Class
Last WLAN MAC
CSE 5345 Fundamentals of Wireless Networks
NAV
DCF
Multi-rate support
Yonghe Liu
[e-mail]
2/16/12
Calculation
Distributed Coordination Function (DCF)
RTS:SIFS+CTS+SIFS+DATA+SIFS+ACK
CTS: RTS duration (CTS+SIFS)
DATA: CTS duration (DATA+SIFS)
ACK: Data duration (ACK+SIFS)
Distributed random access
Carrier sense and virtual carrier sense
NAV
Exponential backoff for collision avoidance
Double Contention Window if retransmission
Fallback to CWmin if success
CWmin (e.g. 31 )<= CW <= Cwmax (e.g. 1023)
Ack to increase reliability
Local retransmission
Short retry (e.g.4) and long retry (e.g., 7)
3
4
1
Today
Some Terminology
PHY
MSDU: MAC Service Data Unit
Link layer frame
Received at MAC via MAC SAP
MPDU: MAC Protocol Data Unit
Mac layer frame
Send to PHY through PHY SAP
MMPDU: MAC Management Protocol
Data Unit
6
Three Types of Frames
Data frames
Control frames
General Frame Format
General frame format
Some frame types may omit some fields
Ack, RTS/CTS
Management frames
(re)Association, beacon, probe
7
Frame control field
8
2
Type Field
To/From DS Field
Type and subtype identify the function of the frame:
Type=00
Management Frame
Type=01
Control Frame
Type=10
Data Frame
Data frame subtypes: data+ack, data+poll, etc
Poll is a data frame
Type=11
Reserved
To DS
From DS
Address 1
Address 2
Address 3
Address 4
0
0
DA
SA
BSSID
N/A
0
1
DA
BSSID
SA
N/A
1
0
BSSID
SA
DA
N/A
1
1
RA
TA
DA
SA
Addr. 1 = All stations filter on this address.
Addr. 2 = Transmitter Address (TA), Identifies transmitter to address the ACK
frame to.
Addr. 3 = Dependent on To and From DS bits.
Addr. 4 = Only needed to identify the original source of WDS
(Wireless Distribution System) frames
9
10
Specific Frame Format
Specific Frame Format
RTS
Ack
CTS
Data (address4 0/6 bytes)
11
12
3
PHY service
PHY Reference Model
Transmit the MAC frame over the wireless
medium
MAC frame (MPDU) is PHY payload
13
Physical Layer Convergence Protocol
14
PLCP Frame Format
Convergence procedure in which MPDUs
are converted to and from PPDUs.
PPDU: PLCP Protocol Data Unit
Prepend a PLCP Preamble and Header
PMD provide real transmission
Sync, (de)modulation, etc.
15
16
4
PLCP Preamble
PLCP Header
Signal
SYNC
128 Bits scrambled ones
For receiver to perform synchronization
Modulation scheme
Differential BPSK (1 Mbps) or Differential QPSK (2 Mbps)
NO coding
Service Field
Start Frame Delimiter (SFD)
Predefined 16 bits value
Indicating start of PHY dependent parameters
Reserved
Length
16 bit unsigned integer to indicate frame length
In number of microseconds
CRC
On PLCP header
17
PLCP Transmission
18
PLCP Receiving
CCA: clear channel assessment
Determines channel state: busy or idle
19
20
5
Clear Channel Assessment Methods
Energy Sequence detection
Direct Spread Spectrum
TX Signal is spreaded with a PN sequence
Energy above threshold
1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1
Carrier sense only
Signal lock
Carrier sense with energy detection
21
Direct Sequence Spread Spectrum
RX Signal is multiplied with the same
sequence
22
Carrier Sense
Measure/count the peaks
1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1
+1 1 +1 +1 -1 +1 +1 +1 -1 -1 -1
+1 1 +1 +1 -1 +1 +1 +1 -1 -1 -1
23
1 1 +1 +1 -1 +1 +1 +1 -1 -1 -1 +1 1 +1 +1 -1 +1 +1 +1 -1 -1 -1
+1 1 +1 +1 -1 +1 +1 +1 -1 -1 -1
24
6
CDMA
Discussion on CCA Methods
Code Division Multiple Access
Energy detection
Each user assigned an orthogonal code
-11-11, 1-1-11
TX signal spreaded using own code
RX signal multiplied with the same code
Carrier sense
Sensing range and transmission range
Cancel out others signal
Achieve simultaneous transmission
Power shared among users
25
Extended Inter-Frame Space (EIFS)
Used if RX an erroneous frame transmission
26
Discussion
What is the rational behind EIFS?
PHY indicated there IS a frame
Preamble locked
Header checked correctly
MAC did not correctly receive frame
MAC CRC
Lost of carrier
Etc.
Will you enforce it when designing a .11
system?
Use EIFS instead of DIFS
EIFS = aSIFSTime + (8ACKSize)
+ aPreambleLength + aPLCPHeaderLength+ DIFS
27
28
7
Operating Frequency
DSSS Channel Allocation
Operates in unlicensed 2.4 GHz ISM band
ISM: Industrial, Scientific and Medical
ISM regulatory range:
2.4 GHz to 2.4835 GHz for North America
Different in other regions
11 channels
20Mhz each
Center frequency 5M apart
Indexed 1 11
1, 6, 11 non-overlapping
Frequency reuse
29
DSSS Channel Allocation
30
802.11 DSSS PHY Parameters
aSlotTime
aSIFSTime
10s
aPreambleLength
144s
aPLCPHeaderLength
48 s
aCWmin
31
aCWmax
31
20s
1023
32
8
Frequency Hopping Spread Spectrum
PLCP frame format
Frequency Hopping Spread Spectrum
Use 1Mhz wide channel
79 channels in North America
Hops over the channels in predefined pattern
Hop sequence
Min hop rate
PSDU Length Word
In number of bytes
PLCP Signaling Field
Data rate
33
802.11b
34
802.11b Long PLCP Frame Format
High data rate enhancement for 2.4GHz
5.5 and 11 Mbps
Use either complementary code keying or
optional packet binary convolutional coding
mode
Based on 802.11 DSSS
Marketed as Wi-Fi
Service Field
35
36
9
802.11b Short PLCP Frame Format (Optional)
Discussion
PLCP Preamble/Header TX at lower rate
1 or 2 Mbps
Payload at higher rate with very different
schemes
5.5 or 11 Mbps
PMD Architecture to support it?
37
38
Discussion
PLCP Preamble/Header TX at lower rate
Carrier sense
range
Interference
range
1 or 2 Mbps
Payload at higher rate with very different
schemes
5.5 or 11 Mbps
DATA
A
B
C
D
E
F
Carrier sense range vs transmission range
Transmit range
39
40
10