MAC Protocols and Security in

1
MAC Protocols and Security in Ad hoc and Sensor
Networks
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A Power Control MAC (PCM) Protocol for Ad hoc
Networks Jung 2002

• A power control MAC protocol allows nodes to vary
  transmit power level on a per-packet basis
• Earlier work has used different power levels for
  RTS-CTS and DATA-ACK, specifically, maximum
  transmit power is used for RTS-CTS and minimum
  required transmit power is used for DATA-ACK
  transmissions
• These protocols may increase collisions, degrade
  network throughput and result in higher energy
  consumption than when using IEEE 802.11 without
  power control
• Power saving mechanisms allow nodes to enter a
  doze state by powering off its wireless network
  interface whenever possible
• Power control schemes vary transmit power to
  reduce energy consumption

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Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol
• Specifies two MAC protocols
• Point Coordination Function (PCF) ? centralized
• Distributed Coordination Function (DCF)
  ?distributed
• Transmission range
• When a node is in transmission range of a sender
  node, it can receive and
• correctly decode packets from sender node.
• Carrier Sensing Range
• Nodes in carrier sensing range can sense the
  senders transmission. It is generally
• larger than transmission range. Both carrier
  sensing range and transmission range
• Depends on the transmit power level.

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Power Control MAC (PCM)
IEEE 802.11 MAC Protocol Carrier Sensing
Zone Nodes can sense the signal, but cannot
decode it correctly. The carrier sensing zone
does not include transmission range
Figure adapted from Jung 2002
5
Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol
• DCF in IEEE 802.11 is based on CSMA/CS (Carrier
  Sense Multiple Access with Collision Avoidance)
• Each node in IEEE 802.11 maintains a NAV (Network
  Allocation Vector) that indicates the remaining
  time of the on-going transmission sessions
• Carrier sensing is performed using physical
  carrier sensing (by air interface) and virtual
  carrier sensing (uses the duration of the packet
  transmission that is included in the header of
  RTS, CTS and DATA frames)
• Using the duration information in RTS, CTS and
  DATA packets, nodes update their NAVs whenever
  they receive a packet
• The channel is considered busy if either physical
  or virtual carrier sensing indicates that channel
  is busy
• Figure 2 shows how nodes in transmission range
  and the carrier sensing zone adjust their NAVs
  during RTS-CTS-DATA-ACK transmission

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Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol

Figure adapted from Jung 2002
7
Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol
• IFS is the time interval between frames and IEEE
  802.11 defines four IFSs which provide priority
  levels for accessing the channel
• SIFS (short interframe space)
• PIFS (Point Coordination Function interframe
  space)
• DIFS (Distributed Coordination Function
  interframe space)
• EIFS (extended interframe space)
• SIFS is the shortest and is used after RTS, CTS,
  and DATA frames to give the highest priority to
  CTS, DATA and ACK respectively
• In DCF, when the channel is idle, a node waits
  for DIFS duration before transmitting
• Nodes in the transmission range correctly set
  their NAVs when receiving RTS/CTS
• Since nodes in carrier sensing zone cannot decode
  the packet, they do not know the duration of the
  packet transmission. So, they set their NAVs for
  the EIFS duration to avoid collision with the ACK
  reception at the source node

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Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol
• The intuition behind EIFS is to provide enough
  time for a source node to receive the ACK frame,
  meaning that duration of EIFS is longer than that
  of ACK transmission
• In PCM, nodes in the carrier sensing zone use
  EIFS whenever they can sense the signal but
  cannot decode it
• IEEE 802.11 does not completely prevent
  collisions due to the hidden terminal problem
  (nodes in the receivers carrier sensing zone,
  but not in the senders carrier sensing zone or
  transmission range, can cause a collision with
  the reception of a DATA packet at the receiver
• In Figure 3, suppose node C transmits packet to
  node D
• When C and D transmit an RTS and CTS
  respectively, A and F set their NAVs for EIFS
  duration
• During Cs data transmission, A defers its
  transmission due to sensing Cs transmission.
  However, since node F does not sense any signal
  during Cs transmission, it considers channel to
  be idle (F is in Ds carrier sensing zone, but
  not in Ds)

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Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
Ds carrier sensing range
Cs carrier sensing range
Figure adapted from Jung 2002
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Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol
• When F starts a new transmission, it can cause a
  collision with the reception of DATA at D
• Since F is outside of Ds transmission range, D
  may be outside of Fs transmission range
  however, since F is in Ds carrier sensing zone,
  F can provide interference at node D to cause
  collision with DATA being received at D

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Power Control MAC (PCM)

• BASIC Power Control Protocol
• Power control can reduce energy consumption
• Power control may bring different transmit power
  levels at different hosts, creating an asymmetric
  scenarios where a node A can reach node B, but
  node B cannot reach node A and collisions may
  also increase a result
• In Figure 4, suppose nodes A and B use lower
  power level than nodes C and D
• When A is transmitting to B, C and D may not
  sense the transmission
• When C and D transmit to each other using higher
  power, their transmission may collide with the
  on-going transmission from A to B

Figure adapted from Jung 2002
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Power Control MAC (PCM)

• BASIC Power Control Protocol
• As a solution to this problem, RTS-CTS are
  transmitted at the highest possible power level
  but DATA and ACK at the minimum power level
  necessary to communicate
• In Figure 5, nodes A and B send RTS and CTS
  respectively with highest power level such that
  node C receives the CTS and defers its
  transmission
• By using a lower power level for DATA and ACK
  packets, nodes can save energy

Figure adapted from Jung 2002
13
Power Control MAC (PCM)

• BASIC Power Control Protocol
• In the BASIC scheme, RTS-CTS handshake is used to
  decide the transmission power for subsequent DATA
  and ACK packets which can be achieved in two
  different ways
• Suppose node A wants to send a packet to node B.
  Node A transmit RTS at power level pmax (maximum
  possible). When B receives the RTS from A with
  signal level pr, B calculates the minimum
  necessary transmission power level, pdesired. For
  the DATA packet based on received power level,
  pr, transmitted power level, pmax, and noise
  level at the receiver B. Node B specifies
  pdesired in its CTS to node A. After receiving
  CTS, node A sends DATA using power level
  pdesired.
• When a destination node receives an RTS, it
  responds by sending a CTS (at power level pmax).
  When source node receives CTS, it calculates
  pdesired based on received power level, pr, and
  transmitted power level (pmax) as
• Pdesired (pmax / pr) x Rxthresh x c
• where Rxthresh is minimum necessary received
  signal strength and c is constant

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Power Control MAC (PCM)

• BASIC Power Control Protocol
• The second alternative makes two assumptions
• Signal attenuation between source and destination
  nodes is assumed to be the same in both
  directions
• Noise level at the receiver is assumed to be
  below some predefined threshold
• Deficiency of the BASIC Protocol
• In Figure 6, suppose node D wants to transmit to
  node E
• When nodes D and E transmits RTS and CTS
  respectively, B and C receives RTS and F and G
  receives CTS, therefore, these nodes defer their
  transmissions
• Since node A is in carrier sensing zone of node
  D, it sets its NAV for EIFS duration
• Similarly node H sets its NAV for EIFS duration
  when it senses transmission from E
• When source and destination decide to reduce the
  transmit power for DATA-ACK, not only
  transmission range for DATA-ACK but also carrier
  sensing zone is also smaller than RTS-CTS

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Power Control MAC (PCM)

• Deficiency of the BASIC Protocol
• Thus, only C and F correctly receives DATA and
  ACK packets
• Since nodes A and H cannot sense the
  transmissions, they consider channel is idle and
  start transmitting at high power level which will
  cause collision with the ACK packet at D and DATA
  packet at E
• This results in throughput degradation and higher
  energy consumption (due to retransmissions)

Figure adapted from Jung 2002
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Power Control MAC (PCM)

• Proposed Power Control MAC Protocol
• Proposed Power Control MAC (PCM) is similar to
  BASIC scheme such that it uses power level, pmax,
  for RTS-CTS and the minimum necessary transmit
  power for DATA-ACK transmissions
• Procedure of PCM is as follows
• Source and destination nodes transmit the RTS and
  CTS using pmax. Nodes in the carrier sensing zone
  set their NAVs for EIFS duration
• The source may transmit DATA using a lower power
  level
• Source transmits DATA at level of pmax,
  periodically, for enough time so that nodes in
  the carrier sensing zone can sense it and this
  would avoid collision with the ACK packets
• The destination node transmits an ACK using the
  minimum required power to reach the source node
• Figure 7 presents how the transmit power level
  changes during the sequence of RTS-CTS-DATA-ACK
  transmission

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Power Control MAC (PCM)

• Proposed Power Control MAC Protocol
• The difference between PCM and BASIC scheme is
  that PCM periodically increases the transmit
  power to pmax during the DATA packet
  transmission. Nodes that can interfere with the
  reception of ACK at the sender will periodically
  sense the channel is busy and defer their own
  transmission. Since nodes reside in the carrier
  sensing zone defer for EIFS duration, the
  transmit power for DATA is increased once every
  EIFS duration
• PCM solves the problem posed with BASIC scheme
  and can achieve throughput comparable to 802.11
  by using less energy
• PCM, like 802.11, does not prevent collisions
  completely

Figure adapted from Jung 2002
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An Energy-Efficient MAC Protocol for Wireless
Sensor Networks (S-MAC) Ye 2002

• S- MAC protocol designed specifically for sensor
  networks to reduce energy consumption while
  achieving good scalability and collision
  avoidance by utilizing a combined scheduling and
  contention scheme
• The major sources of energy waste are
• collision
• overhearing
• control packet overhead
• idle listening
• S-MAC reduce the waste of energy from all the
  sources mentioned in exchange of some reduction
  in both per-hop fairness and latency

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S-MAC

• S- MAC protocol consist of three major
  components
• periodic listen and sleep
• collision and overhearing avoidance
• Message passing
• Contributions of S-MAC are
• The scheme of periodic listen and sleep helps in
  reducing energy consumption by avoiding idle
  listening. The use of synchronization to form
  virtual clusters of nodes on the same sleep
  schedule
• In-channel signaling puts each node to sleep when
  its neighbor is transmitting to another node
  (solves the overhearing problem and does not
  require additional channel)
• Message passing technique to reduce
  application-perceived latency and control
  overhead (per-node fragment level fairness is
  reduced)
• Evaluating an implementation of S-MAC over
  sensor-net specific hardware

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Security in Wireless Ad hoc Networks Buttyan
2002

• Security in wireless ad hoc networks is difficult
  for many reasons
• Vulnerability of channels
• Vulnerability of nodes
• Absence of infrastructure
• Dynamically changing topology
• The problem is broad and there is no general
  solution
• Different applications will have different
  security requirements
• Security aspects can be categorized into four
  groups
• Trust and key management
• Secure routing and intrusion detection
• Availability
• Cryptographic protocols

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References

• Jung 2002 E.-S. Jung and N.H. Vaidya, A Power
  Control MAC Protocol for Ad hoc Networks,
  Proceedings of ACM MOBICOM 2002, Atlanta,
  Georgia, September 23-28, 2002.
• Ye 2002 W. Yei, J. Heidemann and D. Estrin,
  Energy-Efficient MAC Protocol for Wireless Sensor
  Networks, Proceedings of the Twenty First
  International Annual Joint Conference of the IEEE
  Computer and Communications Societies (INFOCOM
  2002), New York, NY, USA, June 23-27 2002.
• Buttyan 2002 L. Buttyan and J.-P. Hubaux,
  Report on a Working Session on Security in
  Wireless Ad Hoc Networks, Mobile Computing and
  Communications Review, Volume 6, Number 4.