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The MegaWatch Wireless PlatformMaster's Thesis - Emanuel Corthay7 The ProtocolsAFrom A to BBBlockedBlockedCDFigure 24 Transmitting Scenario ; C and D blocked (medium busy)Without a collision avoidance mechanism, as soon as the medium is free, node C and D will starttransmitting. Even worse, node B will start transmitting an acknowledgement (Ack) frame too,resulting in a massive collision depicted on Figure 25.AAckB to ABCollisionC to BCollision andReceiver offCD to CDFigure 25 The medium becomes free, multiple transmissions; collisionsTo avoid this collision problem, a collision avoidance mechanism is used. Different types of frameshave different priorities. These priorities are reflected in the Inter Frame Spaces (IFS.Upon medium liberation, the transmitters will wait for the medium to be sensed as idle for a givenIFS according to the frame type.Acknowledgement packets should always have the highest priority to complete a transmission assoon as possible. Thus, the acknowledgement frame has a Short IFS (SIFS). When the mediumhas been sensed as idle for SIFS, the acknowledgement frame will be transmitted.For normal frames a longer IFS or DIFS (Distributed Coordination Function IFS) is used. When themedium has been sensed as idle for a time larger than DIFS, the normal frame can be sent.44 / 83
The MegaWatch Wireless PlatformMaster's Thesis - Emanuel Corthay7 The ProtocolsThis again will lead to collisions when two nodes want to transmit a normal frame. Thus, arandomized back-off time will be selected independently on each node and added to the DIFS timebefore transmission. That method ensures that the probability of collision is now a randomvariable.The randomized back-off time is a multiple of the slot time. The slot time is essentially themaximum time it takes to detect that another station is transmitting.DIFSDIFSSIFSMedium BusyAccess DelayedContention WindowSlot SelectionBackoff WindowSlot TimeMedium BusyFigure 26 CSMA/CA MethodChoice of slot time, SIFS and DIFS:The time duration SIFS and DIFS must be chosen based on the characteristic of the physicalinterface. The DIFS must be long enough to ensure the detection by all nodes within transmissionrange of a unit transmitting an acknowledgement frame using SIFS. Propagation time has to betaken into account; the frame will not be received at the same time by all units if the distancebetween them is different.The SIFS is not equal to zero, because it includes time for the initial transmitter to switch back toreceive mode (also known as turnaround time). In our case, the limiting factor is the switch timefrom receive to transmit mode (warm-up time) that will fix the minimum IFS to at least 500 µS(including margin). In practice, we launch the transmission process of an ACK packet as soon aspossible, knowing that this warm-up time will delay the actual transmission of the frame for at leastthat 500 µS duration. We also have to include the time to transmit the first 40 bits (preamble andSFD), because our system relies on those bits to detect the beginning of a transmission. Thesignal propagation delay must also be taken into account.The maximal propagation delay is calculated based on a 1000m range system. No actual rangemeasurements have been conducted, but at maximum power, in an obstacle-free outdoorenvironment and at a 38.4 Kbps speed, it is very unlikely that such a distance can be covered.The SIFS is then:1000SIFS = 500 µS + 40 · 26 µS + 2 ≈ 500 + 1040 + 7 ≈ 1550 µS6300 ⋅10The DIFS will include the SIFS (1550 µS) plus the time to transmit the preamble and SFD, plus thepropagation time to reach every unit, with a guard time margin:DIFS = SIFS + time to transmit 40 bits + signal propagation delay for 1000 meters1000DIFS = 1550 µS + 40 · 26 µS + 2 ≈ 1550 µS + 1040 µS + 7 µS ≈ 2600 µS6300 ⋅10The slot time is determined by the time to transmit 40 bits, plus the maximum propagation delay,and is equal to 1050 µS, mainly because of the time it takes to synchronize the receiver (preamble)and to send the Starting Frame Delimiter.During this work, the acknowledgement mechanism and CSMA/CA described here were notimplemented in hardware. It will be completed either in hardware or in software during the nextMaster’s Thesis work to start in March 2004 by Max Laager. A random number generator hasbeen implemented to facilitate this improvement.45 / 83
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EPFL ACORT Mon Dec 1 08:47:30 2003
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Component Report MEGAWATCHmegawatch
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Single Node Nets Report MEGAWATCHme
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Qii8i¡,//4$-r G D/- ’KK•-4K*M.
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EPFL ACORT Thu 22.01.2004 11:21 Dra
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EPFL ACORT Thu Jan 22 11:21:21 2004
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Component Report SABUSALIMsabusalim
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BOM Report SABUSALIMsabusalim_gloss
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Signal Page Ref ROKEPXA_Brokepxa Th
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10000000000100NC1510KR119U11VALD19B
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Component Report ROKEPXA_Brokepexa_
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BOM Report ROKEPXA_Brokepexa_gloss
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Power Pins Report ROKEPXA_Brokepexa
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Extensions Milli-BUS RokEPXAA24 MB_
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Extensions Milli-BUS RokEPXAA24 SPI
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24 MHzrf_TRX_moduletxSpeed3 = ctrl3
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-- Modify here if you change the st
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doEmerBut => doEmerBut,inhibitEmer
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xFifoReadEn -- ctrlreg32if (writeE
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futurestate txBusyTmp
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egPayload
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edge)-- If we reached the end of th
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-- ################################
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RxStbis in std-by)TxStbis in std-by
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when idleRx =>rxEn
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uf:process (clk, reset)beginif rese
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ReadPointer
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end process;-- Bufferize the signal
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when emerPressed =>bufferedEmerButt
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process (clk, reset)beginif (reset=
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component sreg8port ( clk : in std_
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-- Signals to and from the TXchip_e
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component IO_Ctrlport ( clkclkTXres
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File: avalon_bus.vhd[…]-- Above:
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eginif reset = '1' thencurrentstate
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The MegaWatch Wireless PlatformMast
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