Medium Access Control for Ad
Hoc Wireless Networks: A
Survey
S. Kumar, V. Raghavan, J. Deng
Ad Hoc Networks 4 (2006) 326-358
Medium Access Control
• Coordinate access from active nodes
– Deal with channel contention
• Challenges
– Wireless communication channel is prone to
errors and problems, e.g., hidden/exposed
node problems & signal attenuation
• This paper provides a comprehensive
survey
Need for MAC Protocols
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•
Popular CSMA/CD (Carrier Sense Multiple Access/Collision Detection)
scheme is not applicable to wireless networks
CSMA suffers hidden node & exposed node problems
•
Collision Detection is impossible in wireless communication
– Hidden node: A sends to B; C sends to B -> Collision at B
– Exposed node: B sends to A; C unnecessarily delays transmission to B
Classification
• Contention-free MAC
– TDMA, FDMA, CDMA: Divides channel by
time, frequency, or code
– More applicable to static networks and/or
networks with centralized control
• Contention-based MAC
– Focus of this survey
Classification
(Partial) Solutions of Hidden/Exposed
Node Problems in CSMA
• Use control packets
– RTS/CTS (Request-To-Send/Clear-ToSend)
– Used by MACA (Multiple Access Control
Avoidance) and MACAW (MACA for
Wireless LANs)
• Use both control packets and carrier
sense
– CSMA/CA, IEEE 802.11
Dynamic Reservation Approaches:
Sender- vs. Receiver-initiated
• Sender-initiated
– A node wanting to send data takes the initiative of
setting up the reservation
– Most existing schemes
• Receiver-initiated
– A receiving node polls a potential transmitting node
for data
– A node can send data after being polled
– MACA-By Invitation
• A bit more efficient than MACA in terms of transmit &
receive turnaround time
Single vs. Multiple Channel
Protocols
• Single channel protocols: Control and
packets use the same channel
• Multiple channel protocols: Frequency
hopping or Separate channels for
control & data transmission
Frequency Hopping Spread
Spectrum (FHSS)
• Transmit radio signals by switching a carrier
among multiple frequency channels using a
pseudo random sequence known to the
transmitter and receiver
– Spread spectrum signals are resistant to noise &
interference
– Difficult to intercept
– Can share a frequency band with other
transmissions
• Efficient bandwidth utilization
Direct Sequence Spread
Spectrum (DSSS)
• Phase modulate a sine wave in a pseudo
random manner
•
•
•
•
– A pseudo random noise code symbols are called
chips
– Chip rate is much higher than the information
signal bit rate
The sequence of chips is known to the receiver
Resistant to jamming
Multiple users can share a single channel
Relative timing correlation
Other criteria for classification
• Power-aware
• Directional or omnidirectional antennas
• QoS-aware
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–
–
–
End-to-end (E2E) delay
Packet loss rate (or the probability)
Available bandwidth
Challenges: lack of centralized control, limited
bandwidth, node mobility, power/computational
constraints, error-prone nature of wireless media
I. Non-QoS MAC Protocols
• General MAC protocols
– MACA (Multiple Access Collision Avoidance)
– IEEE 802.11
– MACA-BI
• Power aware MAC protocols
– PAMAS (Power aware medium access control with
signaling)
– PCM (Power control medium access control)
– PCMA (Power controlled multiple access)
• Multiple channel protocols
– DBMA (Dual busy tone multiple access),
Multichannel CSMA MAC protocol, etc.
MACA
• If node A wants to transmit to B, it first sends an
RTS packet to B, indicating the length of the data
transmission to follow
• B returns A a CTS packet with the expected length
of the transmission
• A starts transmission when it receives CTS
– RTS, CTS packets are much shorter than data packets
• A neighboring node overhearing an RTS defers its
own transmission until the corresponding CTS would
have been finished
• A node hearing the CTS defers for the expected
length of the data transmission
MACA
• MACA can handle hidden node & exposed node problems
unsolved by CSMA
– Hidden node: A sends to B; C sends to B -> Collision at B
-> In
MACA, B sends CTS to A; C can hear the CTS & defer its own
transmission to B in MACA
– Exposed node: B sends to A; C unnecessarily delays transmission to
B -> In MACA, C can overhear B’s RTS sent to A but C cannot hear
CTS from A; So, C transmits to B
MACA
•
Limitations
– MACA does not provide ACK
– RTS-CTS approach does not always solve the hidden node problem
– Example
•
•
•
•
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A sends RTS to B
B sends CTS to A; At the same time, D sends RTS to C
The CTS & RTS packets collide at C
A transmits data to B; D resends RTS to C; C sends CTS to D
The data & CTS packets collide at B
MACAW (MACA for Wireless)
• RTS-CTS-DS-DATA-ACK
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–
–
–
–
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RTS from A to B
CTS from B to A
Data Sending (DS) from A to B
Data from A to B
ACK from B to A
Random wait after any successful/unsuccessful
transmission
• Significantly higher throughput than MACA
• Does not completely solve hidden & exposed
node problems
IEEE 802.11 MAC
• Very popular wireless MAC protocol
• Two modes: DCF (distributed coordination function) & PCF (point
coordination function)
• DCF is based on CSMA/CA ≈ CSMA + MACA
– RTS-CTS-DATA-ACK
– Physical carrier sensing + NAV (network allocation vector)
containing time value that indicates the duration up to which the
medium is expected to be busy due to transmissions by other nodes
– Every packet contains the duration info for the remainder of the
message
– Every node overhearing a packet continuously updates its own NAV
• IFS (inter frame spacing)
– Short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS), Extended
IFS (EIFS)
1.
2.
3.
4.
5.
6.
•
802.11 (DCF mode)
If channel is idle for DIFS, transmit
If busy, initiate back-off counter (Randomly choose a back-off
value between 0 and CW-1)
If channel is idle for DIFS, start decrementing back-off
timer; Stop if channel becomes busy
Transmit the frame when counter = 0
If transmission was successful, set CW = CWmin
If transmission fails (i.e., no ACK), CS = min{2(CW+1)-1, CWmax}
Control packets, i.e., RTS, CTS, and ACK packets, are sent
after the medium has been free for SIFS.
MACA-BI
• Receiver initiated
• Reduce number of control packets
– RTR (Ready To Receive) & DATA rather
than RTS-CTS-DATA
• Receiver needs a traffic prediction
algorithm
• Works well given predictable traffic
patterns
Power aware MAC protocols
• Minimize expensive retransmissions due
to collisions
• Transceivers should be kept in standby
mode as much as possible
• Switch to low power mode sufficient for
the destination to receive the packet
• Two categories
– Alternate between sleep and awake cycles
– Vary transmission power
PAMAS (Power aware medium
access control with signaling)
• RTS-CTS exchanges over a signaling channeling
• Data transmission over a separate data channel
• Receiver sends out a busy tone, while receiving a data
packet over the signaling channel
• Nodes listen to the signaling channel to determine
when it is optimal to power down transceivers
• A node powers itself off if it has nothing to transmit
and its neighbor is transmitting
• A node powers off if at least one neighbor is
transmitting and another is receiving
• Use of ACK and transmission of multiple packets can
enhance performance
• Radio transceiver turnaround time was not considered
PCM: Power Control Medium
access control
• Send RTS & CTS packets using max available power
• Send DATA & ACK with the min power required to
communicate between the sender and receiver
• Based on the received signal strength of the
RTS/CTS packet, adjust the power level for DATA
transmission
• Drawbacks
– Requires rather accurate estimation of the received signal
strength, which is hard in wireless communication
– Difficult to implement frequent changes in the transmission
power level
PCMA: Power controlled multiple
access
• Control transmit power of the sender
– The receiver is just able to receive the packet
– Avoid interfering other neighboring nodes not
involved in the packet exchange
– Two channels: one for busy tone & another for
data
• Request Power To Send (RTPS) & Accept
Power To Send (APTS) on the data channel
• Every receiver periodically sends out a busy
tone
• Sender does carrier sensing
II. QoS-Aware MAC protocols
• Prioritized QoS
– Prioritize network flows
• Parameterized QoS
– Reserve resources for E2E path
– A new stream is not admitted if there’s not enough resources ->
Already admitted streams are not affected
• Soft-QoS: Brief disruptions are acceptable
• Dynamic-QoS: Range of QoS
• Different applications, different QoS requirements
– Audio/video streaming requires reserved share of channel capacity;
Soft-QoS with some transient violations is acceptable
• A lot to do to support audio/video streaming over a wireless channel
– Inter-vehicle communication requires guaranteed delivery of short
bursts of data within a delay bound
• Very little prior work has been done!
QoS-aware MAC protocols
• For real-time (RT) applications, MAC
protocols should support resource
reservation for RT traffic in addition to
addressing hidden/exposed terminal
problems
• Synchronous schemes: TDM variations
requiring time synchronization
• Asynchronous approaches: No need for
global time synchronization
Categories of QoS-aware MAC
protocols
1.
•
2.
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Use shorter inter-frame spacing & smaller backoff contention
window for RT traffic
Extension of 802.11 DCF (e.g., 802.11e)
Black burst contention
RT nodes jam the channel in proportion to waiting time
Observe the channel
Node with the longest jam transmits
3.
Use reserved time slots to provide bounded & required
bandwidth for RT traffic; Non-RT traffic is treated like
802.11
4.
Provide fair channel allocation to different flows unlike 1-3
RT-MAC
• Drop tardy packets
– Check before sending a packet, when its backoff timer
expires, and when a transmission is unacknowledged
– Eliminate possibility of collision
• When a packet is actually sent, include the backoff value in the
packet
• A node hearing the transmission chooses a different backoff
value
– Advantage: Significantly reduced the mean packet delay,
missed deadlines, and collisions compared to 802.11
– Drawbacks
• Contention window may become very large in a network with
many nodes
• No guarantee on E2E delay (or deadline miss ratio)
DCF with priority classes
• Use a shorter IFS and backoff time for higher priority data
(Deng et al)
– Normal node waits for DIFS, but high priority node only waits for
PIFS
– Small contention window for a high priority flow
• EDCF (Enhanced DCF) in 802.11e takes a similar approach to
supporting QoS
– Use AIFS[TC], CWmin[TC] & CWmax[TC] instead of DIFS, CWmin &
CWmax in DCF where AIFS is arbitration inter frame space and TC is
traffic class
– AIFS[TC] ≥ DIFS can be enlarged for lower priority classes
– CWmin[TC] & CWmax[TC] are set differently according to TC
• No deterministic guarantee on delay
• Normal traffic suffers higher delay
Questions?