Introduction To Ad-Hoc Networks Medium Access Protocol (MAC)

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Introduction To Ad-HocNetworksMedium Access
Protocol (MAC)

• 2007
• H. Oh

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What is MEDIUM ?

• The MEDIUM is a denomination for a physical
  support
• The physical support is the electromagnetic
  spectrum
• Usage of the electromagnetic spectrum is
  regulated
• Regulation induces licensing costs and usage
  rules
• Everybody needs capacity and speed simultaneously
• gt Contention!!!

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IR (Infrared)

• Located in the VHF (Very High Frequency) domain
• Needs LOS (Line Of Sight)
• Wall reflection allows multi-hop networks
• Protocol agnostic (ATM, FastEthernet etc.)
• Up to 10 Mbps speed
• Prone to interference problems (sun or
  fluorescent sources)

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MW (Microwave)

• Better performance than IR
• Needs very clear LOS
• Environmental problems

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RF (Radio)

• Allows spread spectrum techniques (DSSS, FHSS)
• DSSS (D)irect (S)equencing (S)pread (S)pectrum
• FHSS (F)requency (H)opping (S)pread (S)pectrum

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Critical Factors that characterize Comm. Tech

• Interference
• Delays
• Throughput
• Capacity
• Coverage
• Robustness
• Power
• Security

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Solution

• MAC (M)EDIUM (AC)CESS (P)ROTOCOL
• Since multiple nodes contend a shared medium, we
  need a certain protocol to control that they
  access the medium
• A MAC protocol is a set of rules or procedures to
  allow the efficient use of a shared medium, such
  as wireless
• Per-link oriented, but not end to end
• Actors the sender, the receiver, the access
  medium

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Classifications

• 1. By operational mode
• synchronous All nodes are synchronized to the
  same reference time
• asynchronous Nodes are coordinated in a more
  distributed manner
• 2. By request origin
• sender-initiated The sending node informs the
  receiving node about the data it has to send
• receiver-initiated The receiving node informs
  potential sending nodes about its availability
  for receiving data

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Shortcomings

• 1. Synchronous approach
• A centralized way of synchronizing the nodes is
  needed
• Example a central time beacon broadcasted to all
  nodes
• 2. Asynchronous approach
• Distributed control mechanism (prone to channel
  contention problems)
• Most of the implemented MAC protocols are
  asynchronous

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Hidden Terminal Problem

• Appears when two nodes are unaware of each
  others attempt to send data to a third node
• Unaware of each other means out of each
  other's signal range
• The result is data collision at the receiving
  node
• The problem exists in contention-based protocols

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A transmits to B
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C transmits to B
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Solution

• Using a handshake protocol would prevent
  collision
• The protocol is used to "reserve" the comm.
  channel
• RTS (R)eady (T)o (S)end
• CTS (C)lear (T)o (S)end
• RTS and CTS are broadcast-type messages
• Still not a bullet-proof solution!

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RTS-CTS Handshake
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A sends RTS to B
RTS
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B broadcasts CTS
CTS
CTS
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A sends DATA to B
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Failure Scenario 1
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A sends RTS to B
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B broadcasts CTS D sends RTS to C (D can not
receive CTS!!)
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A sends DATA to B -D sends RTS to C Again
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A sends DATA to B -C broadcasts CTS
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Failure Scenario 2
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A sends RTS to B
RTS
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B broadcasts CTS C sends RTS to D
simultaneously
RTS
RTS
CTS
CTS
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A sends DATA to B -D broadcasts CTS
CTS
Data
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A sends DATA to B -C sends DATA to D
Data
Data
Data
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Exposed Terminal Problem

• Overhearing a data transmission from a
  neighboring node causes other neighboring nodes
  to stall their transmissions
• The exposed node is within the radio range of the
  transmitter
• ... but at the same time out of receiver's radio
  range

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Scenario

• B is blocked by overhearing C's transmission to D

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Solution Directional Antennas

• Omni-directional antennas increase the
  probability of having "exposed" nodes
• Consequence lowers overall throughput and
  network availability
• Solution 1usage of directional antennas instead
  of omni-directional gt Provide spatial isolation
• Solution 2 separate data and control channels

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Scenario Omni-directional Antenna
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Scenario Directional Antenna
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Sender-Initiated protocols

• The sender initiates communication by explicit
  RTS
• The receiver confirms by explicitly broadcasting
  CTS

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Receiver-Initiated protocols

• The receiver has to contact the sender (passive
  polling)
• Only one control message!

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Existing MAC Protocols

• MACA (M)ultiple (A)ccess with (C)ollision
  (A)voidance
• MACAW MACA (A)ckno(W)ledging
• gt Used in IEEE 802.11
• MACA-BI MACA (B)y (I)nvitation
• DBTMA (D)ual (B)usy (T)one (M)ultiple (A)ccess
• PAMAS (P)ower-(A)ware (M)ulti-(A)ccess with
  (P)rotocols
• MARCH (M)edium (A)ccess with (R)educed
  (H)andshake
• HiperLAN (H)igh (PE)erformance (R)adio

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MACA

• Aims for use in single-freq ad-hoc networks
• Should resolve hidden and exposed node problems
• Three way handshake RTS-CTS-Data
• Built-in power management features
• Prone to collisions in the RTS-CTS handshake
  phase
• If two or more nodes transmit RTS concurrently.
• Solution
• Waits or a randomly chosen interval and try
  again, doubling the average interval on every
  attempt.
• Heavily reduced data packet collisions since RTS
  and CTS are much smaller in size.
• Contention-free period followed by contention
  period

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MACA
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RTS blocks radio range senders
RTS
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CTS blocks radio range receivers
CTS
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Data starts flowing
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Power Control

• If A overhears the CTS, that is node Bs response
  to an RTS from C, A can communicate with D by
  lowering its power level.

CTS
RTS
D
A
B
C
A overhears the CTS
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MACA-BI

• Invented by Fabrizio Talucci
• Uses a two-way handshake
• One control message RTR Preemptive RTR sending
• A node is not allowed to transmit data unless it
  has received an invitation from the receiver.
• Statistic estimators needed for traffic
  prediction
• Variable performance depending on traffic
  characteristics
• Less prone to control packet collisions
• Improved "roundtrip" time

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MACA-BI
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RTR blocks radio range Tx'es
RTR
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Data starts flowing
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Comparison of MACA and MACA-BI
CTS
CTS
CTS
RX
DATA
DATA
DATA
TX
MACA
RTS
RTS
RTS
RTR
RTR
RTR
RX
DATA
DATA
DATA
TX
MACA-BI
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Evaluation of MACA-BI

• The receiver node does not necessarily know that
  the source has data to transmit.
• The timeliness of the invitation by prediction is
  critical
• One possible prediction technique The sender
  piggybacks its packet queue length and data
  arrival rate
• Good for CBR type traffic pattern
• Under non-stationary traffic pattern, still
  employ an RTS if the transmitters queue length
  or packet delay exceeds a certain threshold
  before a RTR is issued.

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Advantages

• Reduced transmit/receive turn around time since
  it uses one control message (RTR)
• Turn around time is aprox. 25 microseconds
• Includes the collision-free feature of MACA
• However, it is less likely to suffer from control
  packet collision since it uses half as many
  control packets as MACA.

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BTMA((B)usy (T)one (M)ultiple (A)ccess)

• Proposed by Fouad Robagi from Stanford Univ 75
• BTMA was intended to solve the hidden terminal
  problem however, it addresses wireless last-hop
  architectures.
• Requires a centralized base station
• When the base station receives data, it sends out
  a busy signal to all other nodes within its radio
  cell
• Hidden nodes sense the busy signal and block
  themselves from transmitting

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DBTMA

• Proposed by Zygmunt Haas from Connel 1999.
• DBTMA extends BTMA to use in ad hoc wireless
  networks
• Uses two out-of-band busy tones over two
  different channels Three channels data,
  sender-busy, receiver-busy

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DBTMA Scheme
Node A Transmitter
RTS
DATA
CTS
Node B Receiver
RTS
CTS
DATA
Transmit BT of Node A
Right before sending data
Receive BT of Node B
Immediately after sending CTS
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DBTMA Scheme

• RTS is sent by the sending node
• If RTS reaches the receiving node, the receiver
  broadcasts a receive-busy tone signal prior to
  CTS
• Neighboring nodes sense the receive-busy tone
  signal and block their Tx'es
• CTS reaches the sending node
• Sending node sends a sender-busy tone signal
  prior to DATA
• Neighboring nodes sense the sender-busy tone
  signal and block their Tx'es
• Superior to pure RTS-CTS MAC schemes

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PAMAS (Power-Aware Multi-Access Protocol with
Signaling)

• Proposed by Suresh Singh and C. S. Raghavendra
  1998
• Based on MACA protocol with the addition of a
  separate signaling channel (RTS-CTS channel)
• Built-in power management
• Shuts off inactive nodes to conserve battery
  power
• Nodes exposed to overhearing shuts off power
  since overhearing can cause power waste
• If data arrives, receiver keep transmitting a
  busy tone over signaling channel

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MAC Interfaces and Power-Aware Logic
Signaling Channel I/F
Data Channel I/F

• - of transmitters
• of receivers
• Length of longest reception and transmission

Data Channel Busy/Idel
Power-Aware Logic
Length
On/Off
Transmit Queue
Power
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PARMAS

• Power-Off conditions
• If a node has no packets to transmit and one of
  its neighbors is active
• If a node has packets to transmit, but at least
  one of the neighboring nodes is transmitting.
• Issue When does a node recover the power?
• Power-off duration affects delay and throughput
  performance
• One solution Circuit design allows to
  selectively power off only the data channel

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MARCH(Mesia Access with Reduced Handshake)

• Proposed by C.-K. Toh 2000
• MARCHgt to walk in a stiff, upright, formal
  manner, usually at a brisk pace and in step with
  others.
• Overhearing causes unnecessary power consumption
• MARCH exploits the overhearing characteristics
  when omni-directional antennas are employed.
• Omni-directional antenna is cheap and simple
• Try to improve performance by reducing the amount
  of control overhead

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MARCH(2)

• Does not resort to traffic prediction
• Overhearing is used to predict data arrivals
• Overheard CTS'es indicate data arrivals at
  neighboring nodes
• Based on overhearing, nodes initiate an
  invitation for data

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MARCH
A
D
C
B
RTS1
CTS1
CTS1
DATA
CTS2
CTS2
DATA
CTS3
CTS3
DATA
Node Cs overhearing of CTS1 indicates data
arrival at node B
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Evaluation of MARCH

• of handshakes needed to send a data packet from
  src to dst
• MACA gt 2l
• MACA-BI gt l
• MARCH gt l 1
• If l is large, l ? l 1

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MARCH Implementation
Z
D
X
C
Interference
B
Route 2
Route 1
A
Y
In order to identify multiple overlapping
routes, RTS and CTS (MAC Addr of sender and
receiver, RTID)
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• The MAC layer has access to routing tables in the
  network layer as well as its upstream and
  downstream neighbors in those routes.
• Performance comparison in 21 of book reference

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MACAW

• Adds a supplementary control message ACK
• ACK is broadcasted after data transmission
• Allows lost packet detection and recovery

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IEEE 802.11(1)

• Specifications for both physical and MAC layers
• Two modes ad-hoc and client-server
• DSSS or IR at the physical layer
• Speeds in between 1-2 Mbps
• MAC layer is based on CSMA/CA protocol

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IEEE 802.11(2)

• Hidden node problem solved like in MACAW
• WEP based encryption
• Power management sleep feature
• Different specifications interoperability
  problems
• Complex standardization process ongoing

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Bluetooth

• Originally designed for P2P radio connections in
  small areas
• Connecting different equipments in a small area
• Prevents collisions by using a quasi-random
  freq-hopping algorithm

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Other candidatesHiperLAN, SEEDEX

• HiperLAN offers higher capacity than 802.11
• Otherwise comparable features with 802.11
• SEEDEX offers better throughput than 802.11
• Delay lower than 802.11
• Adapts throughput to traffic flow characteristic

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Conclusions

• Most spread IEEE 802.11
• Does not operate well in multi-hop nets
• Routing is dependent on the MAC and physical
  layer in multi-hop nets
• Multi-hop MAC protocol still a challenge

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Question 1

• IEEE 802.11 is today's most used MAC protocol in
  wireless mobile devices. Make a comparison study
  between 802.11 and other 4 studied MAC protocols
  from the point of view of common criteria (like
  robustness, throughput, security etc.). Prepare a
  feature sheet list for all five protocols from
  where one can see the advantages and
  disadvantages.

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Question 2

• Omni-directional antennas tend to be the default
  choice in most of today's wireless equipments
  because of cost and simplicity. Suppose that you
  have to organize a game session in your apartment
  involving tens of wireless game consoles having
  omni-directional antennas. Assuming that you
  might be able to select the MAC protocol for
  these consoles, which of the studied protocols
  would be your choice in this case?
• Explain your answer.