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Swayambhoo Presentation (2)
 

Swayambhoo Presentation (2)

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This is a slide presentation giving a nice overview of MAC techniques used in Wireless Sensor Networks.

This is a slide presentation giving a nice overview of MAC techniques used in Wireless Sensor Networks.

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  • Tell something about MAC… why it is needed …. how important it is .. etc.
  • Give Analogy for different students taking different classes in different days … then doing CSMA for asking doubts.Synchronization : Is the heart of TDMA …. problem is how to achieve this .. generally infrastructure is required for thisSlot Assignment: Optimal Slot Assignment is NP-Hard Problem…. Slot Size should be such that it allows for transmission of few data packets… It generally takes into account the delay, communication strategy (should be large enough to accommodate various packets exchanged in transmission. Frame Size: depends on the number nodes in a communication area.
  • In the ambit of non-persistant CSMA .. a protocol may employ various other techniques to avoid collisions like RTS/CTS exchange. Transmit step differs in CSMA/CA
  • One-hop collisions resolved by Carrier Sensing Two-hop collisions (Hidden node / Exposed Terminal node) resolved by control messaging …. But control messaging leads to energy consumptions.Efficient slot assignment is a NP-hard problem. Wireless feature allows slot reuse across independent clusters of nodes. One fixed schedule might not work in because channel conditions change.No collisions only if perfect synchronization (this requires frequent messaging)INTERFERENCE range v/s Communication range you need to find out independent set of nodes
  • *In general fairness is not a issue because network has one single goal.*Fairness over short duration is allowed again it depends on the application.*Over a longer duration of time we need fairness as distributed computation of functions can get held
  • Overhearing : Nodes receive the packets destined for other nodes. (Especially when routes are found … node discard packets ..)Idle listening : Listening to channel to receive the possible traffic.Overemitting : Transmission of message when destination is not ready
  • Scheduling of communication Link : a pair of node are given a slot … no collision , idle listening and overhearing but large over headScheduling of Senders : Senders are given slots… Avoids idle listening , collisions …. less overhead overhearing remains a problem Scheduling of reciever : Recievers are given slots … avoids idle listening , overhearing .. less overhead but collisions remain a big problem.First two suited for periodic and high traffic load and last one for medium traffic load.
  • The high amount of energy is spent on initial network setup. This is based on the assumption that there will not be drastic change in the topology. High energy initially affects the network life time. One advantage is there that amount of spent in the depends only on the local networks size
  • Global Sync. Leads to high energy drain & local sync leads to slot wastage. But what is more pragmatic wrt to WSNs.Some slots are anyways required for control messages so it does not lead to as much a waste as it appears.
  • Note : Slot size affects the performance only under HCL because under LCL it is CSMA. If slot sizes are large then the large network delays.To is chosen by stochastic analysis to maximize throughput under the assumption that sync error is less than the one slot period.
  • The problem is that the decision based noise does not help when the high contention is with the one hop neighbourhood.Two types of ECN : one hop ECN and two hop ECN…. Similar to kind of RTS/CTS …. but uses topology information to avoid two-hop collision. Far better than RTC/CTS because it uses ECN only during HCL.
  • Another question : When do we need global clock synchronization. ? Answer : Needed only once at the beginning. Even with no sync the performance falls back to CSMA/CA
  • Clock drifts lead to frequent running of DRAND 2) Latency and 3) Does not solve hidden node problem
  • Duty Cycle = SD/BDDuration for radio to be on = ½ BI

Swayambhoo Presentation (2) Swayambhoo Presentation (2) Presentation Transcript

  • Swayambhoo Jain
    MSEE, 1st semester
    University of Minnesota, Twin Cities
    Media Access Control (MAC) in Wireless Sensor Networks-II
  • Outline
    Traditional MAC families
    Time Division Multiple Access (TDMA)
    Carrier Sense Multiple Access (CSMA)
    Challenges in MAC design for Wireless Sensor Networks (WSN)
    Taxonomy of MAC protocols in WSN
    Schedule based protocols
    TSMP
    Protocols with common active periods
    S-MAC
    Preamble sampling protocols
    Hybrid Protocols
    Z-MAC
    IEEE 802.15.4 (Zigbee)
  • Traditional MAC: TDMA
    TDMA is a reservation based strategy in which medium is accessed in a time slotted fashion.
    Critical Requirements:
    Synchronization
    Slot Assignment Algorithm (Not easy… Optimal Slot Assignment is NP-Hard problem)
    Choice of frame size and slot size affects the performance
    What should be kept in mind before selecting slot size and frame size ?
    Slot
    1
    2
    3
    5
    4
    6
    Frame
  • Traditional MAC : CSMA
    CSMA is a contention based strategy
    Simple probabilistic MAC protocol in which shared medium is sensed before transmitting
    The rule is “If medium is idle transmit otherwise defer the transmission”.
    Flavors of CSMA :
    Non-persistant CSMA
    p-persistant CSMA
    CSMA/CA used in wireless networks avoids collision by random backoffs.
    Start
    Start
    No
    No
    Medium idle?
    Medium idle?
    Wait for random back off time
    Wait until medium is free
    Yes
    Yes
    Probability 1
    Transmit
    Transmit
    Probability p
    End
    End
    Non Persistant CSMA
    CSMA
  • Reservation based v/s Contention based
    CSMA :
    Requires no infrastructure
    No synchronization and robust to changes in network topology
    High amount of idle listening and overhearing overhead
    Prone to collisions
    Throughput decreases as traffic increases
    TDMA :
    Suited for Base Station/Remote-Station architectures
    Requires synchronization and not robust to topology changes
    No collisions
    Has potential for saving energy
    Low throughput even under low traffic
    One-hop collisions or two-hop collisions ?
    What are the conditions for no collisions ?
  • Reservation based v/s Contention based
    Reservation Based
    1.0
    0.9
    0.8
    0.7
    0.6
    0.5
    0.4
    0.3
    0.2
    0.1
    0
    0.01-persistent CSMA
    Nonpersistent CSMA
    0.1-persistent CSMA
    Throughput
    0.5-persistent CSMA
    1-persistent CSMA
    Slotted Aloha
    Aloha
    0 1 2 3 4 5 6 7 8 9
    Offered Load
  • Challenges in design of MAC for WSNs
    High energy efficiency for network longevity
    Scalability
    Small footprint
    Robustness towards :
    Time varying channel conditions and Dynamic network topology
    Loss in synchronization
    Low latency
    Fairness
    (How can topology change in WSNs?)
    (Is Fairness really an issue ? )
  • Questions / Discussion
    For Wireless Sensor Networks which one is better CSMA or TDMA ?
    Bad
    Good
    Good at low traffic
    Good at high traffic
    Bad
    Good
    Bad
    Good
    Good
    Bad
    Depends on traffic
    Depends on traffic
    Bad
    Good
    Good
    Bad
  • Sources Energy Wastage in WSNs
    Can be solved by Contention
    Energy Efficiency is the prime requirement for MAC design in WSN
    MAC layer is most suitable level to address the issue of energy efficiency
    First we need to identify the possible sources of energy wastage in WSNs.
    There are many-many types of MACs primarily designed to tackle one or more sources of energy wastage
    Can be solved by Reservation
  • Taxonomy of MAC protocols in WSN
    Scheduling based protocols
    Protocols with common active period
    Preamble sensing MACs
    Hybrid MACs
  • Scheduled Based MACs
    Scheduling based protocols
    Medium is shared based on a schedule (requires synchronization)
    Variants of TDMA combined with FDMA
    Suited for periodic and high load
    TSMP (Time Synchronized Mesh Protocol ) is an interesting example of this family
    Types of Scheduling done
    Scheduling of communication link
    Scheduling of senders
    Scheduling of receivers
    Helps in avoiding collisions , idle listening and over hearing
  • TSMP (Scheduling based)
    It is a TDMA based protocol which uses FDMA and frequency hopping. This allows a node to participate in multiple frames at the same time thereby allowing multiple synchronization rates for different tasks.
    Sink generates the scheduling table based on, the list of the nodes, their neighbors and their requirements.
    Precise sense of time is maintained and only offset information is exchanged together with usual data and ACK packets
    ch 1
    ch 2
    ch.1
    ch 3
    ch 4
    ch.2
    ch.3
    ch 5
    t1
    t2
    t3
  • Ch. 15
    Ch. 14
    Ch. 13
    E
    Ch. 12
    F
    A
    Ch. 11
    To Avoid interference
    B
    Ch. 10
    Ch. 9
    Sink
    (G)
    H
    Pros:
    Very less collisions
    No overhearing
    Minimized idle listening
    Cons:
    Complex
    Scalability
    Reduced flexibility
    Memory footprint
    Ch. 8
    C
    Ch. 7
    D
    Ch. 6
    Ch. 5
    Ch. 4
    Ch. 3
    Ch. 2
    Ch. 1
    Ch. 0
    t1
    t2
    t3
    t5
    t7
    t10
    t4
    t6
    t8
    t9
  • Scheduled Based MACs – Various Approaches
  • MACs with Common Active Periods
    Protocols with common active periods
    Energy is saved by common active/sleep periods across a set of nodes
    Suited for periodic traffic
    SMAC (Sensor MAC) typical example of this family
    Radio on
    Radio on
    Radio on
    Radio off
    Radio off
  • SMAC (Common Active Period Protocol)
    Set of nodes periodically become active/sleep in a synchronized fashion. This set of nodes is called Virtual Cluster
    Active periods are divided into two periods, one for exchanging SYNC packets and other for exchanging DATA packets
    Active periods are fixed to a pre-recalculated size optimized for expected traffic.
    Collisions are avoided by RTS/CTS mechanism
    Radio on
    Radio on
    Radio on
    Radio off
    Radio off
    Reduces idle listening
    For Sync
    For Data
    Reduces Over hearing
    RTS/CTS/DATA/ACK
    SYNC
  • SMAC
    Schedule 1
    Schedule 2
    At the start, a node listens to the channel for at least one active period and sleep period and; if it does not receive SYNC packet it chooses its own schedule and broadcasts it to its neighbors.
    There can be different SYNC packets in the network and hence the network is, more often, made up of many virtual clusters.
    The border nodes have to adapt to the schedule of both the neighboring clusters.
  • SMAC
    The long data packets are broken into small packets and transmitted in a burst (RTS/CTS used only in transmitting first fragment)
    Pros:
    Saves energy by avoiding Idle listening, Overhearing.
    Well-designed, complete protocol that addresses deficiencies of 802.11 if applied to a sensor network.
    Cons:
    Suited only for the periodic traffic patterns; irregular traffic patterns may lead to collisions.
    Rigidity due to pre-fixed active periods
    Sleep Delay
  • Common Active Periods – Various Approaches
  • Preamble Sampling MACs
    Preamble Sampling Protocols
    Each node chooses to be active/to sleep independent of others
    Nodes sleep most of the time and wake up periodically to check if there is a transmission
    BMAC is a typical example of this family (Already Covered in detail)
    Sender
    Data
    Preamble
    Check Interval
    Receiver
    Radio Off
    Radio On
    Periodic Channel Sampling
  • Preamble Sampling – Various Approaches
  • Hybrid MACs
    Hybrid Protocols
    Due diverse set of applications the WSNs target Hybrid protocols are needed one particular approach is not perfect.
    As WSNs inherently have variable traffic patterns schemes suitable for one traffic type are not sufficient
    Can’t be classified in any of the above categories as they use the combination of above techniques.
    ZMAC and IEEE 802.15.4 (Zigbee) are typical examples
    # of Contenders
    CSMA
    TDMA
    Channel Utilization
  • Z-MAC (Zebra-MAC) – A hybrid MAC scheme
    Z-MAC is a hybrid MAC scheme :
    Uses CSMA for high throughput at low contention and hints from TDMA schedule for better performance at high contention
    Implemented on the top of B-MAC i.e. uses CCA, LPL etc.
    CSMA is a baseline scheme :
    Robust to Synchronization errors and dynamic topology changes
    At worst it always falls back to CSMA performance
    The design is best understood by the setup phase Z-MAC.
    Neighbor discovery
    Time slot assignment
    Local frame exchange
    Global time synchronization
    The idea is that the high initial setup cost is eventually paid back by improved network performance
  • Z-MAC – Neighbor Discovery
    The sensor nodeinitially broadcasts periodic pings to its 1-hop neighbors
    Ping message contains list of node’s 1-hop neighbors
    List builds up as time passes
    Eventually every sensor has the list of their 1-hop as well as 2-hop neighbors
    Current implementation takes 30 seconds
    The list is used as an input to slot assignment algorithm
  • Z-MAC – The Slot Assignment (DRAND)
    DRAND :
    Distributed implementation of RAND
    Well suited for WSNs, does not require any extra infrastructure
    Two hop list from neighbor discovery is used to come up with a slot assignment such that no two nodes in 2-hop neighborhood have the same slot.
    Slot number assigned does not exceed the number of nodes in a neighborhood
    Scalable.
    Slot assignment is highly efficient.
  • Z-MAC – The Slot Assignment (DRAND)
    E
    A
    C
    D
    B
    F
    E
    E
    A
    A
    D
    C
    D
    C
    F
    B
    F
    B
    Radio Interference Map
    (Energy Cost ) α  (Neighbourhood Size)
     
    DRAND slot assignment
    1
    0
    3
    2
    0
    Time slot
    1
    Input Graph
    1
    2
    3
    4
    5
    6
    7
    How to decide this ?
    Time Frame
  • Z-MAC – Local Frame Exchange
    Time frame Rule:
    “ If for a node i, Maximum Slot Number in  two hop  neighbor is Fi,
         Time frame is 2a where ′a′ satisfies  2a−1 ≤ Fi< 2a ”
    Time frame decided on the basis of local information. (Advantages / Disadvantages ?)
    May lead to slot wastage on a global scale
    Allows for robustness towards local topology changes
    Only local synchronization is needed
     
    1(5)
    E
    C
    3(5)
    A
    F
    2(5)
    0(2)
    B
    D
    1(2)
    5(5)
    G
    4(5)
    5(5)
    H
  • Z-MAC - Transmission Control
    After slot and frame assignment each node sends these details to its two-hop neighbors.
    Two modes of operation :
    HCL (High Contention Load)
    LCL (Low Contention Load)
    Node is normally in LCL until it receives explicit contention notification message from two-hop neighbors within last tECN period.
    Rules:
    Common rule : “Owner of the slot has highest priority”
    LCL Tx. rule : “Any node can compete for a transmission in a particular slot”
    HCL Tx. Rule : “Only owners and their 1-hop neighbors can compete for a slot”
    How these rules are imposed ? …. Answer is in next slide
     
  • Z-MAC - Transmission Control
    Busy
    Owner Accessing Channel
    Busy
    Owner Accessing Channel
    Random Backoff (Backoffs within fixed To)
     
    Busy
    Non-owner Accessing Channel
    To
     
    Busy
    Non-owner Accessing Channel
    To
     
    Random Backoff (Backoffs within Toand Tno)
     
    Owner
    Non Owner
    𝒔𝒚𝒏𝒄 𝒆𝒓𝒓𝒐𝒓≤𝑻𝒐⇒𝒂𝒕 𝒎𝒐𝒔𝒕 𝟐 𝒕𝒐 𝟑 𝒐𝒘𝒏𝒆𝒓𝒔 𝒇𝒐𝒓 𝒂 𝒔𝒍𝒐𝒕
     
    Trade off between slot size and network delay
    𝑻𝒊𝒎𝒆 𝑺𝒍𝒐𝒕>𝒄𝒉𝒆𝒄𝒌 𝒑𝒆𝒓𝒊𝒐𝒅+ 𝑻𝒐+ 𝑻𝒏𝒐+𝑪𝑪𝑨 𝒑𝒆𝒓𝒊𝒐𝒅+𝒑𝒓𝒐𝒑𝒂𝒈𝒂𝒕𝒊𝒐𝒏 𝒅𝒆𝒍𝒂𝒚
     
  • ZMAC – Transmission Control
    Explicit Contention Notification (ECN) messages notify the two hop neighbors not to act as hidden terminal under high load
    Nodes make local decision of high contention :
    By keeping track of ACKs from a particular destination and see packet loss rate
    Since two hop collision are highly correlated to packet loss rate
    By checking the noise level of the channel by measuring the average number of noise backoffs before transmitting a packet
    Based on the fact that there is high correlation between the noise backoffs and traffic
    Flooding ECN is avoided by selective forwarding of ECNs :
    Wait for a random period before transmitting ECN messages.
  • Z-MAC - Synchronization
    Z-MAC performs like CSMA with or without synchronization at low traffic.
    At high traffic , Z-MAC behaves as TDMA , synchronization is necessary for improved performance.
    Synchronization is achieved by sending sync. control message limited to certain fraction of the data sending rate. ( How does this help? )
    Sync. control messages from unsynchronized node should be given less priority.
    Cavg=1−𝛽tCavg+ βtCavg , where βt is trust factor
    Question: In Z-MAC do we need local synchronization or global synchronization ?
     
  • Z-MAC – Performance Evaluation
    Performance was measured for single-hop, two-hop, multiple hop configurations.
    It was compared mainly against BMAC (Default MAC in TinyOS) on the following metrics :
    Throughput
    Energy Efficiency
    Platform:
    ns2
    Mica2
    8-bit CPU at 4MHz
    8KB flash, 256KB RAM
    916MHz radio
    TinyOS event-driven
  • Z-MAC – Performance Evaluation
    Single hop configuration
    Nodes are kept equidistant from the receiver in a circle
    Each node transmits with full transmission power
    Two-hop configuration:
    Nodes are organized into two clusters of 7-8 node (Dumb-Bell shaped topology)
    Aim was to measure performance in presence of a Hidden Terminal
    Transmission Power was 1 dBm (1.3 mW) to control number of hidden terminals
    Multihop Configuration:
    42 Mica2 nodes placed in different rooms of a building
    Routing paths were fixed after one run of Mint (default routing protocol of TinyOS)
  • Z-MAC – Experiment result multi-hop
  • Z-MAC- Experiment result multi-hop
  • Z-MAC – Limitations
    What are the limitations of Z-MAC ?
  • Hybrid Protocols : Various Approaches
  • Zigbee / IEEE 802.15.4 - Introduction
    A group of companies (Zigbee Alliance) started working on a technology for a low data rate, low power consumption, low cost, wireless networking protocol targeted towards control and sensor networks.
    Around same time IEEE 802.15.4 (LR-WPAN MAC Protocol) committee started working on a low data rate standard.
    IEEE 802.15.4 and Zigbee Alliance joined hands to work on this technology and Zigbee is a commercial name of this technology.
  • Zigbee
    Application Interface
    Network Layer
    MAC Layer (IEEE 802.15.4)
    MAC Layer
    PHY Layer
    Application
    Customer
    802.2 LLC
    ZigbeeAlliance
    SSCS
    IEEE
    Silicon
    Zigbee Stack
    Application
  • IEEE 802.15.4 based LR-WPAN
    Device Classification:
    Fully functional device (FFD) :
    Can talk to FFD as well as RFD
    Can function as PAN coordinator or as a device
    Can be used in any topology
    Reduced functional device (RFD):
    Can only talk to FFD
    Does extremely simple tasks.
    Limited to Star topology
    Network is made up many PANs each managed by a PAN Coordinator. PANs are identified by unique PAN ID.
    Network Topologies :
    Star
    Peer to peer
    Mesh
    Cluster Tree
    STAR
    Mesh
    Cluster Tree
    FFD
    PAN Coordinator
    RFD
  • IEEE 802.15.4 based LR-WPAN
    Two types of Communication modes :
    Beacon Enabled
    Beacon is transmitted by FFD periodically after each BI (Beacon Interval)
    Super frame structure is followed
    Suited for higher data rate kind of applications
    Non Beacon Enabled
    Simply reduces to CSMA/CA MAC
    Suited for very simple applications like periodic sensing
    Synchronization is achieved by :
    Beacon tracking mode
    Node simply synchronizes to first beacon and uses the information to switch on just before the next beacon
    Non-Tracking mode
    Sync done only when the data needs to be transmitted
  • IEEE 802.15.4 based LR-WPAN
    IEEE 802.15.4 MAC provides following services:
    MAC Data Service – responsible for transmission and reception of MPDU (MAC protocol data units) across the PHY data service
    MAC Management Services – interfacing to MLME-SAP (MAC Layer Management Entity – Service Access Points)
    Features :
    Beacon and GTS Management
    Channel Access
    Frame Validation and acknowledge frame delivery
    Association & Dissociation
    IEEE 802.15.4 PHY provides following services :
    PHY data service – responsible for transmission and reception of PPDU (PHY protocol data units) across the channel
    PHY management service – interfacing to PLME-SAP (PHY Layer Management Entity – Service Access Point)
    Features :
    ED (Energy Detection)
    LQI (Link quality Indication)
    CCA (Clear Channel Assessment)
  • IEEE 802.15.4 MAC – Super Frame
    Superframe consists of :
    Active period, whose length is defined by SD (Superframe Duration), is divided in 16 equal slots :
    Slot zero is reserved for Beacon
    CAP (Contention Access Period) starts in the next slot after beacon is transmitted and nodes compete using slotted CSMA/CA
    CFP (Contention Free period) starts in the slot after CAP. GTS (Guaranteed time slots ) are used for data transfer.
    Inactive period
    Node sleeps in inactive period
  • IEEE 802.15.4 MAC Layer – The Super Frame
    ( Transmitted by PAN Coordinator, is of variable length for GTS allocation )
    ≥aMinCAPlength
     
    Nodes Sleep here
    Slotted CSMA if Beacon enabled
    At MAX 7 GTS of one or more slots. GTS can uplink or downlink
    𝟎≤𝑺𝑶 ≤𝑩𝑶 ≤𝟏𝟒
     
    Questions: 1) Duty Cycle ? & 2) How should the radio be in sync in non tracking mode ?
  • IEEE 802.15.4 MAC Layer – Data Services
    Direct Data Transfer
    Message Sequence Diagram
  • IEEE 802.15.4 MAC Layer – Data Services
    Indirect Data Transfer
    Message Sequence Diagram
  • IEEE 802.15.4 MAC Layer – MLME
  • IEEE 802.15.4 - Association
    Message Sequence Diagram
  • IEEE 802.15.4 - Dissociation
  • IEEE 802.15.4 - Orphaning
  • Slotted CSMA/CA in IEEE 802.15.4
    SlottedCSMA
    Delay for random(2BE −1) unit backoff periods
     
    NB= 0 , CW= 0
     
    Battery Life Extension ?
    BE = min(2,
     macMinBE)
     
    Perform CCA on backoff period boundary
    Y
    BE = macMinBE
     
    Channel Idle ?
    Y
    N
    N
    Locate backoff period boundary
    CW=2, NB=NS+1, 
    BE=min⁡(BE+1, aMaxBE)
     
    CW=CW−1
     
    N
    NB > macMacCSMABackoffs ?
    CW=0 ?
     
    N
    Y
    Y
    Failure
    Success
    Used in Beacon Enabled mode
  • Unslotted CSMA/CA in IEEE 802.15.4
    UnslottedCSMA
    NB= 0 , BE =macMinBE
     
    Y
    Delay for random(2BE −1) unit backoff periods
     
    NB > macMacCSMABackoffs ?
    N
    Perform CCA
    Failure
    Channel Idle ?
    Y
    Success
    N
    Used Beacon Disabled Mode
    NB=NS+1, 
    BE=min⁡(BE+1, aMaxBE)
     
  • WSN MAC protocols summary
  • References
    [1] InjongRhee, AjitWarrier, Mahesh Aia and Jeongki Min, “ZMAC:aHybrid MAC for Wireless Sensor Networks”, IEEE/ACM Transactions on Networking (TON) Vol. 16 , Issue 3 (June 2008)
    [2] Wei Ye, John Heidemann and Deborah Estrin, “An Energy-Efficient MAC Protocol for Wireless Sensor Networks”, INFOCOM 2002. Twenty-First Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE
    [3] Sunil Kumar , Vineet S. Raghavan and Jing Deng,“Medium Access Control protocols for ad hoc wireless networks: A survey”, Ad Hoc Networks Volume 4, Issue 3, May 2006,
    [4] Abdelmalik Bachir,   Mischa Dohler, Tomas  Watteyne, and  Kin K. Leung, “MAC Essentials for Wireless Sensor Networks”, IEEE Communication Surveys & Tutorials, Vol. 12, No.2, Second-Quarter 2010
    [5] IlkerDemirkol, CemErsoy, and FatihAlagöz, “MAC Protocols for Wireless Sensor Networks: A Survey”, IEEE Communication Magazine, April 2006, Vol. 44 Issue 4
    [6] Anurag Kumar, D. Manjunath and Joy Kuri, “Wireless Networking” , 2008 Edition
    [7] SinemColeriErgen, “ZigBee/IEEE 802.15.4 Summary”