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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

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Medium access control for ad hoc wireless networks a survey l.jpg

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 l.jpg
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 l.jpg
Need for MAC Protocols

  • Popular CSMA/CD (Carrier Sense Multiple Access/Collision Detection) scheme is not applicable to wireless networks

  • CSMA suffers hidden node & exposed node problems

    • 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

  • Collision Detection is impossible in wireless communication


Classification l.jpg
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



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(Partial) Solutions of Hidden/Exposed Node Problems in CSMA

  • Use control packets

    • RTS/CTS (Request-To-Send/Clear-To-Send)

    • 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


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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


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Single vs. Multiple Channel Protocols Receiver-initiated

  • Single channel protocols: Control and packets use the same channel

  • Multiple channel protocols: Frequency hopping or Separate channels for control & data transmission


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Frequency Hopping Spread Spectrum (FHSS) Receiver-initiated

  • 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


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Direct Sequence Spread Spectrum (DSSS) Receiver-initiated

  • 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


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Other criteria for classification Receiver-initiated

  • Power-aware

  • Directional or omnidirectional antennas

  • QoS-aware

    • 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


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I. Non-QoS MAC Protocols Receiver-initiated

  • 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.


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MACA Receiver-initiated

  • 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


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MACA Receiver-initiated

  • 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


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MACA Receiver-initiated

  • Limitations

    • MACA does not provide ACK

    • RTS-CTS approach does not always solve the hidden node problem

    • Example

      • 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 l.jpg
MACAW (MACA for Wireless) Receiver-initiated

  • RTS-CTS-DS-DATA-ACK

    • 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 l.jpg
IEEE 802.11 MAC Receiver-initiated

  • 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)


802 11 dcf mode l.jpg
802.11 (DCF mode) Receiver-initiated

  • 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 l.jpg
MACA-BI Receiver-initiated

  • 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 l.jpg
Power aware MAC protocols Receiver-initiated

  • 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


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PAMAS (Power aware medium access control with signaling) Receiver-initiated

  • 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 l.jpg
PCM: Power Control Medium access control Receiver-initiated

  • 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 l.jpg
PCMA: Power controlled multiple access Receiver-initiated

  • 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 l.jpg
II. QoS-Aware MAC protocols Receiver-initiated

  • 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 l.jpg
QoS-aware MAC protocols Receiver-initiated

  • 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 l.jpg
Categories of QoS-aware MAC protocols Receiver-initiated

  • 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

  • Use reserved time slots to provide bounded & required bandwidth for RT traffic; Non-RT traffic is treated like 802.11

  • Provide fair channel allocation to different flows unlike 1-3


Rt mac l.jpg
RT-MAC Receiver-initiated

  • 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)


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DCF with priority classes Receiver-initiated

  • 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 l.jpg
Questions? Receiver-initiated


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