The authors established a realistic environment for the preliminary performance evaluation of the IEEE 802.15.4 wireless networks. They did several sets of practical experiments to study its features including, 1) the direct and direct data transmission, 2) CSMA-CA mechanism, 3) data payload size, 4) beacon-enabled mode.
A device in an 802.15.4 network can use either a 64-bit IEEE address or a 16-bit short address assigned during the association procedure, and a single 802.15.4 network can accommodate up to 65535 (2^16 – 1) devices (the address 0xFFFF is hold for broadcast).
this paper also provide a brief description about Function devices of IEEE 802.15.4, network topology, beacon and superframe structure. we can check these items from standard IEEE 802.15.4.
the CSMA_CA algorithm shall be used before the transmission of data of MAC command frames transmitted within the CAP. The IEEE 802.15.4 uses two types of channel access mechanism, depending on the network configuration.
- Unslotted CSMA-CA :
Nonbeacon-enabled networks use this channel access mechanism. If a device want to transmit data frames or MAC commands, it shall wait for a random period. If the channel is found to be idle, following the random backoff, the device shall transmit its data. If the channel is found to be busy, following the random backoff, the device shall wait for another random period before trying to access the channel again.
- Slotted CSMA-CA :
Beacon-enabled networks use this channel access mechanism, where the backoff slots are aligned with the start of the beacon transmission. Each time a device want to transmit data frame during the CAP, it shall locate the boundary of the next backoff slot and then wait for a random number of backoff slots. If the channel is busy, following this random backoff, the device shall wait for another random number backoff slots before trying to access the channel again. If the channel is idle, the device can begin transmitting on the next available backoff slot boundary.
in both cases, the algorithm is implemented using units of time called backoff periods, where one backoff period shall be equal to a constant,i.e aUnitBackoffPeriod (20 symbols). The maximum number of backoffs the CSMA-CA algorithm will attempt before declaring a channel access failure,i.e. macMaxCSMABackoffs, can be varied between 0 and 5 (4 in default). CSMA-CA algorithm shall not be used for the transmission of beacon frames, acknowledgment frames, or data frames transmitted in the CFP.
Data Transfer Models
in this paper, data transfer model is divided into two kinds of models : Direct data transmission and Indirect data transmission.
- Direct data transmission :
This is the mechanism to transfer data from a device to a coordinator.
in beacon-enabled mode : when a device wishes to transfer data to a coordinator, it first listens for the network beacon , when the beacon is found, the devicesynchronizes to the superframe structure. At the appropriate point, the device transmit its frame, using slooted CSMA-CA, to the coordinator. The coordinator acknowledges the succesful reception of the data by transmitting an acknowledgment frame.
in nonbeacon-enabled mode : when a device want to transmit data, it directly transmits its data frame, using unslotted CSMA-CA, to coordinator. the coordinator transmits acknowledgment frame to acknowledges the successful reception.
- Indirect data transmission :
This is the mechanism for transferring data from a coordinator to a device.
In a beacon-enabled network : when the coordinator wishes to transfer data to a device, it indicates that data message is pending in the beacon. The device periodically listens to the network beacon and, if a message is pending, transmits a MAC command requesting the data, using slotted CSMA-CA. the coordinator acknowledges the successful reception of data request by transmitting an acknowledgment frame.then, the coordinator sent the pending data frame to device using slotted CSMA-CA. the device acknowledges the successful data reception by transmitting an acknowledgment frame.
In nonbeacon-enabled network :
when a coordinator wishes to transfer data to a device, it stores the data for the appropriate device to make contact and request the data. A device make contact by transmitting a MAC command requesting the data to its coordinator at an appliation-defined polling rate using unslotted CSMA-CA. The coordinator acknowledges the successful reception of the data request by transmitting an acknowledgment frame. If data are pending, the coordinator transmits the data frame to the device 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 the successful reception of data frame by transmitting an acknowledgment frame to the coordinator.
Data Frame Format :
IEEE 802.15.4 define four frame types : beacon, command, acknowledgment and data frame.
the maximum MAC data payload (that is the maximum size of MSDU), aMaxMACFrameSize, is equal to aMaxPHYPacketSize (127 bytes) – aMaxFrameOverhead (25 bytes) = 102 bytes.
Experimental Hardware :
two IEEE 802.15.4 development boards are used as a coordinator and a network device. The boards contain an IEEE 802.15.4 RF transceiver with necessary support components. Also, one evaluation board is used as a packet sniffer the frame to monitor. the other three IEEE 802.15.4 boards are used as the traffic load generators.
Experimental Configuration :
– Direct and Indirect data transmission
– Effects of CSMA-CA Mechanism
– Effects of Data Payload Size
– Effects of Beacon-Enabled Mode
Future Works :
Evaluation of the power consumption, association time, under different duty cycles, longer distances, multi-hop conditions, a tree or a peer-to-peer topology, and more device nodes, the interference issue. evaluate features of the ZigBee platform stack such as the security, binding time, and routing performance.
Performance Evaluation of IEEE 802.15.4 for Low-Rate Wireless Personal Area Networks
IEEE Transactions on Consumer Electronics.
Note : This resume is created for self-learning only. Author and Publisher hold copyrights
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February 22 2008,
High Speed Network Lab