csma.pdf

WLAN, part 2
Wireless Innovative Transmission Lab 1
Contents
IEEE 802.11 MAC layer operation
• Basic CSMA/CA operation
• Network Allocation Vector (NAV)
• Backoff operation
• Wireless medium access example
Usage of RTS / CTS

WLAN, part 2

WLAN, part 2
Medium Access Control (MAC)
LLC
MAC
PHY
:
Medium access control: Different nodes must gain access
to the shared medium (for instance a wireless channel) in
a controlled fashion (otherwise there will be collisions).
FDMA
TDMA
CDMA
CSMA
Assigning channels in frequency domain
Assigning time slots in time domain
Assigning code sequences in code domain
Assigning transmission opportunities in
time domain on a statistical basis
(Randomly chosen numbers)
Access methods:

WLAN, part 2
CSMA/CD vs. CSMA/CA (1)
CSMA/CD (Collision Detection) is the MAC method used in
a wired LAN (Ethernet). Wired LAN stations can (whereas
wireless stations cannot) detect collisions.
Basic CSMA/CD operation:
1) Wait for free medium
2) Transmit frame
3) If collision, stop transmission immediately
4) Retransmit after random time (backoff)
CSMA/CD rule:
Backoff after collision

WLAN, part 2
CSMA/CD
(Flowchart)

WLAN, part 2
CSMA/CD vs. CSMA/CA (2)
CSMA/CA (Collision Avoidance) is the MAC method used in
a wireless LAN. Wireless stations cannot detect collisions
(i.e. the whole packets will be transmitted anyway).
Basic CSMA/CA operation:
1) Wait for free medium
2) Wait a random time (backoff)
3) Transmit frame
4) If collision, the stations do not notice it
5) Collision => erroneous frame => no ACK returned
CSMA/CA rule:
Backoff before
collision

WLAN, part 2
CSMA/CA
(Flowchart)

WLAN, part 2
Basic wireless medium access
AP
transmission in downlink
(from the AP)
and
transmission in uplink
(from a station)
CSMA:
One packet at a time
wired
LAN

WLAN, part 2
Wireless medium access (1)
DIFS SIFS
ACK
(B=>A)
Transmitted
frame
(A=>B)
When a frame is received without bit errors, the receiving
station (B) sends an Acknowledgement (ACK) frame back
to the transmitting station (A).
If the received frame
is erroneous, no ACK
will be sent

WLAN, part 2
DIFS SIFS DIFS
ACK
(B=>A)
Transmitted
frame
(A=>B)
During the transmission sequence (Frame + SIFS + ACK)
the medium (radio channel) is reserved. The next frame
can be transmitted at earliest after the next DIFS period.
Next frame
(from any station)
Earliest allowed
transmission time
of next frame

WLAN, part 2
The two most important interframe spacing times are
SIFS and DIFS:
SIFS (Short Interframe Space) = 10 µs (16 µs)
DIFS (DCF Interframe Space) = 50 µs (34 µs)
When two stations try to access the medium at the
same time, the one that has to wait for the time SIFS
wins over the one that has to wait for the time DIFS.
In other words, SIFS has higher priority over DIFS.
802.11b 802.11g

WLAN, part 2
DIFS SIFS t > DIFS
ACK
(B=>A)
Transmitted
frame
(A=>B)
When a station wants to send a frame and the channel has
been idle for a time > DIFS (counted from the moment the
station first probed the channel) => can send immediately.
Next frame
(from any station)
Channel was idle at
least DIFS seconds

WLAN, part 2
DIFS SIFS DIFS
ACK
(B=>A)
Transmitted
frame
(A=>B)
When a station wants to send a frame and the channel is
busy => the station must wait a backoff time before it is
allowed to transmit the frame. Reason? Next two slides…
Next
frame
Channel was busy
when station wanted
to send frame
Backoff

WLAN, part 2
No backoff => collision is certain
Suppose that several stations (B and C in the figure) are
waiting to access the wireless medium.
When the channel becomes idle, these stations start
sending their packets at the same time => collision!
Station A
Station B
Station C
DIFS
Collision!
ACK

WLAN, part 2
Backoff => collision probability is reduced
Contending stations generate random backoff values bn.
Backoff counters count downwards, starting from bn.
When a counter reaches zero, the station is allowed to
send its frame. All other counters stop counting until the
channel becomes idle again.
Station A
Station B
Station C
DIFS
bn is large
bn is small
Backoff
Remaining
backoff time
ACK

WLAN, part 2
Contention window (CW) for 802.11b
If transmission of a frame was unsuccessful and the frame
is allowed to be retransmitted, before each retransmission
the Contention Window (CW) from which bn is chosen is
increased.
DIFS
… CW = 25-1 = 31 slots
(slot = 20 µs)
Initial attempt
DIFS
…
CW = 26-1 = 63 slots
1st retransm.
DIFS
CW = 210-1
= 1023 slots
5th (and further)
retransmissions
:
…
CW
802.11b

WLAN, part 2
Contention window (CW) for 802.11g
In the case of 802.11g operation, the initial CW length is
15 slots. The slot duration is 9 µs. The backoff operation
of 802.11g is substantially faster than that of 802.11b.
DIFS
… CW = 24-1 = 15 slots
(slot = 9 µs)
Initial attempt
DIFS
…
CW = 25-1 = 31 slots
1st retransm.
DIFS
CW = 210-1
= 1023 slots
6th (and further)
retransmissions
:
…
CW
802.11g

WLAN, part 2
No shortcuts for any station…
DIFS SIFS DIFS
ACK
(B=>A)
Transmitted
frame
(A=>B)
Next
frame
(A=>B)
Backoff
When a station wants to send more than one frame, it has
to use the backoff mechanism like any other station (of
course it can ”capture” the channel by sending a long
frame, for instance using fragmentation).

WLAN, part 2
Hidden Terminal Problem
A
B
X
carrier not free ≠ OK to transmit

WLAN, part 2
Usage of RTS & CTS
The RTS/CTS (Request/Clear To Send) scheme is used as
a countermeasure against the “hidden node” problem:
AP
WS 1
WS 2
Hidden node problem:
WS 1 and WS 2 can ”hear”
the AP but not each other
=>
If WS 1 sends a packet, WS 2 does not
notice this (and vice versa) => collision!

WLAN, part 2
Reservation of medium using NAV
The RTS/CTS scheme makes use of “SIFS-only” and the
NAV (Network Allocation Vector) to reserve the medium:
RTS
SIFS
DIFS
NAV = CTS + Data + ACK + 3xSIFS
CTS
Data frame
ACK
SIFS
SIFS
WS 1
AP
NAV = Data + ACK + 2xSIFS
NAV in RTS
NAV in CTS

WLAN, part 2
CSMA/CA and NAV

WLAN, part 2
Danger of collision only during RTS
WS 2 does not hear the RTS frame (and associated NAV),
but can hear the CTS frame (and associated NAV).
RTS
NAV = CTS + Data + ACK + 3xSIFS
CTS
Data frame
ACK
WS 1
AP
NAV = Data + ACK + 3xSIFS
NAV in RTS
NAV in CTS
Danger of collision

WLAN, part 2
Exposed Terminal Problem

WLAN, part 2
Advantage of RTS & CTS (1)
Usage of RTS/CTS offers an advantage if the data frame
is very long compared to the RTS frame:
RTS
CTS
Data frame
ACK
WS 1
AP
Short interval: collision not likely
Data frame
ACK
WS 1
AP
Long interval: collision likely
(RTS/CTS not used)
(RTS/CTS used)

WLAN, part 2
Advantage of RTS & CTS (2)
A long “collision danger” interval (previous slide) should
be avoided for the following reasons:
Larger probability of collision
Greater waste of capacity if a collision occurs and the
frame has to be retransmitted.
A threshold parameter (IEEE 802.11 frame~Threshold)
can be set in the wireless station. Frames shorter than
this value will be transmitted without using RTS/CTS.

WLAN, part 2
Fragmentation
Fragmentation makes use of the RTS/CTS scheme and the
NAV mechanism:
RTS
SIFS
DIFS
RTS
CTS
Frag 0
ACK 0
SIFS
SIFS
WS 1
AP
CTS
NAV in WS
NAV in AP
Frag 1
ACK 1
SIFS
SIFS
Frag 0
ACK 0

WLAN, part 2
Advantage of fragmentation
Transmitting long data frames should be avoided for the
following reasons:
Larger probability that the frame is erroneous
Greater waste of capacity if a frame error occurs and
the whole frame has to be retransmitted.
A threshold parameter (dot11FragmentationThreshold)
can be set in the wireless station. Frames longer than
this value will be transmitted using fragmentation.

WLAN, part 2
WBAN CSMA/CA
Transmitting long data frames should be avoided for the
following reasons:
Larger probability that the frame is erroneous
Greater waste of capacity if a frame error occurs and
the whole frame has to be retransmitted.
A threshold parameter (dot11FragmentationThreshold)
can be set in the wireless station. Frames longer than
this value will be transmitted using fragmentation.

WLAN, part 2
B
MAP1 MAP2
EAP1 RAP1 EAP2 RAP2 CAP
Beacon period (superframe) n
B2
7
UP
CSMA/Slotted
Aloha
7
UP
CSMA/Slotted
Aloha
s
All UP
CSMA/Slotted
Aloha Polling Mechanisms
s
All UP
CSMA/Slotted
Aloha
s
All UP
CSMA/Slotted
Aloha
Polling Mechanisms
WBAN

WLAN, part 2
Start
Channel is
Busy?
BC = BC-1
BC=0?
Transmit Packet and
Wait for ACK
ACK received
within timeout
period?
Freeze the BC
node has a
packet to
transmit
UPi
Y
Y
N
N
N
Y
N
j=j+1
CW CW
i,j i,max
>
Backoff Counter
(BC)=rand (1,CW )
,min
i
Y
N
BC=rand (1,CW )
i,j
BC=rand (1,CW )
i,max
Collision Counter
j=0
New CW Size;
j
2
CW =2 CW
i,j i,min
 
 
 
 
 
WBAN CSMA/CA
(Flowchart)

WLAN, part 2
Start
Current time is
outside of suitable
access phase?
Channel is
Busy?
No enough time left
in the suitable access
phase(s) to transmit
the packet?
BC = BC-1
BC=0?
Transmit Packet and
Wait for ACK
ACK received
within timeout
period?
Freeze the BC
node has a
packet to
transmit
UPi
Y
Y
Y
N
N
N
N
Y
Y
N
j=j+1
CW CW
i,j i,max
>
Backoff Counter
(BC)=rand (1,CW )
,min
i
Y
N
BC=rand (1,CW )
i,j
BC=rand (1,CW )
i,max
Collision Counter
j=0
New CW Size;
j
2
CW =2 CW
i,j i,min
 
 
 
 
 
End
WBAN CSMA/CA
Flowchart for all the
access phases

csma.pdf

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