Two mechanisms for channel access : contention based and contention free.
contention-based access allows devices to access the channel in a distributed fashion using a CSMA-CA backoff algorithm.
Contention-free access is controlled entirely by the PAN coordinator through the use of GTS.
1. Superframe Structure
A coordinator on a PAN can optionally bound its channel time using a superframe structure. A superframe is bounded
by the transmission of a beacon frame and can have an active portion and an inactive portion.The coordinator may enter a low-power(sleep) mode during the inactive portion.
The structure of this superframe is described by the values of macBeaconOrder and macSuperFrameOrder.
The MAC PIB (PAN information base) attribute macBeaconOrder, describes the interval at which the coordinator shall
transmit its beacon frames.
The value of macBeaconOrder (BO) and the beacon interval (BI) are relates as follows :
for 0<= BO<=14, BI = aBaseSuperframeDuration * 2^BO symbols. The value of macSuperFrameOrder shall be ignored if BO = 15.
macSuperFrameOrder describes the length of the active portion of the superframe, which includes the beacon frame.
The value of macSuperFrameOrder(SO) and the superframe duration(SD), are related as follows : for 0<=SO<=B0<=14, SD = aBaseSuperframeDuration*2^SO symbols.
If SO = 15, the superframe shall not remain active after the beacon.
If BO = 15, the superframe shall not exist
The active superframe shall be devided into aNumSuperframeSlots equally spaced slots of duration 2^SO * aBaseSlotDuration and is composed of three parts : a beacon, CAP and CFP.
The beacon shall be transmitted without the use of CSMA at the start of slot 0 and the CAP shall commence
immediately following the beacon.
the CFP,if present, follows immediately after the CAP and extends to the end of the active portion of the
superframe. Any allocated GTSs shall be located within the CFP.
PAN that wish to use the superframe structure (referred to as a beacon-enabled PAN) shall set macBeaconOrder to a
value between 0 and 14 and macSuperframeOrder to avalue between 0 and the value of macBeaconOrder.
PAN that do not wish to use the superframe structure (referred to as a nonbeacon-enabled PAN) shall set both
macBeaconOrder and macSuperframeOrder to 15.
a coordinator shall not transmit beacons. all transmission shall use an unslotted CSMA-CA mechanism to access the channel. GTSs shall not be permitted.
1.1 Contention Access Period (CAP)
the CAP shall start immediately following the beacon and complete before the beginning of the CFP on a superframe slot boundary. If the CFP is zero length, the CAP shall complee at the end of the active portion of the superframe.
The CAP shall be at least aMinCAPLength symbols, it shall shrink or grow dynamically to accomodate the size of the CFP.
All frames transmitted in the CAP shall use a slotted CSMA-CA mechanism to access the channel.A device transmitting within the CAP shall ensure that its transaction is complete (including the reception of any acknowledgment) one IFS period before the enf of CAP. If this is not possible, the device shall defer its transmission untuil CAP of the following superframe.
1.2 Contention-free period (CFP)
the CFP shall start on a slot boundary immediately following the CAP and it shall complete before the end of the active portion of the superframe. If any GTSs have been allocated by the PAN coordinator, they shall be located within the CFP and occupy contiguous slots.the CFP shall grow or shrink depending on the total length of all of the combined GTS.
No transmission within the CFP shall use a CSMA-CA.
2. Incoming and outgoing superframe timing.
On a beacon-enabled PAN a coordinator that is not the PAN coordinator shall maintain the timing of both the superframe in which its coordinator transmits a beacon (the incoming superframe) and the superframe in which it transmits its own beacon (the outgoing superframe).
The relative timing of these superframe is defined by the StartTime parameter MLME-Start.request primitive.
3. Interframe Spacing (IFS)
The MAC sublayer needs a finite amount of time to process data received by the PHY. To allow for this, two
Successive frames transmitted from a device shall be separated by at least an IFS period.
4. CSMA-CA Algorithm
The CSMA-CA algorithm shall be used before the transmission of data or MAC commad frames transmitted within the CAP.
the CSMA -CA algoruthn shall not be used for the transmission of beacon frames in a beacon-enabled PAN, acknowledgment frames, or data frames trasmitted in the CFP.
Each device shall maintain three variables for each transmission attempt : NB,CW and BE.
NB is the number of times the CSMA-CA algorithm was required to backoff while attempting the current transmission.
This value shall be initialized to zero before each new transmission attempt.CW is the contention window length, defining the number of backoff periods that need to be clear of channel actifity before the transmission can commence, this value shall be initialized to two before each transmission attempt and
reset to two each time the channel is assessed to be busy.
BE is the backoff exponent, which is related to how many backoff periods a device shall wait before attempting to
assess a channel.In unslotted or slooted systems with the received BLE (Battery Life Extended) subfield set to zero,
BE shall be initialized to the value of macMinBE. In slotted systems with the received BLE subfield set to one, this
value shall initialized to the lesser of two and value of macMinBE. if macMinBE is set to zero, collision avoidance
will be disabled during the first iterarion of this algorithm.
Figure of CSMA-CA algorithm and detail explanation can be read from standard on page 170-171.
Darkness in the Jungle,
January 28 2008 – Winter Vacation,
High Speed Network Lab