Medium Access Control protocols for ad hoc wireless networks

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					A Priority MAC Protocol to
Support Real-Time Traffic
   in Ad Hoc Networks


Author: JANG-PING SHEU,CHI-HSUN LIU,
        SHIH-LIN WU, YU-CHEE TSENG
Source: Wireless Networks, Vol. 10, Issue 1,
         pp. 61-69, January 2004
Reporter: Yi-Jen Chiu
Date:2007 / 3/ 22


                                               1
                 Outline
 Introduction
 Priority MAC Protocol
   Priorities Classification
   ID Initialization with CD capability
   ID Initialization with no CD capability
   Transmission
 Simulation Result
 Conclusion
                                              2
                 Introduction
 The IEEE 802.11 has two modes of MAC protocol:
   Point coordination function (PCF) mode
      Based on a centralized polling protocol
      Inappropriate for MANETs since a central
        administrator may not be available

    Distributed coordination function (DCF) mode
      Without central control
      Supports asynchronous data transfer on a
        best-effort basis



                                                    3
               Introduction
 IEEE 802.11 DCF mode channel access
   Carrier Sense Multiple Access/Collision
     Avoidance (CSMA/CA)




                                              4
               Introduction
 InterFrame Spacing (IFS): After a frame has
  been sent, a certain mount of dead time is
  required before any station may send a frame.

   SIFS (Shorter InterFrame Spacing)
   PIFS (PCF InterFrame spacing)
   DIFS (DCF InterFrame Spacing)
   EIFS (Extended Interframe Spacing)



                                              5
            Introduction
 IEEE 802.11 DCF mode does not
  provide a priority mechanism support
  Quality of Service (QoS) transmission.
  QoS guarantee is important for real-
  time traffic, such as video and voice




                                       6
      Priority Mac Protocol
 We assume
  Packets are prioritized into m levels


  Stations are assumed to be able to hear
   each other


  PIFS = 3 × SIFS and DIFS = 5 × SIFS



                                             7
           Classification
 The Black-Burst (BB) scheme is
  adopted here to distinguish the
  priorities of stations.

 BB is a jamming signal. The length of
  BB is proportional to the sending
  station’s priority. Longer BB means
  higher priority.

                                          8
            Classification
 All Stations wait for a DIFS period

 Stations contend for the channel with
  pulses of energy, called the black-
  burst (BB)

 Waits for an “observation” time


                                          9
Black-Burst Scheme




                     10
 Randomized Initialization Protocol
 Given a set of n random stations, the
  purpose of the initialization protocol is
  to assign each of the stations a
  distinct ID number from 1 to n

 We assume for ease of presentation
  that each station has the collision
  detection (CD) capability

                                         11
 Randomized Initialization Protocol
 The stations wait for a PIFS period to
  enter the ID initialization period
 Stations’ requests are denoted by
  REQs
 SIFS is used to separate REQ packets
  during the ID initialization period



                                       12
 Randomized Initialization Protocol
 The basic idea is to construct a binary
  tree called a contention tree
 A station which is able to send a
  request without collision is considered
  successfully obtaining an ID
 If collision occurs, the station will flip
  a fair coin. Incase of head, the station
  will proceed to the left subtree

                                          13
Randomized Initialization Protocol




                                 14
Randomized Initialization Protocol




                                 15
 Randomized Initialization Protocol
 Problem about Protocol
 PIFS = 3 × SIFS and DIFS = 5 × SIFS




                                    16
ID Initialization With CD capability
 Modifies the randomized initialization
  protocol by a “reset” action

 A “reset” action can be taken after
  stations which have not obtained
  their IDs detecting two continuous
  idle rounds. These stations can
  restart a brand new contention tree.

                                           17
ID Initialization With CD capability




                                   18
ID Initialization With CD capability




                                   19
     ID Initialization With No CD
               capability
 We assume the availability of a leader
  election protocol, which can select a station
  as the leader of the network

 The leader serves as the “virtual” collision
  detector in the network.

 The leader will send jamming signal with
  high energy in case that a collision is
  detected

                                                 20
      ID Initialization With No CD
                capability
 The station does not want to send REQ
   Channel busy or silent
      Work as usual
   Channel collision
      Halting for one round
 The station wants to send REQ
   If jamming signals are heard in the next round, it
     knows that its earlier transmission has collision
   As long as no jamming signals are heard, Then it can
     calculate for itself an ID and exit the contention tree.



                                                           21
     ID Initialization With No CD
               capability
 The leader should send jamming signals in the
  SIFS period after the first round

 After the SIFS, the leader should send its REQ
  in the second round to confirm its intention. In
  this case, the leader’s ID is always 1

 All the other stations reset its contention tree
  as a new one



                                                     22
             Transmission
 These stations will transmit their frames in
  a Round-Robin manner

 A piggyback flag should be appended to
  each data frame
   piggyback = 1 means that the sending
    station still has more frames to be sent.
   piggyback = 0 and this station will be
    removed from the list of transmitters

                                                 23
                  Join
 For stations newly joining the network.
   Wait until the next priority
    classification period
   Attach sufficient information on REQ
    frames




                                            24
                      Join
 P: Priority
   L: Collision times
   l: When the station joins the contention
   N: Next available ID number
   Flag: Silent round times
 Initially, L = l = N = 1 and flag = 0




                                              25
               Join


If (“Heads”)
  l=L
Else
  L=L+1
  Flag=0




                      26
                   Join




Initial   Round1          Round2   Round3




Round4    Round5          Round6   Round7

                                            27
          Join




Round8    Round9    Round10




Round11   Round12

                              28
           Simulation Result
 The simulation model is built in a fully
  connected ad hoc network. The channel
  rate is assumed to be 11 Mbps and each
  frame size is 256 bytes

 Following three traffic types are modeled.
   Pure Data – priority 1
   Voice – priority 2
   Video – priority 3


                                               29
Simulation Result




                    30
Simulation Result




                    31
Simulation Result




                    32
Simulation Result




                    33
Simulation Result




                    34
Simulation Result




                    35
               Conclusion
 IEEE 802.11 DCF mode’s MAC protocol does
  not provide a priority mechanism
 This paper Proposes a novel priority MAC
  protocol to support real-time traffic.
   Priority classification mechanism
   ID Initialization
   Transmission with Round-Robin manner
 Simulation results demonstrate that the
  proposed protocol is superior to the DCF
  protocol

                                             36