Medium Access Control in

1
Medium Access Control in Ad hoc and Sensor
Networks
2
Multiple Access Control (MAC) Protocols

• MAC allows multiple users to share a common
  channel.
• Conflict-free protocols ensure successful
  transmission. Channel can be allocated to users
  statically or dynamically.
• Only static conflict-free protocols are used in
  cellular mobile communications- Frequency
  Division Multiple Access (FDMA) provides a
  fraction of the frequency range to each user for
  all the time- Time Division Multiple Access
  (TDMA) The entire frequency band is allocated
  to a single user for a fraction of time- Code
  Division Multiple Access (CDMA) provides every
  user a portion of bandwidth for a fraction of
  time
• Contention based protocols must prescribe ways to
  resolve conflicts- Static Conflict Resolution
  Carrier Sense Multiple Access (CSMA) - Dynamic
  Conflict Resolution the Ethernet, which keeps
  track of various system parameters, ordering the
  users accordingly

3
Frequency Division Multiple Access (FDMA)

• Channels are assigned to the user for the
  duration of a call. No other user can access the
  channel during that time. When call terminates,
  the same channel can be re-assigned to another
  user
• FDMA is used in nearly all first generation
  mobile communication systems, like AMPS (30 KHz
  channels)
• Number of channels required to support a user
  population depends on the average number of calls
  generated, average duration of a call and the
  required quality of service (e.g. percentage of
  blocked calls)

Channel 1
Channel 2
Bandwidth
Channel 3
Channel 4
Time
4
Time Division Multiple Access (TDMA)

• The whole channel is assigned to each user, but
  the users are multiplexed in time during
  communication. Each communicating user is
  assigned a particular time slot, during which it
  communicates using the entire frequency spectrum
• The data rate of the channel is the sum of the
  data rates of all the multiplexed transmissions
• There is always channel interference between
  transmission in two adjacent slots because
  transmissions tend to overlap in time. This
  interference limits the number of users that can
  share the channel

Channel 3
Channel 1
Channel 2
Channel 3
Channel 4
Channel 1
Channel 2
Bandwidth
Time
5
Code Division Multiple Access (CDMA)

• CDMA, a type of a spread-spectrum technique,
  allows multiple users to share the same channel
  by multiplexing their transmissions in code
  space. Different signals from different users are
  encoded by different codes (keys) and coexist
  both in time and frequency domains
• A code is represented by a wideband pseudo noise
  (PN) signal
• When decoding a transmitted signal at the
  receiver, because of low cross-correlation of
  different codes, other transmissions appear as
  noise. This property enables the multiplexing of
  a number of transmissions on the same channel
  with minimal interference
• The maximum allowable interference (from other
  transmissions) limits the number of simultaneous
  transmissions on the same channel

6
Code Division Multiple Access (CDMA)

• Spreading of the signal bandwidth can be
  performed usingDirect Sequence (DS)
• The narrow band signal representing digital data
  is multiplied by a wideband pseudo noise (PN)
  signal representing the code. Multiplication in
  the time domain translates to convolution in the
  spectral domain. Thus the resulting signal is
  widebandFrequency Hopping (FH)
• Carrier frequency rapidly hops among a large set
  of possible frequencies according to some pseudo
  random sequence (the code). The set of
  frequencies spans a large bandwidth. Thus the
  bandwidth of the transmitted signal appears as
  largely spread

7
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

8
(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

9
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

10
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.

11
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
12
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

13
Power Control MAC (PCM)

• IEEE 802.11 MAC Protocol

Figure adapted from Jung 2002
14
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

15
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)

16
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
Ds carrier sensing range
Cs carrier sensing range
Figure adapted from Jung 2002
17
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

18
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
19
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
20
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

21
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

22
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
23
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

24
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
25
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Why STUDY MAC protocols in sensor networks?
• Application behavior in sensor networks leads to
  very different traffic characteristics from that
  found in conventional computer networks
• Highly constrained resources and functionality
• Small packet size
• Deep multi-hop dynamic topologies
• The network tends to operate as a collective
  structure, rather than supporting many
  independent point-to-point flows
• Traffic tends to be variable and highly
  correlated
• Little or no activity/traffic for longer periods
  and intense traffic over shorter periods

26
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Major factors to be considered in the design of
  MAC
• Communication efficiency in terms of energy
  consumed per each packet
• Communication by radio channel consumes the
  highest energy
• Transmit , receive and idle consume roughly the
  same amount of energy
• Fairness of the bandwidth allocated to each node
  for end to end data delivery to sink
• Each node acts as a router as well as data
  originator resulting in two kinds of traffic
• The traffics compete for the same upstream
  bandwidth
• Hidden nodes
• Contention at the upstream node may not be
  detected and results in significant loss rate
• Efficient channel utilization

27
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Major factors to be considered in the design of
  MAC
• The routing distance and degree of intermediate
  competition varies widely across the network
• The cost of dropping a packet varies with place
  and the packet
• Contribution of this paper are as follows
• Listening mechanism
• Listening is effective when there are no hidden
  nodes
• It comes at an expense of energy cost as the
  radio must be on to listen
• Many protocols such as IEEE 802.11 require
  sensing the channel even during backoff
• Shorten the length of carrier sensing and power
  off the node during backoff
• Highly synchronized nature of the traffic causes
  no packet transfer at all in the absence of
  collision detection hardware
• Introduce random delay for transmission to
  unsynchronized the nodes

28
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Backoff Mechanism
• Used to reduce the contention among the nodes
• In the sensor networks, traffic is a
  superposition of different periodic streams
• Apply back off as a phase shift to the
  periodicity of the application so that the
  synchronization among periodic streams of traffic
  can be broken
• Contention-based Mechanism
• Explicit control packets like RTS and CTS are
  used to avoid contention
• ACKS indicate lack of collision
• Use of lot of control packets reduces bandwidth
  efficiency
• ACKS can be eliminated by hearing the packet
  transmission from its parent to its upstream
  which serves as an ACK for the downstream node

29
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Rate Control Mechanism
• The competition between originating traffic and
  route-thru traffic has a direct impact in
  achieving the fairness goal.
• MAC should control the rate of originating data
  of a node in order to allow route-thru traffic to
  access the channel and reach the base station and
  some kind of progressive signaling for route-thru
  traffic such the rate is controlled at the
  origin.
• A passive implicit mechanism is used to control
  the rate of transmission of both traffics

30
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Multi-hop Hidden Node problem
• It avoid the hidden node problem by constantly
  tuning the transmission rate and performing phase
  changes so that the aggregate traffic will not
  repeatedly collide with each other.
• A child can reduce a potential hidden node
  problem with its grand parent by not sending
  packets for t x packet time at the end of
  packet transmission t by its parent

31
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Advantages
• The amount of computation for this scheme is
  small and within networked sensors computation
  capability
• The scheme is totally computational which is much
  cheaper in energy cost than on the radio
• The control packet overhead is reduced

32
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003

• Disadvantages
• The MAC protocol developed here takes into
  consideration the periodicity of the originating
  traffic which doesnt help for non periodic
  traffic

33
A Transmission Control Scheme for Media Access
in Sensor Networks Woo, 2003
Suggestions/Improvements/Future Work
34
An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• T-MAC is a contention based Medium Access Control
  Protocol
• Energy consumption is reduced by introducing an
  active/sleep duty cycle
• Handles the load variations in time and location
  by introducing an adaptive duty cycle
• It reduces the amount of energy wasted on idle
  listening by dynamically ending the active part
  of it
• In T-MAC, nodes communicate using RTS, CTS, Data
  and ACK pkts which provides collision avoidance
  and reliable transmission
• When a node senses the medium idle for TA amount
  of time it immediately switches to sleep
• TA determines the minimal amount of idle
  listening time per frame
• The incoming messages between two active states
  are buffered

35
An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• The buffer capacity determines an upper bound on
  the maximum frame time
• Frame synchronization in T-MAC follows the scheme
  of virtual clustering as in S-MAC
• The RTS transmission in T-MAC starts by waiting
  and listening for a random time within a fixed
  contention interval at the beginning of the each
  active state
• The TA time is obtained using TA gt C R T
• T-MAC suffers from early sleeping problem
• Its overcome by sending Future request to send or
  taking priority on full buffers

36
An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• Advantages
• The T-MAC protocol is designed particularly for
  wireless sensor networks and hence energy
  consumption constraints are taken into account
• The T-MAC protocol tries to reduce idle listening
  by transmitting all messages in bursts of
  variable lengths and sleeping between burst
• T-MAC facilitates collision avoidance and
  overhearing -- nodes transmit their data in a
  single burst and thus do not require additional
  RTS/CTS control packets.
• By stressing on RTS retries, T-MAC gives the
  receiving nodes enough chance to listen and reply
  before it actually goes to sleep -- this
  increases the throughput in the long run

37
An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• Disadvantages
• The authors do not outline how a sender node
  would sense a FRTS packet and enable it to send a
  DS packet
• Also sending a DS packet increases the overhead.
• The network topology in the simulation considers
  that the locations of the nodes are known
• T-MAC has been observed to have a high message
  loss phenomenon
• T-MAC suffers from early sleeping problem for
  event based local unicast

38
An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks Van dam, 2003

• Suggestions/Improvements/Future Work
• If a buffer is full there would be a lot of
  dropped packets decreasing the throughput. A
  method to overcome this drawback is that we could
  have the node with its buffer 75 full broadcast
  a special packet Buffer Full Packet
• MAC Virtual Clustering technique needs to be
  further investigated
• An adaptive election algorithm can be
  incorporated where the schedule and neighborhood
  information is used to select the transmitter and
  receivers for the current time slot, hence
  avoiding collision and increasing energy
  conservation

39
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.
• Woo 2003 A. Woo and D. Culler, A Transmission
  Control Scheme for Media Access in Sensor
  Networks, Proceedings of the ACM/IEEE
  International Conference on Mobile Computing and
  Networking, Rome, Italy, July 2001, pp. 221-235.
  
• Van Dam 2003 T. V. Dam and K. Langendoen, An
  Adaptive Energy-Efficient MAC Protocol for
  Wireless Sensor Networks, ACM SenSys, Los
  Angeles, CA, November, 2003.