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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

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
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
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
  • 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
partial solutions of hidden exposed node problems in csma
(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
dynamic reservation approaches sender vs receiver initiated
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 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
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
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
Other criteria for classification
  • 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
i non qos mac protocols
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.
  • 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 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
  • 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
MACAW (MACA for Wireless)
    • 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
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
    • 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
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
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
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
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
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
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
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
Categories of QoS-aware MAC protocols
  • 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
  • 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
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