgmt MIB DSSS FH IR
WEP
MAC
Mgmt
MAC
MIB
PHY
DSSS FH IR
OFDM
P. Bhagwat

8. 802.11 PHY Sender Receiver 802.11a 802.11g 802.11b MAC PHY
Pravin Bhagwat
PHY
Sender
Receiver
MAC Protcol Data
Unit (MPDU)
MAC Protcol Data
Unit (MPDU)
MAC
PHY
PLCP
header
MAC Protcol Data
Unit (MPDU)
PLCP
header
MAC Protcol Data
Unit (MPDU)
Physical Media
Dependent (PMD) layer
PMD layer
Direct Sequence Spread Spectrum (DSSS) PHY
1,2 Mbps
Frequency Hopping Spread Spectrum (FHSS) PHY
1, 2 Mbps
Infrared (IR) PHY
Orthogonal Frequency Division Multiplexing (OFDM) PHY
6,9,12,18,24,36,48,54 Mbps
802.11a
5.7 GHz
Mention that over SCO link you cannot carry any other real-time traffic. There is no protocol-id field in the SCO header/payload. Is this really true?
High rate (DSSS) PHY
11, 5.5 Mbps
802.11b
Higher rate (DSSS) PHY
20+ Mbps
802.11g
2.4 GHz
P. Bhagwat
Tutorial:

9. DSSS PHY 1 Mbps 1, 2 Mbps 1 Mbps 1, 2 Mbps
Pravin Bhagwat
DSSS PHY
Preamble
Header
MPDU
Preamble
Header
MPDU
1 Mbps
1, 2 Mbps
1 Mbps
1, 2 Mbps
DPSK
modulation
DPSK
de-modulation
Spread the signal using Barker word (11 bits)
+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1
Transmitter
baseband signal
Transmitted signal after spreading
Received signal after despreading
Baseband signal is spread using Barker word (10 dB processing gain)
Spread signal occupies approximately 22 Mhz bandwidth
Receiver recovers the signal by applying the same Barker word
DSSS provides good immunity against narrowband interferer
CDMA (multiple access) capability is not possible
Mention that over SCO link you cannot carry any other real-time traffic. There is no protocol-id field in the SCO header/payload. Is this really true?
P. Bhagwat
Tutorial:

10. DSSS PHY Direct sequence spread spectrum Symbol rate
Ch 1
Ch 6
Ch 11
22 Mhz
. . .
83.5 Mhz
Direct sequence spread spectrum
Each channel is 22 Mhz wide
Symbol rate
1 Mb/s with DBPSK modulatio
2 Mbps with DQPSK modulation
11, 5.5 Mb/ps with CCK modulation
Max transmit power
100 Mw
P. Bhagwat

11. 802.11 MAC Applications Control MAC PHY LLC MAC Mgmt MIB DSSS FH IR
WEP
MAC
Mgmt
MAC
MIB
PHY
DSSS FH IR
OFDM
P. Bhagwat

12. 802.11 MAC : Design goals Single MAC to support multiple PHYs
Support multiple channel PHYs
Robust against interference
Cope with hidden nodes
Support for time bounded service, QoS
Should be scalable and stable at high loads
Need provisions for Power Saving Modes
Need provisions for Privacy and Access Control
P. Bhagwat

13. 802.11 MAC Carrier sensing (CSMA) Collision detection (CD) Rules:
carrier ==> do not transmit
no carrier ==> OK to transmit
But the above rules do not always apply to wireless.
Solution: RTS/CTS
Collision detection (CD)
Does not work over wireless
Therefore, use collision avoidance (CA)
random backoff
priority ack protocol
P. Bhagwat

14. MAC layer
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
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15. 802.11 MAC protocol: CSMA/CA Busy medium Next Frame
contention
window
slot time
DIFS
Busy medium
Next Frame
Defer access
Use CSMA with collision Avoidance
Based on carrier sense function in PHY called Clear Channel Assessment (CCA)
Reduce collision probability where mostly needed
Efficient backoff algorithm stable at high loads
Possible to implement different fixed priority levels
P. Bhagwat

16. 802.11 MAC : Contention window
1023
CW max
For DSSS PHY
Slot time = 20 s
511
255
127 63 31
CW min
Fifth retransmission
Fourth retransmission
Third retransmission
Second retransmission
First retransmission
Initial attempt
P. Bhagwat

17. Backoff procedure A B C D DIFS DIFS DIFS DIFS DIFS CWindow CWindow
Frame
Frame
defer B
Frame
defer C
Frame
defer D
Frame
Immediate access when medium is free >= DIFS
When medium is not free, defer until the end of current frame trasnsmission + DIFS period
To begin backoff procedure
Choose a random number in ( 0, Cwindow)
Use carrier sense to determine if there is activity during each slot
Decrement backoff time by one slot if no activity is detected during that slot
Suspend backoff procedure if medium is determined to be busy at anytime during a backoff slot
Resume backoff procedure after the end of current frame transmission
P. Bhagwat

18. CSMA/CA + ACK protocol Defer access based on carrier sense
DIFS
Data
Src
SIFS
ACK
Dest
DIFS
contention
window
Next Frame
Other
Defer access based on carrier sense
Direct access when medium is sensed free longer than DIFS
Receiver of directed frames to return an ACK immediately when CRC is correct
When no ACK received then retransmit frame after a random backoff
P. Bhagwat

19. Fragments transmission
Fragment burst
DIFS
SIFS
SIFS
SIFS
SIFS
Backoff window
SIFS
Source
SIFS
Fragment 0
Fragment 1
Fragment 2
ACK 0
ACK 1
ACK 2
Destination
Fragment transmission supported to improve transmission reliability under noisy environments
Transmitter holds the channel until the end of fragment transmission burst
If the source does not receive and ACK frame, it will transmit the failed MPDU after performing the backoff procedure and the contention process
Receiver may receive duplicate fragments and is responsible for detecting and discarding duplicate fragments
P. Bhagwat

20. Problems with carrier sensing
Exposed terminal problem Z W
Z is transmitting
to W X Y
Y will not transmit to X
even though it cannot interfere
Presence of carrier ===> hold off transmission /
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21. Problems with carrier sensing
Hidden terminal problem Y Z W
W finds that medium is free
and it transmits a packet to Z
no carrier ===> OK to transmit /
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22. Solving Hidden Node problem with RTS/CTS
listen RTS ==> transmitter is close to me
listen CTS ==> receiver is close to me
- listen RTS
- wait long enough
for the requested
station to respond
with CTS
- if (timeout) then
ready to transmit
- listen CTS
for the transmitter
to send its data Y Z X W
Note: RTS/CTS does not solve exposed terminal problem. In the example above,
X can send RTS, but CTS from the responder will collide with Y’s data.
P. Bhagwat

23. RTS/CTS exchange example
SIFS
DIFS
RTS
Frame
Src
CTS
ACK
Dest
352 µs
304 µs
8192 s
304 µs 10 µs 10 µs 10 µs
Dest
NAV (RTS)
NAV (CTS)
RTS + CTS + Frame + ACK exchange invoked when frame size is large
Overhead estimation
RTS bytes (PCLP Preamble) + 6 bytes (PCLP Header) + 20 bytes (RTS)
192 µs µs = 352 µs
CTS bytes (PCLP Preamble) + 6 bytes (PCLP Header) + 14 bytes (RTS)
192 µs µs = 304 µs
SIFS – 10 µs
NAV (Network Allocation Vector)
NAV maintains prediction of future traffic on the medium based on duration information that is announced in RTS/CTS frames prior to actual exchange of data
P. Bhagwat