BASIC PROPERTIES OF IEEE STD 802.15.4 MAC in .NET

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BASIC PROPERTIES OF IEEE STD 802.15.4 MAC
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TABLE 16.1. Timing Structure of the Slotted Mode MAC Protocol Type of Time Period Modulation symbol Duration 1 Data bit in 860-MHz and 915-MHz bands, 4 data bits in 2.4-GHz band 20 symbols Three unit backoff periods (60 symbols) 16 basic superframe slots (960 symbols) aBaseSuperframeDuration 2SO aBaseSuperframeDuration 2BO 1220 symbols 12 symbols N/A MAC Constant
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Unit backoff period Basic superframe slot (SO = 0) Basic superframe length (SO = 0) (Extended) superframe duration SD Beacon interval BI Maximal time to wait for downlink transmission Rx-to-Tx or Tx-to-Rx maximum turnaround time Timeout value to wait for the acknowledgement
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aUnitBackoffPeriod aBaseSlotDuration aBaseSuperframeDuration = NumSuperframeSlots aBaseSlotDuration macSuperframeOrder, SO macBeaconOrder, BO aMaxFrameResponseTime aTurnaroundTime
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54 symbols
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macAckWaitDuration
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Note: The values of both BO and SO must be less than 15 in the beacon enabled mode.
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that is further spread with the chipping sequence. In the third band, the O-QPSK modulation is used before spreading; in this case, four data bits comprise one modulation symbol that is further spread with the 32-bit spreading sequence. Table 16.1 summarizes the basic timing relationships in the MAC sublayer. Note that the constants and attributes of the MAC sublayer, as de ned by the standard, are written in italics. Constants have a general pre x of a (e.g., aUnitBackoffPeriod), while attributes have a general pre x of mac (e.g., macMinBE). 16.2.1 CSMA-CA Algorithm This algorithm is comprised of downlink data transmission, uplink data transmission, and uplink request transmission states. As is the case with other contention-based access control schemes, transmission will be attempted only when the medium is clear, but withheld if there is channel activity or when contention occurs. The CSMACA protocol, shown as a owchart in Figure 16.3, is invoked when a packet is ready to be transmitted. In this algorithm, three variables are maintained for each packet: 1. NB is the number of times the algorithm was required to experience backoff due to the unavailability of the medium during channel assessment. 2. CW is the contention window that is, the number of backoff periods that need to be clear of channel activity before the packet transmission can begin.
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CLUSTER INTERCONNECTION IN 802.15.4 BEACON-ENABLED NETWORKS
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step (1)
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Yes macBattLifeExt No set BE=macMinBE
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set BE to min(2, macMinBE)
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locate boundary of the backoff period
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step (2)
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wait for a random number of backoff periods
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step (3)
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Yes channel idle No step (4) set c =2, i =i +1, BE=min(BE+1, aMaxBE) set c = c -1 step (5)
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i > macMaxCSMABackoffs
c =0
Yes report failure success proceed with transmission
Figure 16.3. Operation of the slotted CSMA-CA MAC algorithm in the beacon-enabled mode. (Adapted from reference 1.)
MASTER SLAVE BRIDGING ALGORITHM
3. BE is the backoff exponent that is related to the number of backoff periods a device should wait before attempting to assess the channel (see below for a detailed explanation). In step 1, the algorithm begins by setting NB to zero and CW to 2. If the device operates on battery power (as determined by the attribute macBattLifeExt), BE is set to 2 or to the constant macMinBE, whichever is less; otherwise, it is set to macMinBE (the default value of which is 3). The algorithm then locates the boundary of the next backoff period. In step (2), the algorithm attempts to avoid collisions by generating random waiting time in the range 0, . . . , 2BE 1 backoff periods. When the wait period is over, the MAC sub-layer needs to perform CW clear channel assessment (CCA) procedures, transmit the frame, and optionally wait for acknowledgment. The time to wait for an acknowledgment, macAckWaitDuration, is equivalent to 54 or 120 symbols, depending on the currently selected physical channel. If the remaining time within the CAP area of the current superframe is suf ciently long to accommodate all of these, the MAC sublayer will proceed with step (3) and perform the rst CCA to see whether the medium is idle. If the remaining time is not suf cient, the MAC sublayer will pause until the next superframe. If the channel is busy, the values of NB and BE are increased by one (but BE cannot exceed macMaxBE, the default value of which is 5), while CW is reset to 2; this is step (4) in the owchart. If the number of retries is below or equal to macMaxCSMABackoffs (the default value of which is 5), the algorithm returns to step (2), otherwise the algorithm terminates with a channel access failure status. Failure will be reported to the higher protocol layers, which can then decide whether to reattempt the transmission as a new packet or not. If the channel is idle, step (5), the value of CW is decreased by one, and the channel is assessed again. When CW becomes zero, the transmission of the packet may begin, provided that the remaining number of backoff periods in the current superframe suf ces to handle both the packet and the subsequent acknowledgment. If this is not the case, the standard requires that the transmission is deferred until the beginning of the next superframe. Note that the backoff unit boundaries of every device should be aligned with the superframe slot boundaries of the PAN coordinator; that is, the start of rst backoff unit of each device is aligned with the start of the beacon transmission. The MAC sublayer should also ensure that the PHY layer starts all of its transmissions on the boundary of a backoff unit.
16.3 MASTER SLAVE BRIDGING ALGORITHM We consider two interconnected clusters operating in the ISM band around 2.4 GHz. Each cluster operates in a different frequency sub-band so that intercluster interference is avoided (standard prescribes 16 distinct cluster channels). We assume that both clusters operate in beacon-enabled CSMA-CA mode control of their respective