The MAC sublayer provides two services: the MAC data service and the MAC management service interfacing to the MAC sublayer management entity (MLME) service access point (SAP) (MLMESAP). The MAC data service enables the transmission and reception of MAC protocol data units (MPDU) across the PHY data service. The features of MAC sublayer are beacon management, channel access, GTS management, frame validation, acknowledged frame delivery, association and disassociation.
1 Superframe Structure
LR-WPAN allows the optional use of a superframe structure. The format of the superframe is
defined by the coordinator. The superframe is bounded by network beacons and is divided into equally sized slots. The beacon frame is sent in the first slot of each superframe. If a coordinator does not want to use the superframe structure, it may turn off the beacon transmissions. The beacons are used to synchronize the attached devices, to identify the PAN and to describe the structure of superframes.
The superframe can have an active and an inactive portion. During the inactive portion, the coordinator shall not interact with its PAN and may enter a low-power mode. The active portion portion consists of contention access period (CAP) and contention free period (CFP). Any device wishing to communicate during the CAP shall compete with other devices using a slotted CSMACA mechanism. On the other hand, the CFP contains guaranteed time slots (GTSs). The GTSs always appear at the end of the active superframe starting at a slot boundary immediately following the CAP. The PAN coordinator may allocate up to seven of these GTSs and a GTS can occupy more than one slot period.
The duration of different portions of the superframe are described by the values of macBeaconOrder and macSuperFrameOrder. macBeaconOrder describes the interval at which the coordinator shall transmit its beacon frames. The beacon interval, BI, is related to the macBeaconOrder, BO, as follows: BI = aBaseSuperFrameDuration2BO, 0 BO 14. The superframe is ignored if BO = 15.
The value of macSuperFrameOrder describes the length of the active portion of the superframe. The superframe duration, SD, is related to macSuperFrameOrder, SO, as follows: SD = aBaseSuperFrameDuration2SO, 0 SO 14. If SO = 15, the superframe should not remain active after the beacon. The active portion of each superframe is divided into a aNumSuperFrameSlots equally spaced slots of duration 2SOaBaseSlotDuration and is composed of three parts: a beacon, a CAP and
Figure 1: An example superframe structure.CFP. The beacon is transmitted at the start of slot 0 without the use of CSMA. The CAP starts immediately after the beacon. The CAP shall be at least aMinCAPLength symbols unless additional space is needed to temporarily accommodate the increase in the beacon frame length to perform GTS maintenance. All frames except acknowledgement frames or any data frame that immediately follows the acknowledgement of a data request command that are transmitted in the CAP shall use slotted CSMA-CA to access the channel. A transmission in the CAP shall be complete one IFS period before the end of the CAP. If this is not possible, it defers its transmission until the CAP of the following superframe. An example superframe structure is shown in Figure 1. The CFP, if present, shall start on a slot boundary immediately following the CAP and extends to the end of the active portion of the superframe. The length of the CFP is determined by the total length of all of the combined GTSs. No transmissions within the CFP shall use a CSMA-CA mechanism. A device transmitting in the CFP shall ensure that its transmissions are complete one IFS period before the end of its GTS. IFS time is the amount of time necessary to process the received packet by the PHY. Transmitted frames shall be followed by an IFS period. The length of IFS depends on the size of the frame that has just been transmitted. Frames of up to aMaxSIFSFrameSize in length shall be followed by a SIFS whereas frames of greater length shall be followed by a LIFS The PANs that do not wish to use the superframe in a nonbeacon-enabled shall set both macBeaconOrder and macSuperFrameOrder to 15. In this kind of network, a coordinator shall not transmit any beacons, all transmissions except the acknowledgement frame shall use unslotted CSMA-CA to access channel, GTSs shall not be permitted.
2 CSMA-CA Algorithm
If superframe structure is used in the PAN, then slotted CSMA-CA shall be used. If beacons are not being used in the PAN or a beacon cannot be located in a beacon-enabled network, unslotted CSMA-CA algorithm is used. In both cases, the algorithm is implemented using units of time called backoff periods, which is equal to aUnitBackoffPeriod symbols. In slotted CSMA-CA channel access mechanism, the backoff period boundaries of every device in the PAN are aligned with the superframe slot boundaries of the PAN coordinator. In slotted CSMA-CA, each time a device wishes to transmit data frames during the CAP, it shall locate the boundary of the next backoff period. In unslotted CSMA-CA, the backoff periods of one device do not need to be synchronized to the backoff periods of another device. Each device has 3 variables: NB, CW and BE. NB is the number of times the CSMA-CA algorithm was required to backoff while attempting the current transmission. It is initialized to 0 before every new transmission. CW is the contention window length, which defines the number of backoff periods that need to be clear of activity before the transmission can start. It is initialized to 2 before each transmission attempt and reset to 2 each time the channel is assessed to be busy. CW is only used for slotted CSMA-CA. BE is the backoff exponent, which is related to how many backoff periods a device shall wait before attempting to assess the channel. Although the receiver of the device is enabled during the channel assessment portion of this algorithm, the device shall discard any frames received during this time. In slotted CSMA-CA, NB, CW and BE are initialized and the boundary of the next backoff period is located. In unslotted CSMA-CA, NB and BE are initialized (step1). The MAC layer shall delay for a random number of complete backoff periods in the range 0 to 2BE − 1 (step 2) then request that PHY performs a CCA (clear channel assessment) (step 3). The MAC sublayer shall then proceed if the remaining CSMA-CA algorithm steps, the frame transmission, and any acknowledgement can be completed before the end of the CAP. If theMAC sublayer cannot proceed, it shall wait until the start of the CAP in the next superframe and repeat the evaluation. If the channel is assessed to be busy (step 4), the MAC sublayer shall increment both NB and BE by one, ensuring that BE shall be no more than aMaxBE. In slotted CSMA-CA, CWcan also be reset to 2. If the value of NB is less than or equal to macMaxCSMABackoffs, the CSMA-CA shall return to step 2, else the CSMA-CA shall terminate with a Channel Access Failure status. If the channel is assessed to be idle (step 5), in a slotted CSMA-CA, the MAC sublayer shall ensure that contention window is expired before starting transmission. For this, the MAC sublayer first decrements CW by one. If CW is not equal to 0, go to step 3 else start transmission on the boundary of the next backoff period. In the unslotted CSMA-CA, the MAC sublayer start transmission immediately if the channel is assessed to be idle. The whole CSMA-CA algorithm is illustrated in Figure 2.
3 Data Transfer model
Three types of data transfer transactions exist: from a coordinator to a device, from a device to acoordinator and between two peer devices. The mechanism for each of these transfers depend on whether the network supports the transmission of beacons. When a device wishes to transfer data in a nonbeacon-enabled network, it simply transmits its data frame, using the unslotted CSMA-CA, to the coordinator. There is also an optional acknowledgement at the end. When a device wishes to transfer data to a coordinator in a beacon-enabled network, it first listens for the network beacon. When the beacon is found, it synchronizes to the superframe structure. At the right time, it transmits its data frame, using slotted CSMA-CA, to the coordinator. There is an optional acknowledgement at the end as shown in Figure 4.4. The applications transfers are completely controlled by the devices on a PAN rather than by the coordinator. This provides the energy-conservation feature of the ZigBee network. When a coordinator wishes to transfer data to a device in a beacon-enabled network, it indicates in the network beacon that the data message is pending. The device periodically listens to the network beacon, and if a message is pending, transmits a MAC command requesting this data, using slotted CSMA-CA. The coordinator optionally acknowledges the successful transmission of this packet. The pending
Figure 2: The CSMA-CA algorithm.
data frame is then sent using slotted CSMA-CA. The device acknowledged the successful reception of the data by transmitting an acknowledgement frame. Upon receiving the acknowledgement, the message is removed from the list of pending messages in the beacon When a coordinator wishes to transfer data to a device in a nonbeacon enabled network, it stores the data for the appropriate device to make contact and request data. A device may make contact by transmitting a MAC command requesting the data, using unslotted CSMA-CA, to its coordinator at an application-defined rate. The coordinator acknowledges this packet. If data are pending, the coordinator transmits the data frame using unslotted CSMA-CA. If data are not pending, the coordinator transmits a data frame with a zero-length payload to indicate that no data were pending. The device acknowledges this packet a. In a peer-to-peer network, every device can communicate with any other device in its transmission radius. There are two options for this. In the first case, the node will listen constantly and transmit its data using unslotted CSMA-CA. In the second case, the nodes synchronize with each other so that they can save power.
