Collision of wireless signals
The MAC layer in wireless networks
The wireless MAC layer roles
Collision has destructive effect on the receiver
Access control to shared channel(s)
Natural broadcast of wireless transmission
Collision of signal: a time/space problem
Who transmits when? (and where)?
Avoid collisions (no Collision Detection)
Capture effect is possible
Collision domain: set of nodes sharing the same channel
Scarce resources utilization
Channel capacity and battery power
performance and QoS
Frame organization, and intra-, inter-layer
information management
...causes both channel and power waste
Collision detection is not practical in wireless systems
Collision avoidance/resolution + contention control on the sender
Exploited to enhance channel reuse, if possible
Space splitting, transitive relation
System level and (or vs?) user level
Cross layering principles for adaptive behavior?
Risk for “spaghetti design” [Kumar2003]
[Kumar2003] V. Kawadia, P.R. Kumar, "A Cautionary Perspective on Cross Layer Design", Submitted for publication, 2003
(http://black1.csl.uiuc.edu/~prkumar/)
© Luciano Bononi 2007
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Wireless MAC protocols’ classification
© Luciano Bononi 2007
Distributed, contention-based wireless MAC Problem:
Contention free
deterministic access
ID mapping
probabilistic
Contention control
Contention resolution
random access
Distributed dynamic
centralized static
Implicit Reservation
Reservation based
Token based
Fixed TDMA,
FDMA, CDMA
the frame vulnerability (collision risk)
Needs resolution in distributed way (no centralized coordinator)
let’s analyze the time domain first
deterministic
Aloha [Abramson1970]: no coordination
Slotted Aloha
CSMA [Kleinrock1975]: listen before to transmit
Slotted CSMA
CSMA/CD: listen before and while transmitting
No Centralized
Coordinator,
No Centralized
Coordinator,
No Centralized
Coordinator,
Centralized
Coordinator,
static allocation of
resources
Dynamic allocation
of resources
Dynamic allocation
of resources
Static allocation of
resources
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(unpractical in wireless scenarios)
CSMA/CA + contention resolution (reactive resolution of collisions)
CSMA/CA + contention control (preventive/reactive reduction of risk of
collisions)
[Abramson1970]
N. Abramson, ``The ALOHA system - another alternative for computer communications'', Proc. Fall Joint Comput. Conf. AFIPS, 1970
[Kleinrock1975]
L. Kleinrock, F.A. Tobagi ``Packet Switching in Radio Channels: Part I - Carrier Sense Multiple-Access modes
and their throughput-delay characteristics'', IEEE Transactions on Communications, Vol Com-23, No. 12, pp.1400-1416, 1975
Hybrid
Cluster-based
definitions
MAC?
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2
Evolutionary perspective of distributed MAC
Multiple access protocols
Distributed Contention based
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1
The ALOHA protocol
The ALOHA protocol
An integral part of the
ALOHA protocol is feedback
from the receiver
Feedback occurs after a
packet is sent
No coordination among sources
no
Packet
ready?
Frame size
STA i
ok1
frame1
yes
early1
late2
ok2
transmit
wait for a
round-trip time
yes
positive
ack?
Collision domain
delay packet
transmission
k times
late2
ok2
ok1
time
New1+frame1 coll.
Frame1+New2 coll.
© Luciano Bononi 2007
time
collision
Frame1
early1
compute random
backoff integer k
no
time
STA j
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Slotted ALOHA
Frame vulnerability time: twice the frame size
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Slotted ALOHA
STA i Framesize
Packet
ready?
no
frame1
STA j
yes
yes
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positive
ack?
no
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frame2
time
Collision domain
delay packet
transmission
k times
wait for a
round-trip time
quantized in slots
time
time slot
frame2
Wait for start of next slot
transmit
Propagation
frame1
delay
NEW collision
frame1
frame1
frame2
time
Frame1+frame2 collision
compute random
backoff integer k
7
Frame vulnerability time: the frame size (slot + propagation)
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2
CSMA Protocol
CSMA Protocol
STA i
Packet
ready
Frame size
Channel
Busy?
no
frame1
frame1
yes
early2
transmit
delay packet
transmission
k times
Frame1
detected
late2
Collision domain
wait for a
round-trip time
time
Propagation delay
STA j
late2
time
...
collision
frame1
NEW
late2
early2
late2
time
Frame1+late2 vulnerability
positive
ack?
yes
no
compute random
backoff integer k
Frame1+early2 vulnerability
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Slotted CSMA Protocol
STA i
Frame vulnerability time: twice the propagation delay
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Throughput comparison
Frame size
Analytical Results
1
frame1
Pure CSMA
Propagation delay
frame2
0.8
Frame1
detected
0.7
frame2
...
Collision domain
time
collision
NEW
frame2
Slotted CSMA
0.9
time
frame1
frame2
time
0.6
S (Throughput)
STA j
frame1
0.5
0.4
Slotted Aloha
0.3
0.2
Frame1+frame2 vulnerability
0
-3
10
Frame vulnerability time: the propagation delay
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Pure Aloha
0.1
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© Luciano Bononi 2007
10
-2
10
-1
10
0
1
10
Offered Load: G
10
2
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3
10
4
10
5
12
3
CSMA/CA: the IEEE 802.11 Wireless LAN
Evolutionary perspective of distributed MAC
1 Medium Access Control (MAC) protocol:
[Bharghavan et al. 1994]
V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, "MACAW: A Media Access Protocol for Wireless LAN's," proc. ACM SIGCOMM'94,
pp.212-225, London, 1994
[Fullmer et al. 1995]
C.L. Fullmer, J.J. Garcia-Luna-Aceves, "Floor Acquisition Multiple Access (FAMA) for Packet Radio Networks", Proc. ACM Sigcomm'95
Cambridge, MA, 1995
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Hidden and Exposed terminals: RTS/CTS
Hidden terminals: B does not sense traffic, but the receiver C cannot receive its
packet due to a transmission from A to C (A hidden to B)
to seize the channel, according to CSMA/CA, a station transmits a short
RTS (request to send) packet and waits for the CTS (Clear to Send)
response.
© Luciano Bononi 2007
RTS
C
CTS
C
CTS
B
C
A CTS C
CTS B
D
B B to D
RTS
CTS
Adopt RTS/CTS
Time
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A
RTS
Collision on C
A to C
14
RTS/CTS is not a “guaranteed” solution and it is additional overhead
Power asymmetry, detection and interference range >> transmission range
A transmit RTS to C and seizes the coverage area
C respond with CTS to A (B receive the CTS and does not transmit even if it
cannot sense A’s transmission)
A
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RTS/CTS drawbacks
The space domain:
Hidden and exposed terminals: space vulnerability
RTS/CTS mechanism to contrast Hidden terminals
Today under some criticisms
[Karn1990]
P. Karn, ""MACA - A new Channel Access Method for Packet Radio", proc. 9-th Computer Networking Conference, September 1990
Will be analyzed later...
© Luciano Bononi 2007
MACA [Karn1990]: RTS/CTS, no carrier sense (MACA-BI, RIMA...)
MACAW [Bharghavan et al.1994]: RTS/CTS, no carrier sense and
immediate ACK (more reliable and efficient Link Layer Control)
FAMA [Fullmer et al.1995]: RTS/CTS, carrier sense + other stuff
Main solution: RTS/CTS mechanism
Centralized control (Base station)
Polling based access (soft QoS, minimum delay)
minimum bandwidth guarantee
the frame vulnerability (collision risk)
Needs resolution in distributed way (no centralized coordinator)
let’s analyze the Space domain
Ad-Hoc networks (peer to peer)
Distributed control (no base station)
contention based access (no QoS, no minimum delay)
CSMA/CA access protocol with Binary Exponential Backoff
Point Coordination Function (PCF)
Distributed, contention-based wireless MAC Problem:
2 coordination functions co-exist in a superframe structure (time
division)
Distributed Coordination Function (DCF)
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Data
Space
15
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4
MACA: slotted RTS/CTS, no CS
Ad hoc Multi-hop: Time/Space problems
A bi-directional chain of MAC frames
Self-contention (MAC layer problem)
TCP streams (Data + Ack)
Inter-stream self-contention (Data vs. Ack TCP
streams)
Intra-stream self-contention (same TCP stream)
How to obtain coordination?
New proposed solutions
Fast forward
Quick exchange
Flow numbering (pre-routing at the MAC layer???)
Frame transmission by forward invitation
MACA: eliminates the carrier sensing
...because the contention is on the receiver!
Introduces slotted RTS/CTS (30 bytes each) and slot time equals the
RTS (and CTS) duration
Allow exploitation of concurrent spatial transmission if the receiver
is not exposed to two hidden transmitter terminals
Variations: MACA-BI, RIMA (receiver initiated)
RTS
Collision on B and C?
C
A CTS C
CTS B
A
C to A
C
D
B B to D
RTS
CTS
Adopt RTS/CTS
Time
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MACAW: no cs + slotted RTS/CTS + ACK
A to C
C
D
B B to D
A to C
A
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Data
18
RTS
Collision on C
RTS
CTS
Time
Space
FAMA: re-introduces carrier sensing before both slotted RTS/CTS
Introduces lower bound for size of RTS/CTS and CTS-dominance
Floor acquisition: principle for time and space contention
C
A CTS C
CTS B
Adopt RTS/CTS
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RTS
Collision on C
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FAMA: cs + slotted RTS/CTS + ACK
MACAW: fairness of the backoff procedure
MILD + Binary Exponential Backoff
Cooperation-based backoff values (space-issues of contention)
no carrier sensing before both slotted RTS/CTS
Introduces ACK (RTS – CTS – DATA – ACK)
Efficient retransmission policy at Data Link layer
Problem: both sender and receiver act as receiver during frame
transmissions (no concurrent space exploitation of the channel)
A
© Luciano Bononi 2007
Data
C
C
A CTS C
CTS B
D
B B to D
RTS
CTS
Adopt RTS/CTS
Time
Space
19
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Data
Space
20
5
Ad hoc Multi-hop: Time/Space problems
Ad hoc Multi-hop: Time/Space problems
Source to destination stream: TCP DATA
Quick Exchange interinter-stream
Fast Forward intraintra-stream
Destination to Source stream: TCP ACK
RTS
RTS
Sender
Receiver
Receiver’s
Neighbors
DATA
Time
ACK
ACK
Sender’s
Neighbors
CTS
TCP ACK
TCP DATA
Time
RTS
CTS
CTS
ACK + RTS
RTS
ACK + RTS
CTS
CTS
ew Contention
NAV
(RTS)
DATA1
DATA
RTS
NAV
(CTS)
CTS
ACK + RTS
ACK + RTS
DATA
CTS
ACK
DATA
NAV
(DATA1)
ACK1
DATA2
NAV
(ACK1)
ACK2
ACK
ew Contention
RTS
© Luciano Bononi 2007
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© Luciano Bononi 2007
1 Medium Access Control (MAC) protocol:
Difs
Difs
Sifs
Sifs
2 coordination functions co-exist in a superframe structure (time
division)
Distributed Coordination Function (DCF)
DATA
RTS
DATA
RTS
Source
Sifs
Sifs
Sifs
Sifs
ACK
CTS
ACK
CTS
Destination
Destination
NAV (CTS)
Others
© Luciano Bononi 2007
Centralized control (Base station)
Polling based access (soft QoS, minimum delay)
minimum bandwidth guarantee
NAV (DATA)
NAV (DATA)
[ Time ]
NAV (CTS)
Ad-Hoc networks (peer to peer)
Distributed control (no base station)
contention based access (no QoS, no minimum delay)
CSMA/CA access protocol with Binary Exponential Backoff
Point Coordination Function (PCF)
NAV (RTS)
NAV (RTS)
Others
22
CSMA/CA: the IEEE 802.11 Wireless LAN
Modified NAV policy
Source
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Defer Access
Defer Access
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6
IEEE 802.11 MAC protocol architecture
Point coordinated mode (PCF)
• point coordinated mode is a contention free, optional service
contention free
services
Contention based
services
Point Coordination
Function (PCF)
MAC sublayer
• can co-exist with the DCF in a superframe structure.
•central coordinator, i.e. the access point
•manages stations belonging to its access list.
•guaranteed to access the channel in a contention-free environment.
Fundamental
Access Method
Distributed Coordination Function
(DCF)
Physical Layer (PHY)
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DCF and PCF control: IFS
PIFS
Poll
PCF
DCF Trans. 1
ack
Backoff period (contention)
DIFS
Base station
takes control
Poll trans. 1
SIFS
PCF (polling based) time (scenario 2)
ack
DCF stations wait for a DIFS...
DCF Trans. 2
ack
Poll trans. 2
Base Station waits for a PIFS after a transmission and takes control (DCF
stations must wait for DIFS>PIFS)
base station polls stations that reserved the channel
at the end of the PCF period the Base Station releases the channel and
DCF restarts (after a DIFS)
PIFS
Poll
ack
PCF
DIFS: restart DCF
DCF
DCF Trans. 1
DCF time (scenario 1: no PCF takes control)
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during the PCF time the base station has priority in accessing the
channel
Short IFS (SIFS) < Point IFS (PIFS) < Distributed IFS (DIFS)
after a SIFS only the polled station can transmit (or ack)
after a PIFS only the Base Station can transmit (and PCF takes control)
after a DIFS every station can transmit according to basic access CSMA/CA (DCF
restarts)
SIFS
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Point Coordination Function (PCF)
Each station performs a carrier sensing activity when accessing
the channel
priority is determined by Interframe spaces (IFS):
© Luciano Bononi 2007
ack
Backoff period (contention)
DIFS
Base station
takes control
Poll trans. 1
PCF (polling based) time (scenario 2)
ack
DCF stations wait for a DIFS...
DCF Trans. 2
ack
Poll trans. 2
ack
DCF
DCF time (scenario 1: no PCF takes control)
27
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7
Distributed Coordination Function (DCF)
CSMA/CA Access Mechanism
Basic Access mode:
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
access scheme (listen before transmit)
carrier sensing performed to detect ongoing transmissions
Binary Exponential Backoff over slotted idle time
each station randomly selects the transmission slot in a
variable sized Contention Window
no Collision Detection (CD)
SIFS
PIFS
Packet Transmitted
Medium Busy
SIFS
Packet Transmitted
DIFS
Backoff period (contention)
ack
Back-off
Window
DIFS
Backoff period
DCF Trans. 1
Slot Time
DCF Trans. 2
Ack
DCF
DCF time (scenario 1: no PCF takes control)
CSMA/CA is an efficient protocol for data traffic, like
Ethernet
Listen before transmit
Always back-off before a transmission or retransmission
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DCF Backoff procedure
Designed to provide fair access to the medium
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DCF basic access: overview
•Selection of a random Backoff Time
1
15
2
31
3
63
4
127
5
255
7
1023
Destination
ACK
• Collision: no CD
After an idle DIFS period from the last transmission, a station decrements its
Backoff Time by a Slot_time for each slot where no activity is sensed on the
medium.
DIFS
DIFS
Station B
As soon as the medium is determined to be busy, the backoff procedure is
suspended .
Station C
• Transmission
When the Backoff Time reaches zero, the station starts the transmission.
© Luciano Bononi 2007
A,
LB , LC}= lengths of colliding packets
IEEE 802.11 does not
implement
collision detection
LB
LC
COLLISION LENGTH
31
{L
LA
Station A
• Frozen
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DIFS
• Successful transmission
6
511
• Reduction of the Backoff Time
© Luciano Bononi 2007
SIFS
Source
BackoffTime(i)=(Cwi*random())*SlotTime
i
CWi
DATA
DIFS
CWi=contention window size at the i-th transmission attempt. CWi is
doubled after each collision experienced (to reduce the contention)
collision length = maximum{LA , LB , LC}
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IEEE 802.11 Contention Control
IEEE 802.11 Contention Control
Effect of high contention = many collisions
Information known due to carrier sense
...allow to determine the Slot Utilization index:
Slot Utilization = Num Busy Slots / Initial Backoff
Slot Utilization in [0..1]
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IEEE 802.11 Contention Control
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Adoption of Slot Utilization in Distributed
contention control (DCC)
Probability of Transmission = P_T
Num_Att
P_T = (1- S_U)
Probabilistic Distributed Contention Control
Extends 802.11 MAC
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IEEE 802.11 Contention Control
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Stable (avoids collapse)
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IEEE 802.11 Contention Control
IEEE 802.11 Contention Control
DCC => better performance (no overheads)
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IEEE 802.11 Contention Control
DCC => better performance (no overheads)
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IEEE 802.11 Contention Control
DCC distributed priority mechanism
AOB = optimum performance
Num_Att*Prior_Lev
Num_Att*
Prior_Lev
No need to estimate number of stations
P_T=(1--S_U)
P_T=(1
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© Luciano Bononi 2007
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10