802.11 MAC Sublayer
• MAC layer tasks:
–Control medium access
–Roaming, authentication, power conservation
• Traffic services
–DCF (Distributed Coordination Function)
(mandatory): Asynchronous Data Service
•
•
•
•
Only service available in ad-hoc network mode
does not use any kind of central control
exchange of data packets based on “best-effort”
support of broadcast and multicast
–PCF (Point Coordination Function) (optional): TimeBounded Service
• uses the base station to control all activity in its cell
1
802.11
MAC
Sublayer
• PCF and DCF can coexist within one cell by carefully defining
the interframe time interval. The four intervals are depicted:
– SIFS (Short InterFrame Spacing) is used to allow the parties in a
single dialog the chance to go first including letting the receiver
send a CTS and an ACK and the sender to transmit the next
fragment.
– PIFS (PCF InterFrame Spacing) is used to allow the base station to
send a beacon frame or poll frame.
– DIFS (DCF InterFrame Spacing) is used to allow any station to grab
the channel and to send a new frame.
– EIFS (Extended InterFrame Spacing) is used only by a station that
has just received a bad or unknown frame to report the bad frame.
• The result MAC scheme used in 802.11 is carrier sensing
multiple access with collision avoidance (CSMA/CA) that is
based on MACAW.
– Use NAV (Network Allocation Vector) to indicate the channel is
busy.
2
The 802.11 MAC Sublayer Protocol
Interframe spacing in 802.11.
3
802.11 MAC Sublayer
• Access methods
– DFWMAC-DCF (distributed foundation wireless medium
access control- Distributed Coordination Function)
CSMA/CA (mandatory)
• collision avoidance via randomized „back-off“ mechanism
• minimum distance between consecutive packets
• ACK packet for acknowledgements (not for broadcasts)
– DFWMAC-DCF w/ RTS/CTS (optional)
• avoids hidden terminal problem
– DFWMAC- PCF (Point Coordination Function) (optional)
• access point polls terminals according to a list
• Completely controlled by the base station. No collisions occur.
• A beacon frame which contains system parameters is periodically (10 to
100 times per second) broadcasted to invite new stations to sign up for
polling service.
4
802.11 - CSMA/CA access method
DIFS
DIFS
medium busy
contention window
(randomized back-off
mechanism)
next frame
direct access if
medium is free  DIFS
t
slot time
• Station ready to send starts sensing the medium (Carrier
Sense based on CCA, Clear Channel Assessment)
• If the medium is free for the duration of an Inter-Frame Space
(IFS), the station can start sending (IFS depends on service
type)
• If the medium is busy, the station has to wait for a free IFS,
then the station must additionally wait a random back-off
time (collision avoidance, multiple of slot-time)
• If another station occupies the medium during the back-off
time of the station, the back-off timer stops (fairness)
5
802.11 - Competing Stations
DIFS
DIFS
DIFS
boe bor
station1
DIFS
boe bor
boe busy
boe busy
boe bor
boe busy
boe bor
boe busy
station2
busy
station3
station4
boe bor
station5
t
busy
medium not idle (frame, ack etc.)
boe elapsed backoff time
packet arrival at MAC
bor residual backoff time
6
802.11 - CSMA/CA access method
• Sending unicast packets
– station has to wait for DIFS before sending data
– receivers acknowledge at once (after waiting for SIFS) if the packet
was received correctly (CRC)
– automatic retransmission of data packets in case of transmission
errors
DIFS
sender
data
SIFS
receiver
ACK
DIFS
other
stations
waiting time
7
data
t
contention
802.11 – DFWMAC
• Sending unicast packets
– station can send RTS with reservation parameter (transmission
duration) after waiting for DIFS (reservation determines amount of
time the data packet needs the medium)
– acknowledgement via CTS after SIFS by receiver (if ready to receive)
– sender can now send data at once, acknowledgement via ACK
– other stations set its net allocation vector (NAV) in accordance with
the duration field.
DIFS
sender
RTS
data
SIFS
receiver
SIFS
CTS SIFS
DIFS
NAV (RTS)
other
stations
ACK
NAV (CTS)
defer access
8
data
t
contention
Fragmentation
 The deal with the problem of noisy channels, 802.11 allows frames
to be fragmented.
DIFS
sender
RTS
frag1
SIFS
receiver
CTS SIFS
frag2
SIFS
ACK1 SIFS
SIFS
ACK2
NAV (RTS)
NAV (CTS)
other
stations
NAV (frag1)
DIFS
NAV (ACK1)
contention
9
data
t
DFWMAC-PCF
 A super frame comprises a contention-free period and a
contention period.
• D for downstream
• U for upstream
• CF for an end maker
t0 t1
medium busy
PIFS
point
coordinator
SIFS
D1
SIFS
SIFS
D2
SIFS
U1
wireless
stations
stations‘
NAV
SuperFrame
U2
NAV
10
DFWMAC-PCF
t2
point
coordinator
D3
SIFS
D4
t4
CFend
SIFS
U4
wireless
stations
stations‘
NAV
PIFS
t3
NAV
contention free period
11
contention
period
t
• Types
802.11 MAC Frame format
– control frames, management frames, data frames
• Sequence numbers
– important against duplicated frames due to lost ACKs
• Addresses
– receiver, transmitter (physical), BSS identifier, sender (logical)
• Miscellaneous
– sending time, checksum, frame control, data
bytes
2
Frame
Control
bits
2
2
6
Duration/ Address
ID
1
2
Protocol
Type
version
4
Subtype
1
To
DS
6
Address
2
1
6
Address
3
1
1
2
6
Sequence Address
Control
4
1
1
1
0-2312
Data
1
From More
Power More
Retry
WEP Order
DS Frag 12
Mgmt Data
4
CRC
MAC address format
scenario
ad-hoc network
infrastructure
network, from AP
infrastructure
network, to AP
infrastructure
network, within DS
to DS from
DS
0
0
0
1
DS: Distribution System
AP: Access Point
DA: Destination Address
SA: Source Address
address 1 address 2 address 3 address 4
DA
DA
SA
BSSID
BSSID
SA
-
1
0
BSSID
SA
DA
-
1
1
RA
TA
DA
SA
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address
 Ad-hoc network: packet exchanged between two wireless nodes without a
distribution system
 Infrastructure network, from AP: a packet sent to the receiver via the access point
 Infrastructure network, to AP: a station sends a packet to another station via the
access point
 Infrastructure network, within DS: packets transmitted between two access points
over the distribution system.
13
Special Frames: ACK, RTS, CTS
bytes
• Acknowledgement Frame2
ACK
2
Control
bytes
RTS
2
Frame
Control
6
Receiver
Duration
Address
2
6
Receiver
Duration
Address
4
CRC
6
4
Transmitter
CRC
Address
• Request To Send
bytes
CTS
2
Frame
Control
• Clear To Send
14
2
6
Receiver
Duration
Address
4
CRC
802.11
MAC
management
• Synchronization
– try to find a LAN, try to stay within a LAN
– Synchronize internal clocks and generate beacon signals
• Power management
– periodic sleep, frame buffering, traffic measurements
– sleep-mode without missing a message
• Roaming for Association/Reassociation
– integration into a LAN
– roaming, i.e. change networks by changing access points
– scanning, i.e. active search for a network
• MIB - Management Information Base
– All parameters representing the current state of a wireless
station and an access point are stored in a MIB.
– A MIB can be accessed via SNMP.
15
Synchronization using a Beacon
(infrastructure)
 Timing synchronization function (TSF) is needed for:
• Power management
• Coordination of the PCF and for synchronization of the hopping
sequence
 A beacon contains a timestamp and other management
information.
 The access point tries to schedule transmissions according to
the excepted beacon interval (target beacon transmission time).
beacon interval
access
point
medium
B
B
busy
busy
B
busy
B
busy
t
value of the timestamp
B beacon frame
16
Synchronization using a Beacon (adhoc)
 The standard random backoff
algorithm is also applied to the
beacon frames in the ad-hoc networks.
beacon interval
station1
B1
B1
B2
station2
medium
busy
busy
B2
busy
busy
t
value of the timestamp
B beacon frame
17
random delay